1

SPECTRAL BROADENING OF CONJUGATED POLYMERS IN ELECTROCHROMIC DEVICES

BY

BRENDA M. CALDERON

A THESIS PRESENTED TO THE CHEMISTRY DEPARTMENT IN THE COLLEGE OF LIBERAL ARTS AND

SCIENCES AT THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR GRADUATION WITH HONORS.

UNIVERSITY OF FLORIDA

2011

2

Abstract

This thesis reports the spectral broadening of conjugated polymers through synthetic methods,

subtractive color mixing, and blending of materials. These conjugated polymers exhibit

electrochromic properties, switching from a colored state to a transmissive state. Through

structural modifications of the polymer backbone, the position of the HOMO and LUMO levels

are shifted, resulting in changes in the absorption spectrum and the neutral state colors. The

color mixing of conjugated polymers, incorporated in dual polymer or dual active

electrochromic devices, using a subtractive color method also broadens the spectrum and adds

to the color variety of electrochromic polymers. Blending of materials further shows a similar

effect, while minimizing layers in electrochromic devices.

3

Acknowledgements

First and foremost, I would like to thank Dr. Reynolds for taking me in as an undergraduate

researcher, for his guidance and his support. I would also like to thank Dr. Aubrey Dyer for her

mentorship, patience and advice throughout my entire time with the group. She has trusted my

abilities to carry out many responsibilities and helped guide me throughout this entire process.

I am grateful for the entire Reynolds research group, both past and present members, for being

supportive, answering my numerous questions and creating the best work environment I could

hope for. Special thanks to the electrochromics group: Rayford Bulloch, Suhas Rao, and Matt

Nelson, for their help in gathering or processing some of the data presented in this thesis.

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Table of Contents

Abstract 2

Acknowledgements 3

List of Figures and Tables 5

1 Introduction 8

2 Methods 11

3 Results and Discussion

3.1 ECP Magenta 15

3.2 ECP Black 18

3.3 Stacked Devices 22

3.4 Polymer Blends 25

3.5 ECP Deep Purple 29

4 Conclusions and Future Work 33

References 36

5

List of Figures and Tables

Figure 1. Sample energy band formations during polymerization of a monomer 9

Figure 2. Thiophene trimer showing the allowed electronic transitions for the 9

a) neutral, b) polaron and c) bipolaron states

Figure 3. Chemical structure of MCCP (A), neutral state absorbance (B), and reduced 11

(C, Left) and oxidized (C, Right) films of MCCP

Figure 4. Construction schematic of an electrochromic device 14

Figure 5. Construction schematic of a dual active electrochromic device 15

Figure 6. Chemical structure of ECP Magenta, where R=2-Ethylhexyl 16

Figure 7. Spectroelectrochemistry of a film of ECP Magenta 16

Figure 8. Spectroelectrochemistry of a dual polymer MCCP and ECP Magenta device 17

Figure 9. Chemical structure of ECP Random Black, where R=2-Ethylhexyl 18

Figure 10. Spectroelectrochemistry of a film of ECP Random Black 19

Figure 11. Square-wave potential–step chronoabsorptometry of an ECP Random 20

Black film monitored at 550 nm

Figure 12. Spectroelectrochemistry of a dual polymer ECP Random Black and 21

MCCP device

Figure 13. Square-wave potential-step chronoabsorptometry of an ECP Random 22

Black device monitored at 550 nm

Figure 14. A. Normalized spectra of ECP Magenta and ECP Cyan B. Colored and 23

bleached states of films of ECP Magenta (top) and ECP Cyan (bottom)

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Figure 15. Spectroelectrochemistry of dual active Cyan and Magenta devices 25

Figure 16. Neutral state absorbance spectra for blended materials films (A) and 26

colored/bleached states of films of blended materials (B)

Figure 17. Spectroelectrochemistry of ECP Magenta and ECP Cyan blended devices 27

Figure 18. Spectroelectrochemistry of a 2:1 Magenta to Cyan blended device 28

Figure 19. Square-wave potential-step chronoabsorptometry of a 2:1 Magenta to 29

Cyan blended device monitored at 550 nm

Figure 20. Chemical structure of ECP Deep Purple 29

Figure 21. Cyclic voltammetry results for a film of ECP Deep Purple compared to 30

ECP Magenta

Figure 22. Spectroelectrochemistry of a film of ECP Deep Purple 31

Figure 23. Spectroelectrochemistry of a dual polymer Deep Purple and MCCP device 32

Figure 24. Square-wave potential-step chronoabsorptometry of an ECP Deep Purple 33

device monitored at 550 nm

Figure 25. Normalized absorption spectra for electrochromic polymers spray-cast 34

onto indium tin oxide coated glass slides

Figure 26. L*a*b* color space values for electrochromic polymers in their reduced 35

states (left) and their oxidized states (right)

