CAREER: Interfaces and Their Effect on Charge Transfer in Extremely Thin Absorber Solar Cells

这项与可再生能源相关的职业计划的研究目标是调查纳米结构界面、材料特性及其对极薄吸收体(ETA)太阳能电池中电子电荷转移的影响。ETA电池将在经济上与化石燃料竞争，预计效率为15%，因为它们可以通过低成本的解决方案方法生产。然而，ETA电池的效率仅为2.5%。实现的效率和预测的效率之间的差异是由于缺乏对电池内界面和材料性质对电荷转移过程的作用的基本了解。ETA电池采用介孔n型半导体，在界面处涂覆一层薄吸收膜，孔由p型半导体填充，以创建互穿异质结。ETA电池的工作原理是，n型纳米线阵列提供的大结区允许使用薄吸收层，从而跨界面的电荷分离比竞争的体复合更快。纳米线也是理想的几何形状，可以在分离的电荷进行界面复合之前将分离的电荷快速传输到相反的接触。到目前为止，ETA电池的研究主要是通过制造太阳能电池和测量其I-V特性来进行的，对单个材料和界面或电池内的电荷传输的基础研究很少。该方法将结合单个材料和界面的光谱和电子显微镜，以及ETA太阳电池的稳态和微扰研究，以获得对ZnO-纳米线/CdSe/CuSCN材料体系中电荷转移机制的基本了解。包括阻抗光谱、强度调制光电流谱和时间分辨太赫兹光谱在内的技术将被用于(1)测量单个材料和薄膜堆栈中的载流子寿命和迁移率，(2)测量特征时间以比较电荷分离和体复合以及电荷传输和界面复合，(3)确定限制电池性能的工艺，以及(4)设计界面和材料属性以改善电荷转移，从而提高ETA电池的效率。智力优势：本提案中描述的变革性、光谱驱动的方法将首次应用于ETA电池，以了解支配电池性能的整体和界面现象。这项工作将探索纳米结构半导体界面电荷转移的本质，特别是结构、缺陷结构和电子能带结构的作用。将开发沉积高质量和精确厚度的超薄涂层的解决方案。材料和界面的非原位表征将与ETA电池的测量相结合，以确定限制电池效率的特性或工艺。这种使用传统方法无法实现的详细理解，将允许将实验观察与理论预测进行比较，并将有助于设计能够实现更高能量转换效率的材料、界面和分子结构。更广泛的影响：如果成功，拟议的可再生能源相关工作将提供对界面现象的基本理解，这对于将太阳能电池的效率从2.5%提高到理论预测的15%是必要的。这种改进可能会改变ETA电池技术，使其在经济上具有与化石燃料竞争的竞争力。低成本的ETA电池将提供一种清洁、安全、可持续的能源来源，这可能会使美国的投资组合从化石燃料转向能源独立。从CdSe系统学到的设计原理可以应用于其他吸收材料，如CuInGaSe2，以获得更高的潜在效率。此外，对纳米结构半导体异质结电荷转移的深入了解将对许多其他纳米结构系统有用，在这些系统中，控制界面对于实现高性能至关重要。这些应用包括有机-无机太阳能电池、显示器、电致变色和电池，所有这些都对可持续能源或能源效率至关重要。这项建议中描述的新颖的实验方法可以被其他研究人员采用，以使更广泛的科学和技术领域取得新的进展。这项建议的教育目标是通过研究活动和课程开发来教育学生，并使不同年龄和背景的人接触到可再生能源的一般概念。将建立一个“太阳能研讨会系列”，以吸引广大德雷克塞尔社区的参与；并将与费城学区合作，为5-6年级的学生及其教师发起一个“能源与可持续发展研讨会”。由于该学区80%以上是非裔美国人和西班牙裔美国人，将在5年内有100名学生和25名教师参加，因此这个工作坊自然将面向代表性不足的群体。该项目由NSF CBET分部的界面过程和热力学计划和可持续能源计划共同支持。

0846464BaxterThe research objective of this renewable energy related CAREER proposal is to investigate nano-structured interfaces, material properties, and their effect on electron charge transfer in extremely thin absorber (ETA) solar cells. ETA cells will be economically competitive with fossil fuels at their predicted efficiencies of 15% because they can be produced by low-cost solution methods. However, demonstrated ETA cell efficiencies are only 2.5%. The discrepancy between realized and predicted efficiency is due to lack of fundamental understanding of the role of interfaces and material properties on charge transfer processes within the cell. ETA cells employ a mesoporous n-type semiconductor coated at the interface with a thin absorber film, with pores filled by a p-type semiconductor to create an interpenetrating heterojunction. The operating principle of the ETA cell is that the large junction area presented by an n-type nanowire array allows thin absorber layers to be used, such that charge separation across the interfaces is faster than competing bulk recombination. Nanowires are also the ideal geometry to quickly transport the separated charges to opposite contacts before they can undergo interfacial recombination. To date, ETA cells have primarily been studied by making solar cells and measuring their I-V characteristics, with few fundamental studies of either the individual materials and interfaces or charge transport within the cell. The proposed approach will use a combination of spectroscopy and electron microscopy of individual materials and interfaces as well as steady-state and perturbation studies of ETA solar cells to gain fundamental insight into charge transfer mechanisms in the ZnO-nanowire/CdSe/CuSCN materials system. Techniques including impedance spectroscopy, intensity-modulated photocurrent spectroscopy, and time-resolved terahertz spectroscopy will be employed to (1) measure carrier lifetime and mobility in individual materials and thin film stacks, (2) measure characteristic times to compare charge separation vs bulk recombination and charge transport vs. interfacial recombination, (3) determine processes that limit cell performance, and (4) design interfaces and material properties to improve charge transfer and, hence, increase ETA cell efficiency. Intellectual Merit: The transformational, spectroscopy-driven approach described in this proposal will be applied to ETA cells for the first time in order to understand the bulk and interfacial phenomena that govern cell performance. This work will explore the nature of charge transfer across nano-structured semiconductor interfaces, specifically focusing on the role of architecture, defect structure, and electronic band structure. Solution methods of depositing extremely thin coatings of high quality and precise thickness will be developed. Ex situ characterization of materials and interfaces will be combined with measurements of ETA cells to identify the properties or processes that limit cell efficiencies. This detailed understanding, which cannot be achieved using conventional methods, will allow comparison of experimental observations with theoretical predictions and will aid in the design of materials, interfaces, and molecular architectures that enable higher energy conversion efficiencies. Broader Impact: If successful, the proposed renewable energy related work will provide the fundamental understanding of interfacial phenomena that is necessary to increase efficiencies of solar cells from 2.5% toward the 15% predicted by theory. Such an improvement could potentially transform ETA cell technology to be economically competitive with fossil fuels. Low-cost ETA cells would provide a source of clean, secure, sustainable energy that could shift the U.S. portfolio away from fossil fuels and toward energy independence. The design principles learned from the CdSe system can be applied to other absorbers such as CuInGaSe2 for even higher potential efficiency. Additionally, the enhanced understanding of charge transfer at nano-structured semiconductor heterojunctions will be useful for many other nano-structured systems where controlling interfaces is critical to achieving high performance. These applications include organic-inorganic solar cells, displays, electrochromics, and batteries, all of which are important for sustainable energy or energy efficiency. The novel experimental approach described in this proposal can be employed by other researchers to enable new advances in a broad range science and technology fields. The educational objectives of this proposal are to educate students through research activities and curriculum development and to expose people of diverse ages and backgrounds to general concepts in renewable energy. A "Solar Energy Seminar Series" will be established to engage the general Drexel community; and an "Energy and Sustainability Workshop" for 5th-6th grade students and their teachers will be initiated in partnership with the School District of Philadelphia. This workshop will naturally be directed toward underrepresented groups since the district is over 80% African-American and Hispanic and will involve 100 students and 25 teachers over 5 years.This project is jointly supported by the Interfacial Processes and Thermodynamics program and the Sustainable Energy program in the NSF CBET Division.