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Herein, we report a new molecular design based on carbazole for the development of high triplet energy bipolar host materials for blue phosphorescent organic light emitting diodes (PhOLEDs). Two new isomeric host materials named, 11CzCzCN and 14CzCzCN were designed and synthesized by modifying the carbazole core with carbazole donor at C1 and cyano acceptor at C8-position. The effective interchromophoric electronic communications between the attached donors and central carbazole were interrupted by virtue of large dihedral angle at C1 and short-axis linkage. Also, the strong charge transfer interactions between the donor at C1 and acceptor at C8 was hampered due to the broken conjugation. As a result, the compounds showed high triplet energy (ET) over 2.75 eV without compromising the bipolar charge transporting property. Interestingly, computations and single carrier device analysis evidenced good bipolar charge transporting ability for 11CzCzCN over its structural isomer 14CzCzCN. The applicability of these compounds as hosts for blue PhOLEDs was explored by using a well-known Flrpic dopant. The 11CzCzCN hosted device showed higher performance with maximum EQE of 20.6%, current efficiency of 39.6 cd/A and efficiency roll off of 8.2% at 1000 cd/m2 over its analogue 14CzCzCN hosted device attributed to its high ET and balanced charge transporting property. We believe that this study would provide valuable insights for executing new molecular designs on carbazole for the development of efficient host materials for blue PhOLEDs.

Spirobifluorene modified materials developed as electron transport layers lowered device driving voltage, enhanced quantum efficiency, and more than doubled the lifetime of green phosphorescent organic light-emitting diodes.

Asymmetric blue multiple resonance-type thermally activated delayed fluorescence (TADF) materials composed of diphenylamine combined with carbazole and acridine units were synthesized and evaluated to fine-tune the electronic and photophysical properties.

A thermally activated delayed fluorescence (TADF)-type exciplex host employing a novel electron-transport type (n-type) type host managing positive polarons and stabilizing excitons was developed to elongate the device lifetime of deep blue phosphorescent organic light-emitting diodes (PhOLEDs). The bipolar n-type host was designed to prevent hole leakage and secure hole stability while being stabilized under excitons by introducing a CN-modified carbazole moiety as a weak donor. The TADF-type exciplex host-based blue PhOLEDs showed high (above 20 %) quantum efficiency with a deep blue color coordinate of (0.14, 0.16) and elongated device lifetime. The device operational lifetime of the blue PhOLEDs bearing the TADF-type exciplex host was extended by more than twice compared to that of the exciplex-free unipolar host. This work suggested a design concept of the n-type host to develop the TADF-type exciplex host for deep blue phosphors to reach a long lifespan in the deep blue PhOLEDs.

A novel violet emitter, 1,3-bis[10,10-dimethyl-10H-indeno[2,1-b]]indolo[3,2,1-jk]indolo[1',2',3':1,7]indolo[3,2-b]carbazole (m-FLDID), was designed and synthesized by meta-oriented bis-fusion of two 7,7-dimethyl-5,7-dihydroindeno[2,1-b]carbazole (DMID) subunits for use in a pure violet organic light-emitting diode (OLED). Incorporation of the DMID subunits effectively reduced the nonradiative recombination rate, improving the photoluminescence quantum yield of the m-FLDID emitter. The meta-oriented bis-fusion of the two DMID subunits not only triggered an alternative distribution of the frontier orbitals but also effectively locked the π-conjugation chain, which ultimately resulted in a narrow-band, pure violet emission of the m-FLDID emitter. Doped m-FLDID devices possessed an external quantum efficiency (EQE) of > 5%, pure violet emission with a maximum at 407 nm, a narrow full width at half-maximum of 17 nm, and a Commission Internationale de l'eclairage y coordinate of less than 0.03. This is the first work reporting an EQE of > 5% and an extremely narrow emission spectrum for a pure violet emitter.

