Efficiency Enhancement of Silicon Photovoltaic Solar Cells by Passivation

点击翻译按钮获取中文摘要

Increasing energy demands, exhaustion of easily accessible oil resources and fears of climate change make renewable energy sources a necessity. Although it is evident that future power generation will result from a wide mix of technologies, photovoltaic cells have made astounding technical and commercial progress in recent years. Over the last decade renewable energy generation has been stimulated by tax concessions and feed-in tariffs. Large scale manufacturing of photovoltaics has benefited from this and progress along the learning curve necessary to achieve economies of scale in manufacture has been very rapid. However like all renewable energy sources today the cost per kWh of electricity from photovoltaics is greater than that generated by fossil fuels, although the gap has reduced quite dramatically in the last two years. The cost reductions in generation from photovoltaics have been achieved through innovative cell design, the use of lower cost materials, advances in power management electronics and lower profit margins. At the moment, >85% of new installations use wafered silicon cells of multi-crystalline or single crystal material. In these cases a key issue has been developing technologies which use thinner slices (using less silicon for a given area of solar panel) and moving to "solar grade" silicon. This type of silicon is less pure than the electronic grade used for integrated circuits and is cast into multi-crystalline ingots but it is very much cheaper. This is an important issues because before these developments as much as 50% of the cost of a cell could be attributed to the silicon material. An important cost reduction per kWh delivered has been achieved in this way despite solar grade silicon producing cells of lower conversion efficiency than electronic grade material. Further substantial reductions in cost could be achieved by using silicon produced by less energy hungry metallurgical processes, for example starting the manufacturing process by the reduction of quartz with carbon and applying low energy purification processes. This type of silicon, known as upgraded metallurgical silicon, is even less pure containing compensated dopants and metals which can act as important recombination centres so reducing the efficiency further. The aim of this proposal is to develop methodologies which are able to bring the efficiency of cells made from these cheap forms of silicon close to the efficiencies achieved from the higher cost electronic grade material. This could increase the efficiency of multi-crystalline solar grade silicon by around 5% absolute and even more in the case of upgraded metallurgical silicon. Current silicon cell structures work well because hydrogen (usually from the silicon nitride antireflection layer) passivates surfaces and bulk defects. In electronic grade single crystal this reduces recombination to insignificant levels. It doesn't work as well in solar grade multi-crystalline silicon or upgraded metallurgical silicon because there are regions, sometimes entire crystal grains, which are not passivated by the hydrogen. However other regions are of very high quality often as good as electronic grade silicon. We associate the resistance to passivation with specific types of defect observed in lifetime maps of slices. In this project we plan to identify the defects which show resistance to hydrogen passivation by using electronic and chemical techniques (carrier lifetime, Laplace deep level transient spectroscopy, SIMS, Raman spectroscopy and defect modeling). The key part of the proposal is to use our knowledge of defect reactions in silicon to develop alternative passivation chemistries which can be applied, during slice or cell production, to those defect species resistant to hydrogen passivation. In this way we would expect to make a very important improvement to the efficiency of the dominant solar PV technology.