Low driving voltage white organic light-emitting diodes with high efficiency and low efficiency roll-off

Single emission layer white organic light-emitting diodes (WOLEDs) showing high color stability, low turn-on voltage, high efficiency and low efficiency roll-off by incorporating iridium(III) bis[(4,6-difluo-rophenyl)-pyridinato-N,C2] (FIrpic) and bis(2-phenylbenzothiazolato) (acetylacetonate)iridium(III) (Ir(BT)₂(acac)) phosphors dyes have been demonstrated. Our WOLEDs without any out-coupling schemes as well as n-doping strategies show low operating voltages, low turn-on voltage (defined for voltage to obtain a luminance of 1 cd/m²) of 2.35 V, 79.2 cd/m² at 2.6 V, 940.5 cd/m² at 3.0 V and 10 300 cd/m² at 4.0 V, respectively, and achieve a current efficiency of 40.5 cd/A, a power efficiency of 42.6 lm/W at a practical brightness of 1000 cd/m², and a low efficiency roll-off 14.7% calculated from the maximum efficiency value to that of 5000 cd/m². Such improved properties are attributed to phosphors assisted carriers transport for achieving charge carrier balance in the single light-emitting layer (EML). Meanwhile the host–guest energy transfer and direct exciton formation process are two parallel pathways serve to channel the overall excitons to dopants, greatly reduced the unfavorable energy losses.     

Cadmium-free quantum dots based violet light-emitting diodes: High-efficiency and brightness via optimization of organic hole transport layers

Bright and efficient violet quantum dot (QD) based light-emitting diodes (QD-LEDs) with heavy-metal-free ZnSe/ZnS have been demonstrated by choosing different hole transport layers, including poly(4-butyl-phenyl-diphenyl-amine) (poly-TPD), poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB), and poly-N-vinylcarbazole (PVK). Violet QD-LEDs with maximum luminance of about 930 cd/m², the maximum current efficiency of 0.18 cd/A, and the peak EQE of 1.02% when poly-TPD was used as HTL. Higher brightness and low turn-on voltage (3.8 V) violet QD-LEDs could be fabricated when TFB was used as hole transport material. Although the maximum luminance could reach up to 2691 cd/m², the devices exhibited only low current efficiency (∼0.51 cd/A) and EQE (∼2.88%). If PVK is used as hole transport material, highly efficient violet QD-LEDs can be fabricated with lower maximum luminance and higher turn-on voltages compared with counterpart using TFB. Therefore, TFB and PVK mixture in a certain proportion has been used as HTL, turn-on voltage, brightness, and efficiency all have been improved greatly. The QD-LEDs is fabricated with 7.39% of EQE and 2856 cd/m² of maximum brightness with narrow FWHM less than 21 nm. These results represent significant improvements in the performance of heavy-metal-free violet QD-LEDs in terms of efficiency, brightness, and color purity.     

Efficient simplified orange and white phosphorescent organic light-emitting devices with reduced efficiency roll-off

Efficient orange phosphorescent organic light-emitting devices based on simplified structure with maximum efficiencies of 46.5 lm/W and 51.5 cd/A were reported. One device had extremely low efficiency roll-off with efficiencies of 50.6 cd/A, 45.0 cd/A and 39.2 cd/A at 1000 cd/m², 5000 cd/m² and 10,000 cd/m² respectively. The reduced efficiency roll-off was attributed to more balanced carrier injection and broader recombination zone. The designed simplified white device showed much lower efficiency roll-off than the control one based on multiple emitting layers. The efficiency of simplified white device was 40.8 cd/A at 1000 cd/m² with Commission Internationale de I’Eclairage coordinates of (0.39, 0.46).     

All-small-molecule efficient white organic light-emitting diodes by multi-layer blade coating

Blue and white small-molecule organic light-emitting diodes are fabricated by multi-layer blade coating on hot plate at 80 °C with hot wind. Uniform multi-layer structures are made without dissolution due to rapid drying. Only small molecules originally developed for vacuum deposition are used. For hole transport layer of, 4′,4″-tris(carbazol-9-yl)triphenylamine (TCTA), electron transport layer of 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TBPI), emissive layer host of, 6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy), triplet emitters of bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III) (FIrpic), and cathode of LiF/Al, the peak current efficiency for blue emission is 25.1 cd/A (10.8% and 9.3 lm/W). Orange emitter iridium(III)bis (4-(4-t-butylphenyl) thieno[3,2-c]pyridinato-N,C2′)acetylacetonate (PO-01-TB) is added to obtain white emission with CIE coordinate of (0.39, 0.46) [1]. The current efficiency is 34.2 cd/A (11.6% and 12 lm/W) at maximum, 32.4 cd/A at 1000 cd/m², and 31 cd/A at 10,000 cd/m².     

