A Review on Solution-Processable Dopant-Free Small Molecules as Hole-Transporting Materials for Efficient Perovskite Solar Cells

Perovskite solar cells (PSCs) based on conventional hole-transporting materials (HTMs) have achieved power conversion efficiencies comparable to those of typical inorganic solar cells; however, the dopants used to increase the hole mobility or the film-forming ability impart these devices with a poor long-term stability, blocking the industrial commercialization of PSCs. As an alternative, HTMs without any dopants are explored. Herein, dopant-free small molecular HTMs (SM-HTMs) are reviewed and the performance based on the analyses of their molecular structures are evaluated. A summary of the designing principle and an outlook of the development of highly efficient SM-HTMs are presented.