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

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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.