Spiro-linked organic small molecules as hole-transport materials for perovskite solar cells

Organic-inorganic halide perovskite solar cells (PSCs) have attracted great attention as an alternative renewable photovoltaic technology with a power conversion efficiency (PCE) > 22%, which is on par with established technologies. The state-of-the-art spiro-OMeTAD (2,2',7,7' -tetrakis-(N, N-di-p-methoxyphenylamine)- 9,9' -spirobifluorene) is the most successful hole-transport material (HTM) employed in PSCs. Recently, various types of spiro-linked organic small molecules have been reported to overcome the reported disadvantages of spiro-OMeTAD, such as a complex synthetic route, high synthetic cost, and requirement for hygroscopic dopants to improve the charge-carrier mobility and device performance, which significantly limits spiro-OMeTAD for large-scale application in the future. Herein, we provide an overview of recent developments in the design, synthesis, and characterization of spiro-linked organic small molecules as hole-transport materials (HTM) in perovskite solar cells (PSCs).