Ascorbic Acid-Assisted Stabilization of α-Phase CsPbI3 Perovskite for Efficient and Stable Photovoltaic Devices

The all-inorganic alpha-CsPbI3 perovskite with superb thermal stability and suitable band gap for light harvesting has been considered as a promising candidate for efficient perovskite solar cells (PSCs). However, the photoactive black alpha-CsPbI3 is thermodynamically unstable and transforms spontaneously into nonphotoactive yellow delta-phase at room temperature. Herein, a facile method is reported to prepare alpha-CsPbI3 perovskite films with high stability at room temperature by mixing a small amount of ascorbic acid (AA) in the CsPbI3 precursor solutions. It is revealed that the interaction of AA with the CsPbI3 precursors could effectively inhibit the rapid crystallization of CsPbI3 and reduce the size of the coordination colloidal, and thus decrease the grain size of CsPbI3 for preparing long-term stable alpha-CsPbI3 films. The PSCs based on the AA-stabilized CsPbI3 films exhibit reproducible photovoltaic performance with a champion efficiency of up to 11.44% and stable output of 11.30%, along with excellent stability, retaining more than 76% of its initial efficiency after aging in ambient conditions for 250 h without encapsulation. Most importantly, such low-cost, solution-processable inorganic PSCs with high performance also show promising potential for large-scale preparation.