Initial drop in I/sub sc/ of the field test c-Si PV modules in Japan

Stability of photovoltaic (PV) modules has been investigated, based on about 2400 field test PV modules manufactured in 1990´s, which show better reliability than the earlier generation modules. The overall system performance showed no distinct change within the 10 years of field test. Some of the modules showed sign of various degradation modes. Among these, the initial drop in I/sub sc/ indicated that the c-Si modules showed reduction in I/sub sc/ by 1-5% within 1 year of outdoor exposure, then became stable. Experimental results showed that this do not strongly affect the long term stability of the PV modules, as the I/sub sc/ becomes stable in a very early stage of the operation. Spectral response measurement showed that the drop in I/sub sc/ is due to the decrease in the response for the red light and infrared light. Possible contributions from the changes in the optical properties of glass and EVA, and the metastability of c-Si solar cells are discussed. Other modes of instability such as the delamination and increase in R/sub s/ is also discussed.