Triplet harvesting is a candidate technology for highly efficient and long-life white OLEDs, where green or red
phosphorescent emitters are activated by the triplet-excitons diffused from blue fluorescent emitters. We examined two
oxadiazole-based electron transport materials with different horizontal molecular orientation as a triplet-exciton diffusion
layer (TDL) in triplet-harvesting OLEDs. The device characteristics and the transient electroluminescent analyses of the
red phosphorescent emitter showed that the triplet-exciton diffusion was more effective in the highly oriented TDL. The
results are ascribed to the strong orbital overlap between the oriented molecules, which provides rapid electron exchange
(Dexter energy transfer) in the TDL.
Host materials for phosphorescence organic light-emitting diodes (OLEDs) are required to have wide energy gap to
prevent back energy transfer and confine triplet exciton on guest molecules. Carbazole (Cz) has been widely used as a
building block for host materials because of its relatively high energy gap. We found that the energy gap of Cz can be
widened by fluorination at specific positions. A characteristic of the energy gap widening by fluorination is its
controllability by the number and position of fluorine substituents. We synthesized 2,7-difluorocarbazole (F-Cz) and
estimated the energy gap of Cz and F-Cz from absorption spectra to be 3.59 eV and 3.71 eV, respectively. To confirm
the wide-gap effect of F-Cz on OLED device, we synthesized a solution-processable polymer host,
poly(N-vinyl-2,7-difluorocarbazole) (F-PVK), which has F-Cz as pendant groups, and compared it with poly(N-vinylcarbazole) (PVK).
The OLED devices investigated consisted of an ITO/PEDOT:PSS/EML/CsF/Al multilayered structure. The
poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) layer was spin-coated onto an indium tin oxide (ITO)
coated glass substrate. Subsequently, the emission layer (EML) composed of PVK or F-PVK,
1,3-bis[(4-tertbutylphenyl)-1,3,4-oxidiazolyl]phenylene (OXD-7), and a blue phosphorescent dopant, iridium(III)bis
[(4,6-difluorophenyl)-pyridinato-N,C2'] picolinate (FIrpic) was
spin-coated, and the CsF and Al layers were vapor-deposited.
The OLED device with F-PVK showed 1.8 times higher maximum current efficiency (27 cd/A) than that with PVK (15
cd/A). The improved efficiency of F-PVK device can be rationalized by the enhanced triplet confinement effect of
polymer host composed of fluorinated carbazole.