Improving silicon-based photovoltaics

Lead Research Organisation: University of Oxford
Department Name: Materials

Abstract

Climate change requires an urgent shift in the way the world generates and uses electricity. Solar power is the most environmentally favourable and abundant source of renewable energy. This potential is starting to be exploited as evidenced by a global solar capacity now approaching 500 GW. However, further improvements in the technology are required to deploy Tera-Watt levels of solar energy, and sufficiently reduce harmful greenhouse gas emissions. With typical solar panels being in service for 30+ years, even small improvements in the efficiency of solar cells would result in a large increase in the energy produced over their lifetime. Silicon solar cells account for 95% of all solar panels installed today. They dominate the market due to the ease at which they can be manufactured, while maintaining high efficiencies. The efficiency gap between laboratory and commercial cells, however, is still over 4% absolute. Reducing this gap requires the study, understanding, and deployment of new processing technology, which will ensure that TW levels of silicon cells are installed, and help mitigate climate change.

Photovoltaic solar cells work by using the energy from solar photons to excite an electron into a higher energy state. To generated electricity these electrons must reach the metal wires which connect the solar cell to the load, without the electron being lost within the cell. This loss process is called recombination and is caused by defects in the cell. Passivation is required to inhibit this recombination process, and is achieved by either eliminating or preventing electrons from reaching the defect. At the interface between the silicon and the metal contact a large concentration of such defects is formed, but the electron must be able to pass through the metal-silicon interface to carry electricity. To avoid the loss of electrons at such unavoidable interface, a new technology termed passivating selective contacts has been developed. Here an interfacial oxide film separates the metal and the silicon to reduce recombination losses. It has been shown that silicon oxide, SiOx (x=1-3), forms a contact which has significantly fewer defects than a direct metal contact. Despite it being an insulator with a much larger band gap than the Si, if the layer is sufficiently thin, conduction of electrons is still possible through quantum tunnelling.

This project aims to improve the efficiency of silicon solar cells by understanding and developing a novel process to form metal-thin oxide-silicon passivating contacts. The thin oxides are generally produced using nitric acid, though it is of poor quality and as a result some recombination still occurs. Recent research has looked into alternative methods of fabricating the oxide, including Plasma Enhanced Chemical Vapour Deposition (PECVD) and Rapid Thermal Oxidation (RTO). This project, however, is using a new method developed by Tetreon Technologies, where a mixture of Ozone and deionised water can be delivered to the silicon surface to form the oxide using gas rather than liquid processing. Gas processing would make this technique very attractive for industrial mass production. An additional part of the project involves developing a method to impregnate the SiOx layer with ions, resulting in a charge build-up at the interface. Recombination is then reduced at the interface due a reduction in the necessary charge carriers in the silicon. The charged ions can also enable 'trap assisted tunnelling' leading to improvements in contact resistance.

This EPSRC funded project is in collaboration with Tetreon (a UK equipment manufacturer) who will help guide the research and keep it focussed towards improvements, which could foreseeably be implemented on a larger scale and ensure the relevance to industry is maintained.

This project falls within multiple EPRSC themes, including: Energy, Manufacturing for the Future and Physical Sciences.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/R513295/1 01/10/2018 30/09/2023
2113576 Studentship EP/R513295/1 01/10/2018 30/09/2022 Shona McNab