Investigation of Electrical and Optoelectronic Properties of Vanadyl Phthalocyanine for Organic Electronic Devices

Abstract

The characterization of organic semiconducting materials, such as Metal Phthalocyanines (MPcs), has received great attention because of the increased activity in the potential use of these conjugated species in a broad range of electronic and photonic devices. MPcs are used in a variety of applications such as solar cells, field- effect transistors, sensors, etc., due to their thermal and chemically stability. In the research work reported in this dissertation, optimization and characterization of Vanadyl 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine (VOPcPhO) thin films is undertaken for potential applications in organic electronic devices, such as, humidity sensors and solar cell. This study has been carried out in four phases. In the first phase, the effect of annealing is examined on the structural, morphological and optical properties of Vanadyl Phthalocyanine (VOPcPhO) thin films. Phase two comprises spectroscopic absorption studies made on different proportions of the blends VOPcPhO and (6,6)-phenyl C61 butyric acid methyl ester (PCBM). In phase three, symmetrical and asymmetrical electrodes with gaps of 40 μm and 17 μm are fabricated using shadow mask and lithography. Then using these electrodes VOPcPhO based surface type sensors are fabricated and humidity sensing properties of VOPcPhO thin films are investigated. Finally, photovoltaic properties of VOPcPhO films are studied in the fourth phase of this work. The effect of thermal annealing on the structural, morphological and optical properties of Vanadyl Phthalocyanine (VOPcPhO) thin films is investigated for optoelectronic applications. The AFM results show that the surface roughness increases with increasing annealing temperature making the annealed films more prone to high absorption rather than the pristine sample. The thermal annealing processes on the spin-coated VOPcPhO thin films are observed to significantly enhance the features of light absorption and surface morphology. Hence, annealed thin films of VOPcPhO can xAbstract be incorporated in the organic electronic devices, where such features are of prime importance. The spectroscopic studies on a VOPcPhO and (6,6)-phenyl C61 butyric acid methyl ester (PCBM) blend, are performed. The blends of VOPcPhO and PCBM, with three different concentrations, are prepared in chloroform and their thin films are deposited by casting process. Spectroscopic studies have been carried out on these films to study the effect of doping of PCBM on VOPcPhO. An increase in the absorption coefficient has been observed with increasing dopant concentration. Optical energy gap appears to decrease from 3.32 eV to 3.26 eV with a variation of 0–75 % of PCBM concentration in the VOPCPHO films. To examine the sensing potential of VOPcPhO, Ag/VOPcPhO/Ag and Al/VOPcPhO/Pt surface-type structures are fabricated using the techniques of vacuum thermal evaporation, photolithography and spin casting. The effect of humidity is investigated on the electrical properties of these sensors. The symmetric type Ag/VOPcPhO/Ag devices of three different thicknesses are fabricated using thermal evaporator. Thin films of VOPcPhO are deposited on thoroughly cleaned glass substrates with pre-patterned Ag electrodes of 40 μm gap obtained using shadow mask. The effect of humidity is studied on their capacitance and the device with lesser thickness is observed more sensitive towards humidity. The asymmetric type Al/VOPcPhO/Pt device is fabricated using photolithography and spin casting. Thin film of VOPcPhO is spin casted on glass substrate with lithographically pre-patterned Al and Pt electrodes of 17 μm gap. Effect of moisture is examined on the capacitive and resistive response of Al/VOPcPhO/Pt co- planar structure. A 100-folds increase was observed in the capacitance of the VOPcPhO sensing material with an elevation of relative humidity level from 0 – 95 %RH. The resistance of the sensor reduced from 2.9 GΩ to 2.1 MΩ with increasing level of humidity. Adequate sensing properties, such as enough sensitivity, reasonable response and recovery times have been obtained. The humidity-dependent properties of the sensor make it a good match for its potential application in commercial hygrometers. The sensor with asymmetric electrodes has shown better sensitivity which xiAbstract may be ascribed to the relatively smaller gap between the electrodes or different work functions of the electrodes. A thin‐film, single‐layer organic solar cell has been fabricated from the VOPc derivative. The photoactive layer of VOPcPhO has been sandwiched between aluminum (Al) and indium tin oxide (ITO) electrodes to produce photovoltaic device. The device has exhibited photovoltaic effect with the values of J sc , V oc , and FF equal to 5.26 x 10 -6 A/cm 2 , 0.62121 V and 0.33, respectively. However, the device results in low efficiency which can be improved by introducing an acceptor material to form bulk heterojunction solar cell. The VOPcPhO blended with a matched acceptor material has potential application for improved efficiency of the bulk heterojunction type organic solar cells. The electronic parameters of the cell have also been extracted from I-V characteristics in dark. The ideality factor and barrier height were found to be 2.69 and 0.416 eV, respectively.