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

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.

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 gas emissions 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.