Considerations for light soaking of amorphous silicon photovoltaic modules

To estimate the stabilized performance a priori, laboratory tests under controlled, simulated 1-Sun, conditions have been performed on hydrogenated amorphous silicon (a-Si:H) multijunction solar cell modules by various organizations. Such tests generally use an arc light source and maintain the module temperature at some fixed value, e.g. 50°C. Because the spectra of various light sources used for light soaking do not match that of the Sun and because the operating temperature for real systems is not constant and differs from the fixed temperature in laboratory tests, the validity of the laboratory light soaking tests has been questioned. The paper examines the most significant parameters in light-soaking tests to determine their effect and to indicate test strategies to minimize any errors caused by these parameters that may tend to invalidate the results. There is a significant difference (up to a factor of 3) in the relative (to AM 1.5) intensity to which various layers of solar cells are subjected depending on the light source spectra. The effect of temperature during light exposure on the stabilized performance varies between a-Si devices of different structures. The aim of this paper is to determine whether a specific standardized, continuous light-soaking test in principle can predict the stabilized performance that will be observed when modules with different a-Si:H-based layers are deployed outdoors