Yoga Divayana

Synthesis, structure, and optoelectronic properties of a new twistacene 1,2,3,4,6,13-hexaphenyl-7 : 8,11 : 12-bisbenzo-pentacene

We report the synthesis, optical and electrochemical properties, as well as the fabrication of light-emitting devices for a new twistacene 1,2,3,4,6,13-hexaphenyl-7 : 8,11 : 12-bisbenzo-pentacene (HBP 1). Its structure, determined by X-ray crystallography, confirmed that this material has a twisted topology with the torsion angle as high as 23.0°. HBP 1 showed bright green emission both in solution and solid state. The HOMO–LUMO gap of HBP 1 calculated from the difference between the half-wave redox potentials (E1/2ox = +0.74 eV and E1/2red = −1.93 eV) is 2.67 eV, which is in good agreement with the band gap, 2.64 eV, derived from the UV-Vis absorption data. Organic light emitting devices using HBP 1 as the emitters have been fabricated. The results revealed that twistacenes are promising materials to enhance the efficiency of OLEDs.

A bright cadmium-free, hybrid organic/quantum dot white light-emitting diode

We report a bright cadmium-free, InP-based quantum dot light-emitting diode (QD-LED) with efficient green emission. A maximum brightness close to 700 cd/m2 together with a relatively low turn-on voltage of 4.5 V has been achieved. With the design of a loosely packed QD layer resulting in the direct contact of poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (poly-TPD) and 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) in the device, a ternary complementary white QD-LED consisting of blue component (poly-TPD), green component (QDs), and red component (exciplex formed at the interface between poly-TPD and TPBi) has been demonstrated. The resulting white QD-LED shows an excellent color rendering index of 95.

Color tunable light-emitting diodes based on p+-Si/p-CuAlO2 /n-ZnO nanorod array heterojunctions

Wide-range color tuning from red to blue was achieved in phosphor-free p+-Si/p-CuAlO2/n-ZnO nanorod light-emitting diodes at room temperature. CuAlO2 films were deposited on p+-Si substrates by sputtering followed by annealing. ZnO nanorods were further grown on the annealed p+-Si/p-CuAlO2 substrates by vapor phase transport. The color of the p-CuAlO2/n-ZnO nanorod array heterojunction electroluminescence depended on the annealing temperature of the CuAlO2 film. With the increase of the annealing temperature from 900 to 1050 °C, the emission showed a blueshift under the same forward bias. The origin of the blueshift is related to the amount of Cu concentration diffused into ZnO.

Quantum Dot Light-Emitting Diode with Quantum Dots Inside the Hole Transporting Layers

We report a hybrid, quantum dot (QD)-based, organic light-emitting diode architecture using a noninverted structure with the QDs sandwiched between hole transporting layers (HTLs) outperforming the reference device structure implemented in conventional noninverted architecture by over five folds and suppressing the blue emission that is otherwise observed in the conventional structure because of the excess electrons leaking towards the HTL. It is predicted in the new device structure that 97.44% of the exciton formation takes place in the QD layer, while 2.56% of the excitons form in the HTL. It is found that the enhancement in the external quantum efficiency is mainly due to the stronger confinement of exciton formation to the QDs.

Organic light-emitting devices with a hole-blocking layer inserted between the hole-injection layer and hole-transporting layer

A hole-blocking layer (HBL), 2,9-dimethyl-4, 7-diphenylphenanthroline (BCP), was incorporated between the hole-transporting layer (HTL) and hole-injection layer for a tris-(8-hydroxyqunoline) aluminum-based organic light-emitting device. Such a structure helps to reduce the hole-leakage to the cathode resulting in improved current efficiency. Optimum BCP thickness of around 3nm was observed to produce a current efficiency of 3.25cd∕A, which corresponds to a 30% improvement compared to that of the standard device without BCP (2.5cd∕A). Low operating voltage was also achieved by minimizing the thickness of the HTL. Both operating voltage and efficiency can be tuned by varying the thickness of HTL and HBL, respectively.

