Understanding highly mobile excitons in halide perovskites

非技术描述一种名为卤化物钙钛矿的新材料最近受到了极大的关注，因为它们在太阳能电池和发光器件中具有真正有潜力的低成本应用。光可以激发电子到更高的能量，在这些材料上留下电子空位或空穴。相反带电的电子和空穴相互吸引，可以结合成一对，通常被称为激子。这些激子在卤化物钙钛矿中的移动情况对优化它们的应用至关重要，但人们对此知之甚少。研究小组的初步结果表明，这些激子在这些材料中可以高度移动。在这个项目中，首席研究员的目标是应用新颖的空间和时间分辨实验技术来阐明卤化物钙钛矿中激子形成和输运的物理机制。该项目将利用卤化物钙钛矿中的激子为新的电子应用开辟新的机会。该项目还将教育和培训本科生和研究生，包括未被充分代表的少数民族学生，了解迅速发展的纳米和能源科学。首席研究员计划让学生具备技能和知识，与行业合作伙伴一起从事新材料和光伏领域的研究和开发。技术描述激发子在太阳能收集中通常被赋予负面含义，部分原因是它们假定的扩散长度较短。与此相矛盾的是，研究小组最近证明了卤化物钙钛矿中的载流子扩散长度可达200微米，这意味着激子传输可能不会限制这些材料的能量转换过程。基于这些令人兴奋的初步结果，该项目旨在了解三维和低维单晶卤化物钙钛矿纳米和微结构中高迁移率的激子以及强烈的自旋-轨道耦合对激子输运的影响。该项目将利用全面的空间、能量和时间分辨光电子技术来研究单晶卤化物钙钛矿场效应晶体管。依赖温度的光电流作图将提供不同激子结合能和阳离子电偶极子的各种卤化物钙钛矿化合物中激子和自由载流子扩散长度的直接测量。泵浦探测光电流测量将被用来以高时间分辨率确定激子寿命和扩散系数。利用高空间分辨率的圆形光电流效应和表面磁光克尔效应研究卤化物钙钛矿中的Rashba-Dresselhaus效应。具有强量子限制和各向异性光学转变的低维卤化物钙钛矿也将用开发的方法进行研究。这一奖项反映了美国国家科学基金会的法定使命，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。

Nontechnical descriptionA new family of materials, called halide perovskites, have recently received much attention because of their truly promising potentials for cost-effective applications in solar cells, as well as light emitting devices. Light can excite an electron to higher energy, leaving an electron vacancy or a hole in these materials. The oppositely charged electron and hole are attracted to each other and can be bound into a pair, often referred to as an exciton. How well these excitons move in the halide perovskites is critical to optimize their applications but is poorly understood. Preliminary results from the research team suggest these excitons can be highly mobile in these materials. In this project, the principal investigator aims to apply novel spatially and temporally resolved experimental techniques to illustrate the physical mechanisms of the formation and transport of excitons in halide perovskites. The project will open up new opportunities by taking advantage of excitons in halide perovskites for novel electronic applications. This project will also educate and train undergraduate and graduate students, including underrepresented minority students, in the rapidly advancing nanoscale and energy sciences. The principal investigator plans to prepare students with the skills and knowledge to pursue research and development in novel materials and photovoltaics with industrial partners. Technical descriptionExcitons are often given negative connotation in solar energy harvesting in part due to their presumed short diffusion lengths. Contradicting this, the research team has recently demonstrated carrier diffusion lengths up to 200 micrometers in halide perovskites, implying that exciton transport may not limit the energy conversion process in these materials. Based on these exciting preliminary results, the project aims to understand highly mobile excitons as well as the strong spin-orbit coupling effects on exciton transport, in both three-dimensional and low-dimensional single-crystal halide perovskite nano- and micro-structures. The project will investigate single-crystal halide perovskite field effect transistors with comprehensive temperature-dependent spatially, energetically, and temporally resolved optoelectronic techniques. Temperature dependent photocurrent mapping will provide direct measurements of exciton and free carrier diffusion lengths in various halide perovskite compounds with different excitonic binding energy and cation electric dipoles. Pump-probe photocurrent measurements will be used to determine exciton lifetime and diffusivity with high time resolution. Rashba-Dresselhaus effects in halide perovskites will be investigated by circular photogalvanic effect and surface magneto-optical Kerr effect with high spatial resolution. Low dimensional halide perovskites with strong quantum confinement and anisotropic optical transition will also be studied with the developed methodology.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.