Several works report on seasonal fluctuations of power production of amorphous silicon (a-Si). These oscillations are due to two overlapping phenomena (i) spectral and (ii) the Staebler-Wronski effects. It is hence difficult to assess-for a given location and climatic conditions-which one has the largest impact. By means of a straightforward approach based on two sets of singlejunction a-Si photovoltaic modules (stored indoors/exposed outdoors) and on two different I-V measurement set-ups (indoor and outdoor), we were able to separate the different contributions to this phenomenon. For the test-site of Lugano, seasonal oscillations account for performance variations of a-Si of~10% (AE5% around an annual average value with a minimum around the mid of January and a maximum around mid-July). The time-phase of the overall effect lies in between that of the two distinguished phenomena. (i) Spectral variations seem to have the highest impact on the outdoor performance of a-Si with an amplitude corresponding to 10.5% (AE~5.2%). Moreover, the influence of spectral variations on the outdoor performance of a-Si (and for comparison of c-Si) was modeled, and the experimental data were found to be in excellent agreement with the theoretical simulation; (ii) the Staebler-Wronski effect has a slightly lower influence with an amplitude of~8% (AE4% with a minimum at the middle of February and a maximum around mid-August). Because of the position (46 N) and average climatic conditions (southern Alpine climate) of Lugano, these observations are possibly representative of a large part of continental Europe.

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