Ovpd and Ovjp for Electronic Optoelectronic Device Applications

Organic Vapor Phase Deposition and

Vapor Jet Printing for

Electronic and Optoelectronic Device Applications

Max Shtein

A DISSERTATION

PRESENTED TO THE FACULTY

OF PRINCETON UNIVERSITY

IN CANDIDACY FOR THE DEGREE

OF DOCTOR OF PHILOSOPHY

RECOMMENDED FOR ACCEPTANCE

BY THE DEPARTMENT OF

CHEMICAL ENGINEERING

NOVEMBER 2004

UMI Number: 3143423

Copyright 2004 by

Shtein, Max

All rights reserved.

________________________________________________________

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Copyright 2004 ProQuest Information and Learning Company.

All rights reserved. This microform edition is protected against

unauthorized copying under Title 17, United States Code.

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ii

To my Mother

iii

Abstract

Weak van der Waals bonding in molecular organic semiconductors allows depositing

them without lattice matching on a variety of substrates, (e.g. glass, steel foil, and

plastic), for low-cost, large-area device applications. As the device performance

improves, lowering fabrication costs becomes important. Organic Vapor Phase

Deposition (OVPD) and Organic Vapor Jet Printing (OVJP) may accomplish this, while

also presenting scientifically interesting mechanisms of thin-film growth.

In OVPD, a hot inert carrier gas picks up molecular organic vapor and transports

it into a hot-wall chamber, where the vapor selectively physisorbs onto a cooled

substrate. The film deposition rate, uniformity, composition and morphology are

controlled through the source and substrate temperatures, carrier gas flow rate, the source

cell and the deposition chamber pressures. The composition and morphology of the

deposited films bear directly on the electrical and optical device performance. Theory,

simulation, and experiments are used to understand the mechanisms governing OVPD

and demonstrate the method's capabilities.

Applications like full-color displays or transistor circuits require lateral patterning

of the active organic thin films. Because organic semiconductors are typically

incompatible with conventional patterning methods (e.g. photolithography), alternative

techniques are employed. In-situ patterning using shadow-masks is studied. For OVPD,

this involves Monte-Carlo modeling of molecular transport in confined geometries,

where the apparatus dimensions are on the order of the molecular mean free path.

Optimum operating conditions (e.g. pressure, mask-substrate separation) and mask

aperture geometry are suggested and verified by patterning experiments.

iv

Using the knowledge thus gained, OVJP is developed. Here, the light carrier gas

mixes with the heavier organic molecules and is rapidly ejected through a collimating

nozzle onto a proximally located cooled substrate. OVJP proceeds entirely in the gas

phase, eliminating the shortcomings associated with liquid-based ink-jet printing,

enabling high-resolution, rapid and direct printing of molecular organic semiconductors.

A theory of the flow is developed and verified by direct simulation Monte-Carlo models

and printing experiments, showing how pressure gradients, nozzle geometry, distance to

the substrate, and choice of carrier gas control the pattern resolution. A high performance

pentacene TFT is printed at ultra-high local deposition rate.

v

"Straight ahead of him, nobody can go very far..."

"The most important things are invisible to the eye."

- Antoine de Saint-Exupery: The Little Prince

Acknowledgements

To my supervisors, Stephen Forrest and Jay Benziger, I am grateful for stimulating

discussions, a balance of ripe opportunity, vigorous motivation and high standards in

research. Their guidance and more than a pinch of patience saw me through equipment

meltdowns and long detours to eventual progress. From my days at Berkeley I have

valued Enrique Iglesia's sound advice and encouragement. Committee members Sandra

Troian and Morton Kostin have suggested improvements to this thesis. Mark Thompson

has frequently offered insights into the chemistry that is at the heart of what we do, while

much of the "real-world" context was thanks to Julie Brown and Teddy Zhou of

Universal Display. Vicki Paulus and Robin Block helped navigate Princeton's

administrative muddy waters.

Paul Burrows, Vladimir Bulovic, and Marc Baldo were my first mentors in the lab

and in the basics of organic semiconductors. I am indebted to Herman Gossenberger for

teaching me the value of a thorough and careful experiment. In the trenches, I learned

alongside and from: Shashank Agashe, Shubo Datta, Brian D'Andrade, Jon Mapel, Vinod

Menon, Gautam Parthasarathy, Barry Rand, Pavel Studenkov, John Thomson,

Hongsheng Wang, Jian Wei, Fengiang Xia, Jiangeng Xue, and others. Many an evening I

vi

spent with Russ Holmes, Fan Yang, Changsoon Kim, Peter Peumans demystifying

remified data, alternating lively scientific discussions with vociferous commiserations

and assuaging words. My greatest education and fun undoubtedly came from the daily

(and nightly) discussions and collaborations with Peter and Changsoon. The benefits of

their insights, sincere critiques, humor and friendship I cannot emphasize enough.

Outside the lab, Changsoon's nuanced taste in music and Peter's companionship

on the bike made my life infinitely more enjoyable. Thanks to Tanya Nigam for her

hospitality, excellent cuisine and encouragement; to Paru Deshpande for a receptive ear

and excellent jinnantonix; to Audrey Lee for the bright colors and excellent cooking; to

Mohsen Moayer for the flagrant disregard of the speed limit; to Bart Pindor for the

perfectly timed mixture of the sublime and the absurd; to Upma Sharma for the sincere

conversation and the code to Lapidus. Along the way, other friends gave me care and

support: Gwen Barriac, Benjie Chen, Ben Fisher, Joelle Frechete, Katherine Gibson, Nate

Gleason, Phaedon (Steve) Koutsourelakis, Julie Lichty, Simon Mui, Joanna Paja, Thomas

Philip, Laura Stark, Lenka Tucek, Mike Weaver, and others. The enduring ameliorative

counsel and friendship of Helen Shvets through the years deserve more gratitude than I

can express here. While a few personalities on occasion expand to fill the available space,

none do so in a manner as beautiful and engaging as hers.

Finally, I thank my family for their support and faith in me and pride in my

accomplishments. In particular, I dedicate this thesis to my Mother, who was and remains

my greatest teacher.

vii

Table of Contents

Abstract .......................................................................................................................... iii

Acknowledgements......................................................................................................... v

Table of Contents.......................................................................................................... vii

List of Figures ............................................................................................................... xii

List of Tables ............................................................................................................. xviii

Symbols........................................................................................................................ xix

Chapter 1: Introduction to organic semiconductors and devices................................. 1

1.1 Overview............................................................................................................... 1

1.2 Classification of Solids ......................................................................................... 2

1.3 Advantages and disadvantages of organic electronics.......................................... 4

1.4 Electronic structure and properties of molecular organic semiconductors ........... 6

1.4.1 Intramolecular bonding.................................................................................. 7

1.4.2 Intermolecular interactions .......................................................................... 14

1.4.3 Electronic conduction in conjugated organic solids .................................... 17

1.4.4 Electronic excitations in organic solids ....................................................... 24

1.5 Summary ............................................................................................................. 33

Chapter 2: Organic device fabrication technology...................................................... 35

2.1 Overview............................................................................................................. 35

2.2 Spin-on film deposition....................................................................................... 38

2.3 Laser induced thermal imaging........................................................................... 41

viii

2.4 Ink-jet printing .................................................................................................... 42

2.5 Vacuum thermal evaporation.............................................................................. 45

2.6 Organic vapor phase deposition.......................................................................... 52

2.7 Organic vapor jet printing................................................................................... 56

Chapter 3: Organic Vapor Phase Deposition............................................................... 58

3.1 Overview............................................................................................................. 58

3.2 OVPD Concept ................................................................................................... 58

3.3 Theory – Evaporation ......................................................................................... 60

3.4 Vapor transport ................................................................................................... 64

3.5 Deposition ........................................................................................................... 71

Chapter 4: Proof of Concept & Experimental Verification of Theory...................... 75

4.1 Overview............................................................................................................. 75

4.2 Experimental design............................................................................................ 75

4.3 Growth of Alq3 films .......................................................................................... 77

4.4 Evaporation rate and decomposition temperature............................................... 80

4.5 Thermal decomposition of source materials ....................................................... 83

4.6 Deposited film morphology and composition..................................................... 87

4.7 Growth of OLEDs using OVPD ......................................................................... 89

4.8 Control of dopant concentration using temperature............................................ 91

4.9 Roll-to-roll deposition......................................................................................... 93

4.10 Summary ........................................................................................................... 96

Chapter 5: Growth in confined geometries, application to patterning...................... 97

5.1 Overview............................................................................................................. 97

ix

5.2 The need for patterning – OLED example.......................................................... 97

5.3 Ballistic transport in VTE and shadow-masking ................................................ 99

5.4 Shadow-masking and diffusive transport in OVPD.......................................... 100

5.5 Continuum-based analysis is inaccurate for Kn < 10 ....................................... 103

5.6 Modeling gas transport in confined geometries................................................ 105

5.7 Monte-Carlo simulation of OVPD through apertures....................................... 106

5.7.1 Simulation set-up ....................................................................................... 106

5.7.2 First results for patterning of Alq3 thin films............................................. 108

5.7.3 Effects of chamber pressure on deposit shape ........................................... 110

5.7.4 Effects of mask thickness and separation .................................................. 111

5.7.5 Optimizing mask (aperture) shape ............................................................. 113

5.8 Experimental set-up .......................................................................................... 117

5.9 Shadow-masking experiments – results and discussion ................................... 120

5.10 Resolution limits and self-aligned contacts by hybrid VTE-OVPD............... 124

5.11 Summary ......................................................................................................... 126

Chapter 6: Organic Vapor Jet Printing...................................................................... 128

6.1 Overview........................................................................................................... 128

6.2 OVJP Concept................................................................................................... 128

6.3 OVJP theory...................................................................................................... 130

6.4 Simulation of transitional flow regime ............................................................. 134

6.5 Experimental set-up .......................................................................................... 138

6.6 Direct printing of patterned molecular organic thin films ................................ 141

6.7 OVJP of polycrystalline pentacene films and TFTs ......................................... 148

x

6.8 Summary ........................................................................................................... 153

Chapter 7: Growth of pentacene films and thin film transistors ............................. 154

7.1 Overview........................................................................................................... 154

7.2 TFT geometry and operation ............................................................................ 155

7.3 Growth of polycrystalline pentacene thin films from the vapor phase............. 160

7.3.1 Qualitative description of vacuum and vapor phase growth...................... 160

7.3.2 Theory of crystal growth on surfaces......................................................... 164

7.3.3 Role of background carrier gas in crystal growth...................................... 169

7.4 Growth mechanisms for pentacene................................................................... 171

7.5 Evidence for pentacene morphology influencing TFT performance................ 173

7.6 Relative effects of grain size and substrate treatment on device performance . 177

7.7 Effect of surface energy on device performance .............................................. 184

7.8 Further remarks................................................................................................. 186

7.9 Summary ........................................................................................................... 187

Chapter 8: Summary, state-of-the-art, challenges and future directions................ 188

8.1 Organic semiconductors and devices................................................................ 188

8.2 Organic semiconductor processing technology ................................................ 189

8.3 Development and application of OVPD ........................................................... 190

8.4 Development and application of OVJP............................................................. 192

8.5 Future directions in carrier-assisted deposition, novel devices ........................ 193

8.5.1 Carrier-assisted deposition of metals ......................................................... 193

8.5.2 Growth of focal plane arrays using OVJP ................................................. 194

8.5.3 Alternative device form factors, fiber photovoltaics ................................. 196

xi

8.5.4 Large-volume, ultra-purification................................................................ 198

8.5.5 Non-cleanroom device processing............................................................. 199

8.6 Summary ........................................................................................................... 200

Chapter 9: References .................................................................................................. 202

xii

List of Figures

Chapter 1:

1-1 Classification of materials 3

1-2 Examples of organic light emitting technology 5

1-3 Flexible organic electronics 6

1-4 Formation of molecular orbitals 7

1-5 Bond length vs. bond strength 9

1-6 MO energy diagram from methane 11

1-7 sp2 hybridization in ethylene 12

1-8 Energy levels of linear acenes 13

1-9 Van der Waals interactions 15

1-10 Lennard-Jones potential shape 17

1-11 Classical semiconductor energy band diagram 18

1-12 STM image of a Cl-CuPc crystal 20

1-3 Charge traps in anthracene 23

1-14 Frank-Condon shift 25

1-15 Excitons in organic crystals 27

1-16 Energy level diagrams of a heterojunction PV cell and an OLED 28

1-17 Triplet and singlet excitons 29

1-18 CIE diagram of cyclometalated platinum compounds 33

xiii

Chapter 2:

2-1 Cost of flexible electronics and processing temperature 37

2-2 Spin-coating 39

2-3 Laser-induced thermal imaging 42

2-4 Thermal, piezoelectric, and acoustic ink-jet printing 43

2-5 Ink-jet printed full-color display 45

2-6 Schematic of vacuum thermal evaporation (VTE) 46

2-7 Schematic of a full-color display pixel structure 47

2-8 Shortcomings of and remedies for point-source VTE 49

2-9 Linear sources for VTE 50

2-10 Applied Films GMBH linear source VTE system 51

2-11 OVPD Concept 53

2-12 Commercial scale OVPD 55

2-13 OVJP Concept 57

Chapter 3:

3-1 Schematic of OVPD mechanism 59

3-2 Source evaporation cell 60

3-3 Source cell operating regimes 61

3-4 Vapor transport and mixing schematic 67

3-5 Navier-Stokes modeling of mass and heat transfer in OVPD 70

3-6 Velocity, temperature, and concentration profiles in OVPD 72

3-7 Alq3 deposition rate vs. carrier gas flow (Aixtron OVPD) 74

xiv

Chapter 4:

4-1 OVPD system schematic and temperature profile 76

4-2 Photograph of OVPD system assembly 78

4-3 Alq3 deposition rate vs. carrier gas flow rate (Princeton OVPD) 80

4-4 Thermogravimetric determination of vaporization enthalpy 81

4-5 DCM2 deposition rate vs. temperature 84

4-6 Digital scanning calorimetry of common OLED materials 86

4-7 AFM images of α-NPD deposited on Si at different rates 88

4-8 Photoluminescence spectrum of an OVPD-grown Alq3 film 89

4-9 Electrical characteristics of an OVPD-grown OLED 90

4-10 Photoluminescence and composition of DCM2-doped Alq3 films 92

4-11 Clean and efficient growth in OVPD 94

4-12 Roll-to-roll deposition in OVPD 95

Chapter 5:

5-1 Schematic of a pixel in a full-color passive matrix display 98

5-2 Schematic of vacuum thermal evaporation & shadowmasking 100

5-3 Schematic of OVPD and shadowmasking 101

5-4 Geometry definition of the mask aperture and deposit 102

5-5 Pressure-dependence of the patter resolution using Monte-Carlo 109

5-6 Pixel shape factor vs. molecular mean free path 110

5-7 Deposit shape vs. mask thickness 111

xv

5-8 Deposit shape vs. mask-substrate separation 112

5-9 Deposit shape vs. mask aperture shape 114

5-10 Optimization of mask design for pattern sharpness 115

5-11 Schematic of mask & substrate assembly used in the experiment 117

5-12 Micrographs of the masks used in the experiment 119

5-13 Determination of deposit shape using light interference 120

5-14 Experimental deposit profile vs. mask-substrate separation 121

5-15 Comparison of simulated and experimental deposit profiles 122

5-16 Sub-10µm feature experimental and simulated deposit profiles 123

5-17 SEM of OVPD-VTE self-aligned hybrid deposition 125

Chapter 6:

6-1 Organic vapor jet printing (OVJP) apparatus schematic 129

6-2 Nozzle eometry and the OVJP mechanism 131

6-3 Qualitative dependence of pattern size on process variables 134

6-4 Direct simulation Monte-Carlo (DSMC) modeling of OVJP 136

6-5 Deposit profiles vs. process parameters obtained by DSMC 137

6-6 Schematic of the OVJP experimental set-up 140

6-7 Micrographs of the nozzles used for OVJP experiments 141

6-8 Alq3 dots printed on Si vs. nozzle-substrate separation (using N2) 142

6-9 Alq3 dots printed on Si vs. nozzle-substrate separation (using He) 143

6-10 Comparison of pattern resolution achieved with N2 vs. He 144

6-11 Printed pattern resolution vs. downstream pressure 146

xvi

6-12 Alq3 bicyclist figure printed on Si showing 1000 dpi resolution 147

6-13 SEMs and x-ray diffraction pattern of pentacene grown by OVJP 150

6-14 Transfer characteristic of an OVJP-grown pentacene TFT 151

Chapter 7:

7-1 Schematic of a thin-film field effect transistor 156

7-2 Energy band diagram, operation, and transfer curve of a TFT 157

7-3 Top- and bottom-contact device geometry 159

7-4 Vacuum vs. vapor phase growth 162

7-5 Modes and mechanism of crystal growth on surfaces 163

7-6 Micrograph of OVPD-grown pentacene on SiO2 and gold 167

7-7 Influence of carrier gas pressure on crystal growth 170

7-8 X-ray diffraction pattern of OVPD-grown pentacene on SiNx 172

7-9 Transfer characteristic of an OVPD-grown pentacene TFT 174

7-10 Nucleation in the contact regions for bottom-contact TFTs 175

7-11 Hole mobility vs. deposition conditions for bottom-contact TFTs 176

7-12 Pentacene morphology & mobility on SiO2 vs. OTS-treated SiO2 178

7-13 VTE-grown pentacene on OTS / SiO2 179

7-14 Transfer characteristic of a pentacene TFT on OTS / SiO2 181

7-15 X-ray diffraction patterns of pentacene on OTS vs. SiO2 182

7-16 Pentacene TFT performance vs. dielectric surface energy 185

xvii

Chapter 8:

8-1 Flow-through source configuration for a commercial-scale OVPD 191

8-2 Phosphorescent OLEDs grown by a commercial OVPD system 192

8-3 The Petzval condition and retinal implants 195

8-4 Structure of a flexible photovoltaic (solar) fiber 197

8-5 Possible fabrication sequence for the photovoltaic fiber 198

xviii

List of Tables

Table I: Physical property comparison between germanium and anthracene ....................21

Table II: Evaporation enthalpies of typical OLED materials............................................83

Table III: Effects of shadow mask geometry and process conditions on pattern shape..116

Table IV: Summary of electrical characteristics of pentacene TFTs...............................186

xix

Symbols

α sticking coefficient; mask aperture wall angle

a nozzle radius

Ae electron affinity; evaporation area

al lattice constant

c vapor concentration

χ average dispersion distance

δ film or boundary layer thickness

d molecular collision diameter

∆G Gibbs free energy

∆Hdes enthalpy of desorption

∆Hvap evaporation enthalpy

Di diffusivity of i

dm deposition chamber diameter; mixing dimeter

Ds surface diffusion coefficient

∆Sdes entropy of desorption

ε0 permittivity of vacuum

Ec, Ev conduction / valence band energy

Edes desorption activation energy

EF Fermi energy

xx

Esd surface diffusion activation energy

h Planck's constant, source-to-mask distance

ηdep deposition efficiency

IDS drain-source current

Ip ionizatin potential

jorg flux of organic molecules to the substrate

kB Boltzmann constant

Kn Knudsen number

λ mean free path

l evaporation source width

L nozzle length

Lm mixing length

µ viscosity

mcg, mo molecular mass of the carrier gas / organic

me*, mh* electron / hole effective mass

µeff, µh effective / hole mobility

Morg molecular mass of organic species

Ms spin angular momentum

ms spin quantum number

n0, p0 electron / hole concentration

Nc, Nv conduction / valence band density of states

Pcell evaporation source cell pressure

Pcoll probability of molecular collision

xxi

Pdep deposition chamber pressure

Pdyn dynamic pressure

Peq equilibrium vapor pressure

PH, PL upstream and downstream pressure

Porg organic vapor pressure

Porgs organic vapor pressure at the substrate surface

QD net deposition rate

r distance

rcond condensation rate

rdep deposition rate

revap evaporation rate

rout outflow rate

rD distance traveled by molecule due to diffusion

Re Reynolds number

S subthreshold slope

s mask-substrate or nozzle-substrate separation

T temperature

t mask thickness, time

Tcell evaporation cell temperature

τD characteristic diffusion time

Ts surface temperature

U bulk flow velocity

ū mean molecular thermal velocity; average flow velocity in nozzle

xxii

V•

carrier gas volumetric flow rate

VDS drain-source voltage

VT threshold voltage

ω angular velocity

w mask aperture width

w(r) interaction potential

ψ electron wavefunction

ZA,B nuclear charge

1

Chapter 1: Introduction to organic

semiconductors and devices

1.1 Overview

In this chapter the class of materials termed organic semiconductors is defined. The

molecular structure and the resulting electronic and optical properties of thin films and

crystals are reviewed first, followed by a discussion of bonding interactions in these

films. We then briefly review the operating principles of selected organic-based devices,

where control of molecular structure, film morphology and layer thickness determines

device operation. Optical and electronic device physics, coupled with unique material

properties of organic semiconductors drive the development of alternative device

processing methods. In Ch. 2 the existing fabrication techniques are outlined, including

their advantages and shortcomings with regard to precision and potential cost. Solvent-

based polymer device fabrication is typically favored its potential low cost, while vacuum

growth methods for molecular organic devices have typically resulted in high

performance devices through more precise control of the layer structure. Two novel

methods - organic vapor phase deposition and vapor jet printing - are then introduced as

promising alternatives to vacuum growth for the deposition of small molecular organic

electronics. Chapters 3-7 analyze in greater detail these methods. We summarize the

work in Ch. 8, offering some thoughts on new applications and future research directions.

2

1.2 Classification of Solids

Solids can be classified as covalent, ionic, metallic, or molecular, based on the type of

bonding interaction between the constituent atoms or molecules. (Fig. 1-1) Traditionally,

electronic and optoelectronic devices have been fabricated from covalent materials, such

as silicon, germanium, and Group III-V and II-VI elements (Ga, As, N, In, etc.). Metals

are used as electrical contacts, getters and electrical dopants. In recent years organic

semiconductor materials have been attracting increasing scientific and commercial

interest due to their potential for application in electronic and optoelectronic devices.

As Fig. 1-1 indicates, organic semiconductors belong to the broad class of

molecular solids that can be further subdivided into inorganic and organic. An absolute

definition of a molecular solid is lacking, although Kitaigorodsky (Kitaigorodsky 1973)

proposes one where the distance between atoms within a molecule is smaller than the

distance between atoms in different molecules. And while inorganic molecular solids

exist (e.g. I2 crystals), Kitaigorodsky points out that owing to the multitude of organic

compounds, molecular solids are essentially all organic. The latter are carbon-rich

compounds whose virtually infinite variety of chemical structure and composition is due

to the versatility of combination of compounds based on the carbon bond.

Compounds containing a large number of resonantly alternating sequences of

single and double bonds are called conjugated, and exhibit ground-state delocalization of

charge over the conjugation length, resulting in increased electrical conduction compared

to saturated hydrocarbons. These highly conjugated compounds constitute organic

semiconductors, which further differentiate into small molecules (molecular weight <

1000 amu) and polymers (with molecular weight, mw, typically >> 1000 amu). A more

3

Figure 1-1: a) General classification of solids into 4 types based on bonding character. b) Here, molecular materials are of interest, which subdivide into inorganic and organic. c) Organic materials are typically carbon-rich compounds, ranging from simple gases (e.g. methane) to complex biological macromolecules (e.g. DNA). Organic semiconductors belong to a subgroup of organic compounds which contain a large number of conjugated C-C bonds, which can give rise to electrical conduction. d) Organic semiconductors further differentiate into small molecular and polymeric compounds. Small molecular compounds typically weigh <1000 g/mol and exhibit a well-defined molecular structure, with all molecules in a purified sample identical to each other. In contrast, polymers are characterized by the structure of the monomer subunit, while the individual chain length can vary in a single sample; molecular weights are typically >1000 g/mol.

Organic

Polymeric

MolecularCovalent Ionic Metallic

Inorganic

E.g.: Si, Ge, SiO2

BN, Diamond

E.g.: NaCl, LiF

AlCl3, CuSO4

E.g.: Au, Ag

Cu, Al, Mg

E.g.: Ar, Xe, SF6, H2O, HCl, I2 …

PEDOT PSSC60

PPVa-NPD

Small Molecular

Pentacene

a)

b)

d)

Simple moleculesSaturated HCs

Conjugated HCsMulti-ring Aromatics

Biological

E.g.: C6H12, C2H3Cln- & branched alkanes

E.g.: DNA, RNAProteins, Lipids

c)

Alq3

N N

n

4

rigorous distinction between molecular and polymeric compounds is that the constituent

moieties of a purified small molecular weight compound have well-defined, identical

chemical composition and structure; however, the molecular chains of a polymeric

compound can vary significantly in length, without changing the chemical name or the

basic structure. Hence, in this case, the mw is not well defined. We focus on the small

molecular weight materials, which for convenience will be referred to simply as

molecular, in contrast to polymeric. Despite the extended nature of polymer chain

compounds, many of the important electronic properties are shared with molecular

compounds in thin film form, due to the introduction of disorder and coiling of chains in

the course of thin film deposition. Much of the discussion is therefore also applicable to

polymeric semiconductors.

1.3 Advantages and disadvantages of organic electronics

The appeal of organic electronics lies primarily in the potential ease of processing and the

resulting low cost of certain types of devices (e.g. thin film transistors and solar cells), or

improved architectures for emissive devices (e.g. lighting and flat panel displays; see Fig.

1-2). Due to the relatively high permeability of organic thin films to gases, they are also

candidates for chemical sensing.

Organics – both polymers and small molecules – are called “soft materials,”

referring to the weak bonds in the molecular solid. This means that films can be deposited

without concern for lattice match with the underlying substrate. (Forrest 1997) The film-

substrate adhesion strength remains approximately the same as adhesion of the film

molecules to each other. Consequently, low-cost, large-area, light-weight and flexible

substrates can be used, such as glass and plastic. (Fig. 1-3) Since no chemical bonds need

5

to be broken and reformed to make the organic films stick to the substrate, they can be

deposited at much lower temperatures than e.g. silicon or GaAs. Thus, organic device

processing typically has a low thermal budget, which can decrease the cost and

complexity of fabrication compared to inorganic materials.

Polymeric semiconductors in many cases possess analogous optical and electronic

properties, and can be solution-processed, either by spin-coating or ink-jet printing (See

Ch. 2), effectively addressing the low-cost processing requirement. However, while

solution-based fabrication may at first seem less troublesome than vaporization methods,

devices employing molecular organics as active layers often exhibit superior electrical

and optical characteristics, in part due to the ability to deposit sophisticated, high-

performance multi-layer structures with atomically sharp interfaces. Such structures are

generally difficult to obtain using solution techniques, limiting the scope of application,

and/or shifting the burden of design onto synthetic chemistry to precisely control phase

separation on the nanometer-scale. The arguments in favor of polymers have traditionally

Figure 1-2: a) 13- inch diagonal active matrix display built by Sony Corp. using organic LED technolgoy. (From Forrest 2004). b) White emission OLED illuminating a color strip (inset: white OLED in ambient conditions). (From Adamovic et al. 2003).

b)a)

6

been based on the observation that molecular-organic processing involves more costly

techniques, such as vacuum evaporation, particularly in the large-area device

applications. The vapor phase techniques presented here potentially lend the large-area

and low-cost advantages to small molecular semiconductors.

1.4 Electronic structure and properties of molecular organic

semiconductors

Most of the electronic properties of organic semiconductors, as well as those dictating the

choice of processing techniques arise from the interplay between the inter- and intra-

molecular forces and molecular symmetry. In Sec. 1.4.1 covalent bonding is reviewed,

emphasizing the origin of π-bonds, or electronic conjugation. In Sec. 1.4.2 inter-

molecular interactions are reviewed; together with molecular order and intra-molecular

bonding, they determine electronic and energy transport processes (Sec. 1.4.3 and 1.4.4)

in molecular thin films used in device applications such as OLEDs, PV cells, TFTs. A

discussion of device processing methods is deferred until Ch. 2.

Figure 1-3: a) A passive matrix OLED deposited on plastic (Image courtesy of Universal Display Corporation). b) A pentacene transistor circuit deposited on plastic (Image by Jackson group at Penn State University).

a) b)

Figure 1-3: a) A passive matrix OLED deposited on plastic (Image courtesy of Universal Display Corporation). b) A pentacene transistor circuit deposited on plastic (Image by Jackson group at Penn State University).

a) b)a) b)a) b)

7

1.4.1 Intramolecular bonding

Atoms combine to form molecules typically by sharing unpaired valence electrons; one

typical representation is an energy level diagram, illustrated in Fig. 1-4. Here, two atomic

orbitals (AOs) combine to form two energy-split molecular orbitals (MOs) (Fig. 1-4a).

The lower energy MOs are called bonding, while the higher energy MOs are antibonding,

(e.g. σ and σ*, or π and π*, respectively). If several AOs combine, an equal number of

MOs are created; increasing the number of AOs results in decreased energy spacing

within the bonding orbital manifold, and similarly for the antibonding manifold (Fig. 1-

4b). The Aufbau rule dictates that lowest energy orbitals are filled first, while the Pauli

Figure 1-4: Atomic orbitals (AOs) (s, p) combine to form molecular orbitals (MOs) (σ, π). The MOs with energy lower than the AOs are called bonding, while those higher in energy are called antibonding (designated by a *). Multiple AOs of identical or similar energy combine to form degenerate or closely spaced MOs, forming bands when the energy spacing is significantly smaller than the kT energy. The greater the number of starting AOs, the smaller the energy spacing. The electrons supplied by the atoms fill the MOs according to the Pauli exclusion principle and the Aufbau rule, first populating the bonding orbitals, followed by the antibonding. Hence, they are also referred to as highest occupied MOs (HOMOs) and lowest unoccupied MOs (LUMOs).

s, p s, p

σ, π

σ∗, π∗AO1

AO2

HOMO

LUMO

s, p

σ, π

σ∗, π∗AO

HOMO

LUMO

s, p

AO

a) b)

8

exclusion principle ensures that only up to two electrons can reside in a single orbital,

provided their spins are antiparallel. In a thermodynamically stable molecule, more

electrons occupy the low-energy lying bonding MOs, while the higher energy anti-

bonding MOs remain empty. The highest energy bonding orbital is also called the highest

occupied MO (HOMO), and correspondingly, the lowest energy antibonding MO is

called the lowest unoccupied MO (LUMO). Neutral electronic excitations proceed via a

promotion of an electron from HOMO to LUMO, typically by absorption of light or by

thermal excitation. This will be discussed further in Sec. 1.4.3.

Intra-molecular bond strengths can vary significantly, (e.g. from 1.56 to 11 eV for

I2 and CO, respectively), depending on the complexities of short-range quantum

mechanical electronic and nuclear interactions. Experimentally determined interatomic

bond lengths correlate with the bond enthalpy, as shown in Fig. 1-5 for diatomic

molecules. (Oxtoby et al. 1990) These results can also be readily generalized to

polyatomic molecules, since it is known that the covalent bond length and strength

between two atoms varies <10% in virtually all of the compounds containing this bond.

(Oxtoby et al. 1990) For example, the C-C bond is ~1.525 ± 0.025 Å for compounds

ranging from ethane, to diamond, to benzene, to DNA. This can be important when

considering the thermal budget for device processing. For example, a material’s

evaporation rate will increase exponentially with its temperature, until the kT energy

exceeds the atomic bond energy, in which case the material can undergo pyrolysis. In

general, however, decomposition will occur before the point of bond pyrolysis, due to

reactions catalyzed by impurities, source container, or the material itself.

9

Figure 1-5: Plot and table of bond strength (as indicated by enthalpy) vs. bond length for a range of compounds. These typically vary by <10% from compound to compound. (C=- denotes a triple bond, while C…- a conjugated bond as in benzene. (Source: Oxtoby and Nachtrieb, 1990)

Bond Bond Bond Molecule Length Enthalpy Enthalpy

(A) (kJ/mol) (eV)----------------------------------------------------------------H2 0.751 436 4.52N2 1.1 945 9.79O2 1.211 498 5.16F2 1.417 158 1.64Cl2 1.991 243 2.52Br2 2.286 193 2.00I2 2.669 151 1.56HF 0.926 568 5.89HCl 1.284 432 4.48HBr 1.424 366 3.79HI 1.62 298 3.09ClF 1.632 255 2.64BrF 1.759 285 2.95BrCl 2.139 219 2.27Icl 2.324 211 2.19NO 1.154 632 6.55CO 1.131 1076 11.2C-C 1.536 348 3.61C=C 1.337 615 6.37C=-C 1.204 812 8.41C...-C 1.397 505 5.23

0.5 1 51

10

Bond

Stre

ngth

(eV)

Bond Length (A)

H2

HF

I2F2

CO

C-C(diamond)

0.5 1 51

10

Bond

Stre

ngth

(eV)

Bond Length (A)

H2

HF

I2F2

CO

C-C(diamond)

10

We now discuss briefly the formation of double C-C bonds, which are responsible

for much of the semiconducting character of conjugated organic compounds. Figure 1-6

is an energy level diagram illustrating the combination of the 2s and 2p electrons of

carbon with 1s electrons of hydrogen in the simplest organic molecule, methane (CH4).

Here, carbon forms sp3 hybrid orbitals with tetrahedral symmetry, resulting in 4 energy

degenerate bonding orbitals (σ-bonds) and 4 energy degenerate antibonding orbitals (σ*-

bonds). The electrons from C and H pair up in the lowest energy state, i.e. the σ-bond. A

similar situation exists for ethane (C2H3), except one of the hydrogen atoms is substituted

for a methyl group (-CH3).

In ethylene (C2H4), however, the 2pz atomic orbital (AO) of carbon remains

unchanged, while the remaining 3 electrons form planar sp2 hybrid orbitals with a 120°

rotational symmetry axis. As Fig. 1-7 shows, the resulting molecular orbital (MO) is a

combination of σ-bonds between C and H, and σ- and π-bonds between the two C atoms.

Due to the symmetry of the sp2 hybrid MO, all atoms in C2H4 lie in a plane; this is a

general property of the sp2 hybrid MO.

The chemical structure of benzene (C6H6) (Fig. 1-8a) implies an alternating

sequence of single and double bonds. Half of the C-C bonds are π-, while half are σ-

bonds only. However, the vibrational spectrum of benzene has one peak for the C-C

stretch, corresponding to 5.23 eV bond energy and 1.397 Å bond length, indicating that

all of the C-C bonds are equivalent and intermediate of the C-C and C=C bonds (3.61 eV,

1.536 Å and 6.37 eV, 1.337 Å, respectively). To reflect this observation, a resonant

chemical structure is postulated (Fig. 1-8b), where the π-electrons are delocalized over

all 6 C atoms. Furthermore, as illustrated in Fig. 1-8c, polyacenes (compounds with two

11

Figure 1-6: a) energy level diagram of the 1s 2s 2p AOs of carbon undergoing hybridization to form 2sp3 orbitals. b) The spherical 2s and the dumbell-shaped 2p orbitals combine to form the asymetric dubmell-shaped 2sp3 hybrid orbital. c) Energy level diagram of the carbon-hydrogen bond formation, along with (d) the resultant tetrahedral structure of methane (CH4).

Carbon:

1s

2s

2p

E Hybridization

1s

sp3

E

1s

sp3

Carbon:

1s (x4)

Hydrogen:

σ

σ∗

+ 4x 4x

Methane

a)

s p

Methane

sp3 – s or σ bond

b)

c)

d)

12

Figure 1-7: a) energy level diagram of the 1s 2s 2p AOs of carbon undergoing hybridization to form two sp2 and one p orbitals. b) Formation and energy level diagram of σ MOs between C and H and σ and π MOs between C and C. c) Resulting planar structure of ehtylene (C2H4), with 120° angle between bonds characteristic of the sp2 hybridization.

Carbon:

1s

2s

2p

E Hybridization

1s

2sp2

a)

2p

b)

C C

H

H

H

H

C C

H

H

H

H

c)

1s (x2)1s (x2)

σ, πσ σ

Hydrogen HydrogenCarbonCarbon

E

σ∗

π∗

13

or more benzene rings fused together) exhibit even greater electron delocalization, which

is manifested in the decreasing energy spacing between the HOMO and LUMO states,

∆EHOMO LUMO, decreasing ionization potential, and increasing electron affinity. The

combined effect is to decrease the energy associated with accommodation of excess

charge; that is, a dopant may be ionized either by donating its electron to the host matrix

Figure 1-8: a) Resonant chemical structure of benzene, with an alternating sequence of single and double bonds. In actuality, all of the carbon-carbon bonds are equivalent and intermediate of the C-C and C=C types. b) An alternative representation of benzene, reflecting the equivalency of all carbon-carbon bonds. c) Chemical structure of several polyacenes (naphthalene, anthracene, tetracene, and pentacene), where the increasing spatial delocalization of the electron clouds leads to a decrease in the HOMO LUMO transition energy, ∆EHOMO LUMO, a decrease in the ionization potential, Ip, and a simultaneous increase in the electron affinity, Ae. (Data adapted from Pope & Swenberg, p.204)

a) b)

c)

Compound ∆EHOMOLUMO

5.95 eV

4.34 eV

3.31 eV

2.60 eV

2.14 eV

Ip Ae

9.2 eV

8.2 eV

7.5 eV

7.0 eV

6.7 eV

--

1.8 eV

2.0 eV

2.4 eV

2.9 eV

a) b)

c)

Compound ∆EHOMOLUMO

5.95 eV

4.34 eV

3.31 eV

2.60 eV

2.14 eV

Ip Ae

9.2 eV

8.2 eV

7.5 eV

7.0 eV

6.7 eV

--

1.8 eV

2.0 eV

2.4 eV

2.9 eV

14

or taking it away. In the first case Ae of the host matrix is high, while Ip of the dopant is

low; the dopant behaves as a donor. In the second case, the Ip of the host is low, while Ae

of the dopant is high; the dopant is said to be an acceptor.

