Bifunctional Ultrathin PCBM Enables Passivated Trap States and Cascaded Energy Level toward Efficient Inverted Perovskite Solar Cells.

Inverted perovskite solar cells (PSCs) with a C framework are known for their common drawback of low power conversion efficiency (PCE) of <20% because of nonradiative recombination and inefficient charge transport at their perovskite interfaces. Here, we report an ultrathin [6,6]-phenyl-C-butyric acid methyl ester (PCBM) as a cap layer on perovskite films to overcome this issue. Such a functional cap layer efficiently passivates trap states and establishes a gradient energy level alignment onto perovskite, facilitating the efficient charge transfer and extraction. The as-fabricated inverted PSCs capped with such ultrathin PCBM exhibit a record PCE of 20.07%. After the storage under a N atmosphere for more than 500 h, the PCE of PSCs retains over 85% of its initial level. Our work provides an effective method to upgrade inverted PSCs with the C framework with improved efficiency and stability.