The influence of oxygen and nitrogen impurities on the performance of thin-film solar cells based on microcrystalline silicon (μc-Si:H) has been systematically investigated. Single μc-Si:H layers and complete μc-Si:H solar cells have been prepared with intentional contamination by admitting oxygen and/or nitrogen during the deposition process. The conversion efficiency of ∼1.2 μm thick μc-Si:H solar cells is deteriorated if the oxygen content in absorber layers exceeds the range from 1.2× 1019 to 2× 1019 cm-3; in the case of nitrogen contamination the critical impurity level is lower ([N] critical =6× 1018 -8× 1018 cm-3). It was revealed that both oxygen and nitrogen impurities thereby modify structural and electrical properties of μc-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 μc-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 μc-Si:H solar cells are found to be more impurity-sensitive than thinner cells. © 2009 American Institute of Physics.