Understanding the Film Formation Kinetics of Sequential Deposited Narrow-Bandgap Pb-Sn Hybrid Perovskite Films

Developing efficient narrow bandgap Pb-Sn hybrid perovskite solar cells with high Sn-content is crucial for perovskite-based tandem devices. Film properties such as crystallinity, morphology, surface roughness, and homogeneity dictate photovoltaic performance. However, compared to Pb-based analogs, controlling the formation of Sn-containing perovskite films is much more challenging. A deeper understanding of the growth mechanisms in Pb-Sn hybrid perovskites is needed to improve power conversion efficiencies. Here, in situ optical spectroscopy is performed during sequential deposition of Pb-Sn hybrid perovskite films and combined with ex situ characterization techniques to reveal the temporal evolution of crystallization in Pb-Sn hybrid perovskite films. Using a two-step deposition method, homogeneous crystallization of mixed Pb-Sn perovskites can be achieved. Solar cells based on the narrow bandgap (1.23 eV) FA(0.66)MA(0.34)Pb(0.5)Sn(0.5)I(3) perovskite absorber exhibit the highest efficiency among mixed Pb-Sn perovskites and feature a relatively low dark carrier density compared to Sn-rich devices. By passivating defect sites on the perovskite surface, the device achieves a power conversion efficiency of 16.1%, which is the highest efficiency reported for sequential solution-processed narrow bandgap perovskite solar cells with 50% Sn-content.