Hole transporting materials for mesoscopic perovskite solar cells - towards a rational design?

Organic-inorganic halide perovskite solar cells (PSCs) have become one of the most exciting fields in the search for renewable energy sources with the power conversion efficiency (PCE) of PSCs having increased in only a few years from less than 5% to certified values of over 22%. To obtain high efficiencies in PSCs hole transporting materials (HTMs) are needed, and the HTM used in the best devices is so expensive as to make them potentially uncompetitive. In this review we give an overview of the types of HTMs that have been developed and discuss some of the strategies that have been used in designing HTMs for efficient PSCs, including the apparent effects of functional groups in and structures of organic HTMs on device parameters such as the PCE, fill factor (FF), open circuit voltage (Voc), and the stability of the resulting PSCs. We suggest some tentative design principles that might enable the rational design of a costeffective HTM for efficient PSCs.