Numerical study on the optimization of hole injection layers in organic light-emitting devices
It has been reported that hole injection efficiency and thus the performance of organic light-emitting devices can be improved by inserting a hole injection layer (HIL) between the hole transport layer (HTL) and the anode. On the basis of a proposed model, the dependence of injection efficiency on the ratio of the HIL thickness to the total thickness of the HIL and HTL, and on the ratio of the barrier height of the anode/HIL interface to the total barrier height is numerically studied. The results show that for given parameters of the anode and HTL material, there exists an optimal value for the energy level of the highest occupied molecule orbit (HOMO) and the thickness of HIL. For a given total thickness and a total mobility of HIL and HTL, the greater the ratio of HIL thickness to the total thickness, and the smaller the ratio of HIL mobility to the total mobility, the smaller is the optimal ratio of the barrier height of the anode/HIL interface to the total barrier height, and vice versa. For a given total barrier height and a total mobility of HIL and HTL, the greater the ratio of the barrier height of the HIL/HTL interface to the total barrier height, and the smaller the ratio of HIL mobility to the total mobility, the smaller is the optimal thickness ratio of HIL to the total, and vice versa.