  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.
