Polymer Light-Emitting Electrochemical Cells with Embedded Bipolar Electrodes

Polymer Light-EmittingElectrochemical Cells with EmbeddedBipolar Electrodes: Visualizing Bipolar

Electrochemistry in Solid State

by

Shulun Chen

A thesis submitted to the

Department of Physics, Engineering Physics and Astronomy

in conformity with the requirements for

the degree of Master of Applied Science

Queen’s University

Kingston, Ontario, Canada

October 2016

Copyright © Shulun Chen 2016

Abstract

When a floating, conductive object embedded in a medium containing a redox species is polar-

ized by an applied electrical field, redox reactions can occur at the extremities of the floating

object. This is the phenomenon of bipolar electrochemistry and the floating conductive object

is a bipolar electrode (BPE). Bipolar electrochemistry is of increasing interest in many fields

such as material science, analytical chemistry and microelectronics. However, this phenomenon

has not been demonstrated in a solid-state system until recently. In this thesis, we visualized

solid-state bipolar electrochemistry in a polymer light-emitting electrochemical cell (LEEC or

LEC) with embedded bipolar electrodes. LECs are solid state devices containing an active

layer of a luminescent conjugated polymer mixed with a polymer electrolyte. In a planar LEC,

a pair of driving electrodes are evaporated on top of the active layer at some distance. The

fabricated devices operate on in situ electrochemical doping of the active layer. Due to doping

propagation under continuous application of bias current/voltage through electrodes, formation

of p-n junctions eventually occurs and this leads to light emission from the junction region.

The work presented in this thesis examines the properties of BPEs of various configurations

and under different operating conditions in a large planar LEC system. Detailed analysis of

time-lapsed fluorescence images allows us to calculate the doping propagation speed from the

BPEs. By introducing a linear array of BPEs or dispersed ITO particles, multiple light-emitting

junctions or a bulk homojunction have been demonstrated.

In conclusion, it has been observed that both applied bias voltages and sizes of BPEs affected

the electrochemical doping from the BPE. If the applied bias voltage was initially not sufficiently

high enough, a delay in appearance of doping from the BPE would take place. Experiments of

parallel BPEs with different sizes (large, medium, small) demonstrate that the potential differ-

ence across the BPEs has played a vital role in doping initiation. Also, the p-doping propagation

distance from medium-sized BPE has displayed an exponential growth over the time-span of

70 seconds. Experiments with a linear array of BPEs with the same size demonstrate that the

doping propagation speed of each floating BPE was the same regardless of its position between

the driving electrodes. Probing experiments under high driving voltages further demonstrated

the potential of having a much more efficient light emission from an LEC with multiple BPEs.

i

Co-Authorship

Some experimental results in Chapter 4 have been published in Chem. Electro. Chem. on

October 12th, 2015.

ii

Acknowledgements

I would first like to thank my thesis advisor Dr. Jun Gao. The door to Prof. Gao’s office was

always open whenever I ran into a trouble spot or had a question about my research or writing.

He consistently allowed this thesis to be my own work, but steered me in the right direction

whenever he thought I needed it.

I would also like to thank my present group members Faleh and Sirius for sharing their knowl-

edge and helping me out whenever I needed in the lab. My thanks also go out to Loanne

Meldrum for making my graduate life so much easier!

Finally I would like to thank my family and friends for their everlasting encouragement and

support. I love you all.

iii

Contents

Abstract i

Co-Authorship ii

Acknowledgements iii

List of Tables vii

List of Figures viii

List of Abbreviations xv

Chapter 1: Introduction 1

1.1 Historical Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Polymer Semiconductors and Polymer Electrolytes . . . . . . . . . . . . . . . . . 3

1.2.1 Conjugated Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.2 Polymer Electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.2.3 Doping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.3 Bipolar Electrochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.3.1 Basic Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.3.2 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1.3.3 Potential Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1.4 Polymer Light Emitting Electrochemical Cells (LECs) . . . . . . . . . . . . . . . 20

1.4.1 Device Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

iv

1.4.2 Operational Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

1.4.3 Planar LECs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

1.4.4 Bulk Homo-junction LECs . . . . . . . . . . . . . . . . . . . . . . . . . . 27

1.5 Motivation and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

1.5.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

1.5.2 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Chapter 2: Experimentation 30

2.1 Device Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

2.1.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

2.1.2 Solution Making . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.1.3 Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.1.4 Film Casting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

2.1.5 Shadow Masks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

2.1.6 Electrode Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

2.2 Device Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2.2.1 Electrical Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2.2.2 Optical Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

2.2.3 Probe Station and Cryostat . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.2.4 Interferometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Chapter 3: Doping Propagation in a Large Planar LEC 40

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.2 Experimental Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.3 Results and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

v

Chapter 4: Planar LECs with Bipolar Electrodes of Various Sizes 44

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44



4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Chapter 5: Planar LECs with a Linear Array of Bipolar Electrodes 55

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55



5.3.1 Doping Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

5.3.2 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

5.3.3 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

5.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

Chapter 6: Probing Planar LECs with ITO Particle Bipolar Electrodes 69

6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69


6.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

6.3.1 Probing LEC Films without any ITO Particles . . . . . . . . . . . . . . . 70

6.3.2 Probing LEC Films with Dispersed ITO Particles . . . . . . . . . . . . . 72

6.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Chapter 7: Conclusion and Future Work 78

7.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

7.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

Bibliography 80

Appendices 88

vi

List of Tables

1.1 List of Common Conjugated Polymers . . . . . . . . . . . . . . . . . . . . . . . 4

1.2 Work Functions of Potential Electrode Materials . . . . . . . . . . . . . . . . . . 20

1.3 Comparison of PLED and LEC Characteristics . . . . . . . . . . . . . . . . . . . 22

6.1 Summary of Scratch Shapes and Corresponding Areas in Pixels. . . . . . . . . . 75

vii

List of Figures

1.1 Orbital hybridization of ethylene (C2H4) molecule. . . . . . . . . . . . . . . . . . 5

1.2 Formation of π band along the polymer backbone. Hatched regions represent

overlapping between adjacent π-bonds. Hydrogen atoms are omitted here. . . . 5

1.3 Sandwich set-up of a PLED device [23]. . . . . . . . . . . . . . . . . . . . . . . . 8

1.4 PLED variation of current density/luminescence intensity with applied voltages

[24]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.5 Comparison of ionic conductivity vs. temperature of some common electrolytes

[40]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.6 Representation of cation motions in a polymer electrolyte (a) assisted by polymer

chain motion only; and (b) taking account of the ionic cluster contributions [41]. 11

1.7 Cis (top) and trans (bottom) isomers of (CH)x . . . . . . . . . . . . . . . . . . 13

1.8 Conductivity of conjugated polymers at an increasing doping level from left to

right [47]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.9 Scheme of a spherical bipolar electrode in an electric field. . . . . . . . . . . . . 15

1.10 Left: Open bipolar electrode; Right: Closed bipolar electrode. . . . . . . . . . . 16

1.11 Left: Sandwich configuration; Right: Planar configuration (asymmetric) of LECs.

This figure is adapted from [55]. . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

1.12 Potential and field profiles of p-i-n junction model [58]. . . . . . . . . . . . . . . 23

1.13 A breakdown of operation steps of LEC devices in electrochemical doping model

[59]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

viii

1.14 Potential and field profiles of electrodynamic model [58]. . . . . . . . . . . . . . 24

1.15 Potential and field profiles of preferential p-type model [58]. . . . . . . . . . . . 25

1.16 Time-lapse photos of a green-emitting planar LEC with inter-electrode spacing

of 2 mm, (a) 0 min, no bias; (b) 1 min; (c) 10.5 min; (d) 12 min [70]. . . . . . . 26

1.17 Time-lapse photos of a MEH-PPV:PEO:Eu(CF3SO3)3 planar LEC with inter-

electrode spacing of 11 mm, (a) 0 min, no bias; (b) 1.5 min; (c) 2.5 min; (d) 4.0

min; (e) 7.5 min; (f) 6.5 min without UV [69]. . . . . . . . . . . . . . . . . . . . 26

1.18 Left: Red, green, and blue bulk homo-junction LECs incorporating ITO particles

into the film (a) 315 K, 600 V; (b) 320 K, 500 V; (c) 335 K, 700 V. Right: SEM

images of ITO particles in green-emitting bulk homo-junction LEC film [70]. . . 28

1.19 Left: Bulk homo-junction LEC incorporating a layer of gold into film (1000 V,

280 K). Right: AFM images of gold particles in the bulk homo-junction LEC

film [71]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.1 Left: Molecular structure of MEH-PPV. Right: The absorption and PL emission

spectra of MEH-PPV [75]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

2.2 Molecular structure of Lithium Triflate, Cesium Triflate, Potassium Triflate and

PEO. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.3 MBaun labmaster double-glovebox system. . . . . . . . . . . . . . . . . . . . . . 33

2.4 Shadow masks employed to manufacture LEC devices (a), (b), (c) and (d) shown

below in Figure 2.5 (not to scale). . . . . . . . . . . . . . . . . . . . . . . . . . . 34

2.5 Corresponding asymmetric LEC devices (yellow color-gold, silver color-aluminum,

red color-LEC film) made from shadow masks (a), (b), (c) and (d) shown above

in Figure 2.4 (not to scale). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

2.6 Control panel of the LabView program that controls Keithley units and records

measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

2.7 Time-lapse UV fluorescence imaging equipment setup. . . . . . . . . . . . . . . . 38

ix

2.8 Janis ST-500 probe station. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.9 Left: An LEC device mounted side the cryostat by the contact pins. Right:

Cryo-Industries of America custom ST-500 microscopy cryostat. . . . . . . . . . 39

3.1 Fluorescence image of the planar cell with the inter-electrode spacing labelled.

The top electrode is made of aluminum and bottom electrode is made of gold. . 41

3.2 The top row shows grayscale images of time-lapse photos of the LEC planar cell.

The bottom row shows the binary images with outlined boundaries obtained

from grayscale images. From left to right: 10 sec; 50 sec; 100 sec. The n-doping

propagated downwards and the p-doping propagated upwards. The n-doping

front position was measured relative to the top driving electrode and the p-

doping position was measured relative to the bottom driving electrode. . . . . . 42

3.3 p- and n-doping distances in a planar LEC cell as functions of time. Linear

trend-lines are given beside each set of data. 5% error was chosen for both p-

and n-doping distance data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.1 Top: Time-lapse fluorescence images of a planar LEC with gold electrodes. Bot-

tom: Cell current as a function of time since the application of the voltage bias.

Open circles overlaid on the curve indicate the time at which images (a)-(h) were

taken. The inset shows the cross-sectional view of the planar cell [79]. . . . . . . 47

4.2 Phases of current changes and each stage’s equivalent circuit model between

driving electrodes. Notice that current changes from mostly ionic in (a), to

partially ionic and partially electric in (b), to mostly electric in (c) and (d). . . . 48

4.3 Top: Time-lapse fluorescence grayscale images of a 2.12 mm planar cell consist-

ing of gold and aluminum driving electrodes, and a single aluminum disc BPE.

Bottom: Circuitry, voltages and geometries of the cell set-up. Device was tested

at 350 K [79]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

x

4.4 Top: Time-lapse fluorescence images of a 1.94 mm planar cell consisting of gold

and aluminum driving electrodes, and a single aluminum disc BPE. Bottom:

Equivalent circuit of the model cell with multiple light-emitting p-n junctions.

Device was tested at 350 K [79]. . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

4.5 Fluorescence image of a 10.19 mm planar cell with three aluminum disc BPEs of

different sizes. All the dimensions are labelled. Top driving electrode is gold and

bottom driving electrode is aluminum. The cell was tested at 350 K and 100 V

bias. The smaller windows have shown the magnified images of small aluminum

discs [79]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

4.6 Top: Regions of the 10.19 mm cell surrounding the center disc BPE ( = 0.26

mm) The fluorescence image at 50 s (not shown) was subtracted from all of the

fluorescence images shown. Bottom: A plot of the p-doping front position from

the BPE as a function of time since the 100 V bias was applied to the driving

electrodes [79]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

5.1 Fluorescence image of planar cell with an array of aluminum disc BPEs with the

same size. All dimensions are labelled. Top driving electrode is gold and bottom

driving electrode is aluminum. The device was tested at 100 V and 360 K. . . . 56

5.2 Left: From left to right pictures were taken at 15s, 35s, 105s and 370s after

doping started (exp1). Doping, formation of multiple p-n junctions and light-

emission could be clearly observed. Right: Another cell with similar geometry

showed strong light-emission from multiple p-n junctions formed (exp2). An

equivalent circuit model is given to its right. . . . . . . . . . . . . . . . . . . . . 57

5.3 Left: Cell current as a function of time after the application of bias voltage for

exp1. Right: Cell current as a function of time after the application of bias

voltage for exp2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

5.4 Top Left: Image taken under UV illumination. Top Right: Same image converted

to grayscale image. Bottom Left: Same grayscale image converted to a binary

image. Bottom Right: Boundary of the electrode derived from the binary image. 59

xi

5.5 Boundary changes of top BPE from exp1 in chronological order: (a) 0 sec; (b) 5

sec; (c) 10 sec; (d) 15 sec; (e) 20 sec. n-doping from BPE propagates upwards and

p-doping from BPE propagates downwards. Green curves outline the boundaries. 60

5.6 p-doping front positions measured from bottom edges of disc BPEs 5 seconds af-

ter doping started in exp1. Error bar value was obtained from standard deviation

calculation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

5.7 p-doping front positions measured from bottom edges of disc BPEs 10 seconds

after doping started in exp1. Error bar value was obtained from standard devi-

ation calculation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61







5.10 p-doping front positions measured from bottom edges of disc BPEs 5 seconds af-

ter doping started in exp2. Error bar value was obtained from standard deviation

calculation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63




5.12 p-doping front positions measured from bottom edges of disc BPEs as a function

of time since doping started (exp1). Linear trendline was also plotted for p-

doping distances from each disc BPE. . . . . . . . . . . . . . . . . . . . . . . . . 64

5.13 Simulation of the electric and potential fields of the BPE-array LEC device. The

color-map and contour plot illustrate the potential field, the vectors represent

the direction, flux density and strength (vector length) of the electric field. . . . 66

xii

5.14 Line potential profiles close to the disc BPE-array: blue line is through the

centers of disc BPEs, green line is slightly outside of all disc BPEs, red line is

much more outside of disc BPEs. A staircase-like profile could be clearly seen

from the graph. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

5.15 A zoom-in figure on line potential profiles close to one disc BPE: blue line is

through the center of this disc BPE, green line is slightly outside of this disc

BPE, red line is much more outside of this disc BPE. A staircase-like profile

could be clearly seen from the graph. . . . . . . . . . . . . . . . . . . . . . . . . 67

6.1 Top: Fluorescence time-lapse imaging of direct probing of the LEC film between

two deposited metal electrodes. Top Left: 0 sec; Top Right: 35 sec; Bottom Left:

75 sec; Bottom Right: 325 sec. Device was tested at 390 K with a bias of 10 V .

Bottom: The equivalent circuit of the device at 325 sec. Black blocks represent

the deposited electrodes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

6.2 Comparison of doping from plain LEC film probing under two sets of testing

conditions. Left: Probing was done at 380 K and 5 V . Right: Probing was done

at 340 K and 60 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

6.3 Fluorescence time-lapse photos of probing of ITO-LEC. From left to right: 5 sec;

30 sec; 175 sec; 320 sec. Two light-emitting junctions were formed, one between

p- and n-doped regions and another one around the cathode probe. Tested at

320 K and 300 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

6.4 Fluorescence time-lapse photos of probing of ITO-LEC. From left to right: 5 sec;

15 sec; 25 sec; 60 sec. Two light-emitting junctions were formed, one between p-

and n-doped regions and another one around the cathode probe. Tested at 320

K and 400 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

6.5 Left: Current between the probes as a function of time after the application of

300 V bias. Right: Current between the probes as a function of time after the

application of 400 V bias. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

xiii

6.6 Fluorescence time-lapse photos of probing of ITO-LEC with bottom probe ver-

tically bent. From left to right: 5 sec; 10 sec; 45 sec; 185 sec. Two light-emitting

junctions were formed, one between p- and n-doped regions and another one

around the cathode probe. The black shadow from the bottom right corner was

due to the blocking of the probe arm. Tested at 320 K and 500 V . . . . . . . . . 74

6.7 Left: Current between the probes as a function of time after the application of

400 V bias. Right: Current between the probes as a function of time after the

application of 500 V bias. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

6.8 Top: SEM image of the ITO-LEC film surface at one location. Bottom: SEM

image of the ITO-LEC film surface at another location. White objects in the

images are ITO macro-particles. The scale-bar and SEM scan details are listed

in the bottom left corners. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

xiv

Abbreviations

AFM Atomic Force Microscope

BPE Bipolar Electrode

CP Conjugated Polymer

EL Electroluminescence

OLED Organic Light-emitting Diode

PE Polymer Electrolyte

HOMO Highest Occupied Molecular Orbital

ITO Indium Tin Oxide

KTf Potassium Trifluoromethanesulfonate

LEC Light Emitting Electrochemical Cells

LED Light-emitting Diode

LUMO Lowest Unoccupied Molecular Orbital

MEH-PPV Poly[5-(2’-ethylhexyloxy)-2-methoxy-1,4-phenylene vinylene]

Mw Weight average molecular weight

PEO Poly(ethylene oxide)

PDI Polydispersity

PL Photoluminescence

PLED Polymer Light-emitting Diode

PPV Poly(p-phenylene vinylene)

RPM Revolutions Per Minute

SEM Scanning Electron Microscope

xv

Chapter 1

Introduction

1.1 Historical Background

Electroluminescence (EL), the generation of light other than blackbody radiation, was first

observed in silicon carbide (SiC) by Henry Joseph Round in 1907 [1]. He reported that a

current passing through the silicon carbide detector was accompanied by some yellow light. In

1923, Lossev of the Nijni-Novgorod Radio Laboratory in Russia again reported the observation

of yellow light in silicon carbide crystal [2][3]. In 1936, the observation of EL from zinc sulfide

(ZnS) under an alternating current was recorded by Georges Destriau in the laboratories of

Madame Marie Curie in Paris [4]. In 1960, Martin Pope and his co-workers at New York

University first reported EL from an organic single crystal [7] and from an anthracene crystal

doped with tetracene under direct current [8]. They explained the phenomenon as a result of the

recombination of injected electrons and holes [9]. Meanwhile, Helfrich and Schneider achieved

higher injection currents using a pair of liquid injection electrodes [10]. Other researchers such

as Hartman and Armstrong found that light could also be emitted out of polymer (polyethylene,

Nylon 6 and 11, Mylar, Teflon and cellulose acetate) films under a very high electric field [11].

