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Highly Efficient and Color-Stable Deep-Blue OrganicLight-Emitting Diodes Based on a Solution-ProcessibleDendrimer

By Lei Wang, Yang Jiang, Jia Luo, Yan Zhou, Junhong Zhou, Jian Wang,*

Jian Pei,* and Yong Cao

[*] Prof. J. Wang, Dr. L. Wang, J. H. Zhou, Prof. Y. CaoInstitute of Polymer Optoelectronic Materials and DevicesSouth China University of Technology andKey Lab of Specially Functional Materials of Ministry of EducationGuangzhou 510640 (P. R. China)E-mail: jianwang@scut.edu.cn

Prof. J. Pei, Dr. Y. Jiang, Dr. J. Luo, Y. ZhouKey Laboratory of Bioorganic Chemistry andMolecular Engineering of Ministry of EducationCollege of Chemistry, Peking UniversityBeijing 100871 (P. R. China)E-mail: jianpei@pku.edu.cn

DOI: 10.1002/adma.200901039

� 2009 WILEY-VCH Verlag Gmb

Since their discovery,[1] organic light-emitting diodes (OLEDs)have attracted a vast amount of attention due to their applicationsin full-color flat-panel displays (FPDs), solid-state lightingsources, and backlights for liquid-crystal displays.[2] For asuccessful commercialization of OLEDs in such applications,highly efficient, color-stable, and long-lifetime organic materialsand devices emitting saturated blue color are essential, similar tothe development of inorganic blue LEDs.[3] Consequently,considerable work has been carried out to research and develophigh-performance blue-emission materials and devices.

Most research has been focused on either small-moleculeemitters or conjugated-polymer emitters. A number of bluefluorescent small molecules, such as anthracene, quinoline,tetraphenylsilane, distyrylarylene, fluorene, fluoranthene, andpyrene derivatives,[4] and various kinds of conjugated polymer,such as poly(para-phenylene) (PPP), ladder-type poly(para-phenylene) (LPPP), polyfluorene (PF), and their stero-typevariations,[5] have been studied intensively for blue-emittingOLEDs. Among those blue-emission materials, fluorene-basedsmall molecules and polymers have attracted wide interest due totheir high photoluminescence (PL) efficiency.[5i,6] Recently, Penget al. reported a high-efficiency blue OLED with a maximumluminance efficiency (LE) of 5.2 cd A�1 and CommissionInternationale d’Enclairage (CIE) co-ordinates of (0.15, 0.21),by utilizing 2,7-dipyrenyl-909-dimethylfluorene (DPF) as theemissive layer combined with a hole-blocking layer.[7] Thepractical application of polyfluorene-based blue polymer light-emitting diodes (PLEDs) has been impeded by their poor colorstability under thermal annealing, and short operation lifetimecaused by either keto defects[8] or the formation of p-aggregates/excimers.[9] By introducing a dibenzothiophene-S,S-dioxide

isomer into the poly[9,9-bis(4-(2-ethylhexyloxy)phenyl)fluoren-2,7-diyl] (PPF) backbone, Liu et al. obtained an efficientcolor-stable blue PLED with the maximum LE of 6.0 cd A�1,and amaximum external quantum efficiency (EQE) of 5.5% at theCIE co-ordinates of (0.16, 0.19).[10] Besides the fluorescent smallmolecules and polymers, phosphorescent emitters becomeattractive due to their high quantum yields realized by anefficient admixture of singlet and triplet excited states viaspin–orbit coupling.[11] Recently, Kido’s group reported aniridium(III) bis(4,6-(difluorophenyl)pyridinato-N,C2’)picolinate(FIrpic)-based high-efficiency blue phosphorescent OLED witha carrier- and exciton-confining structure. The EQE of their OLEDreached 25% with a power efficiency (PE) of 55 lm W�1 at theluminance of 100 cd m�2.[2c] Independently, So’s group reportedan efficient FIrpic-based blue OLED with a maximum EQE of23% and a PE of 31.6 lmW�1 by incorporating high-triplet-energyelectron-transporting material with high electron mobility as ahole blocker.[12]

Although the above blue-emission OLEDs achieved highefficiency and good color stability, the color purity of the devices isinsufficient for applications in full-color displays, which require asaturated blue emitter with CIE co-ordinates of (0.14, 0.08)specified in the National Television System Committee (NTSC)standard.[13] The development of deep-blue-emission materialsremains a challenge due to their large bandgap and the difficultyto achieve charge balance in devices.[14] Recently, Liu et al.synthesized an asymmetric phenylenevinylene with a cis doublebond 2-(4-(p-tolyl)styryl)-1,4-dip-tolybenzene (cis-TSDTB) as deep-blue emitter, and obtained a device with a LE of 3.4 cd A�1

and a maximum luminance of 9855 cd m�2 at the CIEco-ordinates of (0.15,0.10).[15] However, the device efficiencydeclined rapidly with high current density, indicating that thehole and electron recombination becomes less efficient underhigh operation voltage. Saturated blue-fluorescent OLEDs withstable color co-ordinates and a high efficiency have been rarelyreported.

