Yuichiro Kawamura

100% phosphorescence quantum efficiency of Ir(III) complexes in organic semiconductor films

We demonstrate that three Ir(III) complexes used as principal dopants in organic electrophosphorescent diodes have very high photoluminescence quantum efficiency (ηPL) in a solid-state film. The green emitting complex, fac-tris(2-phenylpyridinato)iridium(III) [Ir(ppy)3], the red-emitting bis[2-(2′-benzothienyl)pyridinato-N,C3′] (acetylacetonato)iridium(III) [Btp2Ir(acac)], and the blue complex bis[(4,6-difluorophenyl)pyridinato-N,C2](picolinato)iridium(III) (FIrpic) were prepared as codeposited films of varying concentration with 4,4′-bis(N-carbazolyl)-2,2′-biphenyl, a commonly used host material. The maximum ηPL values for Ir(ppy)3, Btp2Ir(acac), and FIrpic were, respectively, 97%±2% (at 1.5mol%), 51%±1% (at 1.4mol%), and 78%±1% (at 15mol%). Furthermore, we also observed that the maximum ηPL of FIrpic reached 99%±1% when doped into the high triplet energy host, m-bis(N-carbazolyl)benzene, at an optimal concentration of 1.2mol%.

Energy transfer in polymer electrophosphorescent light emitting devices with single and multiple doped luminescent layers

We study energy transfer in efficient polymer electrophosphorescent organic light emitting diodes (PHOLEDs) using poly(9-vinylcarbazole) (PVK) host doped with one or more phosphorescent cyclometalated Ir(III) complexes. Single dopant double heterostructure PHOLEDs exhibited saturated color luminescence due to emissive triplet metal-to-ligand charge-transfer to ground state transitions of the Ir(III) dopants. Blue PHOLEDs, excited by an endothermic process from the host polymer, exhibited an emission maximum at a wavelength of λmax=474 nm, with an external quantum efficiency of ηext=1.3±0.1% and luminous power efficiency of ηp=0.8±0.1 lm/W. The green PHOLED exhibited ηext=5.1±0.1%, with ηext>2% for both red and yellow emission. Resonant energy transfer between green emitting fac-tris (2-phenylpyridyl)Ir(III) and red emitting bis[2-(2′-benzothienyl)-pyridinato-N,C3′](acetylacetonate)Ir(III) was observed to nearly double the efficiency of red emission when both dopants were simultaneously blended in a PVK ho...

Luminescence of Novel Neodymium Sulfonylaminate Complexes in Organic Media

Luminescence of Nd(III) in an organic solvent having C-H bonds was achieved for the first time by complexing Nd(III) with long-chain perfluoroalkylated ligands such as bis(perfluorooctylsulfonyl)aminate (pos). The complex [Nd(pos)(3)] gave an emission quantum yield of 3.0+/-0.5 % in undeuterated acetone. The bulky pos ligands suppress the excitation of C-H vibrations, energy migration at diffusional collision, and the coordination of acetone molecules (see picture) to the Nd(III) center, which otherwise quench the luminescence.

Observation of neodymium electroluminescence

Organic electroluminescence devices using a neodymium(III) complex as an emitting layer were fabricated. The cell structure of glass substrate/indium–tin–oxide/N,N′-diphenyl-N,N′-di(m-tolyl)-benzidine/tris(dibenzoylmethanato)(monobathophenanthroline)neodymium(III) complex/tris(8-quinolinolato)aluminum(III) complex/Mg:Ag was employed. Sharp near-infrared emission bands assigned to f–f transitions of neodymium ion were obtained at dc bias voltage of over 15 V.

100% fluorescence efficiency of 4,4′-bis[(N-carbazole)styryl]biphenyl in a solid film and the very low amplified spontaneous emission threshold

We examined the amplified spontaneous emission (ASE) characteristics of fluorescent styrylbenzene derivatives doped into a 4,4′-di(N-carbazolyl)biphenyl (CBP) host. In particular, 4,4′-bis[(N-carbazole)styryl]biphenyl (BSB-Cz) demonstrated an extremely low ASE threshold of Eth=0.32±0.05μJ∕cm2. We observed that the 6 wt %-BSB-Cz:CBP film has an ultimate photoluminescence (PL) quantum efficiency of ϕPL=100% and a short transient lifetime of τf=1.0±0.1ns, leading to a large radiative decay rate of kr=1×109s−1. We demonstrated that the extremely low ASE threshold originated from the large radiative decay rate of BSB-Cz.

