Gettering of impurities in silicon: delivering quantitative understanding to improve photovoltaics

Lead Research Organisation: University of Warwick
Department Name: WMG

Abstract

Photovoltaics have the potential to supply all the world's energy needs. The market for photovoltaics is dominated by cells made from crystalline silicon, which account for more than 80% of today's production. Whilst other technologies are being researched, silicon's abundance, chemical stability, density, band gap and non-toxic nature mean that is certain to play a leading role in at least the medium term. More than half of bulk silicon solar cells are fabricated from multicrystalline silicon (mc-Si) wafers. Although mc-Si photovoltaics have lower efficiencies than their single-crystal counterparts, their substantially lower production costs means the technologies have equal commercial viability at present. Mc-Si is produced by casting, often using a low grade feedstock, and is consequently packed with extended defects (dislocations, grain boundaries and precipitates) and transition metal impurity point defects. Recombination of photogenerated charge carriers at such defects is a major reason for the reduced efficiency of mc-Si cells. Gettering processes are routinely used either to redistribute the defects or remove them from the material. However, such processes are not completely effective. One of the major reasons for this is that the interaction between defects prevents them being gettered. This project aims to further the fundamental understanding of defect interactions in mc-Si. The thermodynamics of interactions between transition metals (particularly iron) and extended defects (particularly dislocations and oxide precipitates) will be studied experimentally. Passivation of key extended defects will also be investigated. The fundamental knowledge obtained should allow the development of new or modified gettering processes with the ultimate aim of facilitating the use of dirtier (hence cheaper) feedstocks for silicon photovoltaics.

Planned Impact

There will be many beneficiaries in addition to those in the sizeable academic community in this field. It is clear that the £40bn solar cell industry - most of which is focused on silicon - will benefit directly by being able to produce more efficient products at a reduced cost. The UK has substantial activity in this area (including Sharp and PV Crystalox Solar) and so this research will provide economic benefits to UK industry. The scientific outcomes in the project will also be directly relevant to the £200bn semiconductor industry. Gettering in the production of silicon-based integrated circuits is ubiquitous and the fundamental knowledge gained will result in higher yield production of more reliable electronic devices.

It is generally accepted that global warming is now occurring and although its current extent and future impact are unclear, many scientists believe that a global crisis is a real possibility. Photovoltaics are able to supply all the world's energy needs with minimal carbon emissions. Whilst solar technologies other than silicon are being researched, silicon's abundance, chemical stability, density, band gap, and non-toxic nature mean that it is the only photovoltaic material currently available which is certain to be able to fulfil all our long-term energy needs. At present unsubsidised silicon photovoltaic technologies are marginally economically uncompetitive compared to electricity generated from fossil fuels. The improved understanding arising from my research has the potential to deliver substantial improvements in gettering processes in multicrystalline silicon, allowing more efficient solar cells to be produced from cheaper feedstocks. The impact of this would be more cost-effective solar cell modules which are better able to compete with conventional electricity sources. It is not unrealistic to think that this could be achieved in the next five years. The mass uptake of improved products would result in vastly reduced greenhouse gaemissions and may slow down, or even reverse, global warming. I feel my topic of research has the potential to have major impact in the field and in society.

Publications

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Related Projects

Project Reference Relationship Related To Start End Award Value
EP/J01768X/1 03/09/2012 31/01/2013 £89,533
EP/J01768X/2 Transfer EP/J01768X/1 31/07/2013 28/02/2015 £84,800
 
Description The project furthered the understanding of the fundamental science of the interaction between defects in silicon materials which are used in >90% of solar cells. This includes details of how transition metal impurities interact with extended defects, such as a precipitates, dislocations and grain boundaries. The project developed new low temperature processes for improve minority carrier lifetime in multicrystalline silicon materials, including internal gettering and external gettering to saw damage at the wafer surfaces.
Exploitation Route The processes developed could be used by the silicon solar cell industry to develop lower cost photovoltaic modules. The processes are compatible with low quality (hence low cost) feedstocks which unless improved are currently discarded. Thus, the processes could be used to increase production yields and to improve efficiencies of solar cells. The overall impact will be most cost-effective solar cells.
Sectors Energy

 
Description EPSRC Global Challenges Research Fund (GCRF)
Amount £342,394 (GBP)
Funding ID EP/P511079/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start  
End 03/2017
 
Description EPSRC Supergen Solar Challenge
Amount £1,293,323 (GBP)
Funding ID EP/M024911/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 09/2015 
End 11/2018
 
Description EPSRC Supergen Solar Hub: International and Industrial Engagement Fund
Amount £8,320 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Department SuperSolar Hub
Sector Academic/University
Country United Kingdom
Start 04/2015 
 
Description EPSRC Supergen Solar Hub: Supersolar International Conference Fund
Amount £500 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Department SuperSolar Hub
Sector Academic/University
Country United Kingdom
Start  
 
Description Royal Society International Exchanges Scheme
Amount £11,935 (GBP)
Funding ID IE130110 
Organisation The Royal Society 
Sector Charity/Non Profit
Country United Kingdom
Start 09/2013 
End 08/2015
 
Description ISFH 
Organisation Institute for Solar Energy Research
Country Germany 
Sector Academic/University 
PI Contribution Scientific studies of the influence of defects on silicon materials for photovoltaics.
Collaborator Contribution Access to surface passivation facilities, and intellectual input into scientific studies.
Impact Many scientific papers, including some of those returned with this submission.
Start Year 2009
 
Description SunEdison 
Organisation SunEdison Semiconductor
Country United States 
Sector Private 
PI Contribution Experimental studies of defects in silicon materials for integrated circuits and photovoltaics.
Collaborator Contribution Materials, characterisation and technical expertise.
Impact Many academic papers, including those returned in this submission.