1.4.2 Intermolecular interactions

Figure 1-9 lists the different types of interatomic and intermolecular attractive

interactions, along with their functional form. Molecular solids are typically comprised of

neutral molecules, which interact via electrostatic and polarization forces, grouped

together as van der Waals interactions. (Gutmann et al. 1981; Israelachvili 1992; Pope et

al. 1982) The interaction potential w(r) is written generally:

( ) = −n m

A Bw rr r

(0.1)

where r is the separation between the subunits, A and B are constants, n and m are

typically positive integers. The physical origin of first term is the Coulombic repulsion of

molecular electron clouds, while the second term denotes interactions such as the London

dispersion force that the electrons of one molecule feel for the positively charged nuclei

of another. Typically, n > m; the special case of m = 6 and n = 12 is known as the

Lennard-Jones potential, plotted in Fig. 1-10. The interaction energy is infinitely large for

r 0, reflecting the repulsion of molecular electron clouds, and asymptotes to 0 for

infinite separation. The largest negative interaction energy is the point of strongest net

attractive interaction between the molecules, resulting in an equilibrium separation r = r0,

termed the van der Waals radius. The constant B is typically comprised of several others,

depending on the number and type of VDW interactions involved, including dipole-

15

Fig. 1-9: Common types of interactions between atoms, ions, and molecules in vacuum. w(r) is the interaction free energy (in Joules); Q, electric charge (in Coulombs); u, electric dipole moment (in Coulomb·meters); α, electric polarizability (C·m2/J); r, distance between interacting moieties (in m); k, Boltzmann constant; T, temperature (in K); h, Planck’s constant; ν electronic absorption (ionization frequency) (in s-1); ε0, dielectric permittivity of vacuum (in C2/J·m) (Source: Israelachvilli, 1992)

Covalent, Metallic

Charge - charge

Charge - dipole

Dipole - dipole

Charge - non-polar

Dipole – non-polar

Non-polar –non-polar

Hydrogen bond

Type of Interaction Interaction energy w(r)

Complicated, short range

(Coulomb energy)- Q1 Q2 / 4πε0 r

- Q u cos θ / 4πε0 r2

- Q2 u2 / 6 (4πε0)2 kT r4

- u1 u2 [2 cosθ2 cosθ2 –sinθ1 sinθ2 cosφ] / 4πε0 r3

H H

H2

HO

HH2O

Q1 Q2r

u Q2rθ

Fixed dipoleu Q2r

Rotatingu1 rθ1

Fixedu1 r

Rotating

u2θ2

u2

Q αr

u αrθ

Fixedu αr

Rotatingα1 α2r

φ

- u12 u2

2 / 3 (4πε0)2 kT r6

(Keesom energy)

- Q2 α / 2 (4πε0)2 r4

- u2 α (1 + 3 cos2θ) / 2 (4πε0)2 r6

- u2 α / (4πε0)2 r6(Debye energy)

- ¾ hνα2 / (4πε0)2 r6

(London dispersion energy)

Complicated, short range, Energy ∝ - r -2

HO

H H

HO

O

H

HO

H

H

Covalent, Metallic

Charge - charge

Charge - dipole

Dipole - dipole

Charge - non-polar

Dipole – non-polar

Non-polar –non-polar

Hydrogen bond

Type of Interaction Interaction energy w(r)

Complicated, short range

(Coulomb energy)- Q1 Q2 / 4πε0 r

- Q u cos θ / 4πε0 r2

- Q2 u2 / 6 (4πε0)2 kT r4

- u1 u2 [2 cosθ2 cosθ2 –sinθ1 sinθ2 cosφ] / 4πε0 r3

H H

H2

HO

HH2O

Q1 Q2r

u Q2rθ

Fixed dipoleu Q2r

Rotatingu1 rθ1

Fixedu1 r

Rotating

u2θ2

u2

Q αr

u αrθ

Fixedu αr

Rotatingα1 α2r

φ

- u12 u2

2 / 3 (4πε0)2 kT r6

(Keesom energy)

- Q2 α / 2 (4πε0)2 r4

- u2 α (1 + 3 cos2θ) / 2 (4πε0)2 r6

- u2 α / (4πε0)2 r6(Debye energy)

- ¾ hνα2 / (4πε0)2 r6

(London dispersion energy)

Complicated, short range, Energy ∝ - r -2

HO

H H

HO

O

H

HO

H

H

16

dipole, dipole-induced dipole, induced dipole-induced dipole, and hydrogen bonding (see

Fig. 1-9).

The energy range for typical VDW bonds is 10-3 – 10-2 eV (Israelachvili 1992;

Silinsh et al. 1994), two to three orders of magnitude lower as compared to

intramolecular bonds. For a neutral, nonpolar molecule such as pentacene (Fig. 1-8)

embedded in a molecular solid (e.g. molecular thin film) only the London Dispersion

force contributes significantly to w(r). The nominal intermolecular separation is r0 = 3Å,

and a two-fold increase in r results in an 85% reduction in w(r). The energy required to

remove such a molecule from its matrix into the gas phase, enthalpy of vaporization

(∆Hvap) is typically below ~200 kJ/mol, and increases with the molecular size. For

example, for the series of molecules benzene, naphthalene, anthracene, tetracene, and

pentacene (see Fig.1-8), ∆Hvap = 10.7, 15.7-19.6, 22.8-24.4, 28.1-29.8, 33.5-37.7 kJ/mol,

respectively. (Kitaigorodsky 1973) This is significantly lower than the 600-1000 kJ/mol

lattice energy of typical covalent or ionic solids, such as alkali halides, (Oxtoby et al.

1990; Silinsh et al. 1994) allowing the use of relatively low temperatures for the

evaporation of molecular organic compounds. At the same time, the lower adhesion

energy of VdW bonded materials allows the deposition of ordered thin films on a variety

of substrates, whether or not the film and the substrate are lattice-matched. (Forrest 1997)

While the weaker intermolecular adhesion increases the choice of compatible

substrates, it also can restrict the number of compatible techniques of multi-layer growth.

For example, solvent-processing can be difficult; since the absolute value of interaction

energy is small, the difference in the interaction energy between different types of

molecules and solvents is even smaller. This can lead to unwanted penetration of solvent

17

molecules into the underlying thin films, as well as suboptimum selectivity based on

solubility. Clearly, traditional photolithography relying on solvent-processed photoresist

films is a poor choice for organic semiconductors.

1.4.3 Electronic conduction in conjugated organic solids

In traditional, covalent semiconductors where short-range (~1Å) order can persist over

large length scales (10s of microns to 10s of centimeters). This order and the equivalence

of interatomic bonds lead to formation of wide conduction and valence bands, CB and

VB, respectively. (See Fig. 1-11) The CB and VB are separated by a band of forbidden

states, denoted by the energy band gap, Eg. In the absence of ionized impurities,

Fig. 1-10: The Lennard-Jones potential of the form E(r) = A/r12 – B/r6, where r is the intermolecular separation and A and B are material-specific constants. The first term represents a repulsive interaction due to Coulomb repulsion of the electron clouds, while the second term denotes an attractive interaction due to the induced dipole- induced dipole interactions originating from correlated fluctuations of the molecular electron cloud densities. ∆U is the crystal energy and N is the coordination number of a molecule in the crystal

E

r0 ∞

E

r

0

r0

r0 = (2·A/B)1/6

E = A/r12 – B/r6

∆U/N = B2/4A

∆Uc = ∆Hm + ∆Hvap

r

E

r0 ∞

E

r

0

r0

r0 = (2·A/B)1/6

E = A/r12 – B/r6

∆U/N = B2/4A

∆Uc = ∆Hm + ∆Hvap

rr

18

conductivity is said to be intrinsic, with the concentration of negative charge carriers, n0,

given by:

0 exp −

= ⋅ −

CB FC

B

E En Nk T

(1.1)

where ECB and EF are the CB and Fermi-level energies, respectively, kB is the Boltzmann

constant, T is the temperature, and NC is the conduction band effective density of states

given by:

3/ 2*

2

22 ⋅ ⋅

= ⋅

e BC

m k TNh

π (1.2)

Fig. 1-11: Energy band diagram for a traditional semiconductor, where the conduction and valence bands (CB and VB, respectively) are at their respective energy levels, ECB and EVB, separated by an energy gap Eg. The chemical potential of electrons at thermal equilibrium is the Fermi energy, EF. The energy gained by the addition of an electron is the electron affinity, Ae, while Ip is the ionization potential, or the energy required to remove an electron. All energy levels are measured with respect to the vacuum level at E = 0.

CB

VB

E

Vacuum Level

ECB

E=0

Ae

Ip

EVB

EFEg

19

where me* is the electron effective mass, and h is Planck’s constant. Equivalent

expressions can be written for positive charge carriers, or holes:

0 exp F VBV

B

E Ep Nk T

−= ⋅ −

(1.3)

3/ 2*

2

22 h BV

m k TNh

π ⋅ ⋅= ⋅

(1.4)

where EVB is the VB energy, while Nv is the valence band effective density of states, and

mh* is the hole effective mass. The electrical conductivity is given by σ = q (n0·µe +

p0·µh); it is n-type if the ratio n0/p0 > 1, and p-type if n0/p0 < 1.

Since (ECB-EF) > 0, increasing the temperature exponentially increases n0 and p0,

and hence also the electrical conductivity. Note, however, that the carrier mobility, µe,h,

in traditional semiconductors can also vary with temperature. The increasing lattice

vibrations (phonons) at higher temperatures increase the likelihood of carrier-atom

collisions, lowering the intrinsic carrier mobility via µL ~ T-3/2. On the other hand, the

increasing thermal velocity of carriers at higher temperatures causes them to spend less

time in the vicinity of ionized impurities, thereby decreasing the net Coulombic

interaction. This is called ionized impurity scattering, and causes the mobility of carriers

from ionized impurities, µI to vary as T3/2. The effect on the net mobility can be expressed

as:

,

1 1 1= +

e h I Lµ µ µ (1.5)

The situation in organic semiconductors is often quite different. The weak

intermolecular interactions imply that intermolecular electron sharing in most molecular

solids is weak. Indeed, the electron cloud density typically concentrates on the individual

20

atoms and the intramolecular bonds, dropping to zero in the interstices, (Silinsh et al.

1994) as evidenced in the example of a scanning tunneling micrograph of copper

hexadecachloro-phthalocyanine (Cl-CuPc) (Fig. 1-12). Here, the electron cloud

localization on the individual atoms is apparent (dark regions), with substantial sharing

within intermolecular bonds, but little overlap between the atoms of neighboring

molecules. Such electronic structure of the organic condensed phase determines the

electrical and optical properties of these materials, which retain much of their molecular

nature.

Fig. 1-12: Scanning tunneling electron micrograph of Cl-CuPc (copper hexadecachlorophthalocyanine) molecules in the ab-plane of the crystal. (Image from Silinsh and Čápek, 1994, p. 3) where the dark regions indicate higher electron cloud density, and bright regions their absence.

21

Several in-depth texts and reviews discuss the structure, optical and electronic

properties in organic solids (primarily crystals) (Bulovic et al. 2000; Dimitrakopoulos et

al. 2001; Forrest 1997; Gutmann et al. 1981; Horowitz 1999; Pope et al. 1982; Silinsh et

al. 1994). However, these authors agree that a comprehensive theory of electrical

conduction in organic semiconductors has not been developed, and the subject remains

the topic of vigorous research. Here we outline some key features.

The conduction and valence band picture is inaccurate in the case of organic

semiconductors, which exhibit strong localization and weak coupling. Charges must

overcome a potential barrier when migrating from molecule to molecule, and as a result,

Property

Atomic weight (g/mol)Melting point (ºC) Density (g/cm3)Density (molec/cm3)Crystal structureLattice constant (a, Å)Volume compressibility (cm2/dyne)Dielectric constant (static)Intrinsic ionization energy (eV)Intrinsic conductivity at 300K (σ, ohm-1·cm-1)Electron mobility at 300K (µe cm2/V·s)Hole mobility at 300 (µh cm2/V·s)Concentration of intrinsic carriers (cm-3)Thermal expansion coefficient (ºC-1)Thermal conductivity (k, W/cm·ºC)Specific heat (cal/g·ºC)Vacuum ionization energy (eV)

Germanium

72.639375.3

4.42·1022

Diamond5.66

1.3·10-12

160.780.0238001800

5.2·1013

6.1·10-6

10.074

4.8

Anthracene

178.222171.25

0.42·1022

Monoclinic6.04-11.16

10·10-12

3.43.9

~10-22

0.880.44

~10-4

145·10-6

10-3

0.315.8

Table I: Comparison of some physical properties of germanium and anthracene, covalent and molecular semiconductors, respecitvely (from Gutmann & Lyons)

Property

Atomic weight (g/mol)Melting point (ºC) Density (g/cm3)Density (molec/cm3)Crystal structureLattice constant (a, Å)Volume compressibility (cm2/dyne)Dielectric constant (static)Intrinsic ionization energy (eV)Intrinsic conductivity at 300K (σ, ohm-1·cm-1)Electron mobility at 300K (µe cm2/V·s)Hole mobility at 300 (µh cm2/V·s)Concentration of intrinsic carriers (cm-3)Thermal expansion coefficient (ºC-1)Thermal conductivity (k, W/cm·ºC)Specific heat (cal/g·ºC)Vacuum ionization energy (eV)

Germanium

72.639375.3

4.42·1022

Diamond5.66

1.3·10-12

160.780.0238001800

5.2·1013

6.1·10-6

10.074

4.8

Anthracene

178.222171.25

0.42·1022

Monoclinic6.04-11.16

10·10-12

3.43.9

~10-22

0.880.44

~10-4

145·10-6

10-3

0.315.8

Table I: Comparison of some physical properties of germanium and anthracene, covalent and molecular semiconductors, respecitvely (from Gutmann & Lyons)

22

charge conduction is dominated by the electronic structure of the constituent molecules

and thermally assisted hopping between the localized states (Pope et al. 1982). This is the

situation in virtually all molecular organic solids, except in the case of single crystals.

Table I compares some basic physical properties of an inorganic semiconductor

germanium to those of an archetypal organic semiconductor, anthracene. (Pope et al.

1982) Note the order-of-magnitude lower molecular and densities, three orders of

magnitude lower thermal and dramatically lower intrinsic electrical conductivities for

anthracene.

Consider again the series of polyacenes (naphthalene, anthracene, tetracene,

pentacene) in Fig. 1-8c. Here, π-bond conjugation results in the delocalization of

electrons over yet larger space than in the case of benzene, several times the C-C bond

length. By increasing the spatial extent of the electron wavefunction, the electronic

polarizability, α, of the molecule is increased, which increases the intermolecular

adhesion energy, w(r), as denoted in Fig. 1-9. The ionization potential also decreases,

while the electron affinity increases. The electrical activation of a dopant molecule, D, in

a host matrix, H, can be written as a chemical oxidation or reduction reaction:

H + D H- + D+ ∆G = AeH + Ip

D (1.6)

H + D H+ + D- ∆G = IpH + Ae

D (1.7)

where ∆G is the Gibbs free energy of the reaction, while IpH,D and Ae

H,D denote the

ionization potential and the electron affinity, respectively, of the host and dopant.

Keeping in mind the convention that Ip > 0 and Ae < 0, it is clear that lower Ip and larger

negative values of Ae increase the thermodynamic driving force for the ionization of

dopants.

23

It may be tempting to draw an analogy with traditional semiconductors, to say that

charge conduction in pentacene will thus possess more extrinsic character than in

naphthalene. However, because many dopants form deep-level traps which do not yield

free carriers, in some instances dopants can reduce the conductivity. This has been

observed, for example in anthracene by Karl, as mentioned in (Silinsh et al. 1994)

(p.190), illustrated in Fig. 1-13, where local trapping states are formed by guest

molecules. The above formulae allow the estimation of the depth and character of traps,

but only approximately, due to the changes in effective Ae and Ip from local distortion of

Fig. 1-13: Charge carrier traps formed by guest molecules (a) tetracene, (b) acridine, (c) phenazine, (d) anthraquinone, and (e) phenothiazine in anthracene host crystal (according to the work of Karl). (Figure reproduced from Silinsh and Čápek, 1994, p. 192).

24

the host lattice by the guest molecule. Dipolar molecules may also introduce trapping

effects in proportion to the strength of the dipole via the charge-dipole interaction energy,

as listed in Fig. 1-9. Many of the organic compounds can form oxides and nitrides, such

as ones shown in Fig. 1-13 at ambient conditions, with or without exposure to ultraviolet

(UV) radiation. This can both modify the energy level structure of the molecule, as well

as create molecular dipoles. Therefore care should be taken to avoid exposure to air and

direct intense light during the purification stages, and especially during the growth of thin

films for device applications.

As will be shown in Ch. 7 for pentacene, similarly to traditional semiconductors,

molecular order plays an important role in determining charge conduction in organic

materials. Disorder in the intermolecular spacing will decrease the probability of charge

hopping between molecules due to the variation in intermolecular distance and

orientation. Device processing conditions (e.g. temperature and deposition rate) must be

selected not only with regard to the vapor pressure of the organic material, or the thermal

stability of the substrate, but also for optimal control of the deposited film morphology,

which will in turn affect electrical device performance.

1.4.4 Electronic excitations in organic solids

The structure and dynamic behavior of electronic energy states govern the interaction of

the organic solid with light, and are at the heart of optoelectronic device operation. We

therefore briefly review some features of electronic excitations in organic materials

relevant to device structure and processing.

Optical and thermal excitations in covalent semiconductors result in the

promotion of electrons from CB to VB. However, the excited states are typically very

25

weakly (<0.01eV) bound, having the strong effect of raising the free charge carrier

concentration upon illumination. Strong confinement of the excited state can be achieved

in a structure where a layer of small band-gap material is sandwiched between two layers

of a large band-gap material, forming a quantum well. But organic molecules already

behave as individual quantum wells, retaining many of their properties of the constituent

molecules. Here, electrons are excited from the HOMO to one of the LUMO states

Ee0

Ee1

r0 r0*

Ee

r

A B

Absorption(Eabs)

Internal conversion

Emission(Eem)

Figure 1-14: a) Molecular configuration energy vs. interatomic distance, r, diagram for the electronic ground and first excited states, labeled Ee

0 and Ee1,

respectively. In each electronic state, a series of vibrationally separated states exist. Fast (10-14 s) excitation of a ground state electron into an excited state promotes the molecule into higher vibrational states of Ee

1, which then relax via slower nuclear motion into the lower vibrational modes, which can subsequently can radiatively relax into one of the vibrational states of Ee

0.

26

thermally, optically, or by electrical injection, forming strongly bound (>0.05 eV binding

energy) charge pairs, or excitons. We first examine optical excitations.

A molecular energy diagram is shown in Fig. 1-14 for a hypothetical bonding

situation involving electrons shared by two atoms. The electronic ground state level is

designated by the curve Ee0, and the first excited state by Ee

1; vibronic energy spacings

are designated by Ev0 and Ev

1 for the two electronic states, respectively. The lowest

energy for Ee0 occurs for an equilibrium interatomic distance, r0, and grows when r > r0.

When r << r0, the electrostatic repulsion energy between the nuclei (e2ZAZB/4πε0r, where

ZA,B is the effective charge of atoms A and B, e is the electron charge, and ε0 is the

electrical permittivity of vacuum) grows to infinity, but when r >> r0 the attractive

interaction between atoms and electrons asymptotes to zero. The energy level spacing for

this molecule in its electronic ground state (Ee0) is given by the vibrational energy step

Ev0 proportional to r2 for small r. Due to the antibonding character of the Ee

1 state (i.e. the

LUMO), it will possess an equilibrium separation r0* > r0.

The Born-Oppenheimer approximation states that the much heavier nuclei remain

stationary in the molecular frame of reference on the time scale of electronic motion.

Thus, upon absorption of a photon of energy Eabs, electrons respond to electromagnetic

radiation much faster than the nuclei (on the order of 10-14s), and the optical excitation is

effectively “vertical” in the E vs. r diagram. However, the probability of transition

between two states ψ and ψ* depends on the overlap integral between ψ and ψ*, which is

greater for the states corresponding to higher vibronic levels of the Ee1 manifold than

lower ones. Thus, molecule is excited into an energy level slightly above the first excited

one, as shown in Fig. 1-14. Eventually the nuclear motion dissipates the excess energy,

27

and the molecule partially relaxes into the ground vibrational state of Ee1. Consequently,

radiative decay of the excited state will occur with an emission photon energy Eem < Eabs.

This red-shift of the emission spectrum of a molecule is called the Franck-Condon (or

Stokes) shift, and is characteristic of a majority of molecular organic compounds.

b) c)

d) e)

GroundState

FrenkelExciton

Charge TransferExciton

Wannier-MottExciton

E

Electron (e-)Hole (h+)

. . .

re

re

Frenkel exciton


Wannier-MottExciton

a)

Figure 1-15: a) Three types of electronic excitations (or excitons) commonlyobserved in molecular crystals, including Frenkel, Charge Transfer (CT), and Wannier-Mott, listed in order of decreasing binding energy, Eb, and increasing effective radius, re. b) Energy level representation of the electronic ground state of a molecule, c) the Frenkel, d) CT, and e) Wannier-Mott excitons.

b) c)

d) e)

GroundState

FrenkelExciton


Wannier-MottExciton

E

Electron (e-)Hole (h+)

. . .

re

re

Frenkel exciton


Wannier-MottExciton

a)

Figure 1-15: a) Three types of electronic excitations (or excitons) commonlyobserved in molecular crystals, including Frenkel, Charge Transfer (CT), and Wannier-Mott, listed in order of decreasing binding energy, Eb, and increasing effective radius, re. b) Energy level representation of the electronic ground state of a molecule, c) the Frenkel, d) CT, and e) Wannier-Mott excitons.

28

Owing to weak intermolecular interactions, the electronic excitation is typically

localized on a single molecule, which then constitutes an exciton. An alternative picture

involving electron and hole pairs can also be useful in understanding the operation of

(opto)electronic devices. When an electron is promoted into the LUMO, it leaves behind

a positively charged vacancy, or a hole, in the HOMO. For weakly interacting molecules

in a solid, this electron-hole pair is effectively localized on the single molecule, forming a

bound quasi-particle, or a Frenkel exciton (Fig. 1-15). (Pope et al. 1982) Stronger

intermolecular interactions and lower exciton binding energy, Eb, can increase the exciton

radius, re, relative to the lattice constant, al, forming moderately delocalized charge-

transfer (CT) excitons, or highly delocalized Wannier-Mott excitons. Frenkel excitons

Cathode

Anode

HTL

ETL

LUMO

HOMO

Cathode

Anode

HTL

ETL

LUMO

HOMO

Figure 1-16: a) Principle of operation of an organic photovoltaic (PV) cell, where electrical current generation proceeds via a four-step process: 1. Absorption of photons to generate excitons, 2. Diffusion of excitons to the donor-acceptor (DA) interface, 3. Separation of the electron-hole pair due to energetically favorable HOMO-LUMO alignment of the D and A, and 4. Extraction of charge to the electrodes. (Figure courtesy of Peter Peumans). b) Principle of operation of an organic light emitting diode (OLED), where in contrast to the PV cell, charges are injected from the electrodes, move through the electron and hole-transporting layers (ETL and HTL), and recombine in the ETL-HTL interface. Doping this region with guest molecules having tailored band gap energy, Eg, allows the control of electroluminescence wavelength.

a) b)

29

have large (~1eV) binding energies and have small (<5A) radii, Wannier-Mott excitons

have re >> al. The CT exciton often exists in molecular crystals, having Eb ~10-100 meV,

delocalized over two or more molecules or adjacent polymer chains.

Following their generation, excitons can diffuse throughout the organic solid via

hopping between neighboring molecules (Dexter transfer) or by long-range dipole

interaction (Förster transfer). Exciton and charge dynamics are at the heart of several key

optoelectronic devices. In organic photovoltaic (PV) cells, the photogenerated excitons

are separated into electrons and holes at a donor-acceptor (DA) interface, due to

S = 0

S = 0 S = 1, Ms = 1, 0, -1

= ms = + ½

= ms = + ½

a)

b)

Figure 1-17: a) Two equivalent representations of the electronic ground state of a molecule with two spin-antiparallel electrons in the HOMO. b) Four possible spin-states of the excited state, with the total spin, S = 0 or 1 for the singlet and triplet excitons, respectively. (Figure courtesy of Peter Peumans).

30

energetically favorable HOMO and LUMO level alignment of the D and A species, as

shown in Fig. 1-16a. Organic light emitting diodes (OLEDs) function in reverse of PV

cells, by injecting electrons and holes from the cathode and anode, respectively, and

recombining them at an interface between the electron- and hole-transporting layers (ETL

and HTL, respectively). The charges combine at this interface to form excitons, which

can then radiatively decay. The wavelength of the emitted light can be controlled by

doping the ETL-HTL interfacial region with molecules having specifically tailored

“band-gap” energy (Eg), as illustrated in Fig. 1-16b.

Excitons can be formed in a number of different quantum-mechanical spin states,

having wide-ranging implications for devices like OLEDs. Electrons are Fermions, or

spin ½ particles. The electron spin is denoted as s = ½. The spin angular momentum can

take on two different values with respect to an arbitrarily defined z-axis, denoted by the

quantum number ms = ±½. According to the Pauli exclusion principle, two electrons can

pair up in the same MO, provided their spins are antiparallel, as shown in Fig. 1-17a.

The electronic ground state can be written as ψ1,20 = |+½>0 |-½>0, where the total

electron spin value for both electrons is Ms = ms1 + ms2 = 0. The exciton, on the other

hand, is a quasiparticle comprised of two Fermions and can take on spin values of S = 0

or 1, where S = Ms of both the HOMO and the LUMO electrons. The electron in the first

excited state, ψ1,21, however can take on either ms = + ½ or ms = – ½ values, such that the

new state can have two four possible spin state configurations (Fig. 1-17b):

S = 0, Ms = 0 ψ1,21 = | + ½ >0 | - ½ >1 - | - ½ >0 | + ½ >1 (1.8)

S = 1, Ms = +1 ψ1,21 = | + ½ >0 | + ½ >1 (1.9)

S = 1, Ms = -1 ψ1,21 = | - ½ >0 | - ½ >1 (1.10)

31

S = 1, Ms = 0 ψ1,21 = | + ½ >0 | - ½ >1 + | - ½ >0 | + ½ >1 (1.11)

Where the S = 0 exciton wavefunction is antisymmetric (changes sign) under exchange of

the two electrons (i.e. ψ1,2 = - ψ2,1 = ψ–), while the S = 1 wavefunctions are symmetric

(i.e. ψ1,2 = - ψ2,1 = ψ+), and are triply degenerate in the absence of a magnetic field.

Accordingly, the former are called singlet excitons, while the latter are called triplet

excitons.

The implication for OLEDs arises from the fact that the probability of radiative

transitions (i.e. photoemission) between the excited and the ground states is proportional

to the magnitude of the electronic transition dipole moment between them, |µ1 0|. The

dipole moment for a two-electron system is given by:

1 2e r e rµ = − ⋅ − ⋅ (1.12)

where e is the charge on the electron, while r1 and r2 are the coordinates of the two

electrons. The dipole moment for the transition between two states ψ0 and ψ1 is:

0 11 2( )e r rµ ψ ψ= − + (1.13)

If the ground and excited states are both singlets, under exchange of the electrons we

have:

0 1 0 11 2 2 1( ) ( )e r r e r rµ ψ ψ ψ ψ− − − −= − + − = − − + (1.14)

whereas if the ground state is a singlet, and the excited state is a triplet, we have:

0 1 0 11 2 2 1( ) ( )e r r e r rµ ψ ψ ψ ψ− + − += − + ≠ − − + (1.15)

Since the transition dipole moment cannot depend on the exchange (or reversing the

labeling) of electrons, µ = 0 in the case of the singlet-to-triplet transition, implying that it

is forbidden and non-radiative. The reverse is also true; optical excitations are from the

32

singlet ground state to the singlet excited state. Thus, photoluminescence typically

examines singlet exciton properties. Electrically generated excitons, on the other hand,

can be created in either the singlet or triplet states, but radiative decay is spin-allowed

only to the singlet exciton. Thus, ordinarily, only 25% of the electrically generated

excitons can give photons, while 75% are wasted. (Baldo et al. 1999) In some molecules,

however, the spin selection rules may be broken via the coupling of the electron spin to

the orbital angular momentum of a heavy metal atom (e.g. Pt in platinum

octaethylporphyryn). The singlet and triplet states then become sufficiently mixed, so that

100% of the electrically generated excitons can decay radiatively. (Adachi et al. 2001)

The allowed radiative transition is typically fast (~10-9s excited state lifetime) and is

called fluorescence, while the disallowed transition is therefore typically slow (~10-6 –

10-3 s) and is called phosphorescence. The singlet-triplet dynamics can therefore be

studied by time-resolved spectroscopy in both photo- and electroluminescence (PL and

EL) modes. (Baldo et al. 2000; 2000)

In great part the versatility and appeal of organic semiconductors lay in the

richness of synthetic chemical approaches (Katz et al. 2001) that can be used to tailor the

optoelectronic properties of the organic film to its desired application. In connection to

the singlet / triplet dynamics in organic molecules, for example, heavy metal atom-

chelates (e.g. Pt-and Ir-containing compounds) can be synthesized to increase the EL

efficiency of OLEDs. Furthermore, by substituting electron-rich or electron-withdrawing

ligands in a platinum-containing compound such as shown in Fig. 1-18, enables the

tuning of the compound’s luminescence wavelength. (Brooks et al. 2002)

33

1.5 Summary

In this chapter the class of materials organic semiconductors was defined on the basis of

their intra- and intermolecular bonding characteristics. The latter are governed by

relatively weak van der Waals interactions, broadening the choice of compatible

substrates, but also require the development of novel methods of thin film and multilayer

Figure 1-18: CIE (Commission International d’Eclairaige) diagram of photoluminescence of phosphorescent cyclo-metalated platinum complexes. (From Brooks et al. 2002)

Figure 1-18: CIE (Commission International d’Eclairaige) diagram of photoluminescence of phosphorescent cyclo-metalated platinum complexes. (From Brooks et al. 2002)

34

growth. The electronic and optical properties of conjugated organic solids derive from the

electronic energy structure of individual molecules, dominated by the extended π-

electron system. Molecular structure can be tuned to control the electrical conductivity, as

well as exciton energy, leading to improvements in device applications, exemplified by

the use of emissive triplet excited states of phosphors to quadruple the efficiency of

OLEDs.

35

Chapter 2: Organic device fabrication

technology

2.1 Overview

Molecular organic electronic devices typically consist of thin (<100nm) films of

molecular semiconductors. The films adhere by van der Waals forces, with the strength

of interaction on the order of only a few kT (at room temperature), decreasing quickly

with intermolecular separation. The bonds between the molecular sub-units in van der

Waals solids are weaker than in covalent, ionic, or metallic solids, resulting in a greater

tendency to form micro-crystalline and amorphous films. Thus, even when deposited

onto highly ordered substrates, relaxation of strain due to lattice mismatch occurs after

only a few monolayers (Forrest 1997), obviating the need for lattice-matching between

the deposited layers and the underlying substrate. This considerably broadens the choice

of substrates in the growth of organic devices, including low-cost glass, metal and plastic

foils.

As the electronic and optoelectronic performance of organic devices improves and

reaches commercialization potential, scale-up of processing technology becomes

important (Bardsley 2004; Forrest 2004), and the cost of materials is one of the major

36

considerations. Here it is important to consider both the deposition of organic thin films,

as well as metal-oxides and metals that complete the device structure.

Figure 2-1a shows the sharp decrease in the cost of devices as production volume

rises, estimated from the cost of materials and prior experience of device manufacturing.

(Bardsley 2003) While in traditional semiconductor manufacturing the cost of materials

is relatively small (e.g. 6% in the case of DRAM (Bardsley 2004)), it can be as high as

45% in LCD manufacture (Bardsley 2004). This is partly due to the device structure and

the nature of materials used. For example, an essential part of both LCD- and OLED-

based displays is the conductive transparent ITO anode, fabricated typically by power-

intensive DC-magnetron sputtering of a costly indium-tin target at elevated substrate

temperatures, typically above 200ºC. The important figures of merit for ITO-coated glass

in LCD and OLED applications are electrical conductivity and transparency, which have

been found to improve with higher substrate temperature during sputter-deposition (See

Fig. 2-1b). This is an important consideration in manufacture of OLEDs on flexible

substrates, since, as Fig. 2-1c shows, the cost of plastic substrates capable of

withstanding temperatures above 200ºC can be orders of magnitude higher than the more

fragile plastics.

With regard to the deposition of organic thin films, the same relatively weak

bonding forces that broaden the choice of substrates, also render standard semiconductor

processing techniques (high temperatures and solvents) too damaging of the thin films,

requiring the development of novel device fabrication approaches.(Bardsley 2004;

Blanchet et al. 2003; Forrest 2004; Rogers 2001; Rogers et al. 2002; Sirringhaus et al.

2001) A few particularly promising methods are discussed below. Some of them only

37

Figure 2-1: (a) The cost of flexible displays versus production volume (US Display Consortium). (b) The cost of plastic substrates withmaximum processing temperature. (c) The resistivity and transmission of ITO sputtered onto a glass substrate versus the substrate temperature. (Source: Applied Films) (Source: Hewlett-Packard). (Figures adapted from Bardsley 2003)

Production (ft2/week)0 40,000 80,000

$ / f

t2

0

100

200

300

400

$ 34$ 75

Active Matrix OLED by Deposition

Passive Matrix OLED by Lamination

Production (ft2/week)0 40,000 80,000

$ / f

t2

0

100

200

300

400

$ 34$ 75

Active Matrix OLED by Deposition

Passive Matrix OLED by Lamination

(a)

(b)

(c)

0 100 200 300 400100

200

300

400

500

600

20

40

60

80

100

Resi

stiv

ity(m

Ο/c

m2 )

Substrate Temperature (°C)0 100 200 300 400100

200

300

400

500

600

20

40

60

80

100

Resi

stiv

ity(m

Ο/c

m2 )

Substrate Temperature (°C)

100 140 180 220 2600.1

1

10

100

1000

Temperature (°C)

Subs

trate

cos

t ($/

m2 )

(un

coat

ed)

38

enable the large-area deposition of organic thin films with or without the capability to

deposit heterostructures or pattern the films in the substrate plane. A handful of

approaches permit the simultaneous deposition and patterning of organic thin films, and

are, in principle, more suited to achieving the objective of low-cost of fabrication, from

the standpoint of throughput and less constly materials. (Bardsley 2004)

In many applications (e.g. full-color OLED displays), patterning of organic layers

is a requirement that places additional demands on the fabrication sequence and tooling.

Some of these concerns (e.g. film thickness uniformity across the substrate) are inherent

to the device structure, and thus must be addressed by all fabrication techniques. Other

concerns (e.g. solvent compatibility or mask cleaning) arise from the unique aspects of

each particular method. Several major fabrication techniques are described below, along

with their advantages and disadvantages.

2.2 Spin-on film deposition

A well-established technology already exists in the traditional semiconductor processing

industry for depositing thin films of organic materials – spin-on. (Wolf et al. 1999) Spin-

on (See Fig.2-2a) is an essential step in photolithography, where a solution of polymer

photoresist is dispensed onto a wafer, which is then rapidly accelerated (typically at

~20,000rpm/s ramp rate) to a final angular velocity of ω = 3000-7000 rpm. The

centrifugal force disperses the polymer film across the rotating surface as the solvent

evaporates and the polymer solidifies. The final polymer layer thickness resembles the

profile shown in Fig. 2-2b, while the average coating thickness is given by, ignoring

evaporation, 22 1/ 24( ) (1 )3o ot h t −ρ

δ = δ + ωµ

, (Emslie et al. 1958) where δ0 is the initial

39

coating thickness, ρ is the polymer solution density, µ is viscosity, t is the spinning time,

and ω is the angular velocity. While mathematical formulae have been developed to

predict δ based on solution properties, surface and spin conditions, most users rely either

on the photoresist manufacturer’s data specific to the particular photoresist used, or

calibrate their own set of materials and spinners.