Conjugated dendrimers have emerged as a new kind oflight-emitting organicmaterial for applications in OLEDs becausethey combine the advantages of both the small molecules and thepolymers, for example, the synthetic versatility of organicmaterials, the solubility of polymers in common solvents, andthe ready achievement of a high purity of small molecules.[16]

Although much effort has been dedicated to the development ofblue-fluorescent dendrimer materials and devices, saturated bluedendrimer OLEDs with high efficiency have yet to be achieved.

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The state-of-the-art deep-blue dendrimer OLEDs with the CIEco-ordinates near (0.14, 0.08) have an EQE less than 3%, and amaximum luminance less than 2000 cd m�2.[16k] Herein, wereport color-stable deep-blue-emitting OLEDs with high effi-ciency based on a solution-processible fluorescent p-conjugatedblue dendrimer G0 (Scheme 1). The saturated blue OLED, withthe highest efficiencies of 5.3 cd A�1 (LE) and 6.6% (EQE)obtained at a current density of 10.7mA cm�2 with a luminanceof 550 cd m�2, has CIE co-ordinates of (0.155, 0.086). At themaximum luminance around 17000 cd m�2 and with a currentdensity of 650mA cm�2, the CIE co-ordinates become (0.156,0.091), showing impressive color stability. Furthermore, whenG0is used as a single host material doped with phosphorescent dyes,such as green-emitting iridium tris(2-4(4-toltyl)phenylpyridine)(Ir(mppy)3) or red-emitting iridium bis(1-phenylisoquinoline)(acetylacetonate) (Ir(piq)2acac), an efficient green or red OLED isobtained. The maximum efficiencies reach 35.7 cd A�1 (14.3%)and 6.8 cd A�1 (13.6%) with CIE co-ordinates of (0.351, 0.593) and(0.678, 0.313) for green and red OLEDs, respectively. By dopingG0 with both Ir(mppy)3 and Ir(piq)2acac, color-stable whiteOLEDs with high efficiency and excellent color-rendering index(CRI) have been successfully produced. The white device exhibitsthe highest efficiencies of 16.9 cd A�1 and 7.7 lm W�1 at CIEco-ordinates around (0.312, 0.332) with CRI as high as 90. To thebest of our knowledge, these devices show the highest efficiencyof deep-blue fluorescent OLEDs, and the highest efficiency ofgreen, red, and white OLEDs based on a solution-processiblefluorescent p-conjugated dendrimer as the host matrix.

The molecular structures of the compounds used in theexperiments are illustrated in Scheme 1. The zeroth-generationG0 can be dissolved in many common organic solvents, making itsuitable for solution processes. The photophysical studies showthat G0 films are thermally stable (Supporting Information, Fig.S1). Blue devices with the configuration of ITO/poly(ethylendiox-ythiophene):poly(styrene sulfonic acid) (PEDOT:PSS)(50 nm)/poly(N-vinylcarbazole)(PVK)(50nm)/G0(60nm)/Ba(4nm)/Al(200nm)were fabricated. Since G0 has a highest occupied molecular

Scheme 1. Chemical structures of G0 and the two phosphorescent dyes.

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orbital (HOMO) level at 5.8 eV, the thin layer of PVK wasintroduced on top of PEDOT:PSS as the hole-injection layer. Thecontrol device (Device A) without annealing and two other devicesannealed with different procedures were fabricated to investigatethe thermal-annealing effects on the performances of the devices.For Device B, the PVK layer was annealed at 140 8C for half anhour in nitrogen atmosphere before G0 was spin-coated. ForDevice C, both the PVK layer and the G0 layer were annealed at140 8C for half an hour in nitrogen atmosphere after they werespin-coated sequentially at room temperature. Compared to thespectrum of the control device, the EL spectra of the annealeddevices exhibit no obvious change (Fig. 1a). The CIE co-ordinatesof Devices A, B, and C are (0.155, 0.090), (0.155, 0.087), and(0.155, 0.086), respectively. As the operation voltage increases, theEL spectra of all the devices remain unchanged (Fig. S2).

