Efficient Fluorescent Deep-Blue and Hybrid White Emitting Devices

ARTICLE pubs.acs.org/JPCC

Efficient Fluorescent Deep-Blue and Hybrid White Emitting Devices Based on Carbazole/Benzimidazole Compound Xiaohui Yang,† Shijun Zheng,‡ Rebecca Bottger,‡ Hyun Sik Chae,‡ Takeshi Tanaka,‡ Sheng Li,‡ Amane Mochizuki,*,‡ and Ghassan. E. Jabbour*,†,§ † ‡

Solar and Alternative Energy Engineering Research Center, Physical Science and Engineering, KAUST, Thuwal, Saudi Arabia Nitto Denko Technical Corporation, 501 Via Del Monte, Oceanside, California 92058, United States ABSTRACT: We report the synthesis, photophysics, and electrochemical characterization of carbazole/benzimidazole-based compound (Cz-2pbb) and efficient fluorescent deep-blue light emitting devices based on Cz-2pbb with the peak external
nclairage coordinates quantum efficiency of 4.1% and Commission Internationale d’E of (0.16, 0.05). Efficient deep-blue emission as well as high triplet state energy of Cz2pbb enables fabrication of hybrid white organic light emitting diodes with a single emissive layer. Hybrid white emitting devices based on Cz-2pbb show the peak external quantum efficiency exceeding 10% and power efficiency of 14.8 lm/W at a luminance of 500 cd/m2.

’ INTRODUCTION White organic light emitting diodes (WOLEDs) have been extensively studied for potential applications in displays and solid-state lighting devices.1 Several approaches, such as stacked three primary or two complementary colors light emitting layers, multiple-dopants emissive layer, and utilization of excimer and exciplex emission, have been used to construct WOLEDs.2 Devices with a single emissive layer containing multiple dopants possess simple device structure, show relatively no changes in electroluminescent (EL) spectra due to shift of carrier recombination zone, and do not require emissive materials with specially tailored intra- or intermolecular charge-transfer properties. Nevertheless, multiple energy/charge-transfer processes among the host
dopant and dopant
dopant need to be considered. From the prospects of emitting materials, phosphorescent materials enable the utilization of both single and triplet excited states for light emission and thus are generally used to boost the efficiency of WOLEDs. Intuitively, red, green, and blue phosphorescent emitters would be ideal for construction of WOLEDs. However, the lack of stable phosphorescent blue emitter and the difficulty for carrier injection from an adjacent carrier transporting layer to the large band gap host with higher triplet state energy to confine the excitation on the phosphorescent blue emitter remain unsolved. Recently, hybrid WOLEDs combining a blue emitting fluorophor and low-energy emitting phosphors have been proposed to bypass above-mentioned problems.3 In such devices, triplet excited states of the blue emitting fluorophor can be captured by long-wavelength emitting phosphors, located within a triplet exciton diffusion length from carrier recombination zone. Several groups reported highly efficient hybrid WOLEDs with r 2011 American Chemical Society

stacked fluorescent blue and phosphorescent green/red emitting layers.3 It is noted that doping fluorescent blue emitting host with an orange emitting phosphor at a low concentration, instead of having separated blue and orange emitting layers, not only simplified the device structure but also improved device efficiency due to a decreased distance between blue fluorophor and orange phosphor, which facilitated energy transfer and reduced the loss of excited states.3c In this sense, a single emissive layer composed of fluorescent blue, phosphorescent green, and red emitters is desirable in hybrid WOLEDs. The main difficulty for construction of such single emissive layer hybrid WOLEDs is due to the lack of suitable fluorescent blue emitter. For a material to function as a fluorescent blue emitter and host material for green/red emitting phosphorescent dopants in hybrid WOLEDs, several requirements are needed including (I) high solid-state luminescence yield, (II) balanced hole/electron transporting ability, and (III) large triplet state energy. Though highly efficient fluorescent blue light emitting devices have been reported, such emitters possess relatively low triplet state energy and thus are not ideally suitable for working as host materials for phosphorescent dopants in hybrid WOLEDs.4 Recently, Lai et al.5 reported blue emitting materials composed of hole transporting arylmine and electron transporting (phenyl-2benzimidazolyl) benzene units linked with a fluorene or spirofluorene spacer, which possess well-controlled conjugation and charge transporting property. Using such emitters and Received: April 4, 2011 Revised: June 7, 2011 Published: June 07, 2011 14347

