Functionalized polythiophene thin-film transistors for low-cost gas sensor arrays

180 Pages·2006·2.28 MB·English

Functionalized polythiophene thin-film transistors for
low-cost gas sensor arrays
Josephine Bea Chang
Electrical Engineering and Computer Sciences
University of California at Berkeley
Technical Report No. UCB/EECS-2006-42
http://www.eecs.berkeley.edu/Pubs/TechRpts/2006/EECS-2006-42.html
April 24, 2006
Copyright © 2006, by the author(s).
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FUNCTIONALIZED POLYTHIOPHENE THIN-
FILM TRANSISTORS FOR LOW-COST GAS
SENSOR ARRAYS
by
Josephine Bea Chang
B.S. (University of Texas, Austin) 2001
M.S. (University of California, Berkeley) 2004

A dissertation submitted in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy
in
Engineering-Electrical Engineering
and Computer Sciences
in the
GRADUATE DIVISION
of the
UNIVERSITY OF CALIFORNIA, BERKELEY
Committee in charge:
Professor Vivek Subramanian, Chair
Professor Chenming Hu
Professor Don Tilley
Spring 2006
FUNCTIONALIZED POLYTHIOPHENE THIN-
FILM TRANSISTORS FOR LOW-COST GAS
SENSOR ARRAYS

© Copyright 2006
by
Josephine Bea Chang

The dissertation of Josephine Bea Chang is approved:

Chair Date

Date

Date

University of California, Berkeley
Spring 2006
ABSTRACT
FUNCTIONALIZED POLYTHIOPHENE
THIN-FILM TRANSISTORS FOR LOW-COST
GAS SENSOR ARRAYS
by
Josephine B. Chang
Doctor of Philosophy in Electrical Engineering and Computer Sciences
University of California, Berkeley
Professor Vivek Subramanian, Chair

A low-cost electronic nose solution based on printed arrays of organic thin film transistor gas
sensors is proposed. First, polythiophene organic thin film transistors are shown to be suitable
for use as gas sensors. Good sensitivity to a range of gas analytes including alcohols, thiols,
amines, and acids is observed down to 10 ppm. The alcohol and acid response is also
reversible at low concentrations. Then, a synthetic route is demonstrated for predictably
altering sensor response through the incorporation of functionalized end-capping groups. This
route is used to generate an array of polythiophene sensor inks with differentiated sensor
response, and effect of end-group functionality and placement are investigated. An integrated
gas sensor array is fabricated using a droplet-on-demand technique as a demonstration of the
possibility for low-cost manufacturing. Finally, the design of organic-based logic support
circuitry for the sensor array is considered.

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ACKNOWLEDGMENTS
This work has been the recipient of a generous input of time and effort from many people at
Cal. Dr. Jean Fréchet’s research group in the Chemistry department was an invaluable source
of ideas, chemical expertise, and, most importantly, in-house synthesized materials. In
particular, I would like to thank Kevin Sivula and Amanda Murphy for many useful
discussions and a fruitful collaboration over the years. In my own lab group, Vincent Liu’s
contributions to this research are much appreciated, and Jason Lee was an invaluable resource
for the device modeling and circuit simulation portion of the work. Thanks also to Steve
Volkman, who was always there when I needed advice, and to Alejandro de la Fuente, for
making our lab a bearable place to work. In addition, Frank Liao must be acknowledged as the
true founder of this sensors project. He developed a flexible, robust experimental
infrastructure that I had the good fortune to inherit. And, of course, I’ve been incredibly lucky
to have had the privilege of working with Vivek Subramanian, my advisor, mentor, and role
model over the past four years. Thanks, Vivek, for making my graduate school productive,
rewarding, memorable, and fun. Thank you most of all for football and for Paul.

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Dedicated to my mom and dad, who have always given me only the best,
and have hoped for this even longer and harder than I have.

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TABLE OF CONTENTS
CHAPTER 1: INTRODUCTION..............................................................................................1
CHAPTER 2: BACKGROUND ON ELECTRONIC NOSES...............................................6
2.1 HISTORICAL DEVELOPMENT................................................................................................7
2.2 BIOLOGICAL NOSES..............................................................................................................9
2.3 GAS HANDLING AND DELIVERY........................................................................................11
2.4 GAS SENSING ELEMENTS....................................................................................................12
2.4.1 Metal-oxide sensors.............................................................................................................13
2.4.2 Conducting polymer sensors...................................................................................................15
2.4.3 Polymer composite sensors......................................................................................................17
2.4.4 MOSFET sensors..............................................................................................................19
2.4.5 Gravimetric sensors..............................................................................................................20
2.5 DATA ANALYSIS..................................................................................................................20
2.5.1 Nearest neighbor method.......................................................................................................21
2.5.2 Statistical analysis techniques.................................................................................................21
2.5.3 Artificial neural network......................................................................................................23
2.6 CURRENT APPLICATIONS OF ELECTRONIC NOSES.............................................................23
2.7 REFERENCES.......................................................................................................................25
CHAPTER 3: BACKGROUND ON ORGANIC ELECTRONICS.....................................26
3.1 HISTORICAL DEVELOPMENT..............................................................................................27
3.2 ELECTRICAL CHARACTERISTICS OF CONJUGATED OLIGOMERS AND POLYMERS..............28
3.3 ORGANIC THIN FILM TRANSISTORS (OTFTS).....................................................................33
3.3.1 Device structure and fabrication..............................................................................................33
3.3.2 Current-voltage characteristics................................................................................................36
3.3.3 Field-effect mobility..............................................................................................................38
3.3.4 Turn-on characteristics.........................................................................................................41
3.4 APPLICATIONS OF OTFTS..................................................................................................46
3.4.1 Displays............................................................................................................................46
3.4.2 Radio frequency tag identification (RFID)...............................................................................48
3.5 REFERENCES.......................................................................................................................49
CHAPTER 4: LIFETIME AND RELIABILITY....................................................................50
4.1 CAUSES OF SIGNAL DRIFT...................................................................................................51
4.1.1 Bias stress effect...................................................................................................................51
4.1.2 Sensor response to oxygen and humidity...................................................................................53
4.1.3 Material Degradation..........................................................................................................55
4.2 EXPERIMENTAL SETUP.......................................................................................................56
4.3 RESULTS..............................................................................................................................59
4.3.1 Active layer material dependence.............................................................................................60
4.3.2 Active layer thickness dependence............................................................................................61
4.4 DEALING WITH BIAS STRESS...............................................................................................70
4.4.1 Pulsed operation..................................................................................................................70

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