Electrical Characterization of Semiconducting Polymers Sanda Cea Faculty Mentors: Professor Richard Nelson (EECS) Professor John LaRue (MAE) Graduate student: Chang-hsiu Chen (CheMS) University of California, Irvine

UCI Undergraduate Research Opportunities ProgramOutline  Motivation  Background  Thin Film Fabrication  Electrical Characterization  Data Analysis & Results  Conclusion  Future Work  Acknowledgements 2006 IM-SURE Participants

UCI Undergraduate Research Opportunities ProgramMotivation  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)

UCI Undergraduate Research Opportunities ProgramBackground  Polymer structure  chain composed of monomer units  form weak intermolecular bonds  Emergent properties  solubility  elasticity (Young’s modulus)  tactile strength  electroluminescence  electrical conductivity

UCI Undergraduate Research Opportunities Program 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

UCI Undergraduate Research Opportunities Program Doped polymers studied  Aqueous poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)  Baytron ® P (CPP 105 D)  stable in oxidized state  highly conductive ( S/cm)  Polyaniline (PANI) in xylene  from Ormecon (D 1020)  easy one-step synthesis  conductivity of 200 S/cm PEDOT:PSS Structure Polyaniline Structure Formulation Table for Conductive Baytron P

UCI Undergraduate Research Opportunities Program 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/PVA Solution Stir Plate Setup

UCI Undergraduate Research Opportunities Program  Factors to consider  film continuity  preserving binding structure  Thermal Evaporation  con: causes breakdown of cross-linked chains  Casting on glass  pro: PDMS mold used to control thickness  con: films tend to warp  Spin-coating  pro: ensures even spreading and slow evaporation Thin Film Fabrication PDMS Mold Spin-coater

UCI Undergraduate Research Opportunities ProgramProcess  Cut Si wafer (with an insulating SiO 2 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

UCI Undergraduate Research Opportunities Program Electrical Characterization  Lateral ohmmeter readings with brass strips  contact resistance much higher than bulk resistance

UCI Undergraduate Research Opportunities Program Other techniques  Van der Pauw 4-point probe  damages thin film and SiO 2 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

UCI Undergraduate Research Opportunities Program 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 Collinear Four-Point Probe Prepared Sample

UCI Undergraduate Research Opportunities Program 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)  Data plotted on logarithmic scale is compared against existing data from previous study

UCI Undergraduate Research Opportunities Program  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 Thickness results

UCI Undergraduate Research Opportunities ProgramConclusion  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

UCI Undergraduate Research Opportunities Program 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.

UCI Undergraduate Research Opportunities ProgramAcknowledgements  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

UCI Undergraduate Research Opportunities ProgramQuestions? University of California, Irvine