Direct Optoelectronic Imaging of Nanostructured Halide Perovskites

非技术描述：该项目旨在研究一种很有前途的光伏材料--卤化物钙钛矿中的电荷传输机制。卤化物钙钛矿化合物，包括甲基铵卤化铅，最近显示出巨大的太阳能转换潜力，其功率转换效率超过20%。然而，对材料物理学缺乏基本的了解。该项目旨在提取基础科学，并就电荷传输机制和指导方针提供关键见解，以确定更稳定的钙钛矿型化合物，用于成本效益高的光伏发电。特别是，首席研究人员使用了一种新的实验技术，通过扫描聚焦的激光束来绘制光生电流分布。这种直接成像技术可以提取关于光如何转化为电荷以及电荷如何在这些材料中传输的关键信息。对高转换效率起源的基本理解不仅对基础科学感兴趣，而且对于开发毒性更低、稳定性更高的钙钛矿材料也是至关重要的。这项研究可能会带来地球上丰富的材料，有望开启光伏发电的新范式。该项目在快速发展的纳米尺度和能源科学方面教育和培训本科生和研究生，并提供针对K-12学生的外展活动，这些学生是未被充分代表的少数民族。技术描述：该项目研究单晶卤化物钙钛矿纳米结构中的电荷传输，并区分铁电、离子迁移和电荷陷阱之间的主要电荷传输机制。尽管这些材料显示了高效的能量转换能力，但材料物理，如长载流子寿命和滞后光电流的起源尚不清楚。由单晶卤化物钙钛矿组成的纳米线和纳米板可以消除晶界的卷曲，并允许更好地了解其本征性质。利用波长相关扫描光电流显微镜研究了由单个纳米结构组成的器件，以确定少数载流子扩散长度如何依赖于表面效应和载流子浓度。这些纳米结构在不同温度的外电场下的直接光电成像为区分不同的电荷传输机制提供了关键的见解。研究了单晶纳米结构在外加应变作用下的压电效应。用一种新的液态门控方法研究了氢和锂等不同离子在卤化物钙钛矿中的输运。这项研究加深了对高转化效率来源的基本理解，并允许制定指导方针，以选择更好的分子或原子来取代卤化物钙钛矿中的阳离子或阴离子。

Nontechnical Description: This project aims to investigate the charge transport mechanisms in a promising photovoltaic material, halide perovskites. Halide perovskite compounds, including methylammonium lead halide, have recently demonstrated great potentials for solar energy conversion, with a power conversion efficiency above 20%. However, the fundamental understanding of the material physics is lacking. The project aims to extract fundamental science and provides key insights on the charge transport mechanisms and guidelines for identifying more stable perovskite compounds for cost-effective photovoltaics. In particular, the principal investigator uses a novel experimental technique, which maps the photogenerated current distribution by scanning a focused laser beam. This direct imaging technique allows extraction of key information on how light is converted into charge and how charge transports in these materials. The fundamental understanding of the origins of the high conversion efficiency is not only intriguing to fundamental science, but is also critical to developing better perovskite materials with reduced toxicity and increased stability. The study may lead to earth abundant materials that promise to open a new paradigm for photovoltaics. This project educates and trains undergraduate and graduate students in the rapidly advancing nanoscale and energy sciences, and offers outreach activities targeting K-12 students, underrepresented minorities.Technical Description: The project investigates the charge transport in single-crystal halide perovskite nanostructures and distinguishes the dominant charge transport mechanism among ferroelectricity, ion migration, and charge traps. Despite the demonstration of efficient power conversion capability in these materials, material physics such as the origins of long carrier lifetime and hysteretic photocurrent is not understood. Nanowires and nanoplates composed of single-crystal halide perovskites can remove the convolution of grain boundaries and allow better understanding of the intrinsic properties. Devices consisting of individual nanostructures are investigated by wavelength-dependent scanning photocurrent microscopy to determine how minority carrier diffusion lengths depend on surface effects and carrier concentration. Direct optoelectronic imaging of these nanostructures under external electric field at various temperatures provides key insights for distinguishing among different charge transport mechanisms. Piezoelectric effects are studied in single-crystal nanostructures under an external strain. The transport of different ions such as hydrogen and lithium in halide perovskites is investigated by using a novel liquid gating method. The research deepens the fundamental understanding of the origins of the high conversion efficiency and allows formulating guidelines for choosing better molecules or atoms to substitute the cations or anions in halide perovskites.