Comparing build architecture and electrolyte of ITO-free flexible patterned organic screen-printed ECDs based on a novel PEDOT:PSS compound film post-treated with strong and weak organic acids
Poly(3,4-ethylenedioxythiophene) (PEDOT), a common constituent material used in the preparation of electrochromic devices (ECDs) when doped with the polymer anion poly(styrenesulfonate) (PSS), can also be used to form an electrode layer. Patterned ECDs may thus be fabricated by conventional spin-coating. Due to high wastage and low productivity in spin coating , the demand for low-cost ECDs is difficult to meet. The alternative use of screen printing, however, results in a major challenge due to difficulty in achieving the necessary continuity of the applied print and of the subsequent film forming uniformity when using standard PEDOT:PSS dispersion alone. The film forming property of PEDOT:PSS is shown here to be improved by dispersing in a water methanol mix to form a novel PEDOT:PSS-solvent compound, which, when combined together with the high boiling solvent dimethyl sulfoxide (DMSO) and a thickening binder solution of polyvinyl alcohol (PVOH), allows screen printing to perform well. The sheet conductivity of the printed PEDOT:PSS compound film is then further increased by studying post-treatment using strong and weak organic acids , illustrated by comparing p-toluenesulfonic acid (tosylic acid (PTSA)) with glacial acetic acid (CH 3 COOH (HOAc)) or oxalic acid (OA), respectively, over a range of concentration. In this way, the optimal processing concentration of acid is accordingly obtained to minimize sheet resistance. Screen printed patterned ECDs, based on the post-treated PEDOT:PSS compound electrochromic and electrode films using the optimal acid concentrations, were printed including electrolyte layers, were evaluated. Performance of lateral versus vertical ECD build architecture in respect to chromatic aberration, working voltage, time response, ion mobility in electrolyte type (gel/solution), absorbance at color-switching, as well as mechanical stability during repeated bending, was assessed. The optima of chromatic aberration and absorbance at color-switching were found to coincide with minimal sheet resistance of the novel PEDOT:PSS compound films. Almost equivalent sheet resistance was obtained with both PTSA and HOAc, each at optimal treatment dosage, showing newly that the weak organic acid post-treatment provides an equivalent effect in comparison with strong organic acid if used at sufficient concentration. Furthermore, the electrochromic performance of the experimentally prepared ECDs was shown to be close to the level obtained from conventional indium-tin oxide (ITO)-based ECDs, and retained stable performance for color-switching after repeated bending of 1 000 times.
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