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Oxygen and nitrogen impurities in microcrystalline silicon deposited under optimized conditions: Influence on material properties and solar cell performance

: Kilper, T.; Beyer, W.; Bräuer, G.; Bronger, T.; Carius, R.; Donker, M.N. van den; Hrunski, D.; Lambertz, A.; Merdzhanova, T.; Mück, A.; Rech, B.; Reetz, W.; Schmitz, R.; Zastrow, U.; Gordijn, A.


Journal of applied physics 105 (2009), No.7, Art. 074509, 10 pp.
ISSN: 0021-8979
ISSN: 1089-7550
Journal Article
Fraunhofer IST ()

The influence of oxygen and nitrogen impurities on the performance of thin-film solar cells based on microcrystalline silicon (mu c-Si:H) has been systematically investigated. Single mu c-Si:H layers and complete mu c-Si:H solar cells have been prepared with intentional contamination by admitting oxygen and/or nitrogen during the deposition process. The conversion efficiency of similar to 1.2 mu m thick mu c-Si: H solar cells is deteriorated if the oxygen content in absorber layers exceeds the range from 1.2 x 10(19) to 2 x 10(19) cm(-3); in the case of nitrogen contamination the critical impurity level is lower ([N](critical)=6 x 10(18)-8 x 10(18) cm(-3)). It was revealed that both oxygen and nitrogen impurities thereby modify structural and electrical properties of mu c-Si:H films. It was observed that the both contaminant types act as donors. Efficiency losses due to oxygen or nitrogen impurities are attributed to fill factor decreases and to a reduced external quantum efficiency at wavelengths of >500 nm. In the case of an air leak during the mu c-Si:H deposition process, the cell performance drops at an air leak fraction from 140 to 200 ppm compared to the total gas flow during i-layer deposition. It is demonstrated that oxygen and nitrogen impurities close to the p/i-interface have a stronger effect on the cell performance compared to impurities close to the n/i-interface. Moreover, thick mu c-Si:H solar cells are found to be more impurity-sensitive than thinner cells.