Daisuke Yokoyama

Thermally Activated Delayed Fluorescence from Sn4+–Porphyrin Complexes and Their Application to Organic Light Emitting Diodes — A Novel Mechanism for Electroluminescence

[*] Prof. C. Adachi, A. Endo, Dr. D. Yokoyama Center for Future Chemistry Kyushu University 744 Motooka Nishi, Fukuoka 819-0395 (Japan) E-mail: adachi@cstf.kyushu-u.ac.jp M. Ogasawara, Prof. Y. Kato Department of Materials Science and Engineering Kyushu University 744 Motooka Nishi, Fukuoka 819-0395 (Japan) Dr. A. Takahashi Sogo Pharmaceutical Co., Ltd. Hebinami 28-3, Johban Shimofunao Iwaki, Fukushima 972-8312 (Japan) [+] Present address: Mitsubishi Chemical Group Science and Technology Research Center, Inc., Display Project, Research and Development Division, 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan.

Molecular orientation in small-molecule organic light-emitting diodes

In research on small-molecule organic light-emitting diodes (OLEDs), the molecular orientation in vacuum-deposited amorphous films has been disregarded for around 20 years, and its effects on device performance have not been sufficiently discussed at the microscopic level. Only recently have the intermolecular interaction and subsequent horizontal molecular orientation in OLEDs been investigated and reported. In this article, an overview of the recent studies on molecular orientation in OLEDs is presented. The general properties of molecular orientation of OLED materials are shown, and its significant effects on the electrical and optical properties of devices are discussed to understand device physics and improve the future performance and reliability of OLEDs.

Determination of molecular dipole orientation in doped fluorescent organic thin films by photoluminescence measurements

The orientation of the transition dipole moments of fluorescent organic molecules doped into a matrix material is determined by photoluminescence measurements of the angular dependent emission spectra and by comparison with simulations. The analysis of two small molecular materials doped into a 4,4′-bis(N-carbazole)-biphenyl matrix is demonstrated, yielding a horizontal orientation of 91% for 4,4′-bis[4-(diphenylamino)styryl]biphenyl and a completely random orientation in case of tris-(8-hydroxyquinoline)aluminum (Alq3). This expeditious technique does not require detailed information about the optical properties of the dopant, making this method particularly suitable for characterizing newly developed materials for organic light-emitting diodes with enhanced light-outcoupling efficiency.

Horizontal molecular orientation in vacuum-deposited organic amorphous films of hole and electron transport materials

Using wide-range variable angle spectroscopic ellipsometry, the authors demonstrate large optical uniaxial anisotropy of vacuum-deposited organic amorphous films of hole and electron transport materials having long or planar molecular structures. The ordinary refractive indices and extinction coefficients were higher than the extraordinary ones, revealing that the molecules in the amorphous films are horizontally oriented. The horizontal orientation requires significant modifications in the understanding of both the electrical and optical characteristics of amorphous films when we use materials having long or planar molecular structures.

H2 Evolution from Water on Modified Cu2ZnSnS4 Photoelectrode under Solar Light

Cu2ZnSnS4 (CZTS) thin films were investigated as photoelectrodes for H2 evolution from water under solar light. Surface modifications with Pt, CdS, and TiO2 enhanced the incident photon-to-current conversion efficiency of CZTS thin film electrode for H2 production from water by three orders of magnitude to 40% (λ= 600 nm). The solar energy conversion efficiency in H2 production using modified CZTS reached 1.2%. Our results demonstrate that the photoelectrochemical properties of CZTS were significantly improved by surface modification, which suggests the feasibility of CZTS as a photocathode for a photoelectrochemical water-splitting system.

Synthesis and electroluminescence properties of highly efficient blue fluorescence emitters using dual core chromophores

We describe two novel blue emission materials based on a new type of dual core concept. 1-Phenyl-6-(10-phenyl-anthracen-9-yl)-pyrene (Ph-AP-Ph) and 1-[1,1′;3′,1′′]terphenyl-5′-yl-6-(10-[1,1′;3′,1′′]terphenyl-5′-yl-anthracen-9-yl)-pyrene (TP-AP-TP) were synthesized through boronylation and Suzuki coupling reactions. The Tg values of Ph-AP-Ph and TP-AP-TP were 228 °C and 243 °C, respectively, compared to values of 135 °C and 139 °C for the single core materials 9-(3′,5′-diphenylphenyl)-10-(3′′′,5′′′-diphenylbiphenyl-4′′-yl)anthracene (MAM) and 1,6-bis-[1,1′;3′,1′′]terphenyl-5′-yl-pyrene (TP-P-TP). One of the dual core derivatives, TP-AP-TP, exhibited an ELmax value of 456 nm and a high luminance EQE of 7.51% when used in an EL device. The dual core chromophore materials had narrower PL and EL spectra and better thermal properties than the single core chromophore materials. A device based on TP-AP-TP showed twice the lifetime of a device based on the commercialized material, 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN).

Dual efficiency enhancement by delayed fluorescence and dipole orientation in high-efficiency fluorescent organic light-emitting diodes

To explain the origin of extremely high efficiencies of deep-blue fluorescent organic light-emitting diodes(OLEDs) with anisotropic-shapedanthracene derivatives, the enhancements of singlet-exciton generation efficiency and outcoupling efficiency were investigated by transient electroluminescence measurement and variable angle spectroscopic ellipsometry, respectively. Both the delayed fluorescence from singlet excitons generated via triplet-triplet annihilation and the outcoupling enhancement by dipole orientation of emitters were found to contribute to the high external quantum efficiencies of the devices. This dual efficiency enhancement is important for understanding and further improving high-performance fluorescent OLEDs.

Enhancement of electron transport by horizontal molecular orientation of oxadiazole planar molecules in organic amorphous films

To demonstrate the effect of molecular orientation on charge transport characteristics in vacuum-deposited organic amorphous films, the electrical and optical properties of films of two oxadiazole derivatives were investigated. One of them has bulky tert-butyl terminals and forms an isotropic film, whereas the other has planar bipyridyl terminals and forms a highly anisotropic film. The very large optical anisotropy of the latter means that the planar molecules stack horizontally, leading to large overlaps of π-orbitals and more than 30 times higher electron mobility though the geometric and electronic structures of the main conformers of the two derivatives are quite similar.

In situ real-time spectroscopic ellipsometry measurement for the investigation of molecular orientation in organic amorphous multilayer structures

To investigate molecular orientation in organic amorphous films, in situ real-time spectroscopic ellipsometry measurements were performed during vacuum deposition. Three materials with different molecular shapes were adopted to confirm the generality of the molecular orientation. In all three cases, more than 200 000 values for the ellipsometric parameters measured during deposition were well simulated simultaneously over the entire spectral range and measurement period using a simple model where the films possessed homogeneous optical anisotropy. This demonstrated the homogeneity of the molecular orientation in the direction of film thickness. The molecular orientation can be controlled by the substrate temperature even in multilayer structures. It is also demonstrated that a “multilayer structure” can be fabricated using only one material, where each layer has different optical and electrical properties.

Optical and electrical properties of a squaraine dye in photovoltaic cells

2,4-Bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) was employed as a donor material in organic photovoltaic cells based on planar heterojunctions. We studied optical properties of SQ films, and discussed its photovoltaic performance via numerical fitting and simulation on the photovoltaic cells. Exciton diffusion length (LD) in SQ films (4.5 nm) derived from optical simulation should be the major limitation to efficiency, consistent with external quantum efficiency data. Thermal treatment improved efficiency, which can be ascribed to reduced saturation current of the photovoltaic cells. As a result, a power conversion efficiency of 4.1% was achieved.