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Scheme 1

1,4-bis[(1-naphthylphenyl)amino]biphenyl (NPB) as hole transporting layer, the authors showed an impressive external quantum efficiency (EQE) of 2.8% and Commission Internationale
nclairage (CIE) coordinates of (0.14, 0.11). Furthermore, d’E single layer orange light emitting devices using such fluorescent blue light emitting material as the host for an iridium complex showed an EQE of 6.9%. In this paper, we report a blue emitting bipolar material having electron transporting (phenyl-2benzimidazolyl) benzene moieties attached on 3,6-positions of hole transporting carbazole unit (Cz-2pbb). The partial conjugation between electron donor and acceptor units in Cz-2pbb leads to strong blue emission. In this regard, fluorescent deep-blue emitting devices based on Cz-2pbb show peak EQE of 4.1% and CIE coordinates of (0.16, 0.05). Moreover, hybrid WOLEDs having a single emissive layer composed of Cz-2pbb, fac-tris(2phenylpyridyl)iridium Ir(ppy)3, and iridium(III) bis(2-phenylquinolyl-N,C-20 )acetylacetonate (PQ2Ir(acac)) as blue, green, and red emitters, respectively, show peak EQE higher than 10%. The working mechanism of hybrid WOLEDs is also discussed.

’ EXPERIMENTAL SECTION All nonaqueous reactions were carried out under dry argon atmosphere unless stated otherwise. All reagents and solvents were received from Aldrich or Gelest and were used as received unless indicated otherwise. The NMR spectra data were collected using a JEOL ECL-400. Fluorescence measurements were performed on a Jobin Yvon Fluoromax 3 luminescence spectrometer. Photoluminescence quantum yield (PLQY) was measured in toluene solution using anthracene in EtOH as known standard reference (Φ = 0.27). The phosphorescence spectrum of Cz-2pbb was measured in 2-MeTHF at 77 K. UV
vis spectra were recorded on a Varian Cary 50 Scan spectrophotometer. DSC measurements were carried out on a Seiko Exstar 6000 DSC 6200. TGA was performed on a Perkin-Elmer Pyris. Cyclic voltammetry (CV) was performed using an Autolab typeII potentionsat/galvanostat model. Anhydrous DMF degassed

was used as the solvent under a nitrogen atmosphere, and 0.1 M tertrakis(n-butyl)ammonium hexafluorophosphate (NBu4PF6) was used as the supporting electrolyte. An Ag wire, a Pt wire, and glassy carbon electrode were used as the reference electrode, the counter electrode, and the working electrode, respectively. The redox potentials were referred to a ferrocene/ferrocenium (Cp2Fe/Cp2Fe+) redox couple as an internal standard for calibration. CV scan was recorded with a scan rate of 100 mV s
1. Electroluminescent spectra were measured with an Ocean Optics HR4000 spectrometer, and current density
voltage
light output measurements were taken using a Keithley 2400 SourceMeter, a Newport 2832-C power meter, and 818 UV detector. Unless specified otherwise, all device measurements were done inside a nitrogen-filled glovebox. Material Synthesis. Synthesis procedure: commercially available 4-bromobenzoyl chloride was first reacted with N-phenylbenzene-1,2-diamine in the presence of base to form 4-bromoN-(2-(phenylamino)phenyl)benzamide (1), which then underwent ring-closing condition to yield 2-(4-bromophenyl)-1 -phenyl-1H-benzo[d]imidazole (2). The bromide of the benzimidazole intermediate was converted to the boronic ester (3) first, which was then reacted with 3,6-dibromo-9-p-tolyl-9Hcarbazole under Suzuki coupling condition to yield the final product Cz-2pbb with the overall yield of 36%. The material has been purified by temperature gradient sublimation (Scheme 1). 4-Bromo-N-(2-(phenylamino)phenyl)benzamide (1). To a solution of 4-bromobenzoyl chloride (11 g, 50 mmol) in anhydrous dichloromethane (100 mL) was added N-phenylbenzene-1,2-diamine (10.2 g, 55 mmol) and then triethylamine (17 mL, 122 mmol) slowly. The whole was stirred at room temperature overnight. Filtration gave a white solid (6.5 g). The filtrate was worked up with water (∼300 mL) and extracted with dichloromethane (DCM) (∼300 mL) three times. The organic phase was collected and dried over MgSO4, concentrated, and recrystallized in DCM/hexanes to give another portion of white solid (10.6 g). The total amount of product was 17.1 g, in 93% yield. 1H NMR (400 MHz, CDCl3): 8.32 (s, 1H), 8.23 (d, 1H), 14348

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Figure 1. (a) Chemical structure of Cz-2pbb and HOMO/LUMO distribution of Cz-2pbb calculated by DFT (B3LYP/6-31G). (b) Absorption (squares) and emission (circles) spectra of Cz-2pbb in dilute chloroform solution and phosphorescent spectrum (triangles) of Cz-2pbb in 2-MeTHF at 77 K.

7.48 (m, 4H), 7.24 (m, 5H), 7.15 (m, 1H), 6.89 (t, 1H), 6.79 (d, 2H), 5.56 (s, 1H). 2-(4-Bromophenyl)-1-phenyl-1H-benzo[d]imidazole (2). To a suspension of amide 1 (9.6 g, 26 mmol) in anhydrous 1,4dioxane (100 mL) was added POCl3 (9.2 mL, 100 mmol) slowly. The whole was then heated at about 100 °C overnight. After cooling to room temperature, the mixture was poured into ice (200 g) with stirring. Filtration, followed by recrystallization in DCM/hexanes, gave a pale gray solid (8.2 g, in 90% yield). 1H NMR (400 MHz, CDCl3): 7.86 (d, 1H), 7.51 (m, 3H), 7.43 (bs, 4H), 7.30 (m, 3H), 7.24 (m, 2H). LCMS found: M + H = 349. Calcd for C19H13BrN2: 348. 1-Phenyl-2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-1H-benzo[d]imidazole (3). A mixture of compound 2 (0.70 g, 2 mmol), bis(pinacolate)diborane (0.533 g, 2.1 mmol), Pd(dppf)Cl2 (0.060 g, 0.08 mmol), and anhydrous potassium acetate (0.393 g, 4 mmol) in 1,4-dioxane (20 mL) was heated at about 80 °C under argon overnight. After cooling to room temperature, the whole was diluted with ethyl acetate (∼80 mL) and then filtered. The solution was absorbed on silica gel and then purified by column chromatography (hexanes/ethyl acetate 5:1 to 3:1) to give a white solid (0.64 g, in 81% yield). 1H NMR (400 MHz, CDCl3): 7.90 (d, 1H), 7.71 (d, 2H), 7.54 (d, 2H), 7.46 (m, 3H), 7.30 (3H), 7.24 (m, 2H), 1.32 (s, 12H). Cz-2pbb (4). A mixture of compound 3 (2.40 g, 6.06 mmol), 3,6-dibromo-9-p-tolyl-9H-carbazole (1.245 g, 3.0 mmol), Pd(dppf)Cl2 (0.23 g, 0.31 mmol), and KF (1.05 g, 18.2 mmol) in anhydrous DMF (∼50 mL) was heated at about 120 °C under argon overnight. After the mixture was cooled to room temperature, it was poured into water (∼200 mL) and filtered. The solid was collected and dissolved in DCM (∼200 mL). After removal of water by separate funnel followed by drying over MgSO4, the

