Effect of hole-transporting film thickness on the performance of electroluminescent devices using polymer Langmuir-Blodgett films containing carbazole

Electroluminescence (EL) of a film of poly(N-dodecylacrylamide-co-2-(9-carbazolyl)ethylacrylate) (DDA/CzEA copolymer) as a hole-transporting layer was investigated as a function of the mole fraction of CzEA and the film thickness. The EL devices have double-layer structures consisting of vacuum-deposited tris(8-quinolinolato)aluminum(III) complex (Alq3) as an electron-transporting and emitting layer, and the DDA/CzEA copolymer Langmuir-Blodgett (LB) films on anodic indium-tin-oxide (ITO) electrodes. The EL performance of the double-layer devices was compared with that of the single-layer device of Alq3. The DDA/CzEA copolymers with less than 0.31 mole fraction of CzEA formed stable LB films which acted as a good electron-blocking layer. The current density decreased with increasing film thickness of the DDA/CzEA copolymer LB films. The light conversion efficiency was improved and became almost constant with more than 11 layers because the excitons generated by the recombination of holes and electrons around the interface between the DDA/CzEA copolymer LB film layer and the Alq3 layer was not being quenched by the ITO anodes.