Table 1. Optical properties of films of ECP Random Black 19

Table 2. Optical properties of dual polymer devices of ECP Random Back and MCCP 21

Table 3. Optical properties of dual active devices of ECP Magenta, ECP Cyan and MCCP 24

Table 4. Optical properties of blended materials devices of ECP Magenta, ECP Cyan 28

7

and MCCP

Table 5. Optical properties of films of ECP Deep Purple 30

Table 6. Optical properties of dual polymer devices of ECP Deep Purple 32

and MCCP

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1. Introduction

Electrochromism is the process of reversible optical change in a material caused by an

applied potential which causes electron uptake or release.1 Electrochromic polymers are of

notable interest for their many applications, these polymers change their electro optic

properties when an electric potential is applied. Because these changes usually occur in the

visible region, they can be manipulated in different applications such as windows and displays.2-

7 One type of electrochromic material has a colored state and a bleached state; materials such

as these can be used in transmissive/absorptive devices. Conjugated polymers are an example

of this type of electrochromic material, offering the benefit of color tuning, high optical contrast

and ease of processability.8

Electrochromic polymers can be deposited onto an electrode substrate and used to

form a working system with the addition of a second electrode and an electrolyte to make up a

cell that will allow the passage of current whereby electrons flow.9 These polymers can undergo

reduction by electron uptake or oxidation by electron release causing a color change from their

neutral state. These polymers can exist as colored or transmissive in their neutral states

depending on whether they are cathodically coloring or anodically coloring polymers,

respectively.10 Electrochromic polymers are characterized using different parameters such as

coloration, optical contrast, switching speed and stability.

The coloration, absorption and emission spectra of conjugated polymers is dictated by

their chemical structures; specifically their band gap (Eg). The band gap, shown in Figure 1 for a

sample thiophene polymer, is the energy gap that exists between the highest occupied

molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In conjugated

9

polymers, electrochromism is the result of changes in the π electronic character accompanied

by an insertion and extraction of ions in the film upon oxidation and reduction.11 Electronic

transitions can occur from the valence band to the conductive band in the neutral state as

shown in Figure 2a. Upon oxidation a radical cation is formed, called the polaron, and allowed

electronic transitions are added that include half-filled polaron levels as shown in Figure 2b.

Upon further oxidation a dication is formed, called the bipolaron. As the bipolaron levels are

unoccupied, only the low energy transition from the valence band is allowed shown in Figure

2c, which gives the polymer its transmissive blue tint in the bleached state. The energy gap

between the π and π* states determines the color of the polymer in the neutral state and the

midgap transitions dictate the color of the polymer in the doped state. This band gap becomes

smaller as conjugation is increased, due to the overlap of π orbitals from the multiple repeat

units.12

Figure 1. Sample energy band formations during polymerization of a monomer13

10

Figure 2. Thiophene trimer showing the allowed electronic transitions for the a) neutral, b)

polaron and c) bipolaron states. 11

Since coloration states and the band gap are directly affected by chemical structure, it

has been shown that color control is possible through modifications of the polymer chain and

conjugation length of polymers.14-17 These structural modifications cause the positions of the

HOMO and LUMO levels, and therefore the Eg, to change leading to color changes in the

neutral and doped forms. This color control can take several routes through synthetic methods

or color mixing. One such route is the synthesis of polymers with high optical density in the

transmissive to bleached state, which has uses in windows applications. Another route of

particular interest is color tuning to add to the palette of possible electrochromic polymers for

use in displays. The tuning of these polymers is directed at broadening the absorption

spectrum.

Electrochromic materials can be used as films for characterization of optical properties

by depositing the polymers on an electrode substrate such as indium doped tin oxide (ITO)

which is transmissive in the visible region. Multiple electrochromic polymers can be combined

within devices to produce absorptive/transmissive materials. For these devices, the counter

electrode is composed of a minimally color changing polymer (MCCP) whose structure is shown

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in Figure 3. This polymer absorbs almost entirely in the UV region, giving it a transmissive color

in its neutral state. This specifically makes use of oxidation and reduction potentials of the

electrochromic polymers to complement one another. Here, color tuning and color mixing

greatly expand the variety of possible color combinations and optical density.

Figure 3. Chemical structure of MCCP (A), neutral state absorbance (B), and reduced (C, Left)

and oxidized (C, Right) films of MCCP

2. Methods

2.1 Solutions

The electronic conducting polymers (ECP) referenced in this thesis were synthesized and

purified by previous group members using commercially available chemical reagents and

solvents. The detailed synthesis and polymerization methods will not be discussed, but can be

found in Penjie Shie’s PhD dissertation.18 The conjugated polymers used were solution

processable and as such were dissolved in the appropriate organic solvents. The solvents used

were as follows: ECP Black was dissolved in dichloromethane, MCCP, ECP Magenta and ECP

Deep Purple were dissolved in toluene, and ECP Cyan was dissolved in a 1:1 volumetric ratio of

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chloroform to toluene. For the blended solutions of magenta and cyan, the ECP Magenta was

dissolved in a 1:1 volumetric ratio of chloroform to toluene and this new solution was mixed

with an ECP Cyan solution at the volumetric ratios of 1:1, 2:1 and 1:2 magenta to cyan.

