Dynamics of Charge Transfer and Multiple Exciton Generation in the Doped Silicon Quantum Dot-Carbon Nanotube System: Density Functional Theory-Based Computation.

We use the Boltzmann transport equation (BE) to study time evolution of a photoexcited state, including phonon-mediated exciton relaxation, multiple exciton generation (MEG), and energy-transfer processes. BE collision integrals are derived using Kadanoff-Baym-Keldysh many-body perturbation theory (MBPT) based on density functional theory (DFT) simulations, including exciton effects. We apply the method to a nanostructured p- n junction composed of a 1 nm hydrogen-terminated Si quantum dot (QD) doped with two phosphorus atoms (Si36P2H42) adjacent to the (6, 2) single-wall carbon nanotube (CNT) with two chlorine atoms per two unit cells adsorbed to the surface. We find that an initial excitation localized on either the QD or CNT evolves into a transient charge-transfer (CT) state where either electron or hole transfer has taken place. The CT state lifetime is about 40 fs. Also, we study MEG in this system by computing internal quantum efficiency (QE), which is the number of excitons generated from an absorbed photon during relaxation. We predict efficient MEG starting at 3 Eg ≃ 1.5 eV and with QE reaching QE = 1.65 at about 5 Eg, where Eg ≃ 0.5 eV is the lowest exciton energy, i.e., the gap. However, we find that including energy transfer and MEG effects suppresses CT state generation.

Multiple exciton generation in chiral carbon nanotubes: Density functional theory based computation.

We use a Boltzmann transport equation (BE) to study time evolution of a photo-excited state in a nanoparticle including phonon-mediated exciton relaxation and the multiple exciton generation (MEG) processes, such as exciton-to-biexciton multiplication and biexciton-to-exciton recombination. BE collision integrals are computed using Kadanoff-Baym-Keldysh many-body perturbation theory based on density functional theory simulations, including exciton effects. We compute internal quantum efficiency (QE), which is the number of excitons generated from an absorbed photon in the course of the relaxation. We apply this approach to chiral single-wall carbon nanotubes (SWCNTs), such as (6,2) and (6,5). We predict efficient MEG in the (6,2) and (6,5) SWCNTs within the solar spectrum range starting at the 2Eg energy threshold and with QE reaching ∼1.6 at about 3Eg, where Eg is the electronic gap.

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 calculations based on the density functional theory simulations. We calculate SF exciton-to-biexciton decay rates R1→2 and biexciton-to-exciton rates R2→1 in the (6,2), (6,5), (10,5) SWCNTs, and in the (6,2) SWCNT functionalized with Cl atoms. Within the solar energy range, we predict R1→2∼1014-1015 s-1, while biexciton-to-exciton recombination is weak with R2→1∕R1→2≤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 the (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.

Photoinduced Single- and Multiple-Electron Dynamics Processes Enhanced by Quantum Confinement in Lead Halide Perovskite Quantum Dots.

Methylammonium lead iodide perovskite (MAPbI3) is a promising material for photovoltaic devices. A modification of MAPbI3 into confined nanostructures is expected to further increase efficiency of solar energy conversion. Photoexcited dynamic processes in a MAPbI3 quantum dot (QD) have been modeled by many-body perturbation theory and nonadiabatic dynamics. A photoexcitation is followed by either exciton cooling (EC), its radiative (RR) or nonradiative recombination (NRR), or multiexciton generation (MEG) processes. Computed times of these processes fall in the order of MEG < EC < RR < NRR, where MEG is on the order of a few femtoseconds, EC is in the picosecond range, while RR and NRR are on the order of nanoseconds. Computed time scales indicate which electronic transition pathways can contribute to increase in charge collection efficiency. Simulated mechanisms of relaxation and their rates show that quantum confinement promotes MEG in MAPbI3 QDs.

Abrupt Size Partitioning of Multimodal Photoluminescence Relaxation in Monodisperse Silicon Nanocrystals.

Intrinsic constraints on efficient photoluminescence (PL) from smaller alkene-capped silicon nanocrystals (SiNCs) put limits on potential applications, but the root cause of such effects remains elusive. Here, plasma-synthesized colloidal SiNCs separated into monodisperse fractions reveal an abrupt size-dependent partitioning of multilevel PL relaxation, which we study as a function of temperature. Guided by theory and simulation, we explore the potential role of resonant phonon interactions with "minigaps" that emerge in the electronic density of states (DOS) under strong quantum confinement. Such higher-order structures can be very sensitive to SiNC surface chemistry, which we suggest might explain the common implication of surface effects in both the emergence of multimodal PL relaxation and the loss of quantum yield with decreasing nanocrystal size. Our results have potentially profound implications for optimizing the radiative recombination kinetics and quantum yield of smaller ligand-passivated SiNCs.

Theoretical predictions on efficiency of bi-exciton formation and dissociation in chiral carbon nanotubes.

Efficient multiple exciton generation (MEG) in chiral single-wall carbon nanotubes (SWCNTs) is present within the solar spectrum range as shown by the many-body perturbation theory calculations combined with the density functional theory simulations. To describe the impact ionization process, we calculate exciton-to-biexciton decay rates R1→2 and biexciton-to-exciton rates R2→1 in the (6,2) and (10,5) SWCNTs. Within the solar energy range, we predict R1→2 ∼ 1014 s-1, while biexciton-to-exciton recombination is weak with R2→1/R1→2 ≤ 10-2. Also we calculate quantum efficiency (QE), the average number of excitons created by a single absorbed photon, for which we find QE ≃ 1.2-1.6, that is 20%-60%. However, MEG strength in these SWCNTs varies strongly with the excitation energy due to highly non-uniform density of states at the low energy. We hypothesize that MEG efficiency in the chiral SWCNTs can be enhanced by altering the low-energy electronic spectrum via surface functionalization, or by mixing SWCNTs of different chiralities.