Injection and strong current channeling in organic disordered media

We consider charge injection from a metal into amorphous organic molecular media with correlated disorder. It is shown that correlations, known to be essential for understanding the field dependence of the carrier mobility, also strongly influence the injection current distribution. In particular, we find that the injection hot spots are intrinsic for metal/organic interfaces, even for perfectly flat surfaces. The current density variations reach several orders of magnitude for realistic material parameters. The injection hot spots further induce current channels in the bulk of the material that extend a hundred nanometers beyond the injection surface. For electronic devices based on thin amorphous organic films, as are the organic light-emitting diodes, this current channeling is expected to have a serious impact on device characteristic and performance.

Figure 1

The injection current distribution over the electrode surface for $F=0.5\mathrm{MV}∕\mathrm{cm}$ and $T=300\mathrm{K}$. The injection current density is plotted relative to $j_{\mathit{inj}}^{\mathit{ord}}$, the injection current density in the absence of disorder.

Figure 2

The distribution of sites with respect to the value of the local injection current. The cases correspond to the correlated disorder and uncorrelated Gaussian disorder, with the same disorder strength $\sigma$. Both distributions are of the Gaussian shape when plotted against the logarithm of the current density. While the distributions do not differ at the low-current side, they do differ very much at high currents. The correlated disorder provides better injection paths. The average injected current for the correlated disorder is an order of magnitude bigger than for the uncorrelated disorder.

Figure 3

The current density distribution along the direction of the field ($x$ direction) for $F=0.5\mathrm{MV}∕\mathrm{cm}$. The dark parts on this contour plot correspond to regions with high current density. The left-hand side of the figure corresponds to data in Fig. 1 for $z\approx 40\mathrm{nm}$. More precisely, the present figure shows the current density distribution within the slab between the planes at $z=35\mathrm{nm}$ and $z=45\mathrm{nm}$. The current channels, induced by the injection hot spots near $y=10\mathrm{nm}$ and $y=45\mathrm{nm}$ in Fig. 1 are clearly visible.