N-type hydrogenated amorphous silicon oxide containing a microcrystalline silicon phase as an intermediate reflector in silicon thin film solar cells

To decrease the extent of degradation of a-Si: H/mu c-Si: H tandem solar cells it is important to reduce the thickness of the a-Si: H component cell. A silicon oxide intermediate reflector between top and bottom cell reflects the light back into the a-Si: H top cell and thus increases its current. For this purpose the reflector material has to fulfill certain optical and electrical requirements. Phosphorous doped silicon oxide films (a-SiOx:H < n >) with a certain volume fraction of microcrystalline silicon (mu cSi:H < n >) are used for this purpose and the trade-off between the material's optical and electrical properties is studied in detail. A suitable intermediate reflector is implemented in a tandem solar cell resulting in an increase in top cell current density by 1.3 mA/cm(2) with similar FF and V-oc compared to cells without intermediate reflector. An efficiency of 11.5 % is achieved at a top cell absorber layer thickness of only 250 nm. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim