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

Abstract

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.

Degradation of copper‐plated silicon solar cells with damp heat stress

“…In a module constructed with a breathable backsheet, acetic acid is able to diffuse out of the module, resulting in a module pH environment between 4.76 and 7.0, which seems to agree with attempts to measure the pH of DH‐exposed EVA 59 . Acetic acid concentrations of several thousand ppm have been detected in DH‐aged EVA using various characterization methods 28,60 . Others have found that the precise level of EVA hydrolysis and corrosion depends on the degree of crosslinking of EVA, which seems to impact both the generation and out‐diffusion of acetic acid 39 .…”

Lamination-interface-dependent deacetylation of ethylene vinyl acetate encapsulant in crystalline Si photovoltaic modules evaluated by positron annihilation lifetime spectroscopy

“…This reduction in τ correlated with the reduction in P max as well as the appearance of a dark region in the EL image of a PV cell. 15) The depth profile of τ of the Initial sample indicated that τ at a low E was smaller than that at a high E. This dependence on E may be attributable to the difference in the EVA structure between the surface and the inside of the EVA encapsulants, such as crosslinking density. The depth profile also revealed that a reduction in τ caused by aging was noticeable near the surface region of the Si cell side.…”

“…31,32) Each of the near-surface regions, which were identified as the glass side or the Si side of an EVA encapsulant affixed to the cover glass or the Si cell, respectively, was examined as described in a previous work. 15) To obtain the depth profiles of τ, the incident positron energy was set sequentially at 1, 1.5, 2, 3, 4, 5, 6, and 8 keV. Two million annihilation events were collected for the measurement of each EVA sample.…”

“…We previously reported that the deacetylation of EVA in PV modules induced by a damp heat (DH) test may be evaluated by analyzing of the size of a free-volume hole, which is defined as a sub-nanometer-scale open space among the polymer chains in the amorphous part, by positron annihilation lifetime spectroscopy (PALS). 14,15) PALS can be used to estimate the size of a free-volume hole from the lifetime of ortho-positronium (o-Ps), which is a bound state of a positron and an electron. 16) The deacetylated sites in EVA can be detected by PALS because deacetylation induces a reduction in size of a free-volume hole.…”

Guiding principle for crystalline Si photovoltaic modules with high tolerance to acetic acid

“…[9][10][11] Therefore, much effort has been devoted to developing novel acceleration tests that reproduce the degradation processes occurring in the field. 11,12) One of the origins of chemical corrosion degradation of crystalline Si (c-Si) PV modules is acetic acid [13][14][15][16][17][18][19] generated in the modules by hydrolysis reaction between water vapor 20,21) and an ethylene-vinyl acetate (EVA) encapsulant. It was reported in detail that the glass frit layer at the interface between the Ag finger electrodes and the Si emitter layer is dissolved by acetic acid, 22) and contact resistance is increased with the reduction in fill factor (FF) in the first stage and in short-circuit current (I sc ) in the second stage.…”