Most spin-coating steps are completed within 20-30 seconds, and result in

thickness coating uniformity of ± 100 Å/s, if ω > 6000 rpm. The photoresist spin-on is

most often followed by a baking step to drive off residual solvent and further densify the

film. Exposing the photoresist to ultraviolet (UV) radiation through a patterned optical

z

rδ

vr(z)

Figure 2-2: Coating of substrates using spin-on. A polymer solution is dispensed onto a substrate, which rotates at an angular velocity ω = 3000-7000 rpm. Centrifugal forces spread the solution over the surface, as the solvent evaporates. Spin-on is often followed by a bake to drive off any residual solvent and further densify the coating. Better film thickness uniformity across the substrate is obtained at higher ω, but is typically limited to ±100Å.

(a)

(b)

ω

ω

40

mask induces chemical changes in the resist, which are then chemically developed,

creating a corresponding pattern of openings. Additional material can be then deposited,

or the underlying material can be selectively etched in the open areas. After removing the

photoresist residue, the process can be repeated to generate complex patterns in the

substrate.

Since the photoresist is used only for patterning, the ± 100 Å thickness variation

is adequate for traditional semiconductor manufacture, where the feature size is on the

order of 100nm or larger in the lateral direction. However, since many of the organic

(opto)electronic devices rely on active organic layers as thin as 500 Å, the coating non-

uniformities are thus an unacceptable 20-40% of the total film thickness.

Furthermore, the photoresist is traditionally spun onto covalently-bonded

substrates (e.g. glass, silicon), which are impervious to the organic or aqueous solvents.

In fabricating organic heterostructures, solvents used for the alternating organic layers

must also follow an alternating organic-aqueous sequence. This severely constrains the

choice of solvents and active organic semiconductor materials, especially in view of low

room-temperature solubility of organic semiconductors in common solvents. An

alternative is to design the polymer chemical structure to either perform different

electronic functions (e.g. a p-n junction contained within a single chain of a di-block co-

polymer), or control the phase segregation of two polymers mixed together (e.g. electron-

donating and electron-accepting polymers). (Katz et al. 2001; Moons 2002) The

drawback of these approaches is the dual burden placed on the polymer chemist to

simultaneously and independently tune the rheological and electronic properties of the

41

compound, to ensure electronic performance (e.g. conductivity, luminescence quantum

efficiency), as well as the proper p-n junction orientation with respect to the electrodes.

2.3 Laser induced thermal imaging

Laser induced thermal imaging (LITI) (Blanchet et al. 2003; Blanchet et al. 2003) is

illustrated in Fig.2-3; it is a variant of the dye sublimation printing technique, where

highly localized heating is used to transfer ink from a donor sheet onto a substrate. For

LITI in particular, a large plastic (donor) sheet is coated with an organic semiconductor

(polymer or small-molecule) and brought into intimate contact with the substrate. A laser

pulse impinges onto the back of the donor sheet, vaporizing the semiconductor off the

sheet and onto the underlying substrate. In this fashion, the solvent compatibility issue is

solved, since two different organic slabs can in principle be transferred sequentially from

two separate donor sheets. In addition, the lateral patterning of the active organic layers is

accomplished without exposure to solvents or UV light.

Both polymer and small molecule (pentacene) TFTs have been fabricated using

this method, on notably large (e.g. 1m x 1m) plastic substrates, with pentacene TFTs

exhibiting field-effect hole mobilities of up to 0.3 cm2/V·s. While the precise mechanism

of the transfer is not yet fully understood, the technique holds promise for low-cost

electronics fabrication using much of the existing laser printing equipment infrastructure.

A potential draw-back of LITI is that it still requires the fabrication of the donor

sheet. Spin-on or spray-coating of the donor sheet are well-suited for covering large

areas, but again suffer from poor control of thin-film thickness uniformity. Thus, other

methods of organic layer deposition with better control of film thickness (e.g. vacuum

thermal evaporation (VTE), or organic vapor phase deposition (OVPD)) may be needed

42

to create the donor sheet (they are discussed in Sec. 2.5 and 2.6). Since LITI and

VTE/OVPD do not share the same manufacturing platform, it can be potentially difficult

and costly to integrate these processes. (Bardsley 2004)

2.4 Ink-jet printing

Ink-jet printing is a now-ubiquitous patterning technique, where picoliter droplets of

liquid ink are ejected from microscopic nozzles onto a substrate (e.g. paper or

transparency film). Three variations on this method are illustrated in Fig. 2-4. Thermal

ink-jet (TIJ), developed primarily by Hewlett-Packard (Askeland et al. 1988; Shields

1992), utilizes heat to nucleate and then rapidly expand a bubble in a fluid reservoir,

which then pushes a liquid jet through a tiny nozzle. Another technique developed by

Figure 2-3: a) Illustration of the laser induced thermal imaging (LITI);process. A donor sheet pre-coated with an organic semiconductor and brought into contact with the substrate. A laser pulse impinges on the back of the donor sheet, presumably vaporizing some of the organic material, most of which transfers to the substrate. The donor sheet is then peeled off; (from G. Blanchett, APL 2003) b) Example of a 50cm x 50cm array of polymer TFTs printed on plastic using LITI (from G. Blanchett, NSF Workshop on Organic Electronics, 2003).

(a) (b)

43

Epson uses a piezoelectric element to rapidly generate a pressure wave inside of a small

container of liquid, which also results in a collimated liquid jet exiting the reservoir

through an orifice. (Percin et al. 2003) In a third variation developed by Xerox PARC,

(Elrod et al. 1989) focused pressure waves from a piezoelectrically actuated acoustic lens

eject droplets as small as 5µm in diameter from the surface of a liquid onto a substrate.

By substituting the ink with a solution of conducting or light emitting polymers

(Bharathan et al. 1998; Yang et al. 2000) or small molecular dyes (Hebner et al. 1998),

circuits (Sirringhaus et al. 2001; Sirringhaus et al. 2000), conducting metal films (Huang

et al. 2003) and OLEDs (Bharathan et al. 1998; Yang et al. 2000) can be directly

patterned onto the substrate. Ink jet printing is particularly attractive for organic

electronics fabrication (Calvert 2001), since it is a well-established technology (albeit for

document printing), well suited for large area and flexible substrates. High throughput

can be achieved by utilizing multi-nozzle arrays (Creagh et al. 2003); for example, state-

Heater Piezoelectric actuator

Piezoelectric actuator

Heater Piezoelectric actuator

Piezoelectric actuator

Figure 2-4: Three variatiants of ink-jet printing: a) Thermal ink-jet (Hewlett Packard & Canon); b) Piezoelectric actuation (Epson); c) Acoustic lens (Xerox PARC). (Figure courtesy of Peter Peumans)

(a) (b) (c)

44

of-the-art commercial Hewlett-Packard and Epson printers use arrays of over 500 nozzles

with droplet generation rate in the kHz range. Another significant benefit is that

deposition and patterning are in principle achieved in a single step, with virtually no

wasted material, and only a moderate negative environmental impact.

Several significant challenges are encountered in adapting ink-jet printing to

organic electronics:

1) Since inkjet printing is a solvent-based technique, printing multi-layer

structures suffers from limitations of solubility selectivity, similar to spin-coating.

2) Film thickness uniformity is another major concern (Sirringhaus et al. 2000;

Wang et al. 2004), although on a much smaller length scale. The droplets do not

spread evenly across the substrate, forming beads around the edges and

depressions in the middle as they dry. This problem is addressed by tuning the

wetting angle of the ink on the substrate, typically by patterning confining wells

onto the substrate prior to the ink-jet step. (Sirringhaus et al. 2000) Such wells are

often also necessary to limit the size or edge resolution of the deposited droplet,

since it generally spreads considerably upon impact onto the substrate (Toivakka

2003). This, however, requires the additional step of substrate patterning, which

defeats the direct-deposition advantage of the inkjet.

3) As in the case of spun-on polymer films, inkjet printed films need to be baked

to drive off the residual solvent, which may otherwise lead to degradation of the

device when electrical current is turned on. Thus, substrates need to be able to

withstand the moderate (~100-150°C) baking temperatures without warping or

degradation.

45

4) Slow evaporation of the solvent during the printing process may clog the

nozzle (Lee et al. 2003), requiring additional provisions in the printing sequence

as well as the nozzle design.

Despite all of the above challenges, inkjet printing has been successfully used by

a number of groups to print circuits and OLEDs, and by a number of manufacturers (e.g.

Epson, Toshiba) to print full-color OLED displays (See Fig. 2-5). Some of the current

problems encountered in manufacturing such displays include low yield, limited lifetime

(on the order of hours), and high cost of manufacture, which can be potentially reduced

substantially when production volume increases (Bardsley 2003).

2.5 Vacuum thermal evaporation

The small-molecular multi-layer structures have been traditionally deposited by vacuum

thermal evaporation, VTE (Fig. 2-6). In this method the source materials are evaporated

Figure 2-5: Example of an ink-jet printed 2.5” diagonal full-color polymer LED display (Epson).Figure 2-5: Example of an ink-jet printed 2.5” diagonal full-color polymer LED display (Epson).

46

from heated container (boats or Knudsen cells) onto the substrate placed directly above.

A quartz crystal microbalance and a mechanical shutter are used to control the thickness

of the individual layers. Depositing in vacuum carries several significant advantages with

respect to solution techniques (Bharathan et al. 1998; Elrod et al. 1989; Forrest 1997;

2004; Garnier et al. 1994; Hebner et al. 1998; Le 1998; Lee et al. 2003; Paul et al. 2003;

Rogers 2001; Sirringhaus et al. 2000; Yang et al. 2000). Firstly, VTE allows for very low

levels of impurity incorporation into the deposited films, provided the deposition

chamber pressure is low (e.g. < 10-7 Torr) (Forrest 1997). Additionally, since the

evaporation is carried out in vacuum, the molecular mean free path is long (on the order

of 1m), resulting in rectilinear, direct transport of evaporant molecules from the source to

Figure 2-6: (a) Schematic of vacuum thermal evaporation (VTE). The source materials are evaporated from individually heated source cells onto a substrate placed directly above. A quartz crystal monitor (QCM) and a mechanical shutter are used to control the thickness of individual layers. (b) For film patterning and contact deposition, a shadow-mask is placed in proximity to the substrate. Due to the long mean free path (λ) of molecules in vacuum and small mask-substrate separation (s), the pattern edge resolution can be < 1µm.

λ > 30cm

s

b)

λ > 30cm

s

b)

HeaterTo Pump

Substrate10-6 Torr QCM

Mask

Shuttera)

DopantHost

HeaterTo Pump

Substrate10-6 Torr QCM

Mask

Shuttera)

DopantHost

47

the substrate. Finally, another significant advantage of VTE is being able to evaporate

metals. Since none of the previously discussed methods are capable of depositing

atomically flat metallic thin films on top of the organic layers, they all rely on VTE for

metal contact deposition. And although vacuum fabrication technology is generally

regarded as more costly than, for example, inkjet printing, being able to use the same

platform for the deposition of both organics and metals may reduce the overall cost of

processing, as well as integration with current fabrication facilities (Bardsley 2004).

Figure 2-7: (a) A 13” flat panel display made by Sony Corp. using organic LED technology. (b) Schematic of a picture element (pixel) of the display (shown as a passive matrix architecture for simplicity) consisting of a triad of red-, green-, and blue-emitting OLEDs, each typically ~200µm wide, with as much as 20µm separation. (c) Approximate sequence of steps in the fabrication of a full-color display pixel, wherein different dopants (e.g. btp2Ir(acac), ppy2Ir(acac), FIrpic) are used to control the OLED emission wavelength. Depositing distinct pixels with each dopant thus requires a separate masking step, followed by the deposition of a metal contact (e.g. LiF/Al).

glass / plastic

ITO

SiNx

organic

metal200µm

0.3µm

20µm

glass / plastic

ITO

SiNx

organic

metal200µm

0.3µm

20µm

btp2Ir(acac)

S

N

Ir

2

O

OCH3

CH3N

IrF

F

O

N

O

2

NIrF

F

O

N

O

2

FIrpicppy2Ir(acac)

N

IrO

O

CH3

CH3

2LiF/Al

1 2 3 4

b)

c)

a)

48

The patterning of active organic layers by VTE is accomplished through the use

of shadow-masks. Again, full-color displays serve as an illustrative example. A

simplified full-color display architecture and VTE fabrication sequence are illustrated in

Fig. 2-7. The basic element of the display is a picture element, or a pixel, which is a triad

of red-, green-, and blue-emitting (RGB) OLEDs, micropatterned in the substrate plane.

The OLEDs are ~200µm wide, with up to 20µm gaps between them. The deposition of

the entire display takes place in several steps, each separated by a translation of the

shadow-mask to result in the RGB p`attern (Tian et al. 1999):

Step 1: The hole injection/transport layers can be blanket-deposited for all pixels.

Step 2: A mask is placed over the substrate, covering 2/3 of the pixel locations.

The red-emitting doped layer is then deposited.

Step 3: The mask is translated on the substrate to expose an uncovered 1/3 of the

pixel locations; alternatively, the first mask is replaced by a different second

mask, with openings adjacent to the locations of the previous one. The green-

emitting doped layer is deposited.

Step 4: Step 3 is repeated, this time for a doped layer containing the blue-emitter.

Step 5: The mask from Step 4 is removed, and a different mask with all pixel

locations open is used for depositing the metal cathode.

The last step can be avoided if the substrate contains a pre-fabricated integrated

shadow-mask (Tian et al. 1997).

Since the operating voltage of the OLED depends on the total organic layer

thickness, film thickness non-uniformities across the substrate must be kept less than a

few percent. A point-like evaporation source gives a hemispherical evaporative flux,

49

leading to a thickness profile of the form h = h0·cosmθ, (m≥2), as illustrated in Fig. 2-8a.

Thus, to keep the variation in h to < 1% across the entire substrate, the solid angle θ must

be small, and in most cases this means W/H < 0.01, and most of the source material is

wasted on coating of the chamber walls instead of the substrate. Collimated effusion

sources have been employed in molecular beam epitaxy (MBE), but at the expense of

surface coverage. Typical group III-V devices grown by MBE are small-area devices

used in value-added applications, such as telecommunication lasers and photodetectors,

and the highly collimated, low-coverage sources can still be utilized. On the other hand,

since organic electronic devices are mostly aimed at low-cost, large-area applications

(e.g. flat panel displays, ambient lighting and solar cells), this quickly becomes

Figure 2-8: (a) Schematic of point-source vacuum thermal evaporation (VTE). The distribution of molecular flux from the source leads to varying film thickness across the substrate. To keep thickness non-uniformity < 1%, typically the W/H < 0.01, resulting in ~99% of the source material coating the chamber walls. Inset:empty pocket inside the source forming due to a hot spot and localized evaporation. (b) Shielded and baffled evaporation containers from R.D. Mathis Inc., designed to proved a steady flux of organic vapor.

(a)

hθ

H

W

hθ

H

W(b)

50

problematic due to the cost of the wasted source materials, and the time and effort in

periodic cleaning of the deposition chambers.

Furthermore, automatic control of the deposition process is difficult due to the

nature of the source material. Organic sources are typically crystalline powders at

ambient conditions and do not liquefy, but tend to sublime when heated at pressures

below 1 atm. Since the thermal conductivity of these powders is very low (Kitaigorodsky

1973; Silinsh et al. 1994), evaporation takes place mainly where the powder contacts the

source container. After some time, the organic material in immediate contact with the

heated container wall evaporates, leaving pocket of empty space. The weight of the

powder above the empty pockets tends to periodically collapse them, causing abrupt,

uncorrelated changes in the effective heat transfer area, and hence a somewhat erratic

evaporation rate. And although on the time scale of the growth of a single device in the

laboratory, the rates may appear sufficiently stable to for reproducible device

characteristics, in the scaled-up production environment the problem is exacerbated by

A BA B

Figure 2-9: (a) Photograph of several simple evaporation sources (or boats) made by R.D. Mathis Inc. (b) Schematic representation of a linear evaporation system with adjacent sources for doping. The substrate is translated across the evaporation flux.

(a) (b)

51

Figure 2-10: (a) A fully integrated, vertical evaporation system utilizing linear sources to achieve up to 50% materials use efficiency and uniform coating of substrates up to 400x500mm2 in size. (b) A close-up photograph of one of the evaporation chambers. (Applied Films, GMBH, Germany)

(a)

(b)

Figure 2-10: (a) A fully integrated, vertical evaporation system utilizing linear sources to achieve up to 50% materials use efficiency and uniform coating of substrates up to 400x500mm2 in size. (b) A close-up photograph of one of the evaporation chambers. (Applied Films, GMBH, Germany)

(a)

(b)

52

the requirement of faster evaporation rates, potentially complicating process automation.

Shielded and baffled boats (Fig. 2-8b) can be used to provide a steady organic vapor flux,

but at decreased net evaporation rate, due to the smaller area available for the vapor to

escape.

To improve the film thickness uniformity over large area substrates, as well as

materials use efficiency, a linear source can be used (Fig. 2-9). In this case, the substrate

and the sources are translated relative to each other to provide a uniform film thickness

across the substrate. A commercial-scale, fully integrated evaporation system (Fig. 2-10)

is available from Applied Films GMBH, capable of coating substrates up to 400 mm x

500 mm. Here, to prevent excessive bowing of the substrates and masks under gravity,

the substrate and the sources are vertically oriented. No details of construction were

available at the time of writing; one of the anticipated technological challenges is

retention of the source material, typically a crystalline powder, in the vertically oriented

evaporation cell, as well as the control of doping, due to potentially poor thermal contact

between the heated evaporation container wall and the powder source in the vertical

arrangement.

2.6 Organic vapor phase deposition

Virtually all of the organic materials used in the thin film devices described thus far have

sufficiently high vapor pressures to be evaporated at temperatures below 400°C and then

be transported in the vapor phase by a carrier gas such as argon or nitrogen. This allows

for positioning of evaporation sources outside of the reactor tube (similar to chemical

vapor deposition (CVD)), spatially separating the functions of evaporation and transport,

53

thus leading to more precise control of the deposition process. (Olsen 1982; Stringfellow

1989; Wolf et al. 1999)

Figure 2-11 illustrates the OVPD concept. Here, Alq3, which is a crystalline

powder in its purified state at ambient conditions, may be sublimated by heating at a

reduced pressure. The vapors are picked up by an inert carrier gas such as dry nitrogen,

and are transported to a cold substrate. A temperature gradient arises across the

hydrodynamic boundary layer near the substrate surface. When the organic vapor reaches

a condition of supersaturation while diffusing through the cool boundary layer, the

relatively heavy organic molecules nucleate into a solid film on the substrate surface.

Undesirable condensation can be avoided by keeping the walls of the chamber above the

solid-gas equilibrium temperature of the organic, while directing the flow patterns, to

selectively coat the substrate. Using a high purity carrier gas, the incorporation of

impurities into the film is minimized.

Figure 2-11: Schematic of the Organic Vapor Transport Deposition concept. A hot inert carrier gas (e.g. N2) transports organic vapors through a hot-wall reactor toward a cooled substrate, where the vapor selectively physisorbs.

54

The transport mechanisms, capabilities and commercial implementation of OVPD

will be discussed in more detail in the following chapters. Some of the advantages

include:

1). Selective deposition on the cooled substrate, resulting in > 50% materials use

efficiency;

2). Flow-controlled distribution of the source vapor, enabling uniform coating of

large area substrates by means of appropriately designed gas distributors

3). Precise regulation of the vapor delivery and doping concentration via the flow

rate of a carrier gas saturated with the vapor;

4). Thermally equilibrated source cells by utilizing a pre-heated carrier gas

flowing through the source material;

5). Self-cleaning hot-wall deposition chamber.

The commercially available, fully automated OVPD tool from Aixtron AG (Fig. 2-12)

takes advantage of the above properties of vapor-assisted deposition, enabling the

deposition of high performance OLEDs on large area substrates, with device lifetimes

matching or exceeding those of vacuum-deposited analogs. (Brown 2004)

At the same time, OVPD must overcome several challenges, including:

1). Having to use shadow-masks to pattern the organic layers, where 60% or more

of the source material impinging on the substrate is wasted on coating of the

shadow-mask. Techniques like ink-jet printing have a significant advantage by

virtue of their direct-patterning approach.

55

N2

N2 N2 N2

T1 T2 T3

Load Port(connect to vac. xfer line)

Deposition Chamber

Mixing Unit

Figure 2-12: (a) A mode of OVPD where the carrier gas flows through the source cells for better vapor pick-up, and onto the substrate through a large-area distributor for improved film thickness uniformity. (b) Schematic of a commercial-grade, fully computer-controlled OVPD system available from Aixtron AG, with multiple-evaporation cell furnaces and a remote deposition chamber.

(b)

(a)N2

N2 N2 N2

T1 T2 T3

N2

N2 N2 N2

T1 T2 T3


Deposition Chamber

Mixing Unit


Deposition Chamber

Mixing Unit

Figure 2-12: (a) A mode of OVPD where the carrier gas flows through the source cells for better vapor pick-up, and onto the substrate through a large-area distributor for improved film thickness uniformity. (b) Schematic of a commercial-grade, fully computer-controlled OVPD system available from Aixtron AG, with multiple-evaporation cell furnaces and a remote deposition chamber.

(b)

(a)

56

2). A greater thermal budget relative to VTE, required in continuously heating the

source cells, the chamber walls, and the carrier gas. This also results in additional

heat loads on the substrate, requiring powerful on-board chillers.

3). The current inability of OVPD to deposit atomically flat metal contacts using

carrier gas transport, requiring integration with VTE.

2.7 Organic vapor jet printing

In Ch. 6 organic vapor jet printing (OVJP) is described for the direct patterned deposition

of small molecular organics. This method, illustrated in Fig. 2-9, is at first glance

somewhat of a hybrid between inkjet printing and OVPD. In OVJP, organic molecules

are sublimed into a hot inert carrier gas, which is then expanded through a microscopic

nozzle (or an array of nozzles) forming a highly collimated gas jet. The jet impinges on a

cooled substrate, with organic molecules selectively adsorbing onto the substrate,

forming a well-defined thin film deposit. The deposited pattern size and edge resolution

depend on the nozzle diameter, nozzle-to-substrate separation, and the downstream

ambient pressure.

However, OVJP is distinct from ink jet printing of solution processed polymer

organic semiconductors (Hebner et al. 1998; Paul et al. 2003; Sirringhaus et al. 2000) in

that it uses a hot inert carrier gas, instead of a liquid solvent, to directly print molecular

organic semiconductors. This eliminates meniscus formation, solvent incompatibility,

and other issues limiting inkjet printing. Furthermore, in OVJP no pre-patterning of the

substrate is needed to contain the liquid droplet, whereas in ink jet printing, droplet-

confining wells are required. (Sirringhaus et al. 2001) Unlike OVPD and VTE, OVJP

does not require shadow masks to pattern the organic thin films. A significant

57

disadvantage remains in that atomically flat metal films still cannot be deposited on top

of other organic layers using a carrier transport technique. Despite this, it is anticipated

that OVPD and especially OVJP will prove powerful new methods for highly controlled,

rapid and low-cost organic device growth, especially as it is applied to molecular organic

compounds.

mixing chamber

N2

N2

N2

MAlq ~ 500 g/molMN2 ~ 28 g/mol

Figure 2-13: Schematic of the organic vapor jet printing (OVJP) concept, where a hot inert carrier gas picks up organic vapor and ejects it through a microscopic collimating nozzle, placed in proximity of a cooled substrate. The heavier organic molecules physisorb onto the substrate surface, forming a well-defined deposit, whose size and edge dispersion are directly influenced by the deposition conditions (pressure, temperature, carrier gas molecular mass, distance from the substrate), and the shape of the nozzle.

58

Chapter 3: Organic Vapor Phase

Deposition

3.1 Overview

In this chapter the method of organic vapor phase deposition (OVPD) is analyzed in

greater detail. Material transport regimes are outlined, and a theory is developed that

allows the prediction of source material evaporation and deposition rates. Formulae

describing the control of doping concentration by source temperature and carrier gas flow

rate are also derived. The theory developed here provides a practical understanding of the

basic mechanisms of OVPD, guiding the design and operation of experimental as well as

commercial-scale OVPD systems.

3.2 OVPD Concept

Vacuum thermal evaporation offers some advantages in the deposition of organic thin

films for device applications, including the ability to preserve source material purity, the

ability to deposit films with monolayer precision, and in some cases up to 50% materials

utilization efficiency. At the same time, some characteristics inherent to VTE make

commercial-scale deposition of organic electronics difficult. Organic vapor phase

deposition (OVPD) was developed in part to overcome the limitations of vacuum thermal

evaporation. It decouples the evaporation and deposition events, using a carrier gas to

59

mediate transport of material from the source to the substrate, thereby achieving greater

control of the deposition process.

The concept of OVPD is illustrated in Fig. 3-1. The source material is heated to

generate molecular vapor, which is picked up and transported by a hot inert carrier gas

toward a cooled substrate, where the organic vapor selectively condenses. Parasitic

condensation of the source material is avoided by cooling the substrate only, while

actively heating the walls of the deposition system. To grow doped films, multiple

component streams are mixed en route to the substrate. The process is carried out at

reduced pressure, increasing gas diffusivity and thereby improving mass transfer rates.

(Bird et al. 1996; Wolf et al. 1999) It is helpful to analyze OVPD as a succession of 3

steps: evaporation and pick-up, vapor transport by convection, and deposition. (Shtein et

Figure 3-1: Organic vapor phase deposition (OVPD) concept. A hot inert carrier gas (e.g. N2) transports hot organic source vapor along a heated-wall reactor toward a cooled substrate, on which the organic molecules selectively physisorb.

60

al. 2001) Growth of polycrystalline thin films involves a simultaneous annealing of the

growing film by molecular surface diffusion at elevated substrate temperature. (Shtein et

al. 2002)

3.3 Theory – Evaporation

The source cell is schematically shown in Fig. 3-2. Organic source material, typically in

the form of a crystalline powder, is heated inside of a container, while a hot inert carrier

gas flows over the organic surface. We are interested in predicting the rate of organic

vapor supply based on the source conditions, such as temperature, pressure, carrier flow

rate, and evaporation surface area.

Qualitatively, for a fixed cell temperature, a very low carrier flow rate (V•

), will

result in a vapor-saturated (i.e. vapor-solid equilibrated) cell atmosphere; here, carrier

flow limits how much organic material escapes the cell, which is also said to be in the

Figure 3-2: Schematic of a source cell containing the organic source material, typically a crystalline powder, maintained at P and Tcell. The carrier gas enters the cell picking up the organic vapor, carrying it out at a molar rate rout, which is the balance between evaporation and re-condensation rates, revap and rcond, inside the cell.

V•

Solid / PowderTcell

P

revap rcondcarrier gas

inflowcarrier gas

+ vapor

routV•

Solid / PowderTcell

P

revap rcondcarrier gas

inflowcarrier gas

+ vapor

rout

61

flow-controlled (or equilibrium) operating regime. Increasing V•

will cause organic

molecules to be swept out of the source cell as soon as they evaporate, thereby decreasing

the organic vapor pressure inside the cell and shifting it away from solid-gas equilibrium.

The cell thus enters a kinetic, or evaporation-controlled regime. These two regimes are

depicted in Fig. 3-3a. The same transition can be accomplished by fixing the carrier flow

rate and changing the cell temperature, Tcell, as shown in Fig. 3-3b. For low Tcell,

evaporation rate is low, and the cell is not saturated; it is in the evaporation-controlled

Figure 3-3: a), b) Qualitative depiction of the source cell operating regimes. c) The resulting rate of organic vapor outflow vs. increasing carrier gas flow rate for a series of cell temperatures T1, T2, T3.

Evaporation-Controlled(Kinetic regime)

Flow-controlled(Equilibrium regime)

Constant Tcell

•

V

org

eq

PP

a)



Constant Tcell

•

V

org

eq

PP

a)Evaporation-Controlled(Kinetic regime)


Constant•

V

Tcell

org

eq

PP

b)



Constant•

V

Tcell

org

eq

PP

b)

Kinetic

Equil.

c) Kinetic

Equil.

c)

62

(kinetic) regime. Increasing Tcell leads to vapor saturation, i.e. Porg/Peq 1, where Porg is

the actual vapor pressure of the organic material, while Peq is the equilibrium vapor

pressure; here the system is in the flow-controlled (equilibrium) regime. However, in

practice we are concerned with how much organic material escapes the source cell. And

although for higher V•

, the organic vapor pressure inside the cell, Porg, drops, the

molecules are still being swept out of the container. Thus, for a given Tcell, increasing V•

will result in a monotonic increase in the rate of organic vapor outflow. This situation is

depicted qualitatively in Fig. 3-3c.

A quantitative analysis of the source cell begins with a mass balance on the

organic species:

− =evap cond outr r r (3-1)

where revap and rcond are the evaporation and condensation rates, respectively, while rout is

the molar rate of organic vapor outflow from the evaporation cell. The condensation rate

is given by the molecular collision rate with the surface and the molecular sticking

probability, α:

12

= ⋅ ⋅ ⋅⋅ ⋅cond org e

org B

r P AM k T

απ

(3-2)

where Ae is the effective surface area of the organic material, Morg is its molecular mass,

(2π·Morg·kBT)-1/2 is the surface collision frequency, kB is the Boltzmann constant, and T is

the temperature. Similarly, the evaporation rate is:

12

= ⋅ ⋅⋅ ⋅evap eq e

org B

r P AM k Tπ

. (3-3)

At equilibrium Porg = Peq, the evaporation and condensation rates balance, giving:

63

=org eqP Pα . (3-4)

The net surface evaporation rate is thus given by:

( ) 12

= − ⋅ ⋅ ⋅⋅ ⋅out eq org e

org B

r P P AM k T

απ

, (3-5)

The Clausius-Clapeyron equation states that:

0 exp −∆

=

vap

eqcell

HP PRT

(3-6)

where ∆Hvap is the vaporization enthalpy of the organic material, P0 varies slowly with

Tcell, and the ideal gas constant R = kB·6.02·1023 J·K-1·mol-1. The organic vapor pressure

used in this work is typically < 10-2 Torr (1 Pa), and the carrier gas thus comprises > 99%

of the species present in the deposition chamber. If the vapor and the carrier gas mix

perfectly inside the source cell, the molar outflow of organic material is:

orgout

cell

Pr V

RT

•

= ⋅ (3-7)

where V•

is the volumetric flow rate of the carrier gas through the source cell. Equations

(3-1) – (3-7) combine to give:

0 exp

2

vap

cellorg

org B cell

cell e

HPRT

PM k TV

RT Aπ

α

•

−∆ =

⋅ ⋅+

⋅

(3-8)

0 exp

2

vapi

cellout

org B cell cell

e

HPRT

rM k T RT

A V

πα •

−∆ =

⋅ ⋅+

⋅

(3-9)

64

Since in practice the carrier gas flow in the deposition system is regulated by mass flow

control devices, it will be more convenient to analyze data in terms of the mass flow rate

units, (e.g. standard cubic centimeters per minute, or sccm). The actual volumetric flow

rate of the carrier gas, V•

, can be expressed as:

cell stdsccm

std cell

T PV VT P

• •

= ⋅ ⋅ (3-10)

where sccmV•

is the measured flow rate in sccm, while Tstd and Pstd are the standard

temperature and pressure, 298K and 760 Torr, respectively. Equation (3-9) becomes:

0 exp

2•

−∆ =

⋅ ⋅+ ⋅

⋅

vap

cellout

i B cell std cell

org stdsccm

HPRT

rM k T RT P

A PV

πα

(3-11)

Figure 3-3c in fact shows the same qualitative dependence of rout on the three process

variables, Tcell and sccmV•

, as predicted by Eq. (3-11), assuming an ideal situation where

they do not influence α. For constant sccmV•

and Pcell, raising Tcell will shift the source

closer to the solid-vapor equilibrium (i.e. saturation regime), while at constant Tcell the

source becomes depleted for higher sccmV•

. Equation (3-11) also shows that lower source

cell pressure helps maximize the rate of vapor supply. The cell-loading and material-

specific variables such as ∆Hvap, α, and Ae, would be determined experimentally; this will

be addressed in Ch. 4.

3.4 Vapor transport

65

Next, we analyze Step 2 in the OVPD mechanism, the transport of vapor from the source

to the substrate. The vapor from individual source cells may be further diluted and

blended with the vapor from other sources. Dilution may be necessary for several

reasons, including prevention of vapor supersaturation en route to the substrate, the need

to establish developed flow and heat transfer in the transport lines, and prevent cross-

contamination between sources. Assuming that the vapor streams become well-mixed en

route to the substrate, the resulting molar vapor concentration, c, is given by:

•= out

tot

rcV

(3-12)

where totV•

is the total flow rate of the carrier gas which enters the deposition region.

When two source materials, A and B, are blended, • • • •

= + +tot A B dilV V V V , where •

dilV is the

dilution flow rate. The resulting concentration of the source vapor of material i in the

deposition chamber is given by:

0,

, ,

, ,

exp

2 • •

•

−∆ =

⋅ ⋅ + ⋅ ⋅ + ⋅ ∑

vapi

icell

i

i B cell i cell istdi dil

i org i stdi sccm

HPRT

cM k T PRT V V

A PV

πα

(3-13)

As will be shown in the following sections, the film deposition rate is limited by diffusion

across a boundary layer. Thus, the final concentration of component i in the film will

scale with the vapor concentration given by Eq. (3-13).

Both the boundary layer thickness and the mixing distance depend on the

characteristics of the flow, including velocity, gas species, temperature, pressure, and

whether the flow is laminar or turbulent. The transition between laminar and turbulent

66

flow typically occurs at a Reynolds number of 2,300 for pipe flow, or on the order of 105

for flow past a flat plate. The Reynolds number is:

Re⋅ ⋅

= m f

f

U d ρµ

(3-14)

where U is the linear flow velocity, dm is the diameter of the deposition chamber, ρf and

µf are the density and viscosity of the fluid, respectively. Since the organic vapor is a

minority species in the flow, it can be neglected in calculating Re. For N2 at 270°C and 1

Torr, typical OVPD deposition conditions, ρf = 3.64·10-2 kg/m3 and µf = 2.44·10-5 kg/m·s.

As will be discussed in Ch. 4, the experimental OVPD system consisted of several

components of varying diameter, ranging from 5mm barrel outlets, a substrate 2cm in

diameter, and a 10 cm diameter glass tube. Carrier gas volumetric flow rate was a

maximum of 100 sccm. Converting to deposition temperature and pressure and using the

largest diameter (10cm) this results in U = 0.32 m/s, and Re = 48, well within the laminar

regime for both flow within the deposition chamber and past the substrate. Since at

typical deposition conditions •

totV <30 sccm, the laminar regime assumption holds

throughout.

Given the laminar nature of the flow, the mixing of the separate source vapor

streams in a simple geometry such as depicted in Fig. 3-4 is dominated by diffusion. The

mixing will be complete and uniform, when the vapors diffuse radially to the point of

filling the entire flow diameter, which occurs in characteristic time τD:

2

6= m

Di

dD

τ (3-15)

67

where Di is the gas phase diffusivity of organic vapor of material i. During this time, the

gas molecules entering the main tube will travel a distance Lm:

2

6⋅

= ⋅ = mm D

i

u dL UD

τ . (3-16)

To first order, using a hard-sphere approximation Di is given by the gas kinetic theory:

(Bird et al. 1996)

rd

Lm

dm

•

AV

•

BV

Figure 3-4: Schematic of the source vapors mixing en route to the substrate by diffusion, provided the flow is laminar, which holds true for typical OVPD conditions.

Coolant

Source A

Source B

Carrier gas

Pump

Pump

(a)

Substrate

Heater

(b)

68

2

1 1 83 3 2

⋅= ⋅ ⋅ = ⋅ ⋅

⋅ ⋅ ⋅B B

idep

k T k TD uM P

λπ πσ

(3-17)

where M is the reduced molecular weight (M = MAMB/(MA+MB), where A and B denote

two different gas species), ū and λ are the molecular mean thermal velocity and mean

free path, respectively, σ is the average molecular diameter ((σA+σB)/2), Pdep is the total

deposition chamber pressure, and T is the gas temperature. For Alq3 diffusing in N2, MA =

459 g/mol, and MB = 28 g/mol, giving DAlq ≈ 57 cm2/s, and Lm = 9 cm, or Lm ≈ dm in the

experimental OVPD system. The reactor designed specifically to take this mixing

mechanism into account; it will be also shown in Ch. 4 that the experimentally observed

mixing distance corresponds to the calculated Lm.

The carrier gas flow field can be visualized by numerically solving the complete

Navier-Stokes equations for the deposition geometry. The momentum balance is given

by:

( ) 2f f f f

u u u p u gt

ρ ρ µ ρ∂+ ⋅∇ = −∇ + ∇ +

∂ (3-18)

where g is the gravitational acceleration, which can become important in accounting for

buoyancy effects inside the chamber at very low flow rates. The continuity equation

0u∇ ⋅ = is also included. To obtain the temperature profile of the system, the heat

balance is included:

( )f ff P f PTc k T c T u Qt

ρ ρ∂⋅ ⋅ + ∇ − ⋅∇ + ⋅ ⋅ ⋅ =

∂, (3-19)

where k is the thermal conductivity, Cpf is the isobaric heat capacity, and Q is the net heat

flow to the cooled substrate holder.

