CuIn (Se,Te)₂ Absorbers With Bandgaps <1 eV for Bottom Cells in Tandem Applications

ABSTRACT Thin‐film solar cells reach high efficiencies and have a low carbon footprint in production. Tandem solar cells have the potential to significantly increase the efficiency of this technology, where the bottom‐cell is generally composed of a Cu(In,Ga)Se 2 absorber layer with bandgaps around 1 eV or higher. Here, we investigate CuIn(Se 1 − x Te x ) 2 absorber layers and solar cells with bandgaps below 1 eV, which will bring the benefit of an additional degree of freedom for designing current‐matched two‐terminal tandem devices. We report that CuIn(Se 1 − x Te x ) 2 thin films can be grown single phase by co‐evaporation and that the bandgap can be reduced to the optimum range (0.92–0.95 eV) for a bottom cell. From photoluminescence spectroscopy, it is found that no additional non‐radiative losses are introduced to the absorber when adding Te. However, losses occur in the final solar cell due to non‐optimized interfaces. Nevertheless, a device with 9% power conversion efficiency is demonstrated with a bandgap of 0.97 eV and , the highest efficiency so far for chalcopyrites with band gap <1 eV. Interface recombination is identified as a major recombination channel for larger Te contents. Thus, further efficiency improvements can be expected with improved absorber/buffer interfaces.