Photostable electron-transport-layer-free flexible graphene quantum dots/perovskite solar cells by employing bathocuproine interlayer

Electron transport layer (ETL)-free perovskite solar cells (PSCs) are very useful for the low-temperature process that can reduce the instability caused by the ETL such as TiO₂ layer. There are several publications on ETL-free PSCs, but most of them are based on transparent conductive oxides, not suitable for flexible applications. Here, we first report photostable ETL-free PSCs of maximum 15.72% power conversion efficiency (PCE) by employing an insulating interlayer of bathocuproine (BCP) between the graphene (GR) transparent conductive electrode (TCE) and the graphene quantum dots-added perovskite layer. The BCP interlayer effectively enhances the electron carrier injection by reducing the recombination centers at the active layer/GR TCE interface. Only 8 or 17% of the original PCE is lost under ambient conditions or under light soaking, respectively for 500 h. A flexible-type of the PSC shows a PCE comparable to that of the rigid-type counterpart, and 83% of the initial PCE is maintained even after 1000 repeated bending tests at a radius of 4 mm. These results suggest that the ETL-free PSCs exhibit excellent long-term and mechanical stabilities.