The diverse passivation effects of fullerene derivative on hysteresis behavior for normal and inverted perovskite solar cells

[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is usually used to passivate the trap states and ion defects for suppressing hysteresis in both normal and inverted perovskite solar cells. Here, we find that the normal cell with the perovskite film fabricated on the PCBM film shows an evident hysteresis, while the inverted cell with the perovskite film covered by the PCBM film shows nearly no hysteresis, and the PCBM-Perovskite interface impacts on the hysteresis vastly. The poor PCBM surface wettability results in an inefficient carrier extraction at the interface and a red-shift of photoluminescence emission for the perovskite film coated on the PCBM film, which results from the plentiful trap states and ion defects inside the perovskite film due to its poor crystallization. Thus, the trapping/de-trapping processes and slow ion migration eventually lead to the hysteresis. For the perovskite film covered by the PCBM film, due to the good wettability of the perovskite film, PCBM molecules easily permeate into the perovskite film to passivate the trap states and ion defects, which improves the carrier extraction and thus eliminates the hysteresis of the inverted cell.