Electrolyte solutions were prepared for electrochemistry of films and device

construction. For standalone polymer films, the electrolyte used was 0.5 M Lithium

bis(triflouromethane)sulfonamide (LiBTI) dissolved in propylene carbonate (PC). For the

electrochromic devices, a gel electrolyte was prepared using 0.5 M LiBTI dissolved in PC and

with 12% by weight polymethylmethacrylate (PMMA).

2.2 Polymer film deposition

Many methods of film deposition are available aided by the solution processability of

conjugated polymers. These include inkjet printing, screen printing, slot-die coating and spray

casting among other methods. For our purposes, spray casting of the polymer solutions onto

ITO-coated glass was carried out using a commercial airbrush from Iwata. The ITO-coated glass

slides were of varying sizes, all with a resistance of 5-10 Ω /cm2. The thickness of polymer films

was measured in absorbance at the peak wavelength for each particular polymer.

2.3 Characterization methods

The characterization of the optical properties of the electrochromic polymer films and

devices was done using electrochemistry, spectroelectrochemistry, and colorimetry.

Electrochemistry was done to evaluate the electronic properties of the polymers and measured

by cyclic voltammetry, which gives current density in A/cm2 versus applied potential in V. This

provided information about the onset of oxidation and reduction with applied potentials, each

denoted by a peak. In order to perform these measurements, a three electrode cell was used

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with the polymer films acting as the working electrode aided by copper contacts, a platinum

wire as the counter electrode, and a Ag/AgNO3 reference electrode. An electrolyte solution of

0.5 M LiBTI in PC was used. For the electrochromic devices (ECD), a film of the electrochromic

polymer of choice was used as the working electrode, a film of minimally changing color

polymer (MCCP) was used as the counter electrode and the two were sandwiched between a

layer of gel electrolyte made of 0.5 M LiBTI in PC with 12% by weight PMMA.

The spectral characterization of the conjugated polymers was obtained using a Cary UV-

Vis/NIR spectrophotometer from Varian. Spectroelectrochemistry was done to measure the

changes in the absorption spectrum of the conjugated polymers upon application of a potential

to the films to induce oxidation or reduction of the polymers. These measurements were taken

in the UV/visible range and the spectroelectrochemical data was later converted to percent

transmittance to evaluate optical contrast.

Colorimetry measurements were used to quantify the color perception of the

electrochromic polymers and to evaluate changes due to electrochemical switching. These

measurements were obtained from the spectral data output of an Optronics Visible/NIR

instrument and converted into L*, a* and b* values. The L*a*b* color space was created by the

International Commission of Illumination and is one of the most common methods used to

describe color in a quantitative manner.19 In the L*a*b* color space, the L represents lightness,

a* represents red-green (positive values indicate magenta, negative values indicate green), and

b* represents yellow-blue (positive values indicate yellow and negative values indicate blue).

Kinetic measurements were also taken to evaluate the response time using

chronoabsorptometry, which couples a square-wave potential step method with optical

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spectroscopy.20 The response time is considered the time it takes to reach a full optical switch.

The electrochromic polymers were switched from a negative voltage to a positive voltage at

decreasing switch times, while the percent transmittance was being measured with a Cary

UV/Vis spectrophotometer. These measurements evaluated changes in optical contrast,

measured as percent transmittance at a single wavelength, as switching speeds were

decreased.

2.4 Electrochromic device construction.

Electrochromic devices (ECD) were constructed from polymers spray cast onto ITO-

coated glass substrates sized 0.25 by 0.375 cm2. These ECD consisted of two electrochromic

polymer films, one with highly absorptive properties and one with highly transmissive

properties in the visible range. Figure 4 shows a schematic of the basic device construction. All

devices used in the experiments outlined in this thesis were constructed using MCCP as the

counter electrode as this polymer is essentially colorless in the visible region. The two polymer

films had a layer of gel electrolyte in between to allow for the passage of electrons in the

device. The first sets of devices were made using a silicone sealant and later devices were made

using an ADCO polyisobutyl sealant. Copper contacts were added to the electrodes to facilitate

the application of the potentials.

Figure 4. Construction schematic of an electrochromic device.

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The first series of projects including magenta, black and dual active devices were made

on the bench top open to atmospheric conditions. In contrast, the last series of projects which

included blended and deep purple devices were constructed in a glove box under inert and dry

conditions. For the dual active devices, shown in Figure 5, two separate electrochromic devices

were affixed together using double sided tape. The two layers of MCCP had their copper

contacts shorted, as did the two layers of the working electrodes. This was done to restrict

electrochemistry to the use of one Potentiostat, thereby reducing random errors in using two

different instruments.

