Review of current progress in hole-transporting materials for perovskite solar cells

Recent advancements in perovskites' application as a solar energy harvester have been astonishing. The power conversion efficiency (PCE) of perovskite solar cells (PSCs) is currently reaching parity (>25 percent), an accomplishment attained over past decades. PSCs are seen as perovskites sandwiched between an electron transporting material (ETM) and a hole transporting material (HTM). As a primary component of PSCs, HTM has been shown to have a considerable effect on solar energy harvesting, carrier extraction and transport, crystallization of perovskite, stability, and price. In PSCs, it is still necessary to use a HTM. While perovskites are capable of conducting holes, they are present in trace amounts, necessitating the use of an HTM layer for efficient charge extraction. In this review, we provide an understanding of the significant forms of HTM accessible (inorganic, polymeric and small molecule-based HTMs), to motivate further research and development of such materials. The identification of additional criteria suggests a significant challenge to high stability and affordability in PSC.