Cascade Hole Transport in Efficient Green Phosphorescent Light-Emitting Devices Achieved by Layer-by-Layer Solution Deposition Using Photocrosslinkable-Conjugated Polymers Containing Oxetane Side-Chain Moieties

A hole-injection/transport bilayer structure on an indium tin oxide (ITO) layer was fabricated using two photocrosslinkable polymers with different molecular energy levels. Two photoreactive polymers were synthesized using 2,7-(or 3,6-)-dibromo-9-(6-((3-methyloxetan-3-yl)methoxy)hexyl)-9H-carbazole) and 2,4-dimethyl-N, N-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl) aniline via a Suzuki coupling reaction. When the oxetane groups were photopolymerized in the presence of a cationic photoinitiator, the photocured film showed good solvent resistance and compatibility with a poly(N-vinylcarbazole) (PVK)-based emitting layer. Without the use of a conventional hole injection layer (HIL) of poly(3,4-ethylenedioxythiophene)/(polystyrenesulfonate) (PEDOT:PSS), the resulting green light-emitting device bearing PVK: 5-4-tert-butylphenyl-1,3,4-oxadiazole (PBD):Ir(Cz-ppy)(3) exhibited a maximum external quantum efficiency of 9.69%; this corresponds to a luminous efficiency of 29.57 cd/A for the device K-4 configuration ITO/POx-I/POx-II/PVK:PBD:Ir(Cz-ppy)(3)/triazole/Alq(3)/LiF/Al. These values are much higher than those of PLEDs using conventional PEDOT: PSS as a single HIL. The significant improvement in device efficiency is the result of suppression of the hole injection/transport properties through double-layered photocrosslinked-conjugated polymers. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 388-399, 2012