The Study Of Electrochromic Materials And Device Based On Triphenylamine Derivatives

Eletrochromism is the reversible color changing phenomenon of thin-film materials reduced by external voltage, essentially it is the changes in transmittance or reflectance of electrochromic film because of the occurrence of redox reaction. Organic electrochromic materials having many advantages of such as multicolor change, easy processibility have been studied extensively. Electrochromic devices equiped with superior properties also have broad range of applications, and are gradually developing from rigid to flexible large-scale.In this paper, two types of novel polymer electrochromic material were synthesized based on the two commonly used chromophores carbazole（CBZ） and triphenylamine（TPA） in electrochromism, dithiafulvenyl（DTF） unit was introduced to the polymer structure as a pendant Dithiafulvenyl（DTF, C₆DTF） can be regarded as a smaller version of the fulvene family characterized by terminal electron-donating group（alkyl chains）, which is an excellent electron donor segment. It was the first time to modulate electrochromic film’s performances by introducing strong electron donating unit of dithiafulvenyl. The main contents are as follows:First, two CBZ unit and a DTF unit（characterized by methyl group） were introduced to para-positions of TPA unit respectively, synthesized the DTF-TPA-DCBZ monomer. Conducting polymer films based on 3,6-linked CBZ（poly（DTF-TPA-DCBZ）） were deposited on ITO-coated glass substrate through electrochemical cyclic voltammetry polymerization. The combination of DTF and TPA units forms a double electron-donating DTF substituted hybrid donor, which could reduce the oxidation potentials of corresponding electrochromic film. The obtained polymer film on ITO-coated glass substrate was porous and was the aggregation of numerous particles with a diameter of about 90 nm, which has reversible color change from light yellow to green and to blue grey. Coloring and bleaching response time of poly（DTF-TPA-DCBZ） film were 4 s and 8 s, respectively, and coloration efficiency of the polymer film was calculated to be 23.77 cm2 C-1 at the wavelength of 658 nm, the maximum optical contrast reached to 40%.Second, C₆ DTF unit（characterized by hexyl group） was connected to TPA to synthesis aromatic polyimide material of C₆DTF-TPA-PI, the polyimide film was prepared on ITO-coated glass substrate, then examined its electrochromic properties. We introduced electron-donating C₆ DTF unit to the polyimide structure as the side chain to effectively adjust electrochromic properties of the film. Decomposition temperature of the solid C₆DTF-TPA-PI was 370 °C, and glass transition temperature was 225 °C. Color of the polyimide film could change reversibly from yellow to pink-purple under applied voltages, having the coloring response time of 3 s and bleaching response time of 5 s. There were three characteristic absorption peaks located at 337 nm, 348 nm and 470 nm, while a new absorption peak located at 560 nm appeared after the polyimide film being oxidized under voltages over 0.6 V.Finally, a electrochromic device was fabricated through stacked structure, in which C₆DTF-TPA-PI film was used as the electrochromic layer and ITO-coated glass substrate was the transparent conducting electrode. The polyimide electrochromic device had a reversible electrochemical behavior, when applied voltage the film’s color could change from pale yellow to pink-purple, and coloring response time and bleaching response time was 5 s and 6 s, respectively.