  Doping can profoundly affect the electronic- and optical-structure of
semiconductors. Here we address the effect of surplus charges on non-radiative
(NR) exciton and trion decay in doped semiconducting single-wall carbon
nanotubes. The dependence of exciton photoluminescence quantum yields and
exciton decay on the doping level, with its characteristically
stretched-exponential kinetics, is attributed to diffusion-limited NR decay at
charged impurity sites. By contrast, trion decay is unimolecular with a rate
constant of $2.0\,\rm ps^{-1}$. Our experiments thus show that charged
impurities not only trap trions and scavenge mobile excitons but that they also
facilitate efficient NR energy dissipation for both.
