Charge mobility of discotic mesophases: A multiscale quantum/classical study

A correlation is established between the molecular structure and charge mobility of discotic mesophases of hexabenzocoronene derivatives by combining electronic structure calculations, Molecular Dynamics, and kinetic Monte Carlo simulations. It is demonstrated that this multiscale approach can provide an accurate ab-initio description of charge transport in organic materials

Ab initio probing of the electronic band structure and Fermi surface of fluorine-doped WO3 as a novel low-TC superconductor

First-principles calculations were performed to investigate the electronic structure and the Fermi surface of the newly discovered low-temperature superconductor: fluorine-doped WO3. We find that F doping provides the transition of the insulating tungsten trioxide into a metallic-like phase WO3-xFx, where the near-Fermi states are formed mainly from W 5d with admixture of O 2p orbitals. The cooperative effect of fluorine additives in WO3 consists in change of electronic concentration as well as the lattice constant. At probing their influence on the near-Fermi states separately, the dominant role of the electronic factor for the transition of tungsten oxyfluoride into superconducting state was established. The volume of the Fermi surface gradually increases with the increase of the doping. In the sequence WO3 \rightarrow WO2.5F0.5 the effective atomic charges of W and O ions decrease, but much less, than it is predicted within the idealized ionic model - owing to presence of the covalent interactions W-O and W-F.Comment: 8 pages, 4 figure

Monte Carlo modelling of hole transport in MDMO-PPV: PCBM blends

We propose a model of hole transport in interpenetrating two-phase systems and apply it to blends of poly[2-methoxy-5-3(3′,7′-dimethyloctyloxy) -1-4-phenylene vinylene], (MDMO-PPV), and 1-(3-methoxycarbonyl)-propyl-1-phenyl- (6,6)C 61, (PCBM) with low PCBM content. The main features of the model are that hole transport is mediated by a small polaron tunnelling expression and that the density of states contains a tail of deep traps, which serve to delay carrier transport. The exponential factor governing the depth of these localised states is derived from transient optical measurements. The model is implemented using Monte Carlo simulations and is applied to reproduce both the time of flight hole photocurrent transients and the field dependence of the hole mobilities extracted from the data. We show that the transport behaviour detected by time of flight and transient absorption spectroscopy can be described quantitatively with a single transport mode

Enhanced Quantum Dot Emission for Luminescent Solar Concentrators Using Plasmonic Interaction

Plasmonic excitation enhanced fluorescence of CdSe/ZnS core-shell quantum dots (QDs) in the presence of Au nanoparticles (NPs) has been studied for application in quantum dot solar concentrator (QDSC) devices. We observe that there is an optimal concentration of Au NPs that gives a maximum 53% fluorescence emission enhancement for the particular QD/Au NP composite studied. The optimal concentration depends on the coupling and spacing between neighboring QDs and Au NPs. We show the continuous transition from fluorescence enhancement to quenching, depending on Au NP concentration. The locally enhanced electromagnetic field induced by the surface plasmon resonance in the Au NPs leads to an increased excitation rate for the QDs. This is evidenced by excitation wavelength dependent fluorescence enhancement, where the locally enhanced field around the Au NPs is more pronounced close to the surface plasmon resonance (SPR) wavelength. However, at higher concentrations of Au NPs non-radiative energy transfer from the QDs to the Au NPs particles leads to a decrease of the emission, which is confirmed by detection of both a double exponential lifetime decay in, and a decrease in the lifetime of the QDs. The overall fluorescence emission enhancement depends on these competing effects; increased excitation rate and non-radiative energy transfer

The oxygen vacancy in crystal phases of WO3

The oxygen vacancy in WO3 has previously been implicated in the electrochromism mechanism in this material. Previous theoretical calculations on the oxygen vacancy in WO3 have not considered the full range of crystal structures adopted by the material. Here we report studies of the oxygen vacancy in seven crystal phases. The use of a very accurate tungsten plane-wave pseudopotential means that a byproduct of this study is a more detailed and complete picture of undefected WO3 than previously available. Electronic structures of the crystal phases in both undefected and defected systems have been calculated and are discussed. The band gap in WO3 is dependent upon bonding - antibonding interactions, these being dependent upon overlap in each direction. The effect of an oxygen vacancy is dependent upon the availability of both Op and Wd electrons, this being different for the various phases. A variety of behavior is predicted, which may be explained in terms of O2p-W5d mixing, including the formation of long W-W dimer bonds. It is found that the nature of a polaron in this material is dependent upon both the crystal structure and distribution of oxygen vacancies