Efficient White Organic Light-emitting Devices Research Articles

High-Efficiency Simplified Orange and White Organic Light-Emitting Devices Based on a Platinum(II) Complex.

We demonstrated efficient simplified orange and white organic light-emitting devices based on a platinum(II) complex Tetra-Pt-N. The maximum current efficiency achieved from the optimized orange device was 57.6 cd/A. The emission mechanism for the system of Tetra-Pt-N doped into 4,4'-bis(arbazole-9-yl)biphenyl was discussed. Moreover, a high-efficiency and simplified white device was fabricated by introducing an ultra-thin blue phosphorescent emission layer. The white device with a maximum current efficiency of 41.9 cd/A showed excellent stable spectra and low efficiency roll-off.

Analyses of emission efficiencies of white organic light-emitting diodes having multiple emitters in single emitting layer

Efficient white organic light-emitting devices (OLEDs) are highly required in various applications. Yet analyses of light extraction/out-coupling efficiencies of white OLEDs having multiple emitters are much more complicated and difficult than single-emitter monochromatic OLEDs, as ratios of excitons formed on different emitters, different optical (microcavity) effects on emission of different emitters, and effects of emission characteristics (e.g., emitting dipole orientations) of different emitters etc. need to be considered simultaneously. In this work, we report efficient single-emitting-layer white OLEDs adopting blue and orange-red emitters and a systematic and quantitative optical simulation approach taking into account all these factors, for analyzing optical out-coupling efficiencies of single-emitting-layer multiple-emitter white OLEDs. It is believed to be useful for development of efficient white OLEDs. • Efficient white OLEDs with a single emitting layer having blue and orange-red emitters. • Analyses of exciton partition ratios on different emitters of white OLEDs with considering optical microcavity effects. • Quantitative analyses of optical out-coupling efficiencies of white OLEDs having multiple emitters.

A deep blue fluorescent emitter functioning as host material in highly efficient phosphorescent and hybrid white organic light-emitting devices

Abstract A deep blue fluorescent emitter of 4′, 4‴-(1H-phenanthro [9, 10-d]-imidazole-1, 2-diyl) bis (N, N-diphenyl-[1,1′-biphenyl]-4-amine) (PPI-2TPA) possessing special virtues of high triplet energy, excellent photoluminescence quantum yield and preferable bipolar transporting property was employed as host material by the first time to realize a highly efficient phosphorescent organic light-emitting device (OLED). In comparison with the reference device, the optimized device had better performance, which showed the maximum efficiencies of 40.4 cd/A and 45.3 l m/W and still kept high efficiencies of 39.7 cd/A and 37.1 l m/W at the luminance of 1000 cd/m2. Moreover, the critical current density was as high as 179.6 mA/cm2, which was about 1.8-fold that of the reference device, indicating a significant alleviation of efficiency drop at high current density in the optimized device. The emission of phosphorescent emitter originated from both energy transfer and hole-electron recombination, distinctly revealed by a systematical analysis on the device electroluminescence (EL) characteristics. Simultaneously, an efficient hybrid white OLED as well as a tandem white OLED which demonstrated quite slight color variation over a wide range of luminance were also constructed.

Efficient and chromaticity stable green and white organic light-emitting devices with organic-inorganic hybrid materials.

Efficient inverted bottom emissive organic light emitting diodes (IBOLEDs) with tin dioxide and/or Cd-doped SnO2 nanoparticles as an electron injection layer at the indium tin oxide cathode:electron transport layer interface have been fabricated. The SnO2 NPs promote electron injection efficiently because their conduction band (−3.6 eV) lies between the work function (Wf) of ITO (4.8 eV) and the LUMO of the electron-transporting molecule (−3.32 eV), leading to enhanced efficiency at low voltage. The 2.0% SnO2 NPs (25 nm) with Ir(ddsi)2(acac) emissive material (SnO2 NPs/ITO) have an enhanced current efficiency (ηc, cd A−1) of 52.3/24.3, power efficiency (ηp, lm W−1) of 10.9/3.4, external quantum efficiency (ηex, %) of 16.4/7.5 and luminance (L, cd m−2) of 28 182/1982. A device with a 2.0% Cd-doped SnO2 layer shows higher ηc (60.6 cd A−1), ηp (15.4 lm W−1), ηex (18.3%) and L (26 858 cd m−2) than SnO2 devices or control devices. White light emission was harvested from a mixture of Cd–SnO2 NPs and homoleptic blue phosphor Ir(tsi)3; the combination of blue emission (λEL = 428 nm) from Ir(tsi)3 and defect emission from Cd–SnO2 NPs (λEL = 568 nm) gives an intense white light with CIE of (0.31, 0.30) and CCT of 6961 K. The white light emission [CIE of (0.34, 0.35) and CCT of 5188 K] from colloid hybrid electrolyte BMIMBF4–SnO2 is also discussed.

