Electroluminescence of solution-processed organic light-emitting diodes based on fluorescent small molecules and polymer as hole-transporting layer

White organic light-emitting diodes with three successively spin-coated layers, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate, poly(N-vinylcarbazole) and small-molecule emissive layer (EML) in turn, and a vacuum-deposited electron-transporting layer (ETL) have been prepared. The EML includes a host bis[2-(4-(N,N-diphenylamino)phenyl)vinyl]biphenyl, blue dopant 4,4′-bis[2-(4-(N,N-diphenylamino)phenyl)vinyl]biphenyl (DPAVBi) and yellow dye 5,6,11,12-tetraphenylnaphtacene. The optimized white device shows a current efficiency of 6.7 cd/A (1000 cd/m²) and a maximum luminance of 16 768 cd/m². It was found that the emission spectra of DPAVBi was tuned from blue to greenish blue with increasing of the ETL thickness, which could be attributed to the optical interference effect from the metal cathode. By comparing emission spectra of numerical simulation to tested electroluminescence spectra, the position of the emission zone was determined.