Promotion Strategies of Hole Transport Materials by Electronic and Steric Controls for n–i–p Perovskite Solar Cells

Hole transport materials (HTMs) play a requisite role in n-i-p perovskite solar cells (PSCs). The properties of HTMs， such as hole extraction efficiency, chemical compatibility, film morphology, ion migration barrier, etc., significantly affect the power conversion efficiency (PCE) and stability of PSCs. Up till now, researchers have devoted to developing new types of HTMs as well as promoting pristine HTMs using various strategies. In this review, we comprehensively summarize the design strategies of various common HTMs for n-i-p PSCs from two separate aspects, additive engineering and non-additive engineering. As concluded, additive engineering typically tunes electronic properties of HTMs, whereas non-additive engineering basically modifies their steric structures. A critical analysis and comparison between these design strategies is provided, considering the overall PCE and stability of PSCs. Finally, we give a brief perspective on future promising design strategies for HTMs to fabricate efficient and stable n-i-p devices for the commercialization of PSCs.