Yong-Jin Pu

Addition of Lithium 8-Quinolate into Polyethylenimine Electron-Injection Layer in OLEDs: Not Only Reducing Driving Voltage but Also Improving Device Lifetime

Co-reporter:Takayuki Chiba, Yong-Jin Pu, Takahumi Ide, Satoru Ohisa, Hitoshi Fukuda, Tatsuya Hikichi, Dai Takashima, Tatsuya Takahashi, So Kawata, and Junji Kido

ACS Applied Materials & Interfaces May 31, 2017 Volume 9(Issue 21) pp:18113-18113

Publication Date(Web):May 12, 2017

DOI:10.1021/acsami.7b02658

Solution-processed electron injection layers (EILs) comprising lithium 8-quinolate (Liq) and polyethylenimine ethoxylated (PEIE) are highly effective for enhancing electron injection from ZnO to organic layers and improving device lifetime in organic light-emitting devices (OLEDs). Doping of Liq into PEIE further reduces the work function of zinc oxide (ZnO) by enhancing dipole formation. The intermolecular interaction between Liq and PEIE was elucidated by UV–vis absorption measurement and quantum chemical calculation. The OLEDs with ZnO covered with PEIE:Liq mixture exhibited lower driving voltage than that of the device without Liq. Furthermore, as doping concentration of Liq into PEIE increased, the device lifetime and voltage stability during constant current operation was successively improved.Keywords: electron injection; lithium phenolate complex; long lifetime; organic light-emitting device; polyethylenimine; zinc oxide;

Controlling the excited-state energy levels of 9,9′-bifluorenylidene derivatives by twisting their structure to attaining singlet fission character in organic photovoltaics

Co-reporter:So Kawata;Junki Furudate;Takuya Kimura;Hiroko Minaki;Ayaka Saito;Hiroshi Katagiri

Journal of Materials Chemistry C 2017 vol. 5(Issue 20) pp:4909-4914

Publication Date(Web):2017/05/25

DOI:10.1039/C7TC00315C

9,9′-Bifluorenylidene (BFN) derivatives, in which the two fluorene moieties are connected via a CC double bond at the 9-position, are expected to show singlet fission (SF) character in organic photovoltaics (OPVs). Due to steric repulsion between the hydrogen atoms at the 1,8- and 1′,8′-positions, the two fluorene planes of BFN are twisted by 31° with respect to each other. DFT calculations suggest that this deviation from coplanarity lowers the T1 energy level of BFN to approximately half that of S1. We synthesized a molecular alkylbithiophene-substituted BFN and a copolymer of diketopyrrolopyrrole and BFN. The optoelectronic properties of these compounds were investigated, and OPV devices using these BFNs as a p-type material were fabricated. The device composed of the polymeric BFN derivative achieved a power conversion efficiency of 4.9%. The observed negative dependence of the photocurrent on the magnetic field suggested that the triplet excitons of the molecular BFN derivatives contribute to the photocurrent in these OPV devices.

Influence of solution- and thermal-annealing processes on the sub-nanometer-ordered organic–organic interface structure of organic light-emitting devices

Co-reporter:S. Ohisa;Y.-J. Pu;N. L. Yamada;G. Matsuba;J. Kido

Nanoscale (2009-Present) 2017 vol. 9(Issue 1) pp:25-30

Publication Date(Web):2016/12/22

DOI:10.1039/C6NR06654B

Solution- and thermal-annealing processed organic–organic interface structures were investigated by neutron reflectometry. We revealed the true picture of interfaces, a polymer hole-transporting layer – a small molecule light-emitting layer – a small molecule electron-transporting layer, and discussed influences of those interface structures on organic light-emitting devices.

Dimethylsilyl-linked anthracene–pyrene dimers and their efficient triplet–triplet annihilation in organic light emitting diodes

Co-reporter:Min Jae Sung;Hiroya Chubachi;Ryo Sato;Min-Ki Shin;Soon-Ki Kwon;Yun-Hi Kim

Journal of Materials Chemistry C 2017 vol. 5(Issue 5) pp:1090-1094

Publication Date(Web):2017/02/02

DOI:10.1039/C6TC05308D

New deep blue emitters containing an sp3-hybridised tetrahedral silicon core with dimethyl groups, 9,10-biarylanthracene, and pyrene were synthesised. The pyrene group, with planar π-conjugation and a slightly larger energy gap than that of anthracene, is expected to work as an intramolecular host group in organic light emitting diodes (OLEDs). Density functional theory (DFT-B3LYP) and time-dependent DFT calculations for molecular orbitals and excited states of pyrene and anthracene units showed the possibility of intramolecular energy transfer and a triplet–triplet annihilation (TTA) process. The maximum external quantum efficiencies (EQEs) of non-doped devices using PA1N and PA2N were 4.7% and 4.8%, respectively, while the maximum EQEs of doped devices using PA1N and PA2N as dopants (3 wt%) were 4.5% and 3.6%, respectively. The EQE of the non-doped device with a low photoluminescence quantum yield (PLQY) (14%) was higher than that of the doped device with a high PLQY (74%), which resulted from the existence of a contribution reproducing radiative S1 excitons from nonradiative T1 excitons in the non-doped devices. Both non-doped and doped devices using PA1N and PA2N showed high color pure blue emission. [Commission Internationale de l'Eclairage coordinates, CIE (x, y), of the non-doped device were (0.16, 0.08) for PA1N and (0.15, 0.10) for PA2N.]

Singlet Fission of Non-polycyclic Aromatic Molecules in Organic Photovoltaics

Co-reporter:So Kawata;Ayaka Saito;Yuki Kurashige;Teruo Beppu;Hiroshi Katagiri;Masaki Hada;Junji Kido

Advanced Materials 2016 Volume 28( Issue 8) pp:1585-1590

Publication Date(Web):

DOI:10.1002/adma.201504281

A Solution-Processed Heteropoly Acid Containing MoO3 Units as a Hole-Injection Material for Highly Stable Organic Light-Emitting Devices

Co-reporter:Satoru Ohisa, Sho Kagami, Yong-Jin Pu, Takayuki Chiba, and Junji Kido

ACS Applied Materials & Interfaces 2016 Volume 8(Issue 32) pp:20946

Publication Date(Web):July 26, 2016

DOI:10.1021/acsami.6b06723

We report hole-injection layers (HILs) comprising a heteropoly acid containing MoO3 units, phosphomolybdic acid (PMA), in organic light-emitting devices (OLEDs). PMA possesses outstanding properties, such as high solubility in organic solvents, very low surface roughness in the film state, high transparency in the visible region, and an appropriate work function (WF), that make it suitable for HILs. We also found that these properties were dependent on the postbaking atmosphere and temperature after film formation. When the PMA film was baked in N2, the Mo in the PMA was reduced to Mo(V), whereas baking in air had no influence on the Mo valence state. Consequently, different baking atmospheres yielded different WF values. OLEDs with PMA HILs were fabricated and evaluated. OLEDs with PMA baked under appropriate conditions exhibited comparably low driving voltages and higher driving stability compared with OLEDs employing conventional hole-injection materials (HIMs), poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate), and evaporated MoO3, which clearly shows the high suitability of PMA HILs for OLEDs. PMA is also a commercially available and very cheap material, leading to the widespread use of PMA as a standard HIM.Keywords: gap state; heteropoly acid; MoO3; polyoxometalate; solution process

Solution-Processed White Phosphorescent Tandem Organic Light-Emitting Devices

Co-reporter:Takayuki Chiba;Junji Kido

Advanced Materials 2015 Volume 27( Issue 32) pp:4681-4687

Publication Date(Web):

DOI:10.1002/adma.201501866

Fabrication of Organic Light-Emitting Devices Comprising Stacked Light-Emitting Units by Solution-Based Processes

Co-reporter:Yong-Jin Pu;Takayuki Chiba;Kazushige Ideta;Shogo Takahashi;Naoya Aizawa;Tatsuya Hikichi ;Junji Kido

Advanced Materials 2015 Volume 27( Issue 8) pp:1327-1332

Publication Date(Web):

DOI:10.1002/adma.201403973

Solution-processable electron injection materials for organic light-emitting devices

Co-reporter:Takayuki Chiba, Yong-Jin Pu and Junji Kido

Journal of Materials Chemistry A 2015 vol. 3(Issue 44) pp:11567-11576

Publication Date(Web):04 Sep 2015

DOI:10.1039/C5TC02421H

Solution-processed organic light-emitting devices (OLEDs) have progressed as potential candidates for cost-effective solid-state lighting and flat panel displays. In this highlight, we focus on the recent progress of the state-of-the-art solution-processable electron injection materials: (i) alkali metal-containing compounds, (ii) n-type semiconducting metal oxides, (iii) π-conjugated ionic polymers, and (iv) nonionic polymers. These materials are soluble in water, alcohol, or a water–alcohol mixture solvent and can be formed into a film by a solution process. We discuss the essential characteristics of these electron injection materials and the performance of the solution-processed OLEDs made using them.

Efficient Electron Injection by Size- and Shape-Controlled Zinc Oxide Nanoparticles in Organic Light-Emitting Devices

Co-reporter:Yong-Jin Pu, Norito Morishita, Takayuki Chiba, Satoru Ohisa, Masahiro Igarashi, Akito Masuhara, and Junji Kido

ACS Applied Materials & Interfaces 2015 Volume 7(Issue 45) pp:25373

Publication Date(Web):October 29, 2015

DOI:10.1021/acsami.5b07742

Three different sized zinc oxide (ZnO) nanoparticles were synthesized as spherical ZnO (S-ZnO), rodlike ZnO (R-ZnO), and intermediate shape and size ZnO (I-ZnO) by controlling the reaction time. The average sizes of the ZnO nanoparticles were 4.2 nm × 3.4 nm for S-ZnO, 9.8 nm × 4.5 nm for I-ZnO, and 20.6 nm × 6.2 nm for R-ZnO. Organic light-emitting devices (OLEDs) with these ZnO nanoparticles as the electron injection layer (EIL) were fabricated. The device with I-ZnO showed lower driving voltage and higher power efficiency than those with S-ZnO and R-ZnO. The superiority of I-ZnO makes it very effective as an EIL for various types of OLEDs regardless of the deposition order or method of fabricating the organic layer, the ZnO layer, and the electrode.Keywords: electron injection; nanoparticles; organic light-emitting device; solution process; zinc oxide

Molecular Interdiffusion between Stacked Layers by Solution and Thermal Annealing Processes in Organic Light Emitting Devices

Co-reporter:Satoru Ohisa, Yong-Jin Pu, Norifumi L. Yamada, Go Matsuba, and Junji Kido

ACS Applied Materials & Interfaces 2015 Volume 7(Issue 37) pp:20779

Publication Date(Web):September 2, 2015

DOI:10.1021/acsami.5b05818

In organic light emitting devices (OLEDs), interfacial structures between multilayers have large impacts on the characteristics of OLEDs. Herein, we succeeded in revealing the interdiffusion in solution processed and thermal annealed OLEDs by neutron reflectometry. We investigated interfaces between a polymer under layer and small molecules upper layer. The small molecules diffused into the swollen polymer layer during the interfacial formation by the solution process, but the polymer did not diffuse into the small molecules layer. At temperatures close to the glass transition temperatures of the materials, asymmetric molecular diffusion was observed. We elucidated the effects of the interdiffusion on the characteristics of OLEDs. Partially mixing the interface improved the current efficiencies due to suppressed triplet-polaron quenching at the interface. Controlling and understanding the interfacial structures of the miultilayers will be more important to improve the OLED characteristics.Keywords: asymmetric diffusion; charge accumulation; interface; neutron reflectometry; poly-TPD; thermal diffusion

Instant Low-Temperature Cross-Linking of Poly(N-vinylcarbazole) for Solution-Processed Multilayer Blue Phosphorescent Organic Light-Emitting Devices

Co-reporter:Naoya Aizawa;Takayuki Chiba;So Kawata;Hisahiro Sasabe ;Junji Kido

Advanced Materials 2014 Volume 26( Issue 45) pp:7543-7546

Publication Date(Web):

DOI:10.1002/adma.201402726

Lithium Phenolate Complexes with a Pyridine-Containing Polymer for Solution-Processable Electron Injection Layers in PLEDs

Co-reporter:Takayuki Chiba;Shogo Takahashi;Hisahiro Sasabe ;Junji Kido

Advanced Functional Materials 2014 Volume 24( Issue 38) pp:6038-6045

Publication Date(Web):

DOI:10.1002/adfm.201401060

A series of (vinylphenyl)pyridine-based polymer binders, PVPh2Py, PVPh3Py, and PVPh4Py, are designed and synthesized and it is found that mixtures of Liq and the polymers exhibit superior electron injection characteristics as ultrathin (1.6 nm) electron injection layer (EIL) films. They are comparable to those of EILs composed only of Liq. The addition of the polymers does not deteriorate the performance of Liq EILs. Additionally, when the EIL thickness is increased from 1.6 nm to 16 nm, the driving voltages increase and the external quantum efficiencies decrease. The increase in the voltage and decrease in the EQE are suppressed in the device with mixed EILs compared to those observed for the device composed of 100 wt% Liq. Furthermore, the position of the nitrogen in the pyridine ring is considered to influence the electron transport properties of the EILs. The mixing PVPh4Py with Liq improves the driving voltage of the fabricated devices, even with a thick mixed EIL. This reduced dependence of the performance of EILs on their thickness will be advantageous for the coating of large areas using solution processes.

Bisanthracene-Based DonorAcceptor-type Light-Emitting Dopants: Highly Efficient Deep-Blue Emission in Organic Light-Emitting Devices

Co-reporter:Jian-Yong Hu;Fumiya Satoh;So Kawata;Hiroshi Katagiri;Hisahiro Sasabe ;Junji Kido

Advanced Functional Materials 2014 Volume 24( Issue 14) pp:2064-2071

Publication Date(Web):

DOI:10.1002/adfm.201302907

Deep-blue fluorescent compounds are particularly important in organic light-emitting devices (OLEDs). A donor–accepotor (DA)-type blue-emitting compound, 1-(10-(4-methoxyphenyl)anthracen-9-yl)-4-(10-(4-cyanophenyl)anthracen-9-yl)benzene (BD3), is synthesized, and for comparison, a nonDA-type compound, 1,4-bis(10-phenylanthracene-9-yl)benzene (BD1) and a weak DA-type compound, 1-(10-phenylanthracen-9-yl)-4-(10-(4-cyanophenyl)anthracen-9-yl)-benzene (BD2), are also synthesized. The twisted conformations of the two anthracene units in the compounds, confirmed by single crystal X-ray analysis, effectively prevent π-conjugation, and the compound shows deep-blue photoluminescence (PL) with a high PL quantum efficiency, almost independent of the solvent polarity, resulting from the absence of an intramolecular charge transfer state. The DA-type molecule BD3 in a non-doped device exhibits a maximum external quantum efficiency (EQE) of 4.2% with a slight roll-off, indicating good charge balance due to the DA-type molecular design. In the doped device with 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host, the BD3 exhibits higher EQE than 10% with Commission International de L'Eclairge (CIE) coordinates of (0.15, 0.06) and a narrow full-width at half-maximum of 45 nm, which is close to the CIE of the high definition television standard blue.

Precise Evaluation of Angstrom-Ordered Mixed Interfaces in Solution-Processed OLEDs by Neutron Reflectometry

Co-reporter:Satoru Ohisa;Go Matsuba;Norifumi L. Yamada;Hisahiro Sasabe;Junji Kido

Advanced Materials Interfaces 2014 Volume 1( Issue 9) pp:

Publication Date(Web):

DOI:10.1002/admi.201400097

Excimer-emitting single molecules with stacked π-conjugated groups covalently linked at the 1,8-positions of naphthalene for highly efficient blue and green OLEDs

Co-reporter:Jian-Yong Hu, Yong-Jin Pu, Yusuke Yamashita, Fumiya Satoh, So Kawata, Hiroshi Katagiri, Hisahiro Sasabe and Junji Kido

Journal of Materials Chemistry A 2013 vol. 1(Issue 24) pp:3871-3878

Publication Date(Web):19 Apr 2013

DOI:10.1039/C3TC30319E

Two efficient blue-light-emitting compounds, 1,8-bis(4-(N-carbazolyl)phenyl)naphthalene (BCzPN) and 1,8-bis(4-(10-phenylanthracen-9-yl)-phenyl)naphthalene (BPAPN), are designed and synthesized, in which two phenylcarbazole or diphenylanthracene units are closely stacked through bonding to the 1- and 8-positions of the naphthalene ring, resulting in strong intramolecular excimer emissions in solution or as a film. By utilizing BPAPN as an emitter, high efficiencies of 6 cd A−1 and 5.8% external quantum efficiency (EQE) at 100 cd m−2, and 8 cd A−1 and 5.8% EQE at 1000 cd m−2 are achieved in a non-doped blue device. By using BCzPN or BPAPN as a host, a DPAVBi-doped BCzPN based blue device gave high efficiencies of 15 cd A−1 and 6.5% EQE at 100 cd m−2, and 12 cd A−1 and 5.5% EQE at 1000 cd m−1, and a C545T-doped BPAPN based green device gave high efficiencies of 23 cd A−1 and 6.7% EQE at 100 cd m−2, and 22 cd A−1 and 6.7% EQE at 1000 cd m−2, respectively.

Thermally cross-linkable host materials for enabling solution-processed multilayer stacks in organic light-emitting devices

Co-reporter:Naoya Aizawa, Yong-Jin Pu, Hisahiro Sasabe, Junji Kido

Organic Electronics 2013 Volume 14(Issue 6) pp:1614-1620

Publication Date(Web):June 2013

DOI:10.1016/j.orgel.2013.03.028

•A novel cross-linkable host material, DV-CBP, for use in solution-processed OLEDs.•DV-CBP was thermally cross-linked by curing at approx. 180 °C without initiators.•The PLQEs of the emissive layer were maintained above 75% throughout the reaction.•An additional layer was deposited on top of the cross-linked layer by spin-coating.•The multilayer device showed superior performance to Ca-based single layer devices.A thermally cross-linkable host material, i.e., two vinylbenzyl ether groups containing a carbazole derivative (DV-CBP), was developed for solution-processed multilayer organic light-emitting devices (OLEDs). DV-CBP was thermally cross-linked at styrene end-groups through curing at approximately 180 °C in the absence of a polymerization initiator. This cross-linking reaction rendered the emissive layer insoluble and enabled the subsequent solution deposition of an upper electron-transporting layer. Furthermore, photoluminescence quantum efficiencies of the emissive layer were maintained at greater than 75% throughout the cross-linking reaction. A solution-processed small-molecule electron-transporting layer on top of the cross-linked emissive layer led to lower driving voltages and higher efficiencies in the OLEDs compared to those of a device with a vacuum-deposited Ca electrode on the emissive layer.Graphical abstract

Optimizing the Charge Balance of Fluorescent Organic Light-Emitting Devices to Achieve High External Quantum Efficiency Beyond the Conventional Upper Limit

Co-reporter:Yong-Jin Pu;Go Nakata;Fumiya Satoh;Hisahiro Sasabe;Daisuke Yokoyama ;Junji Kido

Advanced Materials 2012 Volume 24( Issue 13) pp:1765-1770

Publication Date(Web):

DOI:10.1002/adma.201104403

Solution-processed organic light-emitting devices with two polymer light-emitting units connected in series by a charge-generation layer

Co-reporter:Takayuki Chiba, Yong-Jin Pu, Hisahiro Sasabe, Junji Kido and Yang Yang

Journal of Materials Chemistry A 2012 vol. 22(Issue 42) pp:22769-22773

Publication Date(Web):07 Sep 2012

DOI:10.1039/C2JM35344J

We present a solution-based process to fabricate stacked OLEDs consisting of two polymer light-emitting units (LEUs), connected in series by a charge generation layer (CGL). We used Cs2CO3-doped ZnO nanoparticles as an EIL on the LE-polymer to improve the electron injection from the cathode. The surface morphology of a spin-coated metal oxide nanoparticle appears to be rough, with many gaps due to agglutination of nanoparticles. We chose poly(4-vinyl pyridine) (PVPy) as a binder to improve the film morphology of the ZnO:Cs2CO3 mixture and facilitate the formation of a uniform and dense film to prevent the solvent from soaking into the 1st LEU. The efficient solution-based processing of EILs in the 1st CGL containing MoO3/poly-TPD bilayers was employed for the construction of an MPE device. The device exhibited a sum current efficiency of 10 cd A−1, with 4 cd A−1 contributed by the 1st unit and 6 cd A−1 by the 2nd unit.

