Application of microcrystalline Si1−xGex infrared absorbers in triple junction solar cells

In this contribution, we report the first results on the application of hydrogenated microcrystalline Si_1-xGe_x (μc-Si_1-xGe_x:H) as bottom cell absorber in triple junction solar cells. It has been demonstrated that μc-Si_1-xGe_x:H films prepared by PECVD in a composition range x~0.1-0.2 show an enhanced infrared response in a p-i-n single junction structure as compared with the conventional μc-Si:H (x=0) solar cells. Furthermore, in order to enhance the infrared absorption of solar cells, highly-textured front TCO layers with improved infrared transmittance have been developed by performing the chemical-etching of sputtered ZnO:Ga films followed by high-temperature (~600 °C) annealing. By applying these μc-Si_1-xGe_x:H and TCO layers to triple junction devices, so far, we obtained an initial efficiency of 11.6% (J_sc=9.2 mA/cm², V_oc=1.82 V, FF=0.70) in a structure of a-Si:H (0.18 μm)/μc-Si:H (2.0 μm)/μc-Si_0.9Ge_0.1:H (2.2 μm). Spectral response measurement indicates that the photocurrent densities of three component cells are nearly balanced at ~9 mA/cm² with a total photocurrent density of 27.6 mA/cm². Light-soaking test indicates that the degradation ratio is about 5%.