Fluoroalkyl-substituted fullerene/perovskite heterojunction for efficient and ambient stable perovskite solar cells

In this paper, we investigate the feasibility of using a fluoroallryl-substituted fullerene/perovskite heterojunction (f-FPHJ) to realize efficient and ambient stable perovskite solar cells (PVSCs). The hybrid fluoroalkyl-substituted fullerene, DF-C-60, is proven to effectively passivate the defects and grain boundaries in the perovskite film to facilitate charge transport/collection in the derived PVSC. Consequently, the f-FPHJ device yielded an enhanced PCE of 18.11%, outperforming that of the pristine CH3NH3PbI3 device (15.67%). More interestingly, the f-FPHJ PVSC showed a decent PCE of 15.14% (under reverse scan) with small hysteresis even without employing a discrete PCBM electron-transporting layer (ETL), in contrast to the pristine PVSC showing a lower PCE of 14.78% (under reverse scan) accompanied with severe hysteresis. This might arise from the preferential distribution of DF-C-60 nearby the surface region of the f-FPHJ film due to its low surface energy, which serves the similar function of the PCBM ETL in device to reduce hysteresis. More importantly, benefitting from the hydrophobic nature of DF-C-60, the f-FPHJ PVSC shows respectable ambient stability without encapsulation, which can maintain 83% of its initial PCE after being stored in ambient (with a relative humidity of 60 +/- 5%) for 1 month.