Preparation Of PEDOT:PSS-based Conductive Polymers And Its Application In Electrochromic Devices

Electrochromic devices（ECDs）exhibit reversible change in absorption spectrum or color under external stimulus of voltage.ECDs possess a number of advantages,such as low energy cost,rapid response and good cycle stability,and have widely application prospects in the field of smart display.Electrochromic devices are mainly composed of transparent conductive layers,an electrochromic layer,an electrolyte layer and an ion storage layer.The recent studies of ECDs mainly focus on electrochromic layers and electrolyte layers.However,there are few reports about ion storage layers.The ion storage layer plays an important role in the optical contrast,response time and cycle stability of ECDs.Poly（3,4-ethylenedioxythiophene）/poly（styrene sulfonate）（PEDOT:PSS）is one of the alternative materials for the ion storage layer of ECDs because of its high conductivity,simple preparation,good stability,and high ion embedding/migration rate.However,PEDOT:PSS film is hydrophilic and poor flexible,limiting its application in flexible electronics.In order to improve these shortcomings,copolymerization with various flexible hydrophobic monomers was used to modify PEDOT:PSS in the present work.The effects of feed ratio of comonomer and side chain length on PEDOT:PSS morphology,conductivity and hydrophobicity were studied.A series of poly（（styrene sulfonate）-co-poly（ethylene glycol）methacrylate）（P（SS-co-PEGMA））copolymers with different side chain length and composition ratio was successfully prepared.The PEDOT:P（SS-co-PEGMA）were prepared using obtained copolymers as templates.The effect of PEGMA chain segment length and composition ratio on the conductive film performance of PEDOT:P（SS-co-PEGMA）was analyzed by scanning electron microscope,water contact angle,four-point probe resistivity test and ultraviolet spectrometer.Compared to unmodified PEDOT:PSS,mixing 15 mol%of PEGMA-350 in the copolymer,the conductivity of the PEDOT:P（SS-co-PEGMA）conductive film decreased from 3.85x10^-2S/cm to 2.92x10^-2S/cm.The water contact angle increased from 18.5 degrees to 36.0 degrees.The transmission rate remained at 84.9%.The square resistance of the film after bending of 800 times increased from 7.97 kΩ/sq to 9.86 kΩ/sq.The results showed that the hydrophobicity and flexibility of the PEGMA-modified conductive films were improved,but with few effects on conductivity and transmission,indicating that the PEDOT:P（SS-co-PEGMA）-based films can be applied as promising ion storage layers in ECDs.A series of poly（（styrene sulfonate）-co-isopropyl acrylamide）（P（SS-co-NIPAM））copolymers with different NIPAM composition ratios were successfully synthesized,and applied as template to prepare PEDOT:P（SS-co-NIPAM）.The effect of NIPAM composition ratio on the conductive film performance of PEDOT:P（SS-co-PEGMA）was investigated by different approaches.It was found that with the NIPAM molar ratio of 15%in copolymer,the water contact angle of the conductive film reached 39 degrees.The conductivity of the film decreased from 3.85x10^-2S/cm to 2.99x10^-2S/cm.The transmission rate maintained 81.1%.The square resistance after bending of 800 rears only increased from 7.08 kΩ/sq to 9.90 kΩ/sq.It is shown that the hydrophobicity and flexibility of the NIPAM-modified film are also improved,but with few effects on conductivity and light transmission,indicating that the PEDOT:P（SS-co-NIPAM）-based films can also be applied in ECDs.The optical contrastΔT of the ECD with PEDOT:P（SS-co-PEGMA）containing 15 mol%PEGMA-350 as the ion storage layer was 7.8%.The response time for coloring（t_c）was 0.99 s,while the response time for blenching（t_b）was 1.91 s.After 800 cycles of coloring and blenching,the optical modulation range of ECDs attenuated only 1.02%.TheΔT of the ECD with PEDOT:P（SS-co-NIPAM）containing 15 mol%NIPAM as the ion storage layer was7.5%,t_cand t_bwere 1.64 s and 1.87 s,respectively.The optical modulation range of ECDs attenuated only 0.28%.The above properties were basically equivalent to the device with PEDOT:PSS as the ion storage layer,indicating that these two copolymerized modified ion storage layer materials did not affect the discoloration properties of the device,and were feasible in the application of flexible electrochromic devices.