Early organic EL required an operating voltage of a few hundred volts. In 1983 Roger Partridge

demonstrated EL from a device consisted of a poly(N-vinylcarbazole) (PVCz) thin film with a

thickness of 2.2 µm at the National Physical Laboratory in UK [12][13][14][15]. In this direct

current electroluminescent device, the aforementioned PVCz films were placed between an

1

electron-injecting cesium cathode and a hole-injecting anode. A high current density of about

1000 A/m2 was achieved at low voltages. This marked the first functional prototype that could

potentially be commercialized. In fact, not before long, in 1987 Tang and Van Slyke at Kodak

reported the first organic light-emitting diode (OLED) [16]. This device incorporated a two-

layer structure with separate electron and hole transporting layers to enhance the recombination

efficiency and reduce the operating bias voltage. As a result, an external quantum efficiency

(EQE: photons emitted per electron injected) of 1 %, a luminous efficiency of 1.5 lm/W and

brightness larger than 1000 cd/m2 was achieved at bias voltages below 10 V .

Organic light-emitting devices based on the vacuum sublimation of organic thin films possess

decent efficiencies and good emission colors. However, there are still problems associated with

the long-term stability of the organic layer against recrystallization and the high processing

cost. In 1990, Burroughes and others introduced polymer light emitting diodes (PLEDs) based

on amorphous semiconducting luminescent conjugated polymers [17] that are compatible with

solution processing techniques. Typically conjugated polymers are good charge transporters

and of relatively high quantum efficiency [18]. They perform particularly well as conducting

materials when chemically doped. Out of many conjugated polymers, poly(p-phenylene viny-

lene) or PPV (PPhV) can be conveniently made into high-quality films that display strong

photo-luminescence. A typical PLED has a sandwich configuration, with a spin-cast polymer

film sandwiched between an electron injecting metal cathode and a transparent hole-injecting

anode. To maximize EL, electron- and hole-injection need to be efficient and balanced. To

accomplish this, the polymer layer must be ultrathin, and the metal electrode work functions

must align with the lowest unoccupied molecular orbit (LUMO) and highest occupied molecular

orbit (HOMO) of the conjugated polymer.

An alternative approach to achieving EL from a luminescent conjugated polymer was demon-

strated by Pei et al. in 1995 [19]. In a polymer light-emitting electrochemical cell (LEC or

LEEC), the active layer is a mixture of a conjugated luminescent polymer, an ion transport

polymer, and salt. The latter two components of LECs constitute a polymer electrolyte that

contains mobile ions. When a sufficiently large voltage bias is applied to an LEC, the conju-

gated polymer is electrochemically p-and n-doped in situ, and the propagating doping fronts

2

eventually meet to form a light-emitting p-n junction. Electrochemical doping of the conju-

gated polymer renders the polymer film highly conductive. As a result, LECs can be made

in a planar configuration with a large separation between the driving electrodes. In addition,

electrochemical doping creates bandgap states that quench the photoluminescence (PL) of the

polymer film. Therefore, it is possible to visualize the doping propagation process in an LEC

via the time-lapse fluorescence imaging of extremely large planar cells. This thesis exploits

the versatility of extremely large planar LECs to investigate how embedded bipolar electrodes

affects doping patterns and light emission. In recent years, many studies have also been con-

ducted on LECs based on ionic transition metal complexes to study device behaviors [21][22].

Nonetheless, the primary focus of this study is on polymer LECs. The extremely large planar

LEC with embedded BPEs provides a convenient solid-state platform to visualize and study

the phenomenon of bipolar electrochemistry.

1.2 Polymer Semiconductors and Polymer Electrolytes

1.2.1 Conjugated Polymers

A majority of polymers in everyday use, such as polyethylene, polystyrene or poly(ethylene

terephthalate) are colorless insulators. However, there is a special class of polymers that pos-

sess vastly different electrical and optical properties. The so-called conjugated polymers have

alternating single and double bonds in the main chain that allow them to act as semiconductors

or conductors and emit light.

Table 1.1 lists some of the common conjugated polymers along with their structures [23].

3

Table 1.1: List of Common Conjugated Polymers

Structurally, all these polymers have alternating single and double bonds, either in a linear

chain or inside an aromatic ring. The bonding here could be understood with the concept

of orbital hybridization. Figure 1.1 shows the bonding of an ethylene molecule. In it, each

carbon atom is in a sp2pz configuration. Out of the three sp2 orbitals each carbon atom has

one sp orbital that will overlap with the sp orbital from an adjacent carbon atom to form a

C-C σ-bond. The other two sp orbitals will form C-H σ-bond with the s orbitals of hydrogen

atoms. The p orbitals above and below the plane of σ bonds will overlap with the p orbitals of

neighboring carbon atoms to form π-bonds parallel to the same plane. This bond formation is

illustrated in Figure 1.2 below:

4

Figure 1.1: Orbital hybridization of ethylene (C2H4) molecule.

The electrons inside the π-bonds are called π electrons. These electrons are more prone to

delocalization. Along the backbone of the polymer, the overlapping of π bonds will allow the

formation of a band that is composed of delocalized electrons. This band allows the polymer

to become conductive by facilitating charge transport. Therefore metallic behavior is expected

from a long polymer chain with overlapping π bonds as shown in Figure 1.3.

Figure 1.2: Formation of π band along the polymer backbone. Hatched regions represent overlappingbetween adjacent π-bonds. Hydrogen atoms are omitted here.

However, due to Peierls’ theorem [25], which states that: “a one-dimensional equally spaced

chain with one electron per ion is unstable”, dimerization will always occur and lead to the

5

formation of alternating short double bonds and long single bonds between neighboring CH

groups. This process lowers the energy of the entire system thus making it more energetically

favorable. It also creates energy band gaps between π and π∗ bands, rendering a conjugated

polymer semi-conducting. As an analogy to an inorganic semi-conductor, in a conjugated

polymer system the HOMO (highest occupied molecular orbital) or π band resembles a valence

band, the LUMO (lowest unoccupied molecular orbit) or π∗ band resembles a conduction band.

The band gap energies are generally between 1 eV to 4 eV, corresponding to photon energies

of the entire visible spectrum [26].

Conjugated polymers are not only semi-conducting but also semi-crystalline, which means that

they are composed of crystalline and amorphous regions. The crystalline region often has a

high number of imperfections that are from torsional disorders, chemical, and conformational

defects [27][28][29]. Therefore, charges are transported incoherently and experience a diffusive

“hopping”[23] that is characterized by “phonon-assisted quantum mechanical tunneling”[30]

between long range conducting regions. This hopping process is highly dependent on tempera-

ture. Therefore, as a result, an increase in ambient temperature will improve the polymer film

conductivity significantly.

Conjugated polymers also have some optical properties that allow them to absorb and emit

light. Under an external bias, charges are injected into the polymer and transported along

the backbone by hopping. The Coulomb interaction between opposite charges will help form

excitons (coupled electron-hole pairs) and those excitons will also travel by hopping in random

motion due to their charge neutrality [23]. After some time those excitons will spontaneously

annihilate to emit photons or heat depending on the types of excitons. There are two types of

excitons, triplet and singlet excitons, formed during the electron-hole coupling process [31]:

|1, 1〉 = ↑↑

|1, 0〉 = (↑↓ + ↓↑)/√

2

|1,−1〉 = ↓↓

s = 1 (triplet)

|0, 0〉 = (↑↓ − ↓↑)/√

2 s = 0 (singlet)

6

Triplet excitons all have three independent states that correspond to quantum number Sz = 1, 0,

-1 for spins in z-direction while singlet excitons only have one state. This leads to a probability

ratio of 3:1 between triplet and singlet excitons that will recombine radiatively according to

Langevin model [34]. Due to the harvest of both triplet and singlet excitons, highly efficient

LECs [32] and LEDs [33] have been made. Moreover, the color of light emission depends on the

exciton energy, which is closely related to conjugation length and chemical composition of the

polymer. The exciton energy could be influenced by changing the conformation of the polymer,

which alters the conjugation length, or direct chemical substitution [24]. This allows chemists

to design and synthesize different polymers to emit desired colors.

Shining photons with energy larger than Eg (band gap energy) upon the luminescent polymer

will result in electron excitation due to the absorption of photon energy. Electrons are promoted

from valence π band to conduction π∗ band. After some time the excited electrons relax and

fall back down to π band with an emission of photons. As a result, the outgoing photons have

less energy and a longer wavelength compared to incoming photons thus making the emission

red-shifted. This process is called Stokes Shift [35]. During this photon-excited process, the

excitons formed are entirely singlet, thus making the subsequent light emission more efficient.

Therefore, photoluminescence (PL) is generally more efficient than electroluminescence (EL).

However, due to the presence of quenching sites where excitons can migrate to, as well as

interchain interactions, excitons could be demoted to lower energy states, therefore, become

less likely to decay radiatively to emit photons [36]. This phenomenon, named PL quenching,

can reduce the overall luminescent quantum yield [23].

Two primary methods used to synthesize luminescent conjugated polymers are through elec-

trochemical and chemical polymerization means [37]. Even though the electrochemical mean is

a more facile approach but due to its limited amount of yield, chemical synthesis appears more

desirable and is utilized in most cases instead.

An example light-emitting device is illustrated in the picture below:

7

Figure 1.3: Sandwich set-up of a PLED device [23].

A thin film of poly-(phenylene vinylene) (PPV) is sandwiched between a Calcium cathode and

an ITO anode. The electrodes are then connected to an external circuit where a driving voltage

was supplied. Under this setting from Figure 1.3, when the voltage rises above 2 V, both current

and luminescence start to grow exponentially as shown in Figure 1.4. The PPV film starts to

emit light at the onset of current increase as well. This light emission effect, as mentioned in

the previous section, is known as electroluminescence (EL).

Figure 1.4: PLED variation of current density/luminescence intensity with applied voltages [24].

8

1.2.2 Polymer Electrolytes

A polymer electrolyte is a solid-state ionic conductor typically comprised of an ion-conducting

polymer solvent and salt. Ion conductivity in polymer electrolytes (PEs) is governed by the

movement of ions between coordinate sites induced by local movement of polymer host’s chain

segments, in other words, segmental motion in the vicinity of ions. In such systems, it is

commonly believed that it is more energetically favorably that cations form coordinate bonds

to the polymer backbone while the anions diffuse freely [38]. One of the most widely used

polymer solvents for polymer electrolyte is polyethylene oxide (PEO). PEO is a linear polymer

with a structure of -(CH2CH2O)n-. This repeating oxygen group in PEO is a strong electron

donor, and the distance between neighboring oxygen groups is ideal for ion transportation.

By comparison, both -(CH2O)n- and -(CH2CH2CH2O)n- show lower ion conductivity [38].

PEO is capable of strongly coordinating cations and possesses strong coordinating groups to

dissolve salt easily. However, there are also some issues associated with high molecular weight

PEO-based polymer electrolytes:

• Poor ion conductivity in crystalline state

• Loss of mechanical stability in amorphous state

Therefore, switching between crystalline and amorphous states is more or less a tradeoff here.

Due to the need of a higher ion conductivity in the active layer of an LEC, the amorphous state

of a polymer electrolyte is thus preferred. Ion transportation is closely related to the flexibility

of the polymer host’s backbone, which is highly dependent on temperature as previously men-

tioned. Below 60 oC, crystalline sections are still present in the polymer electrolyte, and they

reduce the overall conductivity. When the ambient temperature rises above Tg (glass transi-

tion temperature) of PEO, it becomes flexible and conductive. This conductivity-temperature

relationship could be modeled by Vogel-Fulcher-Tamman (VFT) formulation [39]:

σ(T ) = σ0eEA

R(T−T0) (1.1)

9

σ0 −→ conductivity at temperature T0

EA −→ activation energy

R −→ gas constant

This equation works well with electrolytes above glass transition temperature (T > Tg), how-

ever, in the interval T0 < T < Tg, where T0 is the temperature at which configurational entropy

vanishes, a modified VFT formulation is required [39]. This temperature dependent ion con-

ductivity was exploited in frozen-junction LECs.

Figure 1.5 below illustrates the conductivity-temperature dependence of some common elec-

trolytes used in researches and industrial applications:

Figure 1.5: Comparison of ionic conductivity vs. temperature of some common electrolytes [40].

Polymer solvent, with the addition of salt, forms a polymer electrolyte complex. The best

candidate salts have polyatomic anions with monovalent charge due to weak anion solvation.

For example, large, polarizable, and monovalent anions such as ClO−4 , CF3SO

−3 , (CF3SO2)2N

−,

and (CF3SO2)3C−, together with cations such as Na+, Cs+, Li+, and K+, make the best salts

10

that are highly soluble in PEO.

The mechanism for ion transport suggested by Gray, include two major contributions: (a)

Polymer chain motion; (b) Ionic clustering, is succinctly summarized in Figure 1.6 below:

Figure 1.6: Representation of cation motions in a polymer electrolyte (a) assisted by polymer chainmotion only; and (b) taking account of the ionic cluster contributions [41].

Ion movements are accomplished through polymer segmental motions via making and breaking

of the co-ordination bonds between cations and polymers according to configurational entropy

model. However, the free volume-based model suggests that ion movements occur as a result of

redistribution of free volume within the system under external bias fields. It is also suggested

11

that this mechanism largely depends on the concentration of salt in polymer electrolytes [41][42].

In fact, in low molecular weight polymers, ions are capable of moving together with their

coordinated solvents, but it is not the case when they are in high molecular weight polymers.

For high molecular weight polymer hosts, chain diffusion is relatively small and makes little

contribution to ion transport. In this case the ion transport is heavily dependent on segmental

motions of the polymer host. This entire process is theorized by dynamic bond percolation

theory (DBP) [43] which states that: in microscopic level conductivity of the polymer electrolyte

complex is due to the combination of ion/polymer cooperative motion with the occasional

independent ion movement.

1.2.3 Doping

Doping of conjugated polymers is a reversible process that interchanges the redox state of a

polymer without causing any degradation [44]. For polymers to become electrically conductive,

they need to possess both charge carriers and conduction bands formed by overlapping orbitals.

Since most organic polymers do not have intrinsic charge carriers, the required carriers could

be provided by partial oxidation (p-doping) of polymer with electron acceptors (e:g: I2, AsF5,

AgClO4) or by partial reduction (n-doping) of polymer with electron donors (e:g: Na, K)

[45][46]. With the help of those chemical reactants, more electrons are pushed into the con-

duction band of conjugated polymers. This enhanced electron transportation from intra-chain,

inter-chain and inter-domain contributes to an increased bulk conductivity of the polymers.

Doping could be introduced both chemically and electrochemically. In chemical doping, both

cis- and trans-(CH)x shown in Figure 1.7 below can be partially oxidized (p-doped) or reduced

(n-doped) by adding oxidizing or reducing agents [46].

12

Figure 1.7: Cis (top) and trans (bottom) isomers of (CH)x

The doping in conjugated polymers is different from the doping in traditional silicon semi-

conductors. For doped conjugated polymers, p-doped (CH)x consists of a delocalized polycar-

bonium ion [(CH)y+A−y ]x and a stable counter-anion A−, making the material [(CH)y+A−

y ]x

electrically neutral. n-doped (CH)x consists of a delocalized polycarbanion [(CH)y−]x and a

stable counter-cation M+, making the material [M+y (CH)y−]x electrically neutral.

Conjugated polymers could also be easily electrochemically doped due to their extensive conju-

gation of π electrons. In this case conjugated polymers act as an electron source or sink under an

external bias voltage/current. An example of electrochemical doping will be given in the follow-

ing. This reaction can be accomplished by applying a DC bias between a trans-polyacetylene-

coated positive electrode and a negative electrode (reaction 1) or vise versa (reaction 2). Those

two electrodes are immersed in a solution of LiClO4 in propylene carbonate:

trans− (CH)x + (xy)(ClO4) −→ [CH+y (ClO4)

−y ]x + (xy)e− (reaction 1)

trans− (CH)x + (xy)Li+ + (xy)e− −→ [Li+y (CH)y−]x (reaction 2)

Compared with chemical doping, electrochemical doping enjoys a couple of distinct advantages.

First of all, the doping level could be precisely controlled by monitoring the current level. Sec-

ondly, doping could be reversed without the need of removing any chemical products. Finally,

both p- and n-doping could be achieved with dopants that could not be easily introduced by

13

chemical doping. The drastic increase in conductivity by doping is illustrated in the following

figure:

Figure 1.8: Conductivity of conjugated polymers at an increasing doping level from left to right [47].

By introducing the new charge carriers into the polymer, more energy levels between π-π∗

bands are formed. This effect will lead to smaller energy transitions inside energy band gap

and therefore photoluminescence quenching of the film [48].

1.3 Bipolar Electrochemistry

1.3.1 Basic Principles

Bipolar electrochemistry is an electrochemical phenomenon where a polarized conducting object

inside an electric field will induce redox reactions at its extremities if the potential difference

generated is large enough. The potential difference here between extremities is equal to the

electric field value multiplied by the size of the conducting object. The concepts here are

illustrated in the basic setup shown in Figure 1.9 below:

14

Figure 1.9: Scheme of a spherical bipolar electrode in an electric field.

