Flexible Narrow Bandgap Sn–Pb Perovskite Solar Cells with 21% Efficiency Using <i>N</i>,<i>N</i>′-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 <i>N</i>,<i>N</i>'-carbonyldiimidazole (CDI) passivation strategy. CDI, with strong adsorption energy, preferentially binds to Sn²⁺ compared with oxygen (O₂), thus effectively inhibiting the adsorption of O₂ on perovskite surfaces. The transfer of electron density around Sn²⁺ dramatically decreased, thus suppressing Sn²⁺ 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.