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/Alq3/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.