Thermoelectricity of small-molecular-weight organic semiconductors and titanium oxide nanotubes

Abstract

In this present study, the thermoelectric effect of small-molecular-weight semiconductors and titanium oxide nanotubes has been studied. In thesis studies, the thermoelectric effect is employed as both a characterization method and a target for enhancement by structural modification. The characterization of the effective channel thickness of an organic field effect transistor entails the need for a non-destructive characterization that precisely describes the expansion and shrinking of the channel in response to different operation gate voltages. Based on the Boltzmann Transport Theory, the correlation of the Seebeck coefficient with the charge carrier transport properties, including the areal and volumetric carrier density, the effective density of states, the threshold voltage, and the gate voltage can be numerically derived. The data of the series of experiments conducted in this study have shown that the charge carrier density can be effectively modulated by external fields created by the imposed biases on the gate of the OTFTs, and the carriers tend to concentrate close to the dielectric interface when the field effect is enhanced. The concentration profile deviates from the Debye length prediction because of the layer-by-layer deposition nature of the organic semiconductor molecules. These results are used in evaluating the effective channel thickness of the OTFTs under different operating bias, which can be very useful in production-related applications. In such applications, the film thickness of the channel can be precisely controlled by the effective channel thickness predicted by this method. For titanium oxide nanotubes, the measurement of thermoelectric properties follows the similar scheme, but the focus is on the enhancement of the thermoelectricity of titanium oxide. The Seebeck coefficient of the nanotubes with different annealing conditions are measured and compared. Besides the general trend of the enhancement of Seebeck coefficient by annealing temperature, an interesting change of sign in the Seebeck coefficient is found in the samples annealed in vacuum. The result have shown that the thermoelectric properties of the titanium oxide can effectively be enhance by introducing nanotube structures, and that due to the introduction of extra oxygen vacancies, both the morphology and the thermoelectric properties demonstrates close relationships, especially those annealed in high vacuum environment. The results of this study can serve as a foundation based on which further improvements can be made on the thermoelectric measurement, enhancements and effective-channel-thickness related applications. The relative convenience and precision of this tool is effectively an advantage of thermoelectric effect measurements.