Heavy-Atom-Free Sensitizers for NIR-to-Visible Solar Photon Upconversion

高效、廉价地收集太阳能是生产清洁、可持续能源的一个巨大挑战。太阳发出的光谱范围很广，从远红外到紫外线。然而，只有一小部分的光可以被太阳能电池的有源层吸收。如果能拓宽太阳能电池板所能捕获的太阳光的光谱范围，总发电量就会增加。增加光吸收率的一种策略是将两个低能光子结合起来，并将它们的能量融合成一个高能量光子，这一过程被称为上转换。然后，上转换的光子可以被吸收并用于从光中发电，否则就会被浪费掉。这个项目的目标是识别和设计新的分子，可以用于更高效的太阳能电池的上转换。研究人员将把合成化学和光学表征与理论和机器学习相结合，以实现这一目标。除了科学和技术影响之外，参与者还将与当地和区域社区大学生接触，获得真实和持续的原创研究经验。该团队正在追求的一个具体途径是尝试将光子上转换研究概念注入到当地社区大学的课程学术实验室实验中。这种让社区大学学生接触原始研究的方法旨在帮助克服地理限制，这些限制可能会阻碍这些学生参加更传统的本科研究体验（REU）项目，这些项目需要学生长途跋涉到研究地点。技术描述光子上转换可以显著提高太阳能电池效率，达到或超过肖克利-奎瑟极限。利用光子上转换来提高太阳能光伏发电效率的一个很有前途的策略是三重态-三重态湮灭上转换，因为它可以在太阳光子的低强度非相干照明下发生。从历史上看，用于光子上转换的材料包括基于贵金属配合物、芳基卤化物和量子点的敏化剂。由于金属的高成本，第一类在经济上不可行，第二类在照明下不稳定，第三类受到上转换光被敏化剂寄生吸收的影响。考虑到这些材料的限制，PI的小组确定了基于硫代方胺基材料的无重原子上转换敏化剂，他们将其作为实现近红外到可见光子上转换的潜在候选者，用于太阳能光伏应用。目标是澄清难以捉摸的结构-功能关系，这些关系在历史上使得识别一般的nir吸收三重态敏化剂具有挑战性，更具体地说，迄今为止难以实现nir到可见光光子的UC，而不使用昂贵的贵金属中心或易于光降解的芳基卤化物，这两者对于太阳能应用都没有吸引力。这项工作的一个关键目标是解决具体的问题，如“什么分子描述符是同时实现近红外吸收和高系统间交叉收率的最规范的？”该团队结合了实验和理论方法，基于超快泵探针光谱，从头算预测和强化学习，以识别和评估他们在该项目中合成和测试的材料的新分子设计策略。该项目还支持新的基于课程的本科研究经验（CUREs），供社区大学生进行与下一代太阳能设备相关的先进光子材料研究。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 DescriptionHarvesting solar energy efficiently and inexpensively is a grand challenge to producing clean, sustainable energy. The spectrum of light emitted by the sun spans a vast range, from the far infrared to ultraviolet. However only a fraction of this light can be absorbed by the active layer in a solar cell. If one can broaden the spectral range of sunlight that a solar panel can capture, the overall power generated increases. One strategy to increase the fraction of light absorbed is to combine two low-energy photons and fuse their energy into one higher-energy photon, a process called upconversion. The upconverted photon can then be absorbed and used to generate electricity from light that would otherwise be wasted. The goal of this project is to identify and design new molecules that can be used for upconversion in more efficient solar cells. Researchers will combine synthetic chemistry and optical characterization with theory and machine learning to achieve this goal. Beyond its scientific and technological impacts, participants will engage local and regional community college students with authentic and sustained exposure to original research experiences. One specific avenue the team is pursuing is to attempt to infuse photon upconversion research concepts into course-based academic laboratory experiments at a local community college. This approach to engage community college students with exposure to original research is intended to help overcome the geographic constraints that may hinder these students from taking part in more traditional research experience for undergraduate (REU) programs that require students to travel long distances to the research site.Technical DescriptionPhoton upconversion could significantly enhance solar cell efficiencies to routinely meet or exceed the Shockley-Queisser limit. One promising strategy for deploying photon upconversion to enhance the efficiency of solar photovoltaics is triplet-triplet annihilation upconversion because it can occur under low-intensity, non-coherent illumination from solar photons. Historically, the materials that have been explored for photon upconversion have included sensitizers based on precious metal complexes, arylhalides, and quantum dots. The first class is not economically viable due to the high cost of the metal, the second class is unstable under illumination, and the third class suffers from parasitic absorption of the upconverted light by the sensitizer. Given these materials constraints, the PI’s group identifies heavy-atom-free upconversion sensitizers based on thionated squaraine-based materials, which they explore as potential candidates for achieving NIR-to-visible photon upconversion for solar photovoltaic applications. The goal is to clarify elusive structure-function relationships that have historically made it challenging to identify NIR-absorbing triplet sensitizers in general, and more specifically, hitherto intractable to achieve NIR-to-Visible photon UC without using expensive precious metal centers or arylhalides that readily photodegrade, which are both unattractive for solar applications. One key goal of this work is to address specific questions such as, “What molecular descriptors are most prescriptive for simultaneously achieving NIR absorption and high intersystem crossing yields?” The team combines experimental and theoretical approaches based on ultrafast pump-probe spectroscopy, ab initio predictions, and reinforcement learning to identify and evaluate new molecular design strategies for materials that they synthesize and test in this project. The project also supports new Course-based Undergraduate Research Experiences (CUREs) for community college students to conduct advanced photonic materials research relevant for next-generation solar energy devices.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.