Nitride Photovoltaic Materials for Full Spectrum Utilization

到2050年，全球人口预计将达到120亿，再加上经济适度增长，尽管能源效率有所提高，但全球总能源消耗估计将从目前的13TW增加一倍以上，达到28TW(10亿瓦)。为了限制或降低二氧化碳水平，这些额外能源中的大部分必须来自无碳来源，其中最大的是太阳能(10万太瓦)。然而，太阳能必须以合理的低成本转化为可用的形式。事实上，在英国，由于太阳能发电成本相对较高，最近可再生能源发电量的增加主要依赖于风力发电的增加。降低太阳能发电成本最有希望的方法之一是使用小面积的高效率电池，通过低成本的大面积塑料透镜将光线集中在电池上。到目前为止，效率最高的太阳能电池由三个包含砷化物和磷化物的结III-V半导体器件组成。这些材料的带隙与太阳光谱的波长范围不匹配，限制了所能获得的最大效率。拟议的工作将开发氮化物材料，用于未来展示全光谱超高效率光伏。采用聚光器技术的氮化物太阳能电池有望在效率和成本方面比目前最先进的水平有显著的进步。这是新发现的窄带隙氮化铟(InN)的结果，使三元合金InGaN和InAlN的带隙覆盖整个太阳光谱(InGaN为0.6至3.4 eV，InAlN为0.6至6.2 eV)。由这些材料体系制成的太阳能电池预计将达到双结电池的最大理论效率50%。这几乎是当前一代三结型太阳能电池装置的两倍。具有三个或更多结的氮化物电池可以实现接近60%的效率。然而，对这些材料的研究还不是很先进，材料质量仍在优化中。InGaN和InAlN的外延生长继续发展，项目合作伙伴正在进行改进，他们将为拟议的工作提供样本。在该项目中，将实施一项全面的结构、光学和电学表征计划，以优化这些合金在氮化物光伏器件中的应用。在这些活动的同时，还将对III-氮化物结构进行实验，以实现可重复的n型和p型掺杂，开发隧道结，确定缺陷在光伏性能中的作用，并优化金属接触和透明导电氧化物太阳能电池窗口。太阳能电池模型将使用从实验中确定的材料参数进行，以产生高效氮化物太阳能电池的优化设计，并研究绕过传统电流和晶格匹配限制的新的集成光学/电子太阳能电池设计。拟议的计划最终将使英国在氮化物广泛领域的极高专业知识得到扩展，包括用于低成本、低碳能源生产的富铟氮化物合金。

With the global population projected to reach 12 billion by 2050 coupled with moderate economic growth, and despite increased energy efficiency, the total global energy consumption is estimated to more than double to ~28 TW (1 billion billion watts) from the current use of ~13 TW. To cap or reduce carbon dioxide levels, most of this additional energy must come from carbon-free sources, the largest of which is solar energy (100,000 TW). However, solar energy has to be converted into a useable form at reasonably low cost. Indeed, in the UK, recent increases in renewable energy generation have mostly relied upon increased use of wind power due to the relatively high cost of solar power. One of the most promising approaches to reducing the cost of solar power, is to use small-area high efficiency cells with light concentrated on them by low cost, large-area plastic lenses. The highest efficiency solar cells to date consist of three junction III-V semiconductor devices containing both arsenides and phosphides. The failure of the band gaps of these materials to match the wavelength range of the solar spectrum limits the maximum efficiency obtainable.The proposed work will develop nitride materials for future demonstration of full spectrum super-high efficiency photovoltaics. Nitride-based solar cells with concentrator technologies promise to deliver significant advances in efficiency and reductions in cost over the current state-of-the-art. This potential is a result of the newly discovered narrow band gap of indium nitride (InN), making the band gaps of the ternary alloys indium gallium nitride and indium aluminium nitride span the entire solar spectrum (0.6 to 3.4 eV for InGaN and 0.6 to 6.2 eV for InAlN). Solar cells made from these material systems are predicted to attain the maximum theoretical efficiency of a double-junction cell of 50%. This is almost twice as efficient as the current generation of triple-junction solar cell devices. Nitride based cells with three or more junctions could achieve efficiencies approaching 60%. However, research on these materials is not very advanced and the material quality is still being optimised. The epitaxial growth of InGaN and InAlN continues to be developed, with improvements being made by the project partners who will provide samples for the proposed work. In this project, a comprehensive programme of structural, optical and electrical characterisation will be undertaken to optimise these alloys for application in nitride-based photovoltaic devices. In parallel with these activities, experiments will also be undertaken on III-nitride structures to achieve reproducible n- and p-type doping, to develop tunnel junctions, to determine the role of defects in photovoltaic performance, and to optimise metal contacts and transparent conducting oxide solar cell windows. Solar cell modelling will be performed using material parameters determined from the experiments to produce optimized designs for high-efficiency nitride solar cells and to investigate new integrated optical/electrical solar cell designs which circumvent traditional current and lattice matching constraints. The proposed programme will ultimately allow the UK's exceptionally high expertise in the broad area of nitrides to be extended to include indium-rich nitride alloys for low cost, low carbon energy generation.