Role of Energy Offset in Nonradiative Voltage Loss in Organic Solar Cells

The voltage loss incurred by nonradiative charge recombination should be reduced to further improve the power conversion efficiency of organic solar cells (OSCs). This work discusses the nonradiative voltage loss in OSCs with systematically controlled energy offset between optical bandgap and charge transfer (CT) states. It is demonstrated that the nonradiative voltage loss is a function of the energy offset; it drops sharply with decreasing energy offset. By measuring the quantum yields of electroluminescence from OSCs and decay kinetics of CT states, it is found that the radiative decay rate of CT states becomes larger when the energy offset is negligible compared with those in conventional OSCs with sufficient energy offset. This behavior is rationalized by hybridization between CT and local excited states, resulting in a considerable enhancement of the oscillator strength of CT states. Based on a trend observed in this study, the precise mechanism by which the energy offset affects the nonradiative voltage loss is discussed.