Thin-film Si_1-xGe_x HIT solar cells

We fabricated silicon-germanium (Si_1-xGe_x) based HIT solar cells with x=0, 0.25, 0.41 and 0.56 in order to quantify the effect of germanium fraction on key solar cell performance parameters. The p-type absorber layer consists of 2 and 4μm Si_1-xGe_x layer grown on p+ silicon substrate using a graded buffer layer to reduce the threading dislocation density. The emitter is n+ amorphous-Si. A thin strained-Si layer is grown on the c-Si_1-xGe_x layer prior to a-Si deposition and is believed to improve a-Si-H/c-Si_1-xGe_x interface quality. The short-circuit current (J_sc) increases, from ~14mA/cm² for Si cells to 21mA/cm² for Si_0.44Ge_0.56 cells with 2μm-thick active layers, while open-circuit voltage decreases. The spectral response of the Si_1-xGe_x solar cells improves due to a reduction in absorption depth and smaller band-gap associated with the higher germanium fractions.