Early‐stage identification of encapsulants photobleaching and discoloration in crystalline silicon photovoltaic module laminates

Abstract

Commercially different variants of ethylene‐vinyl acetate (EVA) encapsulants are available in the photovoltaic (PV) market. Photobleaching and discoloration are the two most commonly observed phenomena, and their initiation may be different for different encapsulants. To investigate the EVA encapsulant photobleaching and discoloration, solar cell laminates having different EVA films (UV‐transparent [T], UV‐cut [C], and combination of the two [TC]) were tested in Xenon test chamber. High temperatures are created in the laminates during the aging tests by using a thick insulation layer behind the backsheet. The UV fluorescence images and grayscale profile show clear signs of photobleaching and discoloration. It is found that the oxygen diffusion coefficient of the T sample is four and nine times slower than the TC and C samples, respectively, in the photobleached region. Fluorescence imaging and spectra and Raman spectra were taken before and after the accelerated test and indicate that discoloration causing fluorophores generation is higher after the photobleached region for transparent and combined EVAs, whereas higher at the center for UV‐cut EVA laminates. A colorimeter was used to measure the Yellowness Index of the samples before and after the accelerated aging test. This work will help in the early detection of photobleaching and discoloration of any encapsulant used in the PV modules. This method will also help to study the behavior of encapsulants in different climatic conditions like hot, cold, dry, humid, and their combinations by simulating the same in an accelerated weathering chamber by using the different insulation thickness.

Towards improved cover glasses for photovoltaic devices

“…A similar effect is proposed to occur within these doped glasses because of the shifted absorbance. Yellowing and ultimately browning of C‐EVA has been shown to reduce module efficiency by up to 45% within 5 years of installation, 126 whereas, as noted in Section 1, the long‐term in situ performance of T‐EVA in PV modules has not yet been fully investigated—although it is expected to provide superior capabilities to C‐EVA, its yellowing index is nonzero, 17 and hence, enhanced protection of T‐EVA by the cover glass remains a key requirement.…”

“…Service lifetimes and efficiencies of solar cell components are limited by solar UV radiation damage, which induces degradation of laminate materials, the most frequently used being ethylene vinyl acetate (EVA), eventually leading to delamination and module failure, 15 although in recent years, ‘transparent EVA’ or T‐EVA, with a UV cut‐off wavelength of ~300 nm, has been developed, 16 reducing this effect. However, discolouration and delamination of T‐EVA can still arise at the backsheet interface in PV modules, as recently discussed by Adothu et al 17 They also demonstrated discolouration and photobleaching of T‐EVA in c‐Si PV module tests, with yellowness indices, following UV exposure, that were approximately one‐quarter those of traditional UV‐cutting EVA (C‐EVA) 17 . Although a significant improvement, the yellowness index of the T‐EVA was still nonzero.…”

“…Although a significant improvement, the yellowness index of the T‐EVA was still nonzero. Moreover, the environmental stability of the T‐EVA encapsulant is still not known 17 . Consequently, although T‐EVA represents an important step forward and enables increases in PV module efficiency by comparison with C‐EVA, 17 T‐EVA does not necessarily present a panacea for environmental degradation of PV module encapsulants.…”

“…Figure 1). Despite the availability and recent application of these new EVA materials (see, e.g., Vogt et al 16 and Adothu et al 17 ), they remain susceptible to UV‐ and temperature‐induced discolouration and delamination (see Section 1), and it remains important to have an appropriate level of UV blocking, otherwise degradation of the laminate is a major limiting factor of the service lifetime and lifetime efficiency of PV modules. For traditional C‐EVA, this could lead to annual degradation of 0.6%–2.5% in PV module efficiency because of degradation of the C‐EVA, depending on service conditions and manufacturer 15,30,31 .…”

“…The main effect is discolouration of the EVA layer due to UV damage, resulting in reduced light transmission and thus contributing to reduced module efficiency 32 . The mechanisms and chemical species involved in this discolouration were recently summarized by Adothu et al 17 In addition to its effects on the polymeric encapsulant materials, UV degradation can also impact on the efficiency of the solar cell material itself. This was demonstrated by Shamachurn and Betts, 33 who observed deterioration of solar cell efficiency for bare Si solar cells, which they attributed in part to degradation of the antireflective coating on the cell material.…”