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必要。尽管很明显，未来的发电将来自多种技术的广泛组合，但光伏电池近年来在技术和商业上取得了惊人的进步。在过去十年中，可再生能源发电受到税收优惠和上网电价的刺激。大规模的光伏制造已从中受益，在实现制造规模经济所必需的学习曲线上取得了非常迅速的进展。然而，就像今天所有的可再生能源一样，光伏发电的每千瓦时成本高于化石燃料发电，尽管这一差距在过去两年里大幅缩小。光伏发电的成本降低是通过创新的电池设计、使用成本较低的材料、电力管理电子产品的进步和较低的利润率实现的。目前，85%的新装置使用多晶或单晶材料的晶片硅电池。在这些情况下，一个关键问题是开发使用更薄的薄片的技术(太阳能电池板的特定区域使用较少的硅)，并转向“太阳能级”硅。这种类型的硅比用于集成电路的电子级硅纯度低，可以浇铸成多晶锭，但它要便宜得多。这是一个重要的问题，因为在这些发展之前，高达50%的电池成本可以归因于硅材料。尽管太阳能级硅电池的转换效率低于电子级材料，但通过这种方式实现了每千瓦时交付成本的重要降低。通过使用耗能较低的冶金工艺生产的硅，可以进一步大幅降低成本，例如，通过用碳还原石英和应用低能源净化工艺来启动制造工艺。这种类型的硅被称为升级的冶金硅，其纯度甚至更低，含有补偿的掺杂剂和金属，它们可以充当重要的复合中心，从而进一步降低效率。这项提议的目的是开发能够使由这些廉价形式的硅制造的电池的效率接近于成本较高的电子级材料所获得的效率的方法。这可以将多晶太阳能级硅的效率绝对提高约5%，对于升级的冶金硅来说，效率甚至更高。目前的硅电池结构工作良好，因为氢(通常来自氮化硅减反射层)使表面和体缺陷钝化。在电子级单晶中，这将复合降低到微不足道的程度。它在太阳能级多晶硅或升级后的冶金硅中不起作用，因为有些区域，有时是整个晶体颗粒，不会被氢钝化。然而，其他地区的质量非常高，往往与电子级硅一样好。我们将钝化阻力与在切片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，我们计划使用电子和化学技术(载流子寿命、拉普拉斯深能级瞬变光谱、SIMS、拉曼光谱和缺陷建模)来识别表现出抗氢钝化的缺陷。该建议的关键部分是利用我们对硅中缺陷反应的知识来开发替代钝化化学物质，这些化学物质可以在切片或电池生产过程中应用于那些抗氢钝化的缺陷物种。通过这种方式，我们预计将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.