69

The simultaneous numerical solution of the Navier-Stokes, diffusion and heat

conduction partial differential equations was performed using FEMLab v2.3, using the

damped Newton method. (Deuflhard 1974) Special precautions were taken to ensure

convergence of the non-linear problem. The solver was damped to guard against infinite

Newton iterations. The simulation geometry employed an adaptive mesh, where the

largest grid dimension did not exceed 1/5 of the reactor diameter. No appreciable changes

in the process parameters (deposition efficiency, flowlines, temperature profiles, etc.)

were observed when refining the grid.

For the simulation results shown in Fig. 3-5, the conditions included (in MKS

units) µ = 1.73e-5·(T/300)1/2, Cp=1000 J/K/kg, ρ calculated from ideal gas law at the local

temperature and pressure, k (thermal conductance) = 0.025·(T/300)1/2 W/K/m, D = 0.001

m2/s. In Fig. 3-5, a series of flow fields (white arrows) and a color-coded temperature

map are shown for a simplified OVPD geometry. Nitrogen carrier gas enters the

deposition chamber at room temperature through a single, axially positioned barrel and

exits through the pump port positioned downstream. The walls of the main chamber are

maintained at 500 K, while the substrate, situated downstream of the inlet, is maintained

at 290 K. For very low flow rates (Re = 0.05), the carrier gas quickly thermally

equilibrates with the chamber walls and cools rapidly upon contact with the substrate.

Hydrodynamic and thermal boundary layers appear to form in front of the substrate. As

the flow velocity (i.e. Reynolds number) increases, a greater distance is required to heat

the carrier gas. At the same time, the boundary layer becomes more pronounced, while

shrinking in thickness.

70

Re = 0.05

Re = 5

TT = 500KT = 290K

N2inlet

To pump

T = 290K

Figure 3-5: Modeling the flow field and temperature distribution in an OVPD system using numerical solutions to the Navier-Stokes equations. Unheated N2 is the carrier gas, entering the deposition chamber requires longer distances from the inlet to thermally equilibrate for higher Reynolds numbers (i.e. flow velocity). As expected, the hydrodynamic boundary layer decreases in thickness for higher velocity flows.

Re = 0.15

Re = 0. 5

Re = 1. 5

71

The organic species is introduced via the mass balance:

( )ii i i D

c D c c u Qt

∂+ ∇ − ∇ + ⋅ =

∂ (3-20)

where ci is the organic species concentration, and QD is the net deposition rate. The

numerical solutions to Eqs.(3-18) – (3-20), along with a set of realistic deposition

conditions (Twall = 600 K, Tsub = 300 K, Re = 10) are plotted in Fig. 3-6. The distribution

of species follows the flow, establishing a concentration boundary layer near the

substrate. The close correspondence between the hydrodynamic, thermal, and the

concentration boundary layers is not surprising, considering the dilute gas conditions.

The deposition of organic molecules on the substrate will proceed by diffusion across this

boundary layer.

3.5 Deposition

As the vapor and carrier gas flow over the cold substrate, a stagnant boundary layer of

thickness δ develops above the surface (see Figs. 3-1 and 3-6), across which the organic

vapor must diffuse. Since Peq depends exponentially on temperature, organic molecules

condense on the cold substrate, giving rise to a concentration driving force across the

boundary layer. The flux of organic molecules to the substrate, jorg, obeys Fick’s law of

diffusion:

gas sorg org

sorg org org org

P PRT RTj D c D

−= ⋅∇ = ⋅

δ (3-21)

where Dorg is the gas phase diffusivity of the organic in the carrier gas, Porggas and Porg

s

are the organic species’ vapor pressures in the gas phase and at the substrate surface,

72

Figure 3-6: a) Results of numerical solution of the Navier–Stokes equations for a simplified OVPD system geometry, where carrier gas is introduced in the barrel at a Re=10; the flow field is indicated by the streamlines, while the temperature distribution is shown by the color-map. The flow of the gas around the substrate creates a stagnant boundary layer, evidenced by the flow field. b) Concentration distribution color-map of the organic species carried by N2. The concentration boundary layer near the substrate follows the hydrodynamic boundary layer from (a).

(a)

(b)

73

respectively, evaluated at the deposition chamber and surface temperatures, T and Ts,

respectively. For low surface temperatures, Porgs << Porg

gas, Eq. (3-13) thus becomes:

= ⋅δorg

org org

cj D (3-22)

where corg is the organic vapor concentration in the deposition chamber. Combining Eqs.

(3-13) to (3-22) for the deposition of an organic compound i:

0,,

, ,

, ,

exp

2 • •

•

−∆ = = η ⋅

δ π⋅ ⋅ + ⋅ ⋅ + α ⋅ ∑

vapi

idep i celli

i depsub i cell i cell istd

i dili org i stdi sccm

HPr RTDjA mw RT PRT V V

A PV

(3-23)

where rdep,i is the deposition rate, Asub is the substrate area, ηdep is the deposition

efficiency, which depends on the gas distributor design and the resulting flow pattern

around the substrate. To predict the deposition rate for a given reactor geometry, the flow

equations must be solved to provide information about δ, while more precise values for

Di can be either obtained experimentally, or estimated from correlations such as the

Chapman-Enskog equation (Bird et al. 1996).

It should be noted that if U, Pdep, and T remain constant, then δ does not change.

However, since δ typically varies as U-1/2 (Bird et al. 1996; Schlichting 1968), and the

boundary layer is expected to thin if U (or •

totV ) is increased. Clearly if the vapor

concentration is maintained, the decrease in δ can result in an increased deposition rate.

If all flow conditions in the deposition chamber are kept constant, the changes in

rdep mirror changes in revap. Figure 3-7 shows data from an experiment where •

sccmV was

increased, while •

totV , Pcell, and Tcell were kept constant. The deposition rate behaves

74

analogously to the evaporation rate, as depicted in Fig. 3-3c. The plot of 1/rdep vs. 1/•

sccmV

is linear, as predicted by Eq. (3-15), and can be used to estimate various parameters for

each material source configuration, such as α·Ae and the equilibrium vapor pressure.

Figure 3-7: Experimentally determined deposition rate of Alq3 on Si, matching the qualitative prediction in Fig. 3-4. Equation 3.11 is confirmed by plotting 1/rdep vs. the inverse of the carrier gas flow rate through the source (1/ V) cell, while keeping the total flow rate constant by means of the make-up N2 flow.

Figure 3-7: Experimentally determined deposition rate of Alq3 on Si, matching the qualitative prediction in Fig. 3-4. Equation 3.11 is confirmed by plotting 1/rdep vs. the inverse of the carrier gas flow rate through the source (1/ V) cell, while keeping the total flow rate constant by means of the make-up N2 flow.

75

Chapter 4: Proof of Concept &

Experimental Verification of Theory

4.1 Overview

The basic experimental set-up is described in this chapter, along with results validating

the concept, including growth of OLEDs, roll-to-roll deposition of organic thin films, and

precise molecular doping. Data and computer simulations are presented supporting the

theory developed in Ch. 3.

4.2 Experimental design

As Eq. (3-23) suggests, full control of the deposition process requires ability to control

totV•

, cellV•

, Pdep, Pcell, Ts, Tcell. Figure 4-1a is a schematic of the deposition system used

for most of the OVPD experiments discussed here. The main vessel consists of an 11 cm

diameter by 150 cm long Pyrex® cylinder, with flanges on each end for attachment of the

source baseplate and substrate. The center of the main chamber is positioned inside of a

three-zone tube furnace used to establish a temperature gradient along the chamber axis

(See Fig. 4-1b). Each source material resides inside a round glass container, attached to

the end of a 6mm diameter hollow glass pushrod. The round container is closed at the

back to prevent sudden dislodging of the source powder upon pump-down and sudden

76

increase in the carrier gas flow. Each source cell is further separately encased in a 2.5 cm

diameter by 75 cm long glass barrel. The pushrod enters the barrel through an

UltraTorr™ type Viton™ o-ring seal, enabling it to be moved in and out of the furnace.

The desired organic source temperature is attained by appropriate positioning of each cell

along the temperature gradient within the reactor (Fig. 4-1b). By this means, two or more

Thickness MonitorThermocouplesN2 Inlets Shutter

FurnacePump Port

Glass tubeSource barrels Substrate

holder

Sliding mount

Thickness MonitorThermocouplesN2 Inlets Shutter

FurnacePump Port

Glass tubeSource barrels Substrate

holder

Sliding mount

Figure 4-1: a) Schematic of the OVPD system described in detail in the text. b) Measured axial temperature profile in the system, showing how the temperature of each source may be controlled by its axial position. The temperature profile is regulated by the three zone temperature settings of the furnace, as shown for two Zone 3 settings.

0

50

100

150

200

250

300

-10-5051015202530354045

Dopant

Host

Temp(oC)

Distance (inches)

T3 low

Cooling water

Substrate

0

50

100

150

200

250

300

-10-5051015202530354045

Dopant

Host

Temp(oC)

Distance (inches)

T3 low

Cooling water

Substrate

(a)

(b)T3 high

77

compounds with different vapor pressures can be simultaneously evaporated at

comparable rates. Each barrel terminates in a 10 cm long by 0.6 cm diameter snorkel; this

increases the carrier exit velocity, preventing back-diffusion of organic vapors into the

sources.

Carrier gas flow rates are regulated by mass flow controllers, while a combination

of a 40 lpm scroll pump and a butterfly valve was used to regulate the reactor pressure

between 0.025 and 760 Torr. A liquid nitrogen cold trap is also employed to prevent

contamination of the pump oil with residual organic vapor. Evaporation temperature of

each source is measured by thermocouples positioned behind each source boat, while the

temperature profile of the deposition chamber is obtained using an axially positioned

thermocouple probe. Organic vapors condense onto a rotating water-cooled substrate

positioned behind a mechanically operated shutter. Film thickness and growth rate are

monitored by a quartz crystal microbalance. The tooling factor (i.e. the ratio of actual

deposition rate on the substrate to the rate measured by the balance) is calibrated by

measuring the deposited film thickness with an ellipsometer. The real OVPD system is

pictured as constructed in Fig. 4-2.

4.3 Growth of Alq3 films

The system is first characterized by calibrating the deposition rate for each source for a

set of process conditions. The deposition rate is first determined by depositing a film (e.g.

Alq3) on silicon, while monitoring the rate with a quartz crystal microbalance. The actual

deposited film thickness is measured by ellipsometry, providing a tooling factor for a

given set of process conditions, which can be used for deposition on other substrates (e.g.

78

Figure 4-2: Photograph of the OVPD system, including the clam-shell furnace, open to reveal the 2m long by 10cm diameter glass deposition chamber. Upstream, a flange is fitted holding four source barrels with individual N2 flow inlets, while the cooled rotating substrate holder is positioned downstream (right side of photo), opening into a plexiglass enclosure by means of a sliding bearing mount. Below the enclosure is a chiller used to regulate the substrate temperature. The four frames below illustrate more closely some of the key components of the system.

Figure 4-2: Photograph of the OVPD system, including the clam-shell furnace, open to reveal the 2m long by 10cm diameter glass deposition chamber. Upstream, a flange is fitted holding four source barrels with individual N2 flow inlets, while the cooled rotating substrate holder is positioned downstream (right side of photo), opening into a plexiglass enclosure by means of a sliding bearing mount. Below the enclosure is a chiller used to regulate the substrate temperature. The four frames below illustrate more closely some of the key components of the system.

79

ITO-coated glass) on which ellipsometry is difficult. First, the variation of single-

component (in this case, Alq3) deposition rate with carrier gas flow rate is investigated.

For the reactor and source cell configuration used, the source operates mainly in

the kinetically limited regime, where the flux of organic vapor into the deposition

chamber is limited by the evaporation rate. Keeping all process conditions constant

except the carrier gas flow rate through the source barrel, the vapor will become diluted

with increasing carrier gas flow rate. Thus, since the film deposition rate is diffusion

limited, Eq. (3-23) simplifies to:

1

2

exp1

1•

∆⋅ −

= ⋅δ +

vap

celldep

HkRT

rk V

(4-1)

where k1 and k2 are constants. Eq. (4-1) can be rewritten as:

3 41 exp

vap

dep cell

Hk k Vr RT

• ∆ = + ⋅ ⋅

(4-2)

where k3 and k4 are constants which depend on the loading of the source, diffusivity,

boundary layer thickness, and the relative orientation of the source barrel and substrate.

Figure 4-3 shows the dependence of the Alq3 deposition rate on the carrier gas

flow rate through the Alq3 source barrel for three different values of Tcell. The plot

confirms that for these deposition conditions, the organic vapor is apparently diluted by

the carrier gas, leading to the decrease in rdep with •

V ; this implies that the source is in the

kinetic evaporation regime. The linear fits of the data were performed using ∆H =

162kJ/mol, obtaining k3 = [1.72 +0.5/-0.32]·10-16 s·A-1, and k4 = [6.9 +3.1/-3.58]·10-18

s·A-1·sccm-1. Similar experiments were performed for several other compounds, including

α-NPD, which is frequently used as the hole-transporting material in OLEDs. Subsequent

80

growth of OLEDs in OVPD made use of the deposition conditions determined in the

manner described above, but with in-situ monitoring in place, due to the potential

variations in the source material condition with time or pressure (i.e. k3 and k4).

4.4 Evaporation rate and decomposition temperature

As Eq.(4-2) indicates, the evaporation and hence the deposition rates scale exponentially

with the source temperature, Tcell. This means that using the same source temperature for

materials having very different characteristic vapor pressures can result in dramatically

different deposition rates. Thus, we are also interested in predicting the evaporation

temperature range for a variety of organic semiconductors. In addition to a general

Figure 4-3: Dependence of the Alq3 deposition rate (on silicon) on volumetric carrier gas flow rate for three different source temperatures, Tcell = 269, 276, 278°C.

0 5 10 15 20 25 30 35 40 45 50 550.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

276 C 269 C 278 C

1/r de

p (1/

A)

Barrel Flow (sccm)

0 10 20 30 40 500.0

0.5

1.0

1.5

2.0

r dep (

A/s)

Barrel Flow (sccm)

81

practical importance, this will also set the temperature gradient (See Fig. 4-1b) to be used

in the OVPD system described above. For convenience, thermogravimetric analysis

(TGA) was used.

1.2 1.3 1.4 1.5 1.60.01

0.1

1

∆H vap=176 kJ/mol

∆ H v a p = 1 2 8 k J / m o l

∆ H v a p = 1 6 6 k J / m o l

∆ H v a p = 1 3 9 k J / m o l∆ H v a p = 1 6 2 k J / m o l

C u P c

I r ( p p y )

P t O E P

α - N P D

A l q 3

E v a

p o

r a t

i o n

R a

t e (

m g

/ m i n

)

1 0 0 0 / Tcell ( K - 1 )

Figure 4-4: a) Schematic of the thermogravimetric analysis (TGA) apparatus, where the sample is weighed while being heated in a stream of N2 or other inert gas. b) Plot of the evaporation rate versus inverse temperature for several compounds typically used in OLEDs. The slope of the curves is –∆Hvap/RTcell, where ∆Hvap is the enthalpy of vaporization specific to each compound.

(a)

(b)

82

The TGA technique is illustrated in Fig.4-4a; material is placed into a pan and is

weighed while being heated in a stream of N2. The mass of the sample and its

temperature are recorded as function of time. Fig.4-4b plots revap vs. 1/T for several

commonly used OLED materials. Here, platinum octaethyl porphyrine (PtOEP), fac-

tris(2-phenylpyridine) iridium (Ir(ppy)3) are dopants, Alq3 and α-NPD are the electron

and hole transport materials, respectively, and copper phthalocyanine (CuPc) is the hole

injecting material. Readily apparent is the grouping of materials according to their

evaporation rates and temperature ranges. If a single chamber is to be used to deposit

CuPc, Alq3 and α-NPD, the walls must be kept hot enough to prevent condensation of

CuPc – a temperature much higher than the minimum required to prevent parasitic

condensation of Alq3 and α-NPD on the walls.

Table II lists the enthalpies of evaporation of each material, including those of

DCM2 and 4,4-N,N-dicarbazolebiphenyl (CBP), calculated from the thermogravimetry

and OVPD data. Previous determinations of the enthalpy of vaporization of Alq3 and

CuPc using thermogravimetry and mass spectroscopy (Yase et al. 1995; Yase et al. 1996)

agree with values found here. In addition, the enthalpy of vaporization determined by

OVPD of Alq3 yields ∆Hvap = (162±76) kJ/mol, also in good agreement with the TGA

results. Thermogravimetry of DCM2 above 250°C resulted in decomposition and

formation of non-volatile products, so the TGA results were not representative of DCM2

evaporation. On the other hand, OVPD of DCM2 at temperatures below 230°C yielded

∆Hvap = (110±11) kJ/mol, as shown in Fig. 4-5.

83

4.5 Thermal decomposition of source materials

High rate of device fabrication (e.g. on the scale of m2/s) requires a correspondingly high

evaporation rate; revap = C·exp(-∆Hvap/RT), where C is a constant that depends on factors

like the surface area of the source material, geometry and flow conditions in the source

container. The evaporation rate increases rapidly with temperature, T, and decreases with

the evaporation enthalpy, ∆Hvap which is specific to each material, as shown in Sec. 4.2.

a Measured using OVPD datab Doping into Alq3c Thermogravimetry by Yase et al.d Mass spectrometry by Yase et al.

TABLE II: Vaporization enthalpies of common OLED materials. All values obtained by thermogravimetry in this study, except as otherwise indicated





84

However, Tcell cannot be raised arbitrarily, because when the thermal energy exceeds the

bond strengths, or more likely exceed the activation energy of a chemical reaction (e.g.

polymerization of the conjugated organic molecules), the source material can rapidly

degrade. Thus, there is an upper bound on the specific evaporation rate due to the limited

chemical stability of the source. This is illustrated by the evaporation of DCM2, a red

fluorescent laser dye. Plotted in Fig. 4-5 is the deposition rate of this dye vs. 1000/Tcell,

exhibiting the expected exponential dependence on Tcell. However, as Tcell continues to

increase, the slope of the curve changes abruptly, indicating that the material has been

chemically altered, resulting in a different ∆Hvap. Indeed, examination of the source

1.9 2.0 2.1 2.2 2.30.01

0.1

1

10

N

O CH3

NC CN

DCM2

Figure 4-5: Plot of the deposition rate vs. 1/Tcell for DCM2, where the evaporation of DCM2 was done by OVPD, with the deposition rate being moni-tored by a quartz crystal micro-balance in situ. Increasing the temperature of DCM2 source resul-ted in decomposition and production of a more volatile species, as evi-denced by the sharply increased deposition rate for Tcell > 240°C.

DC

M2

Dep

ositi

on R

ate

(Å/s

)

1000/Tcell (K-1)

1.9 2.0 2.1 2.2 2.30.01

0.1

1

10

N

O CH3

NC CN

DCM2

Figure 4-5: Plot of the deposition rate vs. 1/Tcell for DCM2, where the evaporation of DCM2 was done by OVPD, with the deposition rate being moni-tored by a quartz crystal micro-balance in situ. Increasing the temperature of DCM2 source resul-ted in decomposition and production of a more volatile species, as evi-denced by the sharply increased deposition rate for Tcell > 240°C.

DC

M2

Dep

ositi

on R

ate

(Å/s

)

1000/Tcell (K-1)

85

container after the deposition reveals a hard, frothed, black carbonaceous compound, not

at all resembling the fine powder at the start of the experiment.

Furthermore, as seen from Fig. 4-4, some compounds have similar vaporization

enthalpies, but have dramatically different vapor pressures. If one deposition chamber is

to be used to deposit both CuPc and α-NPD, the wall temperature must be sufficiently

high to prevent condensation of CuPc, but low enough to prevent decomposition of α-

NPD.

The onset of thermal decomposition can be also ascertained by differential

scanning calorimetry (DSC) (Fig.4-6a). In this method, the sample is heated alongside a

“standard” such as gold or aluminum, having a constant heat capacity in the temperature

range of interest. Keeping the temperature of the sample and the standard equal (but

raising it progressively with time), and monitoring the difference in the heat flow through

the materials, phase and chemical changes can be determined. Figure 4-6b shows results

for Alq3, CBP, α-NPD, and CuPc; (samples were loaded into aluminum sample pans

under dry nitrogen). Some materials (Alq3 and CuPc) exhibit greater thermal stability

than others (CBP and α-NPD). By comparing Figs. 4-4b and 4-6b, it becomes apparent

that the high temperatures required to evaporate CuPc at rates comparable to α-NPD

would likely result in decomposition of α-NPD. The argument applies especially in the

case of co-deposition of two components, as in doping of the films for control of the

emission wavelength.

86

100 200 300 400 500 600 700-25-20-15-10-505

1015202530

Q (J

)

Temperature (C)

Alq CBP CuPc NPD

Reference Sample

Tref = Ts

Qs Qr

Q = Qs-Qr

Al pans sealedunder dry nitrogen

(a)

b)

Figure 4-6: a) Schematic of the differential scanning calorimetry (DSC) experiment, where the sample is being heated to the same temperature as a reference (e.g. Al), while monitoring the difference in the heat flow to each container. b) Plotting Q vs. Tcell results in a straight line if no phase change is taking Place (slope ~ heat capacity), a spike if an endothermic phase change is taking place (i.e. melting), and a permanent change in the slope if the material is being chemically transformed (i.e. the heat capacity has been permanently altered.)

(b)

100 200 300 400 500 600 700-25-20-15-10-505

1015202530

Q (J

)

Temperature (C)

Alq CBP CuPc NPD

Reference Sample

Tref = Ts

Qs Qr

Q = Qs-Qr

Al pans sealedunder dry nitrogen

(a)

b)

Figure 4-6: a) Schematic of the differential scanning calorimetry (DSC) experiment, where the sample is being heated to the same temperature as a reference (e.g. Al), while monitoring the difference in the heat flow to each container. b) Plotting Q vs. Tcell results in a straight line if no phase change is taking Place (slope ~ heat capacity), a spike if an endothermic phase change is taking place (i.e. melting), and a permanent change in the slope if the material is being chemically transformed (i.e. the heat capacity has been permanently altered.)

(b)

87

4.6 Deposited film morphology and composition

In addition to characterizing the growth rate, several deposited film properties must also

be characterized. Optoelectronic devices of interest (e.g. OLEDs) typically consist of

thin-film multilayers of organic compounds, with low conductivities that result in

operation under high electric fields. The microscopic variation in the device thickness can

lead to shunt current paths (shorts) and premature death of the OLED in operation.

Typically, this requires the film roughness to not exceed ~5% of the total film thickness.

For example, in a single-heterojunction OLED consisting of 500Å α-NPD and 500Å Alq3

films, individual film roughnesses should be < 25Å.

For typical growth rates (<5Å/s), OVPD obtains molecularly smooth films (Fig.

4-7a). However, because of the diffusion-limited deposition characteristic of OVPD,

increase in the deposition rate is via increase in the vapor concentration at the edge of the

boundary layer. As the organic molecules traverse the stagnant boundary layer, they

gradually cool below the saturation limit, potentially leading to homogeneous nucleation

in the gas phase. The formation of organic particles in the vapor phase can lead to a

dramatic increase in the surface roughness, and should be avoided. For example,

depositing α-NPD on Si, the transition from molecular film growth to gas-phase

nucleation occurs around 10Å/s, worsening as the rate increases (See. Fig. 4-7).

The chemical composition of the deposited film can be verified in a number of

ways, including from their photo- and electroluminescence (PL and EL) spectra. For

example, Figure 4-8 shows the PL spectrum of an Alq3 film grown by OVPD on Si,

exhibiting an intensity maximum at 530nm, similar to what is typically obtained for the

Alq3 source.

88

Figure 4-7: Atomic force micrographs of 2000-Å thick α-NPD films deposited on Si by OVPD. Each scan is a 5µm square, with the vertical range of 500Å. The films were deposited at rates of (top to bottom) 8.4, 10.7, and 11.8 Å/s, exhibiting a corresponding rms surface roughness of 8, 36, and 45 Å.

Figure 4-7: Atomic force micrographs of 2000-Å thick α-NPD films deposited on Si by OVPD. Each scan is a 5µm square, with the vertical range of 500Å. The films were deposited at rates of (top to bottom) 8.4, 10.7, and 11.8 Å/s, exhibiting a corresponding rms surface roughness of 8, 36, and 45 Å.

89

4.7 Growth of OLEDs using OVPD

A heterojunction OLED similar to that reported by Tang and van Slyke (Tang et al. 1987)

was grown by OVPD. It consisted of a 500 Å thick α-NPD hole transport and a 500 Å

thick Alq3 electron transport layer grown by OVPD on top of a pre-cleaned ITO-coated

glass substrate. To deposit the metal cathode, the samples were transferred into a thermal

evaporator (base pressure <10-6 Torr), where 1000 Å thick layer of Mg co-evaporated

with Ag was grown in a 25:1 mass ratio. A 500 Å thick Ag cap was used on top of the

MgAg layer to prevent oxidation of the cathode during device testing. A similar structure

was grown by thermal evaporation of both organic and metal layers, without exposing the

organic layers to the atmosphere prior to deposition of the cathode. The device structure

is illustrated in the inset of Fig. 4-9.

400 500 600 700 800λ (nm)

Alq3

Figure 4-8: Molecular structure (inset) and photo-luminescence spectrum of an Alq3 film grown on Si by OVPD

Nor

mal

ized

PL

Inen

sity

400 500 600 700 800λ (nm)

Alq3

Figure 4-8: Molecular structure (inset) and photo-luminescence spectrum of an Alq3 film grown on Si by OVPD

Nor

mal

ized

PL

Inen

sity

90

Figure 4-9 shows the current density vs. voltage (J-V) and quantum efficiency vs.

current density the OVPD-deposited (1mm diameter) OLED. Its characteristics are

similar to those of the vacuum-deposited control device, also plotted on the same graph.

At a current density of 1 A/cm2, the voltage for a vacuum deposited device is about 2.5 V

lower than for the OVPD-grown device, yet the leakage current (at V<2V) is an order of

magnitude higher. This suggests a slightly thicker organic stack in the case of the OVPD

device and/or the presence of a thin oxide layer between the organic layer and the

Figure 4-9: Electrical characteristics of the hetero-junction OLED comprised of ~500Å thick layers of α-NPD and Alq3 grown by OVPD on ITO-coated glass, followed by VTE of the 1000Å thick Mg:Ag (25:1) ca-thode. The current voltage response and the quantum efficiency of the vacuum-deposited control device are also shown.

J (m

A/c

m2 )

Voltage (V)

J (mA/cm2)

η ext

(%) Alq3

NPD

0.1 1 10

O V P D

V T E

10 -5

10 -4

10 -3

10 -2

10 -1

1

10 1

10 2

0.1

0.2

0.3

0.4

0.50.6

1.0

0.1 1.00.01

Figure 4-9: Electrical characteristics of the hetero-junction OLED comprised of ~500Å thick layers of α-NPD and Alq3 grown by OVPD on ITO-coated glass, followed by VTE of the 1000Å thick Mg:Ag (25:1) ca-thode. The current voltage response and the quantum efficiency of the vacuum-deposited control device are also shown.

J (m

A/c

m2 )

Voltage (V)

J (mA/cm2)

η ext

(%) Alq3

NPD

0.1 1 10

O V P D

V T E

10 -5

10 -4

10 -3

10 -2

10 -1

1

10 1

10 2

0.1

0.2

0.3

0.4

0.50.6

1.0

0.1 1.00.01

J (m

A/c

m2 )

Voltage (V)

J (mA/cm2)

η ext

(%) Alq3

NPD

0.1 1 10

O V P D

V T E

10 -5

10 -4

10 -3

10 -2

10 -1

1

10 1

10 2

0.1 1 100.1 1 10

O V P D

V T E

10 -5

10 -4

10 -3

10 -2

10 -1

1

10 1

10 2

0.1

0.2

0.3

0.4

0.50.6

1.0

0.1 1.00.01

91

cathode that may have formed during the transfer step (Burrows et al. 1996). In addition,

the external quantum efficiency, ηext, of the vacuum evaporated OLED is only marginally

higher than the vapor-deposited OLED. The small difference between these devices

suggests that OVPD preserves the purity of the starting materials and produces high

quality organic thin films, while avoiding air-exposure during processing can help

eliminate the remaining small difference in performance.

4.8 Control of dopant concentration using temperature

For co-deposition of two materials, A and B, the ratio of their respective deposition rates

gives the ratio of their concentration in the deposited film (Shtein et al. 2001):

'

0

'

0

exp

exp

vapA A A

AAA A Avap

BB B B BB

B B

H kP kRTx T V

x T H kP kRT V

•

•

−∆+

= ⋅ −∆ +

, (4.3)

where xA and xB are the mole fractions of A and B, respectively, while kA and kB are

material- and source-specific constants which vary relatively weakly with T and V•

.

Thus, the deposition rate can be simultaneously and independently controlled by both cell

temperature and gas flow rate through each source. The flow is used for fine control,

while the temperature regulation produces greater variation course control of

concentration.

Films of Alq3 doped with DCM2 (Fig. 4-10a) were grown by OVPD on pre-

cleaned glass and silicon substrates. The concentration of DCM2 was varied by means of

adjusting the DCM2 source temperature for each sample. The photoluminescence (PL)

spectra of the resulting films (Fig. 4.10b) were measured under nitrogen (to prevent

92

Figure 4-10: a) Glass slides with 1000Å thick DCM2-doped Alq3 films grown by OVPD; b) corresponding photoluminescence spectra of films in (a), exhibiting a red-shift with increasing DCM2 concentration; c) Plot of the DCM2 concentration determined from its emission spectrum vs. 1/Tcell of the DCM2 source, showing ∆Hvap = 157 kJ/mol, similar to what was found in Fig.4-5.

2.10 2.15 2.20 2.250.1

1

10

∆Hvap = 157 +/- 0.15 kJ/mol

400 500 600 700 800

λ (nm)

10% DCM2Alq3

(a)

(b)

(c)

% D

CM

2 in

Alq

3N

orm

aliz

ed P

L In

ensit

y

1000/TDCM2

93

potential degradation of PL due to O2) using a λ = 400nm wavelength excitation source

with a λ = 400nm filter placed at the spectrometer input to prevent detector saturation by

the excitation source. The Alq3 spectrum, centered at λ = 526nm, was subtracted from the

doped film spectra prior to determining the DCM2 concentration. The peak wavelengths

of the photoluminescence spectra of DCM2-doped Alq3 films were then converted to

dopant concentrations using a previously determined correlation between these variables

(Bulovic et al. 1999; Shtein et al. 2001). The primary source of error in calculating the

doping concentration of OVPD grown films stems from the uncertainty in the deposition

rate of the vacuum-deposited films (±10%), used to develop the spectrum-concentration

correlation. Figure 4-9c depicts the variation in the DCM2 fraction with DCM2 source

temperature. Temperature values in OVPD evaporation are accurate to within (±1)°C.

The data can be fitted using ∆Hvap = (157 ± 36) kJ/mol, close to that determined using a

single DCM2 source in OVPD.

4.9 Roll-to-roll deposition

Figure 4-11 shows the deposition tube with the clamshell furnace open, after several

weeks of deposition experiments and microns of cumulative film growth. The chamber

walls surrounded by the heating elements remained clean, while a small condensation

ring is visible just past the substrate, where the heating elements end. The estimated

materials use efficiency was nearly 50%, or significantly greater than observed with

VTE.

To demonstrate the capability of OVPD to deposit OLED structures on large-area

substrates, a mechanical attachment was built, housing motorized spools and a special

water-cooled susceptor, illustrated in Fig. 4-12a,b. Polyethylene terephthalate (PTFE)

94

film, pre-coated with ITO was cut into long, 5cm wide ribbons and loaded onto the

source spool of the attachment. One end of the film was attached to the take-up spool,

while the center portion was guided into the deposition chamber by the cold susceptor.

The radius of curvature of the susceptor was kept >0.5mm to minimize possible cracking

of the ITO film.

To deposit the α-NPD/Alq3 heterostructure, the PTFE ribbon was first spooled in

and out of the chamber at a rate of ~5cm/min, while the α-NPD source was heated to

270°C, with 15sccm of N2 flowing through the source barrel. After the α-NPD layer was

completely deposited (~800Å from the estimated deposition rate), the Alq3 source was

Figure 4-11: Photograph showing the deposition tube with the clamshell furnace open, after several weeks of deposition experiments and microns of cumulative film growth. The chamber walls surrounded by the heating elements remained clean, while a small con-densation ring is visible just past the substrate, where the heating elements end. The estimated materials use efficiency is ~50%, over 100 times greater than what has been observed in practice with VTE.

Figure 4-11: Photograph showing the deposition tube with the clamshell furnace open, after several weeks of deposition experiments and microns of cumulative film growth. The chamber walls surrounded by the heating elements remained clean, while a small con-densation ring is visible just past the substrate, where the heating elements end. The estimated materials use efficiency is ~50%, over 100 times greater than what has been observed in practice with VTE.

95

30cm

CW

OVPD chamber

Motor-drivenspools & housing

Cooled susceptor

ITO-coated plastic film

(a) (b)

(c) (d)

Figure 4-12: a) Schematic of the mechanical attachment used to deposit the heterostructures shown in the panels below. b) A photograph of the attachment and the downstream end of the deposition chamber in an open configuration prior to loading of the spools with the plastic film. c) Photograph of the α-NPD/Alq3heterojunction deposited by OVPD on a 30cm long segment of a 5cm wide ITO-coated plastic film under UV illumination. d) Photograph of a 2m long by 5cm wide strip of ITO-coated plastic film with an α-NPD/Alq3-DCM2 hetero-structure. Shown under UV illumination, the blue luminescent region is the bare α-NPD, the green region is the Alq3-covered α-NPD, while the orange-red regions are DCM2-doped Alq3 on top of α-NPD. The red-shift in the DCM2 luminescence is due to the intentionally increasing concentration of the DCM2 molecule in the Alq3 film.

30cm

CW

OVPD chamber

Motor-drivenspools & housing

Cooled susceptor

ITO-coated plastic film

(a) (b)

(c) (d)

Figure 4-12: a) Schematic of the mechanical attachment used to deposit the heterostructures shown in the panels below. b) A photograph of the attachment and the downstream end of the deposition chamber in an open configuration prior to loading of the spools with the plastic film. c) Photograph of the α-NPD/Alq3heterojunction deposited by OVPD on a 30cm long segment of a 5cm wide ITO-coated plastic film under UV illumination. d) Photograph of a 2m long by 5cm wide strip of ITO-coated plastic film with an α-NPD/Alq3-DCM2 hetero-structure. Shown under UV illumination, the blue luminescent region is the bare α-NPD, the green region is the Alq3-covered α-NPD, while the orange-red regions are DCM2-doped Alq3 on top of α-NPD. The red-shift in the DCM2 luminescence is due to the intentionally increasing concentration of the DCM2 molecule in the Alq3 film.

96

heated to 275°C under 15sccm of N2 barrel flow, and the spooling direction was reversed,

resulting in an ~800Å thick Alq3 layer. In the case of the DCM2-doped Alq3 layer, the

DCM2 source was progressively heated to higher temperatures to increase its vapor

concentration in the deposition chamber during the deposition of the Alq3 layer.

4.10 Summary

This chapter reviewed the design and construction of an experimental OVPD system

based on the theory and computer models developed in Ch. 3. This deposition system

was then used to verify the theory of growth rate and doped film deposition, and

subsequently to deposit molecular organic thin films and hetorostructures for OLEDs.

The limiting film growth regimes were explored, where the thermal decomposition of the

source material can limit the maximum source evaporation rate, and homogeneous

nucleation due to vapor supercooling near the substrate can introduce roughness in the

deposited films. To demonstrate the capability of OVPD to deposit over large areas, an

attachment to the apparatus was constructed, enabling roll-to-roll deposition of the

archetypal α-NPD/Alq3 and DCM2-doped Alq3 OLED heterostructure on a plastic

ribbon.

97

Chapter 5: Growth in confined

geometries, application to patterning

5.1 Overview

Most practical organic electronic devices require the active organic layers to be patterned

in the substrate plane. As molecular organic semiconductors are deemed incompatible

with traditional semiconductor fabrication methods (e.g. photolithography and plasma

processing), the patterning typically involves the use of shadow-masks. In this chapter we

address the challenge of using OVPD and shadow-masking for thin film patterning in the

substrate plane. Gas phase molecular transport simulations are first presented to outline

process conditions permitting pattern resolution on the order of 1 µm in the deposited

organic films, which is adequate for typical full-color display applications. The

experimental results are presented in Ch. 6.

