Takeshi Komino

Purely organic electroluminescent material realizing 100% conversion from electricity to light

Efficient organic light-emitting diodes have been developed using emitters containing rare metals, such as platinum and iridium complexes. However, there is an urgent need to develop emitters composed of more abundant materials. Here we show a thermally activated delayed fluorescence material for organic light-emitting diodes, which realizes both approximately 100% photoluminescence quantum yield and approximately 100% up-conversion of the triplet to singlet excited state. The material contains electron-donating diphenylaminocarbazole and electron-accepting triphenyltriazine moieties. The typical trade-off between effective emission and triplet-to-singlet up-conversion is overcome by fine-tuning the highest occupied molecular orbital and lowest unoccupied molecular orbital distributions. The nearly zero singlet–triplet energy gap, smaller than the thermal energy at room temperature, results in an organic light-emitting diode with external quantum efficiency of 29.6%. An external quantum efficiency of 41.5% is obtained when using an out-coupling sheet. The external quantum efficiency is 30.7% even at a high luminance of 3,000 cd m−2.

Highly efficient bulk heterojunction photovoltaic cell based on tris(4-(5- phenylthiophen-2-yl)phenyl)amine and C70 combined with optimized electron transport layer

Efficient bulk heterojunction (BHJ) photovoltaic cells (PVs) based on 5 wt. % donors and C70 were fabricated. Tris[4-(5-phenylthiophen-2-yl)phenyl]-amine (TPTPA)-based BHJ PVs show higher power conversion efficiency (ηPCE) than aluminum phthalocyanine chloride-based BHJ PVs. Although the absorption of AlPcCl is complementary to that of C70, TPTPAs high hole mobility and symmetrical molecular structure are likely to be crucial contributing factors to the higher ηPCE. Phase separation occurs in the 5%-TPTPA blend. The device was optimized via replacement of the bathocuproine buffer by a combination of 3,4,9,10-perylenetetracarboxylic bis-benzimidazole and bathocuproine. ηPCE of 5.96% is achieved because of the decreased series resistance.

Horizontal molecular orientation in solution-processed organic light-emitting diodes

Horizontal orientation of the emission transition dipole moments achieved in glassy vapor-deposited organic thin films leads to an enhancement of the light out-coupling efficiency in organic light-emitting diodes (OLEDs). Here, our combined study of variable angle spectroscopic ellipsometry and angle dependent photoluminescence demonstrates that such a horizontal orientation can be achieved in glassy spin-coated organic films based on a composite blend of a heptafluorene derivative as a dopant and a 4,4′-bis(N-carbazolyl)-1,1′-biphenyl as a host. Solution-processed fluorescent OLEDs with horizontally oriented heptafluorene emitters were then fabricated and emitted deep blue electroluminescence with an external quantum efficiency as high as 5.3%.

Tunable and flexible solvent-free liquid organic distributed feedback lasers

We report on optically pumped blue, green, and red liquid organic distributed feedback (DFB) lasers based on solvent-free fluidic organic semiconductors, and prepared on highly flexible corrugated polymeric patterns. By the appropriate selection of laser dyes doping a liquid 9-(2-ethylhexyl)carbazole host, the lasing wavelength is effectively tuned across the visible spectrum via a cascade energy transfer scheme. We also demonstrate a mechanical tunability of the flexible liquid DFB laser emission, which is due to the deformation of the high-aspect ratio DFB grating under bending. Overall, this work provides an important step in the development of flexible liquid organic optoelectronic devices.

Highly Efficient Thermally Activated Delayed Fluorescence from an Excited-State Intramolecular Proton Transfer System

Thermally activated delayed fluorescence (TADF) materials have shown great potential for highly efficient organic light-emitting diodes (OLEDs). While the current molecular design of TADF materials primarily focuses on combining donor and acceptor units, we present a novel system based on the use of excited-state intramolecular proton transfer (ESIPT) to achieve efficient TADF without relying on the well-established donor–acceptor scheme. In an appropriately designed acridone-based compound with intramolecular hydrogen bonding, ESIPT leads to separation of the highest occupied and lowest unoccupied molecular orbitals, resulting in TADF emission with a photoluminescence quantum yield of nearly 60%. High external electroluminescence quantum efficiencies of up to 14% in OLEDs using this emitter prove that efficient triplet harvesting is possible with ESIPT-based TADF materials. This work will expand and accelerate the development of a wide variety of TADF materials for high performance OLEDs.

Droplet manipulation by an external electric field for crystalline film growth.

Combining droplet manipulation by the application of an electric field with inkjet printing is proposed as a unique technique to control the surface wettability of substrates for solution-processed organic field-effect transistors (FETs). With the use of this technique, uniform thin films of 2,7-dioctyl[1]benzothieno[2,3,-b][1]benzothiopene (C8-BTBT) could be fabricated on the channels of FET substrates without self-assembled monolayer treatment. High-speed camera observation revealed that the crystals formed at the solid/liquid interface. The coverage of the crystals on the channels depended on the ac frequency of the external electric field applied during film formation, leading to a wide variation in the carrier transport of the films. The highest hole mobility of 0.03 cm(2) V(-1) s(-1) was obtained when the coverage was maximized with an ac frequency of 1 kHz.

Dipole orientation analysis without optical simulation: application to thermally activated delayed fluorescence emitters doped in host matrix

The dipole orientation of guest emitters doped into host matrices is usually investigated by angular dependent photoluminescence (PL) measurements, which acquire an out-of-plane PL radiation pattern of the guest-host thin films. The PL radiation patterns generated by these methods are typically analysed by optical simulations, which require expertise to perform and interpret in the simulation. In this paper, we developed a method to calculate an orientational order parameter S without the use of full optical simulations. The PL radiation pattern showed a peak intensity (Isp) in the emission direction tilted by 40°–60° from the normal of the thin film surface plane, indicating an inherent dipole orientation of the emitter. Thus, we directly correlated Isp with S. The S − Isp relation was found to depend on the film thickness (d) and refractive indices of the substrate (nsub) and the organic thin film (norg). Hence, S can be simply calculated with information of Isp, d, nsub, and norg. We applied our method to thermally activated delayed fluorescence materials, which are known to be highly efficient electroluminescence emitters. We evaluated S and found that the error of this method, compared with an optical simulation, was less than 0.05.

Enhanced Electroluminescence from Organic Light-Emitting Diodes with an Organic-Inorganic Perovskite Host Layer

The development of host materials with high performance is essential for fabrication of efficient and stable organic light-emitting diodes (OLEDs). Although host materials used in OLEDs are typically organics, in this study, it is shown that the organic-inorganic perovskite CH3 NH3 PbCl3 (MAPbCl3 ) can be used as a host layer for OLEDs. Vacuum-evaporated MAPbCl3 films have a wide band gap of about 3 eV and very high and relatively balanced hole and electron mobilities, which are suitable for the host material. Photoluminescence and electroluminescence take place through energy transfer from MAPbCl3 to an organic emitter in films. Incorporation of an MAPbCl3 host layer into OLEDs leads to a reduction of driving voltage and enhancement of external quantum efficiency as compared to devices with a conventional organic host layer. Additionally, OLEDs with an MAPbCl3 host layer demonstrate very good operational stability under continuous current operation. These results can be extensively applied to organic- and perovskite-based optoelectronics.

Influence of Molecular Orientation in Organic Light-Emitting Diodes

Takeshi KOMINO , Hiroyuki TANAKA, Hiroko NOMURA, Takahiro KOYANAGI and Chihaya ADACHI 2, 3, 4 Center for Organic Photonics and Electronics Research, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395 Education Center for Global Leaders in Molecular System for Devices, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395 Life BEANS Center Kyushu, BEANS Laboratory, 744 Motooka, Nishi-ku, Fukuoka 819-0395 International Institute for Carbon Neutral Energy Research, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395

Investigation of aggregated structures in organic light-emitting diodes: approach from solid-state NMR

The conformations and aggregated structures, such as molecular orientations, of organic molecules in organic light-emitting diodes (OLEDs) are considered to be closely related to the performance of OLEDs. Therefore, their clarification is of great importance in understanding device performance; however, the structures have not been fully analyzed because the organic molecules in OLEDs are often in amorphous states, and conventional methods for structure analysis, such as diffraction methods, do not provide sufficient information. In this study, we have attempted the analysis of molecular orientations of OLED materials by solid-state NMR, which is useful for structure analysis even for amorphous solids.