The influence of the operation temperature on the output properties of solar cells with hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon germanium (a-SiGe:H) photovoltaic layers was investigated. The output power after longtime operation of an a-Si:H single junction, an a-Si:H/a-Si:H tandem, and an a-Si:H/a-SiGe:H tandem solar cell was calculated based on the experimental results of two types of temperature dependence for both conversion efficiency and light-induced degradation. It was found that the a-Si:H/aSiGe:H tandem solar cell maintained a higher output power than the others even after longtime operation during which a temperature range of 25degreesC to 80degreesC. These results confirm the advantages of the a-Si:H/a-SiGe:H tandem solar cell for practical use, especially in hightemperature regions.(C) 2004 Elsevier B.V. All rights reserved.