Doping of a disordered organic semiconductor gives rise to additional energy disorder due to the Coulomb interaction between randomly distributed dopant ions and carriers localized in intrinsic hopping sites. Although the carrier density increases with increasing doping level the additional energy disorder can significantly reduce the carrier hopping mobility. At higher doping levels the filling of deep states takes over, which leads to steeply increasing mobility at high dopant concentrations.

A model of carrier photogeneration in doped conjugated polymers is formulated. The model suggests that dissociation of a relaxed exciton into a Coulombically bound geminate pair (GP) occurs at a charge transfer center that consists of a polymer chain and a nearby electron acceptor. Further dissociation of the GP into free carriers is facilitated either by the kinetic energy of the zero-point oscillation of the on-chain carrier at low dopant concentrations or by the interfacial potential barrier for geminate pair recombination in polymer/electron acceptor blends.

We present the results of studies on the electrical and physical modifications to Poly(3-hexylthiophene), upon thermal annealing. Thermally-induced performance modifications and thermal stability of polythiophene thin film transistors are explored. We observe substantial mobility improvements in devices annealed at low temperatures (>80°C), as well as increases in on/off ratios by two orders of magnitude at moderate anneal temperatures (~120°C). We document changes in conductivity, mobility, on current, and on/off ratio with anneal temperature and total thermal budget. In conjunction with material analysis, we develop qualitative models for the mechanisms involved in the annealing/degradation processes. Hence, this study provides a comprehensive analysis of the effect of thermal cycling of polythiophene TFTs on various device performance metrics, and identifies the relevant thermal limits and failure mechanisms.