Achieving narrow-bandwidth emission and high external quantum efficiency (EQE) simultaneously is a challenge for next-generation blue-emitting organic light-emitting diodes (OLEDs). In this study, novel multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters are developed by fusing an indolocarbazole unit with two carbazole skeletons using para-oriented nitrogen atoms. The resulting rigid and planar π-system without electron-accepting atoms exhibits pure blue photoluminescence at 470 nm, reaching a 100% quantum yield with a full-width-at-half-maximum (FWHM) of 25 nm. Higher-level quantum chemistry calculations confirm an MR effect within the extended π-conjugation and an enhanced triplet-to-singlet crossover (10

A novel electroplex host with two triplet exciton up-converting channels for suppressed triplet exciton triggered degradation mechanisms was developed using an electron transport type host (n-type host) with thermally activated delayed fluorescence (TADF) characteristics to improve the device lifetime of deep blue phosphorescent organic light-emitting diodes (PhOLEDs). The TADF-natured n-type host with high triplet energy was derived from triazine with benzonitrile and carbazole units to induce the TADF characteristics. The TADF natured n-type host generated an electroplex with a hole transport type host and the electroplex-based PhOLEDs revealed an extended device lifetime by more than twice compared to the non-TADF natured n-type host based electroplex host. Transient photoluminescence and electroluminescence analyses revealed that two reverse intersystem crossing (RISC) mechanisms through the n-type TADF host and electroplex host could suppress triplet exciton related degradation and improved the device lifetime. Kinetic modeling of the electroplex supported the RISC mechanisms of the electroplex.

A long lifetime is one of requirements for the commercialization of organic light-emitting diodes (OLEDs). In this paper, we improved the device lifetime of phosphorescent OLEDs by enhancing the oxidation stability of the hole-transport-type host because the host material with steadiness against oxidation has a good positive polaron stability for improved device lifetime. Two p-type host materials, 9-(dibenzo[b,d]thiophen-2-yl)-3′,6,6′-triphenyl-9H-3,9′-bicarbazole (2-TPhDCz) and 9-(dibenzo[b,d]thiophen-4-yl)-3′,6,6′-triphenyl-9H-3,9′-bicarbazole (4-TPhDCz), were synthesized for improved positive polaron stability to obtain a long device lifetime in phosphorescent OLEDs by substituting the active site of a carbazole unit with a phenyl unit. The oxidation stability and positive polaron stability of the host were enhanced by the phenyl protection of the active site, which extended the device lifetime of the phosphorescent OLEDs compared with conventional hole-transport-type hosts. In particular, the 2-TPhDCz host doubled the device lifetime of the phosphorescent OLEDs.

A thermally activated delayed fluorescence (TADF) molecule with high reverse intersystem crossing rate accompanied by high photoluminescence quantum yield was developed by combinatorial donor engineering of bicarbazole and 12H-benzofuro[3,2-a]carbazole. The donor was designed to have strong donor character, rigidity and large twisted angle with the acceptor moiety by combining the merits of bicarbazole and benzofurocarbazole. The TADF emitter derived from the new donor achieved high external quantum efficiencies of up to 23.3% and largely suppressed efficiency roll-off at high luminance relative to the control TADF emitters. Therefore, the combinatorial donor approach was effective to trigger the synergistic effect on the device performances of the TADF devices.

In this study, a multiple resonance (MR) type blue emitter is synthesized, characterized, and evaluated for highly efficient and stable blue fluorescent organic light-emitting diodes (OLEDs). The MR blue fluorescent emitter has a di-tert-butyl benzene substituent in the MR core structure to minimize quenching mechanisms by intermolecular interaction. The emitter shows a high photoluminescence quantum yield and small full width at half maximum of 22 nm, which realize high external quantum efficiency (EQE) of 11.4% in the single unit OLED and device lifetime up to 95% of the initial luminance (LT