Considering this spherical electrode immersed in solution between two driving electrodes, the

current passing through the solution is is1. In the middle, a fraction of the current will flow

through the solution by migration of charged ions, while the rest of the current will flow through

the spherical electrode by electronic conduction. Therefore:

ibe = is1 − is2 (1.2)

Thus the ratio ibe/is2 is correlated with the respective resistance of spherical electrode Rbe and

solution Rs. A highly conductive electrode will result in a large ibe and small is2. Since Va and

Vc are potentials of the corresponding driving electrodes, the value of the imposed electric field

could be expressed as:

E = (Va − Vc)/d (1.3)

where d is the distance between the driving electrodes and the potential drop is assumed to

change linearly throughout the solution. If the length of the conducting electrode across its

extremities is L, then the value of its potential difference ∆V is given by:

∆V = ηa − ηc = E · L (1.4)

15

If ∆V is large enough, redox reactions will occur at electrode extremities. The oxidation

reaction (p-doping for conjugated polymers) will occur at the δ− side and the reduction reaction

(n-doping for conjugated polymers) will occur at the δ+ side.

Bipolar electrochemistry allows different reactions to happen simultaneously at different sites

of the conducting object in the middle. The reactions appear in a wireless fashion and create a

gradient of reactants. This intrinsic dissymmetric reactivity allows modifications with a spacial

selectivity [49].

There are two major configurations of bipolar electrodes, open and closed, as shown in the

figure below:

Figure 1.10: Left: Open bipolar electrode; Right: Closed bipolar electrode.

In the open configuration the device has both currents is2 and ibe. In comparison, the device

in closed configuration only has ibe instead. In our experiments, we mainly studied devices in

an open configuration.

16

1.3.2 Examples

Redox reactions take place at the extremities of the bipolar electrode. In redox process, the

reductant transfers electrons to the oxidant. This process could be broken down into two half

reactions: oxidation and reduction. In oxidation, the substances lose electrons and in reduction

the substances gain electrons. Many redox reactions occur in aqueous solutions or suspensions.

There are many examples of redox reaction in both inorganic and organic chemistry. For

example, in inorganic chemistry:

ClO−3 + 3H2O + 3SO2 −→ 3SO2−

4 + Cl− + 6H+

which could be split into two half reactions:

2H2O + SO2 −→ SO2−4 + 4H+ + 2e− (oxidation)

6e− + 6H+ + ClO−3 −→ Cl− + 3H2O (reduction)

Similarly, in organic chemistry:

3CH3CH2OH + 2Cr2O2−7 + 16H+ −→ 3CH3COOH + 4Cr3+ + 11H2O

which could also be broken down into two half reactions:

CH3CH2OH +H2O −→ CH3COOH + 4H+ + 4e− (oxidation)

Cr2O2−7 + 14H+ + 6e− −→ 2Cr3+ + 7H2O (reduction)

In a solid state bipolar electrochemical system, the bipolar electrode naturally splits the redox

reaction into two half-reactions, one at each extremity. For a p-doping process, the oxidation

17

reaction, its general formula could be written as:

P +X− P+X− + e−

or (P + h+ + A+ P+A−)

P −→ undoped polymer

X− −→ anion from dissolved salt M+X−

P+ −→ a segment of p− doped polymer

A− −→ anion

e− −→ electron

h+ −→ hole

Similarly, for an n-doping process, the reduction reaction, its general formula could be written

as:

P +M+ + e− M+P−

or (P + e− + C+ P−C+)

M+ −→ cation from dissolved salt M+X−

P− −→ a segment of n− doped polymer

C+ −→ cation

1.3.3 Potential Applications

There are many applications that utilize bipolar electrochemistry. Its primary usage could be

classified into the following categories [49]:

• Formation of solid-state gradients

• Formation of molecular gradients

• Electronic device conception

18

The first application comes out of the potential variation from the anode to the cathode in

bipolar electrode devices. For redox reactions at the bipolar electrode, oxidation will occur

from the point where the anodic overpotential ηa is sufficient. Likewise, reduction will occur

from the point where the cathodic overpotential ηc is sufficient. Ramakrishnan and Shannon

immersed a gold wire in a solution containing Cd2+ and S2O2−4 ions [50]. The bipolar electrode

here was polarized using a bias electric field of 425 V/m. The deposition of Cd, CdS, and S took

place at different locations on the wire surface. A Cd layer was deposited close to the cathodic

pole, followed by a CdS layer while moving along the wire surface. Deposition finished with a

S layer right beside the anodic pole. Their work demonstrated that bipolar electrochemistry

could be used to create high value-added objects with solid state material gradients.

The second application comes in handy for biomimetic applications and biosensors. Ulrich et

al. studied the formation of potential gradient at the bipolar electrode surface and utilized this

phenomenon to create self-assembled monolayer gradients that can be post-functionalized with

protein [51]. In fact, it was shown that bipolar electrochemical effects generated molecular

gradients of different shapes and lengths to create enzyme-modified surfaces that could be

applied in biosensing applications.

Bradley and co-workers’ work highlighted the third potential application of bipolar electro-

chemistry [52]. Their work demonstrated the formation of electrical contact between particles.

They employed bipolar electrochemistry to make contacts instead of the traditional industrial

process such as photolithography. This provides an exciting alternative for manufacturing of

small-scale electronic devices in the future.

19

1.4 Polymer Light Emitting Electrochemical Cells (LECs)

1.4.1 Device Characteristics

LECs and PLEDs both feature luminescent conjugated polymers as the emitting material, but

they have very different operating mechanisms. PLEDs are composed of a conjugated polymer

sandwiched between two electrodes with at least one electrode being transparent as shown in

Figure 1.1. Poly(p-phenylene vinylene) (PPV) and its derivatives are common PLED materials

due to their high PL yield and solution processibility. The electrodes are chosen based on

their work functions. The electrode work function needs to match the energy levels of the

luminescent polymer to facilitate charge injection. For anodes, materials with a high work

function, such as indium tin oxide (ITO), are ideal for hole injection. On the other hand,

for the cathode, materials with a low work function, such as calcium, are ideal for electron

injection. The following table lists out the work functions of potential electrode materials [53]:

Table 1.2: Work Functions of Potential Electrode Materials

Material Work Function (eV )

Ag (silver) 4.26-4.74

Al (aluminum) 4.06-4.41

Au (gold) 5.1-5.47

Ca (calcium) 2.87-3.00

Cs (cesium) 2.14

Cu (copper) 4.65-4.70

In (indium) 4.12-4.20

ITO (indium tin oxide) 4.7

Li (lithium) 2.9

Pb (lead) 4.25

Sn (tin) 4.42

20

PLEDs have been demonstrated with an operating lifetime of more than 10,000 hours and

storage lifetime of more than five years [54]. These figures meet the requirements for display

applications. Despite all the benefits, PLEDs possess some drawbacks as well. These include

low work function electrodes being sensitive to humidity and oxidation; different lifetimes of

emitting polymers leading to color shift; short lifetime at high luminescence; ultra-thin films

being prone to pinholes.

Polymer light-emitting electrochemical cells, introduced by Heeger and colleagues in 1995, have

many unique and attractive device characteristics compared to PLEDs. LECs have an active

layer that is a blend of salt, ion-solvating/transport, and conjugated polymers. In the ex-

periments, the luminescent conjugated polymer used was poly[5-(2’-ethylhexyloxy)-2-methoxy-

1,4-phenylene vinylene] (MEH-PPV), the salt used was potassium trifluoromethanesulfonate

(KTf) and the ion-solvating polymer was poly(ethylene oxide) PEO. The active layer of those

simple single-layer LEC devices can be easily prepared by a solution-based method such as

spin-coating. LECs could be manufactured into two configurations, sandwich and planar, as

shown below in Figure 1.11:

Al Cathode

ITO Anode

EL Substrate

EL

EL

Al Au

Substrate

Cathode Anode

Figure 1.11: Left: Sandwich configuration; Right: Planar configuration (asymmetric) of LECs. Thisfigure is adapted from [55].

In sandwich configuration, the active layer blend is spin-coated on top of the substrate covered

with patterned ITO anode. The top cathode is then evaporated onto the polymer film through

a shadow mask. The light is emitted from the active material through ITO layer and substrate.

21

In planar configuration, the substrate is glass or sapphire without the ITO coating. Both

cathode and anode are thermally evaporated on the top surface of the polymer layer. In a

planar cell, EL can be observed either directly from above or from the substrate.

A table summarizing the comparison between PLEDs and LECs is presented below (adapted

from [56]):

Table 1.3: Comparison of PLED and LEC Characteristics

Property PLED LEC

Active layer Conjugated polymer Blend of conjugated polymer and electrolyte

Thickness of active layer ultrathin Not sensitive

Anode High work function Not sensitive

Cathode Low work function Not sensitive

Quantum efficiency Depends on injection barrier and active layer thickness High

Operating voltage Depends on polymer thickness Very low

Power efficiency Low to moderate High

Response speed High Low

Fabrication process Complicated Simple

Light-emitting region Wide Narrow

Lifetimes >10,000 hrs 1,000 hrs

1.4.2 Operational Principles

There had been two competing models of LEC operating mechanism. They are presented and

compared below:

1. Electrochemical Doping Model (EDL) [19]:

When an LEC is at an open circuit condition, the solvated ions are randomly distributed

throughout the active layer. Ions start to redistribute themselves when an external bias is

applied. Ionic and electronic charges start to build up at the polymer and electrode interfaces

[57]. This subsequent “electric double layer”, as shown in Figure 1.13, produces strong electric

fields at the electrode/active material interfaces that allow electronic charges to tunnel through

into the active material. As soon as the applied bias voltage is equal to or higher than Eg/e, with

Eg being the polymer energy bandgap and e being the elementary charge, charges are injected

into the active material. Mobile ions are also compensated by the injected electronic charges

22

as well to allow the electrochemical doping of the conjugated polymer. As charge injection

continues, p- and n-doped regions grow along the direction of current flow. They eventually

meet to form a p-n junction where excess charges recombine radiatively to emit light.

Figure 1.12: Potential and field profiles of p-i-n junction model [58].

The depletion region of the p-n junction accounts for most of the potential drop when the LEC

is fully turned on. The entire LEC operating process is illustrated in Figure 1.13:

Figure 1.13: A breakdown of operation steps of LEC devices in electrochemical doping model [59].

Due to doping and increased conductivity in doped regions, LECs could be turned on at rela-

tively low voltages that are equivalent to Eg/e. For example, as indicated by the band diagrams,

as applied voltage to LEC increases, the electrochemical doping of MEH-PPV first occurs when

23

voltage ≥ 2.1 V [60]. LECs can also operate efficiently under both forward and reverse bias

[61]. This bipolar behavior is significantly different from the diode-like PLEDs.

Since the electrochemical doping of the device is dynamic, removing the applied bias voltage

and leaving the device under a short-circuit condition will lead to dedoping of the LEC film.

Similarly, leaving the device under an open circuit condition will result in a decaying voltage

[62]. Doping concentration could then be calculated by measuring the amount of dedoping

charge.

2. Electrodynamic Model (ED) [63]:

The electrodynamic model does not invoke doping of the polymer layer. The improved and

balanced charge injection are instead attributed to an “ionic space charge” effect [64]. A major

difference between the electrochemical model and the electrodynamic model is their description

of electric field distribution in an LEC under bias. In the former, the electric field is the strongest

in the junction depletion region. In the electrodynamic model, the electric field is screened by

the accumulated ionic charges at the electrode interfaces. The potential and field profiles of

the electrodynamic model are shown below.

Figure 1.14: Potential and field profiles of electrodynamic model [58].

3. Preferential p-type Model [65]:

Finally, one more model is proposed for describing devices with dominating p-doping. In this

model, the bulk of the device undergoes a reversible p-doing compensated by anions while the

n-doping is confined to a small area close to the cathode. Since the conductivity of a p-doped

region is higher than the conductivity of an n-doped region, potential mostly drops across the

n-doped region. Due to little drop of potential across bulk of the device, it appears to be free

of any electric field as shown below:

24

Figure 1.15: Potential and field profiles of preferential p-type model [58].

Extensive experimental observations, including the ones presented in this thesis, overwhelmingly

support the electrochemical doping model.

1.4.3 Planar LECs

In a polymer LEC, high doping level will lead to the formation of ohmic contacts at elec-

trode/active material interfaces. Also, due to the increased conductivity of the doped regions,

polymer LECs are much less sensitive to film thickness than PLEDs. This property allows

large planar polymer LEC devices to be made. This architecture allows researchers to directly

observe the active layer during device operation. The original planar polymer LECs only had

an inter-electrode spacing of 15 µm and required a microscope to be imaged. Our group has

manufactured and imaged devices with extremely large, millimeter-size inter-electrode spacing

in 2003 [66]. Later a large planar light-emitting electrochemical cell with an inter-electrode

spacing of 11 mm was demonstrated [67]. To turn on the polymer LECs in a timely manner,

a high enough voltage bias needs to be applied [68]. By employing time-lapse fluorescence

imaging technique, visualization of the dynamic doping process became possible [69]. Figure

1.16 displays time-lapse photos of a green-emitting planar LEC with an inter-electrode spacing

of 2 mm:

25

Figure 1.16: Time-lapse photos of a green-emitting planar LEC with inter-electrode spacing of 2 mm,(a) 0 min, no bias; (b) 1 min; (c) 10.5 min; (d) 12 min [70].

Before any voltage bias was applied, the LEC film itself has already shown a strong and uniform

green PL under UV illumination. 60 seconds after the application of 400 V , film experienced

PL quenching due to in situ electrochemical doping. Eventually, p- and n-doping fronts met to

form a p-n junction and emitted light. Larger inter-electrode spacing was obtained on MEH-

PPV based LEC device under a bias voltage of 800 V :

Figure 1.17: Time-lapse photos of a MEH-PPV:PEO:Eu(CF3SO3)3 planar LEC with inter-electrodespacing of 11 mm, (a) 0 min, no bias; (b) 1.5 min; (c) 2.5 min; (d) 4.0 min; (e) 7.5 min; (f) 6.5 minwithout UV [69].

26

1.4.4 Bulk Homo-junction LECs

Such extremely large devices mentioned above are made possible due to the increased con-

ductivity of the doped regions. However, doping also introduces heavy PL quenching to the

film and only a single narrow light-emitting region is formed between two doped regions. This

region is only a fraction of the entire inter-electrode spacing [71]. In other words, the width of

the EL emission zone is small compared to the inter-electrode spacing, which renders the device

inefficient in most cases. Also, it is discovered by Tracy that the EL emission zone increases

with decreasing inter-electrode spacing [70]. For example, RE (EL emission zone width/inter-

electrode spacing) is less than 1% for planar LEC devices with inter-electrode spacing of 6 and

11 mm, and around 5% for planar devices with inter-electrode spacing of 1.5 mm. RE appears

to be the highest in sandwich LEC devices, ranging from 10% to 80% determined by impedance

measurements [72].

To improve the device efficiency, micrometer-scale conducting particles, such as Au, Ag and

ITO particles, are introduced into the film of millimeter-scale planar LEC devices [73]. Upon

the application of a large bias voltage, thousands of tiny light-emitting p-n junctions instead

of one single narrow light-emitting p-n junction, are formed in the bulk of the device. Those

junctions significantly improve the device efficiency due to a much larger emission area.

There are two ways of introducing multiple small p-n junctions to fabricate bulk homo-junction

devices. The first method, described in Tracy’s work, was via the direct mixture of conducting

particles into the LEC solutions [70]. The mixture solution was then spin-coated onto the

substrate. After drying, electrodes were then thermally evaporated onto the substrate through

a shadow mask. SEM (Scanning Electron Microscopy) images showed that during the device

processing those particles aggregated to form larger islands that were as large as 50 µm in

diameter. Figure 1.18 illustrates its light-emitting bulk homo-junction LEC and corresponding

particle island sizes:

27

Figure 1.18: Left: Red, green, and blue bulk homo-junction LECs incorporating ITO particles intothe film (a) 315 K, 600 V; (b) 320 K, 500 V; (c) 335 K, 700 V. Right: SEM images of ITO particlesin green-emitting bulk homo-junction LEC film [70].

The second method, introduced in Bonnet’s work, was through thermal deposition of a thin

layer (3.5 nm) of gold on top of polymer LEC film or directly onto the substrate that was later

coated with the film [71]. After drying, electrodes were deposited onto the substrate using a

shadow mask too. Similarly, AFM (Atomic Force Microscopy) images revealed that islands

formed by particle aggregation had an average diameter of 35 nm. Figure 1.19 illustrates its

light-emitting bulk homo-junction LEC and corresponding particle island size:

Figure 1.19: Left: Bulk homo-junction LEC incorporating a layer of gold into film (1000 V, 280 K).Right: AFM images of gold particles in the bulk homo-junction LEC film [71].

Despite the beautiful results, the underlying operating principles of those devices were not

fully understood at the time. Utilizing bipolar electrochemistry, the device behaviors and

mechanisms behind are fully explained in the later chapters of this thesis.

28

1.5 Motivation and Scope

1.5.1 Motivation

My research focused on examining the effects of bipolar electrodes on doping and EL in pla-

nar polymer light-emitting electrochemical cells. The motivation was to explain the operating

mechanism behind bulk homo-junction LECs using the terminology of bipolar electrochemistry.

I have carefully studied how the size of bipolar electrodes, the operating conditions, and the

number of bipolar electrodes affected the doping patterns and EL. All the devices were fabri-

cated inside nitrogen-filled gloveboxes to ensure their quality and all the tests were conducted

inside a vacuumed probe station. Time-lapse fluorescence imaging technique was used to cap-

ture the images of devices in chronological order. My thesis work pointed to a new direction of

research that is solid-state bipolar electrochemistry.

