Optical assessment of lignin-containing nanocellulose films under extended sunlight exposure

Abstract

This study investigates the stability and UV-blocking properties of cellulose nanofibril (CNF) and TEMPO-oxidized cellulose nanofibril (TOCNF) films, with and without lignin, under 1000 h of artificial sunlight. The literature to date provides no quantitative analysis of such films' stability, however such insight is critical for optoelectronic applications for instance solar cells. This contribution examines the films from practical perspectives, considering aging with respect to their optical performance and retention of UV protective qualities. Films containing residual lignin (LignoCNF and LignoTOCNF), and lignin nanoparticles (CNF-LNP and TOCNF-LNP) demonstrated remarkable UV-blocking stability; even after the aging transmittance of LignoCNF and CNF-LNP films remained lower than 1% below 390 nm. Most lignin-containing films exhibited increased transmittance between 400 and 600 nm after aging, except for LignoTOCNF, which showed a decrease in transmittance that was comparable to that displayed by non-lignin films. Nevertheless, long-term light exposure induced a decrease in their mechanical properties. Tensile tests revealed increased brittleness in CNF and LignoCNF, while LNP-containing films showed reduced strain at the break. The observed changes were linked to the potential oxidation of COO- groups and structural modifications in both cellulose and lignin. Overall, the incorporation of lignin into nanocellulose films enhances their durability, UV protection, and mechanical stability, making them promising candidates for sustainable optoelectronic applications.

Supplementary information: The online version contains supplementary material available at 10.1007/s10570-025-06380-7.

Keywords: Biobased solar cells; Mechanical properties; Optoelectronics; Stability; UV protection.

Fig. 1

Optical micrographs of the NC-based films with and without lignin. The top row corresponds to CNF-based films, while the bottom row corresponds to TOCNF-based films

Fig. 2

Digital photographs of nanocellulose films before and after 1000 h exposure to light (Xenon light—corresponding to AM1.5G spectrum)

Fig. 3

The variation of red, green, and blue (RGB) values over time for nanocellulose films with standard deviation. The plots indicate the changes in RGB intensity levels during a 1000-h accelerated aging test, with error bars representing the standard deviation for three distinct regions of each film, ensuring data uniformity. The y-axes of the plots are truncated to display a range from 135 to 240, enhancing the visibility of trends in the data

Fig. 4

a Transmittance and b haze before (solid line) and after (dashed line)1000 h of exposure to artificial sunlight

Fig. 5

Digital imaging of the polyvinyl chloride plastic used behind the films following 1000 h of exposure in comparison to the initial one

Fig. 6

Changes in RGB color values for PVC plastic samples covered by various nanocellulose (NC) films before and after 1000 h of exposure to artificial sunlight. The bar graph compares the initial (solid bars) and final (hatched bars) RGB values for each nanocellulose (NC) film: CNF, TOCNF, LignoCNF, LignoTOCNF, CNF-LNP, and TOCNF-LNP. The RGB color values (red, green, and blue) are presented for each sample, with error bars indicating the standard deviation across three distinct regions

Fig. 7

Mechanical properties of NC cellulose before and after light exposure. All measurements were performed with the same conditions (50% relative humidity, 25 °C and 0.5 mm min^–1 strain rate) and repeated at least five replicates