Figure 5. Construction schematic of a dual active electrochromic device

3. Results and Discussion

3.1 ECP Magenta

In considering broadening the absorption across the visible region, we began with the

conjugated, conducting polymer (ECP) Magenta as a prototype, shown in Figure 6. This

conjugated polymer has electrochromic properties as it switches reversibly from a colored

neutral state, with a maximum absorption at 540 nm, to a bleached state upon application of a

redox potential. ECP Magenta is a polythiophene derivative that is soluble in common organic

solvents due to its alkyloxy chains, allowing processability. A basic characterization of the

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optical properties of the polymer was carried out on spray cast films of the polymer on cuvette

sized ITO 0.07 by 0.5 cm2.

Figure 6. Chemical structure of ECP Magenta, where R=2-Ethylhexyl

Electrochemistry was carried out using cyclic voltammetry to cycle the films from a

potential of -0.4 volts to 0.6 volts vs. Ag/Ag+ reference electrode. The polymer exists in its

neutral state as colored, a bright magenta, and as it is oxidized, becomes fully bleached with a

faint blue tint, making it a cathodically coloring polymer. The changes in the absorption

spectrum of the polymer upon oxidation and reduction were measured in the same voltage

range. The absorption spectroelectrochemical series for ECP Magenta is shown in Figure 7. The

polymer exhibits a large change in its optical properties in the visible range upon increasing

potential.

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Figure 7. Spectroelectrochemistry of a film of ECP Magenta

The conjugated polymer was then incorporated into an electrochromic device (ECD) as

the working electrode paired another electrochromic polymer MCCP. MCCP is a minimally

changing color polymer that absorbs in the UV region in its neutral state and exhibits an

absorption primarily in the near infrared in the oxidized state, allowing it to exist as a nearly

colorless polymer in both oxidation states. Films of these polymers were used in devices as

shown in Figure 4. The films were in a 2:1 ratio of ECP Magenta to MCCP film optical density,

measured by absorbance at the absorption maximum. The cyclic voltammetry of the devices

revealed a larger voltage range tolerated in the devices and this new voltage range of -1.2 volts

to 2.0 volts was used in the spectroelectrochemical series, shown in Figure 8. The absorbance

values at 550 nm were taken and converted to percent transmittance using Beer-Lambert’s law.

The devices achieved high optical contrast with one device having 54.1% transmittance in the

colored state and 0.48% transmittance in the transmissive state.

Figure 8. Spectroelectrochemistry of a dual polymer MCCP and ECP Magenta device

400 450 500 550 600 650 700 7500.0

0.2

0.4

0.6

0.8

1.0

1.2

Absorb

ance (

a.u

.)

Wavelength (nm)

-1.2 V

-0.9 V

-0.6 V

-0.3 V

0 V

0.3 V

0.6 V

0.9 V

1.2 V

1.5 V

1.8 V

2.0 V

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These initial results show ECP Magenta to be useful in absorptive/transmissive devices

as it is capable of achieving a high optical contrast in the visible region. However, the color of

the polymer is not best suited for applications in windows and the absorption spectrum proved

to be narrow with only around 130 nm full width at half max absorption.

The conjugated polymer served as a good model for optical properties and a starting

point for broadening the absorption spectrum of conjugated polymers with electrochromic

properties. Herein, the rest of this thesis discusses the different routes taken to not only

broaden the absorption spectrum, but optimize the optical contrast of the electrochromic

polymers used.

3.2 ECP Black

The synthesis of a polymer with an absorption profile that is broad across the visible

region, ECP Random Black, was carried out by Penjie Shi and the detail of the synthesis can be

found in his PhD dissertation.18 ECP Random Black is a black to transmissive switching

electrochromic polymer, another cathodically coloring polymer, whose structure is shown in

Figure 9. It is composed of donor units, a dioxythiophene, and acceptor units, benzodiathiazole,

incorporated in a random fashion. Due to its electron rich and electron poor moieties it has a

broad absorption across the entire visible spectrum.12 The polymer switches to transmissive

upon oxidation and, as such, is useful in smart windows applications.

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Figure 9. Chemical structure of ECP Random Black, where R=2-Ethylhexyl

Electrochemical studies of films of ECP Random Black spray cast onto cuvette sized ITO,

summarized in Table 1, showed the polymer to change color in a narrow voltage range. The

spectroelectrochemical series is shown in Figure 10, where the electronic transitions from the

valence band to the conducting band are decreasing on oxidation as polaron and bipolaron

absorptions increase on oxidation, seen by absorptions emerging at longer wavelengths. The

arrows indicate the direction of spectral increase or decrease as the film is increasingly oxidized

and the second arrow shows spectral growth for the bipolaron as the film is oxidized.11 The shift

of the absorption peak to the much longer wavelengths is what gives the polymer a highly

transmissive state, as this is beyond the visible spectrum.