Realizing an efficient warm white organic light-emitting device possessing excellent color-stability and color rendering index

Abstract We were successful in realizing an efficient warm white organic light-emitting device simultaneously exhibiting extreme color-stability and high color rendering index (CRI) just by precisely confining the recombination zone and fully utilizing the energy transfer between excitons. The warm white OLED presents maximum efficiencies of 21.4 cd/A, 25.8 lm/W and still remains at high efficiencies of 17.7 cd/A, 15.6 lm/W at the luminance of 1000 cd/m2. As the operating current density increases from 10 mA/cm2 to 60 mA/cm2, the variations in Commission Internationale de l’Eclairage (CIE) coordinates are only (0.004, 0.007) while the general CRI (Ra) maintains at 80 approximately and the special CRI R9 is higher than 20. What is more, the Duv satisfies the criteria for CRI measurements and the gamut area index (GAI) ensures the vividness of the illuminated objects, as renders the fabricated white OLED a promising candidate for high-quality solid-state lighting application. We also studied the origin of the quite slight color variations for the warm white OLED.

Improving the color-rendering index of a tandem warm white organic light-emitting device by employing a simple fabrication process.

We realized an efficient spectra-stable tandem white organic light-emitting device (WOLED) with pleasurable warm white emission and an extremely high color-rendering index (CRI) simultaneously by connecting a fluorescent blue unit and a phosphorescent dual-color unit via an easy-fabrication charge generation unit (CGU). Over a wide range of driving current density, the tandem warm white OLED exhibited general CRI (Ra) value exceeding 85 and special CRI (R9) value close to 57. In addition, Duvs are within the tolerance for CRI measurements, ensuring validity and logicality of the calculated results concerning Ra and R9. The obtained tandem OLED showed the maximum efficiencies of 38cd/A and 22.9lm/W, and still maintained high efficiencies of 35.5cd/A and 17.2lm/W at the luminance of 1000 cd/m2. Moreover, we studied the origins of both extreme color stability in dual-color OLED and extremely efficient electron injection within the CGU. To the best of our knowledge, this is the first report regarding quantitative values of both R9 and the Duv for a high-Ra tandem WOLED.

Efficient and chromaticity-stable flexible white organic light-emitting devices based on organic-inorganic hybrid color-conversion electrodes.

We have developed a novel organic–inorganic hybrid color conversion electrode composed of Ag NWs/poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) via a solution process, which is the first report on a color conversion electrode for applications in flexible optoelectronics. Using the Ag NWs/MEH-PPV composite film as the anode on polyethylene terephthalate substrate and combined with a blue organic light emitting devices (OLEDs) unit employing bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(iii)) (Flrpic) in 1,3-bis(carbazol-9-yl)benzene (mCP) as the emitting layer, a highly efficient and chromaticity-stable color-conversion flexible white OLEDs (WOLEDs) is achieved with a maximum current efficiency of 20.5 cd A−1. To the best of our knowledge, this is the highest efficiency reported for color-conversion based flexible WOLEDs. Our work provides an approach to achieving high-performance flexible WOLEDs devices and demonstrates great potential for lighting and display applications.

Hybrid warm-white organic light-emitting device based on tandem structure.

