Yukiko Hara

Degradation by acetic acid for crystalline Si photovoltaic modules

The degradation of crystalline Si photovoltaic modules during damp-heat test was studied using some test modules with and without polymer film insertion by observing electrical and electroluminescence properties and by chemical analyses. Acetic acid generated by the hydrolysis decomposition of ethylene vinyl acetate used as an encapsulant is the main origin of degradation. The change in electroluminescence images is explained on the basis of the corrosion of electrodes by acetic acid. On the other hand, little change was observed at the pn junction even after damp-heat test for a long time. Therefore, carrier generation occurs even after degradation; however, such generated carriers cannot be collected owing to corrosion of electrodes. The guiding principle that module structure and module materials without saving acetic acid into the modules was obtained.

Consideration on Na diffusion and recovery phenomena in potential-induced degradation for crystalline Si photovoltaic modules

Potential-induced degradation (PID), which brings about a large rapid decrease in output power has been observed in megawatt-scale photovoltaic power plants. Na diffusion from a cover glass to a cell through an encapsulant is possibly the direct origin of PID for p-type crystalline Si photovoltaic modules. On the other hand, PID is suppressed when using an ionomer encapsulant instead of a conventional ethylene vinyl acetate encapsulant. Some researchers consider that the reason is the suppression of Na diffusion when using an ionomer encapsulant. However, there has been no direct observation of Na diffusion behavior to the best of our knowledge for the modules prepared using an ionomer encapsulant. In this study it was found for the first time that Na diffuses in p-type multicrystalline Si photovoltaic modules prepared using an ionomer encapsulant without PID, suggesting that Na diffusion is not a sufficient condition for PID to occur. Another feature of PID is the recovery phenomenon induced by applying reverse voltage after PID occurs. In this study it was also found that reverse voltage application in the initial stage is effective for suppressing PID.

Potential-induced degradation of thin-film Si photovoltaic modules

Potential-induced degradation (PID) of thin-film Si photovoltaic (PV) modules was investigated. The characteristics of PID phenomena of thin-film Si PV modules are markedly different from those of crystalline Si PV modules. Not only performance loss but also linear-shape and spot-shape delamination was observed after negative voltage application. Recovery from PID was also observed after positive voltage application. However, rapid progression of PID was found after the second negative voltage application after recovery from the initial PID. The root cause of PID of thin-film Si PV modules is thought to be the delamination between a transparent conductive oxide film and a glass substrate. Such degradation accompanied by delamination was also observed in thin-film Si PV modules exposed outside for about 5 years.

Development of a pH sensor based on a nanostructured filter adding pH-sensitive fluorescent dye for detecting acetic acid in photovoltaic modules

Acetic acid formed via the hydrolysis of ethylene vinyl acetate (EVA) as an encapsulant in photovoltaic (PV) modules causes a decrease in the conversion efficiency of such modules by grid corrosion. Here, a nondestructive and simple optical method for evaluating the condition of PV modules is proposed. This method uses a dual-wavelength pH-sensitive fluorescent dye to detect acetic acid in PV modules using a change in pH. The change in pH induced by the formation of acetic acid is detected by the change in the ratio of the fluorescent intensities of two peaks of the dye. A pH-sensitive fluorescent dye showed sensitivity for small amounts of acetic acid such as that produced from EVA. Furthermore, a membrane filter dyed with a pH-sensitive fluorescent dye was confirmed to detect acetic acid in aged EVA after a damp-heat test (85 °C, 85%) for 5000 h in PV modules.

Degradation of encapsulants for photovoltaic modules made of ethylene vinyl acetate studied by positron annihilation lifetime spectroscopy

The structure of ethylene vinyl acetate (EVA) encapsulants of crystalline-Si photovoltaic modules after the damp heat (DH) test was evaluated by positron annihilation lifetime spectroscopy (PALS). A reduction in free-volume hole size, which indicates the progress of deacetylation, was observed after the DH test. The difference in lifetime (Δτ) between the initial and DH-tested samples clearly increased after the DH test for 3000 h. The increase in Δτ was correlated with the acetic acid concentration in the EVA estimated by ion chromatography. The depth profile analysis by slow positron beam PALS revealed that Δτ in the near-surface region of the Si-cell side was significantly larger than that of the cover-glass side. This result indicates that deacetylation near the Si cell/EVA interface is accelerated.