Charge Transport and Carrier-Phonon Interactions in Soft Lattice Metal Halide Perovskites

金属卤化物钙钛矿是一类独特的“软”半导体，引起了极大的兴趣。它们可以在相对较低的温度下进行溶液处理，并在太阳能电池、发光二极管和辐射探测器等光电器件中显示出前景。然而，了解这些材料中的电荷输运仍然是难以捉摸的。离子运动和形成高质量电接触的困难使电荷输运的研究变得复杂。此外，钙钛矿在加工过程中或在其顶部沉积金属触点时可能会降解。这会导致接触电阻过大，限制器件性能。通过将电极物理层压到钙钛矿薄膜上，研究小组将避免这种退化，并能够进行可靠的电学研究。他们将把光电流和电容对温度和光强的研究与直接的结构分析结合起来。他们还将改变接触，并使用掺杂来进一步调整载流子密度，并探索这些材料中的独特现象。这项系统的研究将揭示钙钛矿的有趣性质，并开发出设计更高效设备所需的关键见解。相关的研究活动也为学生提供了宝贵的教育机会，以培训下一代相关技术的劳动力。该项目利用独特的范德华集成策略来创建原子清洁触点，大大降低了接触电阻，用于金属卤化物钙钛矿（MHPs）的系统电输运研究。通过将预制的原子平面薄膜金属电极物理层压到钙钛矿薄膜上，而不直接将钙钛矿暴露在任何光刻或沉积步骤中，这种方法可以有效地避免相关材料的降解，从而大大降低接触电阻，从而实现可靠的电输运研究。该项目将探索电荷传输和光载流子诱导的局部晶格畸变，相关的相变，以及它们对载流子动力学和基本输运性质的影响：包括依赖于温度和光照的光导和光电容研究，以探测载流子-声子相互作用，以及它们对载流子复合和输运性质的影响；直接结构分析，研究不同光照或温度下载流子生成的原子结构变化；开发不同的接触或选择性掺杂策略来探测电子和空穴输运特性；利用优化的器件制造和测量方案探测低维MHPs和其他相关材料中的载流子-声子相互作用和铁电性；并通过化学掺杂、静电掺杂和光掺杂的结合来进一步调整载流子密度，以探测载流子-声子或载流子-载流子的相互作用，并探索可能出现的现象。这些研究活动有助于对这类独特材料的基本光物理、电输运性质和有趣的载流子-声子相互作用有一个批判性的理解，这将不仅仅是为了工程上改进光伏或发光二极管，而且还有助于释放MHPs的新技术潜力。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。

Non-technical DescriptionMetal halide perovskites are a unique class of “soft” semiconductors that have attracted enormous interest. They can be solution processed at relatively low temperatures and have shown promise for optoelectronic devices such as solar cells, light emitting diodes and radiation detectors. However, understanding charge transport in these materials remains elusive. Studies of charge transport are complicated by ion movement and the difficulty in forming high-quality electrical contacts. Furthermore, perovskites can degrade during processing or when depositing metal contacts on top of them. This results in excessive contact resistance and limits device performance. By physically laminating electrodes onto perovskite films, the team will avoid such degradation and be able to perform reliable electrical studies. They will combine studies of photocurrent and capacitance on temperature and light intensity with direct structural analysis. They will also vary contacts and use doping to further tailor the carrier density and explore unique phenomena in these materials. This systematic study will unravel the intriguing properties of perovskites and develop critical insights needed to design more efficient devices. The relevant research activities also offer valuable educational opportunities to students for training next generation of workforce in relevant technologies.Technical DescriptionThis project exploits a unique van der Waals integration strategy to create atomically clean contacts with greatly reduced contact resistance for a systematic electrical transport study of metal halide perovskites (MHPs). By physically laminating the prefabricated atomically flat thin film metal electrodes onto the perovskite thin films without directly exposing the perovskites to any lithography or deposition steps, this approach can effectively avoid the associated material degradations to achieve greatly reduced contact resistance for reliable electrical transport studies. The project will probe charge transport and photocarrier induced local lattice distortion, the associated phase transition, and their impact on the carrier dynamics and fundamental transport properties: including temperature- and illumination-dependent photo-conductance and photo-capacitance studies to probe carrier-phonon interactions, and their impact on the carrier recombination and transport properties; direct structural analysis to investigate the atomic structural change associated with the carrier generation under different illumination or temperature; developing different contacts or selective doping strategies to probe both electron and hole transport characteristics; using the optimized device fabrication and measurement protocols to probe the carrier-phonon interactions and ferroelectricity in low-dimensional MHPs and other related materials; and further tailoring the carrier density through a combination of chemical doping, electrical static doping and photodoping to probe carrier-phonon or carrier-carrier interactions and explore possible emergent phenomena. These research activities help develop a critical understanding of the fundamental photophysical, electrical transport properties, and the intriguing carrier-phonon interactions in this unique class of materials, which will not aim engineering improved photovoltaics or light-emitting diodes, but also help unlock new technological potentials from MHPs.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.