1.5.2 Scope

Chapter 2 provides a detailed description of materials, recipes, device processing, testing pro-

cedures, and the equipment used.

Chapter 3 examines the propagation speed of doping in a large planar LEC device. I have used

some simple methods and scripts to calculate the propagation speeds here.

Chapter 4 investigates the effects of bipolar electrode size and applied voltage on the doping

initiation and propagation from the bipolar electrodes.

Chapter 5 examines the doping propagation from a linear array of bipolar electrodes. A simu-

lation of electric potential and fields is presented too.

Chapter 6 demonstrates the formation of bulk homojunction with dispersed ITO macro-particle

bipolar electrode, as driven by a pair of metallic probes.

Chapter 7 presents the conclusion and provides some suggestions for potential future works

that will extend the findings presented here.

29

Chapter 2

Experimentation

2.1 Device Processing

2.1.1 Materials

The devices in this thesis have an active layer that is based on a polymer/salt/PEO mixture.

The luminescent conjugated polymer used here, as mentioned before, is the orange-emitting

MEH-PPV. This is one of the most studied luminescent conjugated polymers and widely used in

many PLEDs and LECs. The peak PL occurs at 585 nm and the CP has an energy bandgap of

2.17 eV [74]. The absorption and PL emission of MEH-PPV in an MEH-PPV/PEO/LiCF3SO3

blend are summarized in Figure 2.1 below:

Figure 2.1: Left: Molecular structure of MEH-PPV. Right: The absorption and PL emission spectraof MEH-PPV [75].

30

The MEH-PPV used in the active material came from Canton Oledking Optoelectric Materials

Co. Ltd., China. Its average molecular weight and PDI (polydispersity index) was determined

to be 3.3× 105 and 1.4 [76].

The ion-solvating polymer used here is poly(ethylene oxide) (PEO). Two types of PEO used

in this study were obtained from Sigma-Aldrich, and they have molecular weights of 1 × 105

g/mol and 2× 106 g/mol.

PEO and salt form polymer electrolyte together. There are many types of salt used in the

polymer electrolyte, such as lithium trifluoromethanesulfanate (triflate) (LiCF3SO3, abbrevi-

ated as LiTf), cesium perchlorate (CsClO4), potassium trifluoromethanesulfonate (triflate)

(KCF3SO3, abbreviated as KTf). After many experiments, KTf was chosen as the primary

salt used in most cases. LECs made from potassium salt tends to have a lower turn-on voltage

and slightly more balanced p- and n-doping speed. KTf salt was purchased from Sigma-

Aldrich with a molecular weight of 188.17 g/mol and an assay of 98%. Molecular structures of

salts and PEO are shown in Figure 2.2:

S OLi

O

CF3

O

Cl OCs

O

O

O

S OK

O

CF3

O

Figure 2.2: Molecular structure of Lithium Triflate, Cesium Triflate, Potassium Triflate and PEO.

2.1.2 Solution Making

The final LEC solution is made by mixing the solution of MEH-PPV with that of the polymer

electrolyte. A common solvent, cyclohexanone, is used to dissolve MEH-PPV and polymer

electrolyte separately to make master solutions of the two. The dry luminescent CP and

31

components of polymer electrolyte are weighed outside in air and dissolved in cyclohexanone

inside the glovebox. MEH-PPV master solution is made in 1%, meaning 10 mg of MEH-PPV

in 1 ml of cyclohexanone solvent. PEO master solution is made in 2%, meaning 20 mg of PEO

in 1 ml of cyclohexanone solvent. KTf is then added to PEO master solution, and the mass

ratio is close to 5.2:1 between PEO and KTf . Finally, LEC solution is made by mixing the

appropriate amount of master solutions to ensure its mass ratio among MEH-PPV:PEO:KTf

is 1:1.3:0.25.

2.1.3 Substrates

The substrates used in this study are mostly sapphire (15 mm by 15 mm) and occasionally high-

quality glass with a thickness of about 1 mm. Both substrates are chemically inert. Sapphire,

with a thermal conductivity of 40 W/m ·K and a high resistivity of 100 TΩ ·m at 25 oC, is also

highly transparent for optical applications [77]. The expensive substrates are always recycled

after each use. The LEC films on top of the square sapphire substrates could be rubbed off, and

the remnants could be etched away by submerging the substrates in cyclohexanone followed

by treatment in a piranha solution. The piranha solution is made by mixing concentrated

sulfuric acid and hydrogen peroxide in a 3:1 volume ratio. Due to the highly caustic and

abrasive nature of the piranha solution, both organics and metals could be thoroughly removed

from the sapphire surface. The safety procedure [78] for handling piranha is strictly followed.

The spent piranha solution is disposed properly by EHS at Queen’s University. After being

thoroughly rinsed with a copious amount of running water, the sapphire substrates are placed

on Teflon racks. These racks are then submerged in different beakers filled with deionized

water, acetone and lastly iso-propanol. Each beaker with racks inside is ultra-sonicated for

10-15 minutes. Finally, substrates are taken out of the racks and blown dry with nitrogen gas

gun immediately and stored in an oven at 120 oC to remove residual solvent. Before casting

films, the substrates are taken out of the oven and treated in a UV-ozone oven for 10 minutes

to remove any organic residue.

32

2.1.4 Film Casting

Due to the air-sensitive or hydroscopic nature of MEH-PPV and polymer electrolyte, the planar

devices are fabricated inside two oxygen- and water-free gloveboxes. Both gloveboxes are filled

with dry nitrogen, and the entire system is shown in Figure 2.3. The left glovebox is used

for wet processing including mixing solutions and spin-coating of polymer films. The right

glovebox has a built-in thermal evaporator to deposit metal electrodes. There is a T-chamber

connecting the gloveboxes to allow transfer of air-sensitive devices in between and move small

objects inside.

Figure 2.3: MBaun labmaster double-glovebox system.

Before casting a polymer film, the LEC solution is stirred on a hotplate at room temperature

for more than an hour. Then 100 µL of the solution is dispensed onto each clean sapphire/glass

substrate. The substrate is then spun by a Chemat Technology KW-4A spin coater at 2000 rpm

(revolutions per minute) for one minute, and 3000 rpm for one minute to dry. After spinning,

the LEC film is dried on the hotplate at 50 oC overnight. This process usually gives a film

thickness ranging from 150 to 250 nm as measured by optical interferometry. Before testing

the edge of the polymer film was wiped away to prevent its contact with the thermal paste and

subsequent short-circuiting between the driving electrodes.

33

2.1.5 Shadow Masks

Shadow masks are specifically designed for fabricating LEC devices into desired configurations.

They are precisely machined and polished brass pieces with pre-designed openings and have a

thickness less than 1 mm. Detailed dimensions of the masks will be included in the appendix.

The major configurations used in experiments are presented below:

(a) (b)

(c) (d)

Figure 2.4: Shadow masks employed to manufacture LEC devices (a), (b), (c) and (d) shown belowin Figure 2.5 (not to scale).

(a) (b)

(c) (d)

Figure 2.5: Corresponding asymmetric LEC devices (yellow color-gold, silver color-aluminum, redcolor-LEC film) made from shadow masks (a), (b), (c) and (d) shown above in Figure 2.4 (not toscale).

34

Every time before evaporation with a shadow mask, it was thoroughly cleaned. First, the front

face of shadow mask was polished using a piece of sandpaper to remove metal deposited on it

from the last evaporation. Second all the small circular openings were cleaned with gold wires to

remove any metal remnants from the last use. The other openings were wiped with cleanroom

paper to remove dust and metal particles. Then the mask was blown using a nitrogen gas gun.

After that step, the mask was immersed in beakers filled with water, acetone and iso-propanol

and each of the beakers was sonicated for 10 ∼ 15 minutes. Finally, the shadow mask was

taken out of the last beaker and blown dry with the nitrogen gas gun.

2.1.6 Electrode Deposition

Once the LEC films are completely dry, they are transferred to the RHS glovebox where metal

electrodes are deposited through physical vapor deposition (PVD). In this process, a metal

source is thermally evaporated and deposited onto the LEC film surface through a shadow mask

inside a vacuum chamber. The thermal evaporator (BOC Edwards AUTO500) is incorporated

inside the nitrogen-filled glovebox, and the high vacuum (down to 10−6 torr) is achieved with a

diffusion pump backed by a BOC Edwards RV12 rotary vane pump. Before starting the diffusion

pump warm-up, a water chiller is turned on to provide cooling water at 20 oC to circulate

through the system. After warming up the diffusion pump for 30 minutes, the vacuuming

process is started. The pump-down sequence is automated, and rest of the process takes about

15-20 minutes. During the sequence, the cold trap behind the pump is manually filled with

liquid nitrogen to prevent oil back-streaming.

Inside the right glovebox, polymer-coated sapphire/glass substrates are put face down into the

shadow masks and then placed on an open support frame. The frame is then positioned above

the evaporation source facing down. The evaporation sources here are tungsten filament for

aluminum and tungsten basket for gold and calcium. High purity aluminum wires are carefully

cut and hung on the filament. High purity gold pellets are placed inside the tungsten basket. A

source shield is screwed on around the filament/basket to prevent unwanted vapor deposition

polluting other surfaces inside the vacuum chamber. The shield itself has a rectangular opening

that could be turned directly facing the coated substrates. Each time only one type of metal

35

could be deposited. Asymmetric devices need to be deposited twice, and openings for electrodes

that require another metal are blocked in the first deposition. In the second deposition openings

for the first metal are blocked while the openings for the second metal remain open.

After mounting the substrates and metal sources, the evaporator bell jar is sealed, and the

pump-down sequence is started. Once the desired vacuum level is reached, the source current

is slowly turned up by the user. When the current goes through the filament/basket, it heats

up the metals inside and vaporizes them. The vapor then travels upwards and is deposited

onto the polymer LEC films through shadow masks. The instantaneous rate of deposition and

accumulated thickness of deposition are measured and monitored by a quartz crystal micro-

balance. Usually 80∼100 nm of aluminum and 30∼60 nm of gold are deposited for metal

electrodes. The current is brought down to zero once the desired thickness is reached. After

waiting for 20 minutes to allow the filament/basket to cool down, the bell jar is vented with

nitrogen and lifted up. Devices are then retrieved, and the evaporation system goes through a

15∼20 minutes turn-off procedure to complete its shut-down.

2.2 Device Characterization

2.2.1 Electrical Measurement

The finished planar LECs or LEC films (without electrodes) are transferred from the RHS

glovebox to the probe station next door in a sealed glass vial to reduce its exposure to air. All

testings are conducted under vacuum with the LECs mounted inside the Janis ST-500 probe

station. The probe station probes are directly connected to a Keithley Source Measurement

Unit (Keithley 237) controlled by a LabView program from a computer. This LabView program

performs measurements of current vs. time under a constant applied voltage bias or voltage

vs.time under a constant applied current. Some silver-based thermal paste is applied to the

sample stage beneath the device under test. This improves the contrast of images by providing

a scattering, black background for the orange-emitting LEC device. The probe station chamber

is sealed off and vacuumed to a low pressure (10−4 mbar) by a turbo-molecular pump backed

36

by a mechanical pump. The temperature of the sample stage is controlled and monitored by

a temperature controller and the LabView program. Figure 2.6 displays the control panel of

the LabView program. This program enables a user to apply constant voltage/current and

simultaneously records current/voltage as a function of time.

Figure 2.6: Control panel of the LabView program that controls Keithley units and records measure-ments.

2.2.2 Optical Imaging

Fluorescence images are taken through a quartz window by a Canon D300 digital camera

equipped with a 90 mm 1:1 macro-lens and positioned vertically above the window. During

the picture-taking, the room remains in darkness, and the probe station chamber is illuminated

by a UV ring light source. An image was taken every five seconds at a shutter speed and an

aperture chosen for the experiments. The general equipment setup for imaging is displayed in

the following:

37

Figure 2.7: Time-lapse UV fluorescence imaging equipment setup.

2.2.3 Probe Station and Cryostat

The probe station (Janis ST-500) mentioned above provides a testing stage for planar LEC

devices. By moving the probes in x, y and z directions using the micrometers, probes could be

brought into contact with the metal electrodes deposited on the LEC film. The image of the

probe station is shown below:

Figure 2.8: Janis ST-500 probe station.

38

Some LECs are tested in a Cryo-Industries of America microscopy cryostat. The cryostat is

mounted on an optical table beneath a Nikon fluorescence microscope fitted with a Q-View

digital camera. The cryostat has a device holder with six gold-plated pins approximately 2.6

mm apart. An LEC device mounted on stage inside the cryostat, and the cryostat itself are

shown in Figure 2.9:

Figure 2.9: Left: An LEC device mounted side the cryostat by the contact pins. Right: Cryo-Industriesof America custom ST-500 microscopy cryostat.

An oil-free Varian Turbo-V 70 pumping station is used to maintain the pressure below 5×10−5

torr inside the cryostat. The temperature inside the cryostat is controlled by Cryocon 32B

temperature controller and low temperature (200.0 ± 0.1 ∼ 300.0 ± 0.1 K) is achieved with

the supply of liquid nitrogen.

2.2.4 Interferometer

The thickness of the polymer films is measured with an Ambios Technology Q-View Inter-

ferometer. The interferometer is calibrated using a standard before measurement each time.

The film of the device that needs to be measured is scratched along its width. This scratch

offers a difference in heights which could be measured by the interferometer. Measurements are

averaged to assure their accuracy and precision.

39

Chapter 3

Doping Propagation in a Large Planar

LEC

3.1 Introduction

Planar LECs without a bipolar electrode were made to demonstrate the functional operation

of the active film. The doping propagation speeds were determined from the time-lapse fluores-

cence images of a large planar cell under operation. The planar cell had asymmetric electrodes

so as to obtain a more even p- and n-doping. Image analysis was carried out in MATLAB.

3.2 Experimental Details

The planar LEC planar was fabricated using the same recipe and procedures as described in

Chapter 2. The left half of shadow mask (b) with the center hole blocked was used to deposit

the electrodes. The planar LEC device had an aluminum cathode and a gold anode. The

separation of the electrodes was 1.78 mm. It was tested in the probe station chamber under

a 10−4 mbar vacuum. A 10 V bias was applied across the driving electrodes to activate the

cell. The planar LEC was heated to 350 K during the activation process. A ring UV light

was placed around the circular quartz window to illuminate the device. A Nikon D300 camera

40

with a 90 mm 1:1 macro-lens was used to image the planar cell with computer control. Those

images were taken every five seconds at a shutter speed of 1/1.3 sec with an aperture of f/16.

3.3 Results and Analysis

Image Processing Algorithm

Image processing was performed in MATLAB to calculate the propagation speeds of p- and

n-doping fronts, from the bottom gold and top aluminum electrodes, respectively. Figure 3.1

shows the photoluminescence image of the planar cell before a voltage bias was applied. The

dimension of the cell is also shown below:

Figure 3.1: Fluorescence image of the planar cell with the inter-electrode spacing labelled. The topelectrode is made of aluminum and bottom electrode is made of gold.

The photoluminescence images were first converted to grayscale images, and an edge-finding

function was employed to identify the edges of electrodes, n-doped and p-doped regions. Because

n- and p-doping fronts are curved and jagged, an average of along the doping front was done to

determined the mean doping front position. The edge-finding function converted each grayscale

image to a binary image with boundaries outlined as even/uneven white lines. The image

processing steps are summarized in the following set of figures in chronological order:

41

Figure 3.2: The top row shows grayscale images of time-lapse photos of the LEC planar cell. Thebottom row shows the binary images with outlined boundaries obtained from grayscale images. Fromleft to right: 10 sec; 50 sec; 100 sec. The n-doping propagated downwards and the p-doping propagatedupwards. The n-doping front position was measured relative to the top driving electrode and the p-doping position was measured relative to the bottom driving electrode.

After finding all the edges, the same MATLAB script recorded the y-coordinate (in pixels) of

each point on the identified boundary lines and reshaped them into a 1D array. Afterward,

a k-means clustering function was utilized to separate data from this 1D array into different

groups and calculate the centroid of each data group. Each data group here ideally centered

around the y-coordinate of one boundary line. For example, in the first column of two images

in Figure 3.2, there should be three data groups of y-coordinates due to the existence of three

boundary lines. However, in the second or third column of two images, there should be four

data groups due to the existence of four boundary lines. The result centroid value was used

as the average y-coordinate of each line boundary. After finding the y-coordinates of lines, the

doping front positions were computed from simple subtractions and a spreadsheet analysis was

conducted on the result data after conversion from pixel to mm. The plots of data with trend-

lines are shown in Figure 3.3. It could be clearly seen that both p- and n-doping propagation

displayed linear growth before 120 seconds since the beginning of doping.

42

Doping Propagation Speed

Using the above method, the p- and n-doping propagation distances were calculated and plotted

in the following figure:

Figure 3.3: p- and n-doping distances in a planar LEC cell as functions of time. Linear trend-lines aregiven beside each set of data. 5% error was chosen for both p- and n-doping distance data.

From the above graph, n-doping appeared 15 seconds later than p-doping in the LEC cell. n-

doping also had a slower speed of propagation, at 0.0042 mm/s, compared to p-doping’s 0.0107

mm/s.

3.4 Conclusion

From the clear doping in the planar LEC cell, the new LEC solution was proven to be fully

functional. From the distance measurements of p- and n-doping propagation, a linear growth

was observed. Finally, p-doped region expanded faster and earlier compared to the n-doped

region.

43

Chapter 4

Planar LECs with Bipolar Electrodes

of Various Sizes

4.1 Introduction

A bipolar electrode (BPE), as introduced in Chapter 1, is a floating conductor that is capable

of driving both redox reactions at its extremities in an electrochemical cell. Several key features

distinguish BPEs from other types of electrodes [79]:

1. A BPE is not directly connected to a power supply and possesses a floating potential

depending on applied bias, its own shape and size;

2. A BPE simultaneously promotes oxidation and reduction reactions at its opposite poles;

3. The potential difference between the BPE and the electrolyte varies continuously along

the length of BPE.

