Hin-Wai Mo

Achieving Highly Efficient Simple-Emission Layer Fluorescence/Phosphorescence Hybrid White Organic Light-Emitting Devices via Effective Confinement of Triplets

Achieving high efficiencies in simple device configurations is a long-standing and meaningful target for organic light-emitting devices (OLEDs). Herein, by utilizing an efficient blue-violet fluorophor (CzS1) that has a high triplet energy of 2.62 eV, the significance of effective confinement of the green triplets in fluorescence/phosphorescence hybrid white devices (F/P-WOLEDs) that have highly simplified emission layers (EMLs) containing only RGB emitters was demonstrated. The non-p-i-n warm-white device exhibited excellent performance with a maximum forward power efficiency high up to 42.1 lm W(-1), and maintaining at 26.3 lm W(1-) at a practical luminance of 1000 cd m(-2).

Efficient and stable deep-red phosphorescent organic light-emitting diodes based on an iridium complex containing a benzoxazole-substituted ancillary ligand.

In the past two decades, organic light-emitting devices (OLEDs) have drawn considerable attention as a promising technology for practical optoelectronic applications. Cyclometalated iridium ACHTUNGTRENNUNG(III) complexes incorporated into OLEDs are one of the most studied class of compounds. Their heavy metal core results in the mixing of singlet and triplet manifolds, which enables harvesting of both singlet and triplet excitons. Furthermore, the iridium complexes are suitable for phosphorescent OLED (PHOLED) applications due to their relatively short phosphorescence lifetimes, easily sublimable feature, and excellent color tunability. For PHOLEDs to be useful in display and lighting applications, red, green, and blue emitters with sufficient luminous efficiencies, saturated color chromaticities, and adequate lifetimes are required. The advances in green phosphors achieved in the past couple of decades is particularly impressive. While the development of red phosphors has also gone through several evolutional steps, their performances are still considerably behind those of green phosphors. For example, reports on high-performance red PHOLEDs with a Commission internationale de l’ clairage ACHTUNGTRENNUNG(CIE) coordinate x 0.67 are still rare because the efficiency and brightness of saturated-red phosphorescent OLEDs are difficult to improve due to the energy gap law and the drop in luminous flux in the saturated-red region. Okada et al. synthesized a widely used Ir ACHTUNGTRENNUNG(C-N)3 complex using 1-phenylisoquinoline as a monoanionic cyclometalating ligand, [tris(1-phenylisoquinolinato-N,C2)iridiumACHTUNGTRENNUNG(III)] (Ir ACHTUNGTRENNUNG(piq)3). A PHOLED devicebased on this complex exhibits a peak emission at 623 nm and a maximum powerefficiency of 8.0 Lm W 1 (9.3 cd A ) at 100 cd m . Sun et al. reported an iridium complex, IrACHTUNGTRENNUNG(piq)2acac, with 1-phenylisoquinoline as the main ligand and acetylacetone as ancillary ligand. The reported Ir ACHTUNGTRENNUNG(piq)2acac-based device exhibits a peak emission at 624 nm and a maximum current efficiency of 7.83 cd A ; In recent work, Kwon et al. reported an iridium complex using a new main ligand (2-(3,5-dimethylphenyl)-4-methylquinoline); a device based on this complex reached a maximum power efficiency of 32 Lm W . So far, most red iridium complexes reported were obtained by changing the main ligands, and there are few reports on the modification of the ancillary ligands. In this study, a deep-red emitter containing a new ancillary ligand, iridiumACHTUNGTRENNUNG(III) bis(1-phenylisoquinolinatoN,C’)(2ACHTUNGTRENNUNG(benzo[d]oxazol-2-yl)phenol) (Ir ACHTUNGTRENNUNG(piq)2bop) has been designed and synthesized. The use of a bulky 2-(benzo[d]oxazol-2-yl)phenol ancillary ligand is expected to suppress molecular aggregation and enhance the performance by reducing self-quenching. By doping Ir ACHTUNGTRENNUNG(piq)2bop into Bebq2 (Bebq2 = bis(10-hydroxybenzo[h] quinolinato)beryllium), an efficient deep-red OLED with a luminous efficiency of 9.1 Lm W 1 (10.3 cd A ) can be obtained at 100 cd m , along with CIE coordinates of (0.67, 0.33). Furthermore, we found that the device has an extrapolated lifetime of over 30 000 h at an initial luminance of 100 cd m . Due to the high efficiency and stability of the device, even in a simple device structure, we conclude that IrACHTUNGTRENNUNG(piq)2bop is a promising organic phosphor for practical OLED applications. The ligand 1-phenylisoquinoline was prepared through a Suzuki coupling reaction between 1-chloroisoquinoline and phenylboronic acid. Synthesis of the final Ir complex involved two key steps: In the first step, IrCl3 was reacted with an excess of the synthesized ligand (1-phenylisoquinoline) to produce a chloro-bridged Ir dimer. This dimer can be easily converted into the monomeric complex IrACHTUNGTRENNUNG(piq)2bop by replacing the bridging chlorides with 2-(benzo[d]oxazol[a] C. Wu, Dr. S. Tao, M. Chen School of Optoelectronic Information University of Electronic Science and Technology of China (UESTC) Chengdu, 610054 (China) Fax: (+86) 28-83201745 E-mail : silutao@uestc.edu.cn [b] C. Wu, Dr. F.-L. Wong, Y. Yuan, Dr. H.-W. Mo, Prof. C.-S. Lee Center of Super-Diamond and Advanced Films (COSDAF) & Department of Physics and Materials Science City University of Hong Kong Hong Kong Special Administrative Region ACHTUNGTRENNUNG(China) Fax: (+852) 27847830 E-mail : apcslee@cityu.edu.hk [c] Y. Yuan Department of Chemistry Shantou University Guangdong, 515063 (China) [d] Dr. W. Zhao Dongguan LITEWELL (OLED) Technology Incorporation Dongguan, 523656 (China)

Effects of graphene defect on electronic structures of its interface with organic semiconductor

Electronic structures of copper hexadecafluorophthalocyanine (F16CuPc)/graphene with different defect density were studied with ultra-violet photoelectron spectroscopy. We showed that the charge transfer interaction and charge flow direction can be interestingly tuned by controlling the defect density of graphene through time-controlled H2 plasma treatment. By increasing the treatment time of H2 plasma from 30 s to 5 min, both the interface surface dipole and the electron transporting barrier at F16CuPc/graphene interface are significantly reduced from 0.86 to 0.56 eV and 0.71 to 0.29 eV, respectively. These results suggested that graphenes defect control is a simple approach for tuning electronic properties of organic/graphene interfaces.