Investigation of the charge transport through disordered organic molecular heterojunctions

We develop a new three-dimensional multiparticle Monte Carlo ({\it 3DmpMC}) approach in order to study the hopping charge transport in disordered organic molecular media. The approach is applied here to study the charge transport across an energetically disordered organic molecular heterojunction, known to strongly influence the characteristics of the multilayer devices based on thin organic films. The role of energetic disorder and its spatial correlations, known to govern the transport in the bulk, are examined here for the bilayer homopolar system where the heterojunction represents the bottleneck for the transport. We study the effects of disorder on both sides of the heterojunction, the effects of the spatial correlation within each material and among the layers. Most importantly, the {\it 3DmpMC} approach permits us to treat correctly the effects of the Coulomb interaction among carriers in the region where the charge accumulation in the device is particularly important and the Coulomb interaction most pronounced. The Coulomb interaction enhances the current by increasing the electric field at the heterojunction as well as by affecting the thermalization of the carriers in front of the barrier. Our MC simulations are supplemented by the master equation (ME) calculations in order to build a rather comprehensive picture of the hopping transport over the homopolar heterojunction.Comment: 26 pages, 11 figures, LaTe

Polarization effects in the channel of an organic field-effect transistor

We present the results of our calculation of the effects of dynamical coupling of a charge-carrier to the electronic polarization and the field-induced lattice displacements at the gate-interface of an organic field-effect transistor (OFET). We find that these interactions reduce the effective bandwidth of the charge-carrier in the quasi-two dimensional channel of a pentacene transistor by a factor of two from its bulk value when the gate is a high-permittivity dielectric such as $(\textrm{Ta}_{2}\textrm{O}_{5})$ while this reduction essentially vanishes using a polymer gate-insulator. These results demonstrate that carrier mass renormalization triggers the dielectric effects on the mobility reported recently in OFETs.Comment: 19 pages, 3 figure

Tunable Frohlich Polarons in Organic Single-Crystal Transistors

In organic field effect transistors (FETs), charges move near the surface of an organic semiconductor, at the interface with a dielectric. In the past, the nature of the microscopic motion of charge carriers -that determines the device performance- has been related to the quality of the organic semiconductor. Recently, it has been appreciated that also the nearby dielectric has an unexpectedly strong influence. The mechanisms responsible for this influence are not understood. To investigate these mechanisms we have studied transport through organic single crystal FETs with different gate insulators. We find that the temperature dependence of the mobility evolves from metallic-like to insulating-like with increasing the dielectric constant of the insulator. The phenomenon is accounted for by a two-dimensional Frohlich polaron model that quantitatively describes our observations and shows that increasing the dielectric polarizability results in a crossover from the weak to the strong polaronic coupling regime

Charge transport across organic-organic interfaces in organic light-emitting diodes

The process of hopping transport across organic heterojunctions is critical to the function of organic light-emitting diodes (OLED) and many other currently developed organic electronic devices. We consider the case of a hole-only or homopolar heterojunction with Gaussian energetic disorder. We cross-compare the results of our previous multiparticle Monte Carlo simulator to results of the pioneering analytic hopping model of Arkhipov et al. [V.I. Arkhipov, E.V. Emelianova, H. Bassler, J. Appl. Phys. 90 (2001) 2352]. This comparison made it possible to point out and correct some shortcomings of the latter model. This includes the new definition of the injection level at the heterojunction, which brings orders of magnitude change to the current with respect to values obtained for previously used injection levels. Further insight, related to the backward-to-forward hopping ratio at the heterojunction, also brings about the order of magnitude correction for the current. We end up with a rather complete and cross-verified analytical description of the charge transport across energetically disordered homopolar heterojunctions. [All rights reserved Elsevier

MOLED: Simulation of multilayer organic light emitting diodes

International audienc

Insights into OLED functioning through coordinated experimental measurements and numerical model simulations

We present some typical applications of our device simulation model "MOLED" which has proven to be a very useful tool for obtaining insight into the functioning of multi-layer organic light-emitting devices (OLED's). Our general approach consists of combining experimental measurement and numerical device modelling in a deliberate and coordinated way. Taking a step-by-step, evolutive approach, starting with simple single layer devices and gradually moving up to more complex multi-layer architectures with a partially doped emission layer, we could progressively determine unknown parameters in the model and at the same time shed light on specific aspects of OLED functioning. For example, electrode characterization was done by measuring and modeling single layer devices with different electrode materials, while the simulation of time of flight signals gave us insight into the transient state and the effects of correlated disorder on the macroscopic mobility. The internal electric field in multi-layer OLED's could also be investigated by varying the thicknesses of the hole transport and electron transport layers. Applying this method to the standard OLED device structure that has received broad attention in the literature, we have found a number of surprising results. From our experiments, we have demonstrated that the average electric field inside the hole transport layer is larger than or equal to the average field in the emission layer over the entire current range. The device simulations fully clarify the situation, giving insight into the space charge effects as well as the hole and the electron current distributions in the device. In particular, we found that there is a leakage of unrecombined holes towards the cathode at low voltages. We also found a strong variation of the electric field in the Alq3 layer due to space charge effects. By using the laser dye derivatives DCM-TPA with electron trapping capabilities and DCM-II with both electron and hole trapping capabilities as dopants in a standard OLED architecture, we could study the effect on transport and emission characteristics. In the case of the exclusively electron trapping dopant, a blue-shift of the emission color with increasing bias is observed which we find is due to a splitting of the recombination zon

MOLED: simulation of multilayer organic light emitting diodes

MOLED solves the dynamics of electrons and holes in multilayer organic light emitting diodes (OLED). The carriers are injected on the positive and negative electrodes of the device by tunneling through a potential barrier. Thermal excitation processes across the barrier are also included. In the interior of the device the electron-hole recombination occurs when the two carriers are close enough, according to a model inspired from the one of Langevin. A fraction of these recombined pairs gives photons. The charge transport inside the organic material occurs through hopping. Several choices of mobility formulae are available in the code. MOLED can be used for OLEDs with an arbitrary number of layers. The output consists of numerous fields that describe the device performance. For example, there are the current, the recombination and the charge density distributions, the electric field distribution, the current-voltage characteristics and the device internal quantum efficienc