Hydrogenated amorphous silicon oxide containing a microcrystalline silicon phase and usage as an intermediate reflector in thin-film silicon solar cells

Abstract

To further improve the stability of amorphous/microcrystalline silicon (a-Si:H/μc-Si:H) tandem solar cells, it is important to reduce the thickness of the a-Si:H top cell. This can be achieved by introduction of an intermediate reflector between the a-Si:H top and the μc-Si:H bottom cell which reflects light back into the a-Si:H cell and thus, increases its photocurrent at possibly reduced thickness. Microcrystalline silicon oxide (μc-SiOx:H) is used for this purpose and the trade-off between the material’s optical, electrical and structural properties is studied in detail. The material is prepared with plasma enhanced chemical vapor deposition from gas mixtures of silane, carbon dioxide and hydrogen. Phosphorus doping is used to make the material highly conductive n-type. Intermediate reflectors with different optical and electrical properties are then built into tandem solar cells as part of the inner n/p-recombination junction. The quantum efficiency and the reflectance of these solar cells are evaluated to find optical gains and losses due to the intermediate reflector. Suitable intermediate reflectors result in a considerable increase in the top cell current density which allows a reduction of the a-Si:H top cell thickness of about 40% for a tandem cell while keeping the current density of the device constant.