5.2 The need for patterning – OLED example

Many thin film devices require the active layers to be patterned in the substrate plane. For

example, full color displays typically consist of millions of pixels, each of which is

composed of three sub-pixels, engineered to emit in the red, green, and blue regions of

the visible spectrum. An example of a passive-matrix full color pixel substructure is

illustrated in Fig. 5-1. Provided the sub-pixels are spaced sufficiently closely, the human

98

Figure 5-1: a) Simplified schematic of a pixel in a full-color passive matrix display, consisting of three sub-pixels, engineered to emit in the red, green, and blue part of the spectrum. b) A typical sequence of fabricating the pixel in (a), involving the deposition in vacuum of a red-emitting sub-pixel through a shadow-mask which covers ~2/3 of the substrate, followed by the same for a green-emitting pixel, followed by the blue emitting pixel. The device is completed by depositing a metal cathode through yet a separate shadow mask. In some cases, the substrate contains an integral shadow mask prior to the organic deposition sequence, eliminating the need for step (5).

glass / plastic

ITO

SiNx

organic

metal150µm

200nm

20µm

1)

2)

3)

4)

5)

(a)

(b)

99

eye will average out the components of light emitted from each; by controlling the

proportional intensities of light from the sub-pixels, any color can be produced.

The electroluminescence wavelength in OLEDs is typically controlled by the

dopant used in the EML; (e.g. PtOEP for red, Ir(ppy)3 for green, FIrpic for blue). Thus,

to avoid unintentional “bleeding” of color between sub-pixels, the doped emissive layers

must be laterally separated.

5.3 Ballistic transport in VTE and shadow-masking

A typical vacuum deposition geometry is illustrated in Fig. 5-2. The organic material is

evaporated from a resistively heated cell onto a substrate positioned directly above it. In

deposition of doped films, two or more materials are co-evaporated with the sources

separated by a baffle, as shown in the figure. The substrate is placed within a distance

less than the molecular mean free path, λ, from the source, and the transport of organic

molecules from the source to the substrate is ballistic. For patterned film deposition, a

shadow mask is placed at a distance s in front of the substrate (Fig. 5-2a). For a mask

aperture with straight walls, thickness, t, and using a source of width l centered on the

aperture axis, the shape of the deposit is approximately trapezoidal, with the width of the

edge taper and, hence, the resolution limit, ρ, given by:

( 2 )2

s t lh

ρ +≈ (5.1)

Here, w is the aperture width, and h is the source-to-mask distance, as defined in Fig. 5-2.

The design guidelines suggested by Eq. (5.1) are intuitive: the mask should be as thin as

possible, while the separation, s, should be small, to produce the sharpest edges.

However, the mask also should not bow under its own weight, and it should withstand

100

multiple mount-remount and cleaning cycles. Thus, the actual mask dimensions represent

a compromise between the pattern resolution and hardware robustness requirements, with

typical values for s, t and w about 10, 70, and 100 µm, respectively. The typical source

diameter, l ≈ 1 cm, and source-to-substrate distance h ≈ 50cm, yielding ρ ≈ 2 µm, which

is adequate for full-color OLED displays.

5.4 Shadow-masking and diffusive transport in OVPD

The OVPD deposition geometry is illustrated in Fig. 5-3a. As shown in Ch.4, the carrier

gas flow creates a hydrodynamic boundary layer at the substrate (typically 1mm < δ <

5cm), across which the organic species diffuse prior to condensation. The organics are a

minority species (< 1% molar concentration) and collide with the carrier gas molecules

en route to the substrate. The carrier gas background pressure far exceeds 10-6 Torr,

leading to the condition λ « δ, where the molecular velocities are completely randomized

pump

substrate

Source A Source B

maskheater

(a) (b) ρ

sw

h

l

substratedeposit

t

source

mask

Figure 5-2: a) Schematic of the VTE process. b) Schematic of pattern formation in the case of ballistic transport occurring in VTE, with definition of the relevant geometrical parameters.

101

near the substrate. This situation is depicted in Fig. 5-3b, equivalent to a source with l »

w and with h « l (i.e. an infinitely wide source positioned very close to the aperture). This

mode of transport is already dramatically different from vacuum thermal evaporation,

where the molecular source-to-substrate distance, h, is large, transport is ballistic, and

molecular trajectories are all nearly perpendicular to the substrate near the aperture.

Furthermore, if λ < s, then the organic molecules undergo scattering collisions in the

space between the mask and the substrate, with a potentially non-trivial dependence on

Coolant

Source A

Source B

Carrier gas

Pump

Pump

(a)

(b)

deposit mask

Substrate

Heater

ts

Figure 5-3: a) Schematic of the OVPD process. b) Illustration of the diffusive mode of transport near the substrate, resulting in randomized molecular trajectories in the vicinity of the mask, ultimately leading to more diffuse patterns than in the case of VTE.

102

the deposition conditions (i.e. Pdep, s, and t). The resulting deposition pattern is likely to

have substantial tails, significantly different from the trapezoidal pattern typically

obtained by VTE. The essential features of the masking geometry and the shape of the

deposited pixel are depicted in Fig. 5-4.

As a means for quantifying pattern shape, we define the shape factor, η, which is

equal to the area bound by -w/2 < x < w/2 (the cross-hatched region in Fig.5-3) divided

by the total deposit cross-sectional area. Then η = 1 for a rectangular deposit, decreasing

as the deposit tails extend beyond the edges of the mask aperture. For example, a deposit

shaped as a Gaussian distribution, with half-width w/2, will have η ≈ 0.67. In analyzing

w

s

tα

mask

α’

z

x

depositprofile

substrate

Figure 5-4: a) Definition of relevant geometrical parameters for the mask aperture and the deposit formed during OVPD. The shaded region corresponds to the “useful” portion of the deposit, defined either by <90% decrease in the deposit height, or by the projected aperture width, w.

w

s

tα

mask

α’α’

z

x

depositprofile

substrate

Figure 5-4: a) Definition of relevant geometrical parameters for the mask aperture and the deposit formed during OVPD. The shaded region corresponds to the “useful” portion of the deposit, defined either by <90% decrease in the deposit height, or by the projected aperture width, w.

103

the effects of aperture geometry on the deposited feature shape, we also consider the

fraction, f, of molecules lost by deposition on the side of the mask facing the substrate,

and on the aperture walls. Then, the deposition efficiency is simply ε = 1 – f.

5.5 Continuum-based analysis is inaccurate for Kn < 10

Assuming simple kinetic theory of a gas and a hard-sphere intermolecular interaction

potential, the molecular mean free path is given by: (Bird et al. 1996)

22B

dep

k Td P

λπ

⋅=

⋅ ⋅ (5.2)

where kB is the Boltzmann constant, T is the gas temperature, and πd2 is the effective

collision area for two molecules, each having a diameter d. At 25°C and a typical VTE

chamber pressure of 10-6 Torr, λ > 50 cm for nitrogen (d = 3.25 Å), while in OVPD at

Pdep = 0.1 – 10 Torr, λ ~ 100 – 1 µm.

The Knudsen number, Kn, is frequently used to characterize different transport

regimes in terms of the ratio of λ to the critical apparatus dimension, L (Kn = λ/L)

(Stechelmacher 1986) The dimension L is typically chosen such that Kn represents the

ratio of intermolecular to molecule-wall collision frequencies. For gas flow between the

source and the substrate regions, L is the length of the transport chamber or the run lines,

so that Kn << 1; thus, continuum models can be used to understand flow dynamics and

the deposition rate on the scale of centimeters or meters. However, when film patterning

is considered, the apparatus dimension must account for the aperture geometry. In this

case, L is 1 – 100 µm, and 0.1 < Kn < 10 for much of the useful deposition pressure

range. As turns out to be the case, the discrete molecular nature of the gas flow cannot be

ignored. (Bird 1994) This can be illustrated as follows.

104

For the geometry depicted in Fig. 5-4, the mask of thickness t and aperture of

width w is separated from the substrate by distance s. For isotropic diffusion, the width of

the deposit base will increase with s. That is, the longer it takes for a molecule to diffuse

to the substrate, the longer it will take (by the same amount) for it to diffuse laterally. The

mutual cancellation of these rates will result in identical patterns at different pressures,

which, as will be discussed in the following sections, does not match the observed

experimental trend. Moreover, in the continuum assumption, the pattern formation does

not depend on the shape of the aperture above the edge closest to the substrate, which is

again contrary to experiment. Due to the discrete nature of transport at very low pressure,

some collimation of flux through the aperture must occur when the organic molecules

condense on the aperture sidewall.

Thus, while the continuum analysis helps visualize transport on the scale of the

chamber dimension, to predict the OVPD growth of patterns whose size and resolution

are on the order of λ, the molecular nature of transport near the substrate must be

considered. The following sections employ Monter-Carlo simulations to obtain

quantitative information about the pattern shape, as well as other merit figures of

practical interest, such as efficiency of material transport through the apertures. The

simulations are then compared with experimental observations for a limited number of

conditions. On the basis of the close correspondence between the simulated and

experimentally obtained results, the former technique is used to examine the potential

improvements of new geometries and conditions that can be difficult to achieve

experimentally without a great expenditure of time and resources. The resolution limits of

shadow masking are explored.

105

5.6 Modeling gas transport in confined geometries

Molecular flow through small channels has been previously studied using several

techniques. For example, Guevremont and coworkers (Guevremont et al. 2000)

experimentally analyzed the shape of molecular fluxes from collimated effusive beam

sources and the nature of gas transport within small-diameter capillary arrays. The beam

shape was obtained by translating the beam across a skimmer, behind which a mass

spectrometer collected the molecular flux. Channel conductance was studied for a range

of Kn. The sources consisted of arrays of capillaries, whose individual diameters were as

small as 10µm. Although λ in their experiments ranged from 1 to 104 µm thereby

covering a wide range of Kn, the overall diameter of the source array, and hence the beam

width, was > 1mm. Furthermore, stagnation of the flow due to the substrate was absent,

in which case the molecular flux is more collimated than when the velocity distribution is

isotropic, as in the case of pure diffusion. Thus, although the beam profiles resemble the

pattern shapes obtained by OVPD, those results are not strictly applicable.

In contrast to the case where collimated beams are employed, transport of species

to the substrate is often diffusive during reactive ion etching and physical vapor

deposition. Semiconductor electronic device processing typically involves etching of, and

deposition in holes and trenches of sub-micron dimensions.(Wolf et al. 1999) The

processing pressure, however, is typically such that Kn ≈ 1, similar to OVPD growth of

patterned films. Studies reveal that Monte-Carlo type simulations can accurately predict

deposition and etch rates when coupled with the proper surface reaction models,

(Akiyama et al. 1995; Griffiths et al. 1998) suggesting their applicability to

micropatterning of thin film deposits by OVPD. Models of OVPD growth, however, are

106

greatly simplified by the absence of gas-phase and surface reactions, and re-evaporation

of adsorbed species that must be considered for reactive deposition or etching (Wulu et

al. 1991).

5.7 Monte-Carlo simulation of OVPD through apertures

The computer simulation models the diffusion of organic molecules through the stagnant

carrier gas layer to the substrate. The boundary conditions include a source of organic

molecules at some distance δ from the substrate, and sinks at the mask and substrate

surfaces. To avoid the shortcomings of the continuum approximation, the molecular

nature of transport in the deposition process is studied using a stochastic (Monte-Carlo)

approach. The objective of the simulations was to examine the trends in deposit shape as

functions of process parameters, such as the deposition pressure, temperature, molecular

diffusivity, mask geometry, and mask-to-substrate spacing.

5.7.1 Simulation set-up

The MC model allows the variation of λ directly, tracking the random path of a molecule

as it diffuses through a background of carrier gas in the vicinity of the substrate. For

simplicity, the molecular sticking probability on the substrate and mask surfaces is

assumed to be unity, which is usually valid for typical multi-ring aromatic semiconductor

molecules and substrate temperatures ≤ 300K. Generally, under OVPD growth at T =

250ºC, the vapor pressure of the organics is in the 10-3 Torr range, (Shtein et al. 2001;

Yase et al. 1995; Yase et al. 1996) while the background carrier gas pressure is on the

order of 0.01 to 10 Torr, allowing for a gradient in the organic vapor concentration at a

relatively constant total pressure. In this case, organic-organic collisions are rare except at

107

very high deposition rates where gas phase nucleation can occur. (Shtein et al. 2001)

Hence the model is valid only for low-to-moderate (≤50 Å/s) film deposition rates.

The rate of molecular transport in gases due to a concentration gradient is

characterized by the diffusivity, D: (Bird et al. 1996)

13

= ⋅D u λ (5.3)

where ū is the molecular mean thermal velocity. Since the organic vapors consist of

molecules that have collision diameters greater than 10Å and are geometrically and

energetically more complex than the hard sphere atoms of kinetic theory, molecular

dynamics simulations should be used for an accurate estimation of the λ. However, since

this study is concerned with the dependence of trends in pattern evolution on process

parameters, simple kinetic theory equations are sufficient for the analysis.

The Monte-Carlo simulation proceeds as follows. The computational space is

divided into an x-z grid extending infinitely in the y-direction (see Fig. 5-4 for definition

of the coordinate space). The purpose of the grid is to locate the substrate and mask

surfaces, and track changes in the thickness of the deposits. A spherical “test organic

molecule” is assigned a random initial location (x0, y0, z0) inside the boundary layer and

above the mask. A random initial direction is then chosen, and the molecule travels a

distance r = [(x1-x0)+ (y1-y0)+(z1-z0)]1/2, where (x1, y1, z1) is its new location. Molecules

escaping the boundary layer are reflected back toward the substrate. The distance r is the

minimum of the grid size, or λ/10. The probability of collision, Pcoll, with a carrier gas

molecule is equal to r/λ, which is checked against a random number, r·n, between 0 and

1. If Pcoll < r·n, the molecule is again allowed to proceed in the same direction for another

step of distance r. If Pcoll > r·n, the particle is assumed to collide with a carrier gas

108

molecule having a velocity chosen randomly from a Maxwell-Boltzmann distribution.

The collision causes the molecule to be deflected with a velocity and an angle consistent

with momentum and energy conservation in the elastic collision of two hard spheres. If

the path of the particle crosses the substrate plane or the aperture wall, the particle is

assumed to stick to the surface with unity efficiency. A single aperture is simulated, with

periodic boundary conditions imposed in the x-direction. Effects of aperture geometry on

deposited pattern formation are modeled by assigning different aperture sidewall angles,

α = 45, 90, 135°, and α’ = 270°.

5.7.2 First results for patterning of Alq3 thin films

Consider the growth of films of the archetype molecule, tris(8-hydroxyquinoline) (Alq3),

having d~10 Å. The mean free path is calculated using Eq. (5.2), assuming that the

collision diameter for a mixture of Alq3 in a N2 carrier gas is the average of the respective

diameters of the two species in self-diffusion viz.: d = ½(dAlq3+dN2).

Figure 5-5a shows a gray-scale map of the organic species concentration from a

simulated deposition of 105 particles, with s = 7µm, t = 3µm, and α = 135°, and λ =

100µm, corresponding to a total deposition pressure of Pdep ~ 0.1 Torr at T = 500K.

While the simulated particle diameter was 10 Å, it was enlarged in the figure to better

illustrate the deposited profile. The initial particle velocities were assigned from a

random thermal distribution, and are superimposed onto a z-directed velocity vector with

a magnitude of 10m/s. The size of the individual particle has been enlarged to show the

deposited film thickness profile. By decreasing the λ by a factor of 10, a more diffuse

pattern is obtained, shown in Fig. 5-5b. Note that the films shown here are unrealistically

thick and t is unrealistically thin for conventional OVPD conditions; a film thickness of

109

~1000 Å and t ≥ 50 µm are more practical. The figure nevertheless illustrates the

increased parasitic deposition on the mask and aperture walls when λ is reduced. For

clarity, subsequent simulation results will provide only the thickness profile without the

(a)

(b)

mfp = 100µm

mfp = 10µm

10µm

10µm

launching surface

mask deposit

mask

substrate deposit

substrate

log[Concentration], normalized0 1

Figure 5-5: a) Monte-Carlo simulation results plotted as a gray-scale map of the organic species, both as a deposit and as gas phase species using 105 particles, with s = 7µm, t = 3µm, and α = 135°, and λ = 100µm, corresponding to a total deposition pressure of Pdep ~ 0.1 Torr at T = 500K. While the simulated particle diameter was 10 Å, it was enlarged in the figure to better illustrate the deposited profile. b) Simulation conditions identical to (a), except λ = 10µm, corresponding to a ten-fold increase in the deposition pressure, leading to a broadened deposit. The time-averaged molecular trajectories are also visible, showing the rapid dispersion of the species past the aperture edge.

110

shadow mask shown. The total number of Alq3 molecules was also increased to 106 to

obtain smoother, more accurate thickness profiles.

5.7.3 Effects of chamber pressure on deposit shape

The effects of deposition pressure on pattern resolution were investigated by varying λ.

Growth through an aperture with w = 80µm, t = 70µm, and with the substrate separated

from the lower mask edge by s = 20µm was considered. The choices for s and t are

consistent with deposition of high resolution display picture elements (pixels) by VTE,

where the substrate-facing-downward evaporation geometry causes the mask to bow

away from the substrate under gravity, leading to s ~ 20µm. To stiffen the mask and

minimize s, usually t > 70µm is required for VTE. Considerably thinner masks (e.g. t ≤

50 µm) can be used with OVPD due to the possibility of top-down deposition. Figure 5-6

1 10 100 10000.60

0.65

0.70

0.75

0.80

Pixe

l sha

pe fa

ctor

, η

Mean free path, λ (µm)

w = 80 µms = 20 µmt = 70 µm

mask

substrate

Figure 5-6: Variation in the pixel shape factor, η, with the mean free path, λ, obtained from the Monte-Carlo simulation. Depositing at lower pressure (or longer λ) results in more sharply defined edges (i.e. larger η, as defined in the text).

1 10 100 10000.60

0.65

0.70

0.75

0.80

Pixe

l sha

pe fa

ctor

, η

Mean free path, λ (µm)

w = 80 µms = 20 µmt = 70 µm

mask

substrate

Figure 5-6: Variation in the pixel shape factor, η, with the mean free path, λ, obtained from the Monte-Carlo simulation. Depositing at lower pressure (or longer λ) results in more sharply defined edges (i.e. larger η, as defined in the text).

111

shows that the shape factor, η, increases only weakly as λ varies over four orders of

magnitude (1µm ≤ λ ≤ 1000µm, corresponding approximately to 10 Torr ≥ Pdep ≥ 0.01

Torr), as would be expected for a system approaching the Knudsen transport regime.

However, η does not reach unity (corresponding to a rectangular deposit profile) due to

parasitic deposition on the sidewalls of the almost square aperture. As expected, the

highest pattern edge resolution (and hence, highest η) is achieved for the largest λ, i.e.

the lowest Pdep. However, as discussed below, the pattern profile exhibits a dome-like

shape due to a relatively large t/w.

5.7.4 Effects of mask thickness and separation

Increasing the aperture thickness can improve collimation of the molecular flux toward

the substrate due to the condensation on the walls of the aperture. However, as Fig. 5-7

0 100 200 300 400 500 600

Thic

knes

s (a

.u.)

Position (µm)

t = 5,10,20,50µm

s = 20µmmfp = 20µm

t=50µm

mask

substrate

Figure 5-7: Deposit profile as a function of mask thickness, t, obtained from the Monte-Carlo simulation discussed in the text, using 106 Alq3 molecules in N2background carrier gas with mfp = s = 20µm. The aperture dimensions are listed on the plot. The pixel shape factor, η, decreases weakly with s for this set of parameters.

0 100 200 300 400 500 600

Thic

knes

s (a

.u.)

Position (µm)

t = 5,10,20,50µm

s = 20µmmfp = 20µm

t=50µm

mask

substrate

Figure 5-7: Deposit profile as a function of mask thickness, t, obtained from the Monte-Carlo simulation discussed in the text, using 106 Alq3 molecules in N2background carrier gas with mfp = s = 20µm. The aperture dimensions are listed on the plot. The pixel shape factor, η, decreases weakly with s for this set of parameters.

112

shows, increasing t from 5 to 50µm for s = λ = 20µm and w = 300µm, has no effect on

the spreading of the lower edge of the deposit. However, as t/w increases, the deposited

pattern becomes significantly more domed due to the scavenging of the organic

molecules by the upper edges of the aperture. The shape factor decreases by less than 5%

over the range 1µm < t < 100 µm.

The mask separation, s, can vary due to bowing under gravity (in VTE), or

possibly heating during the deposition process. Figure 5-8 shows profiles for depositions

where s = 2, 10, 22, and 50µm, and t = 20µm, w = 300µm, and λ = 20 µm. Appreciable

edge broadening arises due to collisions in the mask-substrate gap for s > λ. In addition,

for large s the dome in the middle of the deposit is also pronounced and, in contrast to the

0 100 200 300 400 500 600

t = 20µmmfp = 20µm

Thic

knes

s (a

.u.)

1.0

η

100s0.8

0

Position (µm)

mask

substrate s = 2µm

10µm

22µm

50µm

Figure 5-8: Simulated deposit profiles for a cylindrical aperture mask at various separation distances, s, with dimensions and details of the simulation listed on the plot. The pattern edge becomes more diffuse, while the deposition efficiency decreases with increasing t due to parasitic deposition on the aperture sidewall, as discussed in the text. Inset: The deposit shape factor decreases approximately linearly with s. (Legend: s = [―] 2, [---]10, [···] 22, [–·–] 50µm.)

0 100 200 300 400 500 600

t = 20µmmfp = 20µm

Thic

knes

s (a

.u.)

1.0

η

100s0.8

0

Position (µm)

mask

substrate s = 2µm

10µm

22µm

50µm

Figure 5-8: Simulated deposit profiles for a cylindrical aperture mask at various separation distances, s, with dimensions and details of the simulation listed on the plot. The pattern edge becomes more diffuse, while the deposition efficiency decreases with increasing t due to parasitic deposition on the aperture sidewall, as discussed in the text. Inset: The deposit shape factor decreases approximately linearly with s. (Legend: s = [―] 2, [---]10, [···] 22, [–·–] 50µm.)

113

case of large t/w, the profile is also broadened at the edges. The inset of Fig. 5-8 shows a

linear decrease in η with s for this series of depositions. Thus, optimal pattern resolution

leading to a rectangular deposit is achieved for the smallest values of s and t, as expected.

Since OVPD can, in principle, be carried out with the mask positioned above the

substrate, thin masks can be used while s is kept small, contrary to the case in VTE. By

using thinner masks, the pattern deposition efficiency, ε, is also increased, because less

material will be deposited on the aperture side-wall, as will be shown below. The

drawback of thin masks is that they are more susceptible to thermally and mechanically

induced stresses. Nevertheless, in a “top-down” configuration of OVPD, smaller values

of s and t are possible, potentially leading to higher resolution patterns than is achievable

using conventional VTE deposition.

5.7.5 Optimizing mask (aperture) shape

It is also possible to minimize t at the aperture edge while keeping the mask thick

elsewhere. The variation in η with the aperture shape was investigated by varying the

aperture side-wall angle, α = 45, 90, 135, and α’ = 270° (see Fig. 5-4). The pattern

profiles for these aperture shapes are shown in Fig. 5-9. The aperture with α = 135°

results in the most diffuse edge of the deposit (smallest η) due to the lack of collimation

of the approach angle of the molecules. This occurs to a lesser extent with the biconical

aperture (α’ = 270°), but the sharpest patterns are achieved with α = 90° and α = 45°.

The variation of the shape factor, η, versus t is plotted in Fig. 5-10a for all four aperture

geometries. While apertures with α = 90 and 45° show a minimal decrease in η with t,

114

curves for α' = 270° and α = 135° drop significantly with t, consistent with the profiles in

Fig. 5-9.

In addition to allowing deposition of sharper patterns, it is desirable to maximize

the material flux to the substrate. The pattern deposition efficiency, ε, normalized to that

of the aperture with α = 45° and t = 2 µm, is plotted vs. t in Fig. 5-10b. For all four

aperture shapes, ε decreases for larger t, the effect being most pronounced for α = 90°.

50 100 150 200 250

50 100 150 200 250

50 100 150 200 250

50 100 150 200 250

α=90º

α=45º

α’=270º

α=135º

(a) (b)

(c) (d)

s=10 µm; mfp=20 µm; t=5-75 µm

Increasing t Increasing t

Increasing tIncreasing t

Figure 5-9: a)-d) Effects of aperture wall profile (and mask thickness, t) on deposit cross-section. In each figure, the mask thickness increases from 5 µm (upper curve) to 75 µm (lower curve) in 10 µm increments. The mask-to-substrate separation, s, and mfp are held constant at 10 µm and 20 µm, respectively.

50 100 150 200 250

50 100 150 200 250

50 100 150 200 250

50 100 150 200 250

α=90º

α=45º

α’=270º

α=135º

(a) (b)

(c) (d)

s=10 µm; mfp=20 µm; t=5-75 µm

Increasing t Increasing t

Increasing tIncreasing t

Figure 5-9: a)-d) Effects of aperture wall profile (and mask thickness, t) on deposit cross-section. In each figure, the mask thickness increases from 5 µm (upper curve) to 75 µm (lower curve) in 10 µm increments. The mask-to-substrate separation, s, and mfp are held constant at 10 µm and 20 µm, respectively.

115

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

α = 90ºPi

xel s

hape

fact

or, η

Mask thickness (µm)

Dep

ositi

on e

ffici

ency

, ε

0 20 40 60 80 1000.0

0.2

0.4

0.6

0.8

ε ·η

α’ = 270ºα = 135ºα = 45º

(a)

(b)

(c)

Figure 5-10: a) Deposit shape factor, η, as a function of the mask thickness, t, for the simulated depositions in Fig. 9. b) Plot of the deposition efficiency, ε, vs. t for various aperture shapes. The efficiency for α = 90º is lowest due to parasitic deposition on the aperture walls. c) The figure of merit, ε·η, represents the combined effects of pattern sharpness and deposition efficiency using a particular aperture configuration. Apertures with α = 45º yield the highest ε ·η.

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

α = 90ºPi

xel s

hape

fact

or, η

Mask thickness (µm)

Dep

ositi

on e

ffici

ency

, ε

0 20 40 60 80 1000.0

0.2

0.4

0.6

0.8

ε ·η

α’ = 270ºα = 135ºα = 45º

(a)

(b)

(c)

Figure 5-10: a) Deposit shape factor, η, as a function of the mask thickness, t, for the simulated depositions in Fig. 9. b) Plot of the deposition efficiency, ε, vs. t for various aperture shapes. The efficiency for α = 90º is lowest due to parasitic deposition on the aperture walls. c) The figure of merit, ε·η, represents the combined effects of pattern sharpness and deposition efficiency using a particular aperture configuration. Apertures with α = 45º yield the highest ε ·η.

116

By plotting the product ε·η vs. t, the cumulative effect of aperture shape on the deposited

pattern can be seen. While the aperture with α = 90° yields sharp deposit edges, the drop

in ε with t for this shape makes it less desirable than the aperture with α = 45° for

depositing well-defined patterns with the least amount of material wasted on coating the

mask. However, since ε includes all material deposited on the substrate, ε·η for α = 135°

is greater than for α’ = 270° and α = 90°, although the pattern edges are significantly

more spread out. Table III contains a summary of the relative influences of changes in

the deposition conditions and mask aperture geometry on the resulting pattern shape

factor, η, pattern deposition efficiency, ε, and the combined figure of merit. In the

following sections experimental results are shown confirming the models developed thus

Table III: Effects of masking geometry and process parameters on pattern shape.

Parameter(a)Pixel Shape

FactorDeposition Efficiency

CombinedFigure of Merit

(η) (ε) (η·ε)

s ↑

a ↑

t ↑

λ ↑

↓

↓

↓(b)

↓

↓

-

↓

↓

↓

↓

↓

↓

(a) Here ↑ indicates an increase in the parameter, ↓ a decrease.(b) For a ≤ 90º, h was insensitive to t.

Table III: Effects of masking geometry and process parameters on pattern shape.

Parameter(a)Pixel Shape

FactorDeposition Efficiency

CombinedFigure of Merit

(η) (ε) (η·ε)

s ↑

a ↑

t ↑

λ ↑

↓

↓

↓(b)

↓

↓

-

↓

↓

↓

↓

↓

↓

(a) Here ↑ indicates an increase in the parameter, ↓ a decrease.(b) For a ≤ 90º, h was insensitive to t.

117

far. A novel method for depositing patterned organic films with self-aligned metal

contacts is also demonstrated.

5.8 Experimental set-up

The deposition of thin films of Alq3 was carried out using the horizontal reactor described

in Ch.4. The deposition profiles of organic thin films obtained using OVPD were

compared with those prepared by VTE. The VTE source-to-substrate distance was

approximately 30 cm and the deposition base pressure was maintained at 10-6 Torr. The

mask and substrate configuration is shown in Fig. 5-11. The substrates were placed into

recessed sections in a specially machined copper susceptor, the mask was layed on top of

the substrate, and a retainer was clamped over the mask edges by means of several

Figure 5-11: a) Schematic of the substrate and mask assembly used for the micropatterning experiment. The substrate fits inside of a machined recess in the copper susceptor, while the Molybdenum mask is placed over it. The mask is separated from the substrate by a spacer (Type A) or by a Nickel mesh (Type B). The stack is compressed by a retainer which clamps the edges of the mask & substrate.

Susceptor

RetainerFastening

screw

Machined recess

SubstrateNickel meshMolybdenum mask

SubstrateSpacer

Molybdenum mask

(Type A) (Type B)

Figure 5-11: a) Schematic of the substrate and mask assembly used for the micropatterning experiment. The substrate fits inside of a machined recess in the copper susceptor, while the Molybdenum mask is placed over it. The mask is separated from the substrate by a spacer (Type A) or by a Nickel mesh (Type B). The stack is compressed by a retainer which clamps the edges of the mask & substrate.

Susceptor

RetainerFastening

screw

Machined recess

SubstrateNickel meshMolybdenum mask

SubstrateSpacer

Molybdenum mask

(Type A) (Type B)

118

screws. The first mask (Type A) was a 60 µm thick, 1 cm x 1 cm Mo foil with circular

openings having diameters of 1000, 300, and 100 µm. The aperture profile in this mask

was cylindrical (α = 90°). A second mask (Type B, Fig. 5-12a) was a 75 µm thick Mo

sheet with circular apertures having the same nominal diameters as in Type A. The

openings in this mask had a double-beveled edge, forming a biconical aperture (α’ =

270°). The third mask (Mask C, Fig. 5-12b) was a Ni mesh, 3.5 ± 0.5 µm thick, with

nominally 7.5 µm and 12.5 µm square openings separated by equally wide lines. The

mask-to-substrate separation was controlled by shims of multiple layers of the Ni mesh

placed between the Si substrate and the Mo mask bottom surface. In depositions through

the Ni meshes, 1 cm x 1 cm mesh sheets were fixed to the substrate by sandwiching them

between the substrate and the first or second type of mask, and then were clamped to the

holder by the retainer. Due to the profile of the Ni mesh, the smallest effective separation

was 1.0 ± 0.5 µm.

Analysis of the deposited pattern profiles was performed using scanning electron

microscopy (SEM) and atomic force microscopy (AFM) for the smallest pattern sizes,

and interference microscopy (see Fig. 5-13a) for the larger patterns. The latter method

entailed illuminating the substrates with a monochromatic source (with wavelength λ =

(540 ± 10) nm) and observing the interference fringes formed at the sloping edge of the

bell-shaped deposit (see for example Fig. 5-13b). The thickness profile was extracted

from the digitized pattern image (Fig. 5-13c) by counting the number of fringes from the

edge (Fig. 5-13d) and using H = mλ / 2n, where H is the pattern thickness, m = 0, 1, 2, 3,

etc. is the fringe order, and n = 1.74 is the refractive index of Alq3.

119

Figure 5-12: Scanning electron micrographs (SEMs) of the Molybdenum mask (Type B) and the Nickel mesh (Type C) used in the experiments. The Ni mesh was also used as a spacer, being ~4.5µm thick and flexible enough to be folded over several times for spacer thickness to be increased in small increments. Mask Type A (not shown) was identical to Type B, except the aperture had a cylindrical profile with straight walls, and not the biconical ones of Type A.

(Type B)

(Type C)

Figure 5-12: Scanning electron micrographs (SEMs) of the Molybdenum mask (Type B) and the Nickel mesh (Type C) used in the experiments. The Ni mesh was also used as a spacer, being ~4.5µm thick and flexible enough to be folded over several times for spacer thickness to be increased in small increments. Mask Type A (not shown) was identical to Type B, except the aperture had a cylindrical profile with straight walls, and not the biconical ones of Type A.

(Type B)(Type B)

(Type C)

120

5.9 Shadow-masking experiments – results and discussion

A micrograph with circular pixels deposited by OVPD through Mask A (α = 90º) is

shown in Fig. 5-13b. In Fig. 5-14a, the measured pattern profiles for s = 0µm are plotted

(open circles), while those for s = 40µm are shown in Fig. 5-14b. The open circles in Fig.

5-14a correspond to films nominally 2µm thick, while those in Fig. 5-14b were 1.6µm

(a)

Figure 5-13: a) Illustration of the interference microscopy method used to determine the thick-ness profile of the deposits. b) Micrograph show-ing circular Alq3 patterns deposited on Si using OVPD. The interference fringes observed for each deposit indicate a variation in the thickness near its edge. Here, a cylindrical mask was used with t = 50 µm, s = 0, and w = 100, 300, and 1000 µm. c)Digitized image of a deposit showing the interference fringes near the edge of the deposit d) Plot of the light intensity along the radius of the deposit, and the corresponding thickness profile calculated from the interference pattern similar to the one shown in (b).

dθ

2d sinθ = (m+1/2)λ/n

Thickness Profile

Light intensity profile

Position (µm)100 200 300 400 500 600 7000

(d)

1000µm

(c)(b)

(a)

Figure 5-13: a) Illustration of the interference microscopy method used to determine the thick-ness profile of the deposits. b) Micrograph show-ing circular Alq3 patterns deposited on Si using OVPD. The interference fringes observed for each deposit indicate a variation in the thickness near its edge. Here, a cylindrical mask was used with t = 50 µm, s = 0, and w = 100, 300, and 1000 µm. c)Digitized image of a deposit showing the interference fringes near the edge of the deposit d) Plot of the light intensity along the radius of the deposit, and the corresponding thickness profile calculated from the interference pattern similar to the one shown in (b).

dθ

2d sinθ = (m+1/2)λ/n

Thickness Profile

Light intensity profile

Position (µm)100 200 300 400 500 600 7000

(d)

1000µm

(c)(b)

121

thick. Both plots indicate that the pattern deposition efficiency (inferred from the area

under each profile) decreases with the aperture aspect ratio, t/w. The dome in the center

of each pattern, which is particularly pronounced for large t, is similar to that obtained in

the simulation. Also plotted in Fig. 5-14a is the profile for w = 300µm and s = 40µm

from Fig. 5-14b (filled circles). It is evident that increasing s decreases the efficiency of

pattern deposition (due to condensation on the mask) and edge sharpness. As discussed in

Sec. 5.4, this result is not obtained using the simple continuum diffusion model.

Figure 5-14: Experimental pixel profile for cases with a mask-to-substrate separation of (a) s = 0 and (b) s = 40µm, respectively. The nominal film thicknesses were (a) 2µm and (b) 1.6µm. The thickness profile was obtained for the larger diameter deposits by scanning from the center of the deposit outward, and reflecting the resulting thickness profile about the origin. For smaller deposits, the entire deposit was scanned. Both (a) and (b) indicate that the pixel deposition efficiency decreases with the aperture aspect ratio, t/w, where t and w are the mask thickness and aperture width, respectively. The profiles from both runs for the w = 300µm aperture are scaled by their thicknesses and compared in Fig. 13a (open and solid circles). The experimental comparison shows that greater mask-to-substrate distance results in a more diffuse deposit and lower pattern deposition efficiency.

-1000 -500 0 500 10000.0

0.5

1.0

1.5

2.0

Position (µm)

-500 0 500 1000

Position (µm)

(a) (b)Th

ickn

ess

(µm

) s = 0 µm scaled by ratio of

thicknesses

s = 40 µm

w = ( ) 1000, ( ) 300, ( ) 100 µm w = ( ) 1000, ( ) 300, ( )100 µm

Figure 5-14: Experimental pixel profile for cases with a mask-to-substrate separation of (a) s = 0 and (b) s = 40µm, respectively. The nominal film thicknesses were (a) 2µm and (b) 1.6µm. The thickness profile was obtained for the larger diameter deposits by scanning from the center of the deposit outward, and reflecting the resulting thickness profile about the origin. For smaller deposits, the entire deposit was scanned. Both (a) and (b) indicate that the pixel deposition efficiency decreases with the aperture aspect ratio, t/w, where t and w are the mask thickness and aperture width, respectively. The profiles from both runs for the w = 300µm aperture are scaled by their thicknesses and compared in Fig. 13a (open and solid circles). The experimental comparison shows that greater mask-to-substrate distance results in a more diffuse deposit and lower pattern deposition efficiency.

