Carbonyl functional group assisted crystallization of mixed tin–lead narrow bandgap perovskite absorber in ambient conditions

Tin–lead (Sn–Pb) perovskite solar cells are receiving growing interest due to their applications in tandems and lead mitigation. Nonetheless, fast crystallization and facile Sn2+ oxidation restrict their ambient fabrication, which increases fabrication costs. This Letter presents an experimental study on additive assisted growth of FA0.2MA0.8Sn0.5Pb0.5I2.4Br0.6 narrow bandgap perovskite films employing a Lewis-base molecule, caffeine (1,3,7-trimethylpurine-2,6-dione), having two carbonyl functional groups (C = O) in ambient conditions (relative humidity < ∼10%). The C = O interacts with metallic ions (Sn2+ and Pb2+) via chelation to form an acid–base adduct, slowing down the fast crystallization of FA0.2MA0.8Sn0.5Pb0.5I2.4Br0.6 perovskite films. As a result, the grain size improves resulting in better structural and optical properties. In contrast, Urbach energy values showed higher electronic disorder near the band edges even upon caffeine doping implying Sn4+ doping in an ambient environment. This work accentuates the potential of the acid–base adduction to regulate uncontrolled crystallization of Sn–Pb perovskites in the ambient environment.