Understanding highly mobile excitons in halide perovskites

一种被称为卤化物钙钛矿的新材料最近受到了广泛的关注，因为它们在太阳能电池和发光器件中具有成本效益的应用潜力。光可以激发电子产生更高的能量，在这些材料中留下一个电子空位或空穴。带相反电荷的电子和空穴相互吸引，可以结合成一对，通常称为激子。这些激子在卤化物钙钛矿中的移动情况对优化其应用至关重要，但人们对其了解甚少。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说明卤化物钙钛矿中激子形成和传输的物理机制。该项目将利用卤化物钙钛矿中的激子为新型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，包括代表性不足的少数民族学生，在迅速发展的纳米尺度和能源科学。首席研究员计划为学生提供技能和知识，以便与工业合作伙伴一起研究和开发新型材料和光伏。激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短扩散长度。与此相反，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中的能量转换过程。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观结构中高度可移动的激子以及激子输运的强自旋轨道耦合效应。该项目将研究单晶卤化物钙钛矿场效应晶体管，具有综合的温度依赖空间、能量和时间分辨光电技术。温度相关的光电流映射将提供各种具有不同激子结合能和阳离子电偶极子的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。泵浦探针光电流测量将用于确定激子寿命和扩散率与高时间分辨率。利用高空间分辨率的圆形光电效应和表面磁光Kerr效应研究卤化物钙钛矿中的Rashba-Dresselhaus效应。具有强量子约束和各向异性光学跃迁的低维卤化物钙钛矿也将用所开发的方法进行研究。该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知识价值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.