Using Spacer Molecular Structure to Control Energetics, Stability, Charge-Carrier Transport, and Photovoltaic Performance in 2D Organic Metal Halide Perovskites

非技术描述有机金属卤化物钙钛矿（HPs）是高性能廉价光电器件（如光伏电池和发光二极管）的有吸引力的材料，可以使用高通量方法（如基于溶液的印刷方法）制造。研究规模的基于HPs的光伏电池显示出与最广泛使用的光伏材料硅相当的功率转换效率，但生产成本只有硅光伏电池的一小部分。虽然HPs的效率与硅相似，但商业化和大规模部署的两个主要障碍是稳定性不足和铅（Pb）的存在，铅是一种众所周知的有毒元素。降维HPs是一类HPs，其中三维（3D）钙钛矿结构被大块的有机阳离子分解成二维薄片，是提高稳定性和用毒性小得多的锡（Sn）取代铅的有希望的材料。目前，基于降维HPs的光伏器件的性能明显低于3D，而基于sn的HPs也不如基于pb的HPs。该项目开发了新的降维HPs，重点关注毒性较小的锡基材料，并确定了间隔结构和材料性能之间的关键关系，从而为加速材料开发提供了必要的理解，以实现更稳定、毒性更低的光伏和发光二极管。研究小组在初中和高中阶段促进STEM教育和兴趣，同时强调传统上代表性不足的群体的参与。这一目标很大程度上依赖于肯塔基州各地初中和高中教师的年度研讨会，在那里，教师们在肯塔基大学的有机电子制造实验室进行实验，并与他们的初中和高中班级一起离开制造染料敏化太阳能电池所需的材料。技术描述降维hp的性能受到其高激子结合能和较差的电子输运的限制，而基于Sn的hp的发展通常受到Sn氧化导致的缺陷状态的限制。目前，很难预测间隔分子的分子参数，如静电、极化率或大小，如何影响降维hp的光学和电子性质。在某种程度上，这些预测是困难的，因为间隔分子的分子参数与降维hp的电离能、电子亲和能和激子结合能之间的基本关系在很大程度上是未知的。然而，这些参数对于设计光伏和发光二极管的材料和器件结构至关重要。本研究项目系统地探讨了间隔分子的结构如何影响这些重要的材料特性，以及Pb和sn基降维hp的稳定性、电荷载流子传输和光伏性能。该项目使用低能紫外和逆光电子能谱来揭示降维hp中间隔层结构、晶体结构、电离能和电子亲和之间的关键关系。接下来，研究重点是在降维hp中间隔结构如何影响激子结合能，并利用这些知识来设计具有更低激子结合能的材料。第三，间隔分子被设计为抑制Sn（II）氧化，从而为开发改进的Sn基hp提供了潜在的途径。最后，将选定的降维hp集成到光伏器件中，以确定激子结合能等材料特性如何影响光伏性能。电荷载流子迁移率测量是为了建立更完整的结构-性质关系，有助于推进材料和器件设计。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 DescriptionOrganic metal halide perovskites (HPs) are attractive materials for high-performing inexpensive optoelectronic devices, such as photovoltaic cells and light emitting diodes, that can be fabricated using high-throughput approaches such as solution-based printing methods. Research scale photovoltaic cells based on HPs display record power conversion efficiencies that are on par with the most widely used photovoltaic material, silicon, but can be produced at a fraction of the cost of silicon photovoltaics. Although HPs have similar efficiencies to silicon, there are two major barriers to commercialization and wide scale deployment – inadequate stability and the presence of lead (Pb), a well-known toxic element. Reduced dimensionality HPs, which are a class of HPs where the three-dimensional (3D) perovskite structure is broken into 2D sheets by bulky organic cations, are promising materials for improving stability and enabling the replacement of Pb with tin (Sn), which is much less toxic. At present, photovoltaic devices based on reduced dimensionality HPs show significantly lower performance than their 3D counterparts, while Sn-based HPs also underperform their Pb-based counterparts. This project develops new reduced dimensionality HPs, with a stronger focus on the less toxic Sn-based materials, and determines key relationships between spacer structure and material properties, thus providing the understanding necessary to accelerate material development for more stable and less toxic photovoltaics and light emitting diodes. The research team promotes STEM education and interest at the middle and high school levels while emphasizing participation of traditionally underrepresented groups. This goal relies largely on an annual workshop for middle and high-school teachers from around Kentucky, where teachers conduct experiments in the organic electronics fabrication laboratory at the University of Kentucky and leave with the materials required to make dye sensitized solar cells with their middle and high school classes.Technical DescriptionThe performance of reduced dimensionality HPs is limited by their high exciton binding energies and poor electronic transport relative to their 3D counterparts, while the development of Sn-based HPs in general is limited by defect states resulting from Sn oxidation. Currently, it is hard to predict how molecular parameters of the spacer molecule, such as electrostatics, polarizability, or size, influence the optical and electronic properties of reduced dimensionality HPs. In part, these predictions are difficult because fundamental relationships between the molecular parameters of the spacer molecule and the ionization energy, electron affinity, and exciton binding energy of the reduced dimensionality HPs are largely unexplored. However, these parameters are critical to designing materials and device structures for photovoltaics and light emitting diodes. This research project systematically probes how the spacer molecule’s structure impacts these important material properties as well as stability, charge-carrier transport, and photovoltaic performance in both Pb- and Sn-based reduced dimensionality HPs. The project uses low-energy ultraviolet and inverse photoelectron spectroscopies to uncover critical relationships between spacer structure, crystal structure, ionization energy, and electron affinity in reduced dimensionality HPs. Next, the research focuses on how the spacer structure influences exciton binding energies in reduced dimensionality HPs and uses this knowledge to design materials with lower exciton binding energies. Third, spacer molecules are designed to inhibit Sn(II) oxidation and thereby provide a potential route for developing improved Sn-based HPs. Finally, selected reduced dimensionality HPs are incorporated into photovoltaic devices to determine how material properties such as exciton binding energies impact photovoltaic performance. Charge-carrier mobility measurements are conducted to establish more complete structure-property relationships that help to advance both material and device design.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.