Eco-Friendly and Structurally Efficient Photovoltaic Modules with Transparent Bio-Based Epoxy and CNF Composite Back-Sheets for BIPV Applications

Building-integrated photovoltaic (BIPV) systems require photovoltaic modules that simultaneously ensure structural reliability, optical transparency, and environmental sustainability. Conventional photovoltaic modules typically employ tempered glass as the front cover, which offers excellent optical performance but suffers from inherent drawbacks, including high density, brittleness, and limited design flexibility for BIPV applications. In this study, a glass-free photovoltaic module architecture is proposed, consisting of a transparent bio-based epoxy front cover and a mechanically reinforced cellulose nanofiber (CNF)/bio-based epoxy composite back-sheet. The mechanical properties of CNF/bio-based epoxy composites with CNF contents ranging from 0 to 15 wt% were systematically investigated, and the optical transmittance of the transparent bio-based epoxy was evaluated in comparison with conventional tempered glass. Structural reliability was assessed through finite element analysis under a static mechanical load of 5400 Pa, in accordance with IEC 61215 standards. In addition, the electrical performance of the proposed glass-free module was experimentally compared with that of a conventional glass-based module using identical shingled solar cells. The results reveal that CNF incorporation significantly enhances the stiffness and strength of the bio-based epoxy composite while inducing only a marginal increase in density. Under the IEC mechanical load condition, the glass-free module exhibited up to a 72% reduction in total deformation compared to the conventional glass-based module, demonstrating substantially improved structural efficiency. The transparent bio-based epoxy front layer maintained over 90% optical transmittance in the visible wavelength range at practical thicknesses of 1.0–1.5 mm, comparable to that of 3.2 mm tempered glass. Furthermore, the glass-free module preserved electrical performance, showing less than a 1% difference in maximum power output and stable current–voltage characteristics relative to the reference module. Overall, this study demonstrates that CNF/bio-based epoxy glass-free photovoltaic modules can achieve enhanced structural reliability without compromising optical transparency or electrical performance, highlighting their potential as sustainable and structurally efficient alternatives for next-generation BIPV applications.