Optical, Electronic and Structural Properties of Semiconductor Nanostructures and Optoelectronic...

Optical, Electronic and Structural Properties of Semiconductor Nanostructures and Optoelectronic Devices

Inorganic Semi-conductors & Devices

(Compounds of III-V, I-III-VI2, II-IV-V2)

Organic Semi-conductors & Devices

(Polymers & Functional Polymers)

Solar Components & Systems

(Photovoltaic

und Solar Thermal)

Prof. Dr. Gerhard Gobsch

Department of Experimental Physics I

Organic Optoelectronics

ACOPhys, St. Petersburg, Sept 2006

An Introduction

Materials, Processing, Concepts and Devices

• Introduction• Molecular Energy Levels and

Charges• Materials and Processing• Devices• Summary

Organic Optoelectronics - Outline


Organic Optoelectronics deals with semiconductor devices in which the semiconductor is an organic material.

There are light emitting diodes (OLED), thin film field effect transistors (OFET), solar cells, lasers, detectors, sensors...

Beyond the scope of the present lecture is the field of “Organic Electronics ↔ Polymer Electronics“.

Theoretical basics of electronic properties of organic materials were given in the lecture of Prof. Runge.

The Nobel Prize in Chemistry 2000

"for the discovery and development of conductive polymers"

Alan J. Heeger Alan G.

MacDiarmid Hideki Shirakawa

1/3 of the prize 1/3 of the prize 1/3 of the prize

USA USA and New Zealand

Japan

University of California Santa Barbara, CA, USA

University of Pennsylvania Philadelphia, PA, USA

University of Tsukuba Tokyo, Japan

b. 1936 b. 1927(in Masterton, New Zealand)

b. 1936

“for the discovery and developement of conductive polymers“


A new material class!

“In creating and expanding the 4th generation of polymers, we

attempted to understand nature with sufficient depth that we

could achieve materials with novel and unique properties, that

are not otherwise available.

This was (and is) an elegant and somewhat dangerous exercise;

elegant because it requires the synthesis of knowledge from

chemistry, physics and materials science, and dangerous

because when working on the boundary of three disciplines, one

is always pushing beyond the knowledge and experience of this

background.

To our research in this interdisciplinary field has had sufficient

impact on chemistry to be recognized by the Nobel Prize gives

us, therefore, particular satisfaction.”

Alan J. Heeger (on occasion of giving him the Nobel Prize for Chemistry in 2000)

Advantages

Pros and Cons of polymers for electronic applications

• Metallic and semiconducting properties by doping• Combination of plastic with electronic properties• Property engineering• Solubility in organic solvents, variable processibility• Use of printing technologies• No vacuum and no high temperature processes

Low-cost production

Disadvantages

• Low integrated devices and circuits (in the very near future)

• Degeneration in O2- and H2O-atmosphere

Long-term stability is still a critical point

Application of Functional Polymers

Functional polymers with special / selected electronic properties

Organic field-effect transistor (OFET)

Polymerelektronikmade by semiconducting and dielectric polymers

Organic solar cellsPhotovoltaic & optoelectronic

devices made by polymer photoconductors

Polymer actorsfor microsystem technology

Optical polymers with optimized spectral

transparency

Special elektronic polymers for

Planar antennas

Organic light emitting diodes

(OLED) and lasers Super capacities

Polymer batteries

Polymer sensors

Fuel cells

Integrated polymerelectronic circuits (IPC)

Temperature sensors Humidity sensors

Chemo- & biosensors Pressure / force sensors

More conjugatedpolymers...

Organic Semiconductors

Conductivity of materials

Charge carrier mobilities

comparably small (FET) mobilities*:

*C. D. Dimitrakopoulos and D. J. Mascaro IBM J. Res. & Dev. 45 (1), 2001

low mobilities & large absorption coefficients thin absorber

Charge carrier mobilities of thin films

Absorption overlap with solar spectrum

comparably small absorption range:

only a small fraction of the sunlight is used today!

300 400 500 600 700 800 900 1000 1100 12000.0

0.5

1.0

1.5

2.0

2.5

3.0

0

1x1018

2x1018

3x1018

4x1018

5x1018

MDMO-PPV P3HT PCBM

abso

rbtio

n co

effic

ient

/ 10

5 cm-1

wavelength [nm]

AM

1.5

pho

ton

flux

/m-2s-1

nm-1

Sili

con

Solar Spectrum and Absorption

-bands in conjugated polymers

Molecular Energy Levels

HOMO

LUMO

Optical properties and transitions:Absorption Emission

absorptionfluorescencephosphorescence

luminescence (photo-, or electro-)

Molecular Energy Levels

For oligomers a shift of HOMO or LUMO levels towards smaller band-gaps is observed upon increasing the repeat unit. Compare with quantum-mechanical particle-in-a-box problem.

longer chain/wider box decrease in „band-gap“

CC

CC

CC

CC

H H H H

H H H

CC

CC

CC

CC

H H H H

H H H

Organic Semiconductors: Charges

PositiveSoliton:

NegativeSoliton:

Charge carriers on Polyacetylene:

(degenerategroundstate)

Charge transport in conjugated polymers

Polymer chain with characteristic defects (a);

schematic energy diagram for positive polarons without electric field (b) and

charge transport under applied electric field (c).

