X-ray scattering*

Some transport models in doped organic semiconductors describe electrical conductivity using a static density of states. It is important to consider changes in the DOS due to both carrier/counter-ion Coulombic interactions and microstructural variation at high doping levels. We have explored the structural evolution of poly(3-hexylthiophene), a p-type organic semiconductor, in a thin-film transistor geometry using a polymeric ionic liquid (PIL) dielectric. PILs comprise of charged polymer sidechains and a mobile counter-ion, allowing only the counter-ion to diffuse on experimentally relevant timescales. We observe through in operando X-ray scattering (GIWAXS) that changes in domain structure of P3HT are dependent on the sign and magnitude of the applied gate voltage; bias less than |1.2| V, or 3x10¹⁹ cm^–3, do not cause structural changes, while larger bias swells the polymer crystallites due to ion infiltration. Electrical conductivity follows a similar trend, increasing dramatically at |1.2| V from 5x10^–3 S/cm to 1.5 S/cm. This work shows that substantial differences exist between a doped polymer and its insulating state, signifying the importance of integrating doping-induced disorder into organic semiconductor transport models.