Origin of Photovoltaics in Organic Solar Cells at Negligible Energy Level Offsets─An Insight of the Charge Accumulation Effect

Reducing the energy level offset is one of the key elements of low open-circuit voltage loss in organic solar cells. However, the origin of charge separation driving force at negligible energy level offsets still remains unexplained. Herein, from the perspective of built-in potential caused by charge accumulation, we discuss the nonequilibrium energy level displacement as current passing with distinct variable current densities. Due to the different carrier mobilities of electrons and holes in organic semiconductor materials, carriers with high mobility will be rapidly transmitted to the electrode, while those with low mobility will remain in the materials, resulting in the accumulation of corresponding charges. It is suggested that the higher the carrier mobility, the better the efficiency of photovoltaic devices, with the balance of the charge transport.