Inorganic p-Type Semiconductors as Hole Conductor Building Blocks for Robust Perovskite Solar Cells

Organic–inorganic halide perovskite solar cells (PSCs) have attracted considerable scientific and industrial interest because of their high efficiency and solution processability. As a key component, hole transport materials (HTMs) are critically important for achieving efficient hole extraction and thus high performance. However, the commonly employed organic HTMs such as 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spiro-bifuorene and poly(triarylamine) (PTAA) are quite expensive due to their complicated synthesis procedures and high purity requirements. Moreover, the instability of these organic compounds further hinders the large-scale commercialization of PSCs. To address these issues, various inorganic p-type semiconductors featuring excellent chemical stability, high hole mobility, and suitable energy levels are investigated as promising alternatives to their organic counterparts. Herein, recent advances of inorganic hole conductors for PSCs with a focus on low-cost fabrication and enhanced long-term stability are reviewed. A series of unexplored inorganic p-type semiconductors with desirable optoelectronic properties are also highlighted as promising candidates. In addition, the current challenges and potential strategies are presented to reveal the future opportunities in this research field.