A graphene/ ZnO electron transfer layer together with perovskite passivation enables highly efficient and stable perovskite solar cells

Interface engineering in organometal halide perovskite solar cells (PSCs) has been an efficient tool to boost the performance and stability of photovoltaic (PV) devices. It is known that zinc oxide (ZnO) is one of the promising electron transporting layers for solar cells and is also applicable for flexible devices. However, the utilization of ZnO in PSCs is restricted due to its reactivity with the perovskite film during the annealing process. Here, we demonstrate improved photovoltaic performance and stability by introducing monolayer graphene (MLG) at the interface of the ZnO ETL and perovskite absorber, which results in a stable electric to power conversion efficiency (PCE) of 19.81%. The device based on this modified ETL maintains more than 80% of its initial PCE value after 300h under continuous illumination. Interestingly, we find that the presence of MLG at the ETL/perovskite interface not only improves the carrier extraction and photovoltaic properties but also protects the perovskite film from decomposition at elevated temperatures, which is beneficial for the stability of the device. To improve the stability even further, we have passivated the surface of the perovskite film by using a new modulator, i.e., 3-(pentafluorophenyl)-propionamide (PFPA) to abate the surface trap states of the perovskite. Based on our modification with MLG and PFPA, a stable PSC device with a PCE of 21% was achieved under AM 1.5G illumination with negligible hysteresis. The stability result indicates that the passivated device on MLG/ZnO maintains 93% of its initial PCE value after 300 h under continuous illumination.