Degradation behaviors of EVA encapsulant and AZO films in Cu(In,Ga)Se2 photovoltaic modules under accelerated damp heat exposure
Introduction
The key to the successful commercial production of Cu(In, Ga)Se2 (CIGS) photovoltaic (PV) solar modules is a high conversion efficiency and long lifetime of the order of 20–30 yrs. To verify their long-term stability without actual field data, several types of accelerated tests, such as high temperature, damp heat (DH), thermal cycling, and ultraviolet ray stresses, are employed [1], [2], [3], of which the DH test is a particularly important tool in the qualification of solar cell modules. In general, the DH test for the degradation of solar modules is specified in the IEC 61,646 qualification standard, which requires the units to be subjected to 85 °C and 85% relative humidity for 1000 h [4]. Up to now, many studies have focused on the damp-heat-induced degradation of transparent conducting oxides (TCOs) [5], [6], [7] and CIGS cells [8], [9], [10], [11]. It has been reported that water penetration into CIGS PV cells significantly influences their performance and lifetime.
Most CIGS PV modules are encapsulated by ethylene-vinyl acetate copolymer (EVA) and tempered glass to protect them against knocks, rain, and dust. A well-bonded EVA encapsulation can protect the CIGS cells by physically preventing the accumulation of water at the TCO layers, but EVA is very sensitive to water molecules. Hence, one of the main requirements for the EVA that is used is that it prevents the penetration of water molecules into and within the modules, which would otherwise have serious consequences, such as performance degradation and discoloration of the cells [12], [13], [14]. In previous studies, the degradation of the EVA material mainly occurs through either thermal, thermo-oxidative [15], [16], [17], [18] or photodegradation [19], [20], [21], [22] mechanisms. Recently, the damp heat aging of EVA copolymer has been studied through experimental characterization and simulation [23], [24], [25]. However, to better understand the damp heat-induced degradation behavior of EVA, correlation between the degradation of EVA and the degradation of the AZO films in modules are explained through the failure analysis of modules and their electrical characteristics.
In this study, the degradation behavior of EVA and AZO films in a DH environment is investigated and the discoloration phenomenon that affects the performance of CIGS PV modules was explained by correlating the electrical degradation of the AZO films and the chemical degradation of the EVA. To verify the effect of humidity on the degradation of CIGS modules, the EVA and AZO films in the module were thoroughly investigated through failure analysis, and durability tests of the EVA after DH exposure were conducted in terms of the optical and mechanical properties.
Section snippets
Experimental procedure
Encapsulated CIGS PV modules with a size of 475 mm×375 mm with AZO/i-ZnO/CdS/CIGS/Mo structures were fabricated using an evaporation system [26]. All of the modules were tested at 85% relative humidity and 85 °C for 1000 h, and then a series of light soaking (LS) procedures were performed [27], [28]. The damp heat tests for the encapsulated modules (M1–M5) were carried out in an environmental test chamber (Challenge 250, ACS). After damp heat exposure, the Fill Factor (FF) and efficiency of the PV
Degradation behaviors of AZO films in CIGS modules after the damp heat test
Accelerated damp heat testing of the encapsulated CIGS modules was conducted in an environmental chamber where the temperature (T=85 °C) and relative humidity (RH=85%) conditions could be controlled, and then light soaking was performed following the IEC 61646 standard. Fig. 3 shows the changes in the FF and efficiency of the encapsulated modules as a function of the DH test time. The FF and efficiency for the non-discolored modules (M1, M2, M4) were slightly reduced, and it was thought that
Conclusion
The damp heat stability and degradation mechanism of encapsulated CIGS PV modules were investigated through the failure analysis of the EVA and AZO films in modules. After DH exposure, the FF and efficiency of some of the modules decreased and were accompanied by discoloration. The AZO films in the non-discolored modules exhibited slight changes in their electrical properties, as compared to those before DH exposure. However, the resistivity of the AZO films in the discolored modules changed
Acknowledgment
This research was supported by a Grant from LG Innotek Co., Ltd.
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