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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 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.

Key parameters of the host for long lifetimes in phosphorescent organic light-emitting diodes were investigated by synthesizing three isomeric hosts with a carbazolylcarbazole hole transport moiety and a benzonitrile electron transport moiety. The position of the CN unit was controlled as the ortho, meta and para position to the carbazolylcarbazole to study the relationship between material parameters and lifetime of the phosphorescent organic light-emitting diodes. It was revealed that the bond dissociation energy in the triplet excited state is well-correlated with the device lifetime of the phosphorescent organic light-emitting diodes.

A novel mixed host employing a negative polaron stabilizing p-type host was developed to extend the device lifetime of blue phosphorescent organic light-emitting diodes (PhOLEDs). Instead of a conventional p-type host with only a hole transport unit, a p-type host with a negative polaron resistant unit in addition to the hole transport unit was used in the mixed host consisting of a p-type host and an n-type host. The p-type host had benzonitrile as a negative polaron resistant unit which stabilized the molecule under negative polarons. The use of the p-type host with the benzonitrile unit almost doubled the device lifetime of the blue PhOLEDs due to the improved negative polaron stability of the hosts. Therefore, the p-type host with the weak electron accepting benzonitrile unit can play the role of a lifetime extending p-type host to replace a conventional p-type host.

Lifetime study of blue phosphorescent and thermally activated delayed fluorescent organic light-emitting diodes was carried out to understand the dominant degradation process during electrical operation of the devices. Doping concentration dependence of the phosphorescent and thermally activated delayed fluorescent organic light-emitting diodes was studied, which demonstrated long lifetime at low doping concentration in the phosphorescent devices and at high doping concentration in the thermally activated delayed fluorescent devices. Detailed mechanism study of the two devices described that triplet–triplet annihilation is the main degradation process of phosphorescent organic light-emitting diodes, whereas triplet–polaron annihilation is the key degradation factor of the thermally activated delayed fluorescent devices.

Comprehensive study to understand the degradation mechanism of high efficiency blue organic light-emitting diodes by charge transport layers was carried out by managing the device parameters and charge transport materials. Although it has been generally accepted that the charge transport layers are important for long lifetime, the degradation mechanism was not clear. From this work, it was revealed that exciton quenching of emitters by the degradation products of charge transport layers is the main degradation pathway of blue phosphorescent and thermally activated delayed fluorescent organic light-emitting diodes. It was described that the degradation of the blue devices by the charge transport layers would be largely avoided by separating the exciton generating emitters from the charge transport layers by shifting the exciton formation zone to the center of the emitting layer. Eventually, the lifetime of the blue devices with the emission zone fall apart from the charge transport layers was not affected by the charge transport layers because exciton quenching by the degradation products of the charge transport layers was suppressed.

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.

Degradation of Ir based blue phosphorescent emitters was analyzed from the viewpoint of triplet-polaron annihilation (TPA) of the emitters. Two different classes of Ir emitters, F substituted phenylpyridine type Ir emitter and F substituted phenylimidazole type Ir emitter, were compared as the blue emitters. Aging test of singlet carrier devices of the Ir emitters under polaron, exciton, and exciton-polaron revealed that the phenylimidazole type Ir emitter was relatively more resistant to the TPA degradation than the phenylpyridine type Ir emitter and extended the lifetime of blue phosphorescent organic light-emitting diodes. In addition, the lifetime and degradation of the short-living blue phosphorescent devices were improved by the host material suppressing the TPA under negative polaron using an electron scattering emitting layer structure.

High performance deep blue phosphorescent organic light-emitting diodes (PhOLEDs) were developed using a thermally activated delayed fluorescence type exciplex host. Three isomeric electron transport type host materials based on a triazine core, tetraphenylsilane and benzonitrile groups were synthesized for high triplet energy and good electron transport property in solid state. They formed thermally activated delayed fluorescence type exciplexes with a carbazole based hole transport type host. Among the three hosts, 3-(4,6-bis(3-(triphenylsilyl)phenyl)-1,3,5-triazin-2-yl)benzonitrile host achieved high external quantum efficiency of 21.0% and deep blue color coordinate of (0.14, 0.18). Moreover, the lifetime of the deep blue PhOLEDs was improved by more than 1.6 times using the new exciplex host compared with the state of the art mixed host.

A series of low-bandgap alternating copolymers consisting of electron-accepting [1,2,5]thiadiazolo[3,4-g]quinoxaline (TQ) derivatives and electron-donating fluorene or carbazole were synthesized via the Suzuki coupling reaction. For the purpose to fine-tune the molecular energy level of alternating copolymers and thus to improve charge transfer between polymers and PCBM, two different TQ derivatives substituted with strongly electron-donating butoxy group or weakly electron-donating thienyl group were synthesized and used as a building block of alternating copolymers. Copolymers with butoxy-substituted TQ have proper lowest unoccupied molecular orbital (LUMO) energy levels for effective charge dissociation between polymer and PCBM, whereas the LUMO levels of copolymers with thienyl-substituted TQ are too close to that of PCBM to be effective for charge dissociation. The power conversion efficiency was achieved up to 2.17%, which is the highest value among the TQ-based polymer solar cells, when the blend o...

