Decay of the Lowest Triplet State in Singlet-Fission Molecular Materials: A Case Study on Quinoidal Bithiophenes

Abstract

Singlet fission, a process by which two triplet excitons are generated by fission of one singlet exciton, has attracted considerable interest for its potential to break the Shockley–Queisser theoretical limit for single-junction organic photovoltaic devices. To make full use of the resultant triplet excitons to improve the power conversion efficiency, triplet excitons must have long enough lifetimes to diffuse to the donor/acceptor interface and then dissociate into charge carriers. Thus, the triplet decay dynamics are important for exploiting singlet fission in organic solar cells. In this work, we have theoretically investigated the decay of the lowest triplet excited state (T1) for quinoidal bithiophene derivatives, one kind of promising singlet-fission molecular materials. Our results point to that the rates for radiative phosphorescence and nonradiative intersystem crossing are quite low under the Franck–Condon approximation. Interestingly, the energy barriers from the T1 minima to the T1/S0 minimum ...