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

Abstract

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.