Semester

Spring

Date of Graduation

2022

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Alan D. Bristow

Committee Co-Chair

Cheng Cen

Committee Member

Cheng Cen

Committee Member

Tudor Stanescu

Committee Member

Ian R. Sellers

Abstract

Semiconductor photovoltaics convert light into electricity through the extraction of photo-excited charge carriers. Among the most important parameters for a photovoltaic cell are good optical absorption in the desired region of the electromagnetic spectrum, and sufficient excited-state lifetimes and mobilities of the photocarriers to allow for charge separation and extraction before recombination. For solar cell applications there are significant challenges to overcome to improve the efficiency of the light-to-electricity conversion. The cells are most commonly made of silicon, which has a nearly perfect bandgap for absorbing the most solar radiation, an indirect bandgap to give a long photocarrier lifetime and good carrier mobility to allow for extraction of carriers. However, these single p-n junctions suffer from a thermodynamic limit of about 33% (without solar concentration), due to the detailed-balance of photocarriers excited above the bandgap, giving their energy to heating the device. Alternative methods include expensive multijunction devices with several layers absorbing and providing photovoltage at different energies, inefficient nanoparticle configurations and unproven hot-carrier solar cells (HCSCs). The latter were proposed to take advantage of good quality semiconductor materials and find mechanisms to prolong the photocarrier lifetime and allow them to be extracted via energy-selective contacts before recombination can occur. The detailed balance estimate that some HCSC devices may reach a theoretical maximum of 85%.

In this dissertation, InAs/AlAsSb type-II aligned multiple quantum well (MQW) heterostructures are explored for their potential as HCSCs. III-V semiconductor devices are grown by molecular beam epitaxy and are of high quality, possess a band alignment that generally separates electrons and holes leading to a prolonged photocarrier lifetime and also exhibit low thermal conductivity that can play additional roles in preventing carriers from cooling and thus further prolonging the carrier lifetime. These systems are of great interest because of these known properties, but it is unclear whether or not they are viable for HCSC, because mechanisms for hot-carrier lifetimes are poorly understood and photocarrier transport had not been explored when the results for the this project was started. Specifically, this work uses terahertz time-domain spectroscopy, time-resolved terahertz spectroscopy and transient absorption to addresses the observation of ground-state AC conductivity, excited-state AC photoconductivity and the charge carrier dynamics.

Findings show that lattice temperature significantly influences this MQW’s ground-state carrier transport due to alloy-intermixing at the well interfaces and weak coupling between long-range optical and acoustic phonons. It is also shown that excited photocarrier undergo dynamics that are also dependent on lattice temperature, dominated almost entirely by the availability of defect states in the wells and by localization/delocalization of hole bands with increasing temperature. Further investigation showed that high photocarrier concentration and low lattice temperature revealed a metastable state at early times after photoexcitation due to an intra-subband relaxation bottleneck mediated by reabsorption of optical phonons by the carrier and fast Auger scattering of carriers deeper into their respective bands. Hot-carriers also have long lifetimes outside of this regime, which can be exploited for photovoltaic applications because they generally have high carrier mobilities due to the high quality growth. Moreover, the confinement leads to ambipolar diffusion that can withstand a higher number of scattering events at ambient temperatures where such devices might operate.

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