A correlative indoor-outdoor study of degradation mechanisms in silicon heterojunction and TOPCon modules: Benchmarking against PERC and evaluating bill-of-materials dependence

This paper presents a correlative analysis of the performance and degradation of advanced silicon photovoltaic (PV) technologies, including silicon heterojunction technology (HJT) and tunnel oxide passivated contact (TOPCon), benchmarked against mainstream passivated emitter rear contact (PERC) installed in hot arid environments. The field performance of novel encapsulant materials tailored for these different cell architectures under combined stresses will be presented. After three years of operation in an arid desert climate, the results indicate technology-dependent degradation trends, with HJT and TOPCon modules showing different susceptibility to encapsulant aging, while PERC modules do not show aging. Ultraviolet fluorescence (UVF) imaging reveals early-stage encapsulant degradation that correlates with power loss. Front and rear ethylene-vinyl acetate (EVA) on both HJT-1 and HJT-2 showed UVF degradation patterns. Front polyolefin elastomer (POE) on TOPCon showed distinct UVF degradation patterns. In terms of specific energy yield, TOPCon-2 technology is the best performer, followed closely by PERC-2 and HJT-2. HJT modules exhibit the highest indoor maximum power P _max loss, reaching up to −1.7%/year and showing the largest performance variability. PERC modules demonstrate intermediate degradation rates of −1.0 to −1.7%/year, whereas TOPCon modules consistently achieve the lowest P _max losses, ranging from −0.3 to −1.1%/year. Field degradation analysis shows that PERC and TOPCon strings exhibit consistently moderate and tightly bound degradation rates (−1.1 to −1.3%/year), whereas HJT strings display markedly higher variability, with accelerated degradation of −4.3%/year. This highlights the robust field stability of PERC and TOPCon technologies and the greater sensitivity of HJT to degradation. The comparison between indoor P _max loss and field-derived degradation rates shows a consistent correlation across module types, while HJT-1 exhibits higher outdoor degradation due to field-induced encapsulant degradation and string-level losses.