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Title: Developing high-efficiency compound semiconductor III-V Quantum-Dot solar cells
Author: Lam, P.-M.
ISNI:       0000 0004 8503 7450
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2016
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A concept of utilising discrete energy levels to form an Intermediate Band (IB) within the bandgap of a semiconductor solar cell has been studied. Such a feature can be achieved using nanostructured quantum systems. A nanocrystal Quantum Dot (QD) - forms discrete energy states from its confinement potential, which could be exploited as an IB. The IB acts like a stepping-stone to absorb sub-bandgap energy photons. This concept, of the Intermediate Band Solar Cell (IBSC), was established in 1997. In theory, it could deliver a maximum conversion efficiency of 63%. After the phenomenon called two-step photon excitation had been demonstrated with QD-IBSCs, enthusiasm in this solar cell research followed. IBSC has allowed a proof-of-concept for more efficient absorption of the solar spectrum, bypassing the conventional single-junction solar cell concept. Thus, it is a suitable candidate for high efficiency photovoltaic cells. Fabrication efforts with suitable materials have become one of the main focuses to establish IBSCs in the hope of optimising the conversion efficiency using this concept. This thesis explores the IBSC development using QD materials incorporated into III-V solar cells. The work includes experimental investigations that were carried out to study the effect of QD modification on the QD solar cell performance. The fabrication challenges facing the establishment of IBSC are described here. To tackle them, epitaxial growth as well as post-growth techniques have been studied. This work uses III-V semiconductor QDs that were fabricated using solid-state Molecular Beam Epitaxy technology. Imaging techniques such as Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) have been employed to study the surface morphology and the material quality of the QDs and the grown samples. Optical and electrical characterisation tools have enabled empirical studies on the solar cells. These include Photoluminescence (PL), External Quantum Efficiency (EQE) and current density-voltage (J-V) measurements. The work includes the study of (1) the material quality of submonolayer quantum dot (SML-QD) solar cells; (2) the effect of direct n-type doping on the voltage-recovery characteristic of charged QDs; (3) the effect of post-growth rapid thermal annealing technique used on QD solar cells and; (4) the characteristics of high bandgap barrier InAs/InGaP quantum dot solar cells. At the end of this thesis, the conclusions drawn from the experimental work are presented, followed by a proposal for a new QD solar cell design. The new design consists of a type-II QD configuration embedded in a high bandgap material. It aims to extend the carrier lifetime within the QD as well as creating an efficient multi-step photon absorption pathway. The structural layout of the proposed QD solar cell is outlined with type-II InAs/GaAsSb QD system enclosed with AlGaAs confinement enhancing layer, which could be fabricated in future work.
Supervisor: Liu, H. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available