The Fabrication And Performance Research Of WO₃-based Flexible Electrochromic Devices

Electrochromic devices(ECDs)are multifunctional devices which can trigger redox reactions by ion intercalation,and their color or optical characteristics have a reversible change after applying moderate driving potential.With the rapid development of flexible functional electronics filed,flexible ECDs have been proved to have good application prospects in the next generation of energy storage and conversion systems due to their light weight,transparency and deformability.To date,WO₃ is still the most widely used discoloration material in ECDs due to its excellent reversible chromogenic efficiency and electrochromic stability.WO₃ films are commonly fabricated on transparent flexible substrates by vacuum deposition techniques,including magnetron sputtering.However,the sputtered WO₃ layers are densely packed,which discourages the fast intercalation of cations in the electrolyte.This eventually affects the electrochromic performance of ECDs.Although WO₃nanoparticles pre-synthesized in solution phase are preferred for the fabrication of desired films,the grain boundaries between WO₃ nanoparticles need to be eliminated by high temperature annealing so that the electrons can form smooth pathways.Unfortunately,the high-temperature annealing processing is incompatible with transparent flexible substrates.In this paper,WO₃ films were prepared on flexible substrates by using strategies including small-molecule amine dispersing and cellulose doping.The research is composed as follows:1.The organic amines were used as ligand molecules to auxiliary disperse WO₃nanoparticles,and the WO₃ films were prepared by depositing the dispersed colloidal solution on PET flexible substrate.Compared with the WO₃ suspension liquid,the flexible ECDs which is prepared by uniformly dispersed WO₃ colloidal solution have lower drive voltage(0.2 V),wider visible light modulation range(52%at 850 nm),higher coloring efficiency(33.1?146.7 cm²/C)and better rate of ion diffusion(D_H⁺=9.24×10^-8 cm²s^-1).The results of research show that the charge attraction between the flexible WO₃ film and the electrolyte could act as the driving force of ion intercalation,which not only promote the fast intercalation of cations,but also reduce the driving voltage and ultimately improve the electrochromic performance.2.W₁₈O₄₉ nanowires are synthesized via using hydrothermal method,subsequently adding quantitative methyl cellulose into the suspension of the nanowires to improve its viscosity and film formation.The results show that W₁₈O₄₉ nanowires with uniform morphology and obvious linear characteristics can be synthesized under the reactant concentration of 0.3 mg/m L,and with the increase of the reactant concentration,the agglomeration of products become more serious;moreover,compared with the W₁₈O₄₉ suspension without cellulose,the optimized nanowire suspension can be directly coated on the PET substrate.Because cellulose can improve the stability of grid structure constructed by W₁₈O₄₉ nanowires and therefore strengthen the ion embedding channel,enables the assembled ECDs exhibit preferable electrochemical performance,for instance a wide range of visible light modulation(the light regulation rate is 35%at 700 nm),short response time(10.5/12.4 s)and higher coloration efficiency(68 cm²/C 700 nm).