Investigation of the conductive behavior of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) for organic electronics

Research Doctorate - Doctor of Philosophy (PhD)The conductive behaviour of poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT/PSS), produced by oxidative polymerisation under varying synthetic and doping conditions, and upon structure modification, has been investigated, with the aim of improving the conductivity and processability of PEDOT/PSS films for application in organic photovoltaic (OPV) and organic field electronic transistor (OFET) devices as interfacial conductive layer and gate electrode, respectively. A capillary electrophoresis (CE) method was developed and showed, for the first time, the separation between the PEDOT/PSS complex and free PSS and allowed estimation of the doping efficiency of PSS. The results show that films with conductivity >1.0 S/cm have higher PEDOT/PSS content indicated by shorter migration times (~8 min) and doping efficiency of >80%. This work also demonstrates the potential of PEDOT to be modified to contain functional endgroups and form a more complex architecture. PEDOT was successfully endcapped with thiophene-based endcapping agents 2-thiophenemethanol (2TM) and 2-thienylmethyl methacrylate (2TMM). Crosslinked core star (CCS) PEDOT/PSS was also successfully synthesised via the core first method using 2TMM capable of linking PEDOT (via its thiophene moiety) with the non-conductive crosslinked core (via its polymerisable vinyl group). All PEDOT/PSS films used in this study were generated by spin-coating at 5000 rpm found by formal statistical assessments (correspondence analysis and multi-linear regression analysis) to give the most reproducible results. Conductivity has been found to be affected by the doping efficiency of PSS, the doping condition (in-situ during polymerisation vs post-polymerisation), the combining ratio of EDOT and PSS, the time of reaction, the chain length and the morphology (linear vs CCS) of PEDOT and processability of dispersions. In general, high conductivity was obtain from films generated from highly processable, linear, non-endcapped, high PEDOT/PSS ratios (≤ 1:3) doped in situ dispersions polymerised between 12 and 36 hours. Highest conductivity (5.2 S/cm) was obtained from a linear PEDOT/PSS film resulting from 12 hours reaction. All of the PEDOT/PSS synthesised in house gave conductivity values one to three order of magnitude higher than the commercial sample (Clevious P). The performance of selected PEDOT/PSS films under un-optimised conditions, are comparable to the commercial PEDOT/PSS when used as an interfacial conductive polymer layer and electrode gate in OPV and OFET devices, respectively