The EL performances of the devices are summarized inTable 1. Device A has a maximum LE of 3.3 cd A�1 and amaximum EQE of 4.2% at a current density of 7.8mA cm�2 witha luminance of 260 cd m�2, while the maximum LE of Device Creaches 5.3 cd A�1 and the EQE increases to 6.6% at a current

Figure 1. a) The EL spectra of Devices A, B, and C at a current density of15mA cm�2. b) The EL spectra of G0-based blue, green, red, and whiteOLEDs.

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Table 1. Device performance of the blue, green, red, and white OLEDs based on G0.

Device CIE [x, y][a] Max EQE [%] Max LE [cd A�1] Max PE [lm W�1] Max L [cd m�2] Device performance at 1000 cd m�2

Voltage [V] EQE [%] LE [cd A�1] PE [lm W�1]

A 0.155, 0.090 4.2 3.3 1.7 6900 7.4 3.6 2.9 1.2

B 0.155, 0.087 4.5 3.6 1.8 6900 7.2 3.7 3.0 1.3

C 0.155, 0.086 6.6 5.3 3.0 17000 6.5 6.2 4.9 2.4

D 0.351, 0.593 14.3 35.7 10.1 104000 10.6[b] 14.2[b] 35.6[b] 9.7[b]

E 0.678, 0.313 13.6 6.8 2.9 10700 11.5 10.9 5.4 1.4

F 0.312, 0.332 8.5 16.9 7.7 52000 7.2 8.3 16.6 6.7

[a] Obtained at 15mA cm�2. [b] Device performances at 3000 cd m–2.

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density of 10.7mA cm�2 with the luminance of 550 cd m�2.Moreover, the maximum luminance of Device C reaches17000 cd m�2 compared to 6900 cd m�2 obtained with DeviceA. Although the annealed Device C improves the EL perfor-mances considerably, the EL performances of the other annealedDevice B are on a par with those of the control Device A. Theidentical current-density–bias–luminance (J–V–L) characteristicsof Device A and B (Fig. 2a) reveal that baking the PVK layer didnot improve the device performances. For Device C, first, theturn-on voltage (defined as the operation voltage at the luminanceof 1 cd m�2) decreases from 4V of the control device to 3.5 V;second, the current density is higher than that of the control

Figure 2. a) J–V–L characteristics of Devices A, B, and C. b) Thedependence of the luminance efficiency on the current density for DevicesA, B, and C.

� 2009 WILEY-VCH Verlag Gmb

device after turn-on voltage. By investigating the J–V character-istics of the electron-only and the hole-only devices ofG0with andwithout annealing (Fig. S3), we conclude that the annealing of theG0 layer improves the electron injection from the cathode. Thisresults in more balanced hole and electron densities inside thelight-emitting G0 layer, thereby increasing the device perfor-mances, which is consistent with studies of other groups.[17]

Not only can the thermal annealing of theG0 layer improve theefficiency and the luminance, it also stabilizes the efficiency athigh current density. The dependences of the efficiency on thecurrent density for Devices A, B, and C are plotted in Figure 2b.The LE of Device C remains as high as 2.6 cd A�1 at a currentdensity of 650mA cm�2 with a luminance of 17000 cd m�2, whilethe efficiencies of Devices A and B without G0 annealing drop to1.6 cd A�1 at a current density of 440mA cm�2 with a luminanceof 6900 cd m�2. The results show that the hole and electronrecombination remains efficient even at high operation currentdensities after theG0 layer is annealed. With such high efficiencyat high current density and excellent color stability, the deep-bluefluorescent dendrimer G0 is promising not only for theapplication in full-color FPDs but also for the fabrication ofwhite OLEDs in solid-state lighting.

To realize full color, efficient green and red emissions arerequired in addition to the efficient blue emission. Ir(mppy)3 andIr(piq)2acac have been widely demonstrated as efficient green andred phosphorescent emitters.[18] The energy levels of G0 and thephosphorescent emitters (Fig. S4) suggest thatG0 could be a goodhost material. By doping G0 with Ir(mppy)3 or Ir(piq)2acac andutilizing the same device structure and annealing procedure asthose of Device C, highly efficient green and red OLEDs (DevicesD and E, respectively) are successfully obtained (Fig. S5). The bestdevice performances (Table 1) are achieved for devices with adoping concentration of 4 wt% of both Ir(mppy)3 and Ir(piq)2acacto the G0 matrix. The maximum efficiencies of green and reddevices reach 35.7 cd A�1 (14.3%) and 6.8 cd A�1 (13.6%),respectively, while the maximum luminance reaches104000 cd m�2 for the green device and 10700 cd m�2 for thered device. The emission spectra (Fig. 1b) show that the energytransfer from the G0 host to the phosphorescent dopants iscomplete, leading to excellent green and red emissions with CIEco-ordinates of (0.351, 0.593) and (0.678, 0.313), respectively. Thefact thatG0 itself is not only an efficient deep-blue emitter but alsoa good host to achieve highly efficient green and red emissions,combined with its solubility, makes G0 suitable in fabricatingfull-color FPDs via solution processing.

