Color-Variable Emission in Multilayer Polymer Electroluminescent Devices Containing Electron-Blocking Layer

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Abstract
Color-variable emission in multilayer polymer electroluminescent (EL) devices has been obtained by the use of poly(N-vinylcarbazole) (PVCz) as an electron-blocking layer. The emissive polymers used were poly ( p-phenylenevinylene), poly(3-dodecylthiophene), and poly(2,5-dinonyloxy- p-phenylenevinylene), whose emission colors were green, red, and orange, respectively. When EL devices consisting of two emissive polymer layers separated by a PVCz layer were driven under either forward or reverse bias mode, emission from the negatively biased polymer was observed. PVCz is assumed to act as an electron-blocking layer due to its small electron affinity (2.3 eV) compared with those of the emissive polymers (2.7–3.5 eV), and to facilitate exciton confinement in the negatively biased polymer layer, where radiative recombination of the charge carriers occurs.