-1000 -500 0 500 10000.0

0.5

1.0

1.5

2.0

Position (µm)

-500 0 500 1000

Position (µm)

(a) (b)Th

ickn

ess

(µm

) s = 0 µm scaled by ratio of

thicknesses

s = 40 µm

w = ( ) 1000, ( ) 300, ( ) 100 µm w = ( ) 1000, ( ) 300, ( )100 µm

122

Experimental deposition profiles for Mask B with α’ = 270º, w = 100µm, t =

75µm and s ≈ 0 µm are compared to simulations in Fig. 5-15. The simulation assumed λ

= 20µm and diffusive deposition (i.e. no bulk flow in the z-direction) at Pdep ≈ 0.2 Torr,

as used in the experiment. The simulated aperture geometry matches the experimental

set-up, but s was adjusted to 20µm to match the profile of the deposited pattern. The

discrepancy is possibly due to lack of control of mask-to-substrate separation used in the

experiment. Using microscopic analysis we find that the mask with t = 75µm and α’ =

270º has surface irregularities on the order of several microns in height such that s > 0,

0 50 100 150 200 250 300

0.0

0.2

0.4

0.6

0.8

1.0

w=100µm t=75µms=0,20µm λ=20µm

α’=270°

Position (µm)

Nor

mal

ized

thic

knes

s (a

.u.)

Figure 5-15: Thickness profile of an Alq3 film grown by OVPD through a shadow mask with an aperture width w = 100µm and mask thickness t = 75µm. The simulation (dashed line) was performed with 106 Alq3 molecules assuming s = 20µm and mfp = 20µm for α’ = 270º mask. Experimental depositions employed the same type of mask, but without spacers, to obtain Alq3 patterns having 0.9µm (squares) and 2.1µm (circles) maximum thicknesses for w = 100 µm. The presence of pattern dispersion in both runs, and a close resem-blance to the simulated result indicate inadequate control of the parameter s in the experiment.

0 50 100 150 200 250 300

0.0

0.2

0.4

0.6

0.8

1.0

w=100µm t=75µms=0,20µm λ=20µm

α’=270°

Position (µm)

Nor

mal

ized

thic

knes

s (a

.u.)

Figure 5-15: Thickness profile of an Alq3 film grown by OVPD through a shadow mask with an aperture width w = 100µm and mask thickness t = 75µm. The simulation (dashed line) was performed with 106 Alq3 molecules assuming s = 20µm and mfp = 20µm for α’ = 270º mask. Experimental depositions employed the same type of mask, but without spacers, to obtain Alq3 patterns having 0.9µm (squares) and 2.1µm (circles) maximum thicknesses for w = 100 µm. The presence of pattern dispersion in both runs, and a close resem-blance to the simulated result indicate inadequate control of the parameter s in the experiment.

123

contrary to assumptions used in the simulations. Furthermore, uneven clamping of the

mask to the substrate may lead to warping of the mask, and hence to a larger s.

-10 -5 0 5 10position ( m)

w=6, s=0.5, =45o, t=7, mfp=20 w=2, s=0.5, =45o, t=3.5, mfp=20 w=2, s=0.5, =45o, t=3.5, mfp=40 w=2, s=0.5, =60o, t=3.5, mfp=40 w=2, s=0.5, =90o, t=3.5, mfp=40

(b)

-10 -5 0 5 100.00.20.40.60.81.01.21.4

Position (µm)

α’=270°mfp=20µm

s=0.5µmt=3.5µmw=6µm

0 30(µm)Ni mesh

5µm5µm

(a)

Nor

mal

ized

thic

knes

s (a

.u.)

Figure 5-16: a) Comparison of the experimental and simulated pattern profiles of a patterned Alq3 film, where the edge sharpness was approximately 1-3 µm. Left inset: An atomic force microscope (AFM) image of a patterned Alq3 film deposited by OVPD at 0.1 Torr through a nickel mesh with an aperture width, w = 6µm and mask thickness, t = 3.5µm, with s <1µm. Right inset: SEM side view of the Ni mesh. b) Simulation results for higher resolution patterns that are predicted for OVPD at the conditions listed in the figure.

Dep

osit

heig

ht (a

.u.)

124

The highest resolution patterns achieved by OVPD used Mask C with the bottom

of the Ni mesh in close contact with the substrate, resulting in s ≈ 1µm, due to the profile

of the mesh wires. A patterned Alq3 film deposited by OVPD at Pdep = 0.1 Torr was

imaged using atomic force microscopy (AFM) as shown in Fig. 5-16a. The edge

sharpness is ~3µm, obtained from the thickness profile extracted from the image in the

left inset of Fig. 5-16a. Simulation of these deposition conditions is also shown as a solid

line. The experimental deposit profile is fit assuming w=6.0µm, t=3.5µm, s=0.5µm,

λ=20µm and α’=270°. The simulated and experimental dimensions are nearly identical,

suggesting that the stochastic simulation accurately describes the patterned deposition

mechanism. Similar patterns have been obtained for depositions at pressures of from 0.1

to 2 Torr, with the lower pressures increasing pattern definition, as expected. Figure 5-

16b shows simulation results for several other deposition conditions. One curve (solid

circles) is for an aperture with α = 45º and s = 0.5µm, showing improved pattern edge

sharpness compared to that in Fig. 5-16a Rectangular patterns with η > 90% can be

achieved for w ≤ 2 µm and α = 90º.

5.10 Resolution limits and self-aligned contacts by hybrid VTE-OVPD

Two instances of micron-scale resolution patterning are illustrated by the scanning

electron micrographs in Fig. 5-17: (a) Alq3 patterns deposited on Si by VTE through Ni

meshes at Pdep = 10-6 Torr and s ≈ 1µm without substrate rotation, and (b) analogous

patterns deposited by OVPD at Pdep = 2 Torr. The vacuum-deposited patterns show the

trapezoidal profile discussed in Sec. II where ρ < 1µm, while OVPD patterns have edge

dispersion on the order of 1-3 µm. Simulations in Fig. 5-16b indicate that if apertures

125

10µm

10µm

10µm

(a)

(b)

(c)

VTE growth

OVPD growth

Organic pixel (OVPD)

Metal cathode (VTE)

Hybrid OVPD / VTE growth

Figure 5-17: a) Scanning electron micrograph of an Alq3 film deposited by vacuum thermal evaporation on Si through a mesh with an aperture width, w = 7.5µm and mask-to-substrate separation, s ~ 0µm at 10-6 Torr. b) Scanning electron micrograph showing Alq3 patterns deposited on Si using OVPD at 0.1 Torr through a nickel mesh with t = 3.5µm, w = 7.5 µm, and s < 1µm. c)Scanning electron micrograph of hybrid OVPD-VTE depo-sition showing Alq3patterns deposited on Si by OVPD at 1 Torr, with Ag caps subsequently deposited by VTE at 10-6 Torr, without shifting the shadow-mask between depositions.

126

with α = 45° and minimal s are used, sub-micron resolution is achievable using this

growth technique.

Finally, fabrication of OLED-based full-color displays entails the deposition of an

array of pixels with one color emissive layer followed by the deposition of a second and

third array of different color pixels, forming the red-green-blue full-color triad.(Tian et

al. 1999; Tian et al. 1997) The deposition of organic layers is followed by metal

deposition to form the cathode contacts to the OLEDs. To confine the metal cathode to

coat only the organic films, and hence prevent shorting of the cathode to the anode

contacts, an “integrated” polymer shadow mask consisting of photoresist walls is pre-

patterned onto the substrate.(Tian et al. 1997) The integrated shadow mask can be

eliminated by OVPD at moderate pressure through a conventional shadow-mask, yielding

slightly broadened deposits. This is followed by the deposition of a well-defined metal

contact in vacuum without moving the mask between the growth steps. The controlled

dispersion of the organic coating, therefore, prevents electrical shorts around the edge of

the organic film, since VTE of the electrode will automatically yield a smaller size “cap”

over the organic pixel. An SEM image of the resulting confined electrode structure from

such hybrid deposition is shown in Fig. 5-17c. In the passive-matrix architecture the

confined individual electrode squares are replaced by stripe electrodes. A second set of

pixels may be deposited by simply translating the mask laterally (possibly in-situ),

avoiding the realignment of the mask to a previously deposited pattern.

5.11 Summary

In this chapter we addressed the challenge of in-situ micropatterning the active organic

layers by deposition in the diffusion-limited regime. Since the molecular mean free path

127

of the organic molecules in the vicinity of the substrate is on the order of the apparatus

dimension, (e.g. mask aperture, mask thickness or mask-to-substrate separation),

continuum modeling approaches are inaccurate in describing the deposition. Instead, we

used Monte-Carlo (MC) simulations to more accurately represent the molecular nature of

transport in confined geometries. The quantitative effects of background gas pressure,

aperture geometry, and mask-to-substrate separation were studied. The deposition

experiments confirmed the MC simulations, down to features < 10 µm wide. Based on

the close correspondence between the simulation and the experiment, the former were

used to predict that resolutions and features sizes < 1µm are achievable at appropriate

deposition conditions and mask geometry, i.e. low background gas pressure, small mask-

to-substrate separations, and a re-entrant aperture profile. Furthermore, the Monte-Carlo

simulations permit the quantitative assessment of the efficiency of molecular transport

through the aperture (quantified by the merit figures ε and η), which is of practical

importance in device applications.

128

Chapter 6: Organic Vapor Jet

Printing

6.1 Overview

Both OVPD and VTE require the use of shadow-masks to pattern the active organic

layers in the substrate plane. As a consequence, much of the source material is wasted on

coating of the mask. This chapter introduces a novel method of Organic Vapor Jet

Printing (OVJP), which permits the direct patterned deposition (printing) of the organic

semiconductors onto the substrate. Several potential advantages are thus attained – the

materials utilization efficiency is nearly 100%, and the masking step is eliminated. In this

chapter the OVJP theory is developed, direct simulation Monte-Carlo models are used to

verify the theory, and printing experiments and results are presented, demonstrating the

patterning and device growth capability of OVJP.

6.2 OVJP Concept

The vapor jet printing concept and apparatus are schematically shown in Fig. 6-1. Carrier

gas enters heated cells containing the molecular organic source material, transporting the

saturated source vapor into a mixing chamber. The gas mixture expands through a

microscopic nozzle as a sub-sonic jet, impinging onto a substrate that can be translated

transverse to the nozzle, thereby generating a patterned deposit. The apparatus also has a

129

dilution channel, allowing for the temperature-independent regulation of the organic

vapor concentration in the jet. Although the carrier gas flow field rapidly diverges due to

the proximity of the substrate to the nozzle outlet, the relatively heavy organic molecules

acquire trajectories substantially more collimated than the carrier gas. The heavier

organic molecules condense on the cooled substrate, while the lighter carrier gas escapes

to the sides.

Although somewhat similar in form, OVJP is nevertheless distinct from

conventional ink jet printing of polymer semiconductors.(Hebner et al. 1998; Paul et al.

2003; Sirringhaus et al. 2000) It uses a hot inert carrier gas, instead of a liquid solvent, to

directly print molecular organic compounds, eliminating meniscus formation, solvent

compatibility issues, and other concerns limiting ink jet printing. Furthermore, in OVJP

no pre-patterning of the substrate is needed, whereas in ink jet printing, droplet-confining

wells are required.

Figure 6-1: OVJP apparatus schematic

A B

N2 N2 N2

Substrate

Source cell Heater

NozzleJet

Deposit


A B

N2 N2 N2

Substrate

Source cell Heater

NozzleJet

Deposit


A B

N2 N2 N2

Substrate

Source cell Heater

NozzleJet

Deposit

130

6.3 OVJP theory

For application in printing of organic electronic devices such as LEDs or TFTs, the

desired pattern size is on the order of 1-100 µm, with the required edge resolution

typically 10% of the pattern width. In the work on thin film patterning using shadow

masks, it was found that the aperture width, 2a, must be on the order of the desired

pattern size, while the aperture-to-substrate separation, s, should be smaller than both the

pattern edge resolution and the molecular mean free path, λ. For typical deposition

pressures of 0.1-100 Torr used in organic vapor phase deposition (OVPD), λ is between

500 and 0.5µm, respectively. Thus, often λ ≤ s ≤ a, resulting in transition regime

transport, between the molecular and continuum flow regimes.(John 1984; Roy et al.

2003)

A similar situation is encountered in OVJP, where the vapor is ejected through

microscopic nozzles placed in proximity to the substrate. Previous work (De La Mora et

al. 1989; Eres 1991; Guevremont et al. 2000; Tison 1993; Vasenkov et al. 1995)

suggested that the freely expanding jet diverges monotonically after exiting the nozzle

and into free space.(Shtein et al. 2001) When a substrate is placed close to the nozzle, the

jet is further dispersed relative to the free-expanding beam due to a stagnation front

immediately above the surface. This occurs because the sudden deceleration of the jet

upon impacting the substrate creates a pressure front, which redirects the flow parallel to

the substrate, causing even greater dispersion of the jet relative to the free expansion

situation. Nevertheless, lateral patterning is still possible, because the heavier organic

species retain most of their momentum normal to the substrate, leading to a deposit that is

131

narrower than the carrier gas plume. The deposition geometry and the axial enrichment of

the jet are illustrated in Fig. 6-2.

The pattern resolution depends upon the nozzle shape, and the interplay of

diffusive and convective processes in the region between the nozzle and substrate.

Important parameters include: nozzle radius (a), nozzle-to-substrate separation (s), the

deposition chamber (or background) pressure (PL), and the masses of the carrier gas and

organic molecules (mcg, mp, respectively). Since no accurate analytical theory exists for

flow in this transition regime, predictions of the pattern shape have to derive primarily

from experiment and direct simulation Monte-Carlo (DSMC) techniques.(Bird 1994)

Nevertheless, a scaling argument can be developed based on simple collision dynamics to

provide guidance as to how these parameters influence the deposit edge resolution.

For a sufficiently long nozzle (L/a > 10, where L is the nozzle length) fully

developed flow is established, where the organic molecules are accelerated by the carrier

ux

ū

s

χ

2a

N2

N

OAl3

Figure 6-2: Diagram defining the geo-metry relevant to pattern formation, also depicting the diverging carrier gas (N2) flow streamlines and the collimated tra-jectories of heavier organic molecules (in this case, Alq3.

ux

ū

s

χ

2a

N2

N

OAl3


ux

ū

s

χ

2a

N2

N

OAl3

ux

ū

s

χ

2a

N2

N

OAl3

N

OAl3


132

gas to the bulk jet velocity, ū. If the molecules suffer few collisions en route to the

substrate, they traverse the nozzle-to-substrate gap, s, in:

/t s u= (7.1)

The organic molecules will be displaced radially outward by the diverging carrier gas

flow as well as by diffusion, by a distance χ from their original position at the exit of the

nozzle (Fig. 6-2):

x Dt u rχ = ⋅ + (7.2)

where ux is the radial convective organic molecular velocity, while rD is the distance the

molecules travel due to pure diffusion. The radial convective velocity arises from

molecular collisions with the carrier gas:

2 cgxx

o

m su um λ

= ⋅ ⋅ , (7.3)

where ūx is the carrier gas flow velocity after it is redirected along the substrate plane,

2mcg/mo accounts for the momentum transferred to the organic molecule in each elastic

organic-carrier molecule collision, and s/λ is the total number of collisions occurring

while the molecule is traversing the nozzle-substrate gap. The diffusion travel is:

Dr D t= ⋅ (7.4)

where D is the diffusivity of the organic molecule in the carrier gas. Assuming

incompressible flow and conservation of mass (i.e. 2 2xu a u a sπ π⋅ ⋅ = ⋅ ⋅ ⋅ ), and D = 1/3

c λ, where c is the molecular mean thermal velocity, Eqs. (7.1–4) combine to give:

2

13

cg

o

m s c sa m u aχ λ

λ⋅ ⋅

= ⋅ + ⋅⋅

(7.5)

133

The first term in Eq. (7.5) quantifies the pattern dispersion due to horizontal momentum

transfer to the organic molecules from collisions with the diverging carrier gas, while the

second term represents the scaling of the radial diffusion rate to the ballistic transport rate

normal to the substrate.

Although Eq. (7.5) does not predict the deposit shape, it shows the relative

influence of process conditions on the deposited pattern resolution. In particular, using

/ 2 LkT Pλ σ= , where σ is the cross-section of the molecule, the pattern dispersion is

predicted to have a minimum with respect to PL, as shown in Fig. 6-3a. This pressure

results in maximum resolution and is in the range of 1-50 Torr for typical OVJP

conditions, (e.g. χ ≈ s ≈ 0.2·a ≈ 20µm, mcg/mo ≈ 0.05 in a nitrogen carrier, and nozzle

temperature of ~300ºC). Equation (7.5) also suggests that pattern definition is enhanced

through use of a lighter carrier gas (e.g. He instead of N2). Practically, ū is fixed by the

desired deposition rate via the concentration of the organic vapor. Thus, for a given

nozzle radius a, the remaining adjustable parameters are s and PL. The conditions leading

to maximum pattern resolution can be plotted on a process diagram (Fig. 6-3b), where

the operating line dictates values of s for any given PL. For example, to maintain high

pattern resolution even at large separation, s, the downstream chamber pressure, PL, must

be decreased. The region above the operating line represents printing in the diffusion-

limited regime, while the region below corresponds to convection-limited transport.

This analysis assumes incompressible flow, a situation that is not rigorously true

since both the carrier gas and the organic molecules are first accelerated through the

nozzle, but decelerate upon impact with the substrate. Due to the abrupt change in the

flow direction upon exiting the nozzle and the resulting kinetic impact of the jet on the

134

substrate, the local dynamic pressure in the region between the nozzle and the substrate

generally exceeds PL. Since the effective pressure in the flow stream is always higher

than the chamber pressure, PL, the negative-slope branch of the curve in Fig. 6-3a is not

experimentally observable, and consequently the pattern dispersion curve should have a

positive slope for all PL. Thus, for any chosen value of s, the optimum resolution is

obtained by minimizing PL.

6.4 Simulation of transitional flow regime

Since the molecular mean free path is on the order of nozzle to substrate distance, s, or

nozzle diameter, a, Navier-Støkes (continuum flow) equations do not accurately describe

the flow pattern downstream of the nozzle. In this case, a direct simulation Monte-Carlo

Figure 6-3: a) Plot of the expected dependence of the normalized deposit width (s/a) vs. the downstream pressure (PL). The dispersion is minimized at a given value of PL (indicated by the arrow) due to the counterbalance of convective and diffusive transport rates. b) The conditions for the highest pattern resolution (minimum dispersion) are plotted to give the optimum operating line. Working above or below this line will decrease the pattern resolution; high s and PL result in diffusion-controlled transport, while low s and PL result in convection-controlled transport.

sa

PL

aχ

PL

Ideal operating line

Optimum PL

~PL

~ 1/PL

(a) (b)

135

(DSMC) approach is useful to model the flow field. Thousands of molecules are moved

quasi-simultaneously in discretized space, allowing for molecule-molecule and molecule-

wall collisions. Boundary conditions include upstream and downstream pressures (PH and

PL, respectively), organic molecule concentration (Co), nozzle and substrate temperatures

(TN and TS), and sticking coefficients (0 for the heated nozzle, and 1 for the substrate).

The simulation geometry is shown in Fig. 6-4a. The nozzle was nominally 20µm

in diameter by 50µm in length, slightly fluted at the downstream aperture, and maintained

at 300°C, a temperature sufficiently high to avoid readsorption of the volatilized organics

onto the nozzle walls. The substrate was positioned 25µm below the nozzle, and

maintained at 25°C. Several downstream pressures were investigated: 0.24, 2.4, 24 and

240 Torr, with the corresponding upstream nitrogen pressure set to PH = 400, 600, 800,

and 900 Torr, respectively. A 1% molar upstream concentration of Alq3 was assumed.

Figure 6-4a is a color-map of the vertical velocity component of the calculated

flow field, with the corresponding trajectories of the carrier gas and the organic

molecules shown in Fig. 6-4b. The velocity map shows that the molecules are accelerated

along the nozzle axis, reaching near sonic velocities of ~200m/s at the orifice.

Immediately above the substrate surface, the gas molecules decelerate, giving rise to a

stagnation front, where the dynamic pressure exceeds the ambient pressure (PL) far away

from the nozzle. The heavy organic molecules, however, follow more collimated

trajectories, crossing the carrier gas flow lines. The heavier molecules thus penetrate the

stagnation front and impinge on the substrate in a well-defined adsorbed deposit pattern.

The deposit profiles obtained from DSMC for different printing conditions are plotted in

Figs. 6-5a and b, where the broadening of the deposit due to increasing s and PL is

136

(b)

Position (µm)0 50 100 150

Alq3trajectories

N2flow field

(a)

0

50

Nozzle

Substrate

Vapor jet

Dis

tanc

e (µ

m)

2001000-50z-component of Velocity (m/s)

Figure 6-4: a) Color-map of the z-component (i.e. perpendicular to the substrate) of the flow velocity of the carrier gas, showing the collimation of the jet emerging from the nozzle and the stagnation front just above the substrate surface. b) The corresponding flow field of the carrier gas (red lines) and the trajectories of heavier organic Alq3 molecules (blue lines) are shown. The plots were obtained from direct simulation Monte-Carlo (DSMC) modeling of OVJP using a nozzle diameter a = 20 µm, nozzle length L = 100 µm, nozzle-to-substrate separation s = 30 µm, upstream pressure PH = 0.24 Torr, and downstream pressure PL = 0.24 Torr.

137

-100 -50 0 50 100

0.2

0.4

0.6

0.8

1.0

Thic

knes

s (n

orm

aliz

ed)

Position (µm)

0.24 Torr 2.4 Torr 24 Torr 240 TorrIncrease

PL

(a)

-50 0 50 1000.0

0.2

0.4

0.6

0.8

1.0

Thic

knes

s (n

orm

aliz

ed)

Position (µm)

25 µm 50 µm 75 µm 100 µm

Increases

(b)

Figure 6-5: a)The deposit profiles are calculated by DSMC of Alq3 deposition with s = 25µm, showing a monotonic broadening of the deposit width with increasing PL. This indicates that the dynamic pressure at the nozzle exit dominates transport, forcing the system away from the dispersion minimum, as shown in Fig. 1b. b) Deposit profiles are calculated at PL = 0.24 Torr for s = 25, 50, 75 and 100 µm. The pattern width increases monotonically with s. The solid lines are drawn as a guide to the eye.

138

evident. Pattern dispersion increases with both s and PL. The pattern width is nearly

independent of PL when λ > s, but then increases rapidly with PL in situations when λ < s.

High pattern resolution is achieved when the nozzle is placed within a mean free path

from the substrate, confirming the dynamic pressure effect discussed earlier. The center

of the calculated deposit profile is somewhat domed for all values of s and PL, while the

wings extend beyond the 20µm nozzle orifice diameter. One means to achieve a deposit

profile with a flat top and a sharp edge, a small-diameter nozzle can be rastered, or

dithered at high frequency over the substrate during the deposition.

6.5 Experimental set-up

The OVJP apparatus consists of several components mounted on a six-way cross, as

shown in Fig. 6-6a. The water-cooled copper substrate holder is positioned in the center

of the cross by means of a computer-controlled XYZ-motion stage. Substrates are first

mounted ex-situ onto a copper plate, which is loaded by hand through the hinged view-

port and attached onto the holder by means of integrated spring clips. The pressure in the

chamber is regulated by a combination of a roughing pump (up to 20lpm pumping

speed), throttled by an electronically controlled “butterfly” valve, and metered nitrogen

or helium inflow. The source materials are contained within 5 source cells, positioned

inside of a uniformly heated stainless steel cylinder (Fig. 6-6b).

The source cells (Fig. 6-6c) consisted of a specially machined hollow 0.375-inch

diameter by 1-inch long stainless steel cylinder. The cells were attached to one end of a

hollow stainless steel tube, which transports the carrier gas and acts as the rotating shaft

of a hot-valving arrangement, as indicated in Fig. 6-6c. The source cylinder also includes

139

a central dilution channel, a mixing chamber, and a modular nozzle. Nitrogen and helium

are used as the carrier gases.

Two types of nozzles were used, illustrated in Fig. 6-7. The first (Type I) was a

conical cap stainless steel laser-drilled aperture, having a nominal inside diameter of

20µm and a channel length of 100µm. The second (Type II) nozzle was a polyimide-

coated hollow silica capillary, 4mm long, with 50µm and 350µm inner and outer

diameters, respectively. The capillary was press-fit into a 350µm diameter hole in the

source block. The substrates were mounted onto a water-cooled susceptor attached to a

computer-controlled XYZ-positioning stage. The nozzle-to-substrate distance was

controlled by first moving the stage into contact with the nozzle at three locations,

(detected optically), and subsequently translating the stage from these reference positions.

The substrates used in the patterning experiments consisted of silicon wafers,

precleaned using a procedure described elsewhere.(Shtein et al. 2002) The pattern shapes

were determined by reflected-light interference microscopy.(Shtein et al. 2003) The

substrates used in the printing of the pentacene TFTs consisted of n-type doped (0.001 Ω-

cm) (100) Si wafers with coated with a dry thermally grown, 2100Å thick SiO2 layer

surface treated with octadecyl-trichlorosilane (OTS). The OTS treatment consisted of

sequentially exposing the SiO2 wafers to UV-ozone for 15 minutes, and saturated OTS

vapor at 20°C and 0.1 Torr for 10 minutes immediately prior to printing the pentacene

channel. The 99% pure pentacene powder was further purified twice by vacuum train

sublimation(Forrest 1997) before deposition. The TFTs had gold source and drain

electrodes deposited in vacuum through a shadow mask, while the silicon substrate

140

Source Cylinder

Dilution channel

Mixing Chamber

Source cell

Modular nozzle

Mass flow controllers

N2 / HeN2 / He

RoughingPump

XYZ-stage

6-waycross Hinged Door /

View-/Load-Port

Substrate holder w/ cooling lines

Set screw¼” S.S. Tube

Source Material

(e.g. Alq3)

Qrtz WoolInlet Channel

Outlet Channel

S.S. Source Cell

N2 / He

Rotate for hot valving

Rotate for hot valving

Substrate

Throttle valve

Exhaust

ColdTrap

(a)

(b)

(c)

Figure 6-6: a) OVJP experimental set-up, with the water-cooled substrate attached to a computer-controlled xyz-movable stage, a multi-source print-head, and electronically regulated gas flows and exhaust. b) Schematic of the print-head, containing several source cells, a mixing chamber, and a modular nozzle. c) Schematic of one source cell, showing assembly and hot-valving operation.

141

served as the gate electrode. The TFTs were tested in ambient, in the dark, using a

Hewlett-Packard 4155 semiconductor parameter analyzer.

6.6 Direct printing of patterned molecular organic thin films

Figure 6-8a shows a row of Alq3 dots deposited at PL = 0.24 Torr using nitrogen as the

carrier gas along with a Type I nozzle, an Alq3 source temperature of 270°C, and a

substrate temperature of 15°C. The distance between the nozzle and the substrate was

varied, starting from approximately s = 0, and increasing in ~28µm increments for each

dot. For sufficiently thick deposits, optical interference fringes obtained at a wavelength

Figure 6-7: Nozzles used in the OVJP experiment. a) An example of a conical cap stainless steel screw with a laser-drilled aperture (here shown with a channel 100µm long x 20µm diameter). b) Polyimide-coated silica capillary, inserted into a stainless steel set screw. (Shown here with a 350 and 50µm outer and inner diameters (OD and ID) respectively. The ID of the hole in the set screw is several microns smaller than the OD of the capillary. The polyimide coating is scraped off and the capillary is press-fit into the opening. The channel length is defined by cleaving off appropriate lengths of the capillary.)

Ø 50µm

Proximity sensor

Au wire

Nozzle

Ø 350µm

Ø 350µm

Capillary press-fit into set screw

Nozzle Type I Nozzle Type IIa) b)

142

500µm

(a)

Thic

knes

s (µ

m)

(b)

Position (µm)

s1s2s3s4s5s6

-100 -50 0 50 1000.0

0.5

1.0

1.5

2.0

2.5

3.0

s2

s3

s4

s5

s6

Increase s

Figure 6-8: a) Optical micrograph of Alq3 dots printed on Si using nitrogen carrier gas, and a Type II, 20µm diameter nozzle at PH = 240 Torr and PL = 0.24 Torr in a combinatorial experiment with varying nozzle-to-substrate separation, s = (0, 28, 57, 85, 114, 142 ± 10) µm. The deposit thickness profiles are deduced from the optical interference fringes (see Ref.17). b) Thickness profiles of the Alq3 dots in (a) determined from the interference fringes obtained under 540nm wavelength illumination. The plot of the deposit profiles shows a decrease in pattern resolution with increasing s.

143

-100 -50 0 50 1000.0

0.5

1.0

1.5

2.0

2.5

3.0

Thic

knes

s (µ

m)

Position (µm)

300 µm

s1s2s3s4s5s6s7s8s9s10(a)

(b)

Increase s

Figure 6-9: a) Optical micrograph of Alq3 dots printed on Si using helium carrier gas, and a Type II nozzle at PH = 240 Torr and PL = 0.24 Torr in a combinatorial experiment with varying nozzle-to-substrate separation, s = (0, 15, 30, 45, 60, 75, 90, 105, 120, and 135 ± 10) µm. b) Thickness profiles of the Alq3 dots in (a) determined from the inter-ference fringes obtained as in Fig. 4. The plot of the deposit profiles shows a decrease in pattern resolution with increasing s.

144

of 540nm allow the deposit profile to be determined.(Shtein et al. 2003) The

corresponding thickness profiles in Fig. 6-8b exhibit similar shapes and dimensions to

those obtained by DSMC. This experiment was repeated with helium as the carrier gas,

while all other conditions were kept constant. The resulting Alq3 deposits are shown in

Fig. 6-9a, with the corresponding thickness profiles plotted in Fig. 6-9b. The shape of the

deposits is similar to that in Fig. 6-8, albeit with visibly higher resolution. One

convenient method of quantifying the printing resolution is through the full-width half-

maximum (FWHM) of the bell-shaped deposit profile.

0 50 100 150 2000

50

100

150

200

Alq3 + N2

Alq3 + He

FWH

M (

µm

)

Separation (µm)

Figure 6-10: Plot of the FWHM vs. s from Figs. 6-8b and 6-9b, showing that using a lighter carrier gas (here, He instead of N2) can increase the collimation of the beam and a sharper deposit.

145

Figure 6-10 shows a plot of the FWHM versus s for the deposits in Fig. 6-8. The

data, (open circles), are fit by a line corresponding to Eq. (6.5). Since a, mcg/mo, and c

(evaluated at the nozzle temperature) are known, the fit employs only two parameters, ū

and λ, where λ is a function of the dynamic pressure (Pdyn) above the substrate. From the

fit, we obtain ū = 150 m/s and Pdyn = 75 Torr (with Pdyn calculated from λ evaluated at

145°C, corresponding to the average of the nozzle and substrate temperatures). These

values of ū and Pdyn are in general agreement with the simulation results. Since the fit is

approximately linear with s, the first term in Eq. (6.5) dominates, meaning that transport

is limited by molecular collisions in the nozzle-to-substrate gap.

The plot of the deposit FWHM versus s from Fig. 6-9 is also shown in Fig. 6-10.

The data exhibit a shallower slope than that obtained from the N2 carrier gas experiment,

due to the lower mcg/mp ratio. However, the slope decrease is less than predicted by the

mass ratio change alone. This can be understood in terms of sub-maximal acceleration of

the Alq3 molecules by the He jet, since the nozzle aspect ratio, L/2a ≈ 5, is insufficient to

establish fully developed flow. Indeed, the flow is also not fully developed in the case of

the Alq3+N2 mixture either. Nevertheless, even though the equilibrium thermal velocities

of the Alq3 and N2 in the upstream reservoir are closer than in the case of Alq3+He, fewer

N2-Alq3 collisions are required for full acceleration of Alq3 than He-Alq3 collisions.

Hence, the latter situation results in considerably smaller overall acceleration of the Alq3

during transport along the nozzle length.

The pressure-dependence of the deposit width was also investigated. Figure 6-

11a shows Alq3 dots printed on silicon at PL = 0.27, 1.0, 3.0, 30, 100, and 760 Torr using

N2, at a separation of 25µm, using the Type II nozzle. The FWHM for each pressure is

146

0.1 1 10 100 10000

50

100

150

P7P6P5P4P3P2P1

500µmFW

HM

(µ

m)

(a)

(b)

Figure 6-11: a) Optical micrograph of Alq3 dots printed on Si using nitrogen carrier gas, and a Type II nozzle at s = 25 ± 10 µm in a combi-natorialexperiment with varying downstream pressure, 0.24 < PL < 900 Torr. b) The FWHMs of the deposits are plotted versus PL, showing an increase in the pattern width with pressure (solid line, filled squares). The DSMC results from Fig. 6-5bare also re-plotted for comparison (dashed line, open circles). Although the maxi-mum resolution is somewhat greater at lower pressures in the simulated profiles, the experi-mental and simulation trends cross at PL ≈ 20 Torr. The flat FWHM at low PL, and the rapid increase at PL >200 Torr of the experimental data is attributed, in part, to the roughness and irregularity of the Type II nozzle used in the experiment (see text).

P (Torr)

147

plotted in Fig. 6-11b (squares, solid line). The simulation results from Fig. 6-5b are also

provided for comparison (open circles, dashed line). The simulation shows a monotonic

increase in the pattern dispersion with increasing pressure due to the dynamic pressure

a)

c)

1.5mm

b)

Figure 6-12: A 24x32 pixel bitmap image of a bicyclist figure printed by OVJP using Alq3, with the nozzle of 10µm diameter by 100µm length, nozzle-to-substrate separation of (20 ± 10) µm, a dwell-time of 2s above each pixel location, and <0.2s time interval for translation between each pixel. The pattern resolution in this image varies between 500 and 1000 dpi due to the variation in the nozzle-to-substrate separation across the image. The local Alq3 deposition rate was 1300 Å/s at 270ºC source temperature, and can be increased to >8000 Å/s at 300ºC source temperature, or >18000 Å/s if He is used instead of N2 as the carrier gas. The image was produced by computer-controlled motion of the susceptor, while the nozzle was held steady.

148

effect, as discussed in Sec. 6-2. The experimental data exhibit a similar behavior,

although with a much sharper increase in FWHM at PL ≈ 100 Torr. It is possible that to

accurately simulate transport at PL>100 Torr, the simulation needs to be run for a longer

time period to achieve true steady state representation of the flow profile. Thus, the

simulated Pdyn is lower than experimental, and the simulation consequently underpredicts

the extent of dispersion.

Figure 6-12 shows a pattern of Alq3 (an archetypal electron transport material

used in OLEDs) printed by OVJP using a 20µm diameter nozzle under process conditions

indicated in the caption. The dwell-time of the nozzle above each pixel was 2s, while the

stage moved between pixels in <0.2s. To simplify the printing process, the source was not

turned off during translation between pixels. A magnified portion of the pattern is shown

(Fig. 6-12c), indicating 500-1000 dots per inch (dpi) resolution, which is similar to that

achieved by IJP of polymer-based TFTs.(Paul et al. 2003) The source temperature was

270ºC, resulting in a local Alq3 deposition rate of rdep ≈ 1300 Å/s. Our result suggests

that a 300ºC Alq3 source temperature should result in rdep > 8000 Å/s.(Shtein et al. 2001)

At this growth rate, an array of 800 nozzles can print an SVGA resolution display

(600x800 OLED pixels) in under one minute. This speed is comparable to the current

state-of-the-art inkjet printers, which also use print heads containing in excess of 500

nozzles.

6.7 OVJP of polycrystalline pentacene films and TFTs

Growth of polycrystalline films by OVJP results in an interesting anisotropy which is

closely associated with the non-equilibrium, directional nature of the OVJP process.

149

Molecules condense and diffuse on the substrate surface, nucleating clusters, which then

grow by addition of both surface-diffusing species and molecules arriving at the substrate

from above. The islands do not all grow uniformly at once; since the organic molecules

arrive at the surface at an angle dictated by the flow field, the mutual shadowing of the

islands during growth results in a tilted grain aspect, determined by the incident

molecular velocity vector. This growth process is illustrated in Fig. 6-13a. Figure 6-13b

shows a micrograph of a pentacene pattern printed on SiO2 using the Type II nozzle at a

local deposition rate >300 Å/s and s = (35 ± 15)µm, showing a continuous line printed by

scanning the nozzle over the substrate during the deposition. Scanning electron

micrograph (SEM) images reveal that indeed the pentacene crystallites to the left and

right of the jet centerline tilt in toward the molecular supply. This effect is not observed

in diffusion-limited growth such as in OVPD,(Shtein et al. 2002) and is in agreement

both with the schematic of Fig. 6-2a as well as the DSMC modeling results in Fig. 6-4b.

Somewhat surprisingly, the very high deposition rates achieved in OVJP can still

result in highly ordered crystalline morphologies. This is indicated by x-ray diffraction

patterns in the case of pentacene printed by OVJP, as shown in Fig. 6-13c. Here, up to

four orders of two main peaks are readily observable for pentacene printed on SiO2 at

rates >700Å/s. As will be discussed in Ch. 7, the two peaks correspond to the “thin film”

and “bulk” phases of pentacene typically observed in thin films, 15.5 and 14.5Å plane

spacing, respectively. However, vacuum-deposited pentacene films are typically

amorphous when grown at rates >10Å/s.

