Shallow Iodine Defects Accelerate the Degradation of α-Phase Formamidinium Perovskite

Shallow defects are mostly benign in covalent semiconductors, such as silicon, given that they do not constitute non-radiative recombination sites. In contrast, the existence of shallow defects in ionic perovskite crystals might have significant repercussions on the long-term stability of perovskite solar cells (PSCs) because of the metastability of the ubiquitous formamidinium lead triiodide (FAPbI₃) perovskite and the migration of charged point defects. Here, we show that shallow iodine interstitial defects (I_i) can be generated unintentionally during commonly used post-fabrication treatments, which can lower the cubic-to-hexagonal transformation barrier of FAPbI₃-based perovskites to accelerate its phase degradation. We demonstrate that concurrently avoiding the generation of I_i and the more effective passivation of iodine vacancies (V_I) can improve the thermodynamic stability of the films and operational stability of the PSCs. Our most stable PSC retained 92.1 % of its initial performance after nearly 1,000 h of continuous illumination operational stability testing.