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Monte Carlo simulation of organic-organic interface in OLEDs

General considerations for organic light emitting diodes (OLEDs) suggest that the carrier density is rather low in these devices. Along with the hopping charge transport characterizing the disordered organic materials of which these devices are made, it is clear that the Monte Carlo treatment is a normal way to investigate the functionality of OLEDs. A lot of work in the literature was devoted to theoretical and numerical study of the carrier conduction and injection at the metal/organic interface. However, very few papers address the problem of the organic/organic interface. This is in spite of the fact that the charge transport over organic/organic interfaces often dominates the device behavior and that most of the electron-hole recombination takes place there. Our simulations investigate the hopping transport across the organic/organic interface in a two-layer device. The 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 Miller-Abrahams hopping formula. 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 particle that hops first is the one with the shortest dwelling time. This many particle algorithm is suitable for programming on a parallel machine. The run time for reasonable values of the model parameters is of the order of several hours on eight 1.25GHz processors. Although the full model includes both electrons and holes and the recombination among them, in this communication we concentrate on the problem of the organic/organic interface in the monopolar (hole only) devices. The figure shows the evolution of the current of holes crossing the energetic barrier at the organic/organic interface as a function of disorder strength. The main observation is a decrease of the current with increasing disorder strength. However, when disorder exists only on one side of the interface (i.e. the second layer) the current increases with increasing disorder strength. It is only then that the widely used argument applies: that the disorder brings some energy levels of the second layer very close to the injecting level, thus reducing the effective energy barrier by an amount of $\sigma^2/kT$. However this argument does not apply to the three other cases studied here, where disorder is present on both sides of the interface. This is principally due to the fact that carriers tend to thermalize before jumping over the barrier. This is a consequence of the transversal hopping probability being much higher then the one perpendicular to the barrier leading the carriers to occupy the lower energy states. Thus the effective barrier is not very much altered beyond a certain value of disorder strength ($\sigma=0.02eV$ in the figure). Consequently the disorder does not help conduction over the organic/organic interface.

Monte Carlo; organic; molecular; recombination; light-emiting

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