Enhancement of molecular ordering in seeded molecular beams has been

observed previously. Seeding the organic molecules in a fast-flowing carrier stream

150

(c)

5 10 15 20 25 30

(001

’)(0

01)

(002

’)(0

02)

(003

’)(0

03)

(004

’)(0

04)

2Θ

Sig

nal (

coun

ts)

Figure 6-13: a) Schematic illustration of polycrystalline film growth by OVJP, where the angled approach velocity of each molecule, and the mutual shadowing of growing crystallites can lead to tilted grain growth. b) Micrograph of a pentacene pattern printed by OVJP on SiO2, with scanning electron micrographs below showing pentacene crystallites tilting in the direction of the carrier gas flow on each side of the printed line. c) X-ray diffraction pattern of pentacene deposited on OTS-treated SiO2 at 700Å/s, showing two peaks indicative of the thin-film and bulk phases of pentacene, corresponding to 15.5 and 14.5Å plane spacing, respectively.

(b) 500µm

1µm 1µm

(a)

151

Figure 6-14: a) Scanning electron micrographs of the channel and contact regions of the pentacene TFT printed by OVJP. The bright layer on top of the microcrystalline pentacene film is gold. b) Plot of IDS versus the drain-source voltage (VDS), showing transistor saturation. c) Drain-source current (IDS) versus gate-source voltage (VGS) response of a pentacene channel thin- film transistor (TFT) printed by OVJP. The characteristic was obtained from the drain-source current (VDS) in the saturation regime (at VDS = -40V). The TFT exhibited some hysteresis in IDS, with the threshold voltage, VT, shifting from +10 to +17 V in the forward and reverse VGS sweep directions, respectively, as indicated.

Contact region

Channel region

-40 -30 -20 -10 0 10

0

1

2

3

4

5

-40 -30 -20 -10 0-16-14-12-10

-8-6-4-202

VDS

I DS

(µA

)

I DS

(Am

ps)

(I DS)

1/2

(Am

ps)1/

2·1

03

(a) (b)

(c)

VGS (Volts)

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

152

allows near- to hyper-thermal velocities (i.e. greater than the mean molecular speed of the

heavier molecules at that temperature) to be reached by the adsorbate. Consequently, the

ability to tune the incident molecular kinetic energy via the carrier gas stream allows to

partially decouples the film crystallization dynamics from surface diffusion effects, likely

leading to highly ordered films, even for relatively cold substrates.(Casalis et al. 2003;

Scoles 1988) This effect can be potentially exploited for improving the performance of

devices, such as polycrystalline channel TFTs.(Shtein et al. 2002)

To demonstrate the utility of the very high local deposition rates characteristic of

OVJP for device applications, the channel region of a pentacene TFT was printed to form

a 6mm x 6mm, uniformly filled square by rastering the narrow jet over the substrate at PL

= 0.2 Torr, and at a local growth rate of 700Å/s. The transistor drain-source current (IDS)

versus drain-source voltage (VDS) characteristic is plotted in Fig. 6-14b, showing that IDS

saturates, as observed previously for vacuum and OVPD grown pentacene TFTs (Baude

et al. 2003; Gundlach et al. 1997; Shtein et al. 2002). The IDS vs. gate-to-source bias

(VGS) characteristic is plotted in Fig. 6-14c, indicating an IDS on/off ratio of 7x105, and a

channel field-effect hole mobility of µeff = (0.25 ± 0.05) cm2/V·s in the saturation regime.

The channel hole mobility of a vacuum-deposited control TFT was within the

experimental error of the values obtained by OVJP at PL = 0.2 Torr.

A pentacene TFT was also printed in nitrogen at atmospheric pressure (PL = 760

Torr). The TFT exhibited µeff = 0.2 cm2/V·s but a substantially lower current on/off ratio

of 25, caused mainly by the high off current. The higher off current can be explained by

several factors. The atmospheric pressure-printed pentacene channel was much thicker

(~1µm), making the channel region more conductive in the absence of the field effect.

153

Also, higher pressure in the deposition chamber increases conductive and convective heat

transfer from the nozzle assembly to the substrate, which may disrupt the OTS self

assembled monolayer, or the molecular ordering of pentacene on OTS, which has been

previously observed to reduce device performance (Shtein et al. 2002). Chapter 7

discusses in greater detail the relative effects of film morphology and substrate treatment

on the electrical device performance of pentacene TFTs.

6.8 Summary

In this chapter, we introduced a new method for the direct, solvent-free, patterned

deposition of molecular organic semiconductors – organic vapor jet printing (OVJP). A

semiquantitative theoretical model was developed to predict the influence of process

conditions (e.g. deposition pressure, gas flow rate), materials (e.g. relative masses of the

carrier gas and the organic), and apparatus geometry (e.g. nozzle shape) on the deposit

shape and resolution. This model was verified using a direct simulation Monte-Carlo

(DSMC) approach, where the trajectories of many molecules are tracked simultaneously

in space as they are expanded through the nozzle and deposit on the substrate.

Furthermore, a first experimental OVJP apparatus was designed and constructed,

allowing for experimental verification of both the model and the computer simulations. A

pentacene thin-film transistor was deposited, where despite the ultra-high local deposition

rates (>700 Å/s), molecular order was high and the device electrical performance was

comparable to that in TFTs deposited at much lower rates (<1Å/s) using other methods.

154

Chapter 7: Growth of pentacene films

and thin film transistors

7.1 Overview

Transistors based on organic semiconductors offer a potentially low-cost alternative to

silicon, with greater latitude in the choice of substrates, and adequate device performance

characteristics. In particular, pentacene-channel thin-film transistors (TFTs) have

received considerable attention in recent research in organic electronics with regard to

applications in low-cost electronics (e.g. radio-frequency identification tags (Baude et al.

2003), e-textiles (Boderover et al. 2004)) and LCD-backplane driving circuits (Gundlach

et al. 1999; Kymissis et al. 2001). Pentacene channel TFTs have been demonstrated with

room-temperature field-effect hole mobilities (µeff) on the order of 1 cm2/V·s, and drain

current on-off ratios >106, matching the performance of amorphous silicon transistors

(Gleskova et al. 2001; Wu et al. 2001), though often at a lower substrate temperature

during processing, enabling their deposition on flexible plastic substrates.

(Dimitrakopoulos et al. 2001; Rogers et al. 2002)

It is observed that improved molecular order and reduced density of grain

boundaries can result in improved charge transport in both polymeric (Shaked et al.

2003) and molecular (Shtein et al. 2002) organic semiconductors, similar to what is

155

observed for silicon (Pangal et al. 2001; Wu et al. 2001), where the field effect mobility

is typically on the order of 103, 10-102, and 0.1-1 cm2/V·s in single-crystal,

polycrystalline, and amorphous silicon, respectively. Hence, the candidate fabrication

methods for organic TFTs should enable the control of the deposited film morphology

and, thereby, enhance the electrical characteristics of the organic TFTs.

Polycrystalline pentacene channel TFTs exhibiting field-effect mobility up to 2

cm2/V·s have been fabricated by a number of techniques, including crystallization from

solution, (Brown et al. 1997) organic molecular beam deposition (OMBD),

(Dimitrakopoulos et al. 1996) vacuum thermal evaporation (VTE), (Gundlach et al.

1997) vapor phase growth of single crystals, (Kloc et al. 1997) and OVPD (Shtein et al.

2002). This chapter focuses on OVPD, in which the growth rate, substrate temperature

and carrier gas pressure can affect the deposited film morphology, thereby allowing the

study of how crystalline film order relates to the device properties.

7.2 TFT geometry and operation

The theory of charge transport in organic materials is still a topic of vigorous research.

Nevertheless, many of the concepts of classical semiconductor theory are still helpful in

understanding the electrical behavior of organic semiconductors. Therefore, it will suffice

to review the basic structure and operating principles of a TFT based on the traditional

energy band model of semiconductors.

A thin-film field effect transistor is shown schematically in Fig. 7-1. An electrical

current, IDS, flows in the channel of the thin film semiconductor, under a voltage, VDS,

applied between the drain and source electrodes. The electrical conductivity of the

channel is modulated by the voltage VGS on the gate electrode. Consider an energy band

156

diagram for a traditional metal-oxide-semiconductor (MOS) junction, as shown in Fig. 7-

2; the semiconductor is p-type. When the three different materials are brought into

contact and thermal equilibrium is established, the Fermi levels (i.e. the chemical

potential of the electrons) align. The conduction and valence bands must bend to

accommodate this, resulting in band offset and a built-in electric field near the

semiconductor-organic interface. Positive charges (holes) therefore accumulate at the

oxide-semiconductor interface (Fig. 7-2a). When a negative voltage is applied to the

metal contact, more holes are drawn to the oxide-semiconductor interface, forming a thin

accumulation region of thickness xd (Fig. 7-2b). The extra charge accumulated at the

interface effectively increases the local doping concentration and the number of charge

carriers. Upon applying a voltage perpendicular to the gate-oxide-semiconductor stack,

current can flow along the semiconductor-oxide interface (Fig. 7-2c). The greater the

Source Drain

Gate

Insulator

Semiconductor

LW

tox

VDS

VGS

Source Drain

Gate

Insulator

Semiconductor

LW

tox

VDS

VGS

Figure 7-1: Schematic of a thin-film field-effect transistor (TFT), where current between the source and drain electrodes is modulated by the electric field in the semiconductor channel, which is established by means of a voltage applied to the gate electrode.

157

applied gate voltage, the greater the charge density in the channel, and the higher the

current; this is known as the field effect.

The IDS vs. VDS characteristic of the transistor is plotted in Fig. 7-2d. The initial

increase in IDS is linear with VDS. With current obeying Ohm’s law, this is known as the

linear regime. Here, Ohm’s law is (Sze 1969):

= ⋅ ⋅ ⋅

DS eff DSWI q VL

µ (7-1)

Figure 7-2: a) Energy band diagram of the gate metal-oxide-semiconductor junction in a TFT depicted in Fig. 7-1 at zero gate bias (VGS). b) The same junction under VGS. c) Perspective of the junction in (b) when a drain-source voltage, VDS, is applied, causing current to flow in the direction indicated by the arrow. d) Typical current-voltage response of a field-effect transistor.

MetalGate

EFEF

VG = 0

OxideMetalGate

EFEFEV

EC

EF,i

VG < 0

Oxide

xd

MetalGate

EF

Oxide

xd

VG < 0VDS > 0

SourceDrain

VDS

L

(a) (b)

(c)

Incr. (-VGS)I DS

(µA

)

VDS

0-40

0

-100

-50

-20

LinearRegimeSaturation

(d)

Figure 7-2: a) Energy band diagram of the gate metal-oxide-semiconductor junction in a TFT depicted in Fig. 7-1 at zero gate bias (VGS). b) The same junction under VGS. c) Perspective of the junction in (b) when a drain-source voltage, VDS, is applied, causing current to flow in the direction indicated by the arrow. d) Typical current-voltage response of a field-effect transistor.

MetalGate

EFEF

VG = 0

OxideMetalGate

EFEFEV

EC

EF,i

VG < 0

Oxide

xd

MetalGate

EF

Oxide

xd

VG < 0VDS > 0

SourceDrain

VDS

L

(a) (b)

(c)

Incr. (-VGS)I DS

(µA

)

VDS

0-40

0

-100

-50

-20

LinearRegimeSaturation

(d)

158

where W/L is the channel width-to-length ratio, µeff is the effective mobility of charges in

the channel (expressed in cm2/V·s), and |q| is the magnitude of the charge density in the

channel per unit area.

As VDS increases and approaches VGS, the magnitude of the electric field near the

drain contact vanishes, and xd approaches zero. The conduction channel is said to “pinch-

off”, limiting IDS; this is known as the saturation regime. Transistors are typically

operated in the saturation regime because IDS is independent of VDS, and is modulated

only by changing VGS.

Some of the key performance characteristics of the TFT include the ability to

modulate the drain-source current (IDS) by variations in VGS (known as the

transconductance, or gc = dIDS/dVGS), the magnitude of IDS in the “on” and “off” states of

the device (IDS on/off ratio), and the effective speed of charge carriers in the channel

(expressed through µeff). Due to the built-in potentials between the layers comprising the

FET, it may be necessary to overcome a threshold voltage (VT) to induce charge

accumulation in the channel. In the saturation regime, the current is given by (Sze 1969):

( )212DS ox eff GS T

WI C V VL

µ= ⋅ ⋅ − (7-2)

where Cox is the capacitance of the oxide, given by Cox = 4πε0εox / tox, where tox is the

oxide thickness.

As will be shown below, µeff can depend on the morphology of the semiconductor

channel. This is intuitively evident in view of the discussion in Ch. 1, where molecular

order determines the ease of charge hopping between molecules. Furthermore, grain

boundaries are regions where semiconductor molecules are absent or abruptly change

their relative orientations, thereby disrupting charge transport between grains. Since

159

charge transport in a TFT occurs along the semiconductor-insulator interface, the energy

distribution of charge traps at the interface is also likely to affect IDS. The depth and

density of these traps, in turn, depend on the chemistry of the interface, and parameters

such as VT and µeff will depend on the surface treatment of the insulator prior to the

deposition of the organic semiconductor. (See Sec. 7.5 and 7.6).

Figure 7-3 illustrates the two thin film transistor (TFT) architectures studied in

this work. In the “top contact” geometry, the source/drain contacts are deposited onto the

semiconductor, while in the “bottom contact” TFT, the semiconductor is deposited onto

the source/drain contacts pre-patterned onto the insulator, followed by deposition of the

semiconductor. The "bottom contact" configuration permits the use of conventional

photolithography to define the channel W/L ratio, thereby potentially allowing the

increase of IDS through device geometry modification. However, as will be shown, the

+ + + + + + + + + + + +

- - - - - - - - - - - - - - - -

Source

Drain

Insulator

GateSubstrate

Channel

Figure 7-3: a) “Top-contact” geometry of a MOS type FET. b) “Bottom-contact” geometry TFT. The material used for each component in this study is labeled.

Si

SiNx or SiO2

AuPentacene

(a)

(b)

Pentacene

160

heterogeneous region near the contacts induces morphological irregularities in the

deposited pentacene thin films that become more pronounced for smaller values of L,

partially negating the benefits of shorter channels and masking the influence of the

channel morphology on device performance. After demonstrating these "contact effects,"

we adopt the "top contact" geometry for studying the effects of pentacene morphology

and dielectric surface treatment.

7.3 Growth of polycrystalline pentacene thin films from the vapor phase

7.3.1 Qualitative description of vacuum and vapor phase growth

In OVPD, the growth of polycrystalline pentacene thin films proceeds in the following

sequence:

1) Convective transport of pentacene vapor using a carrier gas (e.g. nitrogen) to

the edge of the boundary layer;

2) Diffusion of pentacene molecules across the boundary layer;

3) Adsorption of pentacene onto the substrate surface;

4) Diffusion of pentacene on the substrate surface;

5) Desorption of pentacene from the substrate back into the boundary layer.

In contrast, deposition in vacuum does not involve gas phase diffusion. Instead, it

proceeds by a simplified process of:

i) Ballistic transport of pentacene molecules from the source to the substrate;

ii) Adsorption of pentacene onto the substrate surface;

iii) Diffusion of pentacene on the substrate surface;

iv) Desorption of pentacene from the surface into vacuum.

161

Convective transport of organic vapor from the source to the substrate in OVPD

was described in Ch. 3. Here, we concentrate on the gas and surface diffusion processes

relevant to polycrystalline film growth from the vapor phase. Figure 7-4 illustrates the

temperature profiles and the corresponding pentacene concentration profiles for growth in

vacuum and in OVPD. In case of vacuum growth, the cooling of the evaporant molecules

is abrupt, where the adsorbed species is forced to equilibrate with the surface

immediately upon contact. In contrast, the gas boundary layer present in OVPD allows

the molecules to cool gradually, dramatically reducing the temperature gradient. Since

the organic vapor was generated at a an evaporation temperature Tevap exceeding the

substrate temperature, Tsub, constitutional supercooling of the vapor will occur near the

substrate, reducing the equilibrium vapor pressure, Peq, of the organic (See Fig. 7-4c).

This provides the driving force for the condensation and growth of crystals.

Crystal growth itself proceeds by a combination of processes, including the

diffusion of molecules on the bare substrate, formation of stable nuclei, growth of the

nuclei by addition of diffusing molecules on the surface, and coalescence of individual

grains when their edges meet. The parameters governing these four processes include the

flux of admolecules to the surface, the critical (minimum stable) nucleus size, the surface

diffusivity (Ds), and the surface tension of the crystallite, γs. The molecular flux is given

by the deposition rate, the critical nucleus for pentacene on SiO2 is four molecules (Ruiz

et al. 2003), while Ds and γσ are more difficult to determine quantitatively, and depend on

the molecular structure and the chemistry and topology of the surface.

At very low deposition rates, the density of nucleation sites is very low, and

surface diffusion will dominate crystallization. This is known as diffusion-limited

162

Figure 7-4: a) Comparison of the molecular temperature profiles in vacuum and vapor phase deposition scenarios; b) Corresponding profiles of the organic vapor pressure, which is a function of temperature, P(T). c) Plot of the vapor pressure vs. temperature, showing the actual vapor pressure Porg(Tevap) of the organic species generated at the source evaporation temperature Tevap. Upon cooling of the vapor from Tevap to Tsub, Porg exceeds the equilibrium vapor pressure Peq(Tsub), driving the condensation and crystal growth on the substrate.

Deposition in Vacuum

Vapor Phase GrowthT

T


Vapor Phase GrowthT

T


Vapor Phase GrowthPorg

Peq

Porg

Driving force



Peq

Porg



Peq

Porg

Driving force

TevapTsub

Peq(Tevap)

Porg(Tevap)

Peq(Tsub) Cooling

CondensationDrivingForce

Equilibrium VaporPressure Curve

Temperature Profile Vapor Pressure Profile

Distance (x) Distance (x)

(a) (b)

(c)

163

aggregation (DLA) (Witten et al. 1981; 1983), and produces highly branched, fractal

crystals (Halsey 2000). In electronic device fabrication, however, high deposition rates

are often required, resulting in multi-nucleation processes, where the size and shape of

the crystals is not governed by the kinetics of surface diffusion alone, but also by the

thermodynamics of the thin-film condensed phase.

Three possible modes of crystal growth on surfaces have been identified,

illustrated in Fig. 7.5a (Venables et al. 1984). In the island (Volmer-Weber) growth

Figure 7-5: (a) Illustration of the three modes of crystal growth on surfaces: (i) layer-by-layer (Frank-van der Merwe), (ii) layer-plus-island (Stransky-Krastanov) and (iii) island (Volmer_weber). (From Venables, et al., 1984). (b) Illustration of the mechanisms governing crystal growth from the vapor phase. (From Irisawa, 2003)

(i) (ii) (iii)

θ < 1 ML

1 < θ < 1 ML

θ > 2 ML

(a)

(b)

θ = coverage

164

mode, clusters of the adsorbate nucleate on the surface and grow into islands. In this case,

the adsorbate-adsorbate interaction exceeds the adsorbate-surface interaction. In layer

(Frank-van der Merwe or epitaxial) mode, the surface interaction is large, and the

admolecules condense into a complete surface-bound monolayer, followed by more

weakly bound monolayers. A common intermediate growth (Stransky-Krastanov) mode

occurs, in which one or few surface-bound, strained adsorbate monolayers are formed,

and become covered with islands of the condensed phase as the growth proceeds.

The structure of the initial monolayer is strongly influenced by the underlying

substrate, while the island density can vary over nine orders of magnitude with substrate

temperature (Venables et al. 1984). For molecular organic quasi-epitaxy (Forrest 1997),

the adsorption energy is typically on the order of 1-2 eV, the surface diffusion barrier is

on the order of 50-100 meV (Casalis et al. 2003; Forrest 1997; Heringdorf et al. 2001;

Heringdorf et al. 2004; Israelachvili 1992; Kitaigorodsky 1973; Krause et al. 2002;

Verlaak et al. 2003). In comparison, the room temperature thermal energy of a molecule

is 26 meV.

7.3.2 Theory of crystal growth on surfaces

Figure 7-5b illustrates the key steps in the growth of crystals on the substrate, once a

stable grain has nucleated (Irisawa 2003). Here, the net flux of molecules at the surface,

jv, is given by the difference between the adsorption and desorption rates. The larger the

energy barrier to desorption, and the lower the surface temperature, the longer the

admolecule will spend on the surface. The average residence time of a molecule on the

surface is inversely proportional to the desorption rate constant. Since desorption is an

activated process, the desorption time, τs, is:

165

1 exp des

s B s

Ek T

ντ

= ⋅ −

(7-3)

where Edes is the energy of desorption, Ts is the surface temperature, and ν is

characteristic of the frequency of desorption "attempts", given by the effective surface

vibration frequency (1011-1013 s-1) (Venables et al. 1984). Note that for endothermic

desorption, Edes > 0, and increasing Ts should decrease τs. During the time τs, the

adsorbed molecules will tend to diffuse along the surface, until finding a kink (labeled as

K in Fig. 7-5) in the crystal facet, where the coordination number is highest and the

energy is minimized with respect to other locations on the surface. Since surface

diffusion is also an activated process, the diffusion coefficient Ds is:

2 exp sds

B

ED ak T

ν

= −

(7-4)

where a is the lattice constant and Esd is the surface diffusion activation energy; Esd > 0,

and increasing T should increase Ds. An effective surface diffusion distance, λs, is

defined following the Einstein relation λs = (Ds·τs)1/2:

exp2

des sds

B

E Eak T

λ −

= ⋅

(7-5)

where it was assumed that ν for τs and Ds is the same. If the admolecule adsorbs within

this distance λs of the site K, it will contribute to the growth of a pre-existing crystal via

surface diffusion, otherwise, it will desorb. Note that since Edes > Esd > 0, increasing T

results in a decrease in λs, because the Boltzmann factor favors desorption events more

than surface diffusion.

166

From the above expressions and the net flux of pentacene molecules to the

surface, the rate of lateral crystallite growth, can be determined. This rate is referred to as

"step velocity" v∞ , and can be expressed as a product of the characteristic area of the

adsorption site (a2), a characteristic length scale (e.g. λs), and the molecular surface flux.

Using Eq. (7-5) (Irisawa 2003):

32 exp2 2

eqorg orgdes sd

B org B

P PE Ev ak T mw k Tπ∞

− −= ⋅ ⋅

⋅ ⋅ (7-6)

where Porg and Porgeq are the actual and the equilibrium vapor pressures of the organic

species (in this case, pentacene), while mworg is the molecular weight (278g/mol for

pentacene). Here, (Porg - Porgeq) is the net driving force for providing fresh admolecules

for the growth of crystallites, and ( ) 2eqorg org org BP P mw k Tπ− ⋅ ⋅ is the net molecular flux

at the surface. Since Esd can vary significantly between different types of surfaces, Eq. 7-

6 predicts different crystallite growth rates and sizes on different materials.

It is tempting to use this mechanism to explain the morphology of OVPD grown

pentacene on SiO2 and gold, as shown in Fig. 7-6. However, a number of uncharacterized

variables and processes (e.g. surface defects, cluster mobility, molecular isomerization)

can affect nucleation and surface diffusion (Venables et al. 1984). Thus, when

experiments are not performed in ultra-high vacuum on cleaved crystalline surfaces, the

theories developed to describe them will contain fitting parameters, which mask the true

mechanism. In Fig. 7-6, for example, the surface roughness of electron-beam sputtered

gold may also enhance nucleation of the pentacene crystals, and thus result in finer

granularity of the grown film. Relief of quasi-epitaxial (Forrest 1997) strain by grain

167

boundary creation is an unlikely mechanism, since the underlying SiO2 surface is not

crystalline.

Furthermore, using Eq. 7-6 for quantitative predictions of crystal growth rates

depends critically on accurate values of Edes and Esd, which can be difficult to obtain. The

desorption free energy change is given by Edes = ∆Hdes - T∆Sdes, where ∆Hdes is the

desorption enthalpy change and ∆Sdes is the desorption entropy change. When the surface

is a crystal or a film comprised of the same species as the admolecule, ∆Hdes = ∆Hvap and

∆Sdes = ∆Svap, where ∆Hvap and ∆Svap are the vaporization enthalpy and entropy changes,

Figure 7-6: A scanning electron micrograph of a 700Å thick pentacene grown by OVPD at 3 Å/s on a SiO2 substrate at Ts = 25°C, with 500Å thick gold source/drain contacts deposited prior to pentacene growth. The micrograph shows the difference in pentacene grain size between the two surfaces.

Pentaceneon Gold

Pentaceneon Gold

Pentaceneon SiO2

p+ Si

SiO2

GoldPentacene

168

respectively. For pentacene, ∆Hvap = 145-180 kJ/mol (1.5-1.8 eV) at 25ºC (Verlaak et al.

2003), and ∆Svap = 560 J/mol·K. However, Esd values are not readily available. A number

of studies have been published on the dynamics of pentacene nucleation and growth, for a

variety of substrates and techniques, including MBE (Heringdorf et al. 2001; Heringdorf

et al. 2004), VTE (Ruiz et al. 2003), seeded noble gas beams (Casalis et al. 2003), as

well as models of nucleation based on first principles and basic thermodynamic data for

pentacene (Verlaak et al. 2003). However, the activation energy for surface diffusion was

not directly addressed. Verlaak et al. (Verlaak et al. 2003) examine the onset of 2- and 3-

dimensional modes of nucleation, finding that the transition from 2-D to 3-D nucleation

requires a jump in the chemical potential of 72 meV or 6.9 kJ/mol for pentacene on SiO2,

yielding λs = 9.3 m for a = 15Å at 25°C. Although the 72 meV activation energy is

consistent with Van der Waals bonding interactions of multi-ring aromatics on a variety

of surfaces (Forrest 1997; Israelachvili 1992; Kitaigorodsky 1973), the calculation of λs is

clearly overlooking some physical mechanism, considering the observed grain size for

these conditions (Fig. 7-6). The presence of surface defects can significantly alter the

surface energy potential, increasing Esd, while the mechanism of desorption from SiO2 is

a complex sequence of steps, varying with molecular shape. A rigid flat molecule such as

pentacene may have to re-orient itself in a step-wise fashion to have fewer and fewer

atoms in contact with the substrate. As it the molecule decreases the number of contact

points with the substrate (e.g. flat-, to edge-, to end-bound states), its binding energy

decreases in a step-wise fashion, while the barrier for surface diffusion decreases with

each re-orientation. Thus, as Ts increases, the actual Edes and Esd in Eq. (7-5) also change.

The binding energy of different molecule-surface configurations can be estimated using

169

the (computationally intensive) atom-atom potential methods (Forrest 1997;

Kitaigorodsky 1973). The accurate prediction of crystal growth would thus depend on

these numbers, coupled with molecular dynamics or Monte-Carlo simulations to model

the surface diffusion.

7.3.3 Role of background carrier gas in crystal growth

The above discussion of growth made little distinction between vacuum and vapor phase

deposition. An important difference arises when the kBT energy of the molecule diffusing

on the surface approaches Edes increasing the probability of its desorption. In vacuum the

desorbed molecules are essentially "lost" from the surface. In contrast, the presence of a

diffusion-limitation of the boundary layer in OVPD allows the molecule to "resample"

the surface via gas phase transport near the substrate surface. This process is illustrated in

Fig. 7-7a, and is essentially the same mechanism as in the depression of vapor pressure

by a background inert gas surrounding the evaporating crystal, as shown by (Kloc et al.

1998; Kloc et al. 1997) for alpha-hexithiophene (α-6T) (Fig. 7-7b). Qualitatively Eq. (7-

6) predicts that raising the surface temperature will favor desorption over diffusion. With

carrier gas blanketing the substrate, this may in fact aid in the growth of crystallites, by

transporting the admolecules in the gas phase directly above the surface to distant

crystallites, instead of waiting for surface diffusion to complete. Since the velocity of

molecules in the gas phase is greater than on the surface, this gas phase-mediated

transport should enhance crystal growth on the surface. In addition, it is possible to add

energy to the organic species on the surface by increasing their impact energy. This can

be achieved using seeded gas beam methods (Casalis et al. 2003), and is a likely

additional difference between OVJP and vacuum or vapor growth.

170

Vacuum Vapor Phase(a)

(b)

Figure 7-7: a) Illustration of the mechanism of crystal growth in vacuum, where if a molecule desorbs from the surface before it can find a favorable site at a crystal facet, it is “lost” and cannot resample the surface. In contrast, deposition from the vapor in the presence of carrier gas molecules (circles) can enhance crystal growth. Molecules which desorb are not lost, but have a higher chance of re-sampling the surface due to the diffusion barrier presented by the surrounding inert gas background. b) Plot of evaporation rate of alpha-hexithiophene (α-6T) vs. the pressure of helium surrounding the α-6T crystals, clearly showing the effect of diffusion limitation on the ability of molecules to escape the surface; (from Kloc et al, 2003).

He

α-6T

Vacuum Vapor Phase(a)

(b)

Figure 7-7: a) Illustration of the mechanism of crystal growth in vacuum, where if a molecule desorbs from the surface before it can find a favorable site at a crystal facet, it is “lost” and cannot resample the surface. In contrast, deposition from the vapor in the presence of carrier gas molecules (circles) can enhance crystal growth. Molecules which desorb are not lost, but have a higher chance of re-sampling the surface due to the diffusion barrier presented by the surrounding inert gas background. b) Plot of evaporation rate of alpha-hexithiophene (α-6T) vs. the pressure of helium surrounding the α-6T crystals, clearly showing the effect of diffusion limitation on the ability of molecules to escape the surface; (from Kloc et al, 2003).

He

α-6T

171

7.4 Growth mechanisms for pentacene

To summarize the discussion thus far, the two extremes of film growth are adsorption-

limited and surface diffusion-limited. When the net rate of arrival of species is low

relative to the surface diffusion rate, larger crystallites are obtained, provided the surface

temperature is low enough for equilibrium to favor the condensed phase. In depositing

molecular organic thin films for electronic device applications, there is a competition

between the need to deposit the films quickly for production cost reasons, and the need to

grow high quality crystalline films to improve device performance. The grain size can be

increased by post-deposition annealing, or by appropriately adjusting the deposition

conditions (e.g. temperature, rate, and pressure). Increasing Ts during growth can improve

the diffusion of admolecules on the surface, but not indefinitely, since the probability of

desorption also increases with Ts.

During our work on pentacene transistors, it was found that adsorption-limited

growth of pentacene in OVPD typically occurs at high (>104 Pa) chamber pressures, low

(<1 Å/s) deposition rate, and Ts > 40°C substrate temperature. When the deposition

pressure and the substrate temperature are both minimized, organic species adsorb on the

substrate upon arrival, typically resulting in small crystals or amorphous films. Vacuum

thermal evaporation at >4 Å/s and Ts < 40°C is one example of surface diffusion-limited

growth. Thus, by changing the deposition conditions, such as deposition rate and

substrate temperature, it should be possible to explore both growth regimes. In addition to

temperature and rate, the background nitrogen pressure can be used to a limited extent in

controlling crystallite growth, as discussed in Sec. 7.3.3.

172

Even at relatively low substrate temperature, Ts = 10°C, the natural tendency of

pentacene molecules to form ordered films coupled with the diffusion-limited transport

result in polycrystalline growth, as indicated by Figs. 7-6 and 7-8a for pentacene grown

at 0.25 Torr and 3 Å/s on PE-CVD SiNx. The x-ray diffraction pattern of the film (Fig. 7-

8b) shows several orders of the peak doublet corresponding to the (001) crystal

orientation having 15.48 and 14.44 Å plane spacing, respectively, indicating that

pentacene molecules "stand up" on the surface. This has been observed by others

(Bouchoms et al. 1999; Dimitrakopoulos et al. 1996; Gundlach et al. 1999), who found

that the 15.5 Å spacing is dominant in thin (<500 Å) films, while the proportion of the

14.44 Å increases with film thickness, as well as substrate temperature during growth.

Similar diffraction patterns were obtained for pentacene on SiO2 (shown later in the text).

Figure 7-8: a) SEM of a polycrystalline pentacene thin film grown by OVPD on PE-CVD SiNx at Pdep = 0.25Torr, Tsub = 10°C, Tsrc = 240°C, rdep = 3Å/s. b) X-ray diffraction pattern of the 2Θ scan of the film in (a), showing 5 orders of the (001) and (001)’ (“thin-film” and “bulk”) peak doublet, corresponding to 15.48 and 14.44 Å plane spacing, respectively. Similar patterns were obtained for pentacene on SiO2, shown later in the text.

Pentacene on SiNx

(a)

5 10 15 20 25 30

0.05 0.10 0.15 0.20 0.25 0.300123456 Pentacene (001)

d1=15.48 Å

d2=14.44 Å

sin(Θ)

peak

ord

er

2ΘC

ount

s

(b)

1µm

Figure 7-8: a) SEM of a polycrystalline pentacene thin film grown by OVPD on PE-CVD SiNx at Pdep = 0.25Torr, Tsub = 10°C, Tsrc = 240°C, rdep = 3Å/s. b) X-ray diffraction pattern of the 2Θ scan of the film in (a), showing 5 orders of the (001) and (001)’ (“thin-film” and “bulk”) peak doublet, corresponding to 15.48 and 14.44 Å plane spacing, respectively. Similar patterns were obtained for pentacene on SiO2, shown later in the text.

Pentacene on SiNx

(a)

5 10 15 20 25 30

0.05 0.10 0.15 0.20 0.25 0.300123456 Pentacene (001)

d1=15.48 Å

d2=14.44 Å

sin(Θ)

peak

ord

er

2ΘC

ount

s

(b)

5 10 15 20 25 30

0.05 0.10 0.15 0.20 0.25 0.300123456 Pentacene (001)

d1=15.48 Å

d2=14.44 Å

sin(Θ)

peak

ord

er

2ΘC

ount

s

(b)

1µm

173

7.5 Evidence for pentacene morphology influencing TFT performance

Figure 7-9 shows the IDS vs. VDS and VGS response of a “bottom-contact” pentacene TFT,

where the 700Å thick pentacene channel was deposited by OVPD, using N2 as the carrier

gas, at a deposition rate rdep = 3Å/s, substrate temperature Ts = 10°C and deposition

pressure Pdep = 0.25 Torr. The substrates were comprised of n-type doped (0.001 Ω-cm)

(100) Si wafers (which also served as substrates and large-area gate contacts), with

2000Å thick SiNx gate dielectric grown by plasma-enhanced chemical vapor deposition

(PE-CVD). For these devices, the source and drain contacts were pre-patterned onto the

SiNx layer using conventional photolithography. Prior to deposition, pentacene source

material was twice purified by vacuum train sublimation (Forrest 1997) to yield ≤1 mm

crystals. Additional purification (i.e. growth from the vapor phase) was done under a flow

of ultrahigh purity N2 at 800 Torr (Kloc et al. 1997) to yield pentacene crystals several

millimeters in diameter, which were used as source material for the OVPD of TFT

channels.

In characterizing pentacene TFTs, it is important to differentiate between factors

arising from charge conduction in the pentacene crystal itself, and factors arising from

device geometry. Figure 7-9a shows the electrical characteristics of a pentacene TFT

with W/L = 100µm/30µm, exhibiting p-type (hole) conduction, with IDS saturating at VDS

< -15V. According to Eq. 7-2, the mobility in the saturation regime is obtained either

from the slope of the (IDS)1/2 vs. VGS curve (Fig. 7-9b), or using (dIDS/dVGS) / (Cox/2·W/L).

Using this latter method, µeff = 0.03 cm2/V·s, while IDS on/off ratio = 105, and VT = -4V. If

µeff represented the true mobility of holes in pentacene, changing the device geometry

(via W/L) should not have altered the measured µeff, because W/L is factored out of the

174

Figure 7-9: a) IDS vs. VDS behavior of a bottom-contact pentacene TFT with Au S/D electrodes, SiNx gate dielectric and W/L = 100µm/30µm, deposited at the same conditions as in Fig. 7-4. b) IDS and IDS

1/2 vs. VGS characteristics of the TFT, exhibiting IDS on/off ratio of 105, VT = -4V and µeff = 0.03cm2/V·s. Inset: It is found that µeff increases with the channel length L.

(a)

-30 -25 -20 -15 -10 -5 0

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Vds (Volts)

I DS

(µA

)

VGS = -30

VGS = +4

(b)

-40 -30 -20 -10 0 10

VGS (Volts)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 5 10 15 20 25 300.00

0.02

0.04

L (µm)

µ eff

(cm

2 /V·s)

[I DS

(µA

) ]1/

2

10-10

10-9

10-8

10-7

10-6

10-5

I DS

(Am

ps)

175

expression for µeff. However, as the Inset of Fig.7-9b shows, decreasing L results in

lower values of µeff, even for the devices deposited measured on the same substrate. This

can be understood by examining the pentacene channel morphology.

Figure 7-10a is an SEM of the TFT channel region for W/L = 100/6. The

pentacene film is polycrystalline, with small grains and a high density of grain boundaries

at the source/drain contacts. Evidently, the gold contact pads act as nucleation sites for

the pentacene vapor, resulting in smaller crystals and a higher grain boundary density per

unit surface area. Thus, when decreasing L (Fig.7-10b) in the bottom-contact geometry,

the pentacene in the channel region contains a higher grain boundary density. Since µeff

varies inversely with L, one concludes that grain boundaries impede charge conduction.

