Energy level-modulated non-fullerene small molecule acceptors for improved VOC and efficiency of inverted perovskite solar cells

The electron transport layer (ETL) plays a significant role in advancing the high efficiency and stability of solution-processed perovskite solar cells (PVSCs). Here, three commercialized n-type non-fullerene small molecule acceptors (IT-4X) with modulated energy levels are introduced as excellent alternative ETLs to replace the commonly used fullerene-based ETL in planar inverted PVSCs. The best-performing PVSCs are constructed by using the IT-4M as the ETL with a champion power conversion efficiency (PCE) of 17.65%, which is superior to that of the typical PCBM-based device. Among the PVSCs based on the three IT-4X ETLs, the differences of photovoltaic performances mainly exist in open-circuit voltage (VOC) and PCE, which are closely related to the built-in potential in PVSCs, directly influenced by the varied LUMOs of the IT-4X molecules. Furthermore, because of the superior hydrophobic properties, all the PVSCs based on IT-4X ETLs show improved long-term stability in the ambient atmosphere compared to the reference devices. In addition, the IT-4X molecules also deliver outstanding performance when used as the interfacial layer with PCEs approaching 19%. The results in this work present the promising prospect of non-fullerene small molecule acceptors for application in the PVSC field, and reveal that regulating the energy levels of ETLs is beneficial to maximize the device outputs of VOC and PCE.