Thermal Physics in Electronic and Optoelectronic Materials and Devices.

Abstract

Operating temperature affects the performance and reliability of most electronic and optoelectronic devices. The aim of this work is to study thermal physics in two particular contexts – thermoelectric devices and organic photovoltaic devices – to gain fundamental understanding of electrical transport and optical processes in these devices that could aid in increasing their efficiency and discovering new applications. Thermoelectric devices convert heat energy to electricity and vice versa. Nanostructured materials offer a means to increase conversion efficiency. In the second part of this thesis we examine a practical means to enable 1D transport (and thus high conversion efficiency) using aligned chains of quantum dots. We show theoretically that this alignment can increase the thermoelectric power factor by a factor of 5 in common semiconductor material systems. In addition, we examine nanostructured thermoelectric materials based on HgCdTe quantum well superlattices. Using a steady state differential technique, we measure Seebeck coefficient and thermal conductivity, deriving a maximum thermoelectric figure-of-merit of 1.4 as compared to a maximum of ~ 0.33 for bulk HgCdTe. Solid state thermoelectric generators can be useful for scavenging waste heat energy, provided they meet the requirements of scalability and low cost. As a potential means to meet the need of scalable fabrication as well as offer mechanical flexibility, we explore the fabrication of thermoelectric power generators based on thin-films deposited on fibers that can be woven into energy-harvesting textiles. Using Ni-Ag metal thermocouples, we experimentally demonstrate the feasibility of this technology, developing a model for optimizing device performance and predicting the maximum power generated for more high-performance material systems. In the third part of my thesis, we show that the strong link between temperature and optical properties of a material can be used to study the generation of excitons in organic semiconductor thin films, with important implications for solar energy conversion. An experimental setup based on a phase-sensitive detection technique is designed and used to measure the temperature dependences of exciton oscillator strength, linewidth, and transition energy. Importantly, this technique can differentiate Frenkel and charge transfer excitons, which play crucial but separate roles in the photovoltaic conversion process.