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

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)Se₂ absorber layer with bandgaps around 1 eV or higher. Here, we investigate CuIn(Se_1 − xTe_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 − xTe_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, $V_{\mathrm{OC}}$ 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 $x=0.07$, 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.