Flexible Narrow Bandgap Sn–Pb Perovskite Solar Cells with 21% Efficiency Using N,N′-Carbonyldiimidazole Treatments

Flexible tin–lead (Sn–Pb) mixed perovskite solar cells (PSCs) are among the promising flexible photovoltaics, owing to the narrow bandgap (NBG) of Sn–Pb perovskites, flexible and wearable features, and their role as a critical component in all-perovskite tandem photovoltaics. However, the flexible Sn–Pb PSCs suffer from a low power conversion efficiency, no higher than 18.5%, along with limited stability. Herein, we reported an efficient and stable flexible NBG Sn–Pb PSC via an N, N ′-carbonyldiimidazole (CDI) passivation strategy. CDI, with strong adsorption energy, preferentially binds to Sn 2+ compared with oxygen (O 2 ), thus effectively inhibiting the adsorption of O 2 on perovskite surfaces. The transfer of electron density around Sn 2+ dramatically decreased, thus suppressing Sn 2+ oxidation. The CDI treatments endowed the Sn–Pb mixed films with fewer defects, improved crystallinity, better morphology, and matched energy-level alignment. The flexible Sn–Pb devices exhibited a high PCE of 21.02%. Besides, the devices showed enhanced stability and promoted flexibility. This work provides a pathway to visibly increase the efficiency and stability of the flexible Sn–Pb mixed photovoltaic cells.