Modern photovoltaic technologies are increasingly evaluated not only in terms of power conversion efficiency and cost, but also with respect to resource origin, toxicity, recyclability, and overall life-cycle impacts. Within this broader sustainability framework, bio-based and bio-inspired materials derived from biomass or mimicking biological structures have emerged as promising candidates for a wide range of photovoltaic components, including active layers, interfacial modifiers, substrates, encapsulants, and natural dyes. This review provides a layer-by-layer overview of such materials implemented or proposed in dye-sensitized, organic, perovskite, biohybrid, and silicon solar cells, linking their molecular structures and optoelectronic properties to representative device performances and key degradation pathways. Cross-cutting challenges related to moisture and thermal stability, barrier performance, feedstock variability, and the risk of “greenwashing” are highlighted, emphasizing that sustainability claims must be supported by quantitative metrics such as life-cycle assessment, circularity indicators, and durability studies. Finally, we outline promising research directions in molecular engineering, hybrid biosynthetic architectures, and advanced encapsulation concepts that could enable bio-based materials to make a meaningful contribution to low-impact photovoltaic technologies.

This study reports the physicochemical characterization and structure–property relationships of rigid sheep wool/phenolic novolac panels developed as bio-based thermal insulation for building envelopes. Mixed Polish sheep wool was washed, mechanically opened, and formed into nonwoven mats, then impregnated with either neat or flame-retardant novolac resin to obtain lightweight boards with a fiber content of about 50 wt%. Elemental analysis, ICP-OES, FTIR spectroscopy, and laser and electron microscopy were used to evaluate the fiber composition, keratin structure, morphology, and fiber–matrix interfaces. Mechanical performance under three-point bending and shear, differential scanning calorimetry, thermogravimetric analysis, and transient hot-probe thermal-conductivity measurements were applied to link microstructure with functional behavior. Novolac impregnation transformed the compliant wool mat into self-supporting panels, increasing the flexural modulus to the 0.8–1.4 GPa range and flexural strength to approximately 48–52 MPa, while the shear modulus and work to failure rose by more than an order of magnitude relative to the loose wool reference. Thermal conductivity remained in a typical range for natural-fiber insulations (λ = 0.061 W·m⁻¹·K⁻¹ for the wool mat and 0.071–0.074 W·m⁻¹·K⁻¹ for the composites), although higher than that of expanded polystyrene. DSC and TGA confirmed that wool fibers remain thermally stable up to about 200–220 °C, that the novolac resin cures around 140 °C, with typical phenolic reaction enthalpies, and that both formulations generate high char residues of roughly 60–80 wt% at 600 °C under nitrogen, evidencing a strong charring propensity rather than directly quantifying fire resistance. Overall, the results position sheep wool/novolac panels between conventional bio-based insulation and structural composites and highlight their potential as sustainable, circular insulation materials for energy-efficient building envelopes.