Helena M. G. Correia

Understanding electron flow in conducting polymer films : injection, mobility, recombination and mesostructure

We survey the current state of models for electronic processes in conducting polymer devices, especially light-emitting diodes. We pay special attention to several processes that have been somewhat neglected in the previous literature: charge injection from electrodes into a polymer sample, mobility of charge-or energy-carrying defects within a single molecule and (more briefly) transfer of carriers between molecules and the interaction between the charge transport and the mesostructure of the polymer. Within all these areas substantial progress has been made in recent years in elucidating the important physics, but further progress is needed to make quantitative contact with experiment.

Change mobility in conjugated polymer molecules

The use of conjugated polymers as active media in organic semiconductor devices demands a deep knowledge of the electronic processes involved at molecular level. Here we report the results obtained from self-consistent molecular dynamics calculations at the complete neglect of differential overlap level concerning intra-chain charge mobility for two well known polymers: polydiacetylene and poly(p-phenylene vinylene).

Modelling the effects of molecular arrangements in polymer light-emitting diodes

In order to understand how to enhance the performance of polymer light-emitting diodes (PLEDs), we used a mesoscopic hopping model, taking into account molecular properties and polymer morphology, to investigate the impact of a number of conjugated polymers and molecular arrangements on the functioning of single-layer devices. The model is applied to devices with the active polymer consisting of poly(p-phenylene vinylene) (PPV) and PPV derivatives with stiff conjugated segments having their long axis oriented parallel and perpendicular to the electrode surfaces as well as randomly oriented, which are three of the molecular arrangements that can be obtained experimentally at microscopic scale in solution-processed conjugated polymer thin films. The model provides insight into current efficiency, charge distribution, internal electric field and consequently recombination throughout the polymer layer. We found that the details of molecular arrangement crucially affect the distribution of recombination events far from the electrodes and its field dependence, which has implications for the efficiency of PLEDs. In particular, we found a variation of recombination efficiency in the bulk of uniform ordered polymer films depending on the molecular alignment relative to the electrode surfaces. It turns out that molecular orientation perpendicular to the electrodes increases recombination in the centre of the polymer film as compared to the case of parallel orientation. We conclude that the orientational alignment perpendicular to the electrode surfaces might be a viable strategy towards efficient polymer-based LEDs.

Electric field induced charge transfer through single- and double-stranded DNA polymer molecules

The charge transfer through single-stranded and double-stranded DNA polymer molecules has been the subject of numerous experimental and theoretical studies concerning their applications in molecular electronics. However, the underlying mechanisms responsible for their different electrical conductivity observed in the experiments are poorly understood. Here we use a self-consistent quantum molecular dynamics method to study the effect of an applied electric field along the molecular axis on charge transfer through single-stranded and double-stranded DNA polymer molecules with an injected electron or hole and assess the consequences for electronic applications. Charge transfer through both single-stranded and double-stranded DNA polymer molecules is predicted, regardless of the sign of the injected charge, the molecular structure and the base sequence. The amount of charge transfer through a double-stranded DNA polymer molecule is slightly lower than through the corresponding two isolated single-strands as a result of the lower charge transport through the purine–pyrimidine base-stacking as compared with through DNA nucleobase-stacking. These results suggest that each DNA polymer strand can act as a molecular wire with both the sugar–phosphate backbone and the bases playing an important role in charge transfer, which opens new perspectives for molecular electronics applications.

Modelling the effect of structure and base sequence on DNA molecular electronics

DNA is a material that has the potential to be used in nanoelectronic devices as an active component. However, the electronic properties of DNA responsible for its conducting behaviour remain controversial. Here we use a self-consistent quantum molecular dynamics method to study the effect of DNA structure and base sequence on the energy involved when electrons are added or removed from isolated molecules and the transfer of the injected charge along the molecular axis when an electric field is applied. Our results show that the addition or removal of an electron from DNA molecules is most exothermic for poly(dC)-poly(dG) in its B-form and poly(dA)-poly(dT) in its A-form, and least exothermic in its Z-form. Additionally, when an electric field is applied to a charged DNA molecule along its axis, there is electron transfer through the molecule, regardless of the number and sign of the injected charge, the molecular structure and the base sequence. Results from these simulations provide useful information that is hard to obtain from experiments and needs to be considered for further modelling aiming to improve charge transport efficiency in nanoelectronic devices based on DNA.

What Can We Learn from Vibrational Analysis Calculations of Defective Polymer Chains

The possibility of using infrared (IR) spectroscopy to determine the concentration of inversion monomer defects in polymers depends on the knowledge of the relationship between the spectral properties and the polymer microstructure. This can easily be achieved by performing vibrational analysis. In order to investigate the changes in IR spectra of poly(vinylidene fluoride) resulting from the presence of monomeric units in “head-to-head” and “tail-to-tail” positions, we calculated the frequencies and intensities of IR-active vibrations for individual molecules in alpha and beta form with a defect concentration up to 15% and compared them with the ones obtained for a defect-free molecule.

Theoretical study of the influence of salt doping in the functioning of OLEDs

One of the strategies to improve the efficiency of organic light emitting diodes (OLEDs) is to dope the active organic semiconducting layer with inorganic salts, leading to the development of a hybrid organic/inorganic hetero-structure. However, it is hard to know from the experiments how each one of the electronic processes underlying the functioning of OLEDs is affected by the accumulation of inorganic ions of different sign at both organic/electrode interfaces. In order to assess these effects, we performed computer simulations by using a multi-scale model that combines quantum molecular dynamics calculations at the atomistic scale with Monte Carlo calculations at the mesoscopic scale. We focus our attention on the main differences obtained between doped and pristine organic layers, when bipolar charge injection occurs. Our results show a significant drop on the turn-on applied electric field while maintaining rapid response to the applied field as well as a clear increase in recombination rate and recombination efficiency far from the electrodes for the doped situation, which are responsible for the dramatic improvement of doped OLED performance found in the experiments.

Modelling Molecular Transformations in Ferroelectric Polymers Induced by Mechanical and Electrical Means

We describe a new approach to model the dynamical structural modifications of ferroelectric polymer molecules induced by uniaxial stretching and by the application of electric fields of various strengths and orientations. Our approach combines a self-consistent quantum mechanical method with molecular dynamics, removing the need to use inter-atomic potentials in the dynamic calculations. Here we present results obtained for individual molecules of poly(vinylidene fluoride) in its alpha and beta form. The effects of structural disorder due to the presence of inverted monomer units in a “head-to-head” and “tail-to-tail” position are also discussed.

Computational study of the presence of defects in semiconducting polymers on exciton formation

Although semiconducting polymers are very attractive to be used in optoelectronic devices due to their molecular structure, they are not pristine semiconductors. After deposition it is possible to find out several structural and chemical defects, with different origins, that strongly influence exciton dynamics since they create deep energetic sites, where excitons can migrate leading to their quenching or reducing exciton diffusion length. By using a self-consistent quantum molecular dynamics method we performed a computational study to understand the influence of well-known polymer defects on excitons dynamics. Our results show that these defects influences mainly intramolecular exciton localization and exciton energy.

Multi-scale modelling of polymer-based optoelectronic devices

The optimization of polymer-based optoelectronic devices such as light-emitting diodes (LEDs), photodetectors and photovoltaic cells requires the understanding how molecular properties and the spatial arrangement of the conjugated strands affect the electronic processes underlying the functioning of these devices. Since some of the important features are determined largely by the individual molecular strands and other features depend strongly on the nanostructure, a multi-scale modelling of materials and device properties is needed. In this work we discuss the atomistic and nanoscale modelling of charge injection, transport and trapping single-carrier diode based on poly(p-phenylene venylene) (PPV), which also applies to other optoelectronic devices.