High Brightness Organic Light-emitting Diodes Research Articles

High Brightness Organic Light-Emitting Diodes with Capillary-Welded Hybrid Diameter Silver Nanowire/Graphene Layers as Electrodes.

The development of silver nanowire electrodes is always limited due to some disadvantages, such as roughness, oxidative properties, and other disadvantages. In this research, a capillary-welded silver nanowire/graphene composite film was used as an electrode for organic light-emitting diode (OLED) devices. As an encapsulation layer, graphene reduced the surface roughness and the oxidation probability of silver nanowires. The composite electrode showed an excellent transmittance of 91.5% with low sheet resistant of 26.4 ohm/sq. The devices with the silver nanowire/graphene composite electrode emitted green electroluminescence at 516 nm, and the turn-on voltage was about 3.8 V. The maximum brightness was 50810 cd/cm2, which is higher than the indium tin oxide-based (ITO-based) devices with the same configuration. Finally, it was proved that the silver nanowire/graphene composite electrodes possessed better heat dissipation than the ITO-based ones under energization. In summary, it means that this novel silver nanowires/graphene electrode has great potential in OLED device applications.

Nano-composites for enhanced catastrophic failure temperature of organic light-emitting diodes

Thermal stability is one of the key technical challenges in developing high brightness organic light-emitting diodes (OLEDs). In this study, thermal catastrophic failure, a process involving temperature-induced abrupt device breakdown, is found to have a high degree of correlation to the glass transition process of the organic molecules. It is also found that C60-organic nano-composites can be used effectively to increase the glass transition temperature of the organic small molecular thin-films leading to the improvement of the thermal stability of OLEDs. In addition, a universal mathematical formula with only one variable, concentration, is discovered to quantify well the glass transition temperatures of all C60 nano-composites. The thermal catastrophic failure temperature of OLEDs with C60 nano-composites as a hole transport layer is found to be proportional to the glass transition temperature of C60 nano-composites.

Exciton–Polaron Interactions in Polyfluorene Films with β-Phase

Fluorescence quenching by electric charges is an important loss mechanism in high-brightness organic light-emitting diodes (OLEDs), but its effect is difficult to quantify in working devices. Here, we combine an electrochemical technique to control the charge density with time-resolved photoluminescence to distinguish between different quenching mechanisms. The material studied was the blue electroluminescent polymer poly(9,9-dioctylfluorene) with β-phase. Our results show that quenching occurs by Forster resonance energy transfer and is mediated by exciton diffusion. We determine the quenching parameters over a wide range of charge concentrations and estimate their impact on the OLED efficiency roll-off at high current density. We find that fluorescence quenching by charges and singlet–triplet exciton annihilation are the two main mechanisms leading to the efficiency roll-off. Our results suggest that hole polarons are not very effective quenchers of singlet excitons, which is important for understanding...

High-brightness organic light-emitting diodes for optogenetic control of Drosophila locomotor behaviour

Organic light emitting diodes (OLEDs) are in widespread use in today’s mobile phones and are likely to drive the next generation of large area displays and solid-state lighting. Here we show steps towards their utility as a platform technology for biophotonics, by demonstrating devices capable of optically controlling behaviour in live animals. Using devices with a pin OLED architecture, sufficient illumination intensity (0.3 mW.mm−2) to activate channelrhodopsins (ChRs) in vivo was reliably achieved at low operating voltages (5 V). In Drosophila melanogaster third instar larvae expressing ChR2(H134R) in motor neurons, we found that pulsed illumination from blue and green OLEDs triggered robust and reversible contractions in animals. This response was temporally coupled to the timing of OLED illumination. With blue OLED illumination, the initial rate and overall size of the behavioural response was strongest. Green OLEDs achieved roughly 70% of the response observed with blue OLEDs. Orange OLEDs did not produce contractions in larvae, in agreement with the spectral response of ChR2(H134R). The device configuration presented here could be modified to accommodate other small model organisms, cell cultures or tissue slices and the ability of OLEDs to provide patterned illumination and spectral tuning can further broaden their utility in optogenetics experiments.

Effects of transparent MPTMS/Ag/MoO3 structure as anode on the performance of green organic light-emitting diodes

A transparent 3-mercaptopropyl trimethoxysilane (MPTMS)/Ag/MoO3 composite anode is introduced to fabricate green organic light-emitting diodes (OLEDs). Effects of the composite anode on brightness and operating voltage of OLEDs are researched. By optimizing the thickness of each layer of the MPTMS/Ag/MoO3 structure, the transmittance of MPTMS/Ag (8 nm)/MoO3 (30 nm) reaches over 75% at about 520 nm. The sheet resistance is 3.78 Ω/□, corresponding to this MPTMS/Ag (8 nm)/MoO3 (30 nm) structure. For the OLEDs with the optimized anode, the maximum electroluminescence (EL) current efficiency reaches 4.5 cd/A, and the maximum brightness is 37 036 cd/m2. Moreover, the OLEDs with the optimized anode exhibit a very low operating voltage (2.6 V) for obtaining brightness of 100 cd/m2. We consider that the improved device performance is mainly attributed to the enhanced hole injection resulting from the reduced hole injection barrier height. Our results indicate that employing the MPTMS/Ag/MoO3 as a composite anode can be a simple and promising technique in the fabrication of low-operating voltage and high-brightness OLEDs.

High-brightness organic light-emitting diodes based on a simplified hybrid structure

High-brightness green phosphorescent hybrid inorganic-organic light-emitting diodes (HyLEDs) based on a simplified WO3/organic layer structure have been demonstrated. The WO3 layer greatly improved hole injection and enabled facile hole transport, leading to balanced charge injection and significant performance enhancement. The HyLEDs reached a luminance of 104 cd/m2 at 7.3 V. At this brightness level, their current efficiency was 55.2 cd/A, 57% higher than that of all-organic LEDs with a N,N-bis-(1-naphthyl)-N,N’-diphenyl-1,1′-biphenyl-4,4′-diamine hole transport layer. The HyLEDs also exhibited markedly improved reliability under constant-current stressing due to the stable indium-tin-oxide/WO3 interface.

High-brightness organic light-emitting diodes

The research of high-brightness organic light-emitting diodes, as an important branch of organic light-emitting diodes (OLEDs), makes it possible for achieving high-brightness lighting source and lasing. Heat dissipation and efficiency roll off, as two main factors, affect the brightness of the OLEDs heavily. In this paper, high-brightness OLEDs are obtained by utilizing pulse voltage, small areas and micro-cavity structure to minimize the effect of the two factors. The major advances, ongoing challenges and future perspectives of this research frontier are also critically discussed.

Multiwalled carbon nanotube sheets as transparent electrodes in high brightness organic light-emitting diodes

We have fabricated efficient organic light emitting diodes using strong, transparent carbon nanotube (CNT) sheets as the hole-injecting anode. These devices show a brightness of 4500 cd/m2 and current efficiency near 2.5 cd/A, which is close to the efficiency we achieve with a similar device, which uses indium tin oxide (ITO) as the anode. We demonstrate that proper planarization of the electrode using the water-soluble polymer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) is necessary for achieving high efficiency and can be accomplished by spin casting multiple layers. We believe that increased conductivity of the sheets could lead to CNT-based devices with efficiencies exceeding those on ITO.