Highly Transparent, Thermally Stable, and Mechanically Robust Hybrid Cellulose-Nanofiber/Polymer Substrates for the Electrodes of Flexible Solar Cells

The polymer substrates of flexible solar cell (FSC) electrodes play a crucial role in determining the electrode performance as well as the device performance and reliability. However, most of the FSC electrode polymer substrates suffer from high coefficients of thermal expansion (CTE) and thermal instability when exposed to thermal-cycling impact. Here, a nanocellulose/epoxy hybrid substrate employing chemically modified cellulose nanofibers, demonstrating significantly improved thermal properties as well as high optical transparency, is presented. Benefiting from nanoscale morphology and surface functional groups of the cellulose nanofibers, which enable excellent compatibility and interfacial interaction with the epoxy matrix, the hybrid substrate’s thermal properties are significantly improved with a decreased CTE of 19 ppm/K, increased glass -transition temperature (Tg) of 71.8 °C, and increased half-life thermal decomposition temperature (Td,50%) of 376 °C. Concurrently, mechanical properties are greatly enhanced with increases in ultimate strength and ultimate strain by 41 and 121.5%, respectively. In particular, the hybrid substrates maintained their high transmittance of 89%@600 nm and demonstrated no transparency loss after the introduction of cellulose nanofibers. Moreover, the conductive layer of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate deposited on the substrate retained a stable conductivity of around 835 S/cm without noticeable electrical degradation after withstanding the environmental thermal-cycling impact. With significantly improved thermal and mechanical properties as well as retained optical transparency and stable electrode conductivity, the use of this newly developed hybrid substrate may open opportunities for the fabrication of high-performance, low-cost FSCs.