These unique aspects of BPEs lead to many applications that could not be conveniently achieved

with conventional electrodes. Some of the applications, such as generation of compositional

gradient and chemiluminescence for sensing applications, as well as selective coating, are briefly

mentioned before. Furthermore, small floating BPEs in an electrolyte could be propelled by

bubbles generated asymmetrically at their poles [80]. Alternatively, a linear motion could be

44

generated when BPE material is dissolved from one pole and deposited on another in a “self-

regeneration” process [81]. These mobile BPEs, acting as tiny motors or cargo lifters, provide

many exciting potential applications.

The bipolar electrochemical system generally involves a liquid electrolyte, in this chapter, I

present polymer light-emitting electrochemical cell with additional metallic discs deposited

between the driving electrodes. I show that these metallic discs function as BPEs in a solid-

state electrochemical cell. Several model cells of various sizes and geometries will be analyzed.

Visualization aided by time-lapse imaging technique demonstrates bipolar electrochemistry in

solid state.


The luminescent conjugated polymer used in the experiments was an orange-emitting polymer

MEH-PPV synthesized by OLED King Ltd., China, with aMw = 3.3×105 and a PDI of 1.4. The

polymer electrolyte consisted of PEO with Mw = 2 M and potassium trifluoromethanesulfonate

(KTf) salt. Both MEH-PPV and PEO were purchased from Sigma-Aldrich and used as

received. The MEH-PPV powder was dissolved in cyclohexanone to make a solution containing

10 mg of MEH-PPV per 1 ml of solvent. PEO and salt were dissolved in cyclohexanone as

well for a second solution containing 20 mg of PEO and 4 mg of KTf per 1 ml of solvent.

Eventually, the solutions were mixed to obtain an LEC solution containing MEH-PPV, PEO,

and KTf in a weight ratio of 1:1.3:0.25. 100 µL of LEC was then dispensed and spun on the

thoroughly cleaned 15 mm × 15 mm sapphire substrate. The substrate was then left on the

hot-plate at 50 oC overnight. The thickness of the film was measured using the Ambios optical

profiler (interferometer) to be 200± 10 nm.

High purity aluminum and/or gold (>99.99%) electrodes were then deposited on top of the

LEC film through the shadow mask ((b) and (c) from Figure 2.4 with the thermal evaporator.

Aluminum and gold electrodes had a thickness around 100 and 60 nm, respectively. The shadow

masks were designed to have rectangular and circular openings for driving electrodes and BPEs,

respectively. All the steps above, except for weighing the chemicals, were carried out inside the

45

nitrogen-filled gloveboxes.

The completed device was transferred to the probe station in a sealed glass vial. The sample

stage of probe station was plastered with a layer of black thermal paste to enhance image

contrast. The LEC device was briefly exposed to air (<5 minutes) as it was mounted onto

the sample stage. The sample stage chamber was then sealed and pumped down to a vacuum

of 10−4 mbar with a turbomolecular/diaphragm pump. The stage was subsequently heated to

350 K at low pressure. The LEC device was facing up and imaged through the quartz window

by a Nikon D300 digital camera with a 90 mm 1:1 macro-lens. A UV ring light was placed

around the quartz window to provide illumination when the room light was turned off. Images

were taken every five seconds at a shutter speed of 1/3 s and an aperture of f/16. A LabView

program controlling a Keithley 237 source measurement unit and a temperature controller was

used to apply a fixed voltage to the device and simultaneously recorded the cell current. The

program was run while the gold/tungsten probes were lowered to contact the driving electrodes.

A sharp increase in current measurement signaled the establishment of contact. This current

flow triggered electrochemical doping of the LEC film that was captured by the camera.

The images were analyzed in Photoshop and MATLAB. Batch processing (cropping, level

adjustment and discarding of color) was performed in Photoshop and cell dimensions and

doping speed propagation calculations were performed using MATLAB on pixelated grayscale

images. MATLAB scripts were written to identify the boundary of doped regions.


An example planar cell is shown in Figure 4.1, which had two gold driving electrodes 2.12

mm apart between their inner edges. This cell did not contain any BPEs between the driving

electrodes. The planar cell was illuminated by a 365 nm UV ring light. The bright orange

PL came from MEH-PPV. The cell was tested at 350 K with an applied bias voltage of 15

V between the driving electrodes. in situ electrochemical doping, which quenches the PL,

caused dramatic darkening of the polymer film, as shown in the figure. Electrochemical p-

doping involves the oxidation of MEH-PPV where π-electrons are removed. This process is

46

accompanied by insertion of anions (CF3SO−3 ) to compensate the oxidized polymer. Similarly,

electrochemical n-doping involves reduction of the MEH-PPV and the insertion of cations (K+).

Figure 4.1: Top: Time-lapse fluorescence images of a planar LEC with gold electrodes. Bottom: Cellcurrent as a function of time since the application of the voltage bias. Open circles overlaid on thecurve indicate the time at which images (a)-(h) were taken. The inset shows the cross-sectional viewof the planar cell [79].

The electrochemically doped polymer is electrically neutral, but contains defect states in the

energy gap, which affect its electrical and optical properties [48].

The effect of PL quenching offers a great way to observe the dynamic doping process in an LEC.

The entire space between the driving electrodes was doped as the doped regions expanded. As

the p- and n-doping of the same polymer caused different defect states, the degrees of PL

quenching in the doped regions were typically different. The p-doped region was more heavily

quenched than the n-doped region. The sharp boundary between the doped regions in Figure

4.1 f indicates the formation of a well-defined p-n junction. The EL shown in Figure 4.1 g and

h was caused by the recombination of injected electrons and holes across the p-n junction. The

EL intensity increased as the doped film became more conductive over time.

47

Figure 4.1 i plots the cell current versus elapsed time in a semi-log graph. The current increased

as soon as the bias voltage was applied. Initially, the cell current was largely ionic. As soon

as the doping fronts met each other, the pathway for electric current was established, and

the current experienced exponential growth. The total current continued to increase until cell

degradation would finally cause the current to drop. This process is illustrated in the following

figure:

=⇒

(a)

=⇒

(b)

=⇒

(c) (d)

Figure 4.2: Phases of current changes and each stage’s equivalent circuit model between drivingelectrodes. Notice that current changes from mostly ionic in (a), to partially ionic and partiallyelectric in (b), to mostly electric in (c) and (d).

Once the cell is fully turned on, the ionic processes are not necessary and ideally absent. This

can be accomplished by cooling the cell below the glass transition temperature of the polymer

electrolyte, so the ionic motion ceases [82]. The resulting “frozen-junction” cells resemble an

inorganic p-n junction LED. The frozen junction could operate as a photovoltaic cell when

it is illuminated with photons of sufficient energy. It is important to note that both EL and

photovoltaic response are only contributed by the junction region. However, in a planar cell

as shown above the p- and n-doped regions cover more than 90% of the cell area, but they are

neither electroluminescent nor photovoltaic.

To increase the total effective cell area and therefore the overall efficiency of the cell, floating

metallic stripes or particles were introduced into the cell. Despite the loss of cell area between

the driving electrodes due to the introduction of extra metal, more junctions were formed. The

cell luminance displayed an order of magnitude improvement over that of a standard cell. This

type of multi-junction cell is called a “bulk homojunction” cell, as many p-n junctions are

formed in the bulk of the active polymer film. For multiple junctions to form, metallic stripes

or particles must serve as both cathode and anode. This had been confirmed by using a model

48

cell with a single floating metallic disc between driving electrodes. It was also realized that

the applied bias voltage determined how many junctions could be formed in series, as each p-n

junction required a minimum voltage corresponding to its bandgap energy. Figure 4.3 below

presents such a model cell consisting of a pair of asymmetric driving electrodes and a single

aluminum disk BPE:

Figure 4.3: Top: Time-lapse fluorescence grayscale images of a 2.12 mm planar cell consisting of goldand aluminum driving electrodes, and a single aluminum disc BPE. Bottom: Circuitry, voltages andgeometries of the cell set-up. Device was tested at 350 K [79].

In this model cell, a gold anode and an aluminum cathode served as the driving electrodes. Gold

has a relatively high work function which is better for p-doping while aluminum has a lower

work function and is thus more suitable for n-doping. The images were cropped and rendered

in grayscale for comparison and clarity. The model cell was tested at 350 K with 20 V applied

across the driving electrodes. In this asymmetric case, the doping appeared to be more even

compared to the cell above. However, doping was not observed from the BPE in the first 35s

after the application of bias voltage. Only after the p-doping from anode has made contact

with the BPE a dark protrusion from the right edge of BPE was observed. Assuming that the

BPE did not disturb the potential distribution between the driving electrodes and the potential

49

drop at the electrodes was negligible, a simple calculation has shown that the initial potential

difference between the extremities of the BPE was 2.17 V . The appearance of p-doping at

the right-hand-side (RHS) of the BPE after 40s suggests that the anode pole polarization was

then sufficient to cause oxidation reaction. As the BPE needed to maintain electrical neutrality

during the experiment, a reduction reaction must have occurred simultaneously. There were two

possibilities as to why the n-doping from the BPE was not observed: first, the n-doped region

could be extremely thin and not resolved in the images; second, electrochemical side reactions

might have reduced the electrolyte instead of the luminescent conjugated polymer. At t=125s

some faint EL could be observed from above and below the BPE but not to its left and right.

In the successful bulk homojunction cells that will be presented shortly, a larger voltage bias

(several hundred volts) was always applied to form more junctions. This experiment featured

by Figure 4.3 illustrates that BPE will not be functional if the applied voltage bias is not

sufficient.

Another cell with similar dimensions to the cell in Figure 4.3 was also tested at the same

temperature but with a different driving voltage of 40 V . This led to a nominal initial potential

difference of 5.36 V across the disc BPE. The time-lapse images are shown in Figure 4.4 below:

Figure 4.4: Top: Time-lapse fluorescence images of a 1.94 mm planar cell consisting of gold andaluminum driving electrodes, and a single aluminum disc BPE. Bottom: Equivalent circuit of themodel cell with multiple light-emitting p-n junctions. Device was tested at 350 K [79].

50

This time p- and n-doping from the BPE could be observed immediately after applying the

bias voltage, and they propagated in opposite directions towards the driving electrodes. The

top and bottom edges of BPE were free of doping just as expected. When the p- and n-doped

regions from the BPE met the doped regions from the driving electrodes, two p-n junctions

were formed in series. Since the p-doped region was much more conductive than the n-doped

region, the right p-n junction had become the main conductive pathway and thus it started light

emission first after 15s. The left p-n junction began to emit after 20s when its EL was strong

enough to be imaged against the background PL. The p- and n-doped region from the driving

electrodes continued to propagate and eventually made contact with each other to form two

more p-n junctions above and below the BPE after 20s. After 30s all four p-n junctions were

emitting strongly. The change in resistance in the doped regions led to the dimming of light

emission from the RHS p-n junction and stronger emission from top and bottom p-n junctions.

This indicates that the cell current mostly conducts through the top and bottom p-n junctions.

The above comparison experiment demonstrated the importance of driving voltages in inducing

redox reactions across a BPE. In addition, the sizes of BPE have also played a key role in

determining the magnitude of polarization at the BPE interfaces.

Figure 4.5: Fluorescence image of a 10.19 mm planar cell with three aluminum disc BPEs of differentsizes. All the dimensions are labelled. Top driving electrode is gold and bottom driving electrodeis aluminum. The cell was tested at 350 K and 100 V bias. The smaller windows have shown themagnified images of small aluminum discs [79].

51

Extremely large planar LEC cell offers a great platform for studying the effect of BPE sizes

under the same applied potential. Figure 4.5 above shows a large planar cell composed of

two driving electrodes that were 10.19 mm apart, and three aluminum disc BPEs of different

diameters in the middle. Three aluminum discs were deposited using the mask (c) shown in

Figure 2.4. They were not perfectly circular owing to shadowing effect of the evaporation

mask. This image shown in Figure 4.5 was taken under UV illumination (365 nm) and 25s

after applying a 100 V bias. P and n-doping from the driving electrodes as well as the largest

BPE could be clearly observed. However, the image did not show any doping from the two

smaller BPEs. Although not shown in the figure, the largest BPE behaved just like the BPE

shown in Figure 4.4 and EL was observed from the p-n junctions formed above and below it.

The smallest BPE, on the other hand, did not show any sign of doping and was eventually

“overrun” by the p-doping from the top driving electrode. The center disc produced visible

doping after 60s while the doping from driving electrodes was still millimeter away.

Figure 4.6: Top: Regions of the 10.19 mm cell surrounding the center disc BPE ( = 0.26 mm)The fluorescence image at 50 s (not shown) was subtracted from all of the fluorescence images shown.Bottom: A plot of the p-doping front position from the BPE as a function of time since the 100 Vbias was applied to the driving electrodes [79].

52

A portion of the cell surrounding the center disc is presented as the time-lapse images in Figure

4.6 above. The image at 50s (not shown) was subtracted from all the images to improve

contrast. In these images, doping appeared as bright regions due to subtraction. Both p- and

n-doping could be observed clearly from the center disc BPE in those images. This delay of

appearance of p- and n-doping from the center disc BPE could be simply explained as follows.

Calculations revealed that the initial potential difference across three BPEs were 9.72, 2.55

and 0.99 V , respectively. The 9.72 V was sufficient to induce doping from the largest BPE at

once. However, 2.55 V , which was slightly larger than the 2.17 V across BPE in Figure 4.3,

was insufficient to induce doping right away. As doping from both driving electrodes continued

to expand, the potential difference across the disc BPEs also changed. Due to the increased

conductivity in the doped region (p region in particular) and thus a stronger apparent electric

field, the potential drop across the center BPE increased. Eventually, the potential difference

across the medium BPE became sufficient to induce p-doping. This increase in potential drop

across the center BPE was also supported by data shown in Figure 4.6, which plotted the

distance of p-doping from center disc versus time. The exponential growth trend suggested an

accelerated growth of p-doping, owing to the increased potential drop across the BPE/polymer

interface with time.

4.4 Conclusion

In this chapter, planar light-emitting electrochemical cells modified with disc bipolar electrodes

were presented. These BPE-LECs are unique electrochemical cells exhibiting bipolar electro-

chemical activities. BPE-LECs are solid state devices manufactured using standard thin-film

techniques. Testing methods and apparatus commonly used to characterize semiconductors

were used to study these BPE-LECs. These BPE-LECs exhibit highly visible electronic changes

during operation. Initially, the active film was strongly photoluminescent. After applying high

enough driving potential, the film was heavily PL quenched due to the dynamic insitu elec-

trochemical doping. Eventually, doping regions met and formed p-n junctions that caused

EL, and its intensity was highly dependent on local current density. BPE-modified LECs ex-

53

hibit highly different doping and EL patterns from standard LECs. The effectiveness of BPEs,

however, strongly depends on the applied potential and the size of the BPE. The solid state

electrochemical cells presented here are described for the first time in the framework of bipolar

electrochemistry.

54

Chapter 5

Planar LECs with a Linear Array of

Bipolar Electrodes

5.1 Introduction

After examining individual BPE of various sizes and the effect of applied driving potential, we

set out to study a linear array of interacting BPEs. The cell in this Chapter was fabricated

using shadow mask (d) from Figure 2.4. The shadow mask also contained a triangular bipolar

electrode. However, the triangular opening was blocked off, and only the linear array BPEs

were studied here. With this shadow mask, a linear array of small disc BPEs with the same

size and equal spacing was deposited between two driving electrodes. Utilizing the time-lapse

fluorescence imaging technique, the doping from BPEs was captured and the propagation speeds

were determined and compared. A simulation using COMSOL on the initial field distribution

of the specific cell geometry was also performed. This simulation revealed the potential and

electric field profiles of the cell and aided in the understanding of the observed cell behavior.


The planar LEC was fabricated using the same materials and procedures as described in Chapter

4. Its detailed dimensions are given in Figure 5.1. The cell had a gold anode and an aluminum

55

cathode. The BPE array was made of aluminum. This device was also tested inside the probe

station chamber pumped down to 10−4 mbar. This time, 100 V was applied across the driving

electrodes, and the sample stage was heated to 360 K. A ring UV light source was placed

around the circular quartz window to illuminate the device inside the chamber. The same

Nikon camera and lens were used. Images were taken every five seconds at a shutter speed of

1.6 sec with an aperture of f/22, which were different from before.


5.3.1 Doping Patterns

Doping from both driving electrodes and the BPE array appeared as soon as the the 100 V

bias was applied.

Figure 5.1: Fluorescence image of planar cell with an array of aluminum disc BPEs with the same size.All dimensions are labelled. Top driving electrode is gold and bottom driving electrode is aluminum.The device was tested at 100 V and 360 K.

In Figure 5.1 above the dimensions of the planar electrochemical cell with the BPE array are

presented. This picture was taken 10 seconds after electrical contact was made. p-doping is

clearly visible from the top driving electrode as well as the array BPEs. N-doping, however,

is only observed from the bottom driving electrode. Due to the charge neutrality requirement,

there must be reduction reactions to accompany the oxidation reaction from the BPE. The

56

delay in the appearance of n-doping can again be elucidated as some unknown side reactions

or the difficulty of imaging the faint n-doping as mentioned in Chapter 4. As time progressed,

the doping pattern changed as well, as shown in Figure 5.2.

+−V

Figure 5.2: Left: From left to right pictures were taken at 15s, 35s, 105s and 370s after doping started(exp1). Doping, formation of multiple p-n junctions and light-emission could be clearly observed.Right: Another cell with similar geometry showed strong light-emission from multiple p-n junctionsformed (exp2). An equivalent circuit model is given to its right.

