Impurities in thin-film silicon: Influence on material properties and solar cell performance

The influence of oxygen and nitrogen impurities on the material properties of a-Si:H and pc-Si:H films and on the corresponding solar cell performances was studied. For intentional contamination of the i-layer the impurities were inserted by two contamination sources: (i) directly into the plasma through a leak at the chamber wall or (ii) into the gas supply line. The critical oxygen and nitrogen concentrations for silicon solar cells were determined as the lowest concentration of these impurities in the i-layer causing a deterioration of the cell performance. Similar critical concentrations for a-Si:H and mu c-Si:H cells in the range of 4-6 x 10(18) cm(-3) for nitrogen and 1-5 x 10(19) cm(-3) for oxygen by applying a chamber leak are observed. Similar increase of conductivity with increasing impurity concentration in the a-Si:H and mu c-Si:H films is found. A more detailed study shows that the critical oxygen concentration depends on the contamination source and the deposition parameters. For a-Si:H cells, the application of the gas pipe leak leads to an increased critical oxygen concentration to 2 x 10(20) cm(-3). Such an effect was not observed for nitrogen. For mu c-Si:H, a new deposition regime with reduced discharge power was found where the application of the gas pipe leak can also result in an increase of the oxygen concentration to 1 x 10(20) cm(-3). (C) 2011 Elsevier B.V. All rights reserved.