In this study, polyethylene naphthalate (PEN) was investigated as a flexible substrate because, compared with polyethylene terephthalate, it achieves a lower root mean square roughness and transmittance, which is favorable for reducing leakage from the bottom of flexible substrates. A flexible device structure composed of tungsten oxide/indium-doped tin oxide/PEN was used in an electrochromic (EC) test. The experimental results show that the flexible EC display device achieved a high transmittance difference of > 40% and color efficiency of 70.2 cm2/C at 560 nm. The transmittance difference was degraded in the visible range after 200 cycles of continuous bending. Furthermore, compared with flat fresh devices, the WO3 device exhibited poor retention properties in a colored state after being subjected to longer bending cycles.
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