Highly efficient green top-emitting organic light-emitting devices with metal electrode structure

In this work, the electrical and optical characteristics of top-emitting organic light-emitting device (TEOLED) using metal Ag as anode with different thicknesses have been investigated. The emission peak of fabricated TEOLED is 512 nm for a full-width at half-maximum (FWHM) of 48 nm in forward direction. The TEOLED turns on at 3 V with luminance of 2.38 cd/m² and reaches 16,300 cd/m² at 9 V. The maximum of current efficiency is 5.2 cd/A at 7 V, corresponding to the external quantum efficiency of 1.72%.     

Color stable phosphorescent white organic light-emitting diodes with double emissive layer structure

In this paper, we report color stable phosphorescent white organic light-emitting diodes (OLEDs) based on a double emissive layer (EML) structure composed of blue and red/green phosphorescent units. Deep hole trapping situation of red and green dopants at the red/green EML could induce less voltage dependent white spectral characteristics by restricting the change of exciton generation zone. A wide band-gap host material, 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy), was used for achieving such deep-trap generation. Fabricated phosphorescent white OLED shows a slight color coordinate change of (?0.002, +0.002) from 1000 cd/m² to 5000 cd/m² with power efficiency of 38.7 lm/W and current efficiency of 46.4 cd/A at 1000 cd/m². In addition, negligible color changes were observed by delaying red dopant saturation time using optimum red dopant concentration.     

Efficient fluorescent white organic light-emitting diodes using co-host/emitter dual-role possessed di(triphenyl-amine)-1,4-divinyl-naphthalene

Efficient fluorescent white organic light-emitting diodes with low carrier-injection barriers were fabricated with device structure of indium tin oxide/N,N′-bis-(1-naphthy)-N,N′-diphenyl-1,1′-biphenyl-4-4′-diamine/white emission layer/1,3,5-tris(N-phenyl-benzimidazol-2-yl)benzene/lithium fluoride/aluminium. By blending in the blue host of 1-butyl-9,10-naphthalene-anthracene in the emissive layer an efficient electro-luminescent greenish-blue co-host of di(triphenyl-amine)-1,4-divinyl-naphthalene, with the doping of a trace amount of red dye of 4-(dicyano-methylene)-2-methyl-6-(julolidin-4-yl-vinyl)-4H-pyran, bright and colour-stable white emission with high power-efficiency of 14.6 lm/W at 100 cd/m² or current efficiency of 19.2 cd/A at 300 cd/m² or 18.7 cd/A at 10,000 cd/m² was obtained. The resulted synergistic increase in brightness and efficiency may be attributed to the presence of cascading new routes with comparatively lower electron injection barrier.     

Fluorene derivatives for highly efficient non-doped single-layer blue organic light-emitting diodes

Diphenylamino- and triazole-endcapped fluorene derivatives which show a wide energy band gap, a high fluorescence quantum yield and high stability have been synthesized and characterized. Single-layer electroluminescent devices of these fluorene derivatives exhibited efficient deep blue to greenish blue emission at low driving voltage. The single-layer OLED of PhN-OF(1)-TAZ shows a maximum current efficiency of 1.54 cd/A at 20 mA cm⁻² with external quantum efficiency (EQE) of 2.0% and CIE coordinates of (0.153, 0.088) in deep blue region, while the single-layer device of oligothienylfluorene PhN-OFOT-TAZ shows a maximum brightness of 7524 cd/m² and a maximum current efficiency of 2.9 cd/A with CIE coordinates of (0.20, 0.40) in greenish blue.     

