Efficiency Enhancement of Silicon Photovoltaic Solar Cells by Passivation

不断增长的能源需求、容易获得的石油资源的枯竭以及对气候变化的担忧，使得可再生能源成为一种必要。虽然很明显，未来的发电将由多种技术混合而成，但近年来，光伏电池在技术和商业上都取得了惊人的进步。在过去十年中，税收优惠和上网电价刺激了可再生能源发电。光伏的大规模生产已经从中受益，并且沿着实现生产规模经济所必需的学习曲线取得的进展非常迅速。然而，像今天所有的可再生能源一样，光伏发电每千瓦时的成本高于化石燃料发电，尽管这一差距在过去两年中已经大幅缩小。光伏发电的成本降低是通过创新的电池设计、低成本材料的使用、电源管理电子技术的进步和更低的利润率来实现的。目前，85%的新装置使用多晶或单晶材料的硅片电池。在这些情况下，一个关键问题是开发使用更薄薄片的技术（在给定的太阳能电池板面积上使用更少的硅），并转向“太阳级”硅。这种硅比用于集成电路的电子级硅纯度低，并且被铸造成多晶锭，但它非常便宜。这是一个重要的问题，因为在这些发展之前，多达50%的电池成本可能归因于硅材料。尽管太阳能级硅生产电池的转换效率低于电子级材料，但以这种方式实现了每千瓦时交付的重要成本降低。进一步大幅降低成本可以通过使用能源消耗较少的冶金工艺生产的硅来实现，例如，通过碳还原石英和应用低能量净化工艺来开始制造过程。这种类型的硅，被称为升级的冶金硅，含有补偿的掺杂剂和金属，它们可以作为重要的重组中心，从而进一步降低效率。这项提议的目的是开发一种方法，使由这些廉价硅制成的电池的效率接近于由成本更高的电子级材料制成的效率。这可以将多晶太阳级硅的效率绝对提高约5%，在升级的冶金硅的情况下甚至更多。目前的硅电池结构之所以工作良好，是因为氢（通常来自氮化硅增透层）钝化了表面和大块缺陷。在电子级单晶中，这将使复合减少到微不足道的水平。它在太阳能级的多晶硅或升级的冶金硅中不起作用，因为有些区域，有时是整个晶粒，没有被氢钝化。然而，其他地区的质量非常高，通常与电子级硅一样好。我们将对钝化的抵抗与在切片的寿命图中观察到的特定类型的缺陷联系起来。在这个项目中，我们计划通过使用电子和化学技术（载流子寿命、拉普拉斯深能级瞬态光谱、SIMS、拉曼光谱和缺陷建模）来识别具有氢钝化抗性的缺陷。该提案的关键部分是利用我们对硅缺陷反应的知识来开发可在片或电池生产期间应用于那些抗氢钝化的缺陷物种的替代钝化化学物质。通过这种方式，我们期望对占主导地位的太阳能光伏技术的效率做出非常重要的改进。

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.