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University of California, Irvine
Electrical Characterization of
Semiconducting Polymers
Sanda Cea
Faculty Mentors:
Professor Richard Nelson (EECS)
Professor John LaRue (MAE)
Graduate student: Chang-hsiu Chen (CheMS)
Outline
 Motivation
 Background
 Thin Film Fabrication
 Electrical Characterization
 Data Analysis & Results
 Conclusion
 Future Work
 Acknowledgements
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2006 IM-SURE Participants
Motivation
 Organic electronics (ICPs)
 easy, low cost processing
 lower Young’s modulus
 durability
 Commercial applications
 antistatic coatings
 corrosion protection for metals
 solar panels
 field effect transistors (FETs)
 organic light emitting diodes (OLEDs)
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Background
 Polymer structure
 chain composed of monomer units
 form weak intermolecular bonds
 Emergent properties
 solubility
 elasticity (Young’s modulus)
 tactile strength
 electroluminescence
 electrical conductivity
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Engineering ICPs
 Naturally-occurring in biological tissues (i.e. melanin)
 Pure conductive polymer = emeraldine base (EB)
 Doped to enhance conductivity = emeraldine salt
 oxidizing agent (removes electrons)
 reducing agent (adds electrons)
 protonic acid (adjusts pH levels)
 Forms of emeraldine salt compound
 powder
 dispersion in solvent
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Doped polymers studied
 Aqueous poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)
(PEDOT:PSS)
Component
% By Weight % By Volume
BAYTRON P
42.92
37.49
 Baytron® P (CPP 105 D)
N-Methyl-2-pyrrolidone (NMP)
2.58
2.19
Silquest A 187
0.86
0.70
 stable in oxidized state
Isopropanol
53.34
59.35
Dynol 604
0.30
0.27
 highly conductive
Formulation Table for Conductive Baytron P
(400-600 S/cm)
 Polyaniline (PANI) in xylene
 from Ormecon (D 1020)
 easy one-step synthesis
 conductivity of 200 S/cm
PEDOT:PSS Structure
Polyaniline Structure
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Mixing the solution
 Solid content of Baytron® P is 1.2%
 Needs host matrix for structural support
 Polyvinyl alcohol (PVA)
 soluble in water
 emulsifying agent
PEDOT/PSS Solution PVA Solution Volume
Volume Ratio
Ratio
PEDOT/PSS Solid Salts Content (Solid Content: 1.2%, (Solid Content: 9%,
in insulating host polymer (wt%) Density=0.87g/cm^3) Density=1.02g/cm^3)
0%
0
1
10%
0.977
1
20%
2.2
1
30%
3.771
1
40%
5.867
1
50%
8.8
1
60%
13.2
1
70%
20.533
1
80%
35.2
1
90%
79.2
1
100%
1
0
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PEDOT/PVA Solution
Stir Plate Setup
Thin Film Fabrication
 Factors to consider
 film continuity
 preserving binding structure
 Thermal Evaporation
 con: causes breakdown of cross-linked chains
PDMS Mold
 Casting on glass
 pro: PDMS mold used to control thickness
 con: films tend to warp
 Spin-coating
 pro: ensures even spreading and slow evaporation
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Spin-coater
Process
 Cut Si wafer (with an insulating SiO2 layer)
into quarters and tape one edge
 provides a step edge for thickness measurement
 Spin-coat at 500 rpm
 not too high or film will be too thin
 Bake in vacuum oven at 90 ºC for 12 hours
 evaporates remaining solvent
 Measure film thickness using the Digital
Dektek 3 Profilometer
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Electrical Characterization
 Lateral ohmmeter readings with brass strips
 contact resistance much higher than bulk resistance
PEDOT:PVA Spin (rpm) Thickness (um) Resistivity (Ohms-cm) Bulk Resistance (Ohms)
Pure (100 %)
500
0.55
1.0926
4.05 k - 19.2 k
1000
0.25
0.1020
1.59 k - 4.11 k
9:1 (90 %)
2000
2.00
4.5000
2.00 k - 18.0 k
4:1 (80 %)
1000
1.00
4.4540
4.20 k - 39.2 k
2:1 (66.7 %)
500
0.70
1.2880
1.00 k - 18.0 k
1000
0.30
1.5456
31.7 k - 53.7 k
1.5:1 (60 %)
500
1.00
0.6438
0.15 k - 7.15k
1000
0.40
1.3680
7.60 k - 35.6 k
1:1 (50 %)
500
1.00
19.8750
79.5 k - 187 k
1.70
1.8931
6.40 k - 22.6 k
1000
0.20
5.4375
45.5 k - 218 k
2.90
16.1414
13.4 k - 58.4 k
1500
0.50
24.5250
248 k - 818 k
2000
0.40
14.3280
70.5 k - 299 k
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Contact Resistance (Ohms)
18.95 k
23.29 k
32.00 k
20.80 k
83.00 k
31.33 k
21.85 k
18.40 k
298.50 k
45.40 k
338.50 k
35.60 k
214.50 k
509.50 k
Other techniques
 Van der Pauw 4-point probe
 damages thin film and SiO2 layer
 Collinear 4-point probe
 soldering or depositing gold electrodes requires high
temperatures
 destroys polymer thin film
 solution: silver epoxy
 cures in less than 10 minutes at 90 ºC
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Measurement procedure
 Cut samples into 1 cm by 4 cm strips and add 4 contacts
 Apply current across outer two terminals and read voltage across
inner two using the Agilent 4156C Semiconductor Parameter
Analyzer
 Calculate
resistance
V
R
I
Collinear Four-Point Probe
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Prepared Sample
Data Analysis & Results
 Resistance, cross-sectional area, and length of sample strip can
be used to calculate resistivity,  (Ω-cm)
 inverse yields conductivity (S/cm)
R
 Data plotted on logarithmic scale is compared against
existing data from previous study
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A
L
A  wt
Thickness results
 Film thickness measurements are plotted as well to highlight
the inverse relationship between
thickness and conductivity
 Sources of error
 deterioration of PEDOT
 contamination
 scratches on film surface
 irregular-shaped strips
 uneven electrode spacing
 internal resistance of silver epoxy and wire leads
 limited sensitivity of measuring equipment
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Conclusion
 Semiconducting polymers are versatile and adaptable
 gives manufacturers and researchers alike more control
 The disparate findings on conductivity for the two forms of
PEDOT/PVA compound indicate that more testing and analysis
is needed to characterize these novel conducting organic
substances
 Work is also needed to compile results found in a
comprehensive manner
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Future Work
 Need to test polyaniline/SU-8 composition
 Mechanical characterization
 micromachine a cantilever beam
 design setup to actuate oscillations
 measure resonance frequency
 calculate Young’s modulus
 0  0.162
.
t
l2
E

0 = resonance frequency (Hz)
E = Young’s modulus
 = film density (kg/cm3)
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Acknowledgements
 Professor Richard Nelson, Electrical Engineering & Comp Science
 Professor John LaRue, Mechanical & Aerospace Engineering
 Chang-hsiu Chen, Chemical Engineering & Materials Science
 Allen Kine, Lab Supervisor
 Said Shokair, UROP Director
 Edward Olano, UROP Undergraduate Research Counselor
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University of California, Irvine
Questions?
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