Mesoscopic modelling of bipolar charge evolution in CN-PPV LEDs

Since various chances are possible in the molecular structure of the repeat unit, substituted poly(para-phenylenevinylene) (PPV) has ben used as active component in light-emitting diodes (LEDs) to obtain light emission in a wide range of colours.A major aspect determining device performance is the competition between current flow, trapping and recombination within the polymer layer. By suitable Monte Carlo calculations, we have performed computer experiments in which bipolar charge carriers are injected at constant rate in polymer networks made of cyano-substituted PPV chains with variable length and orientation. The intra-molecular electronic properties used in these simulations were calculated by a quantum molecular dynamics method. In order to assess the influence of cyano-substitution on the properties of single-layer PPV LEDs, we have focused our attention on bipolar charge evolution in time. Specifically addressed are the differences in electric field strength needed for intra-molecular charge mobility of electrons and holes and their consequences at mesoscopic scale. (C) 2004 Elsevier B.V. All rights reserved

2-Engel relations between subgroups

In this paper we study groups G generated by two subgroups A and B such that is nilpotent of class at most 2 for all a¿. A and b¿. B. A detailed description of the structure of such groups is obtained, generalizing the classical result of Hopkins and Levi on 2-Engel groups

Computer simulation of electron transfer in molecular electronic devices

The study of electron transfer through individual molecules bound to metal electrodes has become important due to the potential application in molecular electronic devices. Since the electronic and atomic motions in these molecules influence each other they need to be treated self-consistently. We have used self-consistent quantum chemistry molecular dynamics calculations to discuss some of the issues related to electron transfer through a spatially symmetric [9,10-Bis((2′-para-mercaptophenyl)-ethinyl)-anthracene] and an asymmetric [1,4-Bis((2′-para-mercaptophenyl)-ethinyl)-2-acetyl-amino-5-nitro-benzene] molecule bound to metal electrodes. Specifically addressed are the effects of voltage inversion on electron transfer between electrodes through both molecules. Our results show an electron transfer behaviour that reproduces the spatial symmetry of the molecules in agreement with experimental current-voltage data. The change in time of electron density and dimerisation at specific atomic sites is also discussed.Fundação para a Ciência e a Tecnologia (FCT) - Programa Operacional “Ciência , Tecnologia, Inovação” - POCTI/CTM/41574/2001, SFRH/BD/11231/2002.Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER)

Products of pairwise totally permutable groups

[EN] In this paper finite groups factorized as products of pairwise totally permutable subgroups are studied in the framework of Fitting classesA.M.-P. and M.D.P.-R. were both supported by Proyecto BMF20001-1667-C03-03, Ministerio de Ciencia y Tecnolog´ıa and FEDER, Spain.Hauck, P.; Martínez Pastor, A.; Pérez-Ramos, M. (2003). Products of pairwise totally permutable groups. Proceedings of the Edinburgh Mathematical Society. 46(1):147-157. https://doi.org/10.1017/S0013091502000299S14715746

Modelling the effects of the anode work function in PPV LED

Transparent conducting oxides are widely used as the transparent electrode in polymer light emitting diodes (PLEDs). The physical properties of these materials and consequently device performance strongly depend on their processing and surface treatment. The injection of charge from the transparent electrode into the polymer layer occurs by tunnelling through a potential barrier from the electrode to molecules close to it. This barrier is influenced by the difference in the relevant energy levels of electrode material and polymer molecules, the external applied potential, the Coulomb potential of the charges present in the polymer layer and the potential of their image charges on the electrodes, and may also be altered by electrode degradation effects. A better understanding of the effect of varying this potential barrier on the functioning of PLED is necessary to achieve further improvements in these applications. Here we present a theoretical study of the influence of changes in the potential barrier at the transparent electrode, on bipolar charge evolution through thin polymer layers, in the absence of defects and impurity states, while the other electrode functions as an ohmic contact. Results of a mesoscopic model provide insight into bipolar charge injection, charge and recombination distribution throughout the polymer layer, and may suggest new materials and processing methods to optimize these optoelectronic devices.FEDER.Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/CTM/41574/2001, CONC-REEQ/443/EEI/2005; SFRH/BD/22143/2005

Mesoscopic modelling of the interaction of infrared lasers with composite materials: an application to dental enamel

The mesostructure and composition of composite materials determine their mechanical, optical and thermal properties and, consequently, their response to incident radiation. We have developed general finite element models of porous composite materials under infrared radiation to examine the influence of pore size on one of the determining parameters of the stress distribution in the material: the temperature distribution. We apply them to the specific case of human dental enamel, a material which has nanometer scale pores containing water/organic, and predict the maximum temperature reached after a single 0.35 μs laser pulse of sub-ablative fluence by two lasers: Er:YAG (2.9 μm) and CO2 (10.6 μm). For the Er:YAG laser, the results imply a strong dependence of the maximum temperature reached at the pore on the area-to-volume ratio of the pore, whereas there is little such dependence for CO2 lasers. Thus, CO2 lasers may produce more reproducible results than Er:YAG lasers when it comes to enamel ablation, which may be of significant interest during clinical practice. More generally, when ablating composite materials by infrared lasers researchers should account for the material’s microstructure and composition when designing experiments or interpreting results, since a more simplistic continuum approach may not be sufficient to explain differences observed during ablation of materials with similar optical properties or of the same material but using different wavelengths

Intra-molecular properties of DMeOPPV studied by quantum molecular dynamics

Introducing methoxy electron donor groups into a poly(para-phenylene vinylene) (PPV) chain will lead to the appearance of unique electronic properties at the molecular scale which should affect the overall properties of light-emitting diodes based on these polymers. Self-consistent quantum molecular dynamics calculations have been used to provide information on intra-molecular properties of poly(2,5-dimethoxy-para-phenylene vinylene) (DMeOPPV), which are relevant for the modelling and characterization of polymer light-emitting diodes at nanometric length scale. We focus our attention on those properties that have been somewhat neglected in previous literature: the charge distribution associated with positive and negative charge carriers and their intra-molecular mobility when an electric field is applied.Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência, Tecnologia, Inovação” - POCTI/CTM/41574/2001, SFRH/BD/11231/2002.Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER

Nilpotent-like Fitting formations of finite soluble groups

[EN] In this paper the subnormal subgroup closed saturated formations of finite soluble groups containing nilpotent groups are fully characterised by means of extensions of well-known properties enjoyed by the formation of all nilpotent groups.This research has been supported by Proyecto PB 97-0674-C02-02 of DGICYT, Ministerio de Educación y Ciencia, Spain.Ballester-Bolinches, A.; Pérez-Ramos, M.; Martínez Pastor, A. (2000). Nilpotent-like Fitting formations of finite soluble groups. Bulletin of the Australian Mathematical Society. 62(3):427-433. https://doi.org/10.1017/S0004972700018943S42743362