Recent progress concerning inorganic hole transport layers for efficient perovskite solar cells

Typically, low cost as well as stability factors of the organo-metal halide perovskite solar cells based on inorganic hole transport layers (HTLs) have been the focus of intense research over the past few years. Accordingly, the power conversion efficiencies have rapidly been improved to ~ 20% with high stabilities. Therefore, this review covers the major advances of inorganic HTLs in perovskite solar cells that have contributed to the recent efficiencies and stabilities, including the evolution of device architecture, the development of hole transport material deposition processes, synthesis, morphology and the interface properties between inorganic HTLs and perovskite layers. Eventually, the challenges and future directions for inorganic HTLs-based perovskite solar cells are also discussed.