DCM solution was absorbed on silica gel, purified by column chromatography (hexanes/ethyl acetate 4:1 to 2:1), and precipitated from ethyl acetate/hexanes to give product Cz-2pbb as an off-white solid (850 mg, in 36% yield). 1H NMR (400 MHz, CDCl3): 8.38 (d, 2H), 7.91 (d, 2H), 7.67 (m, 10H), 7.54 (m, 6H), 7.41 (m, 10H), 7.35 (m, 2H), 7.26 (4H), 2.50 (s, 3H). LCMS found: M + H = 794. Calcd for C57H39N5: 793. Compound Cz-2pbb. 13C NMR (CDCl3, 100 MHz): 21.37, 110.45, 110.65, 118.93, 119.52, 123.49, 123.71, 123.95, 125.60, 126.87, 127.08, 127.60, 128.98, 130.16, 130.70, 132.25, 134.68, 137.89, 137.12, 137.84, 141.37, 142.00, 143.14, 151.99 ppm. MS (APCI positive): calcd for C57H40N5 (M+H): 794. Found: 794. OLED Fabrication. Poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)) (CLEVIOS P VP AI 4083), 4,40 ,400 -tris(N-carbazolyl)triphenylamine (TCTA), and 1,3,5tris(phenyl-2-benzimidazolyl)benzene (TPBI) with thickness of 50, 30, and 40 nm were used as hole injection, hole transporting, and electron transporting layers, respectively. Indium
tin oxide (ITO) with sheet resistance ca. 20 Ω/0 and cesium fluoride/aluminum served as electrodes.6 PEDOT:PSS was spin-coated on precleaned and O2-plasmatreated ITO substrates and then baked at 180 °C for 10 min to remove residual water. All other materials were deposited at a pressure of 10
7 torr (1 torr = 133.322 Pa) in a vacuum deposition system hosted inside a nitrogen glovebox. The deposition rate for organic materials was around 0.06 nm s
1, whereas the deposition rate for CsF and Al was 0.005 and 0.2 nm s
1, respectively. Individual devices had an area of 0.14 cm2. For carrier mobility measurement experiments, devices with the structure of PEDOT:PSS/Cz-2pbb (100 nm)/Al were used to measure the hole mobility, while Al/Cz-2pbb (100 nm)/LiF/Al 14349

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Table 1. Properties of Cz-2pbb solutiona compound Cz-2pbb

Amax(nm)

λ max (nm)

318 ( 1

410 ( 2

solid filmb

T1c

λ max (nm)

λ max (nm)

HOMOd (eV)

LUMOd (eV)

Tg/Tde (°C)

PLQYf

432 ( 2

504 ( 2


5.55 ( 0.02


2.17 ( 0.02

172 ( 0.1/488 ( 1

0.65 ( 0.06

In dilute chloroform solution. b Drop casting neat film. c In 2-MeTHF at 77 K. d HOMO/LUMO was measured by cyclic voltammetry using ferrocene/ ferricenium couple as internal standard. e Glass transition temperature (Tg) measured by DSC, and the temperature in N2 at which 5.0% loss has occurred (Td), was measured by TGA. f In toluene solution: anthracene in EtOH as known standarad reference (Φ = 0.27). a

Figure 2. Current density
voltage characteristics of PEDOT:PSS/ Cz-2pbb (100 nm)/Al hole-only and Al/Cz-2pbb (100 nm)/LiF/Al electron-only devices.

devices were used to measure the electron mobility. The highcurrent end of the current density
voltage curves (6
10 V) was fitted using the space-charge limited current (SCLC) model J = 9εε0μV2/8L3, where ε0 is the vacuum permittivity, ε is the relative permittivity, μ is the carrier mobility, and L is the thickness. The electron and hole mobility can then be derived from the fitting parameters for the electron-only and hole-only devices, respectively.

