Light-induced effects in amorphous silicon alloys - Design of solar cells with improved stability

Abstract

Light-induced effects have been investigated in amorphous silicon and silicon-germanium alloys prepared under a variety of deposition conditions. Studies have been made both on material properties and solar cell performance. The main conclusions are the following: (1) Light-induced defects are primarily caused by the recombination of electron-hole pairs and, to a lesser extent, by hole trapping. (2) The defects create a wide range of gap states distributed in energy over the mobility gap. (3) The defects are of different kinds. (4) The structure of the material plays an important role in the creation and annealing of the defects. Based on the above findings, we have been able to make stable solar cells by a combination of engineering design of cell structure and use of improved materials. We use a tandem cell configuration in which the top cell is very thin. The high field in the top cell reduces recombination thereby minimizing degradation. The bottom cell uses a fluorinated narrow gap material which shows improved stability. This material is also characterized by a high photoconductivity and low sub-band gap absorption. The tandem cells using this material show efficiencies in the range of 8% to 12% with the most stable devices showing virtually no degradation in 2000 hours of AM1 light exposure.