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CAREER提案的研究目标是研究纳米结构界面、材料特性及其对超薄吸收体（ETA）太阳能电池中电子电荷转移的影响。预计ETA电池的效率为15%，与化石燃料相比具有经济竞争力，因为它们可以通过低成本的溶液方法生产。然而，证明的ETA电池效率仅为2.5%。实现和预测的效率之间的差异是由于缺乏基本的理解的作用，界面和材料性能的细胞内的电荷转移过程。ETA电池采用介孔n型半导体，其在界面处涂覆有薄吸收膜，其中孔由p型半导体填充以产生互穿异质结。ETA电池的工作原理是由n型纳米线阵列提供的大结面积允许使用薄的吸收层，使得跨界面的电荷分离比竞争性体复合更快。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它们快速传输到相对的触点。迄今为止，ETA电池主要通过制造太阳能电池并测量其I-V特性来研究，很少对单个材料和界面或电池内的电荷传输进行基础研究。所提出的方法将使用光谱学和电子显微镜的个别材料和界面，以及稳态和微扰研究的ETA太阳能电池的组合，以获得基本的洞察电荷转移机制的ZnO纳米线/CdSe/CuSCN材料系统。包括阻抗光谱、强度调制光电流光谱和时间分辨太赫兹光谱的技术将用于（1）测量单个材料和薄膜堆叠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与体复合以及电荷传输与界面复合，（3）确定限制电池性能的工艺，以及（4）设计界面和材料性质以改善电荷转移，并因此增加ETA电池效率。智力优势：本提案中描述的转换，光谱驱动的方法将首次应用于ETA电池，以了解控制电池性能的体积和界面现象。这项工作将探讨跨纳米结构的半导体界面的电荷转移的性质，特别是专注于架构，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极薄涂层的解决方案方法。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过程。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，使其在经济上与化石燃料竞争。低成本的ETA电池将提供一种清洁、安全、可持续的能源，可以使美国的能源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体，如CuInGaSe 2，以获得更高的潜在效率。此外，在纳米结构的半导体异质结的电荷转移的增强的理解将是有用的许多其他纳米结构的系统中，控制接口是至关重要的，以实现高性能。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，所有这些对于可持续能源或能源效率都很重要。该提案中描述的新实验方法可以被其他研究人员采用，以实现广泛的科学和技术领域的新进展。这项建议的教育目标是通过研究活动和课程开发教育学生，并使不同年龄和背景的人了解可再生能源的一般概念。将建立一个“太阳能研讨会系列”，让德雷克塞尔社区参与进来;将与费城学区合作，为5 - 6年级学生及其教师举办一个“能源和可持续性研讨会”。这个研讨会将自然地针对代表性不足的群体，因为该地区超过80%的非洲裔美国人和西班牙裔，并将涉及100名学生和25名教师超过5年。这个项目是由界面过程和热力学计划和可持续能源计划在NSF CBET司联合支持。

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.