Colloidal Quantum Dot-Light-Emitting Diodes Employing Phosphorescent Small Organic Molecules as Efficient Exciton Harvesters

Cataloged from PDF version of article.Nonradiative energy transfer (NRET) is an alternative excitation mechanism in colloidal quantum dot (QD) based electroluminescent devices (QLEDs). Here, we develop hybrid highly spectrally pure QLEDs that facilitate energy transfer pumping via NRET from a phosphorescent small organic molecule-codoped charge transport layer to the adjacent QDs. A partially codoped exciton funnelling electron transport layer is proposed and optimized for enhanced QLED performance while exhibiting very high color purity of 99%. These energy transfer pumped hybrid QLEDs demonstrate a 6-fold enhancement factor in the external quantum efficiency over the conventional QLED structure, in which energy transfer pumping is intrinsically weak

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

Cataloged from PDF version of article.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/cm(2). 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/cm(2). (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4749278

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

Cataloged from PDF version of article.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/m(2) 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. (C) 2012 American Institute of Physics

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

Cataloged from PDF version of article.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

Transition metal oxides on organic semiconductors

Cataloged from PDF version of article.Transition metal oxides (TMOs) on organic semiconductors (OSs) structure has been widely used in inverted organic optoelectronic devices, including inverted organic light-emitting diodes (OLEDs) and inverted organic solar cells (OSCs), which can improve the stability of such devices as a result of improved protection of air sensitive cathode. However, most of these reports are focused on the anode modification effect of TMO and the nature of TMO-on-OS is not fully understood. Here we show that the OS on TMO forms a two-layer structure, where the interface mixing is minimized, while for TMO-on-OS, due to the obvious diffusion of TMO into the OS, a doping-layer structure is formed. This is evidenced by a series of optical and electrical studies. By studying the TMO diffusion depth in different OS, we found that this process is governed by the thermal property of the OS. The TMO tends to diffuse deeper into the OS with a lower evaporation temperature. It is shown that the TMO can diffuse more than 20 nm into the OS, depending on the thermal property of the OS. We also show that the TMO-on-OS structure can replace the commonly used OS with TMO doping structure, which is a big step toward in simplifying the fabrication process of the organic optoelectronic devices. (C) 2014 Elsevier B.V. All rights reserved

Strukturální, morfologické, optické a magnetické vlastnosti naprašovaného ZnO dopovaného Co jako magnetického polovodiče pro spintronické aplikace

Tento článek popisuje výrobu a charakterizaci tenkých vrstev čistého a kobaltem dopovaného ZnO (obsah Co 4% a 7%), transparentního zředěného magnetického polovodiče (DMS) na Si a skleněných substrátech technikou RF magnetronového naprašování. Krystalická struktura a fáze tenkých vrstev byly analyzovány pomocí rentgenové difrakce (XRD), kterou byla potvrzena hexagonální wurtzitová struktura ZnO s mírným napětím mřížky a změnou orientace atomových rovin. XRD také potvrdilo, že tenké vrstvy vykazují významné linie (1 0 1) a (1 0 3) s polykrystalickou strukturou. Morfologie tenké vrstvy byla zkoumána skenovací elektronovou mikroskopií (SEM), která potvrdila změnu mikrostruktury a povrchu polykrystalických vrstev. Pomocí Energeticky disperzní rentgenové spektroskopie (EDS) v SEM byla změřeno složení vrstev a koncentrace (v%) každého přítomného prvku. Krystalické vlastnosti a morfologie kolmých řezů byl studovány transmisní elektronovou mikroskopií s vysokým rozlišením (HR-TEM). Průměrná tloušťka byla okolo 600 nm změřena v transmisním elektronovém mikroskopu na kolmém řezu. Difrakce elektronů vybrané oblasti (SAED) záznam z TEM pro Co (7%) dopovaný film vypovídá o polykrystalické struktuře vrstev. Optická propustnost vrstev na substrátech z corning skla byla zkoumána pomocí UV-vis spektrometru pro čistý ZnO i 4% a 7% Co dotované ZnO vrstvy, které odhalily optickou průhlednost 85%, 75% a 65%. Feromagnetismus za pokojové teploty dotovaných vrstev byl analyzován pomocí vibrační magnetometrie vzorku a magnetooptického Kerrova efektu. Bylo odhaleno, že feromagnetické chování filmů roste s obsahem „Co“ a výsledky byly podrobně diskutovány.This article reports the fabrication and characterization of thin films of pure and cobalt doped ZnO (Co at 4% and 7%), a transparent diluted magnetic semiconductor (DMS) grown on ‘Si’ and glass substrates by RF magnetron sputtering technique. The crystalline structure and phase of the grown thin films were analyzed by using X-ray diffraction (XRD) method which confirmed the hexagonal wurtzite structure of the ZnO with slight lattice strain and change in orientation of the planes. The XRD also confirmed that, the films exhibit prominent peaks of (1 0 1) and (1 0 3) with polycrystalline nature. The morphology of the grown thin films was investigated by scanning electron microscopy (SEM) which confirmed the variation of micro-structure and size of the polycrystalline film’s surface. The energy dispersive X-ray spectra (EDS) from SEM have confirmed the presence of constituent elements in the films and concentration (in %) of each element. The crystalline properties and morphology of the film’s cross-section were studied by high resolution transmission electron microscopy (HR-TEM). The average thickness of the films was found to be about 600 nm from the cross-section electron microscopic images. The selected area electron diffraction (SAED) pattern from TEM was recorded for the Co (7%) doped ZnO film which has good polycrystalline quality. The optical transmittance of the films coated on corning glass substrates was investigated by UV–Visible spectrophotometer for pure, 4% and 7% Co doped ZnO films, which revealed the optical transparency of 85%, 75% and 65%, respectively. The room temperature ferromagnetism of the doped films was analysed by vibrating sample magnetometry and magneto optic Kerr effect. It was found that the ferromagnetic behaviour of films increases with ‘Co’ content and the results were discussed in detail