Hybrid White Organic Light-emitting Diodes Research Articles

Investigation of triplet harvesting and outcoupling efficiency in highly efficient two-color hybrid white organic light-emitting diodes

We investigate singlet and triplet transfer processes in white triplet harvesting (TH) organic light-emitting diodes (OLEDs), comprising fluorescent and phosphorescent emitter molecules. By analyzing electroluminescence spectra and performing time-resolved measurements, we prove direct TH from the blue fluorescent emitter N,N′-di-1-naphthalenyl-N,N′-diphenyl-[1,1′:4′,1″:4″,1′′′-quaterphenyl]-4,4′′′-diamine (4P-NPD) to the yellow phosphorescent emitter bis(2-(9,9-dihexyluorenyl)-1-pyridine)(acetylacetonate)iridium(III) (Ir(dhfpy)2(acac)). Singlet transfer is identified as a second exciton transfer mechanism in the TH OLEDs under investigation. The CIE coordinates of these devices can be adjusted over a wide range by varying the distance between exciton generation and TH zone. For an OLED with CIE color coordinates of (0.42/0.40), close to the warm white color point A, we obtain nearly perfect Lambertian emission characteristics. This device achieves an external quantum efficiency (EQE) of 9.4% and a luminous efficacy (LE) of 27.1 lm W−1 (at 1000 cd m−2) which can be further increased to 46 lm W−1 using outcoupling enhancement techniques. Optical modeling of the electromagnetic field inside the TH OLED and a comparison to the position of the emitters within the device confirms that the devices operate close to the optical optimum and yields an estimated internal quantum efficiency (IQE) of 48.7% and a singlet/(harvested) triplet ratio of 0.28/0.72 = 0.39.

Studies of fluorescence/phosphorescence hybrid white organic light-emitting diodes

White organic light-emitting diodes (WOLEDs) have attracted more and more attention due to their wide applications in full-color displays, solid-state lighting sources and backlight sources. Amongst, fluorescence/hosphorescence hybrid WOLEDs are considered as promising device structures in lighting application because of the long lifetime of fluorescence materials and the high efficiency of phosphorescence materials. For fluorescence/hosphorescence hybrid WOLEDs, it is very important to resolve the synergic emission from the singlet excitons of fluorescence materials to the triplet excitons of phosphorescence materials. The design of effective device structures is necessary to avoid the quench between the singlet excitons and triplet excitons. In this paper, we introduced the structure design and performances of fluorescence/phosphorescence hybrid WOLEDs with different structures based on two blue fluorescence materials with different triplet levels. It is experimentally demonstrated that the high effective structure design with charge carrier transport balance and the effective control of exciton distribution in a wide range are keys in realizing high performance fluorescence/phosphorescence hybrid WOLEDs.

Highly Efficient White Organic Light-Emitting Diodes with Controllable Excitons Behavior by a Mixed Interlayer between Fluorescence Blue and Phosphorescence Yellow-Emitting Layers

A highly efficient hybrid white organic light-emitting diode (HWOLED) has been demonstrated with a mixed interlayer between fluorescent blue and phosphorescent yellow-emitting layers. The device structure is simplified by using a controllable fluorescence-mixed interlayer-phosphorescence emission layer structure. The electroluminance (EL) performance can be modulated easily by adjusting the ratio of the hole-predominated material to the electron-predominated material in the interlayer. It is found that the HWOLED with a ratio of 3 : 2 exhibits a current efficiency of 34 cd/A and a power efficiency of 29 lm/W at 1000 cd/m2with warm white Commission Internationale de l’Eclairage (CIE1931) coordinates of (0.4273, 0.4439). The improved efficiency and adaptive CIE coordinates are attributed to the controllable mixed interlayer with enhanced charge carrier transport, optimized excitons distribution, and improved harvestings of singlet and triplet excitons.

Hybrid white organic light-emitting diodes with improved color stability and negligible efficiency roll-off based on blue fluorescence and yellow phosphorescence

We report hybrid white organic light-emitting diodes (WOLEDs) based on yellow phosphorescence of Iridium (III) bis(4-phenylthieno[3, 2-c]pyridinato-N, C2′)acetylacetonate (PO-01) and blue fluorescence of p-bis (p-N,N-diphenyl-amino-styryl) benzene (DSA-Ph). By introducing appropriate thickness of tris (phenylpyrazole) Iridium [Ir(ppz)3] as interlayer between the adjacent emission layers, taking advantage of the assistance electron-transporting behavior of guest molecules, efficient WOLEDs with negligible efficiency roll-off and high color stability are obtained. The best device shows a peak current efficiency of 21.0cd/A at 2, 300cd/m2 and the value can be maintained as high as 20.1cd/A at 9, 300cd/m2. Furthermore, the Commission Internationale de L'Eclairage coordinate of the corresponding device only changes marginally from (0.41, 0.46) to (0.40, 0.46) over 103–104cd/m2.

