We present an compact model to describe the charge injection for organic light-emitting diodes. By identifying a critical distance where the concentration of carriers in the extended states equals that of the trapped carriers, we obtain a model for the injection current, which links the drift-diffusion and the multiple-trapping theories. This model yields the injection current as a function of electric field, temperature, and barrier height between metal and organic semiconductor. Good agreement with recent experimental data is observed. The effect of the field-dependent mobility on the injection current is also discussed.

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