Two different kinds of low-bandgap alternating copolymers consisting of 3-hexylthiophene and [1,2,5]thiadiazolo[3,4- g ]quinoxaline were synthesized via the Stille coupling reaction. The copolymers show broad absorption band from visible to infrared region. The bandgaps (1.02–1.35 eV) of the copolymers are successfully tuned by changing the substituent on the acceptor unit. The copolymers are highly soluble in common organic solvents due to three 3-hexylthiophenes in the repeating unit. By optimizing the device parameters such as the blend ratio of copolymer to PCBM and the choice of processing solvent, the power conversion efficiency reaches as high as 1.27% under the standard solar radiation condition (AM 1.5G, 100 mW/cm 2 ).

Magnetic tunnel junctions (MTJ) with the tunnel barrier oxidized in two steps with plasma were fabricated to obtain a structurally uniform AlOx insulator. Plasma oxidation of 10 A-thick Al layer for 5–20 sec formed the initial oxide barrier on top of which a second oxide layer was deposited by oxidizing 13 A-thick Al for 120 sec. The doubly oxidized junctions exhibited a magnetoresistance (MR) ratio of 27–31% depending on the oxidation period of the initial oxide layer while only 24% was obtained for the junction deposited in one-step oxidation with a similar stack configuration. The junction resistance of the MTJ increased monotonically with oxidation time while minimal deterioration of the MR ratio was observed when oxidation time increased from 5 sec to 20 sec. Transmission electron microscopy of the junctions also confirmed that the AlOx thickness was thinner for the doubly oxidized junctions compared to the singly oxidized MTJ. X-ray photoelectron spectroscopy of the double junction also strongly suggested that the initial oxide layer prevents over-oxidation of the bottom electrode. Our results suggest that the AlOx oxidized in two steps produces improved junction performance as well as a wider processing window.

Magnetic tunnel junctions consisting of Ta(50 A)/NiFe(50 A)/IrMn(150 A)/CoFe(50 A)/Al(13 A) –O/CoFe(40 A)/NiFe(400 A)/Ta(50 A) with a 100×100 μm2 junction area were prepared. The AlOx tunnel barrier was produced by oxidizing the 13 A thick Al metal using inductively coupled plasma (ICP) for 30–360 s and the ensuing junction properties were characterized as a function of oxidation time. It was found that a junction oxidized for 80 sec exhibited the highest magnetoresistance ratio, 30.3%, at room temperature. It was also shown that the junctions with an ICP oxidized tunnel barrier maintained the tunneling magnetoresistance ratio over 15% even when the insulator layer was oxidized for a prolonged period, well beyond the optimal oxidation time. The large processing window for the insulator oxidation was attributed to the dense amorphous AlOx structure formed by the ICP oxidation.

The structural and charge transport properties of a low bandgap copolymer poly(3-hexylthiophene -alt-6,7-dimethyl-4,9-bis-(4-hexylthien-2yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline) (P(3HT-MeTDQ)) and its blend with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are investigated. Thermal analysis, X-ray scattering diffraction (XRD), atomic force microscopy and transmission electron microscopy (TEM) of P(3HT-MeTDQ) reveal that the polymer is amorphous in solid state. As the hole mobility of P(3HT-MeTDQ) was measured by the time-of-flight photoconductivity method, the mobility was 3.35 × 10−4 cm2/V s, which is very comparable to that of semicrystalline poly(3-hexyl thiophene). When the mobility of amorphous P(3HT-MeTDQ) was analyzed according to the Gaussian disorder model, the polymer has the energetic and positional disorders with the values of σ = 62 meV and Σ = 1.7, respectively, indicating that the polymer has a relatively narrow Gaussian distribution of transport states. Interestingly, when P(3HT-MeTD...

For the purpose of synthesizing an electron donor polymer with a low-lying highest occupied molecular orbital (HOMO) energy level and thus to achieve high open circuit voltage (VOC) of polymer solar cells, an indolocarbazole derivative, 3,8-bis-(4-octyl-thiophene-2-yl)-5,6-didodecyl-thiadiazolo-indolo[3,2-b]carbazole, was synthesized and used as a building block of a low bandgap alternating copolymer. An alternating copolymer, poly(3,8-bis-(4-octyl-thiophene-2-yl)-5,6-didodecyl-thiadiazolo-indolo[3,2-b]carbazole-alt-2,1,3-benzothiadiazole) (PTICThBT), was synthesized via the Suzuki coupling reaction, and the copolymer exhibited a deep HOMO level of −5.4 eV. When a blend of PTICThBT and PC71BM was used as an active layer in bulk heterojunction polymer solar cells, a high VOC of 0.84 V was obtained with a power conversion efficiency of 2.62% under AM 1.5G condition.

A series of low-bandgap alternating copolymers consisting of electron-accepting quinoxaline derivatives and electron-donating carbazole or fluorene were synthesized via a Suzuki coupling reaction. For the purpose of improving the molecular packing of polymer chains and to enhance the charge carrier mobility in the packing direction, a new quinoxaline derivative, 10,13-bis-(4-octyl-thiophene-2-yl)-dibenzo[a,c]phenazine, which is expected to have planar polycyclic structure, was synthesized and introduced as a new building block of alternating copolymers instead of frequently-used 5,8-dithien-2-yl-2,3-diphenylquinoxaline. Copolymers with the planar quinoxaline derivative exhibited better optical and structural properties compared to copolymers with a less planar quinoxaline derivative. A power conversion efficiency was achieved up to 3.8% when the copolymer with planar quinoxaline derivative was blended with [6,6]-phenyl-C71-butyric acid methyl ester as an active layer material in bulk heterojunction solar cells.