Charge-density-based analysis of the current-voltage response of polythiophene/fullerene photovoltaic devices

A key challenge for organic electronics research is to develop device models that correctly account for the structural and energetic disorder typically present in such materials. In this paper we report an approach to analyze the electrical performance of an organic electronic device based upon charge extraction measurements of charge densities and transient optoelectronic measurements of charge carrier dynamics. This approach is applied to a poly(3-hexyl thiophene) (P3HT)/6,6 phenyl C61 butyric acid methyl ester (PCBM) blend photovoltaic device. These measurements are employed to determine the empirical rate law for bimolecular recombination losses, with the energetic disorder present in the materials being accounted for by a charge-density-dependent recombination coefficient. This rate law is then employed to simulate the current/voltage curve. This simulation assumes the only mechanism for the loss of photogenerated charges is bimolecular recombination and employs no fitting parameters. Remarkably the simulation is in good agreement with the experimental current/voltage data over a wide range of operating conditions of the solar cell. We thus demonstrate that the primary determinant of both the open-circuit voltage and fill factor of P3HT: PCBM devices is bimolecular recombination. We go on to discuss the applicability of this analysis approach to other materials systems, and particularly to emphasize the effectiveness of this approach where the presence of disorder complicates the implementation of more conventional, voltage-based analyses such as the Shockley diode equation.