Plugging the 1 eV band gap gap: GaAsBiN as a highly mismatched alloy for multi-junction photovoltaics.

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In this project, the growth physics of GaAsBiN will be investigated, culminating in a demonstration of the potential of this material for high efficiency photovoltaics.Researchers are striving to achieve solar cell efficiencies over 50 %, with ever more esoteric device designs being proposed to push efficiency limits. The well-established multi-junction device design (which harvests different regions of the solar spectrum in different sub-cells) has a theoretical efficiency limit well in excess of 50 %. However, the current world record efficiency is only 46 %. A lack of high quality sub-cell materials is hindering the development of these devices. A good candidate material will absorb an appropriate region of the solar spectrum and incorporate into existing multi-junction designs without causing strain in the structure, which degrades solar cell performance.The alloy proposed in this research, gallium arsenide bismide nitride (GaAsBiN), is an ideal candidate material. The dramatic impacts of Bi and N on the GaAs electronic structure allow the optical absorption profile of the alloy to be easily tailored; the large size of the Bi atom balancing the small size of the N atom also allows the alloy to be incorporated into existing device designs strain-free. However, the technical challenge of synthesising GaAsBiN has limited its development to date. The size differences between As, Bi and N make GaAsBiN crystal growth problematic, necessitating non-standard growth conditions and techniques. Very few laboratories around the world have reported GaAsBiN growth and only one has demonstrated a device comprising this material.The project will be completed in three work packages:The first work package of this project will involve preliminary growth studies aimed at producing simple GaAsN, GaAsBi, and GaAsBiN test samples. Starting with previously published growth parameters, the material quality will be optimised through a growth condition investigation.In the second work package, the growth parameters derived in work package one will be used to produce several GaAsBiN devices, which will be characterised using standard electronic techniques. In parallel, selected growth parameters will be used to grow thin GaAsBiN layers, which will be imaged with atomic resolution at the University of Sheffield and will have their Bi profiles measured with atomic layer resolution at the University of Huddersfield. Through careful analysis of the surface images and Bi profiles, and comparison of these results with the corresponding electronic device performances, the impact of growth conditions on material composition and quality will be determined.Work package three will use the growth condition understanding developed in WP2 to produce a series of GaAsBiN devices, which will be characterised to determine their opto-electronic performance and applicability for multi-junction solar cells.