Slow Spontaneous Efficiency Enhancement of Single-Crystal Perovskite Solar Cells Due to Trapped Solvent

Perovskite solar cells show spontaneous efficiency enhancement (SEE), i.e., their efficiency improves with time over a few days. Many mechanisms have been proposed for explaining such behavior in polycrystalline perovskite solar cells. However, such studies have not been performed on single-crystal perovskite solar cells. Here, we report our observation of slow (similar to 20 days) SEE in methylammonium lead iodide single-crystal perovskite solar cells. We found that the SEE is due to the slow escape of solvent trapped by the perovskite single-crystal thin films. We show that this process can be substantially accelerated by heat treatment of cells at 40 degrees C. This work provides a ground for fast-tracking the progress of single-crystal perovskite solar cells.