Hybrid white organic light-emitting diodes with a double light-emitting layer structure for high color-rendering index

High-efficiency hybrid white organic white light-emitting diodes (WOLEDs) with a double light-emitting layer structure for high color-rendering index (CRI) have been fabricated. The double light-emitting layer structure is composed of a red phosphorescent layer and a blue-green layer of a blue light-emitting fluorescent host doped with a green phosphor separated by a bipolar spacer. The simultaneous emission of a blue light from the fluorescent host and a green light from the phosphor was realized by controlling the doping concentration of the green phosphor in the fluorescent host. The resulting hybrid WOLEDs achieved a CRI of 90 and kept rather stable spectral emission with Commission Internationale de L'Eclairage coordinates of (0.42, 0.44) independent of driving voltages. Furthermore, the hybrid WOLEDs also exhibited a high efficiency that the maximum current efficiency, external quantum efficiency and power efficiency reached 29.4 cd/A, 13.8% and 34.2 lm/W, and still remained at 25.4 cd/A, 11.9% and 23.0 lm/W at 1000 cd/m2, respectively. Mechanism of the two emission layers and role of the bipolar spacer were systematically studied and we have attributed the significant improvement in device performances to the effective and stable control of exciton recombination in the emissive regions by the bipolar functionality of the used spacer.

Ultra-simple hybrid white organic light-emitting diodes with high efficiency and CRI trade-off: Fabrication and emission-mechanism analysis

We present a simple hybrid white organic light-emitting diodes (WOLED) consisting of only two layers, i.e., a hole-transporting layer and an emitting layer. The emitting layer is formed by simply co-doping a green phosphor and a red phosphor in bis[2-(2-hydroxyphenyl)-pyridine]beryllium (Bepp2), which acts as the blue emitter, electron-transport material, and high triplet energy host for the phosphors, i.e., a multifunctional chromophore. This simple device exhibits a maximum power and quantum efficiency of 46.8lmW−1 and 16.5%, respectively, with a good CRI up to 90. The versatile experimental techniques are performed to gain a deep understanding of the emission mechanism. We believe that this simple design concept can provide a new avenue for achieving ultrahigh performance WOLEDs for lighting application.

Hybrid white organic light-emitting diodes of small molecule and polymer emitters

A hybrid white organic light emitting diode (HWOLED) with a small molecule fluorescent blue emitter and polymer yellow emitter was developed using a stamp transfer printing process. Double emitting layer stack structure was fabricated and the effect of stack sequence on the device performances of HWOLED was investigated. Balanced white emission was observed in the HWOLED with blue/yellow stack structure and a current efficiency of 10.4cd/A was obtained.

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:Bepp2, 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 30.2cd/A, a power efficiency of 32.0lm/W and an external quantum efficiency of 13.4% at a luminance of 100cd/m2, and keeps a current efficiency of 30.8cd/A, a power efficiency of 27.1lm/W and an external quantum efficiency of 13.7% at a 1000cd/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.

56.4: New Deep Blue Fluorescent Materials and Their Application to High Performance OLEDs

AbstractWe developed highly efficient deep blue fluorescent materials. The bottom‐emission OLED showed a CIE1931 coordinate of (0.14, 0.08) and a current efficiency of 6.5 cd/A with using a new efficiency‐enhancement layer. We achieved CIEy of 0.062, a current efficiency of 7.5 cd/A and lifetime (LT95) of 2,000 hrs by depositing a new organic capping layer on a top‐emission blue OLEDs, A hybrid white OLED by stacking a deep blue fluorescence and a phosphorescent red/green stacked yellow was also demonstrated which has an efficacy of 23 lm/W and color temperature of 10,000 K at 10 mA/cm2 and lifetime (LT50) of over 60,000 hrs at a luminance of 1,000 cd/m2.

