Organic Light Emitting Devices Devices Research Articles

Synthesis, aggregation-induced emission, and electroluminescence of AIEgen designed on bis-carbazole platform

The promising applications of organic light emitting devices (OLEDs) as the next-generation lighting technology has boosted global research on small organic fluorescent molecules. However, majority of the organic fluorophores experiences detrimental aggregation caused quenching (ACQ) during their real-world applications. Since the discovery of the AIE phenomena, efforts to develop novel luminescent materials have skyrocketed, creating newer opportunities for the improvement of the properties of the light-emissive materials for OLEDs. Herein we have reported the synthesis of bis-carbazole-derived luminogen by oxidative coupling of the carbazole in good yield. The bis-carbazole luminogen 6 displayed sky-blue-emission in the aggregated state along with excellent electrochemical property and thermal stability, which are considered essential features for constructing high efficiency OLEDs. The luminogen 6 was employed as an emitter for fabricating non-doped OLED and the device resulted in sky-blue emission with CIE coordinates (0.23, 0.41). The light emitting device had a maximum current efficiency of 3.25 cd/A and a maximum external quantum efficiency (EQE) of 5 %, which eventually rolled-off to about 2 % at a J ∼ 50 mA/cm2. Our findings presented a novel way for constructing blue emissive materials for potential application in OLED devices.

Orange-red and saturated red thermally activated delayed fluorescent dendrimers for non-doped solution-processed OLEDs

Two thermally activated delayed fluorescence (TADF) dendritic emitters with orange-red and saturated red emission, respectively, were obtained by coupled diphenylamine-carbazole dendron groups as donor with di(pyridin-3-yl)methanone as accepter. The two TADF dendrimers of tBuDPACz-DPyM and MeODPACz-DPyM exhibited small energy gaps between the singlet and triplet excited states (0.04 vs 0.03 eV). As a consequence, the tBuDPACz-DPyM non-doped solution-processed organic light-emitting devices (OLED) exhibited high current efficiency of 2.36 cd A−1 and external quantum efficiency of 1.82% with low turn-on voltage 3.4 V as well as a lower efficiency roll-off. Moreover, Commission Internationale de L'Éclairage (CIE) coordinates of two OLED devices were (0.56, 0.43) and (0.62, 0.37) for tBuDPACz-DPyM and MeODPACz-DPyM respectively.

Fluorescent pyrene-imidazole material for deep-blue organic light-emitting devices

Numerous endeavors have been exerted to devise deep-blue light-emitting materials (LEMs) for organic light-emitting devices (OLEDs). Nevertheless, deep-blue light-emitting materials exhibiting high efficiency for simple-structured OLEDs are still scarce. Herein, a donor-acceptor based LEM, 4'-(4, 5-diphenyl-1-(3-(pyren-1-yl)phenyl)-1H-imidazole-2-yl)-N,N-diphenylbiphenyl-4-amine (TPA-PyI) is developed by merging imidazole, triphenyl-amine and pyrene units for OLEDs. TPA-PyI is used as the emitting layer in OLED device, which emits a deep-blue fluorescence at 444 nm. With a current efficiency (CE) of 3.05 cd/A and a power efficiency (PE) of 2.95 lm/W, an EQE of 2.97% is achieved. High thermal-stability is achieved with decomposition-temperature (Td) of 516 °C.

Charge Recombination in Polaron Pairs: A Key Factor for Operational Stability of Blue‐Phosphorescent Light‐Emitting Devices

AbstractIrreversible chemical reactions are responsible for limited operational lifetime of organic light‐emitting devices (OLEDs). These reactions are triggered by highly reactive polaron pairs present in the emissive layer of OLEDs. Fast recombination of the polaron pairs is, therefore, crucial for slow degradation and high stability of OLED materials. Here, a study of the formation and annihilation of close polaron pairs in binary mixtures of wide bandgap hosts and a series of blue‐phosphorescent Ir(III) complex dopants, including two novel compounds, is reported. OLED devices containing doped light‐emitting layer are fabricated, and their operational lifetimes are estimated. Although inaccessible in solid films, charge recombination kinetics inside the polaron pairs is measured in liquid solutions using nanosecond laser flash photolysis. Multiscale computer simulations are applied to connect experimental results in different media and predict recombination rates in the device, with proper account taken of the inner‐ and outersphere reorganization in nonpolar materials. Predicted rates correlate with measured operational lifetimes, which demonstrates the key role of polaron pairs in the OLED degradation process. The developed methodology is useful for the rational design of novel OLED materials with higher efficiency and stability.

Synthesis and properties study of a X-type dendrimer based on triphenylamine

A X-type dendrimer DTPA based on triphenylamine was designed, synthesized, and characterized by NMR and mass spectrometry. The obtained dendrimer possesses good thermal stability. Utilizing DTPA as the hole-transporting layer, the organic light emitting devices (OLEDs) were fabricated and the performances were investigated. The devices test showed that DTPA-based devices exhibited higher efficiency and higher external quantum efficiency (EQE), which indicated DTPA possesses good hole-transporting ability. Moreover, the DTPA-based devices also realized an operation lifetime of 265h at 20mA/cm2. The good hole-transporting ability and stability indicated the obtained DTPA is a promising candidate for hole-transporting materials, and has potential application in OLED devices.

ITO-Free OLED and OPV Devices with a Transparent Silver Nanowire Electrode

ABSTRACTCambrios has developed a transparent conductor material based on silver nanowires which can be used to replace the ITO layer in organic photovoltaics (OPV) device and in organic light-emitting devices (OLED).After being deposited from a liquid suspension by conventional coating or printing methods onto a transparent substrate, these nanowires form a transparent conducting network. The sheet resistance of the resulting film is determined by the density of the nanostructures and can thus be easily controlled during the coating process.The elimination of the ITO layer also results in a reduced microcavity effect and thus has a positive impact on the optical performance for OLED lighting devices.This paper will focus on the use of this material as an ITO replacement for OLED devices for lighting applications and for OPV devices. The performance of ITO-free OPV and OLED devices with a nanowire anode will be discussed. We also will present the optical performance data of OLED lighting devices which show the implications of a reduced microcavity effect. In addition, we will show lifetime data for these devices which demonstrate the viability of this technology.

A new near-IR luminescent erbium(III) complex with potential application in OLED devices

We report the synthesis and X-ray structural characterization of the new Er3+ ternary complex [Er(hd)3(bipy)] (where Hhd is 3,5-heptanedione and bipy is 2,2′-bipyridine) as well as its absorption/luminescent properties. X-ray analysis of the novel complex reveals its triclinic centrosymmetric structure with two symmetry independent complexes in the unit cell. Each lanthanide ion is surrounded by 6 oxygen atoms and 2 nitrogen atoms in a square antiprismatic geometry. The solid-state electronic absorption spectra and the luminescence spectrum show long-wavelength 4f–4f transitions which provide a potential use of the compound as a NIR emitting material in organic light-emitting devices (OLEDs).

Electroluminescence properties of poly(3‐hexylthiophene)–cadmium sulfide nanoparticles grown in situ

AbstractIn this work, a simple approach to prepare luminescent poly(3‐hexylthiophene)–CdS nanocomposites to be employed in organic light emitting devices (OLED) devices is reported. The nucleation and growth of CdS nanoparticles were obtained by the thermolysis of a single Cd and S precursor dispersed in the polymer at three different temperatures of annealing: 240, 265, and 300°C. In this way, it was possible to compare the properties of nanocomposites containing nanoparticles with different sizes. X‐ray diffraction and transmission electron microscopy analyses confirmed the formation of CdS nanoparticles and gave information about the size, distribution, and morphology of the nanoparticles; monodispersive and very small nanoparticles with diameters below 2.5 nm were obtained at 240°C. The application of such nanocomposites as emitting layers in OLED devices is discussed. Enhanced electrooptical properties were observed for the device containing the nanocomposite annealed at 240°C with respect to the pure polymer based device. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Excimer emission from a novel ethyne-based fluorescent dye in organic light-emitting devices

A novel ethyne-based fluorescent dye, 10-(4-Dimethylamino-phenyl ethynyl)-anthracene-9-carbonitrile (DMAPEAC), emitting orange/red light was employed as the light emission layer in organic light-emitting devices (OLED). The absorption and photoluminescence (PL) peak of DMAPEAC dispersed in methylene chloride were measured to be 480 nm and 600 nm, respectively. On the other hand, DMAPEAC (3 wt.%) dispersed in tris(8-hydroxyquinoline) aluminum (Alq 3) films deposited on fused silica exhibited two PL peaks at 600 nm and 630 nm. The extra PL peak at 630 nm was shown to be due to the excimer formation by absorption measurement. An OLED device with trilayer structure using N,N′-diphenyl- N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) as the hole transport layer, Alq 3 doped with DMAPEAC as the emission layer, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as the hole-blocking and electron-injection layer, and lithium fluoride (LiF)/aluminum (Al) as the cathode. A strong exciton energy transfer from Alq 3 to DMAPEAC was observed as revealed by the electroluminescence (EL) and PL spectra. Maximum brightness of 19,400 cd/m 2 was obtained at 16 V for the device with 1% DMAPEAC in the Alq 3 layer.

Preparation and Characterization of Organic Light Emitting Devices Using QD Dopant

Two types of the organic light-emitting devices (OLEDs) with different emission structures were prepared using Alq3 (aluminum tris 8-hydroxyquinoline) host material and quinacridone (QD) dopant at the emission layer. One is the OLED device with emission layer consisting of Alq3 host material doped with QD dopant ("codoped OLED"). The another one has a seperated QD dopant film in the Alq3 emission layer ("undoped OLED"). The maximum brightness of the codoped and undoped OLEDs were 3207 cd/m2 and 1570 cd/m2, respectively. The wavelength of the maximum emission peak in the undoped sample was 527 nm and shifted slightly toward longer wavelength with the value of 540 nm for the codoped OLED sample. The maximum luminous efficiency of the undoped OLED was about 1.4 lm/W and increased to 7.0 lm/W for the codoped sample.

Experiment and Modeling of Conversion of Substrate-Waveguided Modes to Surface-Emitted Light by Substrate Patterning

ABSTRACTTo predict the optical power that could be harvested from light emission that is waveguided in the substrate of organic light emitting devices (OLEDs), a quantitative quantum mechanical model of the light emitted into the waveguided modes has been developed. The model was used to compute the exact distribution of energy in external, substrate and ITO/organic modes as a function of the distance of the emission zone from the cathode. The results are compared to the classical ray optics model and to experiments in two-layer OLED devices. Classical ray optics is found to substantially over-predict the light in waveguided modes.