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（HP）是用于高性能廉价光电器件（例如光伏电池和发光二极管）的有吸引力的材料，其可以使用高通量方法（例如基于溶液的印刷方法）来制造。基于HP的研究规模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，但其生产成本仅为硅光伏材料的一小部分。虽然HP具有与硅类似的效率，但商业化和大规模部署存在两个主要障碍-稳定性不足和铅（Pb）（一种众所周知的有毒元素）的存在。降维HP是一类HP，其中三维（3D）钙钛矿结构被大体积有机阳离子破坏成2D片，是用于改善稳定性并能够用毒性小得多的锡（Sn）替代Pb的有前景的材料。目前，基于降维HP的光伏器件显示出比它们的3D对应物显著更低的性能，而Sn基HP也表现不佳它们的Pb基对应物。该项目开发了新的降维HP，重点关注毒性较小的Sn基材料，并确定了间隔结构与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，毒性更小的光致发光材料和发光二极管。该研究小组在初中和高中层面促进STEM教育和兴趣，同时强调传统上代表性不足的群体的参与。这一目标在很大程度上依赖于为来自肯塔基州各地的初中和高中教师举办的年度研讨会，其中教师在肯塔基州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。相对于它们的3D对应物，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，它是很难预测的间隔分子的分子参数，如静电，极化率，或大小，影响光学和电子性能的降维HP。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和电离能，电子亲和势和激子结合能的降维HP之间的基本关系在很大程度上是未开发的。然而，这些参数对于设计用于光致发光器件和发光二极管的材料和器件结构至关重要。该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HPs中间隔结构、晶体结构、电离能和电子亲和势之间的关键关系。接下来，研究的重点是如何间隔结构的影响激子结合能在降维HP和使用这些知识来设计具有较低的激子结合能的材料。第三，间隔分子被设计成抑制Sn（II）氧化，从而为开发改进的Sn基HP提供潜在途径。最后，将选定的降维HP纳入光伏器件中，以确定诸如激子结合能的材料性质如何影响光伏性能。电荷载流子迁移率测量的目的是建立更完整的结构-性能关系，有助于推进材料和器件设计。该奖项反映了NSF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.