Recent Progress in the Semiconducting Oxide Overlayer for Halide Perovskite Solar Cells

Halide perovskite solar cells (HPSCs) contain charge transport layers (CTLs) both above and below the photoactive perovskite layer. These semiconducting CTLs are just as important as the perovskite layer to fully realizing the potential of perovskite materials. In particular, semiconducting oxide overlayer materials (SOOMs) are expected to lower costs and provide better long-term stability compared to the organic semiconducting materials commonly used for the upper layer. However, SOOM-based HPSCs are currently less efficient than conventional devices owing to SOOM's deposition constraints imposed by the underlying perovskite layer. This progress report focuses on the recent evolution of SOOM-based HPSCs by describing the key issues and recent advances in SOOM deposition methods. Finally, remaining challenges and future research directions for SOOMs are discussed to provide guidance toward the commercialization of HPSCs.