Interface modification with PCBM intermediate layers for planar formamidinium perovskite solar cells

Although two-step methods are convenient for producing perovskite layers in mesoporous perovskite solar cells, they usually lead to intensive recombination for planar perovskite devices based on formamidinium ions. In this study, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was employed as the intermediate layer between TiO2 electron transport layers and formamidinium perovskite layers. SEM images indicate that this PCBM intermediate layers encourage the growth of grains in formamidinium perovskite layers. XRD patterns show that these PCBM layers eliminate the yellow δ-phase, thereby improving crystallization. Via optimizing the concentration of PCBM precursor solutions, the average power conversion efficiency of formamidinium planar devices was promoted from 6.8% to 15.1%. Further, it is also noted that devices with optimized PCBM layers are free from the hysteresis effect. Impedance spectra indicate that this improvement comes from the suppression of recombination processes by the PCBM intermediate layers.