Electron transfer in a conjugated polymer after optical excitation calculated within the td-DFT by means of the GAUSSIAN quantum chemistry package

Charge carrier mobilities

comparably small (FET) mobilities*:


low mobilities & large absorption coefficients thin absorber

Organic Semiconductors: Processing

Solution processing Evaporation (polymers): (small molecules):

Molecular Materials: Pigments & Fullerenes

ZnPc

C6

0

Me-Ptcdi

„p-type“: e.g. phthalocyanines

„n-type“: perylenes & fullerenes

used for evaporation

Conjugated Polymers/ substituted Fullerenes

Doctor blading

*

*

O

O

n

*

*

O

O

NCn

S ** n

CN-MEH-PPV PCBM F8BT

MDMO-PPV P3HT PFB

„p-type“ conjugated polymers:

„n-type“ conjugated polymers/fullerenes:

Solution processing:

Spin coating

NN

**

n

*

NN

S

*n

MIM picture of device function

ITO

ITO

ITO

ITO

Al

Al

Al

Al

+ – – +

+ – VOC

(a) (b)

(c) (d)

a.) short circuit condition: solar cell

b.) open circuit condition (VOC): current = 0

c.) reverse bias: photodiode

d.) forward bias: light emitting diodeStrong (up to 105 V/cm) internal electric fields drive charge transport

Very thin (low mobility) absorbers of ~ 50 – 300 nm MIM

Organic Semiconductor Devices

Field effect transistor: bottom gate (usually Si/SiO2)

W. Fix, A. Ullmann, J. Ficker, and W. Clemens, Applied Physics Letters 81, 9, p. 1735 (2002)

Field effect transistor: top gate (polymeric insulator)

Organic Semiconductor Devices: OFET


Organic Semiconductor Devices: OFET

Influence of molecular order on charge carrier mobility:

Organic Field Effect Transistors: RF ID tags

OFET: Applications and Products

Organic light emitting diode (OLED):

Organic Semiconductor Devices: OLED

1.) charge injection2.) charge transport3.) charge recombination exciton formation4.) light emission

single layer device

OLED display structure:

electrode bars one pixel = three devices


http://us1.webpublications.com.au/static/images/articles/i306/30650_2mg.jpg

http://us1.webpublications.com.au/static/images/articles/i306/30650_8mg.jpg

Comparison: advantages of OLED versus LCD (STN/TFT)

• full color (24 bit)• high contrast (3000:1)• wide viewing angle (170°C)• lower power consumption• faster response time• less complicated architecture ( low cost, thinner device)


OLED: Applications and Products

Organic Light Emitting Displays:

(2005, molecule)(2002, polymer)

(2003)

(2004)



Wide viewing angle!



• active layer: conjugated polymer / fullerene blend

• selective contacts:

electrons: Al/LiF

holes: ITO/ PEDOT:PSS

(poly[3,4-(ethylenedioxy) thiophene] : poly(styrene sulfonate))

PEDOT:PSS

Active layer

LiF (6 Å)

Aluminum

ITO

Glass

Evaporation

Spin coating,doctor blading,printing

Etching, laser-etching

Organic Solar Cell: device structure (“plastic solar cell“):

Organic Semiconductor Devices: OSC

or flexibleplasticsubstrates

*N. S. Sariciftci, L. Smilowitz, A. J. Heeger, F. Wudl, Science 258, 1474 (1992)

PPV C60

EEva

c

HOMO

HOMO

LUMO

LUMO

Charge separated state!


Photoinduced Charge Transfer, dissociates the exciton:

The power conversion efficiency is a function of:

in

OCSC

P

VIFF

Where:

1.) VOC is the “open circuit voltage“2.) ISC is the “short circuit current“3.) FF is the “fill factor“


State of the art: ~ 5%

Organic (plastic) solar cell

Organic Solar Cells

OSC: Applications and Products

http://www.newscientist.com/article.ns?id=dn7264&feedId=online-news_rss20

OSC: Sensor applications

Photodiode (arrays) sensors

Combination of OLED illumination and photodiode array detection for personal identification.

Organic Laser (optically pumped):

From left to right: Organic laser emitting red light; structure of the first organic laser using external optical excitation; typical laser emission spectra spanning the visible

from the blue at a wavelength of 450nm, to the infrared at 700nm.

Organic Semiconductor Devices: Laser

Why “ORGANIC“? Disadvantages:


- light weight- flexible- low cost- large area- “tailor-made“ properties- more colors- printing production

- enviromental instability - requires encapsulation- lower performance (charge carrier mobility)

The challenge of today!

Summary

Optical, Electronic and Structural Properties of Semiconductor Nanostructures and Optoelectronic...

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