Modification of ITO Surface for High Performance of Electrochromic Polymer Film

The relatively low reactivity of indium tin oxide (ITO) thin film coated on glass with its good conductivity and transparency make it the widely used electrode in displays and electrochromic devices. However, the inactivity of the surface may cause serious problems for physically deposited electrochromic polymer films from detachment to short durability, especially, in the presence of electrolyte liquids. We here report that significant improvement is made for electrochromic films of poly[3,4-(2,2-dimethylpropylenedioxy)thiophene] by electrodepositing its monomer On ITO surface modified with organic acids. We first designed and synthesized 2-thienyl phosphonic acid which was used to modify ITO glass. ITO glass was immersed in modifier solution vertically on a glass bracket and the solution was allowed to evaporate slowly at ambient condition. The glass was removed from the glass bracket when it was fully exposed to the air, and then baked at 120 degrees C for 24 h in glove box. After modification, the modifier molecules are able to react with hydroxyl groups on ITO surface by losing H2O and self-assemble to high-order monolayer. X-ray photoelectron spectroscopy results indicate that chemical binding is formed between the surface and the modifier. By attaching the modifier molecules to ITO surface with ester bonds, the surface changes from hydrophilic to hydrophobic, but the transmittance and conductivity remain the same as bare ITO. Atomic force microscope results demonstrate that surface roughness of modified ITO showed a little reduction compared with bare one. To test the mechanical strength of electrochromic polymer thin film on different substrate, we treated the films by sonication. The results reveal a much better stability for the film formed on the modified ITO than that on bare one. Meanwhile, color changing property of electrochromic polymer film electropolymerized on modified substrate remains the same as that on bare substrate. This study provides a conventional way to achieve high-performance polymer-based electrochromic films.