Multilayer polymer electroluminescent devices: Color-variable emission through charge carrier confinement

We have succeeded in obtaining polarity-dependent color-variable emission from polymer electroluminescent (EL) devices with a three-layer structure. The emissive polymers used are poly(3-dodecylthiophene), poly(p-phenylenevinylene) and its derivative, and poly(9,9-dihexylfluorene). The emissive layers are separated by a layer of wide-band-gap hole-transporter, poly(N-vinylcarbazole) or an aromatic diamine, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine. When the EL devices are operated under forward or reverse bias, they exhibit the emission from the negatively-biased polymer layer. The energy band diagram of the devices implies that the separating layer confines excitons in the negatively-biased polymer layer