Year

2012

Degree Name

Doctor of Philosophy

Department

School of Chemistry

Abstract

Conducting polymers and their application as electrodes in electrochromics has been widely studied because of their high contrast, low power consumption and ease of processability as compared with inorganic metal oxides. Another component of an electrochromic device is the electrolyte. Organic electrolytes are widely used in electrochromic devices but have limitations such as volatility, flammability and leakage. Therefore, one of the aims of this thesis was to develop polymer electrolytes that afford comparable conductivity to organic electrolytes without the drawbacks mentioned above. The following polymer electrolytes containing polyethylene oxide (PEO), with molecular weight (2,000, 10,000 and 20,000), salt lithium perchlorate (LiClO4), lithium bis(trifluoromethanesulfonyl)imide(LiTFSI), and plasticiser ethylene carbonate/propylene carbonate (EC/PC), ionic liquid were successfully developed. The conductivity of these electrolytes were found to be in the range of 10- 3 to 10-5 S/cm with average photopic transparency between 89% -90%.

In this thesis dual polymer electrochromic devices were prepared using poly (3,4- ethylenedioxythiophene) (PEDOT) and polypyrrole (PPy) using vapour phase polymerisation which have a number of advantages including ease of processing, uniform coating over large surface areas and negligible monomer wastage. The contrast of the PEDOT/PPy device was found to be 43% at the applied potential of ± 1500 mV and 32% at the applied potential of ± 1000 mV. The assembled devices were tested for long term stability using ionic liquid as well as polymer electrolytes. The contrast of the PEDOT/PPy device degraded from 43% to 36% over a period of 15,000 cycles when switched between ±1500 mV. PEDOT/PPy devices were found to be quite stable with photopic contrast of 32% over extended periods of cycling (50,000) when cycled in between appropriate potentials of ±1000 mV.

Device failure or degradation of photopic contrast of the electrochromic device on long term cycling is a commonly observed phenomenon in dual polymer devices. Various techniques such as Impedance, Raman and FTIR were used to study the possible reasons for device failure and aid in the development of ways by which device degradation could be minimised or eliminated.

Degradation of the PEDOT/PPy device was minimised by running at low applied potentials, but there was a loss in contrast in the dual polymer electrochromic device. Alternate electrochromes were investigated in an attempt to improve the device contrast range. Replacing the cathodically colouring polymer from PEDOT to poly(3,4-propylenedioxythiophene) (ProDOT)gave a higher contrast as compared with PEDOT on a single electrode basis. A dual polymer electrochromic device assembled using ProDOT/PPy, when operated at lower potentials of ± 1000 mV, gave contrasts between 39% - 42%. These contrasts were similar to that of a PEDOT/PPy device operated at higher applied potentials of ± 1500 mV. The lower operating applied potentials utilised in the ProDOT/PPy devices resulted in enhanced cycle life of the device. However, it was observed that the contrast of the dual polymer electrochromic devices was limited by the anodically colouring polymer. This finding implied that even if the cathodically colouring polymer gave a high value of contrast as an individual electrode, it does not mean that there would be an increase in contrast for the device.

The PEDOT/PPy device at high applied potential of ±1500 mV gave contrast of 42%, but at low applied potential of ±1000 mV the device gave contrast of 32%. I have tried to match the anion dopant in the conducting polymer with the anion dopant in the electrolyte to study the effect of contrast at low potential. Interestingly, it resulted in the contrast of the PEDOT/PPy device being increased to 42% at ± 1000 mV as compared to 32% at ± 1000 mV, suggesting that anion matching is important for achieving better contrast.

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