Molecular Engineering of the Fullerene-Based Electron Transport Layer Materials for Improving Ambient Stability of Perovskite Solar Cells

It is known that the operation lifetime of perovskite solar cells can be extended by orders of magnitude if properly selected hole-transport and electron transport layers provide good isolation for the perovskite absorber preventing evaporation of volatile species (e.g., photoinduced) from the active layer and blocking the diffusion of aggressive moisture and oxygen from the surrounding environment. Herein, a systematic study of a family of structurally similar fullerene derivatives as electron transport layer (ETL) materials for p-i-n perovskite solar cells is presented. It is shown that even minor modifications of the molecular structure of the fullerene derivatives have a strong impact on their electrical performance and, particularly, ambient stability of the devices. Indeed, an optimally functionalized fullerene derivative applied as an ETL enables stable operation of perovskite solar cells when exposed to air for >800 h, which is manifested in retention of 90% of the original photovoltaic performance. In contrast, the reference devices with phenyl-C-61-butyric acid methyl ester as the ETL degraded almost completely within less than 100 h of air exposure. Most probably, the side chains of the best-performing fullerene ETL materials are filling the gaps between the carbon spheres, thus preventing the diffusion of oxygen and moisture inside the device.