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

混合有机-无机卤化物钙钛矿（HPs）是下一代太阳能电池和发光二极管（led）中很有前途的材料。这些材料和器件的性能和稳定性的进一步改进将有助于实现机械柔性，轻质和廉价的太阳能电池和led，可用于救灾地区，军事应用，并广泛部署，以帮助创造一个更安全和可持续的能源未来。在太阳能电池和led中，HP都夹在电荷传输层之间，用于将电荷载流子（空穴或电子）移出或注入HP。HP和这些输运层之间的界面是有害的非辐射复合过程发生的主要区域，这会降低光的产生效率并增加热损失。因此，有必要更好地了解这些界面并确定降低界面复合率的方法。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，使用瞬态反射光谱直接探测HP界面与各种传输层的重组率。格雷厄姆小组在理解和操纵hp的表面化学方面做了大量工作。通过这笔资助，Graham小组将能够直接探测表面化学变化如何影响界面电荷重组率。这项研究的发现将有助于加速太阳能电池和led的HPs的发展，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机构转移到肯塔基州的研究人员手中。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过程之一，然而这些界面过程仍然比发生在大部分材料中的过程少得多。本研究的目标是利用HPs作为平台，通过瞬态反射光谱的应用来表征能量学和电子耦合如何影响界面复合过程。瞬态反射依赖于超快激光脉冲来监测界面上载流子的重组动态，从而提供了一个强大的工具来区分界面重组和大部分HP内发生的重组。最终，该研究将验证一个假设，即通过使用表面改性剂操纵界面能量景观和不同材料之间界面的电子耦合，可以显著降低界面重组率。先前的研究已经研究了纯hp在单晶和多晶薄膜格式下的表面和体重组，但这些研究还没有扩展到包括电荷传输层的存在。在完整的光伏设备中也研究了重组率，但在这些完整的设备中，不可能确定重组发生的位置；因此，减少重组的有针对性的方法受到阻碍。在这项拟议的研究中，首席研究员和一名研究生将与NREL的Beard博士合作，应用瞬态反射光谱直接探测这些重要界面的重组率。将这些瞬态反射测量与紫外和逆光电子能谱相结合，将揭示界面能量学和电子耦合如何影响界面重组，从而有助于指导基于hp的改进太阳能电池和led的发展。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。

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.