Table 1. Optical properties of films of ECP Random Black

Film Dry Film

Absorbance Δ%T %T

Colored %T

Bleached ΔL* L*

Colored L*

Bleached a*, b*

Colored a*, b*

Bleached

1 0.7 42.3 29.7 72 24.1 64.5 88.6 0.8, -8.2 -4.0, -1.3

2 1.3 35.9 15 50.9 40.8 34.3 75.1 2.8, -12.3 -7.4, -5.1

3 1.5 37 4.7 41.7 45.7 25.8 71.5 3.4, -13.4 -8.2, -4.5

4 2 26.5 1.2 27.7 48.1 15.2 63.3 5.3, -9.6 -9.4, -6.1

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Figure 10. Spectroelectrochemistry of a film of ECP Random Black

To further test the optical properties of the electrochromic polymer, kinetic

experiments were carried out to determine switching speed capabilities. For display and

windows applications a quick switching time from the colored to transmissive state is desired,

but films may lose some optical contrast as switching time is decreased. The films were

switched from the reduced state to the oxidized state at decreasing times ranging from 1

minute to 0.5 seconds. From square-wave potential-step chronoabsorptometry results shown

in Figure 11, it is noted that from a 1 minute switch to a 5 second switch between the two

states, there is relatively little loss of contrast.

Figure 11. Square-wave potential–step chronoabsorptometry of a film of ECP Random Black

monitored at 550 nm.

The polymer was then incorporated into an electrochromic device with MCCP as the

counter electrode, the optical properties of which are summarized in Table 2.

Spectroelectrochemistry, shown in Figure 12, was carried out in the expanded voltage range

focusing on the visible spectrum when considering the use of these ECD for smart windows. The

0 2 4 6 8 10 120

10

20

30

40

Perc

ent T

ransm

itta

nce (

T%

)

Time (min)

v=60 s

v=30 s

v=10 s

v=5 s

v=1 s

v=0.5 s

21

percent transmittance for one of the devices at 555 nm shifted from 12.04% in the reduced

state to 61.5% in the bleached state. For smart windows applications, little transmittance in the

neutral or reduced state and highly transmissive oxidized states are desired. In order to achieve

the latter, thicker films were spray cast up to an absorbance of 3. However, the films lost much

contrast and achieved poor transmissive states.

Table 2. Optical properties of dual polymer devices of ECP Random Back and MCCP

Device Dry Film Absorbance ΔT% T%

Colored T%

Bleached ΔL* L*

Colored L*

Bleached

1 Black=1.0, MCCP=0.5 49 11.1 60.1 41.5 41 82.5

2 Black=1.0, MCCP=0.5 48.1 8.6 56.7 44.1 36.7 80.8

3 Black=2.1, MCCP=1.1 31.3 1.1 32.4 54.8 11 65.8

4 Black=2.2, MCCP=1.1 33 1.2 34.2 56.2 11 67.2

Figure 12. Spectroelectrochemistry of a dual polymer ECP Random Black and MCCP device

The switching speeds results, shown in Figure 13, demonstrated a loss of contrast from

a 60 second switch to a 20 second switch. Although ECP Random black achieved a broadening

400 450 500 550 600 650 700 7500.0

0.2

0.4

0.6

0.8

1.0

1.2

Absorb

ance (

a.u

)

Wavelength (nm)

-1.4 V

-1.0 V

-0.5 V

0.0 V

0.1 V

0.2 V

0.3 V

0.4 V

0.5 V

0.6 V

1.0 V

1.4 V

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of the absorption spectrum in the visible range, the optical contrast was markedly reduced as

compared to the Magenta polymer. Moreover, switch speeds which are important for display-

type applications, proved to have very strict limits for the ECD created from these films. Other

routes were thus taken to achieve both broadening of the absorption spectrum and high optical

density, as shown in later sections.

0 200 400 600 800 1000 1200 1400 1600

10

15

20

25

30

35

40

45

50

55

60

% T

ran

sm

itta

nce

Time (Sec)

60 Sec

30 Sec

20 Sec

10 Sec

%T = 45.7%T = 42.8

%T = 37.8

%T = 26.1

Figure 13. Square-wave potential-step chronoabsorptometry of an ECP Random Black device

monitored at 550 nm.

3.3 Stacked Devices

In order to broaden the absorption spectrum in the visible region, a color combination

approach was taken using subtractive coloring. Since absorption is additive, the construction of

a dual active device with a combination of two electrochromic polymers that are colored in

their neutral state was possible. These devices would also contain a third colorless polymer

MCCP, used in all previous devices. For this project, magenta was chosen as the first polymer

since it has the high optical density desired. In order to broaden the spectrum, a second

polymer, whose absorption peak was in the visible region not absorbed by magenta, was

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needed. ECP Cyan was chosen as it has a max absorption at around 700 nm. The overlay of

absorption spectrum of the two pure polymers is shown in Figure 14, with the polymers in their

neutral colored state.

