New advances in small molecule hole-transporting materials for perovskite solar cells

Organic π-functional molecules are the foundation and basic component of organic optoelectronic devices. For example, for ideal carrier transporting materials, extended π-conjugation and ordered π–π stacking are necessary to enhance the charge mobility and achieve desirable results. As a promising way to convert sunlight into electricity, organometal halide perovskite solar cells (PSCs) have captured a lot of attention due to its predominant merits especially in the aspect of remarkable photovoltaic performance and much potentially low production cost. For conventional planar PSC structure, hole-transporting layer which typically consists of organic π-functional materials plays a key role in suppressing hole–electron pair recombination, promoting charge transporting and ensuring ohmic contact of back electrode. Considering the key roles of HTMs and its soaring progress in recent years, here, we will summarize recent progress in small organic π-functional materials from its diverse functions in PSCs. Besides, aiming to further promote the development of organic π-functional molecules and HTMs, a promising direction toward highly efficient HTMs will also be discussed.