Temperature Activation of the Photoinduced Charge Carrier Generation Efficiency in Quaterthiophene:C60 Mixed Films

We measure photoinduced excitations in a dicyanovinyl end-capped methylated quaterthiophene derivative in blends with the electron acceptor C-60, as already employed in organic photovoltaics. By using DFT calculations and analyzing the recombination characteristics of the excited states revealed by photoinduced absorption (PIA) spectroscopy, the absorption peaks are assigned to triplet exciton, cation, and anion transitions. We determine the temperature dependent generation and recombination behavior of triplet excitons and cations in the mixed layer. At 10 K, we observe an enhanced triplet exciton generation rate compared to the pristine donor layer due to back recombination from a charge-transfer (CT) state at the donor-acceptor interface. With increasing temperature, the triplet generation rate first increases which is ascribed to an enhanced singlet exciton migration to this interface. Above 150 K, the triplet generation rate declines due to the beginning CT exciton separation, leading to the generation of free charge carriers. This temperature activated behavior is ascribed to a temperature activated increase of charge carrier mobility, facilitating CT exciton splitting.