Figure 14. A. Normalized spectra of ECP Magenta and ECP Cyan B. Colored/bleached states of

films of ECP Magenta (top) and ECP Cyan (bottom)

Films of magenta were spray cast onto ITO slides and films of cyan were spray cast at

various thicknesses. This was done to test different ratios of magenta to cyan thicknesses to

probe the resulting optical properties, the results of which are summarized in Table 3. In order

to achieve charge balance, the corresponding films of MCCP were at a 1:2 ratio of optical

density to the working electrode films of magenta or cyan. The two devices were sealed

together with double sided tape. The two working electrodes were shorted together as were

the two counter electrodes such that the cyan-magenta films acted as one common working

electrode and the two MCCP films acted as one common counter electrode, as shown in Figure

5.

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Table 3. Optical properties of dual active devices of ECP Magenta, ECP Cyan and MCCP

Device Dry Film

Absorbance ΔT% T%

Colored

T% Bleache

d ΔL*

L* Colored

L* Bleached

a*, b* Colored

a*, b* Bleached

1 Magenta=1.93

Cyan=0.4 52.8 0.4 53.2 61 17 78 40, -23 -3, -2

2 Magenta=1.89

Cyan=0.84 35.5 0.5 36 57 11 68 28, -20 2, -8

3 Magenta=1.87

Cyan=0.96 30.4 0.4 30.8 54 9 63 24, -18 2, -5

The cyclic voltammetry results showed defined oxidation and reduction peaks, and

spectroelectrochemistry was carried out to observe the new absorption spectrum at the

oxidized and reduced states, the results of which are summarized in Figure 15. The absorbance

of magenta was kept the same for all three devices; however, the cyan absorbance was

increased from device 1 to 3. Device 1, 2 and 3 had magenta to cyan ratios of 2:0.4, 2:0.8, and

2:1 optical density, respectively. As can be seen from the spectroelectrochemical series, the

two peaks combined to produce a broad absorption with an average 238 nm full width at half

max absorption. The absorption is broader for device 1, with the 2:0.4 ratio of magenta to cyan

optical density, resulting in a 328 nm full width at half max absorption. However, device 3 with

a 2:1 ratio of magenta to cyan optical density had the highest contrast ratio, with 53.2%

transmittance in the bleached state and 0.38% transmittance in the colored state. This

demonstrated that high optical density was maintained while simultaneously broadening the

absorption spectrum of the device.

25

400 450 500 550 600 650 700 7500.0

0.5

1.0

1.5

2.0

2.5

Absorb

ance (

a.u

.)

Wavelength (nm)

Device 1 colored/bleached

Device 2 colored/bleached

Device 3 colored/bleached

Figure 15. Spectroelectrochemistry of dual active Cyan and Magenta devices

A set-back to the dual active construct is that the layering of the multiple ITO films

results in some loss of transmission due to light scattering at the various layers: glass, ITO,

polymer film, and gel electrolyte. Furthermore, multiple layers of ITO are unfavorable for

applications such as windows where bulkiness would be problematic. The dual active devices

were constructed with the purpose of testing whether color combinations could broaden the

absorption spectrum and at what cost in optical contrast. The success of the broadened peaks

demonstrates that color mixing is possible with electrochromic polymers that absorb at

different ranges in the visible spectrum. Furthermore, to eliminate the transmission loss due to

extra layers of spray cast films on ITO, a route for color blending of the same cyan and magenta

polymers was proposed.

3.4 Blended Polymers

From the results of utilizing stacked layers (in dual active devices) to obtain a broadened

absorption in the visible region with magenta and cyan films, other combining methods were

26

investigated with blending of the polymers as an obvious route for broadening the absorption

spectrum while minimizing loss of transmittance due to the several layers in dual active devices.

The volume ratios of magenta to cyan were varied to test colors produced and optical contrasts

of devices. The cyan polymer had limited solubility in toluene, a 1:1 mixture of toluene and

chloroform was used for two solutions followed by mixing and co-spraying.

The volumetric ratios tested were a 2:1, 1:1, and 1:2 Magenta to Cyan blended device,

whose neutral state absorbances are shown in Figure 16. As the ratio of cyan is increased,

there’s a reduction in one of the magenta peaks at 545 nm and growth of the two outermost

broad portions at 410 and 700 nm. The spectroelectrochemical series is shown in Figure 17 for

the devices at the two voltage extremes, the colored and bleached states. As can be seen,

although the spectrum is broadened significantly in the 1:2 Magenta to Cyan device, there is

notable loss of optical contrast in the bleached stated. In subsequent experiments, a 2:1

magenta to cyan ratio gave the highest optical contrast at the wavelength of 550 nm.

