Over 28% efficiency perovskite/Cu(InGa)Se₂ tandem solar cells: highly efficient sub-cells and their bandgap matching

The perovskite/Cu(InGa)Se-2 (PSC/CIGS) tandem configuration is an attractive way to achieve an ultrahigh efficiency and cost-effective all-thin-film solar cell. However, the imbalanced efficiencies of the constituent sub-cells and the bandgap mismatch generally limit the achieved overall power conversion efficiency (PCE). Herein, by combining Cl bulk incorporation and piperazinium iodide (PDI) surface treatment for the CsFAPb(IBr)(3) system, we have successfully reduced the bulk and interface defect densities and greatly enhanced the open-circuit voltage and thus the PCE of the 1.67 eV-bandgap PSCs. The accompanied high activation energy of ion migration has also led to high photostability with suppressed phase segregation. By integrating a highly efficient and highly infrared transparent 1.67 eV PSC top cell with various bandgap CIGS solar cells of comparable efficiency, we have demonstrated that a 1.04 eV CIGS bottom cell achieved ideal bandgap matching and realized a four-terminal tandem cell with a calculated tandem efficiency of 28.4%, currently the highest for PSC/CIGS tandem cells. The PSC device showed no degradation after 600 h of operation at 50 degrees C and was attenuated by only B10% at 1300 h. Moreover, the achieved well-matched Jsc between top and bottom sub-cells and the adopted inverted-PSC configuration have paved the way for producing highly efficient monolithic two-terminal PSC/CIGS tandem devices in the near future.