Voltage losses in indoor light harvesting organic photovoltaic devices: a case study of green solvent processed PM6/IT-4Cl devices

Organic photovoltaic (OPV) devices are promising tools for indoor light harvesting and power conversion, but high voltage losses, especially under weak illumination, hinder their development. In this work, the organic active material system PM6/IT-4Cl, which is green solvent processible and highly suitable for indoor light harvesting, is employed to investigate the voltage loss mechanism in indoor OPV devices. Three major problems specific to indoor OPV devices are identified, including a high density of pinholes in the active layer leading to low device shunt resistance, a too-large charge transfer energy loss causing high radiative voltage losses, and a high trap concentration in the active layer limiting the device electroluminescence efficiency and open-circuit voltage. To solve these problems, a strategy based on the use of solvents and solid additives is proposed. This resulted in a significant increase in power conversion efficiency (PCE) for the PM6/IT-4Cl devices from 16% to 23% under white LED illumination (3000 K, 4000 lux). Our findings highlight the importance of developing tailored optimization strategies to reduce voltage losses in order to further improve the performance of indoor OPV devices. Three issues related to green solvent-processed indoor OPV devices, namely pinholes, traps, and charge transfer losses, have been identified. To address these challenges, a strategy involving the use of solvent and solid additives is proposed.