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We have examined charge carrier injection from electrodes to tris(8-hydroxyquinoline) aluminum (Alq3) with insulator insertion layers by displacement current measurement (DCM). The inserted materials we used are LiF and a long chain alkane C44H90 (TTC). First, the device structures of Au (or Al)/TTC (or LiF)/Alq3/SiO2/Si were examined by DCM. In the case of the Au/Alq3 interface, we found that the threshold voltage for hole injection from the Au electrode to the Alq3 layer was much reduced by the insertion of a 10-nm-thick TTC layer. In the case of the Al/Alq3 interface, the threshold voltage for electron injection from the Al electrode to the Alq3 layer was reduced by the insertion of a 1-nm-thick TTC layer. The reduction in the threshold voltage was compatible to that in the case of LiF insertion. From these results, we can expect that TTC insertion can reduce the operation voltage of organic light-emitting diodes (OLEDs). However, no improvement in performance was observed in the Al/Alq3/TPD/indium tin oxide (ITO) device, indicating that the reduction in the threshold voltage for carrier injection does not directly correspond to efficient injection in the high current region.

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