Mobile-Ion-Induced Degradation of Organic Hole-Selective Layers in Perovskite Solar Cells

Organometal halide perovskites are mixed electronic–ionic semiconductors. It is imperative to develop a deeper understanding of how ion-migration behavior in perovskites impacts the long-term operational stability of solar cells. In this work, we found that ion penetration from the perovskite layer into the adjacent organic hole-selective layer is a crucial cause of performance degradation in perovskite solar cells. The monovalent cation, namely, methylammonium (MA+), is the main ion species that penetrates into the organic hole-selective layer of Spiro-MeOTAD because of the built-in electric field during operation. The incorporation of MA+ induces deep-level defects in the Spiro-MeOTAD layer and thereby deteriorates the hole-transporting ability of Spiro-MeOTAD, degrading solar cell performance. Our work points to two ways to improve the stability of perovskite solar cells: one is to insert a compact ion-blocking layer between Spiro-MeOTAD and perovskite, and the other is to find a hole-selective layer that is insensitive to extraneous ions (MA+).