Influence of the arrangement of tungsten oxide nanorods on the mechanical properties of flexible electrochromic thin films

Improving the mechanical durability, particularly the flexibility, of nanocrystal thin films is a critical issue. However, enhancing film flexibility has often required the incorporation of additional materials or complex fabrication processes. Nanostructure control offers a simpler alternative, but its role in mechanical reinforcement remains largely unexplored in electrochromic thin films. In this study, we show that mechanical durability of thin films can be significantly enhanced by simply controlling the arrangement of anisotropic tungsten oxide nanorods. Two types of films are prepared by tuning the randomness of nanorod alignment: densely packed, low-porosity films and randomly packed, high-porosity films. Mechanical properties of both films are characterized using nanoindentation and subjected to bending tests to evaluate crack formation and changes in spectroelectrochemical performance. The randomly packed nanorod films clearly exhibit enhanced crack resistance compared to the densely packed films. Interestingly, both films maintain excellent electrochromic functionality under repeated harsh bending (bending radius = 1.5 mm, 2000 cycles), with the randomly packed films showing slightly better performance retention. These findings highlight nanostructure control as a simple yet effective approach for achieving mechanical robustness in flexible electrochromic devices, with broader applicability to other electrochemical thin films.