Synthesis and characterization of fluorene-based copolymers as electron-transporting materials for PLEDs

Owing to their low cost, easy processing, and the possibility of flexible fabrication, polymer light-emitting diodes (PLEDs) are emerging as an important class of materials. Despite promising characteristics, the relatively easy ionization of the well-known low-work-function cathodes such as Ca and Ba prevents the full usage of these materials. Herein, we report the syntheses of three alcohol-soluble conjugated polymers with different conjugation lengths and electron affinities as electron injection and transport materials for PLEDs: poly[9,9-bis(2-dihexylaminoethoxy)fluorene-co-tetrafluorobenzene] (PFOH-1), poly[9,9-bis(2-dihexylaminoethoxy)fluorene-co-thiophene] (PFOH-2), and poly[9,9-bis(2-dihexylaminoethoxy)fluorene-co-benzo-thiadiazole] (PFOH-3). For comparison, devices using Al, Ca, and Al cathodes were also fabricated. The device based on the Al cathode showed lower performance with a luminescence efficiency of 0.93 cd/A and a luminance of 248 cd/m²; that based on the low-work-function metal Ca as the cathode showed a near-threefold increase in luminescence efficiency at 2.51 cd/A and brightness at 856 cd/m² owing to greatly enhanced electron injection from the cathode; and the device employing the PFOH-3/Al cathode exhibited a luminescence efficiency of 2.35 cd/A and a brightness of 667 cd/m² at a current density of 35 mA/cm², which is comparable with the performance of the device with the Ca cathode.     

A hybrid white organic light-emitting diode with stable color and reduced efficiency roll-off by using a bipolar charge carrier switch

A hybrid white organic light-emitting diode (WOLED) with an emission layer (EML) structure composed of red phosphorescent EML/green phosphorescent EML/spacer/blue fluorescent EML was demonstrated. This hybrid WOLED shows high efficiency, stable spectral emission and low efficiency roll-off at high luminance. We have attributed the significant improvement to the wide distribution of excitons and the effective control of charge carriers in EMLs by using mixed 4,4′,4″-tri(9-carbazoyl) triphenylamine (TCTA) and bis[2-(2-hydroxyphenyl)-pyridine] beryllium (Bepp₂) as the host of phosphorescent EMLs as well as the spacer. The bipolar mixed TCTA:Bepp_2, which was proved to be a charge carrier switch by regulating the distribution of charge carriers and then the exciton recombination zone, plays an important role in improving the efficiency, stabilizing the spectrum and reducing the efficiency roll-off at high luminous. The hybrid WOLED exhibits a current efficiency of 30.2 cd/A, a power efficiency of 32.0 lm/W and an external quantum efficiency of 13.4% at a luminance of 100 cd/m², and keeps a current efficiency of 30.8 cd/A, a power efficiency of 27.1 lm/W and an external quantum efficiency of 13.7% at a 1000 cd/m². The Commission Internationale de l’Eclairage (CIE) coordinates of (0.43, 0.43) and the color rendering index (CRI) of 89 remain nearly unchanged in the whole range of luminance.     

Extremely low driving voltage electrophosphorescent green organic light-emitting diodes based on a host material with small singlet–triplet exchange energy without p- or n-doping layer

In order to achieve low driving voltage, electrophosphorescent green organic light-emitting diodes (OLEDs) based on a host material with small energy gap between the lowest excited singlet state and the lowest excited triplet state (ΔE_ST) have been fabricated. 2-biphenyl-4,6-bis(12-phenylindolo[2,3-a] carbazole-11-yl)- 1,3,5-triazine (PIC–TRZ) with ΔE_ST of only 0.11 eV has been found to be bipolar and used as the host for green OLEDs based on tris(2-phenylpyridinato) iridium(III) (Ir(ppy)₃). A very low onset voltage of 2.19 V is achieved in devices without p- or n-doping. Maximum current and power efficiencies are 68 cd/A and 60 lm/W, respectively, and no significant roll-off of current efficiency (58 cd/A at 1000 cd/m² and 62 cd/A at 10,000 cd/m²) have been observed. The small roll-off is due to the improved charge balance and the wide charge recombination zone in the emissive layer.     

New hole blocking material for green-emitting phosphorescent organic electroluminescent devices

The new amorphous molecular material, 2,5-bis(4-triphenylsilanyl-phenyl)-[1,3,4]oxadiazole, that functions as good hole blocker as well as electron transporting layer in the phosphorescent devices. The obtained material forms homogeneous and stable amorphous film. The new synthesized showed the reversible cathodic reduction for hole blocking material and the low reduction potential for electron transporting material in organic electroluminescent (EL) devices. The fabricated devices exhibited high performance with high current efficiency and power efficiency of 45 cd/A and 17.7 lm/W in 10 mA/cm², which is superior to the result of the device using BAlq (current efficiency: 31.5 cd/A and power efficiency: 13.5 lm/W in 10 mA/cm²) as well-known hole blocker. The ITO/DNTPD/α-NPD/6% Ir(ppy)₃ doped CBP/2,5-bis(4-triphenylsilanyl-phenyl)-[1,3,4]oxadiazole as both hole blocking and electron transporting layer/Al device showed efficiency of 45 cd/A and maximum brightness of 3000 cd/m² in 10 mA/cm².     

