An interesting paper in the last issue (91) of Applied Physics Letters from Ling Li, Gregor Meller, and Hans Kosina.
As they say: "The diffusion-controlled injection process in OLED is investigated and the concept of critical distance is proposed. A compact injection model applicable to OLED is formulated. This model is shown to fit the experimental data well and to explain the dependency of the injection current on the barrier height, the temperature, and the electric field. It was found that the field-dependent mobility plays an important role in the injection model at higher electric field."
Have a look here:
Diffusion-controlled charge injection model for organic light-emitting diodes
Ling Li, Gregor Meller, and Hans Kosina
Appl. Phys. Lett. 91, 172111 (2007) (3 pages)