Overcoming the fill-factor limit of organic solar cells

Abstract

Although organic solar cells have surpassed 20% power conversion efficiency, a persistent trade-off between open-circuit voltage and fill factor (FF) prevents them from closing the gap with inorganic technologies. Here we investigate this trade-off across a wide range of devices and identify an FF limit arising from field-dependent free-charge generation. This limit becomes more severe as voltage losses are minimized, thereby imposing an open-circuit voltage–FF trade-off. To quantitatively describe this limit, we develop an analytical model for field-dependent charge generation, revealing that the underlying cause is the field-sensitive charge-transfer process between excitons and charge-transfer states. This sensitivity originates from the field-induced charge-transfer state energy variations, mainly caused by the Stark effect. Guided by this physics-based model, we highlight that a long exciton lifetime is one of the practical and effective methods to overcome the FF limit.