Ligand‐Mediated Revival of Degraded α‐CsPbI3 to Stable Highly Luminescent Perovskite

Although α‐CsPbI3 is regarded as an attractive optical luminophore, it is readily degraded to the optically inactive δ‐phase under ambient conditions. Here, we present a simple approach to revive degraded (“optically sick”) α‐CsPbI3 through “medication” with thiol‐containing ligands. The effect of different types of thiols is systematically studied through optical spectroscopy. The structural reconstruction of degraded α‐CsPbI3 nanocrystals to cubic crystals in the presence of thiol‐containing ligands is visualized through high‐resolution transmission electron microscopy and supported by X‐ray diffraction analysis. We found that 1‐dodecanethiol (DSH) effectively revives degraded CsPbI3 and results in high immunity towards moisture and oxygen, hitherto unreported. DSH facilitates the passivation of surface defects and etching of degraded Cs4PbI6 phase, thus reverting them back to the cubic CsPbI3 phase, leading to enhanced PL and environmental stability. After ageing under ambient conditions, the photoluminescence quantum yield (PLQY) of as‐synthesized CsPbI3 nanocrystals drastically declined to ≈11 % due to the formation of surface defect states. These “optically sick” CsPbI3 nanocrystals are then revived to their cubic phase through the addition of 1‐dodecanethiol (DSH), and regain a PLQY of approximately 90 %.