We observe that doping from both the driving electrodes and the BPE had propagated and

grown in size. In addition, n-doping is visible at t=35s. The sharp n-doping from the top-most

BPE is seen to penetrate the p-doping from the anode. In between the BPEs, the emergence of

n-doping modifies the tips of p-doping fronts propagating downward. At t=105s, EL is observed

from the p-n junction formed between the BPEs. When the p- and n-doping fronts meet, they

tend to slide past each other sideways. This happens because it is much easier for the doping

front to propagate to an undoped region then to compensation-dope the oppositely doped

polymer. This causes the light-emitting p-n junctions formed to be nearly vertical. Jagged, but

mostly horizontal junctions, are formed above the top-most BPE and below the bottom-most

BPE. This happened because the broad doping from the driving electrodes completely blocked

the doping from the BPE. The last image at the left side in Figure 5.2 taken at t=370s shows

that n-doping from the BPE has merged. As a result, the multiple light-emitting p-n junctions

formed earlier had disappeared. This result shows that the p-n junctions formed from a linear

array of BPEs in an open configuration are unstable.

57

We fabricated and tested multiple cells with a linear array of BPEs deposited on top of the

polymer. Right side of Figure 5.2 shows a cell when all the p-n junctions formed are strongly

emitting. To the right of the cell image, a circuit diagram shows the equivalent circuit of the

emitting cell. We note that the bottom of the cell is equivalent to four p-n junction LEDs formed

between the BPE p-doping and the n-doping from the cathode of the cell. This observation is

similar to that of Figure 4.4. This cell, however, is also unstable and the multiple junctions

eventually dissipated with additional doping propagation. Figure 5.3 shows the cell current

as a function of time. Since the electrical contact was made by lowering the biased probes to

the evaporated electrodes, a sharp increase in current is observed when the contact is made.

In both devices, the overall cell current underwent a sharp increase when the p-and n-doping

fronts from the driving electrodes had made contact. The inclusion of the BPE array did not

have a measurable effect on the cell current due to the large width of the cell. The cell current

was dominated by the broad area without a BPE. The cell current began to drop at about

1,000s due to the degradation of the active layer or the electrical contact.

Figure 5.3: Left: Cell current as a function of time after the application of bias voltage for exp1.Right: Cell current as a function of time after the application of bias voltage for exp2.

5.3.2 Analysis

Image Processing Algorithm

The cell images of Figure 5.2 appear to show that the doping from the BPEs appeared simul-

taneously and propagated at the same speed. In addition, the appearance of n-doping is again

58

delayed (by about 20 seconds) relative to p-doping. A detailed image analysis was performed

in MATLAB to confirm these observations.

The doping front position was calculated in pixels by comparing images from different frames

and was converted to millimeter afterward. For each disc BPE, a sequence of images was

generated and analyzed to calculate the doping front position and the average propagation

speed. The following picture illustrates the analysis process:

Figure 5.4: Top Left: Image taken under UV illumination. Top Right: Same image converted tograyscale image. Bottom Left: Same grayscale image converted to a binary image. Bottom Right:Boundary of the electrode derived from the binary image.

MATLAB programs were written to set up a capture window (width: 200 pixels; height: 220

pixels) for every disc BPE and the picture displayed above featured the top-most disc BPE

of Figure 5.2. The captured picture was first converted to a grayscale image. A grayscale

threshold of 110 was used to convert the grayscale image to a binary image. All white objects

smaller than the threshold size 500 pixels were removed from the obtained binary image. A

“Canny” filter was then applied to attain the boundary. The same steps were applied to images

obtained from other BPEs as well.

59

The size of the binary object (a BPE plus the doped area around it) was calculated in the same

MATLAB script. In this case, since the oval disc BPEs were elongated towards the driving

electrodes, the maximum distances between the two oval extremities were taken as the electrode

size, and the positions of electrode extremities were stored. For the images taken after doping

propagation has begun, the same processing was applied to calculate the maximum distance

between the p-doping front and the edge of the anode, as well as the distance between the

n-doping front and the cathode edge. The BPE size was then subtracted from those obtained

distances to get both p- and n-propagation distances from the BPE extremities. Figure 5.5

illustrates the boundary changes obtained from the scripts:

(a) (b) (c)

(d) (e)

Figure 5.5: Boundary changes of top BPE from exp1 in chronological order: (a) 0 sec; (b) 5 sec;(c) 10 sec; (d) 15 sec; (e) 20 sec. n-doping from BPE propagates upwards and p-doping from BPEpropagates downwards. Green curves outline the boundaries.

Doping Propagation Speed

The approach described above was applied to analyze the rest of the BPEs for the cells of Figure

5.1 and Figure 5.2. Because the bottom-most BPEs in both cells had some minor defects, the

other seven BPEs were examined in a group. Conversion between pixel and millimeter was

done before the analysis, and all the subsequent measurements were converted respectively in

60

the script. The following graphs show the doping front positions at various times since the

application of the driving voltage:

Figure 5.6: p-doping front positions measured from bottom edges of disc BPEs 5 seconds after dopingstarted in exp1. Error bar value was obtained from standard deviation calculation.


61



62


Figure 5.11: p-doping front positions measured from bottom edges of disc BPEs 10 seconds afterdoping started in exp2. Error bar value was obtained from standard deviation calculation.

Figure 5.12 plots the p-doping front position for all seven BPEs as a function of time. We

observe a linear trend in the data. From the trendlines the average doping propagation speeds

were calculated to range from 0.024 mm/s to 0.028 mm/s. These quantitative results confirmed

the visual observations of a uniform doping propagation speed from the linear array of BPEs.

63

Figure 5.12: p-doping front positions measured from bottom edges of disc BPEs as a function of timesince doping started (exp1). Linear trendline was also plotted for p-doping distances from each discBPE.

5.3.3 Simulation

The uniform doping propagation speeds from the linear array of BPEs indicates a uniform

electric field between the driving electrodes. This is expected for a device geometry consisting

of two parallel driving electrodes. To demonstrate that the inclusion of a linear array of BPEs

did not alter the uniform field distribution, I performed a simulation of the actual device con-

figuration using COMSOL at the onset of doping propagation. The dimensions and geometries

used in the simulation correspond to a real device. The simulation relied on the pre-set and

floating boundary conditions to simulate electric potential and electric fields between two driv-

ing electrodes with a linear array of BPEs included. The basic physics for the simulation is

outlined here.

Before the doping starts, the undoped LEC film could be regarded as a dielectric material.

A dielectric material is a substance that is a poor conductor of electricity but an efficient

supporter of electrostatic fields. The LEC layer satisfies this requirement since it is indeed

a poor conductor but it does not impede the electrostatic lines of flux and can function as

64

a capacitor with low capacitance to store energy temporarily [62]. Therefore the boundary

condition of a dielectric/conductor interface applies to the interface between the LEC film

and the BPE or the driving electrodes. For the BPEs the floating boundary condition requires

charge conservation to compute the floating potentials, thus both free and bound charges within

the material need to be included. Moreover, since the edges of the sapphire substrate are in

contact with vacuum, Neumann boundary conditions are applied. For simulating the field

profiles, electric displacement field D is chosen to simplify the formulation. In a dielectric

material:

D = εE (5.1)

where ε is the permittivity of the dielectric material and E is the electric field inside the

dielectric material.

From Gauss’s law,

∇ ·D = ρf (5.2)

where ρf is the density of free charges.

For conductors with no initial charges, ρf = 0, thus,

∇ ·D = 0 (5.3)

The relationship between electric fields and potential fields is:

E = −∇V (5.4)

Therefore, combine the equations above,

∇ · (∇V ) = 0 (5.5)

In a two dimensional system, it means the boundary conditions of the dielectric/conductor

65

interface initially (before the active film is doped) for electric fields are:

∂E

∂x+∂E

∂y= 0 (5.6)

The COMSOL simulation results are shown below:

Figure 5.13: Simulation of the electric and potential fields of the BPE-array LEC device. The color-map and contour plot illustrate the potential field, the vectors represent the direction, flux densityand strength (vector length) of the electric field.

Plotting the line potential profiles through the centers, slightly outside and much more outside

of disc BPEs, the following two figures shown in Figure 5.14 and 5.15 were obtained from the

same simulation. Subtracting the potentials of blue line from potentials of the red line (or vice

versa) the potential difference across extremities of disc BPEs could be calculated.

66

Figure 5.14: Line potential profiles close to the disc BPE-array: blue line is through the centers ofdisc BPEs, green line is slightly outside of all disc BPEs, red line is much more outside of disc BPEs.A staircase-like profile could be clearly seen from the graph.

Figure 5.15: A zoom-in figure on line potential profiles close to one disc BPE: blue line is through thecenter of this disc BPE, green line is slightly outside of this disc BPE, red line is much more outsideof this disc BPE. A staircase-like profile could be clearly seen from the graph.

67

The above simulation reveals the same potential distribution around each BPE. Because the

linear array was positioned well within the driving electrodes, the electric potential/field dis-

tribution was not affected by the fringe fields near the upper or lower edges of the driving

electrodes. The potential differences between the BPE extremities and the LEC film (blue vs.

red) drive the redox reactions observed in the experiments. From Figure 5.15 the potential

difference between the two extremities of a BPE is approximately 3 V . This is larger than 2.55

V of the center disc BPE in Figure 4.5 and is sufficient to induce doping.

5.4 Conclusion

In this chapter, we investigated the doping patterns of a large planar LEC with a linear array

of BPEs. Electrochemical p- and n-doping from all eight BPEs in the array had been observed.

The interaction of the BPE doping fronts led to the formation of multiple light-emitting p-n

junctions. The multiple junctions formed, however, disappeared when the BPE doping merged

to the left and right of the BPE array. P-doping propagated faster than n-doping. The p-

and n-doping from the BPEs appeared to propagate at the same speed. This was confirmed

by image analysis in MATLAB. The p-doping propagation speed for the first 20s of device

operation was calculated to be between 0.024 mm/s and 0.028 mm/s. COMSOL simulation

revealed the electric potential profile of the entire cell. The large potential difference between

the BPE extremities was sufficient to induce redox reactions observed in the experiments.

68

Chapter 6

Probing Planar LECs with ITO

Particle Bipolar Electrodes

6.1 Introduction

In this chapter, we realized doping and light-emitting junction formation with a pair of biased

probes in direct contact with the polymer film. In the first experiment, we positioned the

biased probes between a pair of unbiased driving electrodes. We observed the formation of

five light-emitting p-n junctions. This intriguing phenomenon was again explained by bipolar

electrochemical doping. When probing a polymer film with dispersed ITO macro-particles, we

observed the formation of a bulk homojunction between the biased probes. Eventually, the bulk

homojunction collapsed to a single junction in seres with a much smaller but strongly emitting

bulk homojunction centered around the cathode. We will discuss the interesting observations

here.


The luminescent polymer MEH-PPV used has the molecular weight Mw = 3.3× 105 and PDI

of 1.4. The LEC film had the same composition of 1:1.3:0.25 (MEH-PPV:PEO:KTf, weight

69

ratio) as in Chapter 4 and 5. However, this time, 22.1 mg of ITO (indium tin oxide) macro-

particles was added to 1 mL of the LEC solution to make a new ITO-LEC solution. The

ITO macro-particle powder (ADS10ITO) was obtained from American Dye Source. It had

a nominal particle size less than 2 microns and purity < 99.99%. After thorough stirring and

mixing, 100 µL of this solution was spin-casted unto a 15 mm x 15 mm sapphire substrate

at 2000 rpm and dried at 3000 rpm for one minute each. In the direct probing comparison

experiments, the LECs without ITO particles were dispensed with the original LEC solution

used in Chapter 4 and 5. The devices were spin-casted using the same setting as mentioned

above. All the coated substrates were left on the hot plate at 50 oC overnight to dry. All devices

were then tested using the probe station under vacuum at different temperatures and voltages

chosen for each experiment. The direct probing experiments with plain LECs (without metal

electrodes) generally had testing voltages ranging from 5 to 120 V at a temperature between

340 and 390 K. For plain ITO-LECs the testing voltages generally vary from 100 to 600 V

and the testing temperature was at 320 K. In all the pictures displayed in this section, the top

probe electrode is gold, and it induces p-doping, the bottom probe electrode is tungsten, and

it induces n-doping.

6.3 Results

6.3.1 Probing LEC Films without any ITO Particles

In all the cells we have shown so far, the driving electrodes are of much larger dimensions than

the BPE, be it a single disc or a linear array. Here we show that it is also possible to observe

bipolar electrochemical doping with an inverted geometry. Figure 6.1 shows a planar LEC with

a gap size of 2.07 mm. Instead of applying a voltage bias between the planar driving electrodes,

however, we placed two biased probes between the planar electrodes in direct contact with the

active layer. As a result, the biased probes served as the driving electrodes, and we observed p-

and n-doping from the probes as shown in the top-right image. In addition, we could observe

strong doping from the top and bottom edges of the planar electrodes, in four places. When

70

the various doping fronts had made contact, five light-emitting p-n junctions are formed, as

shown in the lower right image of Figure 6.1. The bottom of Figure 6.1 shows the equivalent

circuit diagram of the cell under operation. These interesting observations can, and can only

be explained by the fact that the floating top and bottom planar electrodes had functioned as

bipolar electrodes. Each of the planar BPE indeed supported both p- and n-doping reactions.

Both reactions, however, occurred on the same flat edges indicating a varying potential along

the horizontal direction.

+−

10V

Figure 6.1: Top: Fluorescence time-lapse imaging of direct probing of the LEC film between twodeposited metal electrodes. Top Left: 0 sec; Top Right: 35 sec; Bottom Left: 75 sec; Bottom Right:325 sec. Device was tested at 390 K with a bias of 10 V . Bottom: The equivalent circuit of thedevice at 325 sec. Black blocks represent the deposited electrodes.

71

Figure 6.2: Comparison of doping from plain LEC film probing under two sets of testing conditions.Left: Probing was done at 380 K and 5 V . Right: Probing was done at 340 K and 60 V .

Removing the planar electrodes, we only observed doping from the biased probes. Interestingly,

the doping pattern showed a strong dependence on the testing conditions. Low voltage bias

(5V) at the high test temperature (380K) led to very compact and uniform p-doping but

finger like n-doping, as shown in Figure 6.2 (left). High voltage bias (60V) at the low test

temperature (340K) led to a more compact n-doping and the observation of branching on p-

doping. We currently do not have an explanation for the observed doping shape. Modeling

of the doping process as coupled ionic/electronic interactions could provide an answer to the

interesting observations.

6.3.2 Probing LEC Films with Dispersed ITO Particles

Next, we introduced ITO macro-particles to the LEC film to create many tiny light-emitting

p-n junctions between the driving electrodes. The resulting bulk homojunction ITO-LEC had

much larger emission area, stronger light emission, and stable operation compared to the devices

in previous chapters. However, these devices did require a large voltage to operate in order to

form multiple p-n junctions in series. The devices would not turn on unless the applied voltage

was more than 300 V . Some of the probing experiment results are presented here. In Figure

6.3 a series of time-lapse photos of the operation of an ITO-LEC at 320 K under a 300 V bias

is shown:

72

Figure 6.3: Fluorescence time-lapse photos of probing of ITO-LEC. From left to right: 5 sec; 30 sec;175 sec; 320 sec. Two light-emitting junctions were formed, one between p- and n-doped regions andanother one around the cathode probe. Tested at 320 K and 300 V .

Devices were also probed at higher voltages. In Figure 6.4 a series of time-lapse photos of the

operation of an ITO-LEC at 320 K under a 400 V bias is shown:

Figure 6.4: Fluorescence time-lapse photos of probing of ITO-LEC. From left to right: 5 sec; 15 sec;25 sec; 60 sec. Two light-emitting junctions were formed, one between p- and n-doped regions andanother one around the cathode probe. Tested at 320 K and 400 V .

Notice that under a higher voltage bias, the junction formation becomes much faster and the

light emission zone is enlarged as well. The current versus time plots for both experiments

above are shown below in Figure 6.5:

73

Figure 6.5: Left: Current between the probes as a function of time after the application of 300 V bias.Right: Current between the probes as a function of time after the application of 400 V bias.

In order to maximize the current density in the doping experiments, the n-doping probe was

bent vertically, the following series of pictures in Figure 6.6 was taken. The device was tested

at 320 K under a 500 V bias.

Figure 6.6: Fluorescence time-lapse photos of probing of ITO-LEC with bottom probe vertically bent.From left to right: 5 sec; 10 sec; 45 sec; 185 sec. Two light-emitting junctions were formed, onebetween p- and n-doped regions and another one around the cathode probe. The black shadow fromthe bottom right corner was due to the blocking of the probe arm. Tested at 320 K and 500 V .

The ITO-LEC film was examined under the optical microscope after testing. A couple of images

of the scratches made by the probes were taken. In those images, one pixel is roughly equivalent

to 1.9 µm. The images obtained were then inspected using MATLAB to estimate the area of

all the scratches.

74

Table 6.1: Summary of Scratch Shapes and Corresponding Areas in Pixels.

Scratch shape Scratch area (pixels)

Circular1 5154.27

Circular2 4778.36

Elliptical1 20959.20

Elliptical2 20611.09

The current profiles for two probings at 400 V and 500 V are show in Figure 6.7. From graphs,

0.5 mA was taken as the peak current since it could be achieved in both tests.

Figure 6.7: Left: Current between the probes as a function of time after the application of 400 V bias.Right: Current between the probes as a function of time after the application of 500 V bias.

Thus, taking 0.5 mA as the operating current and choosing the maximum elliptical area to

calculate current density for the worst case scenario under this setup, the current density was:

J =I

A=

0.5× 10−3

20959.20× (1.9× 10−6)2∼= 6608.27 A/m2 (6.1)

The tested ITO-LEC film was also examined using SEM (Scanning Electron Microscope). The

following pictures were taken:

75

Figure 6.8: Top: SEM image of the ITO-LEC film surface at one location. Bottom: SEM image ofthe ITO-LEC film surface at another location. White objects in the images are ITO macro-particles.The scale-bar and SEM scan details are listed in the bottom left corners.

