SuperSilicon PV: extending the limits of material performance

Climate change attributed to the emission of carbon dioxide from burning coal, oil and gas has stimulated policies which encourage the use of renewable energy via tax concessions or feed-in tariffs. These are necessary because the cost of renewable energy is more than that of energy derived from fossil fuels. The potential of photovoltaics (PV) is enormous, with sunlight delivering the world's annual energy needs every 15 minutes. Unfortunately, in most circumstances, no PV technology yet delivers adequately low cost electricity.

Silicon photovoltaics (PV) are a major renewable technology, accounting for ~90% of the PV market. The present industry view is that silicon will continue to dominate the market for the foreseeable future. Apart from the capital cost, the key parameters affecting cost per kWh are efficiency and working life. The efficiency of a cell is limited by the portion of the spectrum it can use. For a simple (single-junction) cell this fundamental limit is ~30%. Many ideas which aim to go beyond this have been researched but the essential combination of low cost, long life and efficiency have proved very elusive. Commercial modules made from low cost multi-crystalline silicon generally have efficiencies in the range 13 to 16%. Commercial production using high quality (more expensive) silicon reaches 20%, where the world record efficiency for a cell is 25.8%. From our experience of silicon materials research projects over the past five or so years, we believe it will be possible to enhance the carrier lifetime of cheaper forms of silicon to provide substantially higher production conversion efficiencies of ~22%. For domestic installation - where grid parity is regarded as matching the utility supplier's price - latest figures suggest this efficiency is sufficient for parity at latitudes of up to 60 degrees from the equator.

This project unites three UK silicon PV groups with four materials manufacturers, a major cell manufacturer, two materials characterisation companies, and three leading international university groups to work on some of the most pertinent issues in silicon PV materials. We aim to provide underlying science which will enable silicon PV to produce electricity at lower prices than traditional generating plants. The quality of silicon, as characterised by the minority carrier lifetime, places the upper limit on the efficiency that can be achieved. Cell processing is sufficiently mature to be able to make high efficiency cells provided the starting material is of high quality. Simplistically, the aim of this project is to remove defects which act as recombination centres and limit the efficiency of silicon PV cells. We are developing novel new methods of impurity gettering and defect passivation which have the potential to remove recombination centres which remain after existing processes. The project will also further understanding of the fundamental properties of defects in silicon, including the role of nano-precipitates in recombination, factors which prevent the fundamental carrier lifetime of silicon being reached, and the thermodynamics of impurity-dislocation interactions.

If the project is successful it will accelerate the take-up of photovoltaics and hence will lead to reductions in pollution and carbon dioxide emission. In turn, the market expansion will speed the technology along the learning curve and will reduce prices further. The impact will be a wider spread use of photovoltaics to generate electricity. This will benefit the UK, and could also enable more comprehensive electrification of rural areas in developing countries.

If grid parity without subsidy is achieved it will transform the industry. At the moment the market is mostly policy driven. If feed-in tariffs are reduced the market is stalled, an effect magnified by solar financing schemes. This is much less predictable than a simple competitive market and is not good for manufacturers.

The project will directly benefit UK industry. We will address problems identified in collaboration with our project partner (PV Crystalox Solar), who produce multicrystalline silicon for photovoltaics in Oxfordshire. Although much of our research will be generic for the whole industry, Crystalox will clearly be an early beneficiary. Our work will give them a better understanding of the defects which reduce minority carrier lifetime in their materials, and will allow them to develop remedial actions. A better product may facilitate a substantial increase in their market share. Two other UK companies that will benefit from this project are Oxford Instruments and Horiba UK. We will work with them to develop better instrumentation, which should result in new applications for their products.

Our international partners will also accrue benefit from our research. SunEdison Semiconductor (Italy), SunEdison Solar (USA) and CaliSolar (Germany) will learn about defects in their materials, including how they arise and how to deal with their detrimental effects to achieve higher carrier lifetimes. SunPower (USA) may be able to use our knowledge base to create high efficiency cells. Some of the scientific outcomes (e.g. understanding gettering and passivation) in the project will also be directly relevant to the £200bn semiconductor industry. The fundamental knowledge gained will result in higher yield production of more reliable electronic devices.