Rethinking the role of excess/residual lead iodide in perovskite solar cells

It is widely believed that excess/residual lead iodide (PbI${}_2$) can affect the performance of perovskite solar cells . Moderate PbI${}_2$ can enhance efficiency by passivating defects, while extremely active PbI${}_2$ leads to non-negligible hysteresis effects and reduces device stability. Although several efforts are made to investigate the role of excess PbI${}_2$, its impact is still underestimated. Recent advances further demonstrate the extraordinary potential of modifying excess PbI${}_2$; however, a comprehensive study is required to obtain a deeper understanding. Herein, the important breakthroughs regarding excess PbI${}_2$ are reviewed and the mechanism of excess PbI${}_2$ in terms of efficiency and stability is rethought. In addition, the origins, verification, and regulation of residual PbI${}_2$ are summarized. (© 2023 Wiley‐VCH GmbH)