Bifunctional ultrathin PCBM enables passivated trap states and cascaded energy level toward efficient inverted perovskite solar cells

Inverted perovskite solar cells (PSCs) with a C₆₀ 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-C₆₁-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 N₂ 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 C₆₀ framework with improved efficiency and stability.