In this study, we synthesized and characterized multiple resonance (MR) type blue thermally activated delayed fluorescence (TADF) emitters. Unlike many boron-based MR-TADF materials, the blue TADF emitters of this work had an asymmetric molecular structure with one boron, one oxygen, and one nitrogen. The aromatic units linked to the nitrogen were changed into diphenylamine, carbazole, dimethylacridine, and diphenylacridine to manage the light emission properties of the emitters. The TADF emitters exhibited a blue emission due to the weak electron-donating oxygen atom and the emission color was controlled by the aromatic unit connected to the nitrogen. The simple diphenylamine unit was effective in achieving real deep-blue emission for the BT2020 standard with a high external quantum efficiency (EQE), while the electron-rich nitrogen-based dimethylacridine and diphenylacridine accelerated the reverse intersystem crossing for high EQE and small EQE roll-off. Among the emitters, a diphenylamine-substituted emitter, 7-(tert-butyl)-9-phenyl-9H-5-oxa-9-aza-13b-boranaphtho[3,2,1-de]anthracene (B-O-dpa), showed a maximum external quantum efficiency of 16.3%, a small full width at half-maximum of 32 nm, and a real deep-blue color coordinate of (0.15, 0.05).

Benzonitirile modified host materials, 3-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-(triphenylsilyl)benzonitrile (CNmSi-Trz), 3-(4-(dibenzo[b,d]furan-2-yl)-6-phenyl-1,3,5-triazin-2-yl)-5-(triphenylsilyl)benzonitrile (CNmSi-2DBF-Trz) and 3-(4-(dibenzo[b,d]furan-4-yl)-6-phenyl-1,3,5-triazin-2-yl)-5-(triphenylsilyl)benzonitrile (CNmSi-4DBF-Trz), were synthesized for improved efficiency and lifetime in blue phosphorescent organic light-emitting diodes (PhOLEDs). Triazine and benzonitrile units were introduced commonly to the three host materials for electron transporting properties. The three hosts were applied to mixed host as electron transport type hosts with a hole transport type 3,3-di(9H-carbazol-9-yl)biphenyl (mCBP) for blue PhOLEDs. They formed exciplex with mCBP, and the exciplex of CNmSi-2DBF-Trz with mCBP showed high triplet energy above 2.9 eV. As a result, the blue PhOLEDs fabricated with the CNmSi-2DBF-Trz host achieved high EQE over 23% and 650 h lifetime up to 70% of initial luminance at 200 cd/m2 using the exciplex host system with mCBP.

In this report, we unveiled a new molecular design featuring the transformation of inefficient thermally activated delayed fluorescence (TADF) emitters into the efficient TADF emitters by combing them with non-TADF type chromophore units to develop deep-blue TADF emitters for organic light-emitting diodes. The inefficient TADF emitter and non-TADF chromophore were connected directly through a phenyl linker in a sterically hindered configuration by selectively interlocking two donors in the backbone structure. The new TADF emitter demonstrated excellent device performances with maximum external quantum efficiency up to 22% and y color coordinate of 0.15. Based on these results, we believe that the proposed new design strategy is effective with regard to the conversion of inefficient deep-blue TADF emitters to efficient deep-blue TADF emitters and can be diversely applied for the development of efficient TADF emitters.

An phthalonitrile based 3,3''-di(9H-carbazol-9-yl)-[1,1':2′,1''-terphenyl]-4′,5′-dicarbonitrile (IPNCz) was explored as a charge transfer type host of a yellow emitting bis(4-phenyl-thieno[3,2-c]pyridinato-C2,N)(acetylacetonato)iridium(III) (PO-01) dopant. The phthalonitrile unit was an electron deficient unit and 9-phenylcarbazole was an electron rich unit of the IPNCz host. The phthalonitrile unit combined with the phenylcarbazole unit allowed strong charge transfer character by the donor-acceptor structure, delivering good thermal stability, bipolar carrier transport and proper triplet energy. Therefore, the IPNCz host assisted low driving voltage and high quantum efficiency close to 25% in the yellow phosphorescent device.