’ RESULTS AND DISCUSSION Figure 1a shows the chemical structure of Cz-2pbb and HOMO/LUMO distribution of Cz-2pbb. Results from DFT calculation using the B3LYP/6-31G method indicate a small overlap integral between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) orbitals. Figure 1b shows the absorption and emission spectra of Cz-2pbb in dilute chloroform solution, as well as phosphorescence spectrum of Cz-2pbb in 2-MeTHF at 77 K. A noticeable absorption band at wavelength between 200 and 400 nm is observed, while the emission spectrum of Cz-2pbb solution shows a peak at ca. 410 nm. PLQY of Cz-2pbb in toluene solution was measured to be 0.65 referring to anthracene in EtOH. Triplet state energy of Cz-2pbb as estimated from the first peak of phosphorescent spectrum is 2.46 eV, which is higher than that of Ir(ppy)3 (2.4 eV). Electrochemical study of Cz-2pbb indicates that the HOMO and LUMO levels are located at
5.55 and
2.17 eV, respectively. Comparison of energy levels of Cz2pbb with those of Ir(ppy)3 and PQ2Ir(acac)1a suggests that Ir(ppy)3 and PQ2Ir(acac) work as carrier traps in Cz-2pbb host. Properties of Cz-2pbb are summarized in Table 1. The temperature in N2 at which 5.0% weight loss has occurred is 488 °C as measured by TGA. Owing to its V-shaped molecular geometry,

Figure 3. (a) EL spectra of PEDOT:PSS/TCTA (30 nm)/CBP:3% Cz2pbb (30 nm)/TPBI (40 nm)/CsF/Al (squares) and PEDOT:PSS/ TCTA (30 nm)/Cz-2pbb (30 nm)/TPBI (40 nm)/CsF/Al (circles) and (b) external quantum efficiency
current density characteristics (inset: current density
voltage properties).

which prevents tight intermolecular packing, Cz-2pbb possesses a very stable amorphous phase with Tg as high as 172 °C. Averaged carrier mobility of μ(h) = 3  10
8 cm2 V
1 s
1 and μ(e) = 1  10
7 cm2 V
1 s
1 at an electric field of ca.1 MV/cm is derived from the hole-only and electron-only devices (Figure 2), respectively. Figure 3a indicates electroluminescence (EL) spectra of devices with the structure PEDOT:PSS/TCTA (30 nm)/Cz2pbb (30 nm)/TPBI (40 nm)/CsF/Al and PEDOT:PSS/TCTA (30 nm)/CBP:3% Cz-2pbb (30 nm)/TPBI (40 nm)/CsF/Al, where the host material CBP is 4,40 -bis(9-carbazolyl)-2,20 -biphenyl. EL spectrum of Cz-2pbb-doped CBP devices shows an emission peak at ca. 410 nm, similar to photoluminescence spectrum of Cz-2pbb in solution. Due to enhanced intermolecular interactions in neat solid film, emission peak of Cz-2pbb 14350

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Table 2. Summary of Device Characteristicsa @500 cd m
2 bias (V) J (mA/cm2) EQE (%) PE (lm/W) LE (cd/A)

device

CIE

(1) PEDOT:PSS/TCTA/CBP:3% Cz-2pbb/TPBI/CsF/Al

7.0 ( 0.3

48 ( 2.0

2.9 ( 0.1 0.45 ( 0.02 1.04 ( 0.04 (0.16, 0.05)

(2) PEDOT:PSS/TCTA/Cz-2pbb/TPBI/CsF/Al

7.0 ( 0.4 25.6 ( 1.0

2.7 ( 0.1 0.87 ( 0.03 1.95 ( 0.08 (0.16, 0.09)