Solution-Processed Inorganic–Organic Hybrid Electron Injection Layer for Polymer Light-Emitting Devices

Co-reporter:Takayuki Chiba, Yong-Jin Pu, Masakatsu Hirasawa, Akito Masuhara, Hisahiro Sasabe, and Junji Kido

ACS Applied Materials & Interfaces 2012 Volume 4(Issue 11) pp:6104

Publication Date(Web):October 22, 2012

DOI:10.1021/am301732m

A lithium quinolate complex (Liq) has high solubility in polar solvents such as alcohols and can be spin-coated onto emitting polymers, resulting in a smooth surface morphology. A polymer light-emitting device fabricated with spin-coated Liq as an electron injection layer (EIL) exhibited a lower turn-on voltage and a higher efficiency than a device with spin-coated Cs2CO3 and a device with thermally evaporated Ca. The mixture of ZnO nanoparticles and Liq served as an efficient EIL, resulting in a lower driving voltage even in thick films (∼10 nm), and it did not require a high-temperature annealing process.Keywords: electron injection; inorganic−organic hybrid; lithium quinolate; polymer light-emitting device; solution process; ZnO nanoparticles;

A single-molecule excimer-emitting compound for highly efficient fluorescent organic light-emitting devices

Co-reporter:Jian-Yong Hu, Yong-Jin Pu, Go Nakata, So Kawata, Hisahiro Sasabe and Junji Kido

Chemical Communications 2012 vol. 48(Issue 67) pp:8434-8436

Publication Date(Web):16 Jul 2012

DOI:10.1039/C2CC33463A

A pyrene-containing single-molecule excimer-emitting compound, 1,8-bis(pyren-2-yl)naphthalene (BPyN), was synthesized. With BPyN as a host emitter, C545T-based green OLEDs were fabricated, exhibiting high efficiencies of 22 lm W−1, 22 cd A−1 and 6.2% external quantum efficiency (EQE) at 100 cd m−2, and 19 lm W−1, 22 cd A−1 and 6.2% EQE at 1000 cd m−2.

Solution-processable carbazole-based host materials for phosphorescent organic light-emitting devices

Co-reporter:Naoya Aizawa, Yong-Jin Pu, Hisahiro Sasabe, Junji Kido

Organic Electronics 2012 Volume 13(Issue 11) pp:2235-2242

Publication Date(Web):November 2012

DOI:10.1016/j.orgel.2012.06.036

In this study, solution-processable carbazole-type host materials, 1,3-bis(3-(3,6-di-n-butylcarbazol-9-yl)phenyl)benzene (BCzPPh) and 4,6-bis(3-(3,6-di-n-butylcarbazol-9-yl)phenyl)pyrimidine (BCzPPm), were synthesized for use in phosphorescent organic light-emitting devices (OLEDs). Both host materials possess a high solubility in common organic solvents and high triplet energy to confine excitons to the phosphorescent emitter. The two nitrogen atoms in the central pyrimidine ring of BCzPPm have a profound effect on the photoluminescence properties and the electron-accepting capability. When doped with the green phosphorescent emitter tris(2-(4-tolyl)phenylpyridine)iridium (III), BCzPPh exhibited power efficiencies and external quantum efficiencies above 30 lm/W and 13%, respectively, in a simple bilayer OLED.Graphical abstractHighlights► The carbazole hosts showed a high triplet energy to confine triplet excitons. ► The pyrimidine ring gave the electron-accepting capability to the compound. ► The power efficiencies were higher than 30 lm/W in a simple bilayer OLED.

fac-Tris(2-phenylpyridine)iridium (III)s, covalently surrounded by six bulky host dendrons, for a highly efficient solution-processed organic light emitting device

Co-reporter:Yong-Jin Pu, Noriaki Iguchi, Naoya Aizawa, Hisahiro Sasabe, Ken-ichi Nakayama, Junji Kido

Organic Electronics 2011 Volume 12(Issue 12) pp:2103-2110

Publication Date(Web):December 2011

DOI:10.1016/j.orgel.2011.08.015

The fully surrounded complexes by six host dendrons showed high photoluminescence quantum efficiency in a neat film, comparable to in a dilute solution. The surrounding host dendrons efficiently suppressed intermolecular interaction between central Ir complexes and prevented concentration quenching. The complex, (mCP)6Ir, fully surrounded by six carbazole type hosts showed much better performance, compared with the complex, (DAP)6Ir, surrounded by arylamine type hosts, because of well balanced charge injection and transporting in the devices.Graphical abstractHighlights► The dendrimer complexes showed high photoluminescence quantum efficiency. ► Intermolecular interaction between central Ir complexes was suppressed. ► The carbazole complex showed much better OLED performance than the arylamine complex. ► Well balanced charge injection and transporting in the devices are important.

