The Stability Investigation Of Electrochromic Materials And The Development Of Device Fabrication Method

Electrochromic is defined as the material's color change reversibly under the condition of applied voltage.Electrochromic device(ECD)shows a sandwich structure consisting of working electrode(WE),counter electrode(CE)and electrolyte between WE and CE.The WE is made of electrochromic materials,and its main function is switching color;the CE is used in ECD as ion storage.And the match and stability of these two electrodes have a direct impact on the application of ECD.Electrochromic materials can be classified into two categories:organic material and inorganic material.As a kind of organic electrochromic materials,Poly-thiophene and its derivatives shows excellent electrochromic properties.Since the fast response time,large transmittance window and the color can be easily regulated,it has attracted the worldwide attention.Poly 3,4-(2,2-dimethyl-propylene dioxy)thiophene(PProDot-Me2)is a kind of p-type electrochromic materials,the anions such as[ClO4]-inject or eject between PProDot-Me2 film and electrolyte,and complex with polymer.Meanwhile,the redox reactions of PProDot-Me2 takes place,and the film's color switches accordingly.However,while cycling the PProDot-Me2 films,we found the current declines to 36.6%of its origin value after 2 000 cycles,and the charge resistance(Rct)increases.After we measure the morphology and composition by SEM and XPS,we find that the film's structure changes from loose network to compact planar structure.Besides,there appears Cl and Li element on the surface of films.Thus we put forward one deterioration mechanism to explain why the stability of PProDot-Me2 weaken.When the[CIO4]-transfer from the film and electrolyte,parts of[ClO4]-and Li+are trapped and absorbed on the polymer fibers.The covered polymer reduces the electrochemically active sites,and cause the deterioration process.To further prove our explanation,we change the cycling condition to control the complex between[CIO4]-and PProDot-Me2.The results are consistent with our hypothesis.Vanadium pentoxide(V2O5)is a layered structure material,its charge capacity is large enough to be used as counter electrode in electrochromic devices.However,the stability of V2O5 is poor.In this work,we doped metal ions(manganese)into V2O5 to distort its layered structure,then the doped V2O5 film would provide more space for Li+,which was helpful to its cycling stability.We used the method of electrochemical deposition to prepare Mn-doped V2O5 films,Mn ions were doped into the internal structure.Comparing with other methods,the way we used can avoid the generation of precipitation in the precursor solution,and prepared smooth films.After XRD,XPS and Raman test,we proved that the Mn is indeed doped into V2O5's structure,instead of being deposited on its surface.We also investigated the dop ratio and found that when the sample with Mn/V2O5=0.94 showed the best cycling stability.Other metal ions could also be doped into V2O5 layer structure by using this method,which could increase the charge capacity to match with working electrode(such as WO3)in electrochromic devices.For the electrolyte part we developed a new gel composite electrolyte with a strong interfacial adhesion ability based on the modified PMMA-PEG and polyvinyl butyral(PVB).A flexible,transparent free-standing quasi-solid polymer electrolyte(QSPE)was prepared after a UV-initiated polymerization inside the device cell.Through optimizing LiClO4/PC content in QSPE electrolyte,it is found that the QSPE membrane with 30 wt%0.1 M LiC104/PC loading shows a high transmittance of over 70%in the visible region(400-750nm)and large conductivity of 5.23×10-6 S/cm at room temperature.In addition,a PVB membrane with fiber network is designed and prepared via electro-spinning technique.Thanks to the liquid state of QSPE,it can achieve a full penetration with the PVB network structure.The tensile strength of the gel electrolyte to separate electrodes is largely improved from 0.05 to 0.31 MPa,while remaining a transparent state with a high ionic conductivity(2.15x105 S/cm).This novel QSPE-PVB electrolyte is then used to assemble all solid-state devices with PProDot-Me2 and V2O5 electrodes,displaying an excellent electrochromic performance,such as fast response time and long-time stability,indicating the novel gel electrolyte possesses a good compatibility with the electrodes.Incorporating PVB polymer into gel electrolyte is quite meaningful and important in glass devices for safety concern.In the last part,we proposed a new ECD fabrication method,named as inner-electropolymerization.ProDot-Me2 monomer is used to test this new strategy.Namely EC monomers are electrochemical polymerized inside the ECD cell directly.The ECD cell is constructed with one bare ITO glass and V2O5-TiO2 film.The bare ITO is used as the substrate for polymerizing PProDot-Me2.The conductive ITO glasses not only act as a substrate for EC film,but also form a uniform electric field to electropolymerize monomers.Using this method,various shapes and large-area ECDs are designed and fabricated successfully.The obtained ECDs show excellent electrochromic performances,especially for its long cycling lifetime.Thanks to this new idea,polymerization solution could also be used precisely,which avoids a big waste of EC materials in the traditional fabrication process.As far as we know,this method is put forward and applied in ECDs preparation for the first time.