Higher Resistance to Hole Injection and Electric Field Distribution in Organic Light-Emitting Diodes with Copper Phthalocyanine Interlayer

We carried out a displacement current measurement (DCM) study to examine the resistance to hole injection and the electric field distribution in tris(8-hydroxyquinoline) aluminum-based organic light-emitting diodes. The DCM curves of the devices with/without a copper phthalocyanine (CuPc) interlayer of different thicknesses were measured as a function of the sweep rate of applied triangular wave voltage from 1 to 10000 V/s. The presence of a thin CuPc interlayer markedly shifted hole injection voltage to the higher side with increasing sweep rate and resulted in a hump in the backward sweep of the DCM curves. These modulations in DCM curve were reduced or disappeared in the thick CuPc and CuPc-free devices. We found that the modulations originate from the hole injection resistance induced by the CuPc interlayer. The electric field distribution in the devices under operation is estimated taking into account the band bending in the CuPc interlayer.