Exact Wave Packet Dynamics of Singlet Fission in Unsubstituted and Substituted Polyene Chains within Long-Range Interacting Models

TL;DR

This study uses exact wave packet dynamics to analyze singlet fission in polyene chains, revealing how molecular substitution and stacking arrangements influence the efficiency of organic solar cells.

Contribution

It provides a detailed quantum dynamical analysis of singlet fission in polyenes, highlighting the effects of substitution and stacking on fission yield, which was not previously quantified.

Findings

Significant singlet fission yield occurs from the 2^1A state in unsubstituted chains.

Substituting donor-acceptor groups enhances singlet fission by altering excited state character.

Stacking arrangement significantly affects the singlet fission efficiency.

Abstract

Singlet fission (SF) is a potential pathway for significant enhancement of efficiency in organic solar cells (OSC). In this paper, we study singlet fission in a pair of polyene molecules in two different stacking arrangements employing exact many-body wave packet dynamics. In the non-interacting model, the SF yield is absent. The individual molecules are treated within Hubbard and Pariser-Parr-Pople (PPP) models and the interaction between them involves transfer terms, intersite electron repulsions and site-charge--bond-charge repulsion terms. Initial wave packet is constructed from excited singlet state of one molecule and ground state of the other. Time development of this wave packet under the influence of intermolecular interactions is followed within the Schr\"odinger picture by an efficient predictor-corrector scheme. In unsubstituted Hubbard and PPP chains, $2{}^1A$ excited singlet state leads to significant SF yield while the $1{}^1B$ state gives negligible fission yield. On substitution by donor-acceptor groups of moderate strength, the lowest excited state will have sufficient $2{}^1A$ character and hence results in significant SF yield. Because of rapid internal conversion, the nature of the lowest excited singlet will determine the SF contribution to OSC efficiency. Furthermore, we find the fission yield depends considerably on the stacking arrangement of the polyene molecules.