(3) PEDOT:PSS/TCTA/Cz-2pbb:0.4% Ir(ppy)3:0.15% PQ2Ir(acac)/TPBI/CsF/Al 5.3 ( 0.2

2.0 ( 0.08 10.7 ( 0.4 14.8 ( 0.6

25.0 ( 1.0

(4) PEDOT:PSS/TCTA/Cz-2pbb:0.3% Ir(ppy)3:0.07% PQ2Ir(acac)/TPBI/CsF/Al 5.4 ( 0.2

2.8 ( 0.11

9.0 ( 0.4 11.8 ( 0.5

20.3 ( 0.8

(0.38, 0.36)

1.3 ( 0.05 15.6 ( 0.7 17.8 ( 0.8

33.8 ( 1.5

(0.53, 0.43)

(5) PEDOT:PSS/TCTA/Cz-pbb:0.5% Ir(ppy)3:0.25% PQ2Ir(acac)/TPBI/CsF/Al

6.8 ( 0.3

(0.44, 0.41)

Each device is listed by number and structure. Data were measured at which a forward light output of approximately 500 cd m
2 was obtained. Parameters listed are drive voltage (bias), current density (J), external quantum efficiency (EQE), power efficiency (PE), luminous efficiency (LE), and CIE coordinates. The thickness of the emissive layer for devices 3, 4, and 5 is 15 and 25 nm, respectively. a

Figure 4. (a) EL spectra normalized to the red peak at 5 V (solid symbols) and at 8 V (open symbols) of PEDOT:PSS/TCTA (30 nm)/ Cz-2pbb:0.4% Ir(ppy)3:0.15% PQ2Ir(acac) (15 nm)/TPBI (40 nm)/ CsF/Al (squares), PEDOT:PSS/TCTA (30 nm)/Cz-2pbb:0.3% Ir(ppy)3:0.07% PQ2Ir(acac) (15 nm)/TPBI (40 nm)/CsF/Al (circles), and PEDOT:PSS/TCTA (30 nm)/Cz-2pbb:0.5% Ir(ppy)3:0.25% PQ2Ir(acac) (25 nm)/TPBI (40 nm)/CsF/Al (triangles) and (b) external quantum efficiency
current density characterizations (inset: current density
voltage properties).

devices is located at 435 nm, which is red-shifted by ca. 20 nm compared with that of Cz-2pbb-doped CBP devices. It is worth

