The influence of the optical band gap of buffer layers at the p/i- and i/n-side on the performance of amorphous silicon germanium solar cells

Profiled amorphous silicon germanium (a-SiGe:H)-buffer layers which are routinely used at the p/i- and i/n-side of a-SiGe:H solar cells, were replaced by simple constant band gap buffers. The optical band gap E_G of these 9 nm thick buffers was varied over a wide range between 1.3 eV (a-SiGe:H) and 2.0 eV (amorphous silicon carbide, a-SiC:H). The influence of the optical band gap EG of the buffer layer on V_oc and FF was investigated for both p- and n-side to find an optimum band gap. For a-Si:H as buffer material the optimum buffer thickness was found at 9 nm for both sides. Applying these optimised non profiled buffers in a-Si:H/a-SiGe:H tandem cells, a high stabilized efficiency of 9.4% was obtained with a SiGe layer thickness of only 100 nm.