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

Abstract

The perovskite/Cu(InGa)Se₂ (PSC/CIGS) tandem configuration is an attractive way to achieve an ultra-high 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 °C and was attenuated by only ∼10% at 1300 h. Moreover, the achieved well-matched J_sc 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.