From the images above, ITO macro-particles have a size ranging from 10 to 30 µm. When the

probe electrodes are positioned 2 mm apart, the voltage difference across an ITO particle of 10

µm under a bias of 400 V is:

Vd =10× 10−6

2÷ 1000× 400 = 2 V (6.2)

This voltage was almost sufficient to induce doping from the ITO macro-particle BPEs with a

76

size of 10 µm.

6.4 Discussion

From all the results shown above, the addition of ITO particles to the LEC solution has proven

to be an effective approach to enlarge the emission zone. High overall current density, strong

emission from the junction region as well as the region around the cathode probe could be

achieved. Compared to previous single BPE and BPE-array devices, ITO-BPE device has

stronger EL and much larger emission zone. It is interesting to notice that the circular emission

zone around the cathode probe. This zone is formed by multiple p-n light-emitting junctions

from the ITO macro-particle BPEs. The p-n junction which should exist between the bulk p-

and n-doped regions persists alongside this new circular emission zone. It is verified in Hu’s

study [83] that the conductivity in the p-doped region is much higher than the conductivity in

the n-doped region. It is believed that most of the potential drop occurs inside the n-doped

region, especially the region around the negative probe. This leads to a very strong field around

that particular probe when the contact is made. Therefore, a lot more junctions are turned on

in the region around the negative probe.

77

Chapter 7

Conclusion and Future Work

7.1 Conclusion

Previous studies have utilized bipolar electrodes to increase the effective light emission area in

LECs but the underlying operational mechanisms, particularly bipolar electrochemistry, were

never truly realized. In this thesis, bipolar electrodes in a solid state light-emitting electrochem-

ical cell were thoroughly examined. Different geometries and sizes of BPEs, voltage differences

across BPE extremities and doping propagation speeds of doped regions originated from driv-

ing electrodes/BPEs were all carefully studied. It was discovered that a voltage difference of

more than 2.17 V across extremities of BPE is required to overcome the over-potentials of

electrochemical reactions for the doping to start. This voltage difference could be reached by

offering a higher voltage bias across driving electrodes, having BPEs with larger diameters, or

a combination of both. As for the doping propagation speeds, in a planar LEC cell both p-

and n-doping displayed linear growth before 120 seconds after the initial occurrence of doping.

p-doping appeared earlier and grew faster at 0.0107 mm/s than n-doping’s 0.0042 mm/s. In

an extremely large planar LEC with three BPEs of different diameters, BPEs that had large

enough voltage differences across their extremities all induced doping eventually. Moreover,

p-doping from the center medium-sized BPE in this planar cell showed exponential growth

from 60 to 130 seconds after the start of doping from driving electrodes. In the array-BPE

LEC, all the BPEs in the array had p-doping propagation speeds within a range from 0.024 to

78

0.028 mm/s before 20 seconds, regardless of their locations in the cell. Probing experiments on

LEC active films suggested a strong dependence of doping patterns on testing conditions. An

optimal combination of temperature and voltage could potentially attain both uniform p- and

n-doping fronts. In ITO-LECs, under a high voltage bias (> 300 V ), a high current density

of 6608.27 A/m2 and strong EL was achieved from the device operation. ITO macro-particles

acted as small BPEs, and multiple light-emitting p-n junctions were formed in this process. In

experiments, a ring light-emitting zone was formed around n-doping probe in addition to the

conventional p-n junction formed between two bulk doped regions.

7.2 Future Work

This study of bipolar electrochemical effects on device operation provides a pathway to man-

ufacturing more efficient LEC devices with larger emission zones. It introduces bipolar elec-

trochemistry and constructs the theoretical framework to explain the operation of a solid state

LEC device. However, there are still many more improvements that could be made to this study.

Particularly, modeling of the operational BPE-LEC device would help solidify this framework.

The relationship between driving voltages and doping propagation could be further explored

through the modeling process as well. Modeling of the doping process as a coupled ionic/-

electronic process will potentially provide an insight of understanding the doping patterns and

their temperature/voltage dependence. Similar studies could also be conducted with different

conjugated polymer and salt combinations instead of MEH-PPV and KTf to search for the

most effective device that works optimally with BPEs.

79

Bibliography

[1] Round, Henry J., “A Note on Carborundum”, Electrical World, v. 19, p. 309, Feb 1907.

[2] Lossev, O.V., “Wireless Telegraphy and Telephony”, Telegrafia i Telefonia bez provodor,

no. 18, p. 61, 1923 and no. 26, p. 403, 1924.

[3] Lossev, O.V., “Luminous Carborundum Detector and Detection Effect and Oscillations

with Crystals”, Philosophical Magazine, v. 6, no. 39, p. 1024-1044, 1928.

[4] Georges Destriau, “Recherches sur les scintillations des sulfures de zinc aux rayons”, Jour-

nal de Chemie Physique, v. 33, p. 587-625, 1936.

[5] Bernanose, A.; Comte, M.; Vouaux, P., “A new method of light emission by certain organic

compounds”, J. Chim. Phys., v. 50, p. 64, 1953.

[6] Bernanose, A., “The mechanism of organic electroluminescence”, J. Chim. Phys., v. 52, p.

396, 1955.

[7] Kallmann, H,; Pope, M., “Positive Hole Injection into Organic Crystals”, The Journal of

Chemical Physics, v. 32, p. 300, 1960.

[8] Pope, M.; Kallmann, H. P.; Magnante, P., “Electroluminescence in Organic Crystals”, The

Journal of Chemical Physics, v. 38 (8), p. 2042, 1963.

[9] Sano, Mizuka; Pope, Martin; Kallmann, Hartmut., “Electroluminescence and Band Gap

in Anthracene”, The Journal of Chemical Physics, v. 43 (8), p. 2920, 1965.

[10] Helfrich W., Schneider W. G., “Recombination radiation in anthracene crystals”, Physical

Review Letters, v. 14 (7), p. 229-231, 1965.

80

[11] Hartman W. A., Armstrong H. L., “Electroluminescence in Organic Polymers”, Journal

of Applied Physics, v. 38 (5), p. 2393, 1967.

[12] Partridge, R., “Electroluminescence from polyvinylcarbazole films: 1. Carbazole cations”,

Polymer, v. 24 (6), p. 733-738, 1983.

[13] Partridge, R., “Electro-Luminescence from polyvinylcarbazole films: 2. Polyvinylcarbazole

films containing antimony pentachloride”, Polymer, v. 24 (6), p. 739-747, 1983.

[14] Partridge, R., “Electro-Luminescence from polyvinylcarbazole films: 3. Electroluminescent

Devices”, Polymer, v. 24 (6), p.748-754, 1983.

[15] Partridge, R., “Electroluminescence from polyvinylcarbazole films: 4. Electroluminescence

using higher work function cathodes”, Polymer. v. 24 (6), p. 748-754, 1983.

[16] Tang, C. W.; Vanslykem S. A., “Organic electroluminescent diodes”, Applied Physics

Letters, v. 51 (12), p. 913, 1987.

[17] Burroughes, J. H.; Bradley D. D. C.; Brown A. R.; Marks R. N.; Mackay K.; Friend R.

H.; Burns P. L.; Holmes A. B., “Light-emitting-diodes based on conjugated polymers”,

Nature, v. 347 (6293), p. 539-541, 1990.

[18] Sachdev V. K.; Kumar A. Singh; Kumar S.; Mehra R.M., “Electrically conducting poly-

mers: An overview”, Solid State Phenomena, v. 55, p. 104-109, 1997.

[19] Pei Q. B.; Yu G.; Zhang C.; Yang Y.; Heeger A. J., “Polymer light-emitting

electrochemical-cells”, Science, v. 269 (5227), p. 1086-1088, 1996.

[20] Gao J.; Li Y.; Yu G.; Heeger A. J., “Polymer light-emitting electrochemical cells with

frozen junctions”, Journal of Applied Physics, v. 86 (8), p. 4594-4599, 1999.

[21] Costa Ruben; Ortı Enrique; Bolink Henk; Monti Filippo; Accorsi Gianluca; Armaroli

Nicola, “Luminescent Ionic Transition-Metal Complexes for Light-Emitting Electrochem-

ical Cells”, Angewandte Chemie, v. 51 (33), p. 8178-8211, 2012.

81

[22] Kwon Yiseul ; Choe Youngson, “Electroluminescent Properties of LECs Based on Ionic

Transition Metal Complexes Using Tetrazole-Based Ancillary Ligand”, Journal of Solution

Chemistry, v. 43 (9), p. 1710-1721, 2014.

[23] Strobl, Gert R., The Physics of Polymers: Concepts for Understanding Their Structures

and Behavior, Springer-Verlag Berlin Heidelberg, p. 287-312, 2007.

[24] Friend R. H.; Greenham N. C., Handbook of Conducting Polymers, Marcel Dekker, p. 830,

1998.

[25] Fowler Michael, “Electrons in One Dimension: the Peierls Transition”, University of Vir-

ginia, Feb 2007.

[26] Salaneck W. R.; Friend R. H.; Bredas J. L., “Electronic structure of conjugated polymers:

consequences of electron-lattice coupling”, Phys. Rev. Lett., v. 39 (17), p. 1098-1101, 1977.

[27] Baughman R. H.; Chance R. R., “Point defects in fully conjugated polymers”, J. Appl.

Phys., v. 47, p. 4295, 1976.

[28] Heun S.; Mahrt R. F.; Greiner A.; Lemmer U.; Bassler H.; Halliday D. A.; Bradley D.

D. C.; Burn P. L.; Holmes A. B., “Conformational effects in poly(p-phenylene vinylene)s

revealed by low-temperature site-selective fluorescence”, J. Phys. Condens. Matter, v. 5

(2), p. 247-260, Jan 1993.

[29] Bounos Giannis; Ghosh Subhadip; Lee Albert K.; Plunkett Kyle N.; Dubay Kateri H.;

Zhang Joshua C. Rui; Friesner Richard A.; Nuckolls Colin; Reichman David R.; Barbara

Paul F., “Controlling Chain Conformation in Conjugated Polymers Using Defect Inclusion

Strategies”, J. Am. Chem. Soc., v. 133 (26), p. 10155-10160, 2011.

[30] Gagnon D. R.; Capistran J. D.; Karasz F. E.; Lenz R. W.; Antoun S., “Synthesis, Doping,

and Electrical-conductivity of High-Molecular-Weight Poly(Para-Phenylene Vinylene)”,

Polymer, v. 28 (4), p. 567-573, 1987.

[31] Shankar, R., “Chapter 14-Spin”, Principles of Quantum Mechanics (2nd ed.), 1994.

82

[32] Chen Fang-Chung; Yang Yang; Pei Qibing, “Phosphorescent light-emitting electrochemical

cell”, appl. phys. lett., v. 81 (22), p. 4278, 2002.

[33] Adachi Chihaya; Baldo Marc; Thompson Mark; Forrest Stephen, “Nearly 100% internal

phosphorescence efficiency in an organic light-emitting device”, Journal of Applied Physics,

v. 90 (10), p. 5048, 2001.

[34] Friend R. H.; Gymer R. W.; Holmes A. B.; Burroughes J. H.; Marks R. N.; Taliani C.;

Bradley D. D. C.; Lo M.; Salaneck W. R.; Santos A. D.; Bre J. L., Electroluminescence in

conjugated polymers, p. 121-128, 1999.

[35] Gispert J. R., Coordination Chemistry, Wiley-VCH, p. 483, 2008.

[36] Cornil J.; Dos Santos D. A.; Crispin X.; Silbey R.; Bredas J. L., “Influence of Interchain

Interactions on the Absorption and Luminescence of Conjugated Oligomers and Polymers:

A Quantum-Chemical Characterization”, J. Am. Chem. Soc., v. 120 (6), p. 1289-1299, Feb

1998.

[37] Vamvounis George, “Synthesis, Photo- and Electro- Optical Properties of Luminescent

π-Conjugated Polymers”, Simon Fraser University, 2004.

[38] Liu Jiang, “Light-Emitting Electrochemical Transistors”, Linkoping University, 2014.

[39] Garca-Coln L. S.; del Castillo L. F.; Goldstein Patricia, “Theoretical basis for the Vogel-

Fulcher-Tammann equation”, Phys. Rev. B, v. 40, p. 7040, Oct 1989.

[40] Press Release, “Development of a Superionic Conductor Exhibiting the Highest Known

Lithium-Ion Conductivity”, High Energy Acceleration Research Organization, KEK, Dec

2011.

[41] Gray F. M., “Polymer Electrolytes”, The Royal Society of Chemistry, Cambridge Ltd.,

UK, 1997.

[42] Bruce P. G.; Vincent C. A., “Polymer Electrolytes”, J. Chem. Soc., v. 89 (17), p. 3187-

3203, 1993.

83

[43] Webb Michael A.; Savoie Brett M.; Wang Zhen-Gang; Miller Thomas F., “Chemically

Specific Dynamic Bond Percolation Model for Ion Transport in Polymer Electrolytes”,

Macromolecules, v. 48 (19), p. 7346-7358, 2015.

[44] Kar Pradip, Doping in Conducting Polymers: Polymer Science and Plastics Engineering,

John Wiley & Sons, Aug 2013.

[45] Dai Liming, Intelligent Macromolecules for Smart Devices: From Materials Synthesis to

Device Applications, Springer London, p. 41-80, 2004.

[46] MacDiarmid A. G.; Mammone R. J.; Kaner R. B.; Porter S. J., “The concept of ‘doping’

of conducting polymers: the role of reduction potentials”, Phil. Trans. R. Soc. Lond., v.

314, p. 3-15, 1985.

[47] Heeger Alan; MacDiarmid Alan; Shirakawa Hideki, “Popular Information: Information for

the Public”, The Nobel Prize in Chemistry 2000, Oct 2000.

[48] Holt L.; Leger J. M.; Carter S., “Electrochemical and optical characterization of p- and

n-doped polu[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]”, J. Chem. Phys., v.

123 (4), p. 044704, Jul 2005.

[49] Loget Gabriel; Kuhn Alexander, “Shaping and exploring the micro- and nanoworld using

bipolar electrochemistry”, Anal Bioanal Chem, v. 400, p. 1691-1704, Mar 2011.

[50] Ramakrishnan S.; Shannon C., “Display of solid-state materials using bipolar electrochem-

istry”, Langmuir, v. 26, p. 4602-4606, 2010.

[51] Ulrich C.; Andersson O.; Nyholm L.; Bjorefors F., “Potential and current density dis-

tribution at electrodes intended for bipolar patterning”, Anal Chem, v. 81, p. 453-459,

2009.

[52] Bradley J. C.; Crawford J.; Ernazariva K.; McGee M.; Stephens S. G.; “Wire formation

on circuit boards using spatially coupled bipolar electrochemistry”, Adv Mater, v. 9, p.

1168-1171, 1997.

[53] Lide D. R., CRC Handbook of Chemistry and Physics, CRC Press, p. 12-114, 2008.

84

[54] Liedenbaum C.; Croonen Y.; Weijer P.; Vleggaar J.; Schoo H., “Low voltage operation of

large area polymer LEDs”, Synth. Met., v. 91 (1-3), p. 109-111, Dec 1997.

[55] Inayeh A., “Scanning Photocurrent and Photoluminescence Imaging of Frozen Polymer

Light Emitting Electrochemical Cells”, Queen’s University, 2013.

[56] Sun Qingjiang; Li Yongfang; Pei Qibing, “Polymer Light-Emitting Electrochemical Cells

for High-Efficiency Low-Voltage Electroluminescent Devices”, JOURNAL OF DISPLAY

TECHNOLOGY, v. 3 (2), p. 211-224, Jun 2007.

[57] Ouisse T.; Stephan O.; Armand M.; Lepretre J. C.; “Double-layer formation in organic

light-emitting electrochemical cells”, J. Appl. Phys., v. 92 (5), p. 2795, 2002.

[58] Pingree L. S. C.; Rodovsky D. B.; Coffey D. C.; Bartholomew G. P.; Ginger D. S., “Scan-

ning Kelvin Probe Imaging of the Potential Profiles in Fixed and Dynamic Planar LECs”,

J. Am. Chem. Soc., v. 129 (51), p. 1590315910, Dec 2007.

[59] Asadpoordarvish Amir, “The operation of a light-emitting electrochemical cell”, UMEA

University, Oct 2015.

[60] Nishikitani Y.; Takizawa D.; Nishide H.; Uchida S.; Nishimura S., “White Polymer Light-

Emitting Electrochemical Cells Fabricated Using Energy Donor and Acceptor Fluorescent

-Conjugated Polymers Based on Concepts of Band-Structure Engineering”, J. Phys. Chem.

C, v. 119 (52), p. 28701-28710, 2015.

[61] Tasch S.; Gao J.; Wenzl F. P.; Holzer L.; Leising G.; Heeger A. J.; Scherf U.; Mullen

K., “Blue and Green Light-Emitting Electrochemical Cells with Microsecond Response

Times”, Electrochemical and Solid-State Letters, v. 2 (6), p. 303-305, 1999.

[62] Pei Q.; Yu G.; Zhang C.; Heeger A. J., “Polymer Light-Emitting Electrochemical Cells:

In Situ Formation of a Light-Emitting p-n Junction”, J. Am. Chem. Soc., v. 118 (16), p.

3922-3929, 1996.

[63] deMello J. C.; Tessler N.; Graham S. C.; Friend R. H., “Ionic space-charge effects in

polymer light-emitting diodes”, Phys. Rev. B, v.57, p. 12951-12963, May 1998.

85

[64] Reenen S.; Matyba P.; Dzwilewski A.; Janssen R. A. J.; Edman L.; Kemerink M., “A

Unifying Model for the Operation of Light-Emitting Electrochemical Cells”, J. Am. Chem.

Soc., v. 132 (39), p. 13776-13781, Sep 2010.

