Tuning the Spin Texture in Organic-Inorganic Halide Perovskites

有机-无机钙钛矿材料作为一种新型的光电材料，特别是在光伏领域的应用，引起了人们的极大关注。随着钙钛矿太阳能电池的效率接近晶体硅的效率，它们正在成为一种可行的清洁发电来源。与任何新技术一样，需要解决一些根本性的挑战；需要找到替代方法来提高钙钛矿材料的效率和稳定性。当太阳光被光伏材料吸收时，就会产生电荷。提高太阳能电池材料效率的一些因素包括抑制电荷复合，以及增加电荷在收集之前可以在材料中传播的距离(扩散长度)。钙钛矿材料允许通过调节原子间距离来操纵复合路径，这反过来又会影响电荷载流子的磁性行为，即所谓的自旋。这项研究将钙钛矿材料和高压技术结合在一起，并提供了一种通过影响电荷的自旋相关性质来调节电荷载流子寿命和扩散长度的新方法。该项目对提高用于光伏应用的钙钛矿材料的效率和稳定性具有直接影响。通过让本科生参加高级磁光实验来探索钙钛矿薄膜中的自旋依赖现象，加强了科学和教育中先进概念之间的联系。该项目在培养未来的科学家方面发挥着至关重要的作用；本科生、研究生和博士后研究人员获得了一系列多学科技术技能的专业知识。国际视野为美国学生提供了一个与来自欧洲和南非的研究人员合作的令人兴奋的机会。设计了“压力下的科学”的多样性密集研讨会课程，其中包括对高压下材料物理的讨论。技术描述：该项目主要研究调整碱卤化物有机-无机钙钛矿材料中自旋织构效应的高压技术。导带和价带的自旋织构可以通过改变原子间距离来控制，从而产生不同的自旋螺旋度。这项研究的主要目的是对钙钛矿材料中自旋-轨道耦合的后果有一个基本的了解。这项研究证实了Rashba自旋分裂的预测，它导致了自旋允许和自旋禁止的复合通道。该项目采用：a)与南非西开普省大学合作，化学气相沉积铅卤素钙钛矿薄膜，包括用Cs和其他分子阳离子取代铅和有机阳离子；(B)在环境条件下进行光学、结构和磁输运研究；(C)在高压下进行拉曼散射，以监测与钙钛矿结构的八面体笼相关联的声子；(D)在压力下进行X射线衍射，以研究结构和光学性质之间的关联；(E)在高压下进行光反射光谱分析；(F)在高压下进行磁阻测量。该研究小组的目标是开发用于从磁阻测量中分析非平凡的Berry相的协议。该项目的另一个方面解决了在应变场下一类无机卤化物钙钛矿中同时存在铁电和拓扑有序的问题。该项目的结果对各种实际应用具有重要意义。混合碱卤化物钙钛矿中存在的拓扑态和调节自旋织构效应的前景为设计在光伏应用和高速电子设备中具有更高性能的材料开辟了新的方案。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Non-Technical Description: Organic-inorganic perovskite materials are attracting a lot of attention as novel optoelectronic materials, in particular for photovoltaic applications. They are becoming a viable source for clean power generation, as the efficiency of perovskite solar cells gets close to that of crystalline silicon. As with any new technology, there are fundamental challenges that need to be addressed; there is a need to find alternate ways of improving the efficiency and stability of perovskite materials. As sunlight is absorbed in a photovoltaic material, charges are produced. Some factors that improve the efficiency of solar cell materials involve suppression of charge recombination and increasing the distance that charges can travel in the material (diffusion length) before being collected. Perovskite materials allow the manipulation of recombination pathways by tuning interatomic distances, which in turn impacts the magnetic behavior of the charge carriers, known as spin. This research interfaces the areas of perovskite materials and high pressure techniques, and provides a novel way of tuning charge carrier lifetimes and diffusion lengths by influencing the spin-dependent property of the charges. The project has a direct impact on improving the efficiency and stability of perovskite materials for photovoltaic applications. The connection between advanced concepts in science and education is reinforced by involving undergraduate students in advanced magneto-optical experiments to probe spin-dependent phenomena in perovskite films. The project plays a vital role in training future scientists; undergraduate, graduate, and postdoctoral researchers gain expertise in a multidisciplinary range of technical skills. The international scope provides US students an exciting opportunity to work with researchers from Europe and South Africa. A diversity intensive seminar course on "Science under Pressure" is designed, which includes discussions on the physics of materials under high pressure. Technical Description: The project focuses on high pressure techniques for tuning the spin texture effect in alkali halide organic-inorganic perovskite materials. The spin texture of the conduction and valence bands may be controlled by changing the interatomic distances, resulting in different spin helicities. The main objective of the research is a fundamental understanding of the ramifications of spin-orbit coupling in perovskite materials. The research validates the prediction of the Rashba spin-splitting, which result in both spin-allowed and spin-forbidden recombination channels. The project employs: a) chemical vapor deposition of methylammonium Pb-halogen perovskite films including replacement of Pb and organic cations with Cs and other molecular cations in collaboration with the University of Western Cape, South Africa; (b) optical, structural, and magneto-transport studies under ambient conditions; (c) Raman scattering under high pressure to monitor the phonons associated with the octahedral cage of the perovskite structure; (d) X-ray diffraction under pressure for correlating structural and optical properties; (e) photoreflectance spectroscopy under pressure; and (f) magnetoresistance measurements under high pressure. The research team aims to develop protocols for the analysis of a non-trivial Berry's phase from magnetoresistance measurements. An additional facet of the project addresses the simultaneous presence of ferroelectric and topological order in a class of inorganic halide perovskites under a strain field. The results of this project are relevant for various practical applications. The presence of topological states and the prospect of tuning the spin texture effect in hybrid alkali halide perovskites open up schemes for designing materials with improved performance in photovoltaic applications and high speed electronics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.