We realized an efficient hybrid tandem warm white organic light-emitting device with extremely stable color purity in forward direction by employing an easy-fabrication charge generation unit (CGU) combining 1, 4, 5, 8, 9, 11-Hexaazatriphenylene-hexacarbonitrile with ultrathin bilayers of LiF and Al. The tandem white device exhibits maximum efficiencies of 37.3 cd/A, 23.3 lm/W and still remains at high efficiencies of 34.4 cd/A, 17 lm/W at the luminance of 1000 cd/m2. When the operating current density increases from 5 mA/cm2 to 30 mA/cm2, the variations in Commission Internationale de l'Eclairage (CIE) coordinates are only (0.007, 0.003). Moreover, we also studied the cause of color variation in tandem white device by performing systematic optical simulation and discovered the origin of alleviation in efficiency roll-off for tandem white device by applying triplet-polaron quenching (TPQ) model.

Simplified efficient warm white tandem organic light-emitting devices by ultrathin emitters using energy transfer from exciplexes

Abstract To realize energy-saving and environmental green illumination by simplified white organic light-emitting devices (WOLEDs) structure and fabricate process, herein, we successfully fabricate a novel simplified efficient WOLED structure using energy transfer from interface exciplexes to complementary color dyes as undoped ultrathin emissive layers (UEMLs) based on tandem OLEDs structure. (Acetylacetonato)bis[2-(thieno[3,2-c]pyridin-4-yl)phenyl]iridium(III) (PO-01) and iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′]-picolinate (FIrpic) are applied as yellow and blue UEMLs, respectively. Particularly, the designed WOLED with Commission Internationale De L'Eclairage chromaticity coordinate (0.36, 0.41) obtaining a peak of 41.5 cd/A (EQE = 18.59%) without light out-coupling technology, achieving high current efficiency among reported WOLEDs all emitters applied UEMLs. Moreover, the designed white OLEDs generally emit warm white spectrum with low correlative color temperature of 3686 K–4700 K, which show are available for healthy illumination. Therefore, we believe our work help to further develop ideal simplified WOLEDs structure and fabricate process for energy-conservation and environment-friendly healthy illumination with high efficiency, simplified fabrication, low reagent consumption and cost, simultaneously.

Interfacial engineering of graphene for highly efficient blue and white organic light-emitting devices

Graphene as anodes of flexible organic light-emitting devices (OLEDs) has intrinsic drawbacks of a low work function and a high sheet resistance although it can eliminate the brittle feature of ITO. Chemical doping as a conventional approach is universally used to decrease the sheet resistance and adjust the work function of graphene electrodes, but it suffers from instability problems due to the volatility of chemical species. Here, an insulated poly(4-styrenesulphonate) (PSS) modification layer is firstly coated on the graphene surface along with improved air-stability and hole-injection ability via interfacial dipoles. Besides, the utilization of PSS is beneficial to reduce the leakage current of OLEDs. Then a gradient injection layer of poly(3,4-ethylenedioxythiophene):PSS (PEDOT:PSS)/tetrafluoroethyleneperfluoro-3,6-dioxa-4-methyl-7-octenesulphonic acid copolymer-doped PEDOT:PSS is covered onto the PSS-modified graphene to further promote hole injection and improve carrier balance inside OLEDs. With above interfacial modification technique, very high efficiencies of 201.9 cd A−1 (76.1 lm W−1, 45.2%) and 326.5 cd A−1 (128.2 lm W−1, 99.5%) for blue and white emissions are obtained, which are comparable to the most efficient display and lighting technologies so far.

Efficient and angle-stable white top-emitting organic light emitting devices with patterned quantum dots down-conversion films

Abstract Efficient white top-emitting organic light-emitting devices (TEOLEDs) are fabricated by combining blue TEOLEDs with down-conversion films. Quantum dots (QDs) are integrated with polymethyl methacrylate (PMMA) to obtain freestanding composite films, and the films are used as down-conversion films to produce red emission by converting blue emission from blue TEOLEDs. The blue and white TEOLEDs exhibit the maximum current efficiency of 37.4 and 22.2 cd/A, respectively. Through integrating a microstructure QDs down-conversion film by transfer printing on blue TEOLED, the light extraction performance of white TEOLEDs is improved and the angular distribution of the white emission is almost Lambertian. The CIE color coordinates of white TEOLEDs with patterned QDs down-conversion films shift only (0.025, 0.005) from 0° to 60°.