To approach the true mobility of holes in single-crystal pentacene the average pentacene

grain size should be increased relative to L. Larger grains may be obtained by increasing

both the substrate temperature and the deposition pressure. Higher substrate temperature

1µm 1µm (b)(a)1µm 1µm (b)(a)

Figure 7-10: a) SEM of a bottom-contact pentacene TFT with SiNx gate dielectric, with L = 6 µm. b) SEM of another bottom-contact TFT on the same substrate, but with L = 2.3 µm, both deposited at the same condition as the device in Fig. 5a. In both cases pentacene growth appears to nucleate at the Au S/D contacts, leading to finer grains and higher grain boundary density near the current injection region. This is what ultimately limits µeff for smaller channels, explaining the trend shown in the Inset of Fig. 5b.

176

provides the thermal energy to anneal defects out of the deposited film, while higher

pressure in the deposition chamber decreases the likelihood of desorption of pentacene

molecules from the surface at the elevated temperature by impeding transport through the

boundary layer.

Using Ts = 65°C and Pdep = 8 Torr, pentacene crystallites are obtained on the

order of 1 µm or greater in size (Fig. 7-11c). However, even for L ~ 1 µm, the observed

Figure 7-11: Plot of µeff vs. deposition pressure, Pdep for “bottom-contact” pentacene TFTs deposited onto SiNx with pre-patterned Au S/D contacts; pentacene was grown at several substrate temperatures. For Ts = 20°C, there is a peak in µeff at ~1 Torr. For Ts = 10°C, increasing the channel length, L, results in an increase in µeff, similar to what is observed for Ts = 20°C (see Fig. 7-5) and for Ts = 50°C. At Ts=65°C, µeff drops to 2·10-3cm2/V·s, explained by de-wetting of pentacene in the S/D region (see SEM in (c)). For pentacene deposited on unpatterned SiNx at 65°C, with top-deposited S/D contacts, pentacene crystallization is unaffected by the contact, and µeff = 0.6 cm2/V·s

0.1 1 1010-3

10-2

10-1

100

incr L

10ºC L=30,10,6,2 µm

Pressure (Torr)

1µm1µm

1µm1µmPentacene on SiNx; Top Contact

Pentacene on SiNx; Bott. Contact

µ eff

(cm

2 /V·s

)

Ts = 10°CTs = 10°C Ts = 20°CTs = 20°C Ts = 50°CTs = 50°C

Ts = 65°CTs = 65°C Ts = 65°C (Top contact)Ts = 65°C (Top contact)

(a) (b)

(c)

177

µeff drops by almost an order of magnitude from that in a TFT deposited at Ts = 10 or

20°C. The answer again lies in the pentacene morphology. As shown in Fig. 7-11c, at

elevated Ts pentacene de-wets the gold source/drain contacts, resulting in large voids and

poor overlap between the semiconductor and the contacts. One solution is to grow

pentacene on blank SiNx substrates and deposit the source/drain contacts on top. This

requires the use of shadow-masks, which typically limits L to lengths >10µm.

Nevertheless, using such a shadow-mask, µeff = 0.6 cm2/V·s was obtained for pentacene

TFTs having >5µm average pentacene crystallite size. These findings are summarized in

Fig. 7-11a, where µeff is plotted vs. the deposition pressure for various substrate

temperatures and device geometries.

7.6 Relative effects of grain size and substrate treatment on device

performance

As PE-CVD SiNx is generally rougher and less homogeneous than dry thermally-grown

SiO2, the latter was chosen for the next set of experiments, to decrease substrate-induced

grain nucleation. Top-deposited 500Å thick gold source/drain contacts were used. As

Fig.7-12a-c shows, the simultaneous increase in Ts and Pdep, and a decrease in rdep result

in larger average grain size for 700-1000Å thick pentacene films grown by OVPD on

SiO2. For pentacene TFTs made from the samples shown in Fig.7-12a-c having W/L =

1000µm / 45µm, µeff = 0.06, 0.12 and 0.58 cm2/V·s, increasing with the average

pentacene grain size. Here, pure island growth dominates, and the grain boundaries

penetrate down to the pentacene-insulator interface (see Fig. 7-5a). Since charge

transport in the TFT likely takes place in the first few monolayers of pentacene

178

(Pratontep et al. 2004), the increasing mobility is a logical outcome of the decreasing

density of grain boundaries at the pentacene-SiO2 interface.

Still, the grain size (~5–10 µm) does not exceed L (45µm), and grain boundaries

in the channel arguably result in µeff < µcrystal, where µcrystal is the mobility of holes in

single-crystal pentacene. At the time, several studies (Gundlach et al. 1999; Gundlach et

al. 1997; Kymissis et al. 2001; Lin et al. 1997) reported improved device characteristics

1 µm(a) 10°C, 0.25 Torr, 3.0 Å/s

µeff=0.06cm2/V·s

1 µm(b) 40°C, 6.0 Torr, 1.0 Å/s

µeff=0.12cm2/V·s

(c) 65°C, 10.5 Torr, 0.3 Å/s2 µm

µeff=0.58cm2/V·s

(d) 10°C, 0.25 Torr, 3.0 Å/s1 µm

µeff=1.2cm2/V·s

(e) 40°C, 6.0 Torr, 1.0 Å/s1 µm

µeff=1.2cm2/V·s

1 µm(f) 65°C, 10.5 Torr, 0.3 Å/s

0.6 < µeff< 0.9cm2/V·s

Figure 7-12: (a)-(c) SEMs of 1000Å thick pentacene films grown by OVPD on SiO2for “top-contact” TFTs at the deposition conditions indicated below each frame. The average crystal size increases as Ts and Pdep increase, with concurrently decreasing rdep. The “top-contact” TFTs using these films exhibited increasing µeff as the pentacene film grain size increased, indicating that grain boundaries indeed increase the overall channel resistance. (d)-(f) SEMs of pentacene grown by OVPD on OTS-treated SiO2 at the deposition conditions matching those in (a)-(c). Unlike films deposited on SiO2, pentacene on OTS has finer grain size, relatively unaffected by growth conditions, except when Ts exceeds 60°C, possibly due to degradation of OTS at high temperature. The µeff is thus relatively unaffected by growth conditions, but is much greater than in devices with untreated SiO2; this is somewhat counterintuitive, due to the much higher grain boundary density in OTS-treated devices.

179

when chemically treating the substrate to result in a hydrophobic surface prior to the

deposition of pentacene. Octadecyl-trichlorosilane (OTS) was one such surface treatment

of choice. The polar –SiCl3 head groups chemically react with the underlying SiO2

substrate and neighboring –SiCl3 molecules in the presence of water to form a self-

assembled monolayer chemically bound to the substrate, while the saturated hydrocarbon

backbone results in an ordered hydrophobic surface. It was thought that the non-polar

pentacene molecules would form a better-ordered film on this type of surface, thus

explaining the observed increase in µeff and IDS on/off ratios. Thus, to investigate the

Figure 7-13: SEMs of 1000Å thick pentacene grown by VTE on OTS-treated SiO2, where the deposition rate was < 2Å/s. The micrographs shows a polycrystalline layer of pentacene having grains on the order of 0.5 µm in diameter, covered by twig-like islands embossing the grain boundaries in the wetting layer. The polycrystalline layer is too thick to be the wetting layer typically observed in Stransky-Krastanov growth mode, while the rod-like crystals may rise from the underlying film under the pressure of the merging grain boundaries. The measured µeff for TFTs comprised of vacuum-deposited pentacene on OTS-treated SiO2 were also approximately 1 cm2/V·s. (Figure courtesy of Changsoon Kim).

180

effects of surface modification on pentacene crystallization and TFT performance, the

substrates in Fig. 7-7a-c were also treated with OTS. The OTS treatment consisted of a

15min exposure to O2-Ar plasma, followed by a 24 hour soak in DI H2O, followed by a

3-hour or longer soak in a 700 µM solution of octadecyltrichlorosilane (OTS) in

chloroform and hexane (3:7 by volume).

Scanning electron micrographs of 700-1000Å thick pentacene thin films grown on

the OTS-treated substrates are shown in Fig.7-12d-f. Surprisingly, pentacene formed

substantially smaller grains on the OTS-treated SiO2 compared to the untreated

substrates. More surprisingly, in view of the smaller pentacene grain size, the µeff

measured for these TFTs exceeds 1.0 ± 0.1 cm2/V·s. Figure 7-14 shows the IDS vs. VGS

and VDS characteristics of a typical OVPD-grown pentacene TFT with an OTS-treated

SiO2 gate dielectric. The saturation-regime µeff = 1.4 cm2/V·s, IDS on/off ratio = 108-109,

in reverse and forward VGS sweeps, respectively, while VT = 5V.

The surprising observations may be reconciled by a different surface growth

mode for pentacene on OTS. If a layer-plus-island (Stransky-Krastanov) mode is

assumed, the thin wetting layer of pentacene on OTS participates in charge conduction,

while the grain boundaries visible in the top-most layers does not penetrate down to the

pentacene-OTS interface. The wetting typically being only one or a few monolayers,

direct observation of it can be done using atomic force microscopy (AFM). Here, its

existence is inferred from the mobility behavior of the TFTs. Provided that charge

transport is indeed confined to the few pentacene monolayers immediately adjacent to the

gate dielectric, further verification of the Stransky-Krastanov growth mode can

181

potentially be done by varying the channel thickness. If the µeff is invariant with thickness

for > 2 or 4 monolayers, the wetting layer is indeed present.

For vacuum-deposited 1000Å thick pentacene films on OTS (Fig. 7-13), a

polycrystalline but relatively flat pentacene layer grows on top of OTS, but becomes

covered in smaller twig-like crystallites embossing the grain boundaries. The measured

µeff for the VTE-grown TFTs is also on the order of 1 cm2/V·s, indicating once again that

the channel mobility is largely unrelated to the top film morphology when the OTS

-80 -60 -40 -20 010-1310-1210-1110-1010-910-810-710-610-510-410-3

I DS (

Amps

)

VGS

(Volts)

-40 -30 -20 -10 0

-20

-10

0

VDS (Volts)

-40 = VGS

-30

-20

W / L = 1000 / 45VDS = -40 V

I DS

(µA

mps

)

On SiO2

On OTS/SiO2

Figure 7-14: IDS vs. VGS characteristic of an OVPD-grown 1000Å thick pentacene channel TFT with W/L = 1000µm/45µm, obtained in the saturation regime at VDS = -40V. The device contained an OTS-treated SiO2 gate dielectric; it is compared to a TFT with no OTS treatment. The OTS-treated device exhibited µeff = 1.4 cm2/V·s, IDS on/off ratio 108-109 (forward and reverse VGSsweeps, as indicated), and VT = -5V. Inset: IDS vs. VDS characteristic of the device, showing classical current saturation behavior. The TFT without the OTS exhibited lower IDS on/off ratio and µeff.

182

treatment of SiO2 is present. However, at the higher growth temperature, Ts, the adhesion

between pentacene and OTS can be disrupted in favor of pentacene-pentacene

interaction, resulting in island growth and larger grains (Fig. 7-12f). Here, the grain

boundaries may penetrate down to the OTS surface, resulting in the lower measured µeff.

Figure 7-15 shows the x-ray diffraction pattern for a 1000Å thick pentacene layer

deposited onto SiO2 at 1-2 Å/s, a chamber pressure of 6.0 Torr, and a substrate

temperature of 40ºC. The peaks arise from the previously observed "bulk" and "thin-film"

Figure 7-15: X-ray diffraction data for pentacene films grown by OVPD on SiO2(bottom curve) and OTS-treated SiO2 (top curve). Both samples exhibit the peaks corresponding to thin-film and bulk phases of pentacene (15.5 and 14.5Å (001) plane spacing, respectively). The OTS-containing sample also shows a pronounced peak at 2Θ ≈ 18°, corresponding to ~6Å (1’11, 110) plane spacing; a similar feature can be seen in the curve for the untreated substrate, albeit at much lower intensity. The smaller plane spacing corresponds more with the short axis of the pentacene unit cell. It is thus likely that a larger fraction of pentacene molecules on the OTS-treated SiO2 are oriented with the long molecular axis parallel to the substrate plane, compared to those on non-treated SiO2.

2Θ5 10 15 20 25 30

Pentacene / SiO2

Pentacene / OTS / SiO2log(

Inte

nsity

) (ar

b. u

nits

)(002)

(002’)

(003)

(003’)

(004

)(004’)

(005)(005’)

(111,110) Pentacene

15 ±

0.5

Å

183

phases of pentacene.(Gundlach et al. 1999) The thin-film phase has an interplanar

spacing of 15.5 ± 0.1 Å, while the bulk-phase, characteristic of thicker films and higher

substrate temperatures, has an interplanar spacing of 14.4 ± 0.1 Å. The x-ray diffraction

pattern for a 700 Å thick pentacene film on OTS-treated SiO2 is also shown in Fig. 7-15.

The pentacene film on the OTS-treated SiO2 substrate exhibits a Bragg reflection at 19 ±

0.25º, which is less pronounced in the case of untreated SiO2. This peak is likely an

unresolved doublet arising from (110) and (111) reflections of pentacene (Campbell et

al. 1961), with an interplanar spacing of approximately 3 Å. Observation of these features

in the 2-Θ scan indicates that a significant fraction of the pentacene molecules are

oriented with the long molecular axis parallel to the substrate. Pentacene in this

orientation has also been observed when deposited onto copper (Lukas et al. 2002;

Schuerlein et al. 1995), where the surface morphology is similar to that observed for

pentacene on OTS. However, the depth distribution of this pentacene phase is not yet

clear, but would be a useful measurement.

For the devices in Fig. 7-12, the subthreshold slope, S, for TFTs on SiO2 and

OTS-treated SiO2 was found to be S = (2.1 ± 0.1) and (0.6 ± 0.1) Volts/decade,

respectively. The µeff = (0.12 ± 0.02) cm2/V·s and (0.6 ± 0.1) cm2/V·s, while the peak IDS

on/off ratios were 5x105 and 5x107 for SiO2 and OTS-treated SiO2 substrates,

respectively. In one possibility, the IDS at high VGS is increased in the case of OTS may be

attributed to the tighter packing of pentacene crystallites in the channel, or, alternatively,

to the increased strain of pentacene on OTS. In the first instance, the improved physical

contact between pentacene grains increases the overall electrical conductivity of the

channel during charge accumulation in the “on”-state. In the second instance, the

184

decreased intermolecular distance can improve the π-π electron overlap. The increased

density of grain boundaries does not seem to affect IDS in the "off"-state, which can be

seen from the smaller S values for OTS-treated TFTs. This is particularly pronounced in

the case of samples grown under conditions used in Figs. 7-12a and d, where µeff

increases by a factor of 20, but the average grain size decreases by a factor of two.

However, the smaller S values may also be due to the lower energy and number of

charge traps present in the regions between the grain boundaries, directly underneath the

crystals. In this case, the lowering of the dielectric surface defect or dipole concentration

may contribute.

7.7 Effect of surface energy on device performance

Another possible explanation for the improvement of various device performance

parameters can be based on changes to the surface energy of the dielectric with the

various treatments. Figure 7-16b shows the IDS vs. VGS response for variously treated

SiO2 substrates. The treatments included: i) O2 plasma exposure, ii) baking in air at

150°C, iii) OTS treatment following the bake, and iv) OTS treatment following the O2

plasma exposure. The contact angle of water on these substrates (Fig.7-16a) increases

from (i) to (iv), indicating a decreasing hydrophilic character of the surface, i.e. a

lowering of the surface energy of the oxide. Concomitantly, the TFTs exhibited a

simultaneous increase in the IDS on/off ratio, increase in the IDS-VGS subthreshold slope, S,

a shift from VT > 0 to VT < 0, and a less-pronounced hysteresis in the IDS vs. VGS behavior.

While the exact physics of this is not yet understood, it is possible that the lowering of the

dipole strength at the dielectric-pentacene interface (evidenced by increasing water

contact angle and decreasing VT) may in fact contribute to decreased charge trapping,

185

both in the depth of traps (evidenced by a sharper sub-threshold IDS rise) as well as their

number (evidenced by milder IDS-VGS hysteresis).

-70 -60 -50 -40 -30 -20 -10 0 10 2010-1210-1110-1010-910-810-710-610-510-410-3

W=45µmL=1000µmtop Au contacts

2100A SiO2 with surface treatmentsNon-Polar Surface

Polar Surface

Non-Polar Surface

Polar Surface

O2 plasma bake bake + OTS plasma + OTS

I DS (A

mps

)

VGS @ VDS = - 40 V

(a)

(b)

(i) (ii) (iii) (iv)

(i)

(ii)(iii)

(iv)

Figure 7-16: (a) Photograph showing water droplets on 4 different SiO2 substrates, where each substrate received a different treatment: (i) 10 min in O2 plasma, (ii) same as i, followed by 15 min bake at 150°C in air, (iii) same as iii, followed by an OTS treatment, and (iv) same as i, followed by an OTS treatment. The substrates are shown in order of increasing hydrophobicity, as evidenced by the increasing wetting angle of the droplet. (b) IDS vs. VGS saturation-regime characteristics of pentacene TFTs grown on the substrates used in (a), having top-deposited contacts and the identical channel dimensions. The data show that IDS on/off ratio, µeff, VT, and the subthreshold slope all increase with the dielectric surface hydrophobicity.

186

7.8 Further remarks

Despite a large body of work focusing on charge transport in organic semiconductors,

there can be a lack of reproducibility of results from laboratory to laboratory, and even

within the same laboratory from one time to another. In the course of this study, some of

these patterns have been observed. For example, using freshly purified pentacene to

deposit channels at Ts = 40ºC, Pdep = 6.0 Torr, and rdep = 1 Å/s, µeff = (1.5 ± 0.2) cm2/V·s

was obtained. However, similar conditions more commonly yielded µeff = (0.9 ± 0.3)

cm2/V·s for pentacene stored under nitrogen for longer than 24 hours. Devices with

pentacene deposited on identically treated substrates in the same run showed substrate-to-

substrate variations in µeff on the order of 15%, although freshly-purified source material

almost always yielded the higher µeff values. Table IV lists some observed trends with

respect to TFT electrical characteristics and process conditions, which should be minded

in the preparation of devices and specified in reports.

Table IVTable IV

187

7.9 Summary

Pentacene thin films with controlled morphology were grown on a variety of substrates

by OVPD. The main objective was to use vapor phase growth to control pentacene

morphology and to understand the relative effects of morphology and the gate dielectric

surface treatment upon the electrical characteristics of the TFTs.

To decouple the influence of the heterogeneous source/drain contact region on

pentacene morphology, top-deposited contacts were used. For pentacene grown on SiO2

larger crystallites were obtained at lower deposition rates and higher the substrate

temperatures. The average pentacene grain size increased by a factor of ~10, causing the

field-effect hole mobility, µeff, to increase by a similar factor. Up to a 20-fold

improvement in µeff was obtained by pre-treating the SiO2 surface with OTS – a

surfactant that forms a chemically bound self-assembled monolayer (SAM) on the SiO2

surface. Somewhat surprisingly, the improvement in µeff occurred despite the higher

density of pentacene grain boundaries in the channel region of OTS-treated devices.

Furthermore, several TFT performance parameters improved with hydrophobicity of

SiO2. We conclude that surface treatment had a much stronger effect on the electrical

characteristics of the TFT than grain morphology, and that the observed surface

morphology may not provide a clear picture of the molecular order of the buried

pentacene layers closest to the dielectric surface. Grazing-incidence neutron scattering

(Scoles 1988) or scanning probe techniques (Mccarty et al. 1999) can be used to study

the packing of pentacene not exceeding several monolayers of coverage on OTS.

188

Chapter 8: Summary, state-of-the-art,

challenges and future directions

8.1 Organic semiconductors and devices

This thesis began with an introduction to some basic properties of a class of compounds

called organic semiconductors. These are highly conjugated organic small molecules and

polymers, where excess positive or negative charge is readily distributed over much of

the molecule due to the extended nature of the π molecular orbital (Kitaigorodsky 1973;

Pope et al. 1982). However, due to the weak intermolecular interactions dominated by

van der Waals forces, electron and hole mobilities in organic compounds rarely exceeds

~1 cm2/V·s, orders of magnitude lower than in their inorganic, covalently bonded

cousins, e.g. silicon, germanium, gallium-arsenide, etc (Pope et al. 1982). In addition, or

perhaps as a consequence, electronic excitations (excitons) in organic solids are typically

contained on a single molecule, or shared by no more than the nearest neighbor

molecules (Pope et al. 1982; Silinsh et al. 1994). Thus, much of the optoelectronic

behavior of organic solids is dictated by their molecular structure, thereby opening up a

powerful toolbox of synthetic chemistry for precise control of light-matter interactions in

these materials.

189

As the understanding of the optical and electronic processes in organic solids

grows, practical device applications have emerged, including light emitting diodes, solar

cells, and transistors. And although charge mobility in organic solids is orders of

magnitude lower than in inorganic semiconductors, by using extremely thin (on the order

of 101-103 Å) layers, certain key performance characteristics could be improved (e.g.

power efficiency of OLEDs and solar cells, speed of photodetectors, etc.). At the same

time, it has been realized that the weak intermolecular forces enable the deposition of

organic semiconductors on a wide variety of substrates (e.g. glass, plastic), without

attempting to lattice-match the substrate and the active layers (Forrest 1997). This has

opened up a broad range of device applications, such as light-weight, flexible, wearable,

and low-cost electronics.

8.2 Organic semiconductor processing technology

Since most of the existing methods of semiconductor device fabrication are geared

toward inorganic materials, and are costly, considerable research has been thus directed

toward developing novel methods of organic semiconductor device fabrication (Forrest

2004). Low-cost alternatives such as ink-jet printing (Hebner et al. 1998; Sirringhaus et

al. 2000), dye sublimation (Blanchet et al. 2003), and vacuum evaporation (Forrest 1997;

Tang 1986; Tang et al. 1987) emerged. The polymer community favors solvent-based

methods, while the small-molecular community prefers molecular beam (or vacuum

evaporation) techniques. The room-temperature solubility of small molecules in most

solvents is very poor, while the thermal energy required to evaporate polymers or

molecules with >1000 molecular weight generally exceeds their inter-atomic bond

strengths. Thus, small-molecular weight devices possess an advantage over the polymer-

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based devices in the greater control of thickness, doping and sophistication of multi-layer

device structures, while being unable to efficiently scale up for low cost manufacturing

by vacuum evaporation. Organic vapor phase deposition has thus been developed to

address some of these issues for small molecular weight semiconductors.

8.3 Development and application of OVPD

In the course of recent research in OVPD, our understanding of the physical mechanisms

governing OVPD increased (Shtein et al. 2001), enabling the controlled growth of

OLEDs (Baldo et al. 1998; Baldo et al. 1997; Shtein et al. 2001), TFTs (Shtein et al.

2002), and more recently solar cells. The specific requirements of organic thin-film

patterning were also addressed (Shtein et al. 2003), thus setting the stage for

commercialization of the OVPD technology. Figure 8-1 shows a schematic of a

commercial-grade OVPD system built by a semiconductor equipment manufacturer,

Aixtron AG (Aachen, Germany) in collaboration with Princeton University (Princeton,

New Jersey) and Universal Display Corporation (Ewing, New Jersey). At least four such

systems have been built, with systems installed and operating in Ewing and Princeton,

while OVPD is becoming recognized as a key enabling technology in the low-cost

fabrication of organic electronics. High performance phosphorescent OLEDs have been

deposited on substrates up to 6-inch x 6-inch, with film thickness uniformity >95%,

excellent quantum efficiency and operation lifetime characteristics (Fig. 8-2). Current

research in OVPD focuses on utilizing the unique aspects of OVPD to control nucleation

and growth of bulk heterojunctions (e.g. CuPc/PTCBI) for efficient solar cell

applications.

191

Load Port

Deposition Chamber

Control Module

Heated SourceModule

Mixing Unit

N2

N2 N2 N2

T1 T2 T3

N2

N2 N2 N2

T1 T2 T3

N2

N2 N2 N2

T1 T2 T3

Figure 8-1: (Top) Schematic of OVPD more suited to commercial-scale production, where carrier gas flows through the source containers for more efficient pick-up, and is subsequently evenly spread over the substrate using an appropriately engineered disbributer. (Bottom) Illustration of a commercial-grade OVPD system built by Aixtron AG in collaboration with Princeton and Universal Display Corporation.

192

8.4 Development and application of OVJP

Despite these achievements, it was realized that using shadow-masks to pattern the

organic layers posed an inherent limitation. While >50% efficiency for delivering the

source material to the substrate can be achieved in OVPD, up 60% of that can be wasted

on coating of the mask in any given step of depositing a full-color OLED display.

Furthermore, shadow masks can only be used ~10 times before coatings become thick

enough to flake off and cause particulate contamination, and coated masks could undergo

only ~10 cleaning cycles before needing to be replaced. (Bardsley 2004) To address this

problem, organic vapor jet printing was developed. (Shtein et al. 2004) In OVJP, the

organic molecules are printed directly onto the substrate, by seeding a hot inert carrier

gas (e.g. N2 or He), and expanding the gaseous mixture through microscopic collimating

Figure 8-2: Photograph of a 6”x6” glass substrate with an array of green-emitting phosphorescent OLEDs grown on it using the OVPD system illustrated in Fig. 8-1. The uniform brightness of the devices under operation implies uniform film thickness across the entire substrate.

193

nozzles at near-sonic average velocity. When the seeded gas jet impinges on a cooled

substrate, the organic molecules selectively physisorb onto the substrate, forming a well-

defined deposit. This highly non-equilibrium mode of film growth and patterning enables

the printing of high-performance polycrystalline pentacene TFTs at ultra-fast (>700A/s)

local deposition rates. Current research on OVJP focuses on the printing of organic

heterostructures (e.g. α-NPD/Alq3) for OLED applications.

8.5 Future directions in carrier-assisted deposition, novel devices

8.5.1 Carrier-assisted deposition of metals

As mentioned in Ch. 2, both OVPD and OVJP cannot be used to deposit metals using the

carrier gas-assisted mechanism. Metals typically require very high temperatures

(>800°C) to evaporate; upon contact with inert gases such as Ar or He, the rapidly

increasing probability of 3-body collisions leads to the formation of a metal aerosol. The

aerosol particle size can vary substantially, depending on the process conditions, but

generally exceeds 10nm, making it difficult to deposit atomically flat metal contacts.

An alternative technique may involve starting with metal-organic precursors,

transporting them using a carrier gas toward the substrate, and driving off the ligands in

the vapor phase just before the molecules strike the surface. The scission of ligands can

be initiated by intense (laser) light, a plasma discharge, or gas-phase chemistry. The

drawback of these approaches is that if the precursor breaks up, degradation of the pre-

deposited organics can occur. A detailed study of the relative bond strengths of several

classes of compounds may be helpful. Furthermore, to resolve this problem research is

194

needed to understand the nature of the metal-organic charge injecting contact to fully

understand and optimize the range of physical properties leading good electrical contacts.

8.5.2 Growth of focal plane arrays using OVJP

Curved focal plane photodetector arrays (Wang et al. 2003) offer potential advantages

over flat imaging arrays, which are currently used for cameras and sensors. The focal

surface of a simple thin lens is curved, whose shape is given by the Petzval condition.

The curvature leads to out-of-focus regions when flat imaging arrays are used. Figure 8-3

(top) shows that either the edges or the center of an image can be in focus at any given

time. Correcting for this type of aberration requires complex objective optics, which

increase the bulk and cost of the imaging apparatus. In contrast, the human eye solves

this imaging problem by using a curved focal plane – the retina as shown in Fig. 8-3

(bottom).

One potential application is in portable (e.g. cell-phone) cameras. If a curved

focal-plane array can be deposited onto an inexpensive plastic lens, the image sharpness

may improve. Alternatively, curved very large-area arrays for use in telescopes can

substantially improve image quality and reduce the complexity of the optics. Even

replacing the retina itself with prosthetic photodetector implants is a potential application,

as has been shown with silicon photovoltaic arrays, which are forced to be small, due to

the high curvature of the eye.

Unfortunately, existing methods of inorganic semiconductor fabrication and

patterning cannot easily produce such an array, since a curved surface cannot be created

out of a flat sheet without creasing. One approach involves assembling and joining large

(~1cm) arrays of silicon on a curved backing (Wang et al. 2003). In another method,

195

Figure 8-3: (Top) Illustration of the Petzval condition, whereby a simple thin lens always produces a curved focal surface, which is impossible to image perfectly using flat imaging arrays, requiring the use of complex and bulky corrective optics in the objective. (Bottom) Illustration of the human eye, where the retina is nature’s own curved photodetector, matching perfectly the Petzval surface produced by the lens. To the right is illustrated the placement of a prosthetic retinal implant consisting of an array of photovoltaic elements on a silicon chip, designed to partially restore vision in certain cases of blindness (e.g. retinitis pigmentosa and macular degeneration), where only the retina has lost its photodetection capability, while the rest of the visual apparatus remains intact.

196

photoresist is deposited onto a flat substrate, which is subsequently plastically deformed,

while metal deposition and lift-off are used to pattern flexible metal interconnects

between stiff semiconductor islands on the curved substrate (Hsu et al. 2002).

Alternatively, OVJP can be used to deposit the photodetectors and other components onto

a pre-curved substrate. This approach would employ an OVJP nozzle mounted on a

computer-controlled goniometer. Contact deposition may potentially limit this approach,

although metal sputtering can be used for conformally coating curved surfaces. For

retinal implants, understanding the behavior of substrates and organic materials in

biological media is an obvious practical concern.

8.5.3 Alternative device form factors, fiber photovoltaics

As OLED flat panel display technology becomes commercially viable, novel applications

and yet-lower price-points become important for both OLEDs and other organic devices.

Electricity generation using photovoltaic cells is one example. Silicon solar cells are

simply too expensive to be used in all but the most specialized of applications, given that

conventional methods of electricity generation, and even wind-power, can provide

electricity at $0.02 or $0.04/kWhr. With an average solar flux of ~1kW/m2 in favorable

locations, it can be shown that to be competitive, solar cells must achieve a $30/m2 price

point, while also substantially reducing the cost of the solar module installation. The first

can be difficult, considering that ITO-coated glass which is used for organic solar cell

fabrication already costs ~$30/m2. There is little hope to decrease the cost of this

substrate, since the ITO deposition process has been optimized for several decades,

concurrent with already tremendously high production volumes. The second is difficult

197

due to the fact that both Si and glass are both relatively fragile and heavy. Unfortunately,

ITO-coated light-weight plastic substrates are even more expensive.

An alternative may involve fabrication of organic PV cells on fibers, (Fig. 8-4 and

8-5) and weaving them into cloth. In this way, installation costs can be drastically

reduced, while the range of application/deployment can also increase (e.g. backpacks and

Figure 8-4: a) Structure of the proposed solar fiber, consisting of a nylon strand (radius r1) metallized (thickness d1) and coated with the photogeneratingpolymer blend (radius r2). The primary photogenerating core contacts a metal anode wire (diameter d2), and both are encased in a protective nylon sheath (radius r3).

MDMO-PPV

PCBM

PEDOT PSS

CuPc

C60

hνLi/Al

Nylon core

Inner conductor (e.g. Al)

Outer / Auxiliary conductor (e.g. Al)Barrier coatingPhotoactive layer

Figure 8-4: a) Structure of the proposed solar fiber, consisting of a nylon strand (radius r1) metallized (thickness d1) and coated with the photogeneratingpolymer blend (radius r2). The primary photogenerating core contacts a metal anode wire (diameter d2), and both are encased in a protective nylon sheath (radius r3).

MDMO-PPV

PCBM

PEDOT PSS

CuPc

C60

hνLi/Al

Nylon core

Inner conductor (e.g. Al)

Outer / Auxiliary conductor (e.g. Al)Barrier coatingPhotoactive layer

198

tents made from such “solar cell cloth”). The cost of manufacture can be further

decreased by virtue of adapting existing low-cost fiber-drawing technology to a new set

of photoactive materials. It is anticipated that liquid-processable materials may be most

suited for this application.

8.5.4 Large-volume, ultra-purification

With the increasing volume of device production, the need for larger quantities of

purified source material also grows. Current methods of organic material purification

(e.g. solvent fractionation for polymers, or train sublimation for small molecules) are

(a) (b) (c) (d) (e)

(g)

(h)

(i)

(j)

(f)

(k)

Process flow

Unspoolnylon wire

Al coat Coat with active layers

Co-wind Aux. anode

Barrier coating

Spool upand ship

Figure 8-5: A possible process flow diagram for manufacturing a solar cell on a continuous fiber, which can be subsequently woven into “solar cell cloth” with a wide range of applications.

199

simply inadequate with regard to their slow speed and small quantities of purified

material obtained. For example, it can take a week or more to complete 3 purification

stages for relatively small quantities (~3g) of pentacene.

An inherent limitation exists for small molecular weight organics. Due to the

weak intermolecular bonding, the energy required to disrupt the crystalline order is

relatively small, allowing for a high density of defects and impurities to be incorporated

(Kitaigorodsky 1973; Silinsh et al. 1994). To achieve high quality crystals, growth in an

equilibrium-limited regime may be preferred, for example, employing small temperature

gradients and boundary layer diffusion (Kloc et al. 1997). Since the transport from the

vapor to the solid phase is driven primarily by the temperature gradient across the

boundary layer, rapid growth of high quality crystals is thus difficult.

Another solution may employ nucleation and growth of crystals from a

supercritical fluid. Supercritical CO2 has been used extensively in the chemical, food, and

pharmaceutical industries for extraction, owing to the chemical inertness and high

solubility of the constituent organics in it. Thus, high concentrations of organic molecules

in CO2 can in principle be achieved. Furthermore, this fluid is interesting from a mass

transport point of view, since compressibility, viscosity, mass and thermal diffusivity all

tend to be much higher in the supercritical phase, or diverge at the critical point. Thus,

large nucleation driving forces, as well as rapid molecular transport are possible upon the

transition from the super- to the sub-critical phases, all at relatively low temperatures,

albeit by means of pressure change at very high (>70atm) pressures.

8.5.5 Non-cleanroom device processing

200

Keeping with the theme of low-cost device processing, it becomes obvious that

traditional approaches to semiconductor manufacturing involving cleanrooms have

limited applicability. The high costs of cleanroom floor space and highly skilled labor are

prohibitive when trying to shift device fabrication from the high-value-added, low-yield

to the commodity-scale modes. Therefore, it is necessary to develop materials, device

structures, and processing methods whereby dust or ambient contamination does not

affect device performance. Alternatively, fully automated processes must be designed

whereby the samples are fully contained within a controlled, inert environment, from

loading of the substrates to their emergence in the final packaged state.

8.6 Summary

This thesis described the novel methods of organic vapor phase deposition (OVPD) and

organic vapor jet printing (OVJP) for the deposition and patterning of molecular organic

thin film electronics. The range of applications presently includes organic LEDs, thin-

film transistors, lasers, and photovoltaic cells on a variety of substrates and form factors,

including light-weight flexible plastics and fibers.

Theory, computer simulations, and OVPD experiments were reviewed, outlining

the material transport regimes in OVPD, as well as the equations predicting deposition

rate and doping concentration. The presence of a hydrodynamic boundary layer at the

substrate leads to diffusion-limited deposition, requiring a stochastic approach in

modeling the molecular nature of transport in confined geometries used for in-situ

patterning of active organic thin films. Monte-Carlo simulations were performed and

confirmed using shadow-masking experiments in OVPD.

201

Based on the knowledge of gas-phase dynamics in OVPD, a novel method –

OVJP – was developed for the direct patterned deposition of molecular organics. This

method potentially enables high-resolution printing of molecular organic electronics, at

nearly 100% efficient use of the source material. It avoids the use of shadow-masks for

organic layer deposition, which may greatly simplify the fabrication sequence and bill of

materials, lowering the overall cost of organic electronic devices.

After developing a semi-quantitative theory of OVJP, direct-simulation Monte-

Carlo simulations were used to predict the deposited pattern shape and resolution. An

experimental OVJP system was constructed and used to demonstrate the concept, by

printing high (>1000 dots per inch) resolution patterns of Alq3, α-NPD, and pentacene at

ultra-high local deposition rates. A pentacene TFT was printed at >700Å/s local

deposition rate, having device performance characteristics similar to TFTs with highly

crystalline pentacene layers deposited at rates orders of magnitude lower.

As the optical and electrical performance of organic-based devices improves, and

their complexity increases, the development of low-cost fabrication methods becomes

important. Many novel processing methods have been developed, geared specifically to

organic semiconductors. The two carrier-assisted vapor-phase techniques described here

show significant promise, with OVPD already entering the commercial market, after

nearly 10 years of its scientific development. While it is hoped that OVJP will meet a

similar fate, many exciting scientific and technological challenges still await. There is

plenty of room for improvements, innovations, and radical new concepts in device

processing.

202

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