The role of water in the synthesis and performance of vapour phase polymerised PEDOT electrochromic devices

Abstract

Parameters affecting the quality of vapour phase polymerised (VPP) PEDOT and their influence on electrochromic device performance were investigated. Specifically, the role of water during synthesis was examined and a polymerisation mechanism proposed. Paradoxically, water vapour is essential for PEDOT polymerisation, however, too high a loading leads to crystallite formation in the oxidant layer, rendering the oxidant inactive. Changes in water vapour affect the doping level of the polymer, presumably due to poor conjugation along the polymer's backbone during synthesis. The addition of a surfactant, PPG-ran-PEG, was studied using XPS. The surfactant inhibited oxidant crystal growth and slowed the rate of PEDOT polymerisation, reducing film defects and improving PEDOT conductivity. Controlling and optimising the levels of water vapour and surfactant during synthesis resulted in reproducible, high conductivity, high optical switch, PEDOT films. Finally, complementary dual-polymer electrochromic devices utilising (pre-and post-process-enhanced) VPP PEDOT and PMAS (control) were fabricated and changes in switching transmission evaluated.

Patterning and Conductivity Modulation of Conductive Polymers by UV Light Exposure

“…[29,30] Mobility of the oxidant is important to the VPP process, given the requirement for transport of these ions to the liquid-vapor interface to facilitate further polymerization to occur. [31,32] Changes in the coordination between iron, tosylate and water have been shown to have large effects on the polymerization kinetics.…”

Micron-thick highly conductive PEDOT films synthesized via self-inhibited polymerization: roles of anions

“…[25][26][27][28][29][30][31][32][33] These oxidants are also easily crystallized during the reaction due to their low solubility, and the crystallized oxidants cause the formation of pin-hole defects in the film. 34 The loading of PEPG further reduces the film thickness due to a severe concentrating effect. A low thickness results in both a high internal resistance that limits the power output as a TE device 36 and a high square resistance that is not viable for electrode materials.…”

“…A low thickness results in both a high internal resistance that limits the power output as a TE device 36 and a high square resistance that is not viable for electrode materials. Although multiple fabrications could be used to obtain thicker films, that process may not only generate layer-layer interfaces due to the disconnected polymer chains and nonuniformity along the layer growth direction 31,33,34 but also cause the decomposition of the formerstep-deposited layers by re-heating.…”

“…30 However, the unequal evaporation of pyridine leads to uneven polymerization and a severe deterioration of σ when fabricating thicker films. 31 In addition to pyridine, copolymer inhibitors, such as poly(ethylene glycol)-blockpoly(propylene glycol)-block-poly(ethylene glycol) (PEPG), are also commonly used to reduce the oxidant reactivity through steric effects and suppress the crystallization of oxidants, 22,[26][27][28][32][33][34][35] but the copolymer phase remaining inside PEDOT causes the deterioration of σ. 33 For both applications as TE devices and electrode materials, PEDOT films with a reasonable thickness are required.…”

Vapor phase oxidative synthesis of conjugated polymers and applications

“…Fabretto et al have reported on the role of water during PEDOT polymerization via VPP. 40 Water was introduced into the polymerization chamber to vary relative humidity from 10 to 70% and films were compared according to conductivity. Substrates pretreated with Fe(III) tosylate as an oxidizing agent were unable to grow polymer films without the presence of any water vapor.…”