Photovoltaic effect and quadratic electro-optic modulation in specific iodine-doped nonconjugated conductive polymers

Abstract

In this thesis, photovoltaic effect and quadratic electro-optic modulation in specific iodine-doped nonconjugated conductive polymers (NCP) have been reported. Exceptionally large quadratic electro-optic effects in specific iodine-doped nonconjugated conductive polymers have been previously reported. In the present work, an intensive research has been undertaken in order to enhance the electro-optic modulation-depth for the electro-optic device applications. Quadratic electro-optic modulation has been measured at 633nm using field-induced birefringence method. Photovoltaic cells involving specific iodine-doped nonconjugated conductive polymers have been fabricated and enhanced photo-voltages and photocurrents have been measured. Iodine-doped nonconjugated conductive polymers such as poly(β-pinene), cis-1,4-poly(isoprene) and styrene-butadiene-rubber (SBR) upon doping with iodine were used to fabricate photovoltaic cells. The cells formed using SBR polymer did not show as much voltage and current compared to the other two polymers. Photovoltaic cells have been fabricated using titanium dioxide/doped NCP/carbon on ITO glass-substrates. Photocurrents and photo-voltages for different intensities of light (from a white illuminant light bulb, emission at 300-700 nm) have been measured. Use of iodine-doped nonconjugated conductive polymer film has led to significant enhancement of photocurrent compared to previous reports involving undoped polymer-C60 composites in a different structure. A maximum photocurrent of about 0.3 mA was observed for a light intensity of ~ 5 mW/cm2 and maximum photo-voltage (open-circuit) was about 0.6 V for the photovoltaic cell fabricated using doped poly(β-pinene) whereas the maximum photocurrent of about 0.25 mA and maximum photo-voltage (open-circuit) was about 0.75 V for the same light intensity for photovoltaic cell fabricated using cis-1,4-poly(isoprene). Quadratic electro-optic modulation in iodine-doped cis-1,4-poly(isoprene) has been measured at the wavelength of 633nm using field-induced birefringence method. A modulation depth of about 5% has been observed at 633nm at an applied field of ~ 1 V/μm for about 50μm thick film. The modulation depth had a quadratic dependence on applied field. Enhanced modulation has also been observed in a optical fiber waveguide coated with iodine-doped nonconjugated conductive polymer. The Kerr coefficients measured are exceptionally large and have been attributed to the subnanometer size metallic domains (quantum dots) formed upon doping and charge-transfer.