Quantum Mechanical Assessment of Optimal Photovoltaic Conditions in Organic Solar Cells

Recombination losses contribute to reducing J_SC, V_OC, and the fill factor of organic solar cells. Recent advances in non-fullerene organic photovoltaics have shown, nonetheless, that efficient charge generation can occur under small energetic driving forces (ΔE_DA) and low recombination losses. To shed light on this issue, we set up a coarse-grained open quantum mechanical model for investigating the charge generation dynamics subject to various energy loss mechanisms. The influences of energetic driving force, Coulomb interaction, vibrational disorder, geminate recombination, temperature, and external bias are included in the analysis of the optimal photovoltaic conditions for charge carrier generation. The assessment reveals that the overall energy losses are not only minimized when ΔE_DA approaches the effective reorganization energy at the interface but also become insensitive to temperature and electric field variations. It is also observed that a moderate reverse bias reduces geminate recombination losses significantly at vanishing driving forces, where the charge generation is strongly affected by temperature.