Investigations of high-performance GaAs solar cells grown on Ge-Si1-xGex-Si substrates

High-performance p(+) /n GaAs solar cells were grown and processed on compositionally graded Ge-Si1-xGex-Si (SiGe) substrates. Total area efficiencies of 18.1% under the AM1.5-G spectrum were measured for 0.0444 cm(2) solar cells. This high efficiency is attributed to the very high open-circuit voltages (980 mV (AMO) and 973 mV (AM1.5-G)) that were achieved by the reduction in threading dislocation density enabled by the SiGe buffers, and thus reduced carrier recombination losses. This is the highest independently confirmed efficiency and open-circuit voltage for a GaAs solar cell grown on a Si-based substrate to date. Larger area solar cells were also studied in order to examine the impact of device area on GaAs-on-SiGe solar cell performance; we found that an increase in device area from 0.36 to 4.0 cm(2) did not degrade the measured performance characteristics for cells processed on identical substrates. Moreover, the device performance uniformity for large area heteroepitaxial cells is consistent with that of homoepitaxial cells; thus, device growth and processing on SiGe substrates did not introduce added performance variations. These results demonstrate that using SiGe interlayers to produce "virtual" Ge substrates may provide a robust method for scaleable integration of high performance III-V photovoltaics devices with large area Si wafers.