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Trapped and interfacial charges have significant impact on the performance of organic light-emitting devices (OLEDs). We have studied devices consisting of 20 nm copper phthalocyanine (CuPc) as the buffer and hole-injection layer, 50 nm N,N';-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB) as the hole transport layer, and 65 nm tris(8-hydroxyquinolinato)aluminum (Alq 3 ) as the electron transport and emitting layer sandwiched between a high-work-function metal and a semitransparent Ca electrode. Current-voltage measurements show that the device characteristics in the negative bias direction and at low positive bias below the built-in voltage are influenced by trapped charges within the organic layers. This is manifested by a strong dependence of the current in this range on the direction and speed of the voltage sweep. Low-frequency capacitance-voltage and static charge measurements reveal a voltage-independent capacitance in the negative bias direction and a significant increase between 0 and 2 V in the given device configuration, indicating the presence of negative interfacial charges at the NPB/Alq 3 interface. Transient experiments show that the delay time of electroluminescence at low voltages in these multilayer devices is controlled by the buildup of internal space charges, which facilitates electron injection, rather than by charge-carrier transport through the organic layers. To summarize, our results clearly demonstrate that the tailoring of internal barriers in multilayer devices leads to a significant improvement in device performance.
Rieß et al. (Mon,) studied this question.