Application of Organic Hole-Transporting Materials in Perovskite Solar Cells

Organic-inorganic halide perovskite solar cells (PSCs) have attracted increasing attention because of their desirable properties. A key advance has been the replacement of the liquid electrolytes by solid-state hole-transporting materials (HTMs), which not only improves the power conversion efficiency (PCE) but also enhances the cell stability. HTMs are now an integral part of PSCs. Both organic and inorganic HTMs have found application in PSCs. However, inorganic HTMs are hampered by the limited choice of materials and the relatively low PCE of the solar cells based on them. The development of new organic HTMs is therefore necessary to improve the PCE and stability of PSCs. This has become a focus of various research fields, and new HTMs continue to emerge in large numbers. In this paper, we give an overview of the use of organic HTMs in PSCs. According to their molecular weight, organic HTMs are classified as either molecular or polymeric. We discuss in detail the effects of the functional groups and structures of organic HTMs on the PCE, fill factor, open circuit voltage, and stability of the resulting PSCs, as developed in recent years. The paper also covers the highest occupied molecular orbitals, the hole mobility, and the use of additives in HTMs. Finally, forecasts of the future development of organic HTMs are reviewed.