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TitlePolymer Light-Emitting Electrochemical Cells with Embedded Bipolar Electrodes
LanguageEnglish
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Total Pages116
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Page 1

Polymer Light-Emitting
Electrochemical Cells with Embedded
Bipolar 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

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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

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Figure 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.

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.

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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 are
given 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.

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