Growth of (Ag,Cu)(In,Ga)Se₂ Absorbers under Band Gap Variation and Characterization with a Focus on Optical Spectroscopy

Whilst Cu(In,Ga)Se2 (CIGSe) is an extremely promising material for solar cell fabrication, the widening of the band gap beyond the standard 1.1 eV is highly desirable for semitransparent applications. By replacing Cu with Ag and increasing the Ga content, we fabricate ACIGSe absorbers with band gaps ranging from 1.27–1.55 eV. An Ag/(Ag + Cu) ratio from 0.36–1.00 is chosen, as well as a Ga/(Ga + In) ratio from 0.25–0.59. The larger Ag and Ga contents lead to the expected band gap widening, which is, together with high sub-gap transparency, essential for semitransparent applications. The crystalline properties are confirmed by Raman spectroscopy and X-ray diffraction, which both reveal peak shifts according to the composition variations: a higher Ag content results in lower Raman shifts as well as in lower angles of X-ray diffraction for the main peaks due to the larger mass of Ag compared to Cu and the larger lattice constant of Ag-rich compounds. Increased open circuit voltages and decreased short circuit current densities are confirmed for higher band gaps. An overall trend of increased power conversion efficiency of the related devices is promising for future research of wide band gap Ag-chalcopyrites and their semitransparent application