Color-variable emission in multilayer polymer electroluminescent devices containing electron-blocking layer

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.