Reprint, 21 European Photovoltaic Solar Energy Conference and Exhibition, Dresden 2006 FABRICATION OF HIGH EFFICIENCY MICROCRYSTALLINE AND MICROMORPH THIN FILM SOLAR CELLS ON LPCVD ZnO COATED GLASS SUBSTRATES

The fabrication of high efficiency (9-10%) microcrystalline silicon (μc-Si:H) p-i-n solar cells on glass has been so far reported mostly for cells deposited on sputtered and etched zinc oxide (ZnO). In this paper, we describe the preparation of μc-Si:H devices with efficiencies close to 10%, using glass coated with ZnO deposited by low pressure chemical vapor deposition (LPCVD). These results are achieved both by improving the transparency of the TCO in the infrared part of the spectrum and by changing the surface morphology. The LPCVD front and back contacts are first optimized for p-i-n microcrystalline silicon solar cells by decreasing the free carrier absorption (FCA) of the layers and by increasing the surface roughness of the front contact. These modifications result in an increase of the current density but also in a reduction of the fill-factor (FF) and of the opencircuit voltage (Voc). To avoid these reductions, a surface plasma treatment of the ZnO is introduced. It modifies the surface morphology by turning the typical V-shaped valleys of the LPCVD ZnO into U-shaped valleys and by erasing from the surface small-sized pyramids and asperities. As a result, for fixed deposition parameters, the p-i-n μc-Si:H silicon solar cell efficiency increases from 3.3% to 9.2%. This increase is caused by the disparition of microcracks and damaged areas which appear otherwise at the bottom of the V-shaped valleys. Further optimization of the modified ZnO front contact and of the cell process lead to an efficiency of 9.9%. Finally, we report the fabrication of micromorph cells with 11.8% initial efficiency on the modified ZnO.