Ultrathin GaAs Solar Cells With a Silver Back Mirror

We report on the fabrication and characterization of ultrathin GaAs solar cells with a silver back mirror and absorber thicknesses of only t = 120 nm and t = 220 nm. The silver back mirror is combined with localized ohmic contacts. Without antireflection coating, Fabry-Perot resonances lead to strong enhancement over single-pass absorption (up to 4), and external quantum efficiency reaches 0.8 at resonance wavelengths. An analytical model is used to determine the resonance wavelengths and the absorption maxima. A short-circuit improvement of 27% results from the enhanced absorption induced by the Fabry-Perot resonances. By implementing an additional antireflection coating, short-circuit currents reach 16.3 mA/cm2fort = 120 nm and 20.7 mA/cm2for t = 220 nm, corresponding to efficiencies of 8.7 % and 12.9 %, respectively.