High triplet energy exciplexes which can improve the lifetime of blue phosphorescent organic light-emitting diodes were developed by mixing a carbazole based hole transport type host with a CN modified carbazole based electron transport type host.
Novel
hosts strategically designed to have high triplet energy
and bipolar carrier-transport features were derived by substituting
carbazole and CN-modified carbazole at the ortho position of the ortho-connected
biphenyl core with a CN functional group. The position of the CN unit
in the biphenyl core was controlled, and the effect of the CN position
on the photophysical properties and device performances of the blue
phosphorescent organic light-emitting diodes (PHOLEDs) was studied.
The CN substitution position changed the dihedral angle between the
aromatic units of the host materials, which managed the singlet energy,
triplet energy, and carrier-transport properties of the hosts. Four
hosts were synthesized to study the effect of the CN substitution
position, and the hosts with the CN unit at a sterically hindered
position increased the triplet energy, whereas the hosts with the
CN unit at other positions with less steric hindrance improved the
charge-transport properties. The management of the CN position could
increase the triplet energy of the hosts over 3.0 eV, and the best
blue PHOLED showed over 20% external quantum efficiency.
positive and negative polarons are key parameters to be managed for the hosts. Rapid progress of the EQE and lifetime of PhOLEDs by the hosts was already exemplified in many research works. [12][13][14][15] Especially, the EQE and lifetime of red and green PhOLEDs were promoted with the help of hosts. [16][17][18] In the case of blue PhOLEDs, the EQE was advanced to that of red and green PhOLEDs by assistance from various hosts like exciplex hosts and bipolar hosts. [19,20] However, the lifetime level of blue PhOLEDs is far below that of red and green PhOLEDs partly due to lack of host materials stabilizing electrical operation of the blue PhOLED by suppressing degradation of the devices. Although several hosts demonstrated advanced lifetime in the blue PhOLEDs, the lifetime is still very short and lifetime extending host is to be explored for further extension of lifetime.Herein, we explain material design, molecular simulation, synthesis, photophysical characterization, device evaluation, and device analysis of host materials for blue PhOLEDs comprehensively. Two hosts were synthesized using dibenzothiophene and carbazolylcarbazole building units by connecting them with or without a biphenyl linker. It was revealed that the new hosts extended the lifetime of the blue PhOLEDs by more than five times relative to conventional 1,3-bis(N-carbazolyl)benzene (mCBP) host while enhancing the EQE.
Results and DiscussionTwo host materials with or without a biphenyl linker between dibenzothiophene and carbazolylcarbazole were designed for high EQE and improved lifetime in blue PhOLEDs. 9-(Dibenzo[b,d]thiophen-4-yl)-9H-3,9′-bicarbazole) (4DBTCz) has a chemical structure with a directly connected dibenzothiophene and carbazolylcarbazole through 4-position of dibenzothiophene. Whereas,d]thiophen-4yl)-[1,1′-biphenyl]-3-yl)-9H-3,9′-bicarbazole (4DBTBPCz) has an additional metalinked biphenyl between dibenzothiophene and carbazolylcarbazole. The 4DBTCz and 4DBTBPCz have a common backbone structure utilizing 4-position of dibenzothiophene because the 4-position connection of dibenzothiophene was reported to be advantageous for improved lifetime. [21] High triplet energy materials based on dibenzothiophene and carbazolylcarbazole are developed as hosts for lifetime extension in blue phosphorescent organic light-emitting diodes. Two host materials with or without a biphenyl linker between dibenzothiophene and carbazolylcarbazole are synthesized to get insight about the effect of aromatic linkers on the photophysical properties of the hosts and the device performances of a blue triplet emitter doped phosphorescent organic light-emitting diodes. It is described that the host design with dibenzothiophene and carbazolylcarbazole at 4-position of dibenzothiophene is effective for both high efficiency and improved lifetime. In particular, the new hosts improved the lifetime of the blue devices by more than five times while enhancing the quantum efficiency compared to a common triplet host.
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