Theoretical approach to weakly bound triplet-triplet multiexciton in intramolecular singlet fission chromophores

NONTECHNICAL SUMMARY This award supports theoretical and computational research on physical processes to convert solar energy to electricity in organic materials. Solar cells, which absorb sunlight and generate electricity, are playing and will continue to play a strong role in reducing the demand for fossil-fuel derived energy. Solar cells constructed out of organic molecules or polymers are of interest because they are inexpensive, easy to process, lightweight 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. A promising approach is to use singlet fission, a photophysical process within which the electrical current generated by light absorption is in principle doubled. Practical utilization of singlet fission requires further research. Charge generation here occurs in two steps, and while impressive success has been reached in the development of chromophores in which the first step is very efficient, in nearly all cases the second step takes too long, and electric charge, and so, energy, is lost to the process of recombination. The PI will perform quantum chemical calculations that will lead to new molecular materials in which the second step of singlet fission, charge separation, will occur ultrafast, leading to organic solar cells with higher quantum efficiencies. A significant portion of the research is in collaboration with groups that specialize in experimental research on singlet fission. The broader impacts of this work include interdisciplinary training of a graduate student and a postdoctoral fellow, outreach efforts fostering interest in chemistry and physics among members of underserved groups, and increased scientific literacy and public awareness. TECHNICAL SUMMARY This award supports theoretical and computational research and education on the process of singlet fission in organic solar cell materials. Research on singlet fission until recently focused on designing materials that give rapid singlet to triplet-triplet conversion. Rapid triplet-triplet generation; however, is a consequence of strong interchromophore coupling that also leads to strong triplet-triplet multiexciton binding and fast triplet recombination instead of generation of free triplets. Overcoming this impasse requires clear understanding of the relationship between bonding motifs and the generation of triplet-triplet multiexcitons with weak binding energy, that have been discovered recently in different families of intramolecular singlet fission materials. The PI will perform high order multiple reference configuration interaction-calculations within the semiempirical Pariser-Parr-Pople Hamiltonian using a diagrammatic exciton basis that will yield precise structure-property correlations with predictive capability. The goal of this research is to arrive at structural features that will give fast generation of free triplets in ultrafast times. The research contributes to fundamental knowledge that underlies solar energy and other technologies, particularly those that could be based on organic materials. Junior researchers involved in this project are gaining experience in state-of-the-art computational methods, including quantum many-body physics and chemistry. 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.