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

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.

This research will produce a new advanced material for infrared opto-electronics and demonstrate its potential as a candidate for multijunction photovoltaics (PV). IQE is a world leader in semiconductor manufacture for industry. Their collaboration on the proposed research will expand their portfolio, and provide a pathway to the commercial development of GaAsBiN both in the UK and further afield. This will be a competitive advantage to UK companies specialising in opto-electronic device innovation-an industry worth > £10bn, employing > 70,000 people and growing at ~ 10 % annually (Innovate UK, Emerging Industries and Technologies Strategy 2014-2018).
The UK's engineering people pipeline is critical for maintaining the UK's position as a leader in compound semiconductor research, with an engineering skills shortage identified in the Royal Academy of Engineering's 2017 Engineering a Future that Works for All document. The host department has promised a PhD studentship in support of this research. This student will develop excellent semiconductor fabrication and characterisation skills. The UK has recently made a heavy investment in the Compound Semiconductor Catapult in Cardiff and a highly trained workforce will be required to maximise the impact of this investment.
My collaborators will directly benefit from this research. The pressure dependent material characterisation at the University of Surrey will enable Professor Sweeney to validate his theoretical predictions of the properties of GaAsBiN. The transmission electron microscopy characterisation at the University of Cádiz will provide a vehicle to test the N composition profiling techniques developed by Dr Reyes. The medium energy ion scattering analysis in collaboration with Dr Gavin Bell will complement the 6.1 Å family dilute bismide work at the University of Warwick. The characterisation of GaAsBiN layers grown by metal-organic vapour phase epitaxy by Professor Volz's group at Philipps University Marburg will initiate a new collaboration capable of the exploration of other materials under development in Marburg, such as InPBi.
The application targeted by this research is high efficiency PV, identified by the government as an area of opportunity for innovation (HM Government, Clean Growth Strategy, 2017). Concentrator PV (CPV) is a technology suited to climates with consistently high levels of direct sunlight. Many such countries are developing countries and would greatly benefit from utilising this natural resource for clean energy production. Surpassing 50 % efficiency from multi-junction solar cells will make this prospect significantly more economically viable, and the development of GaAsBiN has the potential to achieve that. As well as bolstering their economies, CPV development would allow developing countries to contribute to a cleaner global electricity mix. This will reduce our current dependence on fossil fuels, improving air quality and contributing to a climate change mitigation strategy. It also promises enhanced satellite capabilities through increased solar cell areal power density.
Through the National Epitaxy Facility, GaAsBiN will be made available to research groups across the UK, enabling them to use this alloy to further their own research agendas. It will also enhance the UK's standing in the area of highly mismatched alloy research, which is a rapidly developing avenue of research globally.
Dr Richards will participate in planned and existing engagement activities organised by the University of Sheffield, such as Festival of the Mind 2020, Pint of Science, and Café Scientifique to raise awareness of the importance of semiconductor research and of PV as part of a sustainable energy portfolio. Dr Richards is training as a Royal Academy of Engineering STEM ambassador and will use this position to promote STEM careers to school students and address the future skills shortage predicted by the UK government's 2017 industrial strategy.