Rupasree Ragini Das

Small molecule based and solution processed highly efficient red electrophosphorescent organic light emitting devices

The authors report the very high luminous efficiency in solution processed red electrophosphorescent organic light emitting devices using small molecular host and guest materials. The maximum luminous efficiency reached 12.7cd∕A, corresponding to external quantum efficiency of 15.1%, with its emission peak wavelength of 620nm and the Commission Internationale de l’Eclairage coordinates of (0.65, 0.33). Along with these excellent performances of the solution processed device, which were comparable to those of the vacuum deposited counterpart device with similar structure and materials, the comparative study on both devices suggests the merits of the solution process adopting robust small molecular materials only.

Highly efficient red electrophosphorescence from a solution-processed zwitterionic cyclometalated iridium(III) complex

A mononuclear red-emitting zwitterionic iridium(III) complex, bis(1-(phenyl)isoquinolinato) iridium(III) (2,2′-bipyridine-3-ol-3′-olate), was synthesized and used as a triplet emitter doped in a polymeric host in solution-processed electrophosphorescent light-emitting diodes. We achieved a high luminous effieciency (LE) of 12.62cd∕A with fast response time in the device using the dopant unlike in the device using cationic irridium(III) phosphorescent materials. A hole transporting polymeric interlayer between the anode and the emitting layer improved the LE and decreased the exciton quenching at high electric fields.

Optical and electroluminescent properties of a new green emitting Ir(III) complex

Abstract We have synthesized and characterized a new green Ir(III) complex, Ir(mpp) 3 , with the ligand 3-methyl-2-phenylpyridine (Hmpp) and fabricated phosphorescent light emitting devices with the complex as a triplet emissive dopant in PVK. The PL and EL spectra of the PVK film doped with the complex confirm an efficient energy transfer from carbazole excimer to Ir complex. The device showed a maximum external quantum efficiency of 4.5% for 2% Ir(mpp) 3 doping concentrations, and a peak luminance of 25,000 cd/m 2 . The device demonstrates the effect brought in by the fine-tuning of the ligand, 2-phenylpyridine in the form of the methyl substitution in the pyridine ring.

Spin-assembled nanolayer of a hyperbranched polymer on the anode in organic light-emitting diodes: the mechanism of hole injection and electron blocking.

We introduced a spin-assembled nanolayer of hyperbranched poly(ether sulfone) with sulfonic acid terminal on top of an indium-tin oxide anode in organic light-emitting diodes. This results in great improvement in luminous efficiency, better than that of devices using a commercially available conducting polymer composition as a hole-injection layer. The effect of the nanolayer was investigated by impedance spectroscopy, photovoltaic measurement for built-in-potential, and transient electroluminescence. We concluded that the high luminous efficiency resulted from the efficient electron-blocking by the nanolayer and hole-injection assisted by the accumulation of electrons at the interface. This result implies that, for an efficient hole-injection layer, the electron-blocking capability should be incorporated in addition to the hole-injection and -transport capability.

Synthesis and Characterization of Bis-Orthometalated Ir(III) Complex Consisting of Non-Carbon-Coordinating Ligand

We have synthesized a new green emitting bis-orthometalated Ir(III) complex, Ir(mpz)2hbz, (where mpz is 3-methyl-1-phenyl pyrazole and hbz is the anion of 2′-hydroxy-2-phenylbenzoxazole) and investigated the device performance by doping it in a mixture of PVN (poly-vinylnapthalene) (60 wt%) and PBD (2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole) (40 wt%) or PVK (poly-vinylcarbazole) with 8 wt% doping concentration. The Ir(mpz)2hbz doped PVN:PBD device demonstrates a better device performance than that of Ir(mpz)2hbz doped PVK device due to better hole and electron balance in the emissive layer. The maximum external quantum efficiency (ηext) of 8 wt% Ir(mpz)2hbz doped PVN:PBD device is ∼1.2% at 12 mA/cm2. The maximum brightness is 5,800 cd/m2 at 18 V. This work shows that phosphorescent Ir(III) complex which emits the luminescence from the MLCT transition based on a non-carbon-coordinating ligand can comply with the device demand.

P-105: New Electrochromic Systems having Controllable Color and Bistability

Here we describe the newly designed electrochromic(EC) materials to achieve better RGB colors and improved bistability. We also report the color images of the printed reflecting and transparent EC devices showing good reversibility and memory. A reflectance value of above 50% was obtained by using the white reflector. The simple structure and easy driving method will allow the device application in various areas, such as simple segment driving devices, display information (DI) board, and paper like display (PLD).

Polymer phosphorescence device using a new green emitting Ir(III) Complex

Abstract We have synthesized a new green Ir(III) complex fac‐tris‐(3‐methyl‐2‐phenyl pyridine)iridium(III) Ir(mpp) 3 and fabricated phosphorescent polymer light‐emitting device using it as a triplet emissive dopant in PVK. Ir(mpp) 3 showed absorption centered at 388 nm corresponding to the 1 MLCT transition as evidenced by its extinction coefficient of the order of 10 3 . From the PL and EL spectra of the Ir(mpp) 3 doped PVK film, the emission maximum was observed at 523 nm, due to the radiative decay from the 3 MLCT state to the ground state, confirming a complete energy transfer from PVK to Ir(mpp) 3 . The methyl substitution has probably caused a red shift in the absorption and emission spectrum compared to Ir(ppy) 3 . The device consisting of a 2 % doped PVK furnished 4.5 % external quantum efficiency at 72 cd/m 2 (current density of 0.45 mA/cm 2 and drive voltage of 13.9 V) and a peak luminance of 25,000 cd/m 2 at 23.4 V (494 mA/cm 2 ). This work demonstrates the impact of the presence of a methyl substituent at the 3‐position of the pyridyl ring of 2‐phenylpyridine on the photophysical and electroluminescence properties.

Effects of Symmetry of Ir (III) Complex on the Photophysical Properties and Device Performances

The optical, electrochemical properties of two isomers of Ir(mpp)2acac (mpp = anion of 3-methyl-2-phenyl-pyridine and acac = acetylacetonate) and the device performance of polymer light emitting devices using two isomers as a triplet emitter have been studied. The two isomers show the symmetric and asymmetric coordination of two mpp ligands to Ir(III), which is confirmed from the 1H NMR spectra. The ligand based absorption is red shifted by 35 nm in asymmetric Ir(mpp)2acac compared to the symmetric Ir(mpp)2acac. The MLCT absorption peak positions are similar in both cases, but the extinction coefficients of the asymmetric Ir(mpp)2acac are higher than those of symmetric Ir(mpp)2acac. The two isomers exhibit similar PL emission spectra but PL lifetimes of two isomers are different. The lifetime of the symmetric Ir(mpp)2acac is 1.74 μs, whereas that of the asymmetric Ir(mpp)2acac is 0.9 μs. The asymmetric Ir(mpp)2acac shows higher electrochemical oxidation potential than that of the symmetric Ir(mpp)2acac. The polymer light emitting devices are fabricated by blending 8 wt% of Ir(mpp)2acac in the PVK. The polymer light emitting device with symmetric Ir(mpp)2acac shows a maximum external quantum efficiency (ηext) of 0.48% at 73 mA/cm2 and a luminance of 12,300 cd/m2. The polymer light emitting device with asymmetric Ir(mpp)2acac shows a maximum external quantum efficiency of 0.32% at 100 mA/cm2 and a luminance of 7,500 cd/m2. This results show that the symmetry of metal complex can affect the photophysical properties as well as the device performance.

4.8″ QVGA electrochromic displays driven by oxide TFTs

We have developed 4.8″ QVGA active matrix electrochromic displays(AMECD) based on oxide TFTs backplane. For the high resolution(100PPI) and gray scalable(16 levels) display, Cross-talk free structure and a unique process to deposit selectively titanium dioxide on each pixel electrode were presented.

New Electrochromic Systems for the Color e-Paper

Electrochemical materials showing color changes from clear to the three primary colors (RGB) and bistability have been required to realize the full-color electronic paper. To fabricate a multicolor ECD (electrochromic device), we have examined the electrochromic properties of various types of electrochromism from the polymer to low molecular weight organic materials such as viologen derivatives and other derivatives with conventional ITO-sandwich cell. Even though, the polymer electrochromism has shown the progress on primary colors recently [1 ,2 ], it has still problems on the transition speed and whiteness on the transparency. Low molecular weight electrochromic materials show the better processability and color purity for the color E-paper applications until now. Highly reflective blue color was reported by using viologen derivatives but the green and red color materials of high color purity and bistability need to be still developed [3]. Here we describe the new structured electrochromic(EC) materials to improve the three primary colors (RGB) and bistability.