Recent progress in improving strategies of inorganic electron transport layers for perovskite solar cells

Inorganic electron transport layers (ETLs) are considered the most promising materials for the construction of efficient and stable perovskite solar cells (PSCs) for commercialization owing to their distinct advantages, such as high physicochemical stability, simple preparation process, excellent photovoltaic properties, and low cost. Nonetheless, the unbalanced carrier transport between inorganic ETL and hole transport layer, a high trap state density at the interface between inorganic ETL and perovskite, and the probably mismatched energy levels with the absorbing layer lead to heavy photocurrent density-voltage hysteresis, inferior device stability, and low power conversion efficiency. Herein, strategies for improving inorganic ETLs are systematically reviewed, and the related microscopic mechanisms of advance are analyzed in detail. In addition, we summarize the inadequacies of current research and provide a perspective on how future research should be carried out to achieve the goal of commercialization of PSCs.