66.3: Hybrid White OLEDs for General Lighting

AbstractOLEDs for general lighting require both high efficacy and good lumen maintenance. Hybrid stacks combining fluorescent blue and phosphorescent red and green emitters are a very good choice for both long lifetime and good efficacy values: 31 lm/W are demonstrated for a cold white color point. 44 lm /W are measured with improved out‐coupling (ILO) using a scattering foil and 60 lm/W are demonstrated using a macro‐extractor for light out‐coupling. For warm white we realized 34 lm/W (with ILO 45 lm/W and 64 lm /W using a macro‐extractor). LT50 lumen maintenance is for both stacks better than 30,000 hours.

Effect of the interlayer composition on the lifetime and color change of hybrid white organic light-emitting diodes

The effect of the interlayer composition on the lifetime and color change after lifetime measurement of the hybrid white organic light-emitting diodes (WOLEDs) was systematically studied. The hybrid WOLEDs with different interlayer compositions were fabricated and the interlayer composition was correlated with the lifetime and color stability of the devices. It was found that the interlayer composition is critical to the color stability of the WOLEDs before and after lifetime measurement, while the lifetime was not affected by the interlayer composition.

Hybrid white organic light-emitting diode with a mixed-host interlayer processed by organic vapor phase deposition

Abstract. Organic light-emitting diodes (OLEDs) are a key technology in solid state lighting.Withoutalong-livedphosphorescentblueemitter,ahybridconceptbasedonphosphorescentredandgreenemittersandafluorescentblueemitterinawhiteOLEDstackisapromisingapproachfor pure-white emission. Several challenges such as exciton recombination on all emitters andtripletdiffusion,aswellasquenching,havetobeovercome.Toaddresstheseissues,amixed-hostphosphorescent emission layer is employed. The mixture ratio is locally varied in the emissionlayer. An interlayer separates the phosphorescent and fluorescent emission layer. Strategies totune the color coordinates are presented. The lifetime and color stability versus luminance areinvestigated. At Commission Internationale de I’Eclairage color coordinates of 0.44/0.44, acurrent efficacy of 28.0 cd/A (at 1000 cd/m 2 ), and a luminous efficacy of 20.6 lm/W can bemeasured. C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE) . [DOI: 10.1117/1.3545966]

Codoped spacer ratio effect of hybrid white organic light-emitting diodes

The authors have demonstrated hybrid white organic light-emitting diodes (HWOLEDs) on electroluminescent characteristics for codoped spacer ratio effect using N,N′-dicarbazolyl-3,5-benzene (mCP) and 4,7-diphenyl-1,10-phenanthroline (BPhen). The optimized HWOLEDs of spacer ratio (3:1) which had proper recombination balance of holes and electrons, mCP:BPhen, showed the external quantum efficiency of 6.01%, the power efficiency of 8.12 lm/W, and Commission Internationale de I’Eclairage coordinates of (0.37, 0.41) at 1000 cd/m 2.

Efficient and colour-stable hybrid white organic light-emitting diodes utilizing electron–hole balanced spacers

High-efficiency two-colour white organic light-emitting diodes (WOLEDs) comprising a newly synthesized iridium complex orange phosphor ((impy)2Ir(acac)) and a blue fluorophor (BD012) have been realized by placing several kinds of thin spacers between two emitters. Hybrid WOLEDs with a spacer composed of a hole-transporting N,N-dicarbazolyl-3,5-benzene (mCP) and an electron-transporting 4,7-diphenyl-1,10-phenanthroline (Bphen) exhibit a high external quantum efficiency (EQE) of up to 8.4% and a negligible colour change (the colour coordinate of (0.39, 0.41) at 1000 cd m−2) with increasing brightness, whereas the device using a hole-transporting mCP spacer shows a relatively low EQE of 6.2% and a large shift of emitting colour with increasing brightness. Device performance is further characterized based on the charge transport behaviour of the spacers inserted between the two emitters.

Codoped Spacer Ratio Effect on Electroluminescence Characteristics of Hybrid White Organic Light-Emitting Diodes for Reduced Efficiency Roll-Off

We demonstrated hybrid white organic light-emitting diodes (HWOLEDs) using 1,4-bis{2-[7-N-diphenyamino-2-(9,9-diethyl-9H-fluoren-2-yl)]vinyl} benzene as the fluorescent blue emitter and iridium(III) bis(2-phenylquinoline) acetylacetonate as the phosphorescent red emitter. We also investigated the codoped spacer ratio (CSR) effects of 4,4',4''-tris(N-carbazolyl)-triphenylamine (TCTA) and 2,2',2''-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBI) on the electroluminescence characteristics of HWOLEDs. The optimized HWOLED with a CSR of 1:3 (TCTA:TPBI) exhibited a lower efficiency roll-off of 32% than the control device (67%) with a CSR of 1:1 (TCTA:TPBI). The HWOLED also showed a current efficiency of 12.76 cd/A, an external quantum efficiency of 6.95%, and Commission Internationale de I'Eclairage coordinates of (0.38,0.39) at 1000 cd/m2.

