Antisolvent‐Free Heterogenous Nucleation Enabled by Employing 4‐Tert‐Butyl Pyridine Additive and Two‐Step Annealing for Efficient CsPbI₂Br Perovskite Solar Cells

Abstract All inorganic perovskite based on CsPbI₂Br has attracted significant attention due to its relatively thermal stable structure compare to its hybrid counterparts. With a wide bandgap of 1.9 eV and excellent light absorption capability, it has been extensively explored for applications in indoor photovoltaics and as a front absorber in tandem devices. However, the uncontrollable crystallization process during solvent evaporation and thermal annealing leads to both macroscopic defects like cracks and microscopic defects such as voids. In this study, a metastable adduct with lead (II) halides by incorporating 4‐tert‐butyl pyridine as a volatile Lewis base monodentate ligand in the precursor solution is formed. The strategic preferential decomposition of the adduct during the early‐stage low‐temperature annealing facilitated the desorption of lead (II) halides, inducing antisolvent‐free heterogenous nucleation. This, in turn, promoted crystal growth during subsequent high‐temperature annealing, resulting in dense films with low defect density. As a result, a maximum open‐circuit voltage of 1.30 V is achieved with the champion power conversion efficiency of 16.5% in CsPbI₂Br‐based perovskite solar cell. The work reveals a new mechanism of using Lewis acid‐base adduct to obtain high quality perovskite films other than hindering crystallization in traditional way.