The Air Effect in the Burn-In Thermal Degradation of Nonfullerene Organic Solar Cells

Solution-processed organic solar cells (OSCs) have become a promising photovoltaic technology in recent years. However, OSCs suffer from poor stability, and most of the OSCs exhibit dramatic burn-in degradation at the initial stage of device operation. The burn-in degradation has become an ineluctable barrier for OSCs to achieve long-time stability and future commercialization, where an in-depth understanding of the mechanism behind burn-in degradation has become the precondition to conquer this barrier. To date, unlike the well-studied air effect in the burn-in light degradation, the air effect in the burn-in thermal degradation is rarely studied. Herein, the air effect in the burn-in thermal degradation in the nonfullerene OSCs is investigated. The air effect is shown to be closely related to the energy transfer and exciton dissociation process in the device, where the involvement of air can cause excessive degradation in both these processes. The atomic force microscopy (AFM) and scanning electron microscopy (SEM) results point out that the air-induced phase separation is the leading cause for the excessive degraded exciton dissociation process in the thermal burn-in degradation with air.