The causes of seasonal variations on the performance of an amorphous silicon solar module were clearly separated using long-term outdoors I(V)-measurements. We normalized the data to a standard temperature, by using measured temperature coefficients of the characteristic parameters of the I(V)-curve, rather then extrapolating the curve itself. The resulting data were interpreted using a new model containing an effective mu tau-product in the i-layer of the device (Merten et al. 1997), This mu tau-product is accessed by variable illumination measurements (VIM) of the I(V) characteristic, which can be easily performed outdoors, making use of the natural variation in the illumination levels. The effective mu tau-product of the module remains constant throughout its second year of outdoor exposure. We conclude that the enhanced efficiency in summer is, therefore, mainly a spectral effect, and operating temperatures exceeding the winter value of 60 degrees C do not further increase the module's performance. (C) 1998 Elsevier Science B.V. All rights reserved. [References: 17]
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