White light electroluminescence from a hole-transporting layer of mixed organic materials

White light emission is important for full color display and back-lighting applications of organic electroluminescence devices. In the present work, a white light organic electroluminescent device has been fabricated by employing a hole-transporting layer (HTL) of mixed light-emitting organic materials. The device has a structure of glass substrate\indium tin oxide (ITO)\mixed HTL\Tris(acetylacetonato) (monophenanthroline) (Tb(ACAC)3Phen)\Mg:Ag. The mixed HTL was prepared by spin coating of a solution of poly(N-vinylcarbazole) (PVK), N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1′-biphenyl-4,4′-diamine (NPB) and 1,1-bis(4-tolylaminophenyl) cyclolexane (TAPC). The Tb complex layer plays several roles in the present device. It is an electron-transporting layer as well as a green light-emitting layer. In addition, it also functions as a hole-blocking layer such that recombination of electrons and holes can also take place in the HTL. Blue and red emissions are due to the NPB and TPAC in the HTL. Together with the green emission from the Tb complex, white light emission above 1000 cd/m2 has been achieved at a driving voltage of 20 V.