The role of copper-phthalocyanine in multilayer organic LEDs based on small molecules

The role of copper-phthalocyanine (CuPc) as intermediate layer between the anode and the hole-transport layer in multilayer organic light-emitting devices (OLEDs) was studied. The OLEDs consisted of CuPc, N,N′-di(naphtalene-1-yl)-N,N′-diphenyl-benzidine (NPB) as hole-transport layer and tris-(8-hydroxyquinolinato)-aluminum (Alq3) as electron-transport and emitting layer sandwiched between a high-work-function metal and a semi-transparent calcium cathode. A combinatorial approach that allows the simultaneous fabrication of 10 × 10 individual devices was used to vary the thicknesses of CuPc and NPB over a broad range from 0 to 45 nm and from 10 to 100 nm, respectively. Systematic current-voltage and impedance measurements revealed a redistribution of the internal electric field of the CuPc/NPB/Alq3 three-layer structure compared to that of the NPB/Alq3 bilayer OLED. It was demonstrated that the hole transport is mainly controlled by the internal energy barrier at the CuPc/NPB interface. The fact that CuPc strongly impedes hole injection into NPB also has a significant impact on the frequency-dependent behavior of the capacitance, especially the cutoff frequency.