noting that EL spectra of both devices are independent of drive voltage. The CIE coordinates of Cz-2pbb and Cz-2pbb-doped CBP devices are (0.16, 0.09) and (0.16, 0.05), respectively. Inset of Figure 3b shows the current density
voltage curves, indicating an operating voltage for both devices of about 2.5
3 V. At 500 cd/m2, the drive voltage for Cz-2pbb and Cz-2pbb-doped CBP devices is 7 V. As shown in Figure 3b, the peak EQE of Cz2pbb and Cz-2pbb-doped CBP devices is 3.1 and 4.1%, respectively. At 500 cd/m2, EQE/power efficiency (PE) values of 2.7%/ 0.87 and 2.9%/0.44 lm/W are measured for Cz-2pbb and Cz2pbb-doped CBP devices, respectively. Table 2 summarizes some of the device characteristics. The good EL properties of Cz-2pbb and high triplet state energy render it as an efficient fluorescent blue emitter, and a suitable host material for green and red emitting phosphors, critical attributes for the construction of hybrid WOLEDs with a single emissive layer. Figure 4a presents EL spectra of devices with the structure PEDOT:PSS/TCTA (30 nm)/Cz-2pbb:0.4% Ir(ppy)3:0.15% PQ2Ir(acac) (15 nm)/TPBI (40 nm)/CsF/Al, PEDOT:PSS/TCTA (30 nm)/Cz-2pbb:0.3% Ir(ppy)3:0.07% PQ2Ir(acac) (15 nm)/TPBI (40 nm)/CsF/Al, and PEDOT: PSS/TCTA (30 nm)/ Cz-2pbb:0.5% Ir(ppy)3:0.25% PQ2Ir(acac) (25 nm)/TPBI (40 nm)/CsF/Al. As shown in Figure 4a, the EL spectra of devices are composed of blue emission from Cz-2pbb, green emission from Ir(ppy)3, and red emission from PQ2Ir(acac), respectively. The emission intensity of Ir(ppy)3 and PQ2Ir(acac) with respect to that of Cz-2pbb increases with increasing Ir(ppy)3 and PQ2Ir(acac) concentration. In addition, the emission intensity of Cz-2pbb increases with increasing drive voltage. Devices with 0.3% Ir(ppy)3 show CIE coordinates of (0.33, 0.32) and Color Rendering Index (CRI) of 82 at 8 V. The operational voltage of devices with a 15 nm thick emissive layer is around 3 V, as shown in inset of Figure 4b. Figure 4b displays the EQE-current density characteristics. For devices with Ir(ppy)3 concentration of 0.4%/0.3%, the peak EQE is 12.2%/10.7%, respectively. These values drop to 10.7% and 9.0% at a luminance of 500 cd/m2. In this regard, assuming a lambertian emission pattern, PE of 0.4%/0.3% Ir(ppy)3 devices is 14.8/11.8 lm/W, respectively. There are several channels to excite Ir(ppy)3 and PQ2Ir(acac) including the following: (I) singlet energy transfer from Cz-2pbb, followed by rapid intersystem crossing of singlet
triplet manifolds in Ir(ppy)3 and PQ2Ir(acac); (II) direct carrier recombination on phosphor molecules; (III) capture of Cz-2pbb triplet excited states. Process I is not significant due to small F€orster radii of iridium complexes3c,7 and low doping concentration used 14351

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The Journal of Physical Chemistry C in devices. Process II is manifested in the more intense Ir(ppy)3 and PQ2Ir(acac) emission observed in EL spectra than in photoluminescence spectra. We do not have a direct evidence to confirm the occurrence of process III. Devices with a 25 nm thick Cz-2pbb:0.5% Ir(ppy)3:0.25% PQ2Ir(acac) emissive layer show the peak EQE of 17.5% (Figure 3) and preserve significant Cz-2pbb emission in the EL spectra at the same time, which may indirectly imply that triplet excited states of Cz-2pbb can be harvested by Ir(ppy)3 and PQ2Ir(acac) to achieve high EQE.3c At a given current density, blue emission from Cz-2pbb in devices with a 25 nm emissive layer is reduced compared to that of devices with a 15 nm emissive layer and having identical Ir(ppy)3 and PQ2Ir(acac) concentration. Decrease of EQE with the reduction of Ir(ppy)3 and PQ2Ir(acac) concentration, as well as a fast roll-off of EQE with drive current density, suggests that device efficiency is probably limited by the capture of triplet excited states of Cz-2pbb by the phosphors.

’ CONCLUSION In conclusion, the synthesis of Cz-2pbb was reported. OLEDs based on an emissive layer of Cz-2pbb showed peak EQE of 4.1% and CIE coordinates of (0.16, 0.05). Hybrid WOLEDs having a single emissive layer with EQE exceeding 10% were enabled by efficient deep-blue emission and bipolar carrier transporting ability along with high triplet state energy of Cz-2pbb.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (G.E.J.); amane_mochizuki@ gg.nitto.co.jp (A.M.). Present Addresses §

School of Materials, Arizona State University, Tempe, Arizona 85287, United States.

’ ACKNOWLEDGMENT The authors acknowledge the FiDiPro program, Academy of Finland, and funding from Nitto Denko Technical Corporation. ’ REFERENCES (1) (a) D’Andrade, B. W.; Forrest, S. R. Adv. Mater. 2004, 16, 1585. (b) Xiao, L