Mechanism of Water Effect on Enhancing the Photovoltaic Performance of Triple-Cation Hybrid Perovskite Solar Cells.

The water effect on the performance of perovskite solar cells has been intensively studied in recent years. However, the conflicting conclusions derived from different studies make it impossible to fully understand the mechanism involved. Besides, all studies have concentrated on single methylammonium cation perovskites. As a consequence, the effects of water on formamidinium and cesium perovskites are still unclear. Herein, we introduce water during the fabrication of triple-cation hybrid perovskites. By controlling the water content, we demonstrate that an optimal concentration of water contributes to a better crystallized and more uniform hybrid perovskite film without impurities, resulting in significant enhancement in power conversion efficiency and long-term stability. In addition, two forms of water (hydrate water and bulk water) are found in the hybrid perovskite film. Hydrate water induces a recrystallization process, whereas bulk water leads to decomposition of the perovskite. These distinct phases are considered to form the basic mechanism affecting the performance of the cells.