EAGER: TDM solar cells: Bifacial III-V nanowire array on silicon tandem solar cells

摘要：传统的串联结光伏太阳能电池由多个III-V型化合物半导体亚电池组成，能够以比所有其他类型的太阳能电池更高的效率将太阳入射辐射转化为电能。这些器件的高性能部分是由于使用了高质量的单晶III-V材料，部分是由于多个子电池的耦合，这些子电池可以共同吸收宽带太阳光谱范围。然而，开发III-V级串联结器件所需的材料和制造成本对于大规模地面消费应用来说过高。因此，世界上性能最高的太阳能电池被限制在小众市场使用，如地面高浓度和空间电力应用。本文提出了一条高风险、高回报的探索性研究路径，旨在提供一种非传统的、但具有潜在变革性的纳米技术解决方案，以解决当前最先进的串联结太阳能电池所面临的上述消费者市场渗透挑战。该项目旨在通过将由垂直纳米线阵列组成的III-V亚电池单片集成到中央硅亚电池上，从而大幅降低制造成本，从而同时消除主要的III-V衬底成本驱动因素，同时与传统技术相比，将III-V晶体生长体积减少高达95%。该EAGER项目更广泛的意义在于有可能实现低成本、高效率的可再生能源创新，为国家提供更大的能源独立性和清洁电力。这项研究还影响和推进了物理学、纳米材料生长和表征、纳米电子学和光电子学等领域的科学和工程基础知识。直接预期的社会影响包括向公众推广科学、技术、工程和数学概念的推广活动，培训高技能劳动力的新成员，以及直接纳入来自代表性不足社区的高中、本科生和研究生。技术：该EAGER项目的技术方法是通过金属有机化学气相沉积在薄Si （1.1 eV）亚电池的顶部表面上的GaAsP （1.75 eV）纳米线阵列的选择性面积异质外延。通过单片集成背面InGaAs （0.5 eV）纳米线阵列，形成了一种双面、三种不同材料的串联结器件。垂直纳米线包括顶部和背面阵列，将包含径向分段的p-i-n结，并将通过外延隧道结连续连接到中央硅亚电池。这种设计能够吸收宽带入射太阳能以及反照率辐射。通过沿纳米线自由表面的应变松弛克服了标准晶格匹配约束。因此，在不需要梯度缓冲层或位错调解策略的情况下，实现了理想的光谱匹配。使用同轴p-i-n结几何形状的垂直纳米线阵列具有关键优势，包括在正常和倾斜入射下太阳辐照度的接近统一吸收，而不使用抗反射涂层，光子吸收和载流子收集方向的去耦，以及外延体积的显着减少95%。严格的器件参数建模将迭代耦合广泛的材料表征和性能相关实验，以优化单纳米线和集成阵列水平上的III-V亚电池结构。这项工作的最终目标是展示一种功能双面，三种不同材料，纳米线为基础的串联结太阳能电池，一次太阳能转换效率达到30%或更高。

Abstract:Non-Technical:Conventional state-of-the-art tandem junction photovoltaic solar cells, composed of multiple sub-cells of III-V compound semiconductors, are capable of converting incident radiation from the Sun to electricity with greater efficiency than all other types of solar cells. The high performance of these devices is enabled in part due to the use of high quality monocrystalline III-V materials and in part due to the coupling of multiple sub-cells that collectively allow for absorption of a broadband solar spectral range. However, the materials and manufacturing costs required for the development of III-V tandem junction devices is prohibitively high for use in wide-scale terrestrial consumer applications. As a consequence, the world's highest performance solar cells are limited to use in niche markets such as terrestrial high concentration and space power applications. A high-risk, high-payoff exploratory research path is proposed here that aims to provide an unconventional, yet potentially transformative nanotechnology-enabled solution to the above consumer market penetration challenges faced by state-of-the-art tandem junction solar cells. The project aims to dramatically reduce manufacturing costs by monolithically integrating III-V sub-cell composed of vertical nanowire arrays to a central silicon sub-cell, thereby simultaneously eliminating the primary III-V substrate cost-driver while cutting III-V crystal growth volumes by up to 95% compared to conventional technologies. The broader significance of this EAGER project lies in the potential realization of a low-cost and high-efficiency renewable energy innovation that provides greater national energy independence and clean power. This research also impacts and advances fundamental knowledge in science and engineering in the fields of physics, nanomaterials growth and characterization, nanoelectronics, and optoelectronics. Immediate anticipated societal impacts include outreach activities that promote science, technology, engineering, and mathematics concepts to the general public, training new members of a highly-skilled workforce, and direct inclusion of high school, undergraduate, and graduate students from under-represented communities. Technical:The technical approach of this EAGER project relies on selective-area heteroepitaxy of a GaAsP (1.75 eV) nanowire array on the top surface of a thinned Si (1.1 eV) sub-cell by metal-organic chemical vapor deposition. A bifacial, three dissimilar materials, tandem junction device is formed via monolithic integration of a back-side InGaAs (0.5 eV) nanowire array. The vertical nanowires comprising the top- and back-surface arrays will contain radially-segmented p-i-n junctions and will be serially connected to the central Si sub-cell via epitaxial tunnel junctions. This design enables absorption of broadband incident solar energy as well as albedo radiation. Standard lattice-matching constraints are overcome via strain relaxation along nanowire free surfaces. Therefore, ideal spectral matching is realized without a need for graded buffer layers or dislocation mediation strategies. Use of vertical nanowire arrays with coaxial p-i-n junction geometries permits key advantages, including near-unity absorption of solar irradiance at normal and tilted incidence without the use of anti-reflection coatings, decoupling of photon absorption and carrier collection directions, and dramatic reduction of 95% in epitaxial volumes. Rigorous modeling of device parameters will be iteratively coupled with extensive materials characterization and property correlation experiments for optimization of III-V sub-cell structure on the single nanowire and ensemble array levels. The ultimate target of this work is demonstration of a functional bifacial, three dissimilar materials, nanowire-based tandem junction solar cell with one Sun power conversion efficiency of 30% or better.