Near-Infrared Photoluminescence and Electroluminescence of Neodymium(III), Erbium(III), and Ytterbium(III) Complexes

Tris(dibenzoylmethanato)(monobathophenanthroline)lanthanide(III) complex [Ln(DBM)3 bath (Ln: Nd, Er and Yb)] both in solutions and thin films at room temperature showed narrow band photoluminescence (PL) due to the f–f transitions in the near-IR region: 890, 1070 and 1350 nm for Nd(III), 980 and 1540 nm for Er(III), and 985 nm for Yb(III). The PL efficiencies in solution were determined [φPL=3.3×10-3 for Nd(III), 7.0×10-5 for Er(III), and 1.4×10-2 for Yb(III)]. Organic electroluminescent (EL) devices having the structure of glass substrate/indium-tin oxide/N,N′-diphenyl-N,N′-di(m-tolyl)benzidine/Ln(DBM)3bath(Ln: Nd, Er and Yb)/bathocuproine/Mg:Ag/Ag were fabricated, giving the EL bands around 900–1600 nm at room temperature. The external near-IR EL efficiencies at low current density were estimated by comparing with that of the Eu(III) device having the same structure. The saturation of near-IR EL intensity observed at the high current density suggested that the near-IR EL should suffer the T–T annihilation.

Injection and Transport of High Current Density over 1000 A/cm2 in Organic Light Emitting Diodes under Pulse Excitation

We succeeded in injecting and transporting a maximum high current density of J=1163 A/cm2 in organic light-emitting diodes using short-pulse excitation combined with a highly thermally conductive silicon substrate (thermal conductivity: 148 W m-1 K-1) and a small cathode configuration (cathode radius r=50 µm). A maximum current density almost 20 times higher than that associated with direct current (DC) operation was observed by driving an OLED with a short pulse voltage. With short-pulse excitation, the decrease in external quantum efficiency (ηext) obeyed a typical singlet–singlet exciton annihilation model well, indicating that the generation of Joule heat in OLEDs can be suppressed under pulse operation.

Ultraviolet amplified spontaneous emission from thin films of 4,4′-bis(9-carbazolyl)-2,2′-biphenyl and the derivatives

We demonstrate 394 nm ultraviolet amplified spontaneous emission (ASE) with a low pumping power threshold of Eth=1.3±0.2 μJ/cm2, from a thin film of an organic semiconductor 4,4′-bis(9-carbazolyl)-2,2′-biphenyl (CBP) under the pulse excitation of a N2 gas laser (337 nm). 3-methyl and 3,6-dimethyl substituted CBP derivatives also exhibited pronounced ASE in the deep-blue region of 401 and 406 nm and ASE thresholds of less than Eth=2 μJ/cm2. We also examined the ASE characteristics of N,N′-di(m-tolyl)-N,N′-diphenylbenzidine (TPD), N,N,N′,N′-tetraphenylbenzidine (DPABP) and N,N′-di(α-naphtyl)-N,N′-diphenylbenzidine (α-NPD). While TPD and DPABP showed low ASE thresholds, α-NPD did not show any ASE. We show that the large radiative decay rate (kf) of DPABP and TPD, which is derived from their short fluorescence lifetime (τf) and large quantum efficiency (ηf), leads to a low ASE threshold. On the other hand, the lack of ASE from α-NPD is ascribable to the small kf of 0.8±0.1×108 s−1, which is due to the rather ...

39.4: Highly Efficient Fluorescent Blue OLEDs with Efficiency‐enhancement Layer

We found a new device structure which has an efficiency- enhancement layer between emitting layer and electron transporting layer. We applied the structure to a pure blue fluorescent device with a CIE1931 coordinate of (0.14, 0.12) and successfully obtained very high EQE of over 10% with low driving voltage of 3.7 V at 10 mA/cm2 and long lifetime of 10,000 hrs at an initial luminance of 1,000 cd/m2. Results of transient EL measurements indicated the contribution of triplet excitons to the high EQE.

56.4: New Deep Blue Fluorescent Materials and Their Application to High Performance OLEDs

We developed highly efficient deep blue fluorescent materials. The bottom-emission OLED showed a CIE1931 coordinate of (0.14, 0.08) and a current efficiency of 6.5 cd/A with using a new efficiency-enhancement layer. We achieved CIEy of 0.062, a current efficiency of 7.5 cd/A and lifetime (LT95) of 2,000 hrs by depositing a new organic capping layer on a top-emission blue OLEDs, A hybrid white OLED by stacking a deep blue fluorescence and a phosphorescent red/green stacked yellow was also demonstrated which has an efficacy of 23 lm/W and color temperature of 10,000 K at 10 mA/cm2 and lifetime (LT50) of over 60,000 hrs at a luminance of 1,000 cd/m2.