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 (FAPbI(3)) 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 FAPbI(3)-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.