Elimination of Efficiency Degradation Mechanisms in Silicon Photovoltaic Solar Cells

光伏太阳能电池似乎肯定会对21世纪的世界能源需求做出重大贡献。目前，这种装置只将落在地球表面的太阳辐射每平方米1kW的一小部分转化为电能。使用非集中太阳光的典型商用硅设备的效率不到15%，尽管已经制造出效率高达24%的实验室设备。对于效率更高的第三代电池，已经提出了许多非常有趣的想法，但其中大多数离大规模商业实现还有很长的路要走。目前，90%的生产是基于硅片(直拉单晶或铸造多晶)。使用现有硅技术预测的2010年产量将达到26GW的峰值。不幸的是，在最初的几个小时内，大多数硅太阳能电池都会退化，在相对效率降低10%(即20%的电池变成18%的电池)后才稳定下来。在世界范围内光伏发电总损失的背景下，这是非常重要的。人们认为，效率的降低是由于硅中的缺陷反应，在这个反应中，氧二聚体扩散到硼掺杂中，形成了一个强大的复合中心。这个问题并不局限于未来十年的商业设备，也与一些最有希望的第三代(高效率)电池高度相关，这些电池使用硅作为有源结构的一部分。该研究方案旨在通过在形成复合中心之前从硅中移除反应路径来消除退化过程。一个重要的先决条件是对缺陷中心及其形成有一个详细的了解。我们以前研究过氧二聚体，它们目前被认为是复合中心的前体。可以使用光学吸收测量来检测这些物质，理想情况下是在低温(~10K)下。初步工作表明，应该有可能开发出对硅材料的处理方法，将成品电池中二聚体的浓度降低到可以忽略的水平，并且因为二聚体不是在正常工作温度下形成的，所以可以消除缺陷的形成。使用这种方法来保持间隙氧的浓度是非常可行的，这为硅提供了机械强度，从而获得了产量和成本效益。在这项工作中，我们将利用少数载流子拉普拉斯深能级光谱研究复合中心，并通过施加单轴应力来确定其结构。当复合中心形成时，将使用光学吸收技术实时研究氧二聚体的反应。这项工作将与领先的太阳能硅制造商之一MEMC、德国太阳能能源研究所(ISFH)和葡萄牙阿韦罗大学(University Of Aveiro)合作完成，前者最近在太阳能电池降解方面进行了大量实验工作，后者将合作进行理论计算，以支持曼彻斯特的工作。

Photovoltaic Solar Cells seem certain to make a significant contribution to the world's energy needs in the 21st Century. At the present time such devices only convert a small part of the 1kW per square meter of radiation from the sun which falls on the earth's surface into electricity. Typical commercial silicon devices that use un-concentrated sunlight are less than 15% efficient although laboratory devices with efficiencies as high as 24% have been made. Many very interesting ideas have been put forward for higher efficiency 3rd generation cells but most of these are a very long way from commercial realization on a large economic scale. At the present time 90% of production is based on wafered silicon (Czochralski single crystal or cast poly-crystalline). The predicted production for 2010 using existing silicon technology will have a peak output of 26GW. Unfortunately in their initial few hours of operation most silicon solar cells suffer from degradation which stabilizes after a reduction in efficiency of 10% relative ie a 20% cell becomes an 18% cell. In the context of total photovoltaic power lost on a world scale this is very significant. It is believed that the reduction in efficiency is due to a defect reaction in the silicon in which oxygen dimers diffuse to the boron dopant to form a powerful recombination centre. The problem is not restricted to the commercial devices of the next decade but is also highly relevant to some of the most promising third generation (high efficiency) cells which use silicon as part of the active structure. This research proposal aims to eliminate the degradation process by removing the reaction path from the silicon prior to the formation of the recombination centre. An essential pre-requisite to this is to achieve a detailed understanding of the defect centres and their formation.We have previously studied the oxygen dimers which are currently thought to be the precursors of the recombination centre. These can be detected using optical absorption measurements, ideally at low temperatures (~10K). Preliminary work indicates that it should be possible to develop treatments of the silicon material which reduce the concentration of the dimers to a negligible level in the finished cell and, because the dimers do not form at normal operating temperatures, so eliminate the formation of the defect. It would be quite feasible using this approach to maintain the concentration of interstitial oxygen which provides mechanical strength to the silicon with consequent yield and cost benefits. In this work the recombination centre will be studied using minority carrier Laplace Deep Level Spectroscopy and its structure determined by the application of uniaxial stress. The reaction of the oxygen dimer will be studied as the recombination centre forms, in real time, using optical absorption techniques. The work will be done in collaboration with MEMC who are one of the leading manufactures of solar silicon, the Institut fr Solarenergieforschung Hameln/Emmerthal (ISFH) in Germany who have undertaken much experimental work recently on solar cell degradation and the University of Aveiro in Portugal who will collaborate on theoretical calculations to support the Manchester work.