Inorganic Hole-Transporting Materials for Perovskite Solar Cells

In the last few years, inorganic-organic metal halide perovskite solar cells (PSCs) have attracted a great deal of attention as a promising next-generation solar-cell technology because of their high efficiencies and low production cost. Hole-transporting materials (HTMs) play an essential role in effective charge extraction and thus in achieving high overall efficiency. Therefore, searching for an efficient, stable, and low-cost HTM in PSCs has been one of the hottest research topics in this field. Inorganic p-type semiconductors that possess several appealing characteristics, such as suitable energy levels, high hole mobility, and high chemical stability, as well as low production cost, etc., are promising HTM candidate materials in PSCs. Here, specific attention is paid to the recent progress in inorganic HTMs being explored for PSCs. A variety of methods developed for the fabrication of these inorganic HTMs are summarized in detail, together with their corresponding performance in PSCs. Finally, an outlook on further enhancements of highly efficient PSCs based on inorganic HTMs is presented.