pi-conjugated polymers: Synthesis and application in devices

The first part of the work embodied in this thesis is towards modifying the electronic band-gap of conjugated polymers consisting of thieno[3,4- b]thiophene as one of the repeat units. Two different approaches were undertaken to prepare copolymers of thieno[3,4-b]thiophene in order to accomplish this. First, a simple simultaneous electropolymerization of two monomers from a solution mixture, and second, an oligomeric approach, wherein an oligomer of thiophene and thieno[3,4-b]thiophene with cyanovinylene spacer units was electrochemically polymerized. Several electroanalytical and spectroscopic techniques were used to characterize these polymers. ^ The major part of the thesis is focused on the electrochromic properties of conjugated polymers both in liquid and solid-state electrochemical cells. Several polymers reported in literature were re-evaluated on the basis of its photopic transmissivity. It is necessary that the contrast in a device is more pronounced at wavelengths, wherein the human eye is most sensitive at a given time of the day. At the same time, it is also important to take into account that the eye is sensitive to a broad spectral range. Hence, this photopic approach is very essential in understanding how the intensity of light can be controlled using electrochromic windows. ^ Furthermore, the optical switchability of solid-state electrochromic windows were also studied. Several issues, such as electrodeposition onto indium doped tinoxide (ITO) surfaces with areas ≥ 30cm2, device stability were addressed during the course of this research. The photopically weighted contrast of the device as a whole was found to be enhanced using derivatives of 3,4-propylenedioxythiophene, belonging to the 3,4-alkylenedioxythiophene family. ^ Finally, new device architecture was studied, wherein very low band-gap conjugated polymer with little/no visible electrochromism was used as the ion-storage layer in a dual polymer configuration. Colorimetric analysis of these devices was compared with the earlier devices incorporating a high band-gap anodically coloring conjugated polymer and low band-gap cathodically coloring polymer. These devices were found to be switching between a deep blue to a sky blue, which is a neutral color, as opposed to deep blue to yellow in the earlier configuration. Yellow filter is known to alter the way colors are perceived by the human eye.