[65] Leger J. M.; Carter S. A.; Ruhstaller B., “Recombination profiles in poly[2-methoxy-5-(2-

ethylhexyloxy)-1,4-phenylenevinylene] light-emitting electrochemical cells”, J. Appl. Phys.,

v. 98, p. 124907, 2005.

[66] Gao Jun; Dane Justin, “ Planar polymer light-emitting electrochemical cells with extremely

large interelectrode spacing”, Appl. Phys. Lett., v. 83, p. 3027, 2003.

[67] Gao Jun; Dane Justin, “Imaging the doping and electroluminescence in extremely large

planar polymer light-emitting electrochemical cells”, Appl. Phys. Lett., v. 98, p. 063513,

2005.

[68] Dane, J.; Gao J., “Imaging the degradation of polymer light-emitting devices”, Appl. Phys.

Lett., v. 85 (17), 2004.

[69] Hu Yufeng; Tracy Corey; Gao Jun, “High-resolution imaging of electrochemical doping

and dedoping processes in luminescent conjugated polymers”, Appl. Phys. Lett., v. 88, p.

123507, 2006.

[70] Tracy Corey; Gao Jun, “Polymer bulk homojunction light-emitting electrochemical cells”,

J. Appl. Phys., v. 100, p. 104503, 2006.

[71] Bonnet Wayne; Tracy Corey; Wantz Guillaume; Liu Guojun; Gao Jun, “Bulk Electrolumi-

nescence from Conjugated Polymer Thin Films via the Formation of Gold Nanoislands”,

Small, v. 4 (10), p. 1707-1710, Oct 2008.

[72] Li Yongfang; Gao Jun; Yu Gang; Cao Yong; Heeger Alan J., “ac impedance of polymer

light-emitting electrochemical cells and light-emitting diodes: a comparative study”, Chem.

Phys. Lett., v. 287 (1-2), p. 83-88, Apr 1998.

[73] Tracy Corey; Gao Jun, “Polymer bulk homojunction photonic devices”, Appl. Phys. Lett.,

v. 87, p. 143502, 2005.

86

[74] Li Y.; Cao Y.; Gao J.; Wang D.; Yu G.; Heeger A. J., “Electrochemical properties of

luminescent polymers and polymer light-emitting electrochemical cells”, Synthetic Metals,

v. 99 (3), p. 243-248, Feb 1999.

[75] Ishii Yuya; Murata Hideyuki, “True photoluminescence spectra revealed in electrospun

light-emitting single nanofibers”, J. Mater. Chem., v. 22, p. 4695-4703, 2012.

[76] Li X.; Gao J.; Liu G., “Reversible luminance decay in polymer light-emitting electrochem-

ical cells”, Appl. Phys. Lett., v. 102 (22), p. 223303, 2013.

[77] “Sapphire Products: Properties and Benefits”, Saint-Gobain, 2014.

[78] “Piranha Solution Information & Safety:Ortiz Lab 03/04”, Massachusetts Institute of

Technology, 2016.

[79] Chen Shulun; Wantz Guillaume; Bouffier Laurent; Gao Jun, “Solid-State Bipolar Electro-

chemistry: Polymer-Based Light-Emitting Electrochemical Cells”, Chem. Electro. Chem.,

v. 3 (3), Mar 2016.

[80] Loget G.; Kuhn A., “Electric field-induced chemical locomotion of conducting objects”,

Nat. Commun., v. 2, p. 535, 2011.

[81] Loget G.; Kuhn A., “Propulsion of Microobjects by Dynamic Bipolar Self-Regeneration”,

J. Am. Chem. Soc., v. 132 (45), p. 15918-15919, 2010.

[82] Wantz G.; Gautier B.; Dumur F.; Phan T.N.T.; Gigmes D.; Hirsch L.; Gao J., “Towards

frozen organic PN junctions at room temperature using high-Tg polymeric electrolytes”,

Organic Electronics, v. 13 (10), p. 1859-1864, Oct 2012.

[83] Hu Yufeng; Jun Gao, “Direct Imaging and Probing of the pn Junction in a Planar Polymer

Light-Emitting Electrochemical Cell”, J. Am. Chem. Soc., v. 133 (7), p. 2227-2231, Jan

2011.

87

Appendix A

The following section includes all the MATLAB codes written to perform the image analysis

tasks. They are organized based on chapters. The version of MATLAB used is R2013a. The

codes are presented in an example case format and the code itself could be generalized to

analyze other images by simply changing the image file-name within the first couple lines of

codes. Detailed comments are also provided in-line.

The following codes were written to identify the boundaries and calculate the doping propaga-

tion speeds in Chapter 3. Script was implemented on ‘Img030.jpg’:

1 close all; clear all; imtool close all; clc;

2 % filename here

3 str = 'Img030';

4 img = imread(strcat(str, '.jpg'));

5 imwrite(img, strcat(str, '.tif'));

6 % find the conversion ratio from pixel to mm

7 m1 = 1445.53; m2 = 1440.05; m3 = 1438.40; m4 = 1438.36;

8 conv = 15/((m1 + m2 + m3 + m4)/4);

9 fprintf('Conversion ratio: %.5f mm/pixel\n', conv);

10 img1 = imread(strcat(str, '.tif'));

11 img2 = imrotate(img1,-2.2,'bilinear');

12 imwrite(img2, strcat(str, ' rotate.tif'));

13 %imtool(img2)

14 % window size

15 width = 290;

16 height = 290;

88

17 % starting position of capture window

18 start row = 615;

19 start col = 840;

20 end row = start row + height;

21 end col = start col + width;

22 rows = [start row, end row];

23 cols = [start col, end col];

24 img3 = imread(strcat(str, ' rotate.tif'),'PixelRegion',rows,cols);

25 img4 = rgb2gray(img3);

26 imtool(img4)

27 % gsVal (grayscale value) range classification

28 % gsVal < 12: electrode

29 % 12 <= gsVal < 80: p-doped region

30 % 80 <= gsval < 100: n-doped region

31 % identify edges and convert the result images to binary

32 BW = edge(img4,'Canny');

33 % remove objects that are less than 120 pixels, size is changed based on

34 % the stage of the picture to remove discontinuous objects

35 BW1 = bwareaopen(BW,120);

36 imtool(BW1)

37 % find size of the img4 matrix

38 [row,col] = size(img4);

39 % storage matrices for boundaries, assume 4 lines in total

40 boundary = zeros(4,col);

41 %number of lines in BW1

42 numOfLines = 4;

43 for j = 1:col

44 k = 1; % indicator of boundary line #

45 for i = 1:row

46 if BW1(i, j) == 1 && k <= numOfLines

47 boundary(k,j) = i;

48 k = k + 1;

49 end

50 end

51 end

89

52 % find size of new boundary matrix

53 [row1,col1] = size(boundary);

54 % reshape boundary matrix into a 1D array

55 boundary1D = reshape(boundary,[1,row1*col1]);

56 boundaryNew = sort(boundary1D);

57 % remove zero cols

58 boundaryNew(:,~any(boundaryNew,1)) = [];

59 % use k-means clustering on the 1D array to find y-coordinates of lines

60 [idx,ctrs] = kmeans(boundaryNew,numOfLines,'Replicates',5);

61 % print out the averaged y-coordinate of each boundary line

62 fprintf('Boundary y-ccordinate: %.2f pixel\n', sort(ctrs));

The following codes were written to identify and calculate positions of electrodes in a starting

image of a series of time-lapse photos of the doping process from exp1 in Chapter 5. Script

was implemented on ‘Img017.jpg’:

1 close all; clear; imtool close all; clc;

2 % read the picture file and rewrite to tif for resolution

3 img = imread('Img017.jpg');

4 imwrite(img, 'Img017.tif');


6 m1 = 2575.05; m2 = 2570.55; m3 = 2571.58; m4 = 2576.29;

7 conv = 15/((m1 + m2 + m3 + m4)/4);


9 % number of metal dots

10 numOfDots = 8;

11 % set up window size and interval between dots

12 col int = 200;

13 row int = 220;

14 space = 1.2/conv - 4; % 1.2 mm between the center of each metal dot

15 % setup initial postion of window

16 col0 = 2684;

17 row0 = 645;

18 % set up threshold

90

19 threshold intensity = 110;

20 threshold size = 500;

21 % print out results

22 fprintf('Electrode Size(mm) Confirm(mm) Point1 Point2\n');

23 % save to txt file

24 fileID = fopen('Img017 Electrode Tip Position Measurement.txt', 'w');

25 fprintf(fileID, '%18s %11s %6s %6s\r\n', 'Electrode Size(mm)', '

Confirm(mm)', 'Point1', 'Point2');

26 % iteration through each metal dot (here used to point to dot of measurement

)

27 % current metal dot:

28 for iter = 1:numOfDots

29 % image processing and analysis

30 % set up the capturing window for each metal dot

31 start col = col0 + 8*(iter-1);

32 end col = start col + col int;

33 start row = row0 + space*(iter-1);

34 end row = start row + row int;

35 rows = [start row end row];

36 cols = [start col end col];

37 % capture the image based on window defined above

38 img1 = imread('Img017.tif','PixelRegion',rows,cols);

39 grayScale = rgb2gray(img1);

40 % obtain binary image from grayscale image through threshold intensity

41 bw = grayScale < threshold intensity;

42 % remove salt and pepper noise to clean up image

43 bw1 = medfilt2(bw);

44 % remove objects on image that are less than the threshold pixel size

45 bw2 = bwareaopen(bw1, threshold size);

46 bw edge = edge(bw2, 'Canny');

47 % produce the plots containing the stages of image processing

48 figure(iter);

49 subplot(2,2,1), hImg1 = subimage(img1);

50 XDataInMM1 = get(hImg1,'XData')*conv;

51 YDataInMM1 = get(hImg1,'YData')*conv;

91

52 set(hImg1,'XData',XDataInMM1,'YData',YDataInMM1);

53 set(gca,'XLim',XDataInMM1,'YLim',YDataInMM1);

54 xlabel(subplot(2,2,1), 'x (mm)');

55 ylabel(subplot(2,2,1), 'y (mm)');

56 subplot(2,2,2), hImg2 = subimage(grayScale);







63 subplot(2,2,3), hImg3 = subimage(bw2);







70 subplot(2,2,4), hImg4 = subimage(bw edge);







77 % boundary identification

78 B = bwboundaries(bw2,'noholes');

79 % trace the boundary out on the grayscale image

80 figure(numOfDots+iter);

81 imshow(grayScale);

82 hold on;

83 for k = 1:length(B)

84 boundary = Bk;

85 plot(boundary(:,2),boundary(:,1),'g','LineWidth',3);

86 end

92

87 hold off;

88 % calculate the coordinates of top an bottom electrode points

89 [m, n] = size(boundary);

90 maxDistance = 0;

91 maxIndex = zeros(1,2);

92 for i = 1:m

93 for j = i+1:m

94 newMaxDistance = sqrt((boundary(i, 2) - (boundary(j, 2))).ˆ2 +

...

95 (boundary(i, 1) - (boundary(j, 1))).ˆ2);

96 if maxDistance < newMaxDistance

97 maxDistance = newMaxDistance;

98 maxIndex(1,1) = i; maxIndex(1,2) = j;

99 end

100 end

101 end

102 % compute the maxDistance using Matlab function to confirm

103 confirm = max(pdist(boundary))*conv;

104 % report to command window

105 fprintf('%.4f %.4f (%d, %d) (%d, %d)\n', ...

106 maxDistance*conv, confirm, boundary(maxIndex(1,1),:), boundary(

maxIndex(1,2),:));


108 fprintf(fileID,'%.4f %.4f (%d, %d) (%d, %d)\r\n

', ...

109 maxDistance*conv, confirm, boundary(maxIndex(1,1),:), boundary(

maxIndex(1,2),:));

110 end


112 % maxDistance*conv;

113 % maxIndex;

The following codes were written to capture and calculate the doping propagation distance of

each BPE electrode from one of a series of time-lapse photos of the doping process from exp1

in Chapter 5. Script was implemented on ‘Img018.jpg’:

93

1 close all; clear; imtool close all; clc;

2 % analysis of metal dots

3 % data are obtained from ImageMarking2.m

4 electrodeTopPoint = [98 66; 95 69; 94 68; 94 70;91 72; 90 71; 86 69; 86 75];

% in order of [x y]

5 %electrodeBottomPoint = [109 116]; % in order of [x y]

6 electrodeSize = [0.2970 0.2987 0.3005 0.3005 0.2901 0.3009 0.3103 0.3131];

7 % read the picture file and rewrite to tif for resolution

8 img = imread('Img018.jpg');

9 imwrite(img, 'Img018.tif');


11 m1 = 2575.05; m2 = 2570.55; m3 = 2571.58; m4 = 2576.29;

12 conv = 15/((m1 + m2 + m3 + m4)/4);


14 % number of metal dots

15 numOfDots = 8;

16 % set up window size and interval between dots

17 col int = 200;

18 row int = 220;

19 space = 1.2/conv - 4; % 1.2 mm between each metal dot

20 % setup initial postion of window

21 col0 = 2684;

22 row0 = 645;

23 % set up threshold

24 threshold intensity = 110;

25 threshold size = 500;

26 % print out results

27 fprintf('P-doping(mm) N-doping(mm)\n');


29 fileID = fopen('Img018 Doping Distance Measurement.txt', 'w');

30 fprintf(fileID, '%12s %12s\r\n', 'P-doping(mm)', 'N-doping(mm)');

31 % iteration through each metal dot (here used to point to dot of measurement

)

32 % current metal dot:

33 for iter = 1:numOfDots-1

94

34 % set up the capturing window for each metal dot

35 start col = col0 + 8*(iter-1);

36 end col = start col + col int;

37 start row = row0 + space*(iter-1);

38 end row = start row + row int;

39 rows = [start row end row];

40 cols = [start col end col];

41 % capture the image based on window defined above

42 img1 = imread('Img018.tif','PixelRegion',rows,cols);

43 grayScale = rgb2gray(img1);

44 % obtain binary image from grayscale image through threshold intensity

45 bw = grayScale < threshold intensity;

46 % remove salt and pepper noise to clean up image

47 bw1 = medfilt2(bw);

48 % remove objects on image that are less than the threshold pixel size

49 bw2 = bwareaopen(bw1, threshold size);

50 bw edge = edge(bw2, 'Canny');

51 % boundary identification

52 B = bwboundaries(bw2,'noholes');

53 % trace the boundary out on the grayscale image

54 figure(iter);

55 imshow(grayScale);

56 hold on;

57 for k = 1:length(B)

58 boundary = Bk;

59 plot(boundary(:,2),boundary(:,1),'g','LineWidth',3);

60 end

61 hold off;

62 % calculate the p-doping propagation distance

63 % calculate the distance between two tips, the total distance

64 MaxDistance = max(pdist(boundary));

65 % calculate the combined distance of electrodeSize and pDistance

66 pMaxDistance = 0;

67 [m, n] = size(boundary);

68 for i = 1:m

95

69 new pMaxDistance = sqrt((electrodeTopPoint(iter,1) - boundary(i,2)).

ˆ2 + ...

70 (electrodeTopPoint(iter,2) - boundary(i,1)).ˆ2);

71 if pMaxDistance < new pMaxDistance

72 pMaxDistance = new pMaxDistance;

73 pMaxIndex = i;

74 end

75 end

76 pDistance = pMaxDistance*conv - electrodeSize(1, iter);

77 nDistance = (MaxDistance - pMaxDistance)*conv;


79 fprintf('%.4f %.4f\n', pDistance, nDistance);


81 fprintf(fileID,'%.4f %.4f\r\n', pDistance, nDistance);

82 end

96

Appendix B

In the following pages the mechanical drawings with detailed dimensions of three shadow masks

are included.

97

PROJECT TITLE

1

REVISIONSREMARKSMM/DD/YY

2345

Shadow_Mask_Mod1 001

A

_ _ /_ _ /_ _ ..._ _ /_ _ /_ _ ..._ _ /_ _ /_ _ ..._ _ /_ _ /_ _ ..._ _ /_ _ /_ _ ...

18.3 mm

18.3

mm

2.1

mm

1.2 mm 1.2 mm 1.2

mm

4.0 mm

7.9 mm 4.0 mm

5.3

mm

12.5 mm

1.1

mm

1.9

mm

14.1 mm

15.9 mm

15.9 mm

12.7 mm

1.8 mm1.8 mm 2.1 mm

3.9

mm

4.8 mm

0.13 mm

0.3 mm

1 mm

PROJECT TITLE

1


2345


A

_ _ /_ _ /_ _ ..._ _ /_ _ /_ _ ..._ _ /_ _ /_ _ ..._ _ /_ _ /_ _ ..._ _ /_ _ /_ _ ...

18.3

mm

18.3 mm

2.1

mm

3.9

mm

4.9

mm

6.2 mm

1.2

mm

1.2 mm 1.2 mm

1.2

mm

2.5 mm

2.5

mm

5.0 mm

6.7

mm

1.9

mm

1.1

mm

17.5 mm

15.9 mm

14.1 mm 2.1 mm

1.9

mm

15.9 mm12.6 mm

0.3 mm14.7 mm 1.8 mm

1.1

mm

0.3 mm

PROJECT TITLE

1


2345


A

_ _ /_ _ /_ _ ..._ _ /_ _ /_ _ ..._ _ /_ _ /_ _ ..._ _ /_ _ /_ _ ..._ _ /_ _ /_ _ ...

18.0

mm

18.0 mm2.8 mm 12.4 mm

2.0

mm

5.4

mm

2.4

mm

3.1

mm2.0

mm

2.4

mm

2.6 mm

1.3 mm

1.3

mm1.3 mm

10.3 mm

6.4

mm

10.3 mm 1.9

mm

1.1

mm

17.0 mm15.4 mm

2.8 mm2.8 mm

0.2 mm

0.2 mm

Polymer Light-Emitting Electrochemical Cells with Embedded Bipolar Electrodes

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Transcript of Polymer Light-Emitting Electrochemical Cells with Embedded Bipolar Electrodes