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

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 (T-1) 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 T-1 minima to the T-1/S-0 minimum energy crossing points are just ca. 49-63 meV; furthermore, the spin-orbit couplings between T-1 and S-0 at the crossing points are very strong. This indicates that the singlet-fission-generated T-1 excitons will decay rapidly to the ground state through the crossing points. Therefore, to fully utilize the potential of singlet fission, efforts should be made to suppress the triplet decay via the T-1/S-0 crossing points.