A 19.9%-efficient ultrathin solar cell based on a 205-nm-thick GaAs absorber and a silver nanostructured back mirror

Conventional photovoltaic devices are currently made from relatively thick semiconductor layers, similar to 150 mu m for silicon and 2-4 mu m for Cu(In,Ga)(S,Se)(2), CdTe or III-V direct bandgap semiconductors. Ultrathin solar cells using 10 times thinner absorbers could lead to considerable savings in material and processing time. Theoretical models suggest that light trapping can compensate for the reduced single-pass absorption, but optical and electrical losses have greatly limited the performances of previous attempts. Here, we propose a strategy based on multi-resonant absorption in planar active layers, and we report a 205-nm-thick GaAs solar cell with a certified efficiency of 19.9%. It uses a nanostructured silver back mirror fabricated by soft nanoimprint lithography. Broadband light trapping is achieved with multiple overlapping resonances induced by the grating and identified as Fabry-Perot and guided-mode resonances. A comprehensive optical and electrical analysis of the complete solar cell architecture provides a pathway for further improvements and shows that 25% efficiency is a realistic short-term target.