Study On Fabrication Of Flexible Transparent Conductive PEDOT/PET Films By Liquid Phase Deposition Polymerization

Currently, indium tin oxide(ITO) coated polymeric film dominates the market of flexible transparent conductive films(FTCF). Due to the brittleness of ITO and shortage of noble indium, ITO FTCF exhibits poor lifetime and has high price. It can not meet the requirements of developing optoelectronic devices. PEDOT possesses high electrical conductivity and optical transparency. It is a good candidate to replace ITO and fabricate new FTCF. However, PEDOT is infusible and insoluble. It is difficult to convert virgin PEDOT powder to thin films. How to fabricate PEDOT film is a hot topic in polymer science.In this thesis, in situ synthesis of PEDOT coating on PET by liquid phase depositional polymerization was studied and transparent conductive PEDOT/PET film was fabricated. Firstly, transparent PET film was irradiated with 172 nm VUV to improve the wetting ability and adsorbing capacity; then, VUV irradiated PET film was dipped in Fe(OTs)3 solution to adsorb Fe(OTs)3; finally, PET film with adsorbed Fe(OTs)3 was suspended in EDOT solution to synthesize PEDOT coating. The surface compositions of PET film were analyzed with FTIR after VUV irradiation. The wetting ability and adsorbing capacity to Fe(OTs)3 were evaluated with goniometer and iodometry method, respectively. The effect of VUV on the morphology of Fe(OTs)3 adsorbed on PET was revealed with SEM. The sheet resistance and thickness of synthesized PEDOT coatings were measured with four-point probe meter and surface profiler. The structures of synthesized PEDOT were characterized with FTIR, UV-vis and XPS spectroscopes and C-AFM. The effects of polymerization temperature, oxidant contents, retarding contents, EDOT solvents and processes on structures were investigated. The conductivity, transparency and adhesion of synthesized PEDOT coatings were discussed in terms of conjunctive length, doping level and condensed structure. To find new etching method to pattern PEDOT coating and fabricate transparent PCB,the effect of 172 nm VUV on molecular structure and electrical conductivity of PEDOT was investigated.VUV irradiation introduced carboxylic groups to PET surface and improved the wetting ability and adsorbing capacity as well as the uniformity of adsorbed Fe(OTs)3. As PET film with adsorbed Fe(OTs)3 was suspended in EDOT solution, EDOT deposited onto PET surface and joined one another to form positive charged PEDOT coating. After exposed to VUV for 4 min, dipped in 80 mmol/L Fe(OTs)3 n-butyl alcohol solution and suspended in 80mmol/L EDOT cyclohexane solution for 20 hrs, a PEDOT/PET film with sheet resistance 870 ?and transmittance 80 % could be fabricated.As temperature increased, the defects in synthesized PEDOT decreased whereas the conjunctive length and doping level increased because of the increase of solvation and the dissociation of Fe(OTs)3 to Fe3+ and OTs-. The conductivity and transparency of synthesized PEDOT/PET film increased.At reaction temperature 70 oC,the doping level reached 32% and the sheet resistance dropped to 370Ω; the conductivity and transmittance was 400 S/cm and 86 %, respectively.IM depressed the oxidative reactivity of Fe3+ ions and reduced the doping level of synthesized PEDOT but increased the conjunctive length. As the IM content increased, the polymerization rate decreased; the conductivity, transparency and interface bonding increased. At IM: Fe(OTs)3 mole ratio of 1:8, the sheet resistance and transmittance was 230Ωand 88 %, respectively; the interface bonding graded 3A. But excess IM would retard the oxidative polymerization of EDOT seriously.As the concentration of Fe(OTs)3 solution increased, the adsorption of Fe(OTs)3 on PET increased, which resulted in thicker PEDOT coating and higher surface conductivity but lower transparency and poorer interface bonding. As the PEDOT/PET film was immersed in diluted Fe(OTs)3 solution after depositional polymerization, the EDOT oligomer on top layer would be oxidized by Fe3+ further and the conjunctive length and doping level could beenhanced; thereby, the conductivity, transparency and interface bonding of resultant PEDOT/PET film increased remarkably. After immersed in 20mmol/L Fe(OTs)3 solution for 15 min at 60 oC, the sheet resistance decreased 30 % to 165 Ωand the transmittance increased 4 % to 84%.EDOT solvent has remarkable influence on the structure and performance of synthesized PEDOT coating. In petroleum ether, the adsorbed Fe(OTs)3 would not leaked to solution phase; however, it led to PEDOT coating with more defects and short conjunctive length; the conductivity, transparence and interface bonding of the coating were very poor. In acetonitrile solution, the adsorbed Fe(OTs)3 would defuse to solution phase largely and it became difficult to synthesize PEDOT coating. This problem could be solved by adding small amount of acetonitrile to petroleum ether.As the amount of added acetontrile increased, the conjunctive length of synthesized PEDOT increased whereas the doping level decreased; the conductivity, transparence and interface bonding of PEDOT/PET films got maximums as acetonitrile increased. Toluene could improve the operation range of acetonitrile/petroleum system. In toluene/acetonitrile/petroleum system, as the volume percentage of toluene and acetonitrile was 3‰ and 4‰, respectively, the sheet resistance of synthesized PEDOT/PET film was 200Ω, the conductivity and transmittance was 830 S/cm and 87%, respectively; the interface bonding reached 3A grade.After exposure to 172 nm VUV, PEDOT coating would photolysis and photo-oxidize. Transparent conducting PEDOT coating could be patterned directly with 172 nm VUV and mask.