Highly efficient white organic light-emitting devices with optimized electron transporting layers

We fabricated three phosphorescent blue organic light-emitting devices based on blue phosphor of iri-dium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′](Firpic) with different electron transporting layer(ETL) materials. By analyzing efficiency curves and spectral characteristics, the significant effect of ETL on many aspects of device performance was demonstrated. With optimized ETL, the characteristics of devices, such as voltage and efficiency were significantly improved. Combined with a yellow phosphor of iridium(III) bis(4-phenylthieno[3,2-c]pyridinato-N,C2′) acetylacetonate(PO-01), phosphorescent white organic light-emitting devices(PhWOLEDs) were obtained. Then, with an aim to promote the performance of the PhWOLEDs, a thin layer of 4,4′,4″-tri(N-carbazolyl)-triphenylamine(TCTA) was introduced between two light emission layers, and the diffusion of excitons was confined. The outperformance device fabricated with 4,7-diphenyl-1,10-phenanthroline(Bphen) as the electron transporting layer exhibited a peak current efficiency of 36.6 cd/A, a peak power efficiency of 19.2 lm/W, and the International Commission on Illumination coordinates(0.37, 0.46).

Color stable and highly efficient hybrid white organic light-emitting devices using heavily doped thermally activated delayed fluorescence and ultrathin non-doped phosphorescence layers

Blue/orange complementary fluorescence/phosphorescence hybrid white organic light-emitting devices with excellent color stability and high efficiency have been fabricated, which are based on an easily fabricated multiple emissive layer (EML) configuration with an ultrathin non-doped orange phosphorescence EML selectively inserted between heavily doped blue thermally activated delayed fluorescence (TADF) EMLs. Through systematic investigation and improvement on luminance-dependent color shift and efficiency deterioration, a slight Commission Internationale de 1′Eclairage coordinates shift of (0.008, 0.003) at a practical luminance range from 1000 to 10000 cd/m2, a maximum power efficiency of 45.8 lm/W, a maximum external quantum efficiency (EQE) of 15.7% and an EQE above 12% at 1000 cd/m2 have been achieved. The heavily doped blue TADF emitters which act as the main charge transport channels and recombination sites in the host with high-lying lowest triplet excited state, take advantage of the bipolar transport ability to broaden the major charge recombination region, which alleviates triplet energy loss. The selectively inserted ultrathin non-doped orange EML makes its emission mechanism dominated by Förster energy transfer, which is effective to keep color stable under different drive voltages.

Efficient white organic light-emitting devices based on a novel phosphine oxide host with low driving voltage and excellent color stability

Efficient phosphorescent white organic light-emitting devices (WOLEDs) based on a novel phosphine oxide host, 4,6-bis(diphenylphosphoryl)dibenzofuran (DBFDPO), with particularly low turn-on voltage and high color stability are reported. Without any electrical doping (p-doing or n-doping) strategies, the simplified optimal WOLEDs utilizing DBFDPO as the same host for blue/yellow emitters show low operating voltages, 1cd/m2at 2.5V (turn-on voltage), 46cd/m2 at 3V and 2787cd/m2at 5V, and 12840cd/m2 at 7V, respectively. A forward-viewing current efficiency (CE) of 41.3cd/A (power efficiency (PE) of 34.0lm/W) is achieved at the luminance of 673.4cd/m2 without any out-coupling enhancement structures. Furthermore, improved color stability is achieved through adjusting the yellow emitting layer. The Commission Internationale de I’Eclairage coordinates (CIE) of the optimal WOLED merely shift from (0.330, 0.405) to (0.327, 0.400) over a wide range of luminance (10–10,000cd/m2). We have revealed that the competition of the two mechanisms of energy transfer and carrier-trapping/-assist transport severely impact on the spectra stability.

