Stability diagnostics for thin-film photovoltaic modules

Photovoltaic (PV) modules create electricity from solar radiation by converting photon energy to electrical potential energy. The use of photovoltaic electricity is growing rapidly particularly in building-integrated, grid-connected applications. The useful lifetime of PV modules is an important determinant of the competitiveness of PV electricity. Lifetime improvement and prediction requires detailed information on degradation mechanisms in the field and in accelerated aging tests. In this thesis, new methods for studying and predicting the stability of thin-film PV modules have been developed. Stability is a greater challenge for thin-film devices than for established PV technologies, and good stability diagnostics are therefore crucial for improvements in thin-film device lifetime. The diagnostics developed in this thesis focused on two areas of lifetime research. The first was the study of water transport in photovoltaic module encapsulants. It was shown that a TiO2 film of micrometer thickness can be used as a sensor structure to measure water transport in the encapsulant with high accuracy without affecting the transport process. The sensor concept was applied in a study of absorption and desorption of water in ethylene-vinyl-acetate (EVA) films laminated between two glass sheets. The rate of desorption at a temperature difference of 25°C between the sample and the surroundings was 16 times higher than the rate of absorption at ambient temperature. This result indicates that unframed, EVA-encapsulated modules are likely to dry out in sunny conditions. The degradation of thin-film modules in outdoor operation was the second area of interest in this thesis. A de-encapsulation method for characterizing field degradation in thin-film modules was presented and applied to CdTe modules. It was observed that small-area sampling is especially well suited for characterizing module fill factor degradation. A data filtering methodology was also developed to improve the accuracy of data analysis in field tests. The method was applied to CIGS modules and was found to be especially useful in the analysis of low-irradiance data and current parameters. Additionally, thermal modeling of building integrated a-Si modules was used to predict thermal stress in different European locations. The diagnostics developed in this thesis open up possibilities for improving thin-film module lifetime by enabling precise testing of the moisture-protection properties of encapsulants and by providing methods for identifying degradation mechanisms in field-tested modules.