Ultra-high efficiency by multiple emission from stacked organic light-emitting devices

Co-reporter:Takayuki Chiba, Yong-Jin Pu, Ryoichi Miyazaki, Ken-ichi Nakayama, Hisahiro Sasabe, Junji Kido

Organic Electronics 2011 Volume 12(Issue 4) pp:710-715

Publication Date(Web):April 2011

DOI:10.1016/j.orgel.2011.01.022

We investigated the charge generation characteristics of intermediate layer, consisting of an organic acceptor material 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN6) and an organic donor material N,N′-di(naphthalene-1-yl)-N,N′-diphenylbenzidine (NPD), for a stacked organic light-emitting device (OLED). A stack consisting of an ultra-thin 1 nm of LiF and of Al was used as an electron-injection layer (EIL) from HAT-CN6 to an adjacent electron transporting layer (ETL). The orange-emitting fluorescent stacked OLED with the charge generation layer and the electron injection layer exhibited twice higher current efficiencies and a longer operational lifetime than those of the corresponding unstacked device under a high luminance. We also fabricated a green phosphorescent OLED showing an extremely high current efficiency of 256 cd/A.Graphical abstractResearch highlights► Multi-stacked OLEDs with vertically stacked light-emitting units were fabricated. ► Charge-generating layer (CGL) consists of HAT-CN6 and the organic donor materials. ► Green PHOLED with the CGL showed an extremely high current efficiency of 250 cd/A. ► Orange OLED exhibited a long operational lifetime with a high luminance.

Bifluorene compounds containing carbazole and/or diphenylamine groups and their bipolar charge transport properties in organic light emitting devices

Co-reporter:Keiji Noine, Yong-Jin Pu, Ken-ichi Nakayama, Junji Kido

Organic Electronics 2010 Volume 11(Issue 5) pp:717-723

Publication Date(Web):May 2010

DOI:10.1016/j.orgel.2010.01.010

Novel bipolar bifluorene compounds containing carbazole and/or diphenylamine groups were synthesized by C–C and C–N coupling using a palladium catalyst. The ionization potentials of the compounds reflected the electron withdrawing or donating nature of the substituents. The charge transport properties of these compounds were evaluated from time-of-flight transient photocurrent measurements. Large transient currents (10−4–10−3 cm2/Vs) based on holes and electrons were observed. Hole or electron-only devices containing these compounds with p-type doping with MoO3 and n-type with Cs showed Ohmic current density–voltage characteristics. Organic light emitting devices with homo-junction structures containing the bifluorene compounds (ITO/bifluorene:MoO3 (50 mol%, 20 nm)/bifluorene (10 nm)/bifluorene:rubrene (60 nm)/bifluorene (10 nm)/bifluorene:Cs (30 wt.%, 20 nm)/Al) exhibited external quantum efficiencies of 1.5–2.0%.

Dipyrenylpyridines for electron-transporting materials in organic light emitting devices and their structural effect on electron injection from LiF/Al cathode

Co-reporter:Yong-Jin Pu, Makoto Yoshizaki, Takahiro Akiniwa, Ken-ichi Nakayama, Junji Kido

Organic Electronics 2009 Volume 10(Issue 5) pp:877-882

Publication Date(Web):August 2009

DOI:10.1016/j.orgel.2009.04.020

3,5-Dipyrenylpyridine (PY1) and 2,6-dipyrenylpyridine (PY2) were synthesized to achieve not only efficient electron injection from cathode but also high electron mobility. The both of compounds showed much higher electron mobilities than that of Alq3, and have a similar ionization potential and electron affinity. However, the barrier height of electron injection from cathode to PY1 in an organic light emitting device was much smaller than that to PY2, probably due to the steric hindrance to the central pyridine group. These results suggest that the chemical affinity of electron-transporting materials with cathode is more important than their own electron affinity to improve the electron injection.

Lithium phenolate complexes for an electron injection layer in organic light-emitting diodes

Co-reporter:Yong-Jin Pu, Masashi Miyamoto, Ken-ichi Nakayama, Toshiro Oyama, Yokoyama Masaaki, Junji Kido

Organic Electronics 2009 Volume 10(Issue 2) pp:228-232

Publication Date(Web):April 2009

DOI:10.1016/j.orgel.2008.11.003

We synthesized π-conjugated lithium phenolate complexes, lithium 2-(2-pyridyl)phenolate (LiPP), lithium 2-(2′, 2′′-bipyridine-6′-yl)phenolate (LiBPP), and lithium 2-(isoquinoline-1′-yl)phenolate (LiIQP). These complexes showed lower sublimation temperatures of 305–332 °C compared to 717 °C of LiF. The organic light-emitting devices (OLEDs) using these complexes as an electron injection layer exhibited high efficiencies which are comparable to that of the device using LiF. Especially, a 40-nm thick film of LiBPP or LiPP was effective as an electron injection material, providing low driving voltages, while such a thick film of LiF serves as a complete insulator, resulting in high driving voltages.

Solution-processable organic fluorescent dyes for multicolor emission in organic light emitting diodes

Co-reporter:Yong-Jin Pu, Makoto Higashidate, Ken-ichi Nakayama and Junji Kido

Journal of Materials Chemistry A 2008 vol. 18(Issue 35) pp:4183-4188

Publication Date(Web):24 Jul 2008

DOI:10.1039/B806160B

Four novel fluorescent dyes, bis(difluorenyl)amino-substituted carbazole 1, pyrene 2, perylene 3, and benzothiadiazole 4, were synthesized by C–N cross-coupling with a palladium catalyst. These dyes are soluble in common organic solvents, and their uniform films were formed by spin-coating from their solutions. Their glass transition temperatures were sufficiently high (120–181 °C) to form amorphous films for organic light emitting diodes. These solution processable dyes exhibited strong photoluminescence (PL) in the film form (1: sky blue, 2: blue-green, 3: yellow, and 4: deep red). Optical and electrochemical properties of the compounds were investigated with photoelectron spectroscopy and cyclic voltammetry. The energy levels obtained from both measurements were in good agreement, and those levels were related to the electronic properties of the central core; the electron-donating carbazole compound showed the lowest ionization potential and the electron-withdrawing benzothiadiazole compound showed the largest electron affinity. Simple double layer devices were prepared with these fluorescent dyes as emitting layer and bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminium(III) (BAlq) as a common hole blocking layer for each color. Electroluminescence colors were the same as those of the PL spectra in each compound. These multicolor electroluminescences show that these conjugated oligomers can be candidates for solution processable light emitting materials for OLEDs as well as conjugated polymers or dendrimers.

Solution-processable electron injection materials for organic light-emitting devices

Co-reporter:Takayuki Chiba, Yong-Jin Pu and Junji Kido

Journal of Materials Chemistry A 2015 - vol. 3(Issue 44) pp:NaN11576-11576

Publication Date(Web):2015/09/04

DOI:10.1039/C5TC02421H

Solution-processed organic light-emitting devices (OLEDs) have progressed as potential candidates for cost-effective solid-state lighting and flat panel displays. In this highlight, we focus on the recent progress of the state-of-the-art solution-processable electron injection materials: (i) alkali metal-containing compounds, (ii) n-type semiconducting metal oxides, (iii) π-conjugated ionic polymers, and (iv) nonionic polymers. These materials are soluble in water, alcohol, or a water–alcohol mixture solvent and can be formed into a film by a solution process. We discuss the essential characteristics of these electron injection materials and the performance of the solution-processed OLEDs made using them.

Excimer-emitting single molecules with stacked π-conjugated groups covalently linked at the 1,8-positions of naphthalene for highly efficient blue and green OLEDs

Co-reporter:Jian-Yong Hu, Yong-Jin Pu, Yusuke Yamashita, Fumiya Satoh, So Kawata, Hiroshi Katagiri, Hisahiro Sasabe and Junji Kido

Journal of Materials Chemistry A 2013 - vol. 1(Issue 24) pp:NaN3878-3878

Publication Date(Web):2013/04/19

DOI:10.1039/C3TC30319E

Two efficient blue-light-emitting compounds, 1,8-bis(4-(N-carbazolyl)phenyl)naphthalene (BCzPN) and 1,8-bis(4-(10-phenylanthracen-9-yl)-phenyl)naphthalene (BPAPN), are designed and synthesized, in which two phenylcarbazole or diphenylanthracene units are closely stacked through bonding to the 1- and 8-positions of the naphthalene ring, resulting in strong intramolecular excimer emissions in solution or as a film. By utilizing BPAPN as an emitter, high efficiencies of 6 cd A−1 and 5.8% external quantum efficiency (EQE) at 100 cd m−2, and 8 cd A−1 and 5.8% EQE at 1000 cd m−2 are achieved in a non-doped blue device. By using BCzPN or BPAPN as a host, a DPAVBi-doped BCzPN based blue device gave high efficiencies of 15 cd A−1 and 6.5% EQE at 100 cd m−2, and 12 cd A−1 and 5.5% EQE at 1000 cd m−1, and a C545T-doped BPAPN based green device gave high efficiencies of 23 cd A−1 and 6.7% EQE at 100 cd m−2, and 22 cd A−1 and 6.7% EQE at 1000 cd m−2, respectively.

Solution-processed organic light-emitting devices with two polymer light-emitting units connected in series by a charge-generation layer

Co-reporter:Takayuki Chiba, Yong-Jin Pu, Hisahiro Sasabe, Junji Kido and Yang Yang

Journal of Materials Chemistry A 2012 - vol. 22(Issue 42) pp:NaN22773-22773

Publication Date(Web):2012/09/07

DOI:10.1039/C2JM35344J

We present a solution-based process to fabricate stacked OLEDs consisting of two polymer light-emitting units (LEUs), connected in series by a charge generation layer (CGL). We used Cs2CO3-doped ZnO nanoparticles as an EIL on the LE-polymer to improve the electron injection from the cathode. The surface morphology of a spin-coated metal oxide nanoparticle appears to be rough, with many gaps due to agglutination of nanoparticles. We chose poly(4-vinyl pyridine) (PVPy) as a binder to improve the film morphology of the ZnO:Cs2CO3 mixture and facilitate the formation of a uniform and dense film to prevent the solvent from soaking into the 1st LEU. The efficient solution-based processing of EILs in the 1st CGL containing MoO3/poly-TPD bilayers was employed for the construction of an MPE device. The device exhibited a sum current efficiency of 10 cd A−1, with 4 cd A−1 contributed by the 1st unit and 6 cd A−1 by the 2nd unit.

Solution-processable organic fluorescent dyes for multicolor emission in organic light emitting diodes

Co-reporter:Yong-Jin Pu, Makoto Higashidate, Ken-ichi Nakayama and Junji Kido

Journal of Materials Chemistry A 2008 - vol. 18(Issue 35) pp:NaN4188-4188

Publication Date(Web):2008/07/24

DOI:10.1039/B806160B

Four novel fluorescent dyes, bis(difluorenyl)amino-substituted carbazole 1, pyrene 2, perylene 3, and benzothiadiazole 4, were synthesized by C–N cross-coupling with a palladium catalyst. These dyes are soluble in common organic solvents, and their uniform films were formed by spin-coating from their solutions. Their glass transition temperatures were sufficiently high (120–181 °C) to form amorphous films for organic light emitting diodes. These solution processable dyes exhibited strong photoluminescence (PL) in the film form (1: sky blue, 2: blue-green, 3: yellow, and 4: deep red). Optical and electrochemical properties of the compounds were investigated with photoelectron spectroscopy and cyclic voltammetry. The energy levels obtained from both measurements were in good agreement, and those levels were related to the electronic properties of the central core; the electron-donating carbazole compound showed the lowest ionization potential and the electron-withdrawing benzothiadiazole compound showed the largest electron affinity. Simple double layer devices were prepared with these fluorescent dyes as emitting layer and bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminium(III) (BAlq) as a common hole blocking layer for each color. Electroluminescence colors were the same as those of the PL spectra in each compound. These multicolor electroluminescences show that these conjugated oligomers can be candidates for solution processable light emitting materials for OLEDs as well as conjugated polymers or dendrimers.

Controlling the excited-state energy levels of 9,9′-bifluorenylidene derivatives by twisting their structure to attaining singlet fission character in organic photovoltaics

Co-reporter:So Kawata, Junki Furudate, Takuya Kimura, Hiroko Minaki, Ayaka Saito, Hiroshi Katagiri and Yong-Jin Pu

Journal of Materials Chemistry A 2017 - vol. 5(Issue 20) pp:NaN4914-4914

Publication Date(Web):2017/04/26

DOI:10.1039/C7TC00315C

9,9′-Bifluorenylidene (BFN) derivatives, in which the two fluorene moieties are connected via a CC double bond at the 9-position, are expected to show singlet fission (SF) character in organic photovoltaics (OPVs). Due to steric repulsion between the hydrogen atoms at the 1,8- and 1′,8′-positions, the two fluorene planes of BFN are twisted by 31° with respect to each other. DFT calculations suggest that this deviation from coplanarity lowers the T1 energy level of BFN to approximately half that of S1. We synthesized a molecular alkylbithiophene-substituted BFN and a copolymer of diketopyrrolopyrrole and BFN. The optoelectronic properties of these compounds were investigated, and OPV devices using these BFNs as a p-type material were fabricated. The device composed of the polymeric BFN derivative achieved a power conversion efficiency of 4.9%. The observed negative dependence of the photocurrent on the magnetic field suggested that the triplet excitons of the molecular BFN derivatives contribute to the photocurrent in these OPV devices.

A single-molecule excimer-emitting compound for highly efficient fluorescent organic light-emitting devices

Co-reporter:Jian-Yong Hu, Yong-Jin Pu, Go Nakata, So Kawata, Hisahiro Sasabe and Junji Kido

Chemical Communications 2012 - vol. 48(Issue 67) pp:NaN8436-8436

Publication Date(Web):2012/07/16

DOI:10.1039/C2CC33463A

A pyrene-containing single-molecule excimer-emitting compound, 1,8-bis(pyren-2-yl)naphthalene (BPyN), was synthesized. With BPyN as a host emitter, C545T-based green OLEDs were fabricated, exhibiting high efficiencies of 22 lm W−1, 22 cd A−1 and 6.2% external quantum efficiency (EQE) at 100 cd m−2, and 19 lm W−1, 22 cd A−1 and 6.2% EQE at 1000 cd m−2.

Dimethylsilyl-linked anthracene–pyrene dimers and their efficient triplet–triplet annihilation in organic light emitting diodes

Co-reporter:Min Jae Sung, Hiroya Chubachi, Ryo Sato, Min-Ki Shin, Soon-Ki Kwon, Yong-Jin Pu and Yun-Hi Kim

Journal of Materials Chemistry A 2017 - vol. 5(Issue 5) pp:NaN1094-1094

Publication Date(Web):2016/12/30

DOI:10.1039/C6TC05308D

New deep blue emitters containing an sp3-hybridised tetrahedral silicon core with dimethyl groups, 9,10-biarylanthracene, and pyrene were synthesised. The pyrene group, with planar π-conjugation and a slightly larger energy gap than that of anthracene, is expected to work as an intramolecular host group in organic light emitting diodes (OLEDs). Density functional theory (DFT-B3LYP) and time-dependent DFT calculations for molecular orbitals and excited states of pyrene and anthracene units showed the possibility of intramolecular energy transfer and a triplet–triplet annihilation (TTA) process. The maximum external quantum efficiencies (EQEs) of non-doped devices using PA1N and PA2N were 4.7% and 4.8%, respectively, while the maximum EQEs of doped devices using PA1N and PA2N as dopants (3 wt%) were 4.5% and 3.6%, respectively. The EQE of the non-doped device with a low photoluminescence quantum yield (PLQY) (14%) was higher than that of the doped device with a high PLQY (74%), which resulted from the existence of a contribution reproducing radiative S1 excitons from nonradiative T1 excitons in the non-doped devices. Both non-doped and doped devices using PA1N and PA2N showed high color pure blue emission. [Commission Internationale de l'Eclairage coordinates, CIE (x, y), of the non-doped device were (0.16, 0.08) for PA1N and (0.15, 0.10) for PA2N.]

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