RII Track-4: Applying Transient Reflectance Spectroscopy to Decipher the Impact of Energetics and Electronic Coupling on Interfacial Recombination in Hybrid Halide Perovskites

有机-无机混合卤化物钙钛矿(HPS)材料在下一代太阳能电池和发光二极管(LED)中具有广阔的应用前景。这些材料和器件在性能和稳定性方面的进一步改进将有助于实现机械灵活、重量轻、价格低廉的太阳能电池和LED，这些电池和LED可用于救灾领域和军事应用，并广泛部署，以帮助创造更安全和可持续的能源未来。在太阳能电池和LED中，HP被夹在电荷传输层之间，用于移除或向HP注入载流子(空穴或电子)。HP和这些传输层之间的界面是发生有害的非辐射复合过程的主要区域，这会降低光产生效率并增加热损失。因此，更好地了解这些界面并确定降低界面复合速率的方法是至关重要的。这项研究将通过与国家可再生能源实验室(NREL)的Matthew Beard博士合作，使用瞬时反射光谱直接探测HP与各种传输层的接口的复合速率。格雷厄姆小组在理解和操纵HPS的表面化学方面进行了广泛的工作。通过这笔拨款，格雷厄姆研究小组将能够直接探索表面化学变化如何影响界面电荷复合速率。这项研究的结果将有助于加快太阳能电池和LED的HPS的开发，而与NREL的合作将有助于将知识从美国首屈一指的可再生能源研究设施转移到肯塔基州的研究人员。界面上的电荷转移过程是决定电子和光电子器件性能的最相关的过程之一，然而，与发生在材料中的那些过程相比，这些界面过程仍然知之甚少。这项研究的目标是以HPS为平台，通过应用瞬时反射光谱来表征能量和电子耦合如何影响界面复合过程。瞬时反射依赖于超快激光脉冲来监测电荷载流子复合动力学，特别是在界面处，从而提供了一个强大的工具来区分界面复合和发生在高压体内的复合。最终，这项研究将检验这一假设，即通过使用表面改性剂操纵不同材料之间的界面能谱和电子耦合，可以显著降低界面复合速率。以前的研究已经研究了单晶和多晶薄膜形式的纯HPS中的表面和体复合，但这些研究还没有扩展到包括电荷传输层的存在。在完整的光伏器件中也对重组率进行了调查，但在这些完整的光伏器件中，无法确定发生复合的位置；因此，阻碍了减少复合的有针对性的方法。在这项拟议的研究中，首席研究员和一名研究生将与NREL的比尔德博士合作，应用瞬时反射光谱直接探测这些重要界面的复合速率。将这些瞬时反射率测量与紫外线和反向光电子能谱相结合，将揭示界面能量学和电子耦合如何影响界面复合，从而帮助指导基于HP的改进型太阳能电池和LED的开发。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。

Hybrid organic-inorganic halide perovskites (HPs) are promising materials for use in next-generation solar cells and light-emitting diodes (LEDs). Further improvements in the performance and stability of these materials and devices will help enable mechanically flexible, lightweight, and inexpensive solar cells and LEDs that may be used in disaster relief areas, in military applications, and broadly deployed to help create a more secure and sustainable energy future. In both solar cells and LEDs, the HP is sandwiched between charge-transporting layers that serve to remove or inject charge carriers (either holes or electrons) into the HP. Interfaces between the HP and these transporting layers are the dominant areas where harmful non-radiative recombination processes occur, which can lower light generation efficiency and increase heat loss. Thus, it is essential to better understand these interfaces and identify means to reduce interfacial recombination rates. This research will directly probe recombination rates at HP interfaces with various transport layers using transient reflectance spectroscopy through a collaboration with Dr. Matthew Beard at the National Renewable Energy Laboratory (NREL). The Graham group has worked extensively on understanding and manipulating the surface chemistry of HPs. Through this grant, the Graham group will be able to directly probe how changes in surface chemistry impact interfacial charge recombination rates. The findings of this research will help accelerate the development of HPs for solar cells and LEDs, while the collaboration with NREL will help transfer knowledge from the nation's premier renewable energy research facility to researchers in Kentucky. Charge-transfer processes at interfaces are one of the most relevant processes determining the performance of electronic and optoelectronic devices, yet these interfacial processes remain much less understood than those occurring within the bulk of the materials. The goal of this proposed research is to use HPs as a platform to characterize how energetics and electronic coupling influence interfacial recombination processes through the application of transient reflectance spectroscopy. Transient reflectance relies on ultrafast laser pulses to monitor charge carrier recombination dynamics specifically at interfaces, thereby providing a powerful tool to differentiate interfacial recombination from recombination occurring within the bulk of the HP. Ultimately, the research will test the hypothesis that interfacial recombination rates can be significantly reduced through manipulating the interfacial energy landscape and electronic coupling at interfaces between dissimilar materials using surface modifiers. Previous research has investigated surface and bulk recombination in pure HPs in both single crystal and polycrystalline film formats, but these investigations have not been extended to include the presence of charge transport layers. Recombination rates have also been investigated within completed PV devices, but in these complete devices it is not possible to identify where recombination is occurring; thus, targeted approaches to reduce recombination are hindered. In this proposed research the principal investigator and a graduate student will work with Dr. Beard at NREL on applying transient reflectance spectroscopy to directly probe recombination rates at these important interfaces. Combining these transient reflectance measurements with ultraviolet and inverse photoelectron spectroscopy will uncover how interfacial energetics and electronic coupling impact interfacial recombination, thereby helping to guide the development of improved solar cells and LEDs based on HPs.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.