An exciton stabilizing electron transport type (n-type) host material was developed for application in long lifetime blue phosphorescent organic light-emitting diodes (PhOLEDs). A conjugation breaking diphenylsilyl backbone structure combined with a diphenyltriazine unit was a basic molecular platform to obtain high triplet energy and electron transport property. Two hosts with and without carbazole were synthesized as the bipolar and unipolar n-type hosts derived from diphenylsilyl backbone and diphenyltriazine unit. The blue PhOLEDs using the two hosts showed high external quantum efficiency above 20% and blue color coordinate of (0.14, 0.17). Particularly, the bipolar n-type host with the carbazole unit extended the device lifetime by more than 1.4 times compared with the unipolar host and by more than three times relative to a conventional diphenyltriazine based host without the diphenylsilyl core because of the exciton stabilizing function of the host. This work proved the potential of the diphenyltriazine and carbazole merged diphenylsilyl skeleton as a chemical platform of the n-type hosts.

A unipolar charge device was utilized to gain information of the charge balance in organic optoelectronic devices to enhance efficiency and operational stability. Usually, a unipolar charge device is designed as a multilayer structure to improve charge transport and block injection of counter charges from electrodes. In this paper, the correlation between charge transport across the interlayer and corresponding bulk electrical parameters from quasi-static analysis was investigated. Although only the interlayer material was modified, the charge carrier mobility and drift charge carrier density of the light-emitting layer were totally changed, which originated from large series resistance due to charge accumulation at the interface. This result indicates that quantitative analysis of electrical properties from unipolar charge devices should be performed as background analysis to ensure efficient interface charge transport during operation.

In this work, we report a novel spirobifluorene carbazole (SBFCbz) donor developed by fusion of carbazole with 9,9′-spirobi[fluorene] (SBF) unit. The newly synthesized SBFCbz donor combined with a diphenyl triazine acceptor to offer an 5'-(4,6-diphenyl-1,3,5-triazin-2-yl)-5′H-spiro[fluorene-9,11′-indeno[1,2-b]carbazole] (SBFTrz) host which played a role of an n-type host. An exciplex host of SBFTrz: 9-([1,1′-biphenyl]-4-yl)-9′-phenyl-9H,9′H-3,3′-bicarbazole (BPBPCz) was developed by combining it with a p-type BPBPCz host. The developed exciplex host showed triplet energy of 2.59 eV, making it a suitable host for the green and red phosphorescent organic light-emitting diodes (PhOLEDs). The PhOLEDs fabricated using the exciplex host system offered excellent charge balance in the emitting layer, which resulted in above 20% external quantum efficiency (EQE) and resistance to the efficiency roll-off at high luminance. The green and red PhOLEDs developed showed the maximum EQE/EQE at 1000 cd/m2 of 22.0/21.7% and 26.2/23.2%, respectively. Besides, the SBFTrz: BPBPCz exciplex host-based green and red devices showed three- and five-fold enhancement, respectively, in the operational lifetime compared to their corresponding SBFTrz single host devices. Thus, the above results confirmed the suitability of the SBFTrz: BPBPCz exciplex as a host to develop PhOLEDs with high efficiency, improved stability at high luminance, and a long operational lifetime.

White organic light-emitting diodes (WOLEDs) with four wavelengths were fabricated by using three doped layers, which were obtained by separating recombination zones into three emitter layers. Among these emitters, blue emissions with two wavelengths (456 and 487 nm) were occurred in the 4,4′-bis(carbazoyl-(9))-stilbene (BCS) host doped with a perylene dye. Also, a green emission was originated from the tris(8-quinolinolato)aluminum (III) (Alq 3 ) host doped with a green fluorescent of 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-[1]benzopyrano [6,7,8- ij ]-quinolizin-11-one (C545T) dye. Finally, an orange emission was obtained from the N , N ′-bis(1-naphthyl)- N , N ′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) host doped with a 4-(dicyanomethylene)-2- tert -butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) dye. The white light could be emitted by simultaneously controlling the emitter thickness and concentration of fluorescent dyes in each emissive layer, resulting in partial excitations among those three emitter layers. Electroluminescent spectra of the device obtained in this study were not sensitive to driving voltage of the device. Also, the maximum luminance for the white OLED with the CIE coordinate of (0.34, 0.34) was 56,300 cd/m 2 at the applied bias voltage of 11.6 V. Also, its external quantum and the power efficiency at about 100 cd/m 2 were 1.68% and 2.41 lm/W, respectively.