Microscopic theory of singlet exciton fission. I. General formulation

TL;DR

This paper develops a comprehensive microscopic theory of singlet fission, linking quantum chemistry with quantum relaxation theory, to better understand and interpret the process in organic materials for solar energy applications.

Contribution

It introduces a unified framework connecting excited state calculations with quantum dynamics, and advocates Redfield theory for practical simulations of singlet fission.

Findings

Redfield theory balances speed and accuracy for singlet fission dynamics

Model systems validate the numerical approach

Distinction between diabatic and adiabatic bases enhances interpretation

Abstract

Singlet fission, a spin-allowed energy transfer process generating two triplet excitons from one singlet exciton, has the potential to dramatically increase the efficiency of organic solar cells. However, the dynamical mechanism of this phenomenon is not fully understood and a complete, microscopic theory of singlet fission is lacking. In this work, we assemble the components of a comprehensive microscopic theory of singlet fission that connects excited state quantum chemistry calculations with finite-temperature quantum relaxation theory. We elaborate on the distinction between localized diabatic and delocalized adiabatic bases for the interpretation of singlet fission experiments in both the time and frequency domains. We discuss various approximations to the exact density matrix dynamics and propose Redfield theory as an ideal compromise between speed and accuracy for the detailed investigation of singlet fission in dimers, clusters, and crystals. Investigations of small model systems based on parameters typical of singlet fission demonstrate the numerical accuracy and practical utility of this approach.