Conventional photovoltaic devices are currently made of relatively thick semiconductor layers, about 150 µm for silicon, and 2-4 µm for CIGS, CdTe or III-V direct bandgap semiconductors. Ultrathin solar cells using 10 times thinner absorbers could lead to considerable material and processing time savings. 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 multiresonant absorption in planar active layers, and we report a 205 nm-thick GaAs solar cell with a certified 19.9% efficiency. 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 the pathway for further improvements and shows that 25% efficiency is a realistic shortterm target.

Doping is a fundamental property of semiconductors and constitutes the basis of modern microelectronic and optoelectronic devices. Their miniaturization requires contactless characterization of doping with nanometer scale resolution. Here, we use low-and room-temperature cathodoluminescence (CL) measurements to analyze p-type and n-type GaAs thin films over a wide range of carrier densities (2 × 10 17 to 1 × 10 19 cm −3 ). The spectral shift and broadening of CL spectra induced by shallow dopant states and band filling are the signature of doping. We fit the whole spectral lineshapes with the generalized Planck law and refined absorption models to extract the bandgap narrowing (BGN) and the band tail for both doping types, and the electron Fermi level for n doping. This work provides a rigorous method for the quantitative assessment of p-type and n-type carrier density using CL. Taking advantage of the high spatial resolution of CL, it can be used to map the doping in GaAs nanostructures, and it could be extended to other semiconductor materials.