Figure 16. Neutral state absorbance spectra for blended materials films (A) and

colored/bleached states of films of blended materials (B)

27

The colors produced by the different ratios ranged from a purple tinted magenta in the

2:1 magenta to cyan device to a purple blue hue in the 1:2 magenta to cyan. This demonstrated

the possibility of creating new colors by blending polymers and co-spraying. As we have the full

subtractive color palette available (cyan, magenta, yellow, red, blue) this color tuning can be

expanded further by using the complimentary color wheel to determine blends needed to

achieve a certain desired color.21

400 450 500 550 600 650 700 7500.0

0.5

1.0

1.5

2.0

2.5

Absorb

ance (

a.u

.)

Wavelength (nm)

1:1 M:C colored/ bleached

2:1 M:C colored/bleached

1:2 M:C colored/bleached

Figure 17. Spectroelectrochemistry of ECP Magenta and ECP Cyan blended devices

From the data given by the spectroelectrochemical series of the different blends,

summarized in Table 4, it was determined that a 2:1 ratio of magenta to cyan gave the best

optical contrast. In the dual active series, similar results had been obtained as the devices with

greatest absorption ratio of magenta to cyan gave the best results. Devices of this volumetric

ratio were constructed to optimize conditions and further test limits of optical properties. The

spectroelectrochemical series for one such device is shown in Figure 18. As can be seen, there is

a broadening on the 500 to 700 nm range, due to the inclusion of cyan.

28

Table 4. Optical properties of blended materials devices of ECP Magenta, ECP Cyan and MCCP

Blended Ratio (V:V) ΔT%

T% Colored

T% Bleached ΔL*

L* Colored

L* Bleached

a*, b* Colored

a*, b* Bleache

d

1:1 (M:C) 40.1 0.9 41 56.4 13.9 70.3 10.8, -29.6 -2.9, -5.6

2:1 (M:C) 43.8 0.8 44.6 58.5 14.3 72.8 19.3, -29.9 -2.9, -6.0

1:2 (M:C) 27.2 0.9 28.1 48 11.8 59.8 -0.5, -24.8 -6.9, -6.9

Figure 18. Spectroelectrochemistry of a 2:1 Magenta to Cyan blended device

Kinetics experiments were performed on these devices in order to test their switching

capabilities and the results are shown in Figure 19. There is relatively little loss in transmittance

going from a 2 minute switch to a 1 second switch. In ECP Black, switching speeds lost

significant optical contrast proving to be a limiting factor in the use of those devices for displays

or windows. Here, the response time to achieve a full switch from the neutral to oxidized state

provides an advantage of these devices for applications.

300 400 500 600 700 8000.0

0.5

1.0

1.5

2.0

Ab

so

rba

nce

(a

.u.)

Wavelength (nm)

-1.2 V

-0.6 V

0.0 V

0.5 V

1.0 V

1.4 V

1.6 V

1.8 V

1.9 V

29

Figure 19. Square-wave potential-step chronoabsorptometry of a 2:1 Magenta to Cyan blended

device monitored at 550 nm.

3.5 ECP Deep Purple

One of the final routes to broadening the spectrum was through the synthesis of a new

broadly absorbing polymer utilizing structural modification of the magenta polymer by the

addition of 3,4-ethylenedioxythiophene (EDOT) units randomly, the synthesis of which was

carried out by Mike Craig. The resulting ECP Deep Purple, whose structure is given in Figure 20,

has absorption in the 400 to 700 nm range. This broadened absorption gives it a purple color,

rather than the bright pink hue of the magenta polymer.

Figure 20. Chemical structure of ECP Deep Purple

0 10 20 30 40

0

5

10

15

20

25

30

35

40

Perc

ent T

ransm

itta

nce (

T%

)

Time (min)

v= 120 s

v= 60 s

v= 30 s

v= 10 s

v= 5 s

v= 1 s

30

Cyclic voltammetry results showed the typical reduction and oxidation peaks of

electrochromic polymer. Figure 21 shows an overlay of the cyclic voltammogram for ECP

Magenta and the new polymer ECP Deep Purple. Films of varying thicknesses were spray cast

onto ITO coated glass slides, whose characterization results are summarized in Table 5. The

spectroelectrochemical series of the polymer is shown in Figure 22. Much like magenta, a high

optical contrast was achieved, 75.8% transmittance in the transmissive state and 17.3%

transmittance in the reduced state. Kinetics experiments revealed little loss of optical contrast

going from a one minute switch to a 5 second switch, which proved promising compared to the

ECP Black films.

