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Abstract
This thesis seeks to addresses a question raised in the photovoltaic literature: “How can the kinetic energy of hot photogenerated carriers in a semiconductor be utilized to obtain larger open-circuit voltages than possible in a conventional pn-junction photovoltaic composed of the same material?”
This thesis shows by theory and experiment that by using light to drive a photothermoelectric heat engine, larger open-circuit voltages than possible in a conventional pn-junction photovoltaic composed of the same material can be achieved. Specifically, it is demonstrated that hot photogenerated carriers in a single InAs nanowire can be used to obtain an open-circuit voltage in excess of the InAs detailed balance limit. This is accomplished by employing a highly resistive InP thermionic barrier to block the transit of holes and lattice-temperature electrons. The InP thermionic barrier, which is located between two identical, degenerately n-type InAs regions, allows high-energy electron photocurrents to pass, while simultaneously ensuring that bias-induced drift currents of lattice-temperature carriers are blocked. This enables achievement of high open-circuit voltages in excess of the InAs detailed balance limit.
In the course of this work, two other findings are made. Firstly, it is shown theoretically that a thermally driven light emitting diode is thermodynamically allowed and that it is the reverse mode of operation of a hot carrier photovoltaic. Secondly, it is found that the wavelength dependent location of strong photon absorption in a metal-contacted, heterostructure single-nanowire can be exploited to obtain a photothermoelectric device in which the polarity of the photovoltage and photocurrent depend upon the wavelength of illumination. Methods for numerically modelling this optoelectronic phenomenon are detailed and possible uses are suggested.