Colloquium: Dr. Andrei KryjevskiDr. Andrei Kryjevski Department of Physics North Dakota State University Fargo, ND
Multiple exciton generation in chiral single-walled carbon nanotubes: DFT-based study including competition between carrier multiplication and phonon-mediated relaxation, energy and charge transfer
The conclusion about multiple exciton generation (MEG) efficiency in a nanoparticle can only be made from a comprehensive study that uncludes different relaxation channels, such as phonon-mediated thermalization, carrier multiplication, energy and charge transfer. Here, we study time evolution of a photo-excited state using Boltzmann transport equation (BE) that includes phonon emission/absorption together with the exciton multiplication and recombination. BE rates are computed using non-equilibrium finite-temperature many-body perturbation theory (Kadanoff-Baym-Keldysh technique, or NEGF) based on DFT simulations, including exciton effects using RPA-screened Coulomb interaction. First, we compute rates for both all-singlet MEG and Singlet Fission channels, which are of order 1014 s-1. Then, for all-singlet MEG we use BE with MEG and phonon relaxation included to calculate internal quantum efficiency (QE), the number of excitons generated from a single absorbed photon. We predict that efficient MEG in chiral single-wall carbon nanotubes (SWCNTs), such as (6,2), both pristine and Cl-doped, (6,5) is present within the solar spectrum range. We predict QE≈1.3-1.4 at the excitation energy of 3*Egap in (6,2) and (6,5). However, QE=1 is found in CNT (10,5) which suggests strong chirality dependence of MEG. MEG efficiency in functionalized SWCNTs is enhanced compared to the pristine case. Also, we study doped CNT – QD system and find a long-lived (lifetime is about 0.1 ps) charge transfer state.