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To achieve high-quality white emission, mixing threeprimary-color emissions, that is, red, green, and blue, has beendemonstrated to be the best choice.[19] White OLEDs with threeemission peaks of blue (430nm), green (510nm), and red (620nm)color, and CIE co-ordinates of (0.312, 0.332), were successfullydeveloped by precisely co-doping G0 with 0.25 wt% of Ir(mppy)3and 0.25 wt% of Ir(piq)2acac (Fig. 1b). The white OLED exhibits amaximum power efficiency of 7.7 lm W�1, a maximum LE of16.9 cd A�1, and a maximum brightness of 52000 cd m�2

(Table 1). At a luminance of practical applications, for example,1000 cd m�2, the forward-viewing PE and LE are as high as 6.9 lmW�1 and 16.6 cd A�1, respectively. In addition, when the appliedvoltage increases from 6 to 11V, the brightness increases from 53to 52000 cd m�2, and the current density increases from 0.6 to660mA cm�2, the CIE co-ordinates change from (0.312, 0.332) to(0.285, 0.317), showing a very stable white emission. For theapplication of white OLEDs in solid-state lighting, a keyparameter to characterize the quality of the white emission isthe CRI value.[20] The CRI value of the dendrimerG0-based whiteOLED reaches 90 at the practical brightness of 1000 cd m�2,much higher than values between 65 and 75 obtained by whiteelectroluminescent devices based on an orange chromophoreincorporated into a blue-emission host.[17b,19b,21] The combina-tion of high efficiency, stable white emission, and high CRI valuemakes theG0-based white OLED an ideal candidate for solid-statelighting.

In summary, highly efficient and color-stable deep-blue OLEDsbased on a fluorescent p-conjugated dendrimer G0 have beensuccessfully developed. At CIE co-ordinates of (0.155, 0.084), theblue dendrimer OLEDs show the highest efficiencies of 5.3 cdA�1 (LE) and 6.6% (EQE) with a luminance of 550 cd m�2. Highlyefficient green, red, and white OLEDs have been successfullydeveloped by doping G0 with the phosphorescent dyes Ir(mppy)3and/or Ir(piq). The CRI value of the white OLEDs reaches 90because of the three emission peaks in the spectrum. Thestable photophysical properties of the G0 solid-state film, due tothe carefully designed rigid stilbenoid architectures with largebranches, ensure the color stability and the efficiency stability ofthe devices under high temperature, high applied current density,and high operation luminance. Combining high device efficiencyand good stability with solution processibility, the p-conjugateddendrimer G0 becomes one of the best deep-blue-emittingmaterials for the fabrication of FPDs and white-light lightingpanels via solution processing at low cost.

Experimental

All chemicals and materials were purchased and used as received. PVKwaspurchased from Aldrich, Ir(mppy)3 and Ir(piq)2acac were purchased fromAmerican Dyes Sources. PEDOT:PSS was Baytron P 4083 from Bayer AG,H. C. Starck, Inc. G0 was synthesized in our laboratory [16h]. Devicefabrication followed the well-established standard processes describedelsewhere [17c]. G0 films were deposited by spin-coating from p-xylenesolution with a concentration of 16mg mL�1. The J–V–L data werecollected using a Keithley 236 source meter and a silicon photodiode.The EQEs were obtained in an integrating sphere (IS-080, Labsphere).The luminance was calibrated by a PR-705 SpectraScan Spectrophotometer(Photo Research) with simultaneous acquisitions of the EL spectra and theCIE co-ordinates.

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Acknowledgements

The authors are deeply grateful to the Ministry of Science and Technology(973 Program 2009CB623601, 2009CB623604, 2009CB930604, and 863Program 2008AA03A311) for financial support. Supporting Information isavailable online from Wiley InterScience or from the author.

Received: March 26, 2009

Published online: July 16, 2009

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