Interfacial Degradation in Bifacial Glass/Glass Silicon Photovoltaic Modules Under Applied Bias and Humidity

As silicon solar cells approach their theoretical maximum efficiency, recent focus is towards increasing photovoltaic (PV) module power output and reliability. Bifacial silicon modules can achieve greater power densities from additional rear side illumination when the opaque backsheet is replaced by a transparent back covering, such as glass or a transparent backsheet. Glass/glass (G/G) PV modules are more rigid due to the symmetry of the structure and less permeable to moisture and oxygen compared to conventional glass/backsheet (G/B) modules. The aim of this work is to understand the distinct degradation pathways in Si PV modules of different structures (G/G and glass/transparent backsheet (G/TB)) and encapsulation type. Historical field data from G/G Si PV modules encapsulated with poly(ethylene-co-vinyl acetate) (EVA) suggest these modules suffer from additional degradation causing more severe power losses. New-generation G/G bifacial modules are emerging with better packaging and robust mounting. Alternative encapsulants to EVA have been proposed to mitigate the degradation of G/G PV modules and maintain long-term field durability. Polyolefin elastomer (POE) is a promising candidate to replace EVA due to the lack of the degradation byproduct acetic acid. Furthermore, POEs with higher volume resistivity and lower water vapor transmission rate (WVTR) are likely to limit ionic and moisture migration through the encapsulant and reduce the risk of potential induced degradation (PID) in Si cells.<br/>In this project, we observed different degradation modes that occurred under stress in bifacial Si p-PERC mini-modules in G/G and G/TB configurations with either EVA or POE encapsulants. Mini-modules of all 4 types were subjected to accelerated stress testing consisting of 85°C and 85% relative humidity and either zero, positive, or negative system voltage of 1000 V for 1000 hours. Module performance testing and spatially-resolved imaging revealed PID of type shunting (PID-s), polarization (PID-p), and corrosion (PID-c) depending on module configuration and applied stress. POE-based modules experienced the lowest degradation at all test conditions, with less than 5% power drop compared to a maximum of 35% for EVA-containing modules. The extent of power degradation in EVA-containing modules depends on the bias polarity, with most degradation being observed in G/G modules under damp heat with positive bias.<br/>The aim of this work is to provide a better understanding of the chemical processes responsible for the observed degradation modes by performing detailed chemical and mechanical analyses of the cell/encapsulant and encapsulant/glass interfaces. The characterization methods include X-ray photoelectron spectroscopy (XPS) that confirms ionic migration from glass to the cell and vice-versa as well as any changes in chemical bonding at the interfaces. Preliminary results indicate presence of Na near the cell gridline in EVA-containing G/G modules, whereas lower and no Na was present in POE-containing G/G modules and pristine encapsulants respectively. The adhesion testing at the interfaces suggests that even though the interfaces weakened considerably in all module configurations, mechanical degradation through loss of adhesion was unlikely related to the power loss observed here, but can cause long-term packaging issues such as delamination. Fourier transform infrared spectroscopy (FTIR) compares chemical changes and degradation of the encapsulants. Greater changes in the polymer structure were observed in the encapsulants from G/TB modules, which can be related to enhanced moisture ingress through the backsheet compared to glass. This study will provide a better understanding of degradation pathways in bifacial Si modules depending on the module configuration and provide guidance on the choice of packaging materials for bifacial modules.