Distinct photophysics of the spin-entangled triplet-triplet versus free triplets in organic semiconductors

太阳能电池吸收阳光并产生电能。由有机分子或聚合物构成的太阳能电池之所以引起人们的兴趣，是因为它们便宜、易于加工、重量轻且灵活。由于热力学的限制，今天的有机太阳能电池产生的电力是有限的。目前，科学界正在努力克服这些限制，制造出功率更高的太阳能电池。一种可能的方法是使用单线态裂变，这是一种光物理过程，在这种过程中，光吸收产生的电流原则上增加了一倍。在这个由化学部化学结构动力学和机制（CSDM-A）项目资助的项目中，亚利桑那大学的Sumitendra Mazumdar教授正在进行量子力学计算，以了解单线态裂变的详细机制。我们需要了解这一过程，以找到新的分子材料，在吸收光时产生更多的电荷，这反过来可能会导致有机太阳能电池的效率显著提高。这项工作的广泛影响包括对本科生和研究生的前沿跨学科培训，以及计算化学和物理学的博士后。外展工作正在培养服务不足群体成员对化学和物理的兴趣。目前很难确定目标材料的单线态裂变是否完全。区分初始光学自旋单重态和作为最终产物的自由三重态，以及活跃的中间三重三重态双激子的唯一方法是瞬态吸收光谱，而这三种物质的可见光瞬态吸收是重叠的。需要在近红外和中红外进行实验，以区分不同的激发态。该项目着重于计算光学自旋-单重态激子、最低自旋-三重态激子和三重-三重态双激子的激发态吸收，使用高水平配置相互作用方法和多电子模型哈密顿量中的激子基础，产生多体本征态和激发态吸收的物理、图像解释。该项目的目标是开发三种有机半导体家族的结构-性能相关性，这些家族是分子内单线态裂变的候选者。这三个家族是(i)共价连接的二聚体和相关分子，（ii）共轭聚合物，（iii）低带隙给受体共聚物。计算正在产生诊断工具，用于实验区分三重态-三重态和自由三重态的瞬态吸收，并确定三重态-三重态的协同双电子转移是否发生在体异质结构环境中，正如最近提出的那样。这项研究项目对太阳能和其他可能基于有机材料的技术具有启示意义。参与该项目的学生和博士后研究人员正在获得最先进的计算方法的经验，包括多体量子化学和物理处理。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。

Solar cells absorb sunlight and generate electricity. Solar cells constructed out of organic molecules or polymers are of interest because they are cheap, easily processible, lighweight and flexible. The electric power generated by today's organic solar cells is limited because of thermodynamic constraints. There is currently intense scientific effort to overcome these constraints and to construct solar cells that yield higher power. One possible approach is to use singlet fission, a photophysical process within which the electrical current generated by light absorption is in principle doubled. In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Sumitendra Mazumdar of the University of Arizona is performing quantum mechanical calculations to understand the detailed mechanism of singlet fission. Understanding of the process is needed to find new molecular materials that yield larger amounts of charge upon light absorption, which in turn may lead to organic solar cells with significantly higher efficiencies. The broader impacts of this work include cutting-edge interdisciplinary training of undergraduate and graduate students, and a postdoctoral fellow in computational chemistry and physics. Outreach efforts are fostering interest in chemistry and physics among members of underserved groups.Currently it is difficult to ascertain whether singlet fission in targeted materials is complete. The only way to distinguish between the initial optical spin-singlet and the free triplets that are end products, as well as the active intermediate triplet-triplet biexciton is from transient absorption spectroscopy, and transient absorptions in the visible from all three species are overlapping. Experiments in the near and mid infrared need to be performed to distinguish between the different excited species. The project focuses on computations of excited state absorptions from the optical spin-singlet exciton, the lowest spin-triplet exciton and the triplet-triplet biexciton, using high level configuration interaction approach and an exciton basis within a many-electron model Hamiltonian that yield physical, pictorial interpretations of many-body eigenstates and of excited state absorptions. The goal of the project is to develop structure-property correlations with predictive capability of three families of organic semiconductors that are candidates for intramolecular singlet fission. The three families are, (i) covalently linked dimers of acenes and related molecules, (ii) conjugated polymers, and (iii) low bandgap donor-acceptor copolymers. Computations are yielding diagnostic tools for experimentally distinguishing between transient absorptions from the triplet-triplet and free triplets, and determination of whether or not concerted two-electron transfers from the triplet-triplet occur in a bulk heterostructure environment, as has been proposed recently. This research project has implications for solar energy and other technologies that could be based on organic materials. The students and post-doctoral researchers involved in this project are gaining experience in state-of-the art computational methods, including many-body quantum chemistry and physics treatments.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.