engleski

Investigation of the organic/organic interface in OLEDs by a Monte Carlo simulation

Considerations based on the magnitude of the relevant parameters in organic light emitting diodes (OLEDs) suggest that the carriers density is rather low in these devices. Along with the hopping charge transport characterizing the disordered organic materials which make these devices, it is clear that the Monte Carlo treatment is a normal way to investigate the functionality of OLEDs. A lot of work in the letterature was devoted to theoretical and numerical study of the carrier injection at the metal electrodes of the device. However very few papers treat the problem of the organic/organic interface where most electron-hole recombinations take place. This simulation investigates the crossing of the organic/organic interface in a two layer device including Coulomb interactions and effect of correlated energetic disorder. Hopping transport is considered through the Miller-Abrahams hopping formula. After computing the probabilities for each possible hop, the dwelling time for each electron and hole in the device is derived. The particle that hops first is the one with the shortest dwelling time. The Coulomb interactions are taken into account through the update of the HOMO and LUMO energy levels after each hop. This many particle algorithm is suitable for programming on a parallel machine and the run time for reasonable values of the parameters is several hours on eight processors. Recombination is relevant only when the pair electron-hole is on the same site. But in order for this to work in this MC algorithm the energy of LUMO(HOMO) must be shifted due to the presence of hole(electron) by an amount of the order of the binding energy of the exciton. Injection of electrons and holes from the electrodes is modeled by a themoionic process and the forward and backward currents through the organic/organic interfaces are deduced. To see clearly the effect of disorder on the energy barrier crossing at the organic/organic interface we considered first a hole only device with a cross-section of 200x200 sites and 100 sites width. Periodic boundary conditions are applied to the lateral dimensions to take into account the extent of the real devices. For typical working fields and currents, we have in such a lattice 130 holes. The figure shows the evolution of the hole current crossing the energetic barrier at the organic/organic interface as a function of disorder strength. The main behaviour is a decrease of the current when the disorder increases. But for the case when there is disorder only on one side of the interface (i.e. the second layer) the current increases with increasing disorder. This due to the generally accepted argument that disorder brings some energy levels of the second layer very close to the injecting level, reducing by the same way the effective energy barrier by an amount of $\sigma^{; ; 2}; ; /kT$. However, this conclusion do not apply to the three other cases where disorder is present on both layers. This is principally due to the fact that before jumping over the barrier carriers will thermalize because of the higher transversal hopping probability and as a consequence they will occupy the lower energy states in the gaussian distribution. Thus the effective barrier is not very much altered beyond a certain value of disorder strength ($\sigma=0.02$eV on the figure). We also studied the effect of the correlated disorder on the recombination efficiency in the electron-hole device. We considered two types of correlations: short range (three sites) correlated gaussian disorder as was done for the single carrier device and long range dipole induced correlations.

organic heterojunction; disorder; Monte Carlo; recombination

nije evidentirano