Flexible Monolithic 2‐Terminal Kesterite Cu ₂ ZnSnSe ₄ /Perovskite Tandem Solar Cells

Growing demand for flexible photovoltaics has intensified interest in high‐efficiency tandem solar cells. This study demonstrates flexible monolithic kesterite/perovskite tandems by addressing the poor morphology and limited long‐wavelength response of low‐bandgap kesterite bottom cells. To enable flexible and tandem device applications, high‐quality Cu 2 ZnSnSe 4 (CZTSe) films were grown at 480 °C using a low‐temperature, single‐step coevaporation process compatible with flexible polyimide (PI) substrates. The incorporation of Ag further enhances crystallization and enables the defect passivation in the CZTSe absorber layer. Admittance spectroscopy reveals that Ag incorporation reduces the defect density of states from 1.4 × 10 18 to 8.7 × 10 17 cm −3 eV −1 . Consequently, the single‐junction efficiency improves from 5.6% to 8.7% on glass and reaches 7.3% on flexible PI substrates. Integration with a perovskite top cell yields a tandem efficiency of 16.4% on glass and a record 11.35% on flexible PI. Furthermore, the flexible device retains ~70% of its initial efficiency after 1000 bending cycles. These results highlight the potential of Ag‐doped CZTSe fabricated via low‐temperature coevaporation for high‐performance, earth‐abundant, and flexible tandem photovoltaics.