Dual role of LiF as a hole-injection buffer in organic light-emitting diodes

It is demonstrated experimentally that the effect of a LiF buffer layer inserted at the $\mathrm{ITO}\backslash {\mathrm{N,N}}^{\prime }-\mathrm{bis}\text{(1-}\mathrm{naphthyl}\mathrm{)-}{\mathrm{N,N}}^{\prime }-\mathrm{diphenyl}{\text{-1,1}}^{\prime }$ biphenyl ${\text{4,4}}^{\prime }-\mathrm{dimaine}$ (NPB) interface on the hole injection is greatly dependent on the initial barrier height (IBH) existing at the interface. Only for a large IBH, will the introduction of the LiF show improvement effect. For small one, it will weaken the hole injection. These phenomena are explained in terms of tunneling model and calculations based on this model show a good agreement with the experimental results. This further confirms that the energy level realignment and the change in carrier tunneling probability are mainly responsible for the variation of current injection induced by the insulating buffers in organic light-emitting diodes.