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Title: Investigation of material and device properties of GaAsSbN for multi-junction solar cell applications
Author: Thomas, Tomos Daniel
ISNI:       0000 0004 7427 6974
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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Multi-junction solar cells (MJSCs) have achieved the highest solar power conversion efficiency to date of any type of solar cell and still have considerable potential for improvement compared to their theoretical efficiency. The bandgap combination of sub-cells is an extremely important factor in cell efficiency, with new materials with particular bandgaps necessary to increase efficiency further. Possible improved multi-junction solar cell designs require sub-cells with bandgaps between 1.2 and 0.8 eV. The dilute nitride alloy GaAsSbN can provide adjustable bandgaps in this region while remaining lattice-matched to common Ge and GaAs substrates. This thesis presents measurements and analysis of GaAsSbN 1 eV material and devices. Firstly, simulations of overall MJSC designs for concentrator photovoltaic applications are presented, including the importance of bandgap tunability in the 1 eV region. The material and device properties of 1 eV GaAsSbN subcells are studied experimentally and their performance compared to that necessary for inclusion in MJSCs. Electroreflectance spectroscopy and a model dielectric function are used to examine the absorption properties of GaAsSbN. Time-resolved photoluminescence spectroscopy is used to examine carrier recombination times and mechanisms, finding a typical recombination lifetime of 400 ps. Solar cell devices are electrically and optically characterised using current-voltage, capacitance-voltage and quantum efficiency measurements. The devices are modeled in order to investigate transport, finding a 30 nm hole diffusion length and 500 nm electron diffusion length. Improved sub-cell designs are proposed. Practical limits to the performance of GaAsSbN sub-cells are discussed.
Supervisor: Ekins-Daukes, Nicholas Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral