Type-II hot carrier solar cells: control and manipulation of non-equilibrium carriers using band engineering

摘要：控制和操纵量子工程结构中的热损失：高效热载子太阳能电池的实用途径。非技术：改进的太阳能电池不仅对美国经济和能源独立至关重要；它们对减缓全球变暖和促进整个发展中国家的经济增长至关重要。太阳能电池提供了一种免费且丰富的清洁能源，但目前的运行效率水平限制了其经济可行性。核心问题是，太阳能电池将太阳能转化为可用能源的能力远远低于从根本上实现的能力。一个主要的损耗机制是当高能量光子被吸收时，通过产生热量而迅速损失能量。该计划通过对量子工程太阳能电池架构的基础研究来解决这些热载流子损失的缓解，这些架构在原理验证研究中已被证明可以抑制热的产生。改进版本的太阳能电池结构将有助于控制和操纵热载流子，以增加太阳能转化为电能的比例。这种结构的开发和运行为实用的热载体太阳能电池提供了真正的潜力，它有可能影响公用事业规模的发电，在高峰运行期间支持现有的公用事业基础设施，增加全球容量，减少对传统化石燃料的依赖。通过参与这个项目，研究生和本科生在多学科的技术技能范围内发展专业知识，并获得基础研究的独特视角，同时获得对新技术发展微妙之处的欣赏。技术：该项目主要研究基于锑化物和砷化物异质结构的量子工程结构。尽管这些半导体有一些潜在的重要优势，但对于下一代太阳能电池来说，它们还相对未被开发。这项研究建立在俄克拉何马大学的最新进展的基础上，该进展显示了在高温下砷化铟(InAs)/砷化铝锑化（AlAsSb）超晶格中稳定而强大的热载流子种群。一般来说，“热”高能载流子的产生在量子阱中更稳定，特别是在热损失被抑制的低温下。然而，这些效应在更高的温度下会迅速消失，而真正的太阳能电池必须在更高的温度下工作。InAs/AlAsSb超晶格是研究半导体结构中热载流子物理的理想材料。这些结构中的大约束势直接在量子阱中产生热载流子。简并价带改善了正电荷载流子（空穴）的提取，通过增加超晶格导带中的电子寿命来减缓热载流子的弛豫。在设计利用热载流子的太阳能电池中，直接吸收将在电池上部发射区域的量子阱中产生热载流子，然后通过超晶格和共振隧道结构快速提取。约束可以用来调节能量间隙以优化工作电压，同时有效地利用太阳光谱中uv -可见光光子产生的热载流子。利用分子束外延生长半导体异质结构，并利用光谱学和光电子测量对其热载子特性进行表征。太阳能电池装置将从优化的结构中制造出来，目标是提高效率。对这些结构和器件的研究将使人们更好地理解实际系统中的热载流子动力学，这对光伏社区非常感兴趣，对太阳能电池在国内能源市场的成本效益实施至关重要。

Abstract: Control and manipulation of thermal losses in quantum-engineered structures: a practical route to high efficiency hot carrier solar cells.Non-technical: Improved solar cells are vital not only to the U.S. economy and energy independence; they are essential to the reduction in global warming and for economic growth throughout the developing world. Solar cells offer a free and abundant source of clean power, but currently operate at efficiency levels that limit their economic viability. The central issue is that solar cells convert considerably less of the sun's energy to useable power than fundamentally possible. One major loss mechanism is the rapid loss of energy through heat generation when high energy photons of light are absorbed. This program addresses mitigation of these hot carrier losses via a fundamental investigation of quantum-engineered solar cell architectures that have been shown to inhibit heat generation in proof-of-principle studies. Improved versions of the solar cell structures will facilitate the control and manipulation of hot carriers to increase the fraction of solar energy that is converted into electricity. The development and operation of such structures offer a real potential for practical hot-carrier solar cells, which have the potential to impact utility-scale power generation supporting existing utility infrastructure during peak operating periods, increasing global capacity, and reducing dependence on traditional fossil fuels. Through involvement in this program, graduate and undergraduate students develop expertise in a multidisciplinary range of technical skills, and gain a unique perspective of fundamental research while acquiring an appreciation of the subtleties of novel technology development. Technical: The project focuses on quantum-engineered structures based on Antimonide and Arsenide heterostructures. Despite several potentially important advantages, these semiconductors are relatively unexplored for next generation solar cells. The research builds on recent advances at the University of Oklahoma showing stable and robust hot carrier populations at elevated temperatures in Indium Arsenide (InAs)/Aluminium Arsenide Antimonide (AlAsSb) superlattices. The generation of "hot" high energy carriers has been observed to be more stable in quantum wells, in general, particularly at low temperatures where thermal losses are inhibited. These effects, however, are rapidly quenched at higher temperatures, conditions in which real solar cells must operate. InAs/AlAsSb superlattices are ideal for studying the physics of hot carriers in semiconductor structures. The large confinement potential in these structures facilities hot carrier generation directly in the quantum wells. The degenerate valence band improves the extraction of the positive charge carriers (holes), which serves to slow hot carrier relaxation through increased electron lifetimes in the conduction band of the superlattice. In solar cells designed to harness hot carriers, direct absorption will create hot carriers in quantum wells in the upper emitter region of the cell, which are then rapidly extracted via superlattice and resonant tunnelling architectures. Confinement can be used to tune the energy-gap to optimize the operating voltage, while effectively harnessing the hot carriers generated by the UV-visible photons of the solar spectrum. The semiconductor heterostructures will be grown by molecular beam epitaxy and their hot carrier properties will be characterized by optical spectroscopy and optoelectronic measurements. Solar cell devices will be fabricated from optimized structures with the goal of improved efficiency. Investigation of these structures and devices will enable a better understanding of hot carrier dynamics in practical systems, which are of great interest to the photovoltaics community and important for the cost-effective implementation of solar cells in the domestic energy market.