Singlet Fission in Chiral Carbon Nanotubes: Density Functional Theory Based Computation

Singlet fission (SF) process, where a singlet exciton decays into a pair of spin one exciton states which are in the total spin singlet state, is one of the possible channels for multiple exciton generation (MEG). In chiral single-wall carbon nanotubes (SWCNTs) efficient SF is present within the solar spectrum energy range which is shown by the many-body perturbation theory (MBPT) calculations based on the density functional theory (DFT) simulations. We calculate SF exciton-to-biexction decay rates ${\rm R}_{1\to 2}$ and biexciton-to-exction rates ${\rm R}_{2\to1}$ in the (6,2), (6,5), (10,5) SWCNTs, and in (6,2) SWCNT functionalized with Cl atoms. Within the solar energy range, we predict ${\rm R}_{1\to2}\sim 10^{14}-10^{15}~s^{-1}$, while biexciton-to-exction recombination is weak with ${\rm R}_{2\to 1}/{\rm R}_{1\to 2}\leq 10^{-2}.$ SF MEG strength in pristine SWCNTs varies strongly with the excitation energy, which is due to highly non-uniform density of states at low energy. However, our results for (6,2) SWCNT with chlorine atoms adsorbed to the surface suggest that MEG in the chiral SWCNTs can be enhanced by altering the low-energy electronic states via surface functionalization.