Hybrid White Organic Light-Emitting Diodes with Reduced Efficiency Roll-Off

The authors have demonstrated hybrid white organic light-emitting diodes (HWOLEDs) with reduced efficiency roll-off. It was shown that HWOLEDs fabricated with {9,9-dimethyl-7-[10-(naphthalen-2-yl)anthracen-9-yl]-9H-fluoren-2-yl} triphenylsilane (ANFTPS) as the blue host have the maximum external quantum efficiency of 9.92%, power efficiency of 14.51 lm/W, and reduced efficiency roll-off 2.0 times that of the control white device. The deep highest occupied molecular orbital of ANFTPS of the optimized white device adequately brocks holes and confines excitons in emitting layers. The white device also exhibited a more stable color shift from low to high current density than did the control white device.

Ratio Effect of Codoped Spacer on Electroluminescent Characteristics of Hybrid White Organic Light-Emitting Diodes

The authors have demonstrated hybrid white organic light-emitting diodes (WOLEDs) using 1,4-bis[2-(7-N-diphenyamino-2-(9,9-diethyl-9H-fluoren-2-yl))vinyl] benzene as a fluorescent blue emitter and iridium(III) bis(2-phenylquinoline) acetylacetonate as a phosphorescent red emitter. As the spacer ratio of 4,4',4''-tris-(N-carbazolyl) triphenylamine to 4,7-diphenyl-1,10-phenanthroline changed from 1:1 to 1:3, WOLEDs showed changes from cold to warm electroluminescent characteristics. The optimized WOLEDs of 1:3 spacer ratio showed an external quantum efficiency of 7.40%, a luminous efficiency of 14.79 cd/A, and Commission Internationale de I'Eclairage coordinates of (0.43, 0.39) at 1000 cd/m2.

Effect of host and interlayer structures on device performances of hybrid white organic light-emitting diodes

The effect of host and interlayer structures on device performances of hybrid white organic light emitting diodes was studied by changing the energy level of host and interlayer materials. A mixed layer of hole transport type and electron transport type materials was used as a host and an interlayer. In the red:green/interlayer/blue stacked structure, a red shift of emission color was observed by increasing the highest occupied molecular orbital of the electron transport type material in the mixed layer. An optimization of the device structure gave a high current efficiency of 32.4 cd/A with a color coordinate of (0.44, 0.45).

The Spacer Thickness Effects on the Electroluminescent Characteristics of Hybrid White Organic Light-emitting Diodes

The authors have demonstrated the various characteristics of hybrid white organic light-emitting diodes (HWOLED) using fluorescent blue and phosphorescent red emitters. We also demonstrated that two devices showed different characteristics in accordance with thickness of the 4,4′-N,N′-dicarbazole-biphenyl (CBP) spacer (CS) inserted between the blue and the red emitting layer. It was found that the device with a CS thickness of 70 A showed a current efficiency 2.5 times higher than that of the control device with a CS thickness of 30 A by preventing the triplet Dexter energy transfer from the red to the blue emitting layer. The HWOLED with the CS thickness of 70 A exhibited a maximum luminance of 24500 cd/m, a maximum current efficiency of 42.9 cd/A, a power efficiency of 37.5 lm/W, and Commission Internationale de I'Eclairage coordinates of (0.37, 0.42). [DOI: 10.4313/TEEM.2009.10.6.208]

Color stable and interlayer free hybrid white organic light-emitting diodes using an area divided pixel structure

Color stable and interlayer free hybrid white organic light-emitting diodes were fabricated by using an area divided pixel structure. The area divided pixel structure was realized by stacking red and blue emitting layers using a fine metal mask. A phosphorescent red emitting layer was patterned by a metal mask and a blue fluorescent emitting layer was commonly deposited on the patterned red emitting layer. The blue fluorescent emitting layer could play a role of a hole-blocking layer and a white emission could be obtained due to separate emission of red and blue emitting layers. The interlayer free hybrid WOLEDs showed color stability from 100cd/m2 to 10,000cd/m2.