Highly efficient orange and white phosphorescent organic light-emitting devices with simplified structure

We successfully developed efficient orange and white phosphorescent organic light-emitting devices based on simplified structure. The maximum power efficiency and current efficiency of the orange device with only 5nm emission layer are 34.1lm/W and 38.0cd/A. Performances of the simplified orange devices can be greatly improved to 62.6lm/W (79.7cd/A) by introducing one blue phosphorescent dye iridium(III)[bis(4,6-difluorophenyl)-pyridina-to-N,C2′]picolinate into the orange emission layer. The emission mechanism of the above orange devices was also discussed. White light emission with Commission International de L’Eclairage coordinates of (0.36, 0.45) and efficiency of 40.6lm/W (38.7cd/A) was obtained by exactly adjusting the doping concentration of orange phosphorescent dye.

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 46.5lm/W and 51.5cd/A were reported. One device had extremely low efficiency roll-off with efficiencies of 50.6cd/A, 45.0cd/A and 39.2cd/A at 1000cd/m2, 5000cd/m2 and 10,000cd/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 40.8cd/A at 1000cd/m2 with Commission Internationale de I’Eclairage coordinates of (0.39, 0.46).

A novel nicotinonitrile derivative as an excellent multifunctional blue fluorophore for highly efficient hybrid white organic light-emitting devices

A multifunctional compound that serves as both an efficient blue fluorescent emitter and a high performance host for various phosphors simultaneously.

Efficient non-doped phosphorescent orange, blue and white organic light-emitting devices.

Efficient phosphorescent orange, blue and white organic light-emitting devices (OLEDs) with non-doped emissive layers were successfully fabricated. Conventional blue phosphorescent emitters bis [4,6-di-fluorophenyl]-pyridinato-N,C2′] picolinate (Firpic) and Bis(2,4-difluorophenylpyridinato) (Fir6) were adopted to fabricate non-doped blue OLEDs, which exhibited maximum current efficiency of 7.6 and 4.6 cd/A for Firpic and Fir6 based devices, respectively. Non-doped orange OLED was fabricated utilizing the newly reported phosphorescent material iridium (III) (pbi)2Ir(biq), of which manifested maximum current and power efficiency of 8.2 cd/A and 7.8 lm/W. The non-doped white OLEDs were achieved by simply combining Firpic or Fir6 with a 2-nm (pbi)2Ir(biq). The maximum current and power efficiency of the Firpic and (pbi)2Ir(biq) based white OLED were 14.8 cd/A and 17.9 lm/W.

Efficient fluorescent white organic light-emitting devices with a reduced efficiency roll-off based on a blue ambipolar fluorescent emitter

White organic light-emitting devices (WOLEDs) with fluorescent donor-acceptor-substituted spirobifluorene compounds (red 2-diphenylamino-7-(2,2-dicyanovinyl)-9,9′-spirobifluorene and blue 2-diphenylamino-7-(2,2-diphenylvinyl)-9,9′-spirobifluorene) have been fabricated. The optimized WOLEDs shows a maximum current efficiency 5.9 cd/A and very low efficiency roll-off. From the brightness at maximum current efficiency to high brightness of 10000 cd/m2, the current efficiency roll-off is only 0.4%. It can be attributed to the ambipolar blue fluorescent emitter with voltage-independnet mobility which makes the device having a broader charge recombination zone and balance of carrier transport.

Highly color-stable and efficient hybrid white organic light-emitting devices by compensating a recombination zone shift in a single host layer

We have developed highly color-stable and efficient hybrid white organic light-emitting devices (WOLEDs) with multi-emission layers structure consisted of a single host layer doped with fluorescent and phosphorescent guest molecules. The blue fluorescent, red and green phosphorescent molecules were doped into the separated layers of a single host. The color stability of the WOLEDs is substantially improved by embedding an additional blue fluorescent layer which compensates the color change due to the recombination zone shift. The Commission Internationale de L’Eclairage coordinates of the hybrid WOLED change from (0.45, 0.46) to (0.44, 0.45) as the luminance varies from 128 to 23,460cd/m2. The color-stable WOLED also exhibits a high maximum external quantum efficiency of 15.2% and a current efficiency of 38.8cd/A.