Construction And Application Of Polythiophene-based Long-life Electrochromic Devices

Polythiophene and its derivatives are a class of conductive polymers with excellent performance,which can be widely used in electrochromic display,thermal control,dynamic camouflage,supercapacitors,sensors,organic or dye sensitized solar cells,and other fields.However,electrochromic films made from polythiophene and its derivatives have shortcomings such as low optical contrast and poor cyclic stability,which limits their application in electrochromic devices(ECDs).In order to further improve the cyclic stability of their electrochromism,a series of polythiophene-based electrochromic films were prepared by potentiostatic deposition and spray-coating methods in this paper.The morphologies of polythiophene films were adjusted by nanomodification of electrode surface and changing the side chain length of polymer.The mechanism of influence of nanoparticle modification of electrode surface and change of side chain length on the electrochromic cycle life of devices was investigated.The main research contents are as follows:(1)Poly(3-hexylthiophene)(P3HT)/ITO was prepared by potentiostatic deposition.Then,the mixed dispersions of multi-walled carbon nanotubes(MWCNTs)and poly(3,4-ethylenedioxythiophene)(PEDOT)were sprayed onto the surface of ITO glass to prepare the PEDOT-MWCNTs composite films.Additionally,the PEDOT/MWCNTs bilayer film was obtained by coating MWCNTs and PEDOT on ITO glass using alternating spray method.The ECDs were assembled using P3HT/ITO as the working electrode and PEDOT-MWCNTs/ITO or PEDOT/MWCNTs/ ITO as the counter electrode.The composition,structure of films and device performance were investigated in detail.The results showed that in the PEDOT/MWCNTs bilayer films,PEDOT nanoparticles were coated on the surface of MWCNTs to form a relatively loose surface morphology.The PEDOT/MWCNTs bilayer films had larger specific surface area,lower charge transfer resistance,and higher ion diffusion rate.The device(ECD-3)assembled with PEDOT-MWCNTs/ITO as the counter electrode had excellent cycle stability(after 510 cycles,the decrease of optical contrast was about 1% of initial ΔT).The device(ECD-4)prepared with PEDOT/MWCNTs/ITO as the counter electrode exhibited better cycle stability(after 510 cycles,there was almost no change in optical contrast).(2)Poly(2,2’-dithiophene)(PBTh)films were prepared by potentiostatic deposition,and a layer of helical multi-walled carbon nanotubes(H-MWCNTs)was coated on ITO glass by spraycoating method.Then,a layer of PEDOT film was sprayed on the surface of H-MWCNTs to prepare the PEDOT/H-MWCNTs bilayer films.The ECDs were assembled by using PBTh/ITO as the working electrode and PEDOT/H-MWCNTs/ITO as the counter electrode.The influence of composition and structure of films on the device performance was studied in detail.The results indicated that the PEDOT/H-MWCNTs bilayer film was composed of PEDOT nanoparticle aggregates and nanosheets coated on the surface of H-MWCNTs.The surface of films was relatively rough,and had larger specific surface area and more electrochemical active sites.The device(ECD-PBTh3)prepared with PEDOT/H-MWCNTs/ITO as the counter electrode had an optical contrast of 15%,fast response speed,and good cycle stability(after 500 cycles,the decrease of optical contrast was about 45% of initial ΔT).(3)Poly(3-octylthiophene)(P3OT),poly(3-dodecylthiophene)(P3DDT),and poly(3-hexadecylthiophene)(P3HDT)films with different side chain lengths were prepared on ITO glass by potentiostatic deposition.The ECDs were assembled using bare ITO as the counter electrode,and the composition,structure of films and properites of devices were characterized.The results showed that with the increase of side chain length(from 8 C to 12 C,and then to 16 C),the distance between the polymer molecular chains became larger,and the surface morphology of films changed from smaller nanoparticles to network porous structure and loose large particle structure.The electrochemical activity of films first decreased and then increased.Their charge transfer resistance increased,and the ion diffusion ability enhanced.The ECD-P3 OT based on P3 OT film had relatively large optical contrast(33%)and poor cycle stability(after 510 cycles,the decrease of optical contrast was about 72% of initial ΔT).The initial optical contrast of ECDP3 HDT based on P3 HDT film was relatively low(23%),but its cycle stability was good(after510 cycles,the decrease of optical contrast was about 22% of initial ΔT).The above research indicates that the length of polymer side chains can affect the cyclic stability of devices,which provides a new idea for the preparation of polythiophene-based long-life ECDs.