We present an ab initio approach to investigate the interaction between single excitons and non-interacting biexcitons from first-principles using many-body perturbation theory within the GW approximation and the Bethe-Salpeter Equation approach. We apply our model to explore singlet-fission in different polymorphs of crystalline pentcene. For these systems, we calculate the electronic coupling between singlet excitations and non-interacting but spin-correlated triplet pairs. We find a purely-Coulombic symmetry-allowed fission process involving a bright singlet exciton initial state and a final state consisting of two non-equivalent nearly-degenerate triplets of nonzero, equal and opposite, center-of-mass momenta.  For such a process, we predict a singlet-fission rate in very good agreement with available experimental data, indicating the importance of this channel in the underlying mechanism. Our analysis elucidates the role of crystal symmetry and structure in singlet-fission, and shows that not only energy conservation, but inversion symmetry, the number of symmetry-inequivalent molecules in a unit cell, and the parity of each finite-momentum excited state play a central role in determining the singlet fission rates in crystals. This work was supported by the Center for Computational Study of Excited-State Phenomena in Energy Materials at the Lawrence Berkeley National Laboratory, which is funded by the U. S. Department of Energy.<br/>

Singlet-fission from first-principles: Role of crystal symmetry and structure

254th National Meeting (2017)

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