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8 Magenta

Purple

E(Volts)

Cu

rrent D

ensity (

mA

/cm

2)

Figure 21. Cyclic voltammetry results for a film of ECP Deep Purple compared to ECP Magenta

Table 5. Optical properties of films of ECP Deep Purple

Film Dry

Absorbance Δ%T

%T Colored

%T Bleached

ΔL* L*

Colored L*

Bleached a*, b*

Colored a*, b*

Bleached

1 0.95 62.1 13.7 75.8 38 51 89 24, -38 -3, -6

2 1.49 61.8 6.6 68.4 47 39 86 28, -44 -5, -8

3 1.96 52.9 2.9 55.8 51 28 79 30, -45 -6, -9

31

Figure 22. Spectroelectrochemistry of a film of ECP Deep Purple

Films of ECP Deep Purple and MCCP were incorporated into an electrochromic device at

a 2:1 optical density ratio, respectively. Cyclic voltammetry revealed a much broader voltage

range in the devices. Devices were created in two separate environments. The first set of

devices was made on the bench top with atmospheric conditions, while the second and third

set of devices were created in argon glove-box which minimized oxygen and water content to 5

ppm O2 and 0.7 ppm H2O. The latter devices showed much larger voltage ranges tolerated in

the cyclic voltammetry and had larger optical contrasts. The spectroelectrochemical series for a

glove-box constructed device is shown in Figure 23 and the optical properties of these devices

are summarized in Table 6.

32

Figure 23. Spectroelectrochemistry of a dual polymer Deep Purple and MCCP device

Table 6. Optical properties of dual polymer devices of ECP Deep Purple and MCCP

Device Dry Film

Absorbance ΔT% T%

Colored T%

Bleached ΔL L*

Colored L*

Bleached a*, b*

Colored a*, b*

Bleached

1 1 32.8 9.6 42.4 30 42 72 18, -23 1, -3

2 1.52 34.8 2.4 37.2 33 35 68 28, -30 -3, -4

3 1.96 27.3 1 28.3 44 16 60 29, -28 -3, -1

Switching speeds experiments for Deep Purple devices, shown in Figure 24, showed

improvement on the stand alone films. There is little contrast lost from a one minute switch to

a half second switch, exceeding rates for previous polymers, and polymer combinations, used.

A quick response time to switch from a neutral or reduced state to a transmissive one is desired

for display applications. Deep purple offers the high optical contrast of Magenta and the

broader absorption spectrum due to EDOT monomer copolymerized with the magenta

monomer. Furthermore, it exemplifies the correlation between chemical structure and the

absorption spectra which allows for color tuning in conjugated polymers.

400 600 8000.0

0.5

1.0

1.5

Ab

so

rba

nce

(a

.u.)

Wavelength (nm)

-1.2 V

-0.9 V

-0.7 V

-0.5 V

-0.3 V

0 V

0.3 V

0.5 V

0.7 V

0.9 V

1.1 V

1.4 V

33

0 5 10 15 200

10

20

30

40

50

Perc

ent T

ransm

itta

nce (

T%

)

Time (min)

v=60 s

v=30 s

v=10 s

v=5 s

v=1 s

v=0.5 s

Figure 24. Square-wave potential-step chronoabsorptometry of an ECP Deep Purple device

monitored at 550 nm.

4. Conclusions and Future Work

In conclusion, the development of highly absorptive/transmissive devices using

conjugated polymers that exhibit electrochromism was reported. The wavelengths at which

these polymers absorb incident light, and therefore the colors they reflect, were shown to be

adjustable by synthetic methods, color mixing and materials blending approaches. Synthetic

approaches focused on exploiting the energy gap differences between the HOMO and LUMO

levels of the polymers and the color mixing and color blending approach used subtractive color

mixing. The absorption spectrum of these polymers is summarized in Figure 25 and the colors

achieved in the reduced and oxidized states are summarized by the L*a*b* color space of the

polymers in Figure 26.

This color tuning was aimed at not only broadening the absorption spectrum, but also

increasing the color variety of electrochromic polymers. This yields electrochromic polymers

34

useful in display and smart windows applications due to their easy processability, color variety,

high optical contrast and quick response times. Other endeavors currently focus on optimizing

electrochromic devices constructed using these polymers. These studies focus on the stability

of the polymers and devices constructed from them, the ability of devices to maintain a certain

optical contrast through memory experiments, and the lifetime of such devices. Further work

will also focus on obtaining desired transmittance values in both the neutral state and the

oxidized states.

400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

No

rmaliz

ed A

bsorb

ance (

a.u

.)

Wavelength (nm)

Magenta

1:2 M:C

1:1 M:C

2:1 M:C

Black

Deep Purple

Cyan

Figure 25. Normalized absorption spectra for electrochromic polymers spray cast onto indium

tin oxide coated glass slides

35

Figure 26. L*a*b* color space values for electrochromic polymers in their reduced states (left)

and their oxidized states (right)

36

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