engleski

Computer simulation of multilayer organic diodes

Understanding the hopping transport in amorphous organic materials is a key ingredient to efficient operation organic light emitting diodes. By coordinated model simulation and ex-periment we got valuable insights into the OLED functioning and device optimization. We present first the computational tool MOLED for simulation of OLEDs . MOLED is available to the scientific community through the CPC Program Library (http://cpc.cs.qub.ac.uk/summaries/ADSG). It solves the transient dynamics until steady state, of electron and hole transport in multilayer OLEDs. Although the model is one-dimensional due to the planar symmetry of the OLED devices, it takes into account the discrete 3D nature of charge carrier distribution close to the electrodes. Both tunneling and thermionic mecha-nisms are considered for electron and hole injection at the electrodes. The charge transport inside the organic material occurs through nearest-neighbor hopping whereas the model al-lows choosing between different field dependent behaviour of the mobility law. The electron-hole recombination occurs when the two carriers are close enough, according to a generalized Langevin model. In fact, exciton formation originates from both drift and the diffusion cur-rent. The latter is far from being negligible at the organic-organic energy barrier. An adjust-able fraction of these electron-hole pairs leads to radiative recombination. Due to its 1D nature MOLED is enable to address three-dimensional effects of the discrete charges which are particularly important at organic heterojunctions. In fact such interfaces form the bottlenecks for charge transport in multilayer diodes. We discuss, in a second part, a microscopic simulation model based on a Monte Carlo treatment. This model includes the Coulomb interactions among carriers and the effect of correlated Gaussian energetic disorder in the organic material. Hopping processes are modelled through the various hopping formu-lae. The Monte Carlo procedure goes as follows: After computing the probabilities for each possible hop, the dwelling time for each electron and hole in the device is derived. The partic-le that hops first is the one with the shortest dwelling time. This three-dimensional multipar-ticle Monte Carlo (3DmpMC) algorithm is suitable for programming on a parallel machine. For a bilayer homopolar device we study the effects of disorder on both sides of the hetero-junction, the effects of the spatial correlation within each material and among layers. Most importantly, the 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 parti-cularly important and the Coulomb interaction mostly pronounced. The Coulomb interaction enhances the current by increasing the electric field at the heterojunction as well as by affec-ting the thermalization of the carriers in front of the barrier. The consideration of the bilayer bipolar system shows deviations of the recombination crossection from the Langevin behaviour.

multilayer; organic; light-emitting; diode; simulation; numerical model

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