Hole transporting materials in inorganic CsPbI_3−xBr_x solar cells: Fundamentals, criteria and opportunities

Inorganic cesium lead halide (i.e., CsPbI_3−xBr_x) perovskite solar cells have made great breakthroughs in the last years with power conversion efficiency beyond 20%, thermal and photo stability reaching hundreds of hours. Hole transporting materials, as important building blocks in perovskite solar cells, present significant influences on both performance and stability. Understanding the energy loss mechanisms and failure pathways of inorganic perovskite solar cells that are originated from the hole transporting layer and the adjacent interfaces paves the way to enhance the efficiency towards Shockley-Queisser limit and to approach the long-term stability requirement. In this review, we first briefly overview the fundamentals of inorganic perovskites and solar cells, particularly on the criteria for designing and engineering efficient hole transporting materials. Second, we give a comprehensive review of recent advances on inorganic, small molecular and polymeric hole transporting materials. Finally, we discuss the challenges of state-of-the-art inorganic perovskite solar cells in view of the hole transporting materials and conclude this review by providing perspective on development of advanced hole transporting materials towards next-generation efficient and stable inorganic PSCs.