Improved Inverted Organic Solar Cells With a Sol–Gel Derived Indium-Doped Zinc Oxide Buffer Layer

We studied sol-gel derived indium-doped zinc oxide (IZO) with various indium contents as a functional buffer layer in inverted polymer:fullerene bulk-heterojunction solar cell. The short-circuit current density was observed to increase by doping indium in pure ZnO buffer layer. The maximum current density was obtained with a 1 at.% indium doping. Although the open-circuit voltage and fill factor reduced slightly, the inverted organic solar cell with 1 at.% IZO buffer layer showed a power conversion efficiency of 3.3%, which is higher compared to that (2.94%) of the device with undoped ZnO buffer layer under illumination of AM1.5G. The better performance is due to combined effects of improvement in charge collection and higher optical transmittance of electrode/buffer layer stack.

An efficient non-Lambertian organic light-emitting diode using imprinted submicron-size zinc oxide pillar arrays

We report phosphorescent organic light-emitting diodes with a substantially improved light outcoupling efficiency and a wider angular distribution through applying a layer of zinc oxide periodic nanopillar arrays by pattern replication in non-wetting templates technique. The devices exhibited the peak emission intensity at an emission angle of 40° compared to 0° for reference device using bare ITO-glass. The best device showed a peak luminance efficiency of 95.5 ± 1.5 cd/A at 0° emission (external quantum efficiency—EQE of 38.5 ± 0.1%, power efficiency of 127 ± 1 lm/W), compared to that of the reference device, which has a peak luminance efficiency of 68.0 ± 1.4 cd/A (EQE of 22.0 ± 0.1%, power efficiency of 72 ± 1 lm/W).

On the triplet distribution and its effect on an improved phosphorescent organic light-emitting diode

We reported phosphorescent organic light-emitting diodes with internal quantum efficiency near 100% with significantly reduced efficiency roll-off. It was found that the use of different hole transporting layer (HTL) affects the exciton distribution in the emission region significantly. Our best device reaches external quantum efficiency (EQE), current, and power efficiency of 22.8% ± 0.1%, 78.6 ± 0.2 cd/A, 85 ± 2 lm/W, respectively, with half current of 158.2 mA/cm2. This considerably outperforms the control device with N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB) (HTL) and 4,4′-N,N′-dicarbazole-biphenyl (host) with maximum EQE, current and power efficiency of 19.1% ± 0.1%, 65.6 ± 0.3 cd/A, 67 ± 2 lm/W, respectively, with half current of only 8.1 mA/cm2.

Efficient blue organic light-emitting device based on N, N′-di(naphth-2-yl)-N, N′-diphenyl-benzidine with an exciton-confining structure

A blue organic light-emitting device with improved efficiency and excellent color purity is reported (Commission Internationale de’l Eclairage coordinates of x=0.1659 and y=0.0772 at 5V), where N, N′-di(naphth-2-yl)-N, N′-diphenyl-benzidine (NPB), a traditional hole-transporting layer, was used as the emission layer. A significant increase in efficiency was achieved by confining the excitons within the NPB layer by two wide-band-gap hole-blocking layers sandwiching the NPB layer. This structure also increases the direct exciton formation at the NPB layer by promoting electrons to cross the NPB layer, responsible for further efficiency improvement. Optimized structure showed an external quantum efficiency of 1.38%, which accounts for a 25% increase compared to a standard device.

Improving organic light-emitting devices by modifying indium tin oxide anode with an ultrathin tetrahedral amorphous carbon film

The characteristics of organic light-emitting devices based on tris-(8-hydroxyqunoline) aluminum with an ultrathin tetrahedral amorphous carbon (ta-C) film on indium tin oxide have been investigated. The device with a 1.0-nm ta-C layer has the highest current and power efficiency. The current efficiency of a device with a ta-C layer thickness of 1.0nm is 3.7cd∕A at 20mA∕cm2, however, the current efficiency of a standard device without a ta-C layer is 2.56cd∕A at the same current density. The current efficiency is improved by 46% compared to the standard device. Although there is a 1.2-V increase in driving voltage for 100-cd∕m2 luminance, the power efficiency is still improved by 22% compared to that of the standard device. The improvement of the efficiency is due to smoothing indium tin oxide surface, blocking hole injection from anode and balancing hole and electron currents. The optimal thickness of ta-C layer for hole injection mechanics can be understood by tunneling.