Light Trapping in Ultrathin CIGS Solar Cells with Nanostructured Back Mirrors

Novel architectures for light trapping in ultrathin Cu(In,Ga)Se $_2$ (CIGS) solar cells are proposed and numerically investigated. They are composed of a flat CIGS layer with nanostructured back mirrors made of highly reflective metals. Multi-resonant absorption is obtained for two different patterns of nanostructured mirrors. It leads to a dramatic increase in the short-circuit current predicted for solar cells with very thin CIGS layers. We analyze the resonance phenomena and the density of photogenerated carriers in the absorber. We discuss the impact of the material used for the buffer layer (CdS and ZnS) and the back mirror (Mo, Cu, Au, and Ag). We investigate various CIGS thicknesses from 100 to 500 nm, and we compare our numerical results with experimental data taken from the literature. We predict a short-circuit current of $J_{\text{sc}}$ = 33.6 mA/cm $^2$ for a realistic solar cell made of a 200-nm-thick CIGS absorber with a copper nanostructured mirror. It opens a way toward ultrathin CIGS solar cells with potential conversion efficiencies up to 20%.