Non-Annealing Treatment Mechanism of Electron Transport Layers for High-Performance Inverted Perovskite Solar Cells

Inverted perovskite solar cells (IPSCs) have presented great potential in the photovoltaic field due to their outstanding optoelectronic properties. However, the widely used electron transport layer of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in IPSCs presents some issues such as poor passivation effect, insufficient film coverage, and suboptimal energy-level alignment, and the influence of annealing treatment of PCBM on device performance has rarely been investigated. Therefore, in this work, we systematically study the influence of PCBM with or without the thermal annealing process (e.g., annealed PCBM or non-annealed PCBM) on the device performance of IPSCs. Surprisingly, it is found that the annealing treatment of PCBM potentially leads to collapse of the perovskite lattice and generation of PbI2, whereas the non-annealed PCBM exhibits uniform surface morphology and higher coverage integrity at the perovskite surface. Moreover, the device with non-annealed PCBM presents higher electron mobility and lower trap state density. Ultimately, IPSCs based on non-annealed PCBM achieve the highest power conversion efficiency (PCE) of 25.37%, accompanied by prolonged air stability, retaining 88.4% of the initial efficiency after being stored for 1360 h under ambient air.