Geraldo Magela e Silva

Exciton dissociation and charge carrier recombination processes in organic semiconductors.

Exciton dissociation and charge recombination processes in organic semiconductors, with thermal effects taken into account, are described in this paper. Here, we analyzed the mechanisms of polaron-excitons dissociation into free charge carriers and the consequent recombination of those carriers under thermal effects on two parallel π-conjugated polymers chains electronically coupled. Our results suggest that exciton dissociation in a single molecule give rise to localized, polaron-like charge carrier. Besides, we concluded that in the case of interchain processes, the bimolecular polaron recombination does not lead to an usual exciton state. Rather, this type of recombination leads to an oscillating dipole between the two chains. The recombination time obtained here for these processes are in agreement with the experimental results. Finally, our results show that temperature effects are essential to the relaxation process leading to polaron formation in a single chain, as in the absence of temperature, this process was not observed. In the case of two chains, we conclude that temperature effects also help the bimolecular recombination process, as observed experimentally.

Theoretical Temperature Dependence of the Charge-Carrier Mobility in Semiconducting Polymers†

We present a theory of the temperature and electric field dependence on the mobility of polarons in conjugated polymers in terms of a tight-binding and stochastic approach. The polaron mobility is shown to have a strong dependence on the electric field, with two distinct regimes of temperature dependence. Lattice thermal oscillations enhance polaron mean velocity for electric fields of 1.0 mV/A or higher. In contrast, its mobility is damped by thermal oscillations under weaker electric fields. This semiconductor/metallic analogous behavior comes from the difference between the inertial content acquired by polarons under stronger/weaker electric fields. These new results and their analysis shed new light on several experimental controversies.

Effects of temperature and electric field induced phase transitions on the dynamics of polarons and bipolarons

The stability of charge carriers in conjugated polymers is investigated in terms of a nonadiabatic evolution method by using an extended version of the Su–Schrieffer–Heeger (SSH) model that includes the effects of an external electric field and temperature. On the basis of this physical picture, different patterns of applied electric field and temperature dependence of polaron and bipolaron kinematics as well as the transitions between different regimes are found. Phase transitions from subsonic to supersonic velocities are also discussed in terms of the system conditions. We were able to describe at which thermal regime each quasi-particle loses its stability and also to determine under which circumstances do the electric field and temperature rise or dampen its motion. The results indicate that thermal effects on polaron and bipolaron stability may provide guidance for improving the charge carrier conduction in organic optoelectronic devices.

Dynamics of charge transfer in molecular switches

With impurity molecules working as switches, the charge transfer on a single conducting polymer chain is studied. The chain is modeled by a modified tight-binding Hamiltonian extended to include the effects of an external field and the parameters of the switching molecules. The charge transfer through the sites that work like a switch is analyzed by the numerical integration of the equations of motion. Two basic types of molecular switches are studied: single and pairs of donor-acceptor molecules bonded to the chain. The main differences between these two models of switches are determined. We have found that the single radical switch has an anisotropic character and only works for solitons with the same parity of bonding site. For the donor-acceptor pair we have encountered that the chain offers a wider range of devices, from simple switches to perfect molecular rectifiers. The influence of the parameters of the molecules on the charge transfer and the changes they must undergo to characterize the molecular switch are obtained. The role of the length of separation between the sites where the donor and acceptor molecules bond is clarified. The optimum switch configuration is determined.

Estimating correlation energy of diatomic molecules and atoms with neural networks

The electronic correlation energy of diatomic molecules and heavy atoms is estimated using a back propagation neural network approach. The supervised learning is accomplished using known exact results of the electronic correlation energy. The recall rate, that is, the performance of the net in recognizing the training set, is about 96%. The correctness of values given to the test set and prediction rate is at the 90% level. We generate tables for the electronic correlation energy of several diatomic molecules and all the neutral atoms up to radon (Rn). © 1997 by John Wiley & Sons, Inc. J Comput Chem 18: 1407–1414, 1997

CO2 adsorption on single-walled boron nitride nanotubes containing vacancy defects

The adsorption of a CO2 molecule on the vacancy defect type of armchair (5,5) and zigzag (10,0) single-walled boron nitride nanotubes was studied based on Density Functional Theory (DFT). Vacancy defects were studied and the geometrical modifications implemented on the original hexagonal lattice yielded a considerable level of changes in the electronic properties. These changes are reflected in a greater level of CO2 reactivity in relation to the adsorption over a pristine structure. For all types of studied CO2 molecule interaction, we have found a chemical adsorption process based on binding energy. Furthermore, the CO2 adsorption takes place on the top of the vacancy region. A decomposition state was observed when the CO2 molecule interacted with the armchair nanotube with a vacancy on the nitrogen site. By comparing the values of the adsorption energies with those from other defect approaches present in the literature, we conclude that the proposed protocol presents a possible tool to develop stable and sensible carbon dioxide sensors.

Carbon dioxide adsorption on doped boron nitride nanotubes

Boron nitride (BN) nanotubes are promising structures as far as the gas adsorption process is concerned. The electronic and vibrational properties of pristine and cobalt doped single walled boron nitride nanotubes of different chiralities interacting with a carbon dioxide molecule are investigated through the use of density functional theory (DFT) and the discrete variable representation method. When compared to similar simulations concerning carbon nanotubes, a stronger interaction is observed between the carbon dioxide molecule and the functionalized BN nanotube. A density of state investigation suggests that the doping induces major changes in the electronic structure pattern in the sense of critically reducing the original gap. From the vibrational point of view, we note that the zig-zag chirality tends to present higher values of vibrational frequencies for most of the states considered, regardless of the nanotubes being doped or not. Our results suggest that doped zig-zag BN nanotubes are among the best possible candidates for adsorption purposes.

Impurity effects on polaron-exciton formation in conjugated polymers

Combining the one-dimensional tight-binding Su-Schrieffer-Heeger model and the extended Hubbard model, the collision of two oppositely charged polarons is investigated under the influence of impurity effects using a non-adiabatic evolution method. Results show that electron-electron interactions have direct influence on the charge distribution coupled to the polaron-exciton lattice defect. Additionally, the presence of an impurity in the collisional process reduces the critical electric field for the polaron-exciton formation. In the small electric field regime, the impurity effects open three channels and are of fundamental importance to favor the polaron-exciton creation. The results indicate that the scattering between polarons in the presence of impurities can throw a new light on the description of electroluminescence in conjugated polymer systems.

Charge carrier untrapping by temperature effects in conjugated polymers

The temperature influence on charge carrier dynamics is fundamental for the description of the performance of organic diodes and transistors. In this work we treat the temperature on charge transport of both intrachain and interchain process, considering situations where the charge carriers are initially bound in some aspect. We obtain in both cases that the temperature increase can raise the carriers mobility by the same mechanism. The reasons for mobility increase are charge carrier delocalization as well as energy gain for charged polarons. The mechanisms are studied in the scope of the SSH model modified to include temperature effects, impurities and interchain interactions.

Transport of Polarons in Graphene Nanoribbons

The field-induced dynamics of polarons in armchair graphene nanoribbons (GNRs) is theoretically investigated in the framework of a two-dimensional tight-binding model with lattice relaxation. Our findings show that the semiconductor behavior, fundamental to polaron transport to take place, depends upon of a suitable balance between the GNR width and the electron-phonon (e-ph) coupling strength. In a similar way, we found that the parameter space for which the polaron is dynamically stable is limited to an even narrower region of the GNR width and the e-ph coupling strength. Interestingly, the interplay between the external electric field and the e-ph coupling plays the role to define a phase transition from subsonic to supersonic velocities for polarons in GNRs.