# Origins of singlet fission in solid pentacene from an ab initio Green's-function approach

## TL;DR

This paper introduces a first-principles Green's-function method to predict singlet fission rates in crystalline pentacene, accurately matching experimental lifetimes and offering a general framework for multiexciton interactions in solids.

## Contribution

A novel ab initio Green's-function approach for predicting singlet fission rates based on many-body perturbation theory and exciton coupling calculations.

## Key findings

- Predicted singlet lifetime of 40-150 fs in pentacene matches experimental data.
- Identified symmetry conditions for Coulombic singlet fission decay.
- Framework applicable to multiexciton interactions in various solids.

## Abstract

We develop a new first-principles approach to predict and understand rates of singlet fission with an ab initio Green's-function formalism based on many-body perturbation theory. Starting with singlet and triplet excitons computed from a GW plus Bethe-Salpeter equation approach, we calculate the exciton--bi-exciton coupling to lowest order in the Coulomb interaction, assuming a final state consisting of two non-interacting spin-correlated triplets with finite center-of-mass momentum. For crystalline pentacene, symmetries dictate that the only purely Coulombic fission decay from a bright singlet state requires a final state consisting of two inequivalent nearly degenerate triplets of nonzero, equal and opposite, center-of-mass momenta. For such a process, we predict a singlet lifetime of 40 to 150 fs, in very good agreement with experimental data, indicating that this process can dominate singlet fission in crystalline pentacene. Our approach is general and provides a framework for predicting and understanding multiexciton interactions in solids.