Improved surface passivation for semiconductor solar cells

The world is currently undergoing one of the biggest transformations in energy usage since the industrial revolution. From the poorest to the richest nations, our planet has shown the consequences of climate change, and the exhaustion of some fossil fuels is now in the foreseeable future. We have started to change the way we generate, distribute and use energy throughout the world. Solar power is one of the most environmentally favourable sources, which in principle could provide all the energy required for the planet. Solar cells use the photovoltaic effect to convert solar energy to usable electrical energy, and thus are a key technology to provide the world with renewable, inexpensive and reliable electricity. Photovoltaics research and industry have experienced enormous advancements in the last two decades. The most important material by far in the photovoltaics field is silicon. Silicon today accounts for over 85 % of the photovoltaics market, and has over 140 GW of installed capacity. Current silicon solar cell systems have an energy payback time of only 2-4 years with 30-years lifetime. Their cost of power generation is now falling below 0.5 $/W, and in some areas of the world they are already cost effective for supplying grid electricity. Silicon photovoltaics is therefore an extremely promising technology where significant technological improvements are still possible which will ensure further price reductions and increased deployment.

Silicon solar cells capture solar energy when light is absorbed near the cell's surface and it creates electrical charge carriers. These carriers then diffuse through the cell, get collected at one of the contacts and are then able to deliver electricity. In this process many carriers are lost due to the imperfections of the material. The conversion efficiency of a solar cell is therefore limited by this loss of charge carriers at imperfections and defects. The surface of the cell represents a major material defect. The reduction of charge loss at the surface, termed passivation, is hence a critical feature requiring improvement. This project aims to improve the efficiency of silicon solar cells by optimising passivation using the cost-effective technologies proposed and patented as part of my previous research work. It is rare that a newly proposed technique could produce a step-change in the efficiency of passivation in commercial solar cells. This grant application will specifically enable that step-change to be developed. My research programme includes the fabrication, processing and characterisation of different passivation coatings used in solar cell manufacture. Different methods of producing the coatings and enhancing their passivation properties will be studied. Techniques to deposit the coating will include chemical and physical vapour deposition. In each case the key importance will be the characteristics of the layer with respect to storing excess electric charge that will be especially introduced. The research will be carried out at the Oxford Materials department, in close partnership with four UK manufacturing companies and a leading overseas research centre, the Fraunhofer Institute for Solar Energy Systems ISE. This institute will provide processing and characterisation facilities, staff time and state-of-the-art custom-made solar cells, and will also help interface the outcomes of this collaboration to the solar cell industry worldwide. Overall, this project will combine a strong team of academics and industry to improve efficiency and reduce the cost of semiconductor solar cells, thus paving the way for wide deployment and uptake of a technology with the potential to provide the world with abundant renewable and reliable energy.

It is expected that this research will produce impacts in many different ways as follows:

1. An increase in scientific understanding of the passivation process on silicon surfaces. Such understanding is of interest to researchers in photovoltaics, and also optoelectronics, lasers, MEMS, semiconductor detectors and actuators. This understanding will enable improvements in the performance of solar cells and different devices with applications to consumer electronics, aero-space technology, scientific instruments, biomedical imaging and diagnostics, and security and military.

2. The production of the best stable passivation of silicon surfaces to date. This research is expected to demonstrate what it possible in terms of passivation processes compatible with industrial manufacturing.

3. The development, in collaboration with our industrial partners, of commercial processes to produce excellent, cheap and stable passivation. If this goes into production in the UK it will result in high tech UK jobs, exports and increasing the UK manufacturing capability. If the work is commercialised elsewhere, then at the very least, a revenue stream through licensing will return to the UK.

4. A technology which enables economically viable production of back contact cells -which are 20% more efficient relative to their competitors- through the provision of a cost effective passivation technique. The best case scenario is that over the next 5 to 10 years, as back contact cells become standard, the extrinsic field effect passivation techniques developed here will be applied to the majority of solar cells manufactured. By such time it is possible that hundreds of giga watts of new capacity will be installed each year most of which will have the potential to benefit from this research.

5. Environmental impact. If the output of the research is commercialised then it will increase the uptake of solar power via the improvements in costs and efficiency of solar cell manufacture. Wider solar power uptake allows a direct reduction of CO2 emissions and long-term energy security. A reduction in the rate of global warming will have huge beneficial impacts for mankind across the world.

6. The skilled researchers trained and the involvement of UK business will promote the competitiveness of the UK in the quickly growing renewable energy industry.

7. The work will also have wider commercial implications in areas such as semiconductor sensors. Such sensors are widely used in many high technology areas where the UK already has a strong presence, such as detectors in high performance cameras used in space applications, scientific experiments and medical imaging instruments. In the case of sensors, the research is very likely to be commercialised and exploited within the UK because there are already UK based, world-leading, companies in these fields. Through the close collaboration with UK companies in this project, they will be ideally placed to develop the results as soon as they are produced.