Effects of Solvent and Annealing on Photophysical Properties of Polythiophene Photovoltaic Cells

The impact of different preparation conditions and post-deposition annealing treatments on pristine poly(3-hexyl-thiophene) (P3HT) thin-film photovoltaic devices are investigated with respect to device parameters, surface roughness, and exciton dynamics. Devices were realized from the slow evaporating solvent orthodicholorobenzene (ODCB) and the fast evaporating solvent trichloroethylene (TCE). Different annealing conditions (thermal, solvent-atmosphere, post-production thermal after electrode evaporation) were employed to affect the self-organization of the P3HT chains. All thermal-annealed films led to devices with higher short-circuit currents (I-sc) and lowered open-circuit voltages (V-oc). Solvent-annealed films casted from ODCB (roughest surface) led to highest I-sc but ones from TCE (smoothest surface) showed the lowest I-sc. Monotonic relationship between roughness and I-sc is ascribed to an increase in (a) P3HT-Al interface for charge collection and (b) trapping/scattering assisted light absorption in the region close to Al electrode. The correlations reported herein provide insights about further optimization of the polymeric phase for plastic solar cells.