Researchers from the University of Milano-Bicocca, Polytechnic University of Catalonia (UPC), Istituto di Struttura della Materia (CNR), University of Rome “Tor Vergata” and New Energies have demonstrated solution-processed perovskite/kesterite tandem solar cells that surpass 22% power conversion efficiency in a 4‑terminal architecture on rigid substrates and exceed 20% on flexible devices.
Their work focuses on kesterite Cu2ZnSn(S,Se)4 (CZTSSe) absorbers, which are based on earth-abundant, non-toxic elements and combine high stability, tunable bandgap and mechanical flexibility, making them attractive for integrated photovoltaic applications. In this study, the team employs selenium-rich CZTSSe with a bandgap of 1.1 eV as the bottom cell, an energetically well-suited choice for tandem operation because it captures the near‑infrared portion of the spectrum that passes through a wide-bandgap top cell.
The CZTSSe bottom absorbers are fabricated via an effective solution-based route on both rigid Mo-coated soda-lime glass and flexible Mo foil, showing that the same chemistry can be adapted to conventional and flexible platforms. To optimize absorber morphology and grain size, the researchers introduce Na doping and Ag alloying into the kesterite layer, which promotes larger grains, improved crystallinity and a more homogeneous microstructure, all of which are beneficial for charge transport and reduced recombination. On top of these optimized kesterite cells, they assemble mechanically stacked 4‑terminal tandem devices in which the bottom and top subcells operate independently at their respective maximum power points, avoiding strict current-matching constraints.
For the top cell, the team employs a solution-processed Cs0.17FA0.83Pb(I0.90Br0.10)3 perovskite with a bandgap of 1.63 eV, chosen to provide good bandgap complementarity with the 1.1 eV CZTSSe absorber. This spectral splitting scheme allows the perovskite to harvest higher-energy visible photons while the kesterite absorbs lower-energy near‑infrared photons, thereby broadening light harvesting and reducing thermalization losses.
As a result, the 4‑terminal tandem devices achieve power conversion efficiencies exceeding 22% on rigid substrates and 20% on flexible substrates, clearly outperforming what either subcell could deliver alone. This proof-of-concept, fully solution-processed tandem approach highlights a promising route toward cost-effective, sustainable solar technologies that combine high efficiency with the advantages of earth-abundant materials, scalable processing and mechanical flexibility.
