• I–V characteristic;
  • metastability;
  • thermal annealing;
  • light soaking;
  • performance ratio (PR);
  • spectral factor (SF)


The performance of photovoltaic (PV) modules is generally rated under standard test conditions (STC). However, the performance of thin-film photovoltaic modules is not unique even under STC, because of the “metastability”. The effects of the light soaking and thermal annealing shall be incorporated into an appropriate energy rating standard. In this study, the change in I–V characteristics of thin-film PV modules caused by the metastability was examined by repeated indoor measurements in addition to round-robin outdoor measurements. The investigated thin-film modules were copper indium gallium (di)selenide (CIGS), a-Si : H, and a-Si : H/µc-Si : H (tandem) modules. The increase in the performance of the CIGS module between the initial and final indoor measurements was approximately 8%. Because of light-induced degradation, the indoor performance of the a-Si : H and a-Si : H/µc-Si : H modules decreased by approximately 35% and 20%, respectively. The performance was improved by about 4–6% under high temperature conditions after the initial degradation. The results suggest that the performance of thin-film silicon modules can seasonally vary by approximately 4–6% only due to thermal annealing and light soaking effects. The effect of solar spectrum enhanced the outdoor performance of the a-Si : H module by about 10% under low air mass conditions, although that of the a-Si : H/µc-Si : H modules showed a little increase. The currents of these a-Si : H/µc-Si : H modules may be limited by the bottom cells. Therefore, it is required to optimize the effect of solar spectrum in addition to the effects of light soaking and thermal annealing, in order to achieve the best performance for thin-film silicon tandem modules. Copyright © 2013 John Wiley & Sons, Ltd.