Optoelectronic characteristics of MEH-PPV polymer LEDs with thin,doped composition-graded a-SiC:H carrier injection layers

The optoelectronic characteristics of poly(2-methoxy-5-(2′ethyl-hexoxy)-1,4-phenylene-vinylene) (MEH-PPV) polymer LEDs (PLEDs) have been improved by employing thin doped composition-graded (CG) hydrogenated amorphous silicon–carbide (a-SiC:H) films as carrier injection layers and O₂-plasma treatment on indium–tin-oxide (ITO) transparent electrode, as compared with previously reported ones having doped constant-optical-gap a-SiC:H carrier injection layers. For PLEDs with an n-type a-SiC:H electron injection layer (EIL) only, the electroluminescence (EL) threshold voltage and brightness were improved from 7.3 V, 3162 cd/m² to 6.3 V, 5829 cd/m² (at a current density J = 0.6 A/cm²), respectively, by using the CG technique. The enhancement of EL performance of the CG technique was due to the increased electron injection efficiency resulting from a smoother barrier and reduced recombination of charge carriers at the EIL and MEH-PPV interface. Also, surface modification of the ITO transparent electrode by O₂-plasma treatment was used to further improve the EL threshold voltage and brightness of this PLED to 5.1 V, 6250 cd/m² (at J = 0.6 A/cm²). Furthermore, by employing the CG n[p]-a-SiC:H film as EIL [hole injection layer (HIL)] and O₂-plasma treatment on the ITO electrode, the brightness of PLEDs could be enhanced to 9350 cd/m² (at a J = 0.3 A/cm²), as compared with the 6450 cd/m² obtained from a previously reported PLED with a constant-optical-gap n-a-SiCGe:H EIL and p-a-Si:H HIL.     

High-efficiency and multi-function blue fluorescent material for organic electroluminescent devices

We demonstrate one high-efficiency blue fluorescent material, N-(4-((E)-2-(6-((E)-4-(diphenylamino)styryl)naphthalen-2-yl)vinyl)phenyl)-N-phenylbenzenamine, with an emissive peak of 472 nm and the hole-transporting property speculated from different devices. It can function either as the single emissive layer or as the dye doped in N,N′-dicarbazolyl-4-4′-biphenyl (CBP). The former shows a maximum current efficiency and luminance of 7.06 cd/A (0.04 mA/cm²) and 16 930 cd/m², in contrast to 11.5 cd/A (4.35 mA/cm²) and 25 690 cd/m² for the latter. The better performance of the latter can be attributed to the bipolar carrier transport property of CBP and the hole-blocking and electron-transporting characteristic of 4,7-diphenyl-1,10-phenanthroline (BPhen), which resulting in a good balance of holes and electrons. Moreover, the Commission Internationale De L’Eclairage coordinates of the latter change slightly from (0.162, 0.3) to (0.148, 0.268) upon increasing the voltage from 3 V to 14 V.     

High-efficiency low color temperature organic light emitting diodes with solution-processed emissive layer

Low color temperature (CT) lighting provides a warm and comfortable atmosphere and shows mild effect on melatonin suppression. A high-efficiency low CT organic light emitting diode can be easily fabricated by spin coating a single white emission layer. The resultant white device shows an external quantum efficiency (EQE) of 22.8% (34.9 lm/W) with CT 2860 K at 100 cd/m², while is shown 18.8% (24.5 lm/W) at 1000 cd/m². The high efficiency may be attributed to the use of electroluminescence efficient materials and the ambipolar-transport host. Besides, proper device architecture design enables excitons to form on the host and allows effective energy transfer from host to guest or from high triplet guest to low counterparts. By decreasing the doping concentration of blue dye in the white emission layer, the device exhibited an orange emission with a CT of 2280 K. An EQE improvement was observed for the device, whose EQE was 27.4% (38.8 lm/W) at 100 cd/m² and 20.4% (24.6 lm/W) at 1000 cd/m².     

Fabrication of high efficiency and color stable white organic light-emitting diodes by an alignment free mask patterning

Small molecule based white organic light-emitting diodes were fabricated by using an alignment free mask patterning method. A phosphorescent red/green emitting layer was patterned by a metal mask without any alignment and a blue phosphorescent emitting layer was commonly deposited on the patterned red/green emitting layer. A white emission could be obtained due to separate emission of red/green and blue emitting layers. A maximum current efficiency of 30.7 cd/A and a current efficiency of 26.0 cd/A at 1000 cd/m² were obtained with a color coordinate of (0.39, 0.45). In addition, there was little change of emission spectrum according to luminance because of balanced red/green and blue emissions.     

High-performance inverted top-emitting green electrophosphorescent organic light-emitting diodes with a modified top Ag anode

Green electrophosphorescent inverted top-emitting organic light-emitting diodes with a Ag/1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) anode are demonstrated. A high current efficacy of 124.7 cd/A is achieved at a luminance of 100 cd/m² when an optical outcoupling layer of N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl-4,4′-diamine (α-NPD) is deposited on the anode. The devices have a low turn-on voltage of 3.0 V and exhibit low current efficacy roll-off through luminance values up to 10,000 cd/m². The angle dependent spectra show deviation from Lambertian emission and color change with viewing angle. Hole-dominated devices with Ag/HAT-CN electrodes show current densities up to three orders of magnitude higher than devices without HAT-CN.     

Highly efficient Organic Light-Emitting Diodes from thermally activated delayed fluorescence using a sulfone–carbazole host material

Organic Light-Emitting Diodes (OLEDs) using the thermally activated delayed fluorescence (TADF) emitter (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) are demonstrated using a novel ambipolar host 3,5-di(carbazol-9-yl)-1-phenylsulfonylbenzene (mCPSOB). When doped in a 5 wt.% concentration, OLEDs with EL efficiency values of more than 81 cd/A for current efficacy and 26.5% for external quantum efficiency are reported. These devices exhibit a low turn-on voltage of 3.2 V at 10 cd/m², as well as reduced efficiency roll-off at high current densities. To the best of our knowledge, these are among the highest ever reported efficiencies for TADF OLEDs, and are even comparable to the highest reported efficiencies for phosphorescent OLEDs.     

High current conduction with high mobility by non-radiative charge recombination interfaces in organic semiconductor devices

We report a unique non-radiative p-n-p junction structure to provide high current conduction with high mobility in organic semiconductor devices. The current conduction was improved by increasing p-n junctions made with intrinsic p-type hole transport layer and n-type electron transport layer. The excellent hole mobility of 5.3 × 10^?1 cm²/V s in this p-n-p device configuration is measured by the space charge limited current method with an electric field of 0.3 MV/cm. Enhanced current conduction of 248% at 4.0 V was observed in fluorescent blue organic light-emitting diodes with introduction of non-radiative p-n-p-n-p junction interfaces. Thereupon, the power efficiency at 1000 cd/m² was improved by 22% and the driving voltage also was reduced by 17%, compared to that of no interface device. Such high current conduction with high mobility is attributed to the carrier recombination at p-n-p interfaces through coulombic interaction. This non-radiative p-n-p junction structure suggested in this report can be very useful for many practical organic semiconductor device applications.     

High performance top-emitting and transparent white organic light-emitting diodes based on Al/Cu/TcTa transparent electrodes for active matrix displays and lighting applications

It is challenging to obtain broadband emission covering as much of the visible light spectrum as possible in top-emitting white organic light-emitting diodes (TEWOLEDs) due to the well known microcavity effects. In this work, we achieved TEWOLED with three separate peak and negligible angular dependence by employing a high transmittance stack cathode Al (2 nm)/Cu (18)/TcTa (60 nm). The TEWOLED shows an efficiency of 25.6 cd/A, 20.1 Lm/W at 1000 cd/m², and low voltage of 4.2 V for 1222 cd/m². Synchronously, we achieved transparent white organic light-emitting diode (TWOLED) using this high transmittance stack cathode, the TWOLED exhibits similar spectrum and comparable luminance from both sides, and the maximum total efficiencies of the TWOLED are 28.6 cd/A, 24.9 Lm/W.