Self-assembled monolayers as hole-transporting materials for inverted perovskite solar cells

The emerging perovskite solar cells (PSCs) have been explored as the most promising photovoltaic technology in the past decade, with the sharp increase of the power conversion efficiency (PCE) from 3.8% to certified 26.1%, comparable to that of crystalline silicon solar cells. Compared to conventional PSCs, inverted PSCs show attractive advantages, such as high device stability, negligible hysteresis and excellent compatibility with flexible and tandem devices. Self-assembled monolayers (SAMs) have been considered as one of the most promising hole-transporting materials (HTMs) for inverted PSCs owing to their low costs and material consumption and simple device fabrication with high PCEs. This review summarizes the recent developments in highly efficient SAMs as HTMs for inverted PSCs. On the basis of the anchoring group, three categories of SAMs are identified and discussed: SAMs with phosphonic acid, SAMs with carboxylic acid, and SAMs based on other anchoring groups. Finally, a future outlook of SAMs for high-performance inverted PSCs is provided. We hope that this review will be useful for the further design of SAMs toward the eventual commercialization of PSCs.