Thermoelectric, Electrical and Photonic Properties of Semiconductors

Abstract

Thermal energy harvesting devices have many promising applications due to no emission of greenhouse gases during the process. Thermoelectric generators convert heat (temperature gradient) to electricity and thus the waste heat from different energy sources such as heat engines, automobiles and industry can be recovered to generate electricity. Therefore, thermoelectric devices are promising candidates for energy harvesting technologies. The characterization of thin film based organic semiconducting materials is a rapidly developing research and has received great attention because of their applications in low cost fabrication of electronic and photonic devices, such as, light sensitive junction diodes, sensors, solar cells, field-effect transistors, etc. In the research work reported in this dissertation thermoelectric, electrical, and photonic properties of the semiconducting thin films are undertaken for potential applications in the development of cost effective thermal energy harvesting technologies and electronic devices. In part I of this dissertation, energy harvesting from solution grown semiconductors via thermoelectric effect are discussed. For thermoelectric applications, three materials are investigated. The first material used is lead sulfide (PbS) grown from solution due to its inherent property of scalability. The solution grown PbS film consists of nanoclusters and has a high Seebeck coefficient, 450μV/K. The high Seebeck coefficient may be the result of increased contributions from junctions and boundary scattering of carriers occurring at interfaces between nanocrystallites, consistent with the scanning electron microscopy study of the film morphology. The solution-grown PbS technology may be well suited for energy-harvesting applications. The second and third materials are carbon nanotubes (CNTs) and organic polymer Poly(3,4- ethylenedioxythiophene) Polystyrene sulfonate (PEDOT:PSS). In a homogeneous system (CNTs or PEDOT:PSS alone), the Seebeck coefficient and electrical conductivity are interlocked by the Boltzman transport equation. Thus a heterogeneous layered system on a nano scale is developed from CNTs and PEDOT:PSS film to weaken the Boltzman transport and thus change the Seebeck coefficient and electrical conductivity somewhat independently. Thus the thermoelectric properties are enhanced by making heterogeneous system. viiIn part II, the electrical properties of sandwich type heterojunctions are presented. Thin film of organic material cobalt phthalocyanine (CoPc) were deposited by vacuum thermal evaporation on p- and n-Si with aluminum (Al) as top electrode. The charge transport mechanism was studied and the mobility of the organic thin films, with p- and n-Si as substrate, was calculated. Different parameters of the junctions were extracted from the electrical characteristics such as rectification ratio, reverse saturation current, series/shunt resistances, ideality factor and barrier height. The effect of light on current-voltage characteristics were also studied to show the photonic behavior of CoPc/n-Si heterojunction. The electrical characterization is useful as preliminary studies for further utilization of the CoPc in other electronic devices.