Demonstration of overcoming 20% efficiency in kesterite/perovskite tandem solar cells on rigid and flexible substrates

Among emerging thin-film solar cells, kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) absorbers offer several advantages: they are based on earth-abundant, non-toxic elements and combine high stability, tunable bandgap (E g ), and flexibility, key features for integrated photovoltaic (PV) applications. Selenium-rich CZTSSe (E g of 1.1 eV) is an ideal bottom-cell candidate for flexible tandem devices with perovskites, thereby maximising efficiency. However, CZTSSe/perovskite tandems are still underexplored. Here, we report an effective solution - based route for producing high-efficiency CZTSSe bottom cells on both rigid Mo-coated soda-lime-glass and flexible Mo-foil substrates. To optimise absorber morphology and grain size, Na-doping and Ag-alloying were performed. Then, 4-Terminal (4T) tandem devices combining kesterite- and perovskite-based subcells were designed. Solution-processed Cs 0.17 FA 0.83 Pb(I 0.90 Br 0.10 ) 3 perovskites (E g of 1.63 eV) ensured optimal bandgap matching and broadened light harvesting, yielding efficiencies exceeding 22% and 20% for rigid and flexible 4T tandem devices, respectively. This proof-of-concept solution-processed tandem approach represents a promising step toward developing cost-effective and sustainable PV technologies, with promising results for future solar energy applications.