Progress of research on new hole transporting materials used in perovskite solar cells

Perovskite solar cells with a solid-state thin film structure have attracted great attention in recent years due to their simple structure, low production cost and superb photovoltaic performance. Because of the boost in power conversion efficiency (PCE) in short intervals from 3.8% to 19.3% at present, this hybrid cells have been considered as the next generation photovoltaic devices. It is expected that the efficiencies of individual devices could ultimately achieve 25%, which is comparable to the single-crystal silicon solar cell.In this article, the perovskite absorber, its basic device structure, and operating principles are briefly introduced. Since most of the high efficiency perovskite solar cells employ hole transporting materials (HTM), they could benefit the hole transport and improve the metal-semiconductor interface in the cells. This perspective gives analyses of some effective hole transporting materials for perovskite solar cell application. The hole transporting materials used in perovskite solar cell are classified into six categories according to their structures, including triphenylamine-based small molecule HTM, small molecule HTM containing N atom, sulfur-based small molecule HTM, sulfur-based polymer HTM, polymer HTM containing N atom and inorganic HTM. Emphasis is placed on the interplay of molecular structures, energy levels, and charge carrier mobility as well as device parameters. A critial look at various approaches applied to achieve desired materials and device performance is provided to assist in the identification of new directions and further advances.