High-quality perovskite in thick scaffold: a core issue for hole transport material-free perovskite solar cells

Organic-inorganic perovskite solar cells (PSCs) have attracted intense attention in the last few years due to the phenomenal increase in power conversion efficiency (PCE), but their low stability has greatly hindered their practical application. By removing unstable hole transport materials (HTM), the device stability of HTM-free PSCs has been greatly improved. However, the PCE has largely lagged behind those of HTM-based PSCs. We contend that deposition of high-quality perovskite into a thick scaffold is the key to achieving high-performance, HTM-free PSCs. Indeed, a few deposition methods have been used to successfully deposit a high-quality perovskite layer into a relatively thick TiO2 scaffold, hence producing PSCs with relatively high PCEs. In this review, we will introduce the basic working principle of HTM-free PSCs and analyze the important role of thick TiO2 scaffold. Most importantly, the problems of the conventional perovskite deposition methods in thick TiO2 scaffold will be examined and some recent successful deposition methods will be surveyed. Finally, we will draw conclusions and highlight some promising research directions for HTM-free PSCs.