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Degradation phenomena of photovoltaic cells and modules were systematically and in detail studied by observing those exposed outdoors and subjecting those to various indoor acceleration tests. Those degradation phenomena were roughly classified into three categories; less incident light into photovoltaic cells, less collection of photogenerated carriers, and less photovoltaic ability itself. Degradation mechanisms of photovoltaic cells and modules were clarified by microscopic physical and chemical analyses. Based on the mechanisms, it was found that key materials for improving reliability are encapsulants and electrodes. Improvement methods of reliability for photovoltaic cells and modules were also proposed. Prediction of lifetime by indoor acceleration test methods is also important although the prediction is quite difficult using the artificial test methods with only one or two degradation factor(s). Remaining issues of reliability and durability will be also presented.

Recently there has been increased interest in using thermoplastic encapsulant materials in photovoltaic modules, but concerns have been raised about whether these would be mechanically stable at high temperatures in the field. Recently, this has become a significant topic of discussion in the development of IEC 61730 and IEC 61215. We constructed eight pairs of crystalline-silicon modules and eight pairs of glass/encapsulation/glass thin-film mock modules using different encapsulant materials, of which only two were formulated to chemically crosslink. One module set was exposed outdoors with thermal insulation on the back side in Mesa, Arizona, in the summer (hot-dry), and an identical module set was exposed in environmental chambers. High-precision creep measurements (±20 μm) and electrical performance measurements indicate that despite many of these polymeric materials operating in the melt or rubbery state during outdoor deployment, no significant creep was seen because of their high viscosity, lower operating temperature at the edges, and/or the formation of chemical crosslinks in many of the encapsulants with age despite the absence of a crosslinking agent. Only an ethylene-vinyl acetate (EVA) encapsulant formulated without a peroxide crosslinking agent crept significantly. In the case of the crystalline-silicon modules, the physical restraint of the backsheet reduced creep further and was not detectable even for the EVA without peroxide. Because of the propensity of some polymeric materials to crosslink as they age, typical thermoplastic encapsulants would be unlikely to result in creep in the vast majority of installations.