Seasonal Variations in Amorphous Silicon Solar Module Outputs and Thin Film Characteristics

Hydrogenated amorphous silicon (a-Si:H) solar modules exposed to outdoor conditions exhibit, over a long-time scale, an efficiency pattern which improves during summer months and decreases in winter time. The variations are usually attributed to two main mechanisms: (a) thermal annealing effects enhanced by summer month temperatures, which might partly offset the efficiency decrease caused by light-induced changes in the amorphous silicon material (known as the Staebler-Wronski effect), and (b) seasonal spectral variations in the solar radiation reaching the earth's surface, which are quite marked in the wavelength region in which amorphous silicon solar cells respond. While both factors might contribute to a more severe performance degradation in winter, we show, by exposing both commercial a-Si:H solar modules and thin films to the same AM 1.0 spectrum while keeping them at temperatures corresponding to extreme summer and winter operating cell temperatures, that the second effect might be the major factor in the overall seasonal efficiency changes. Light-induced annealing, enhanced by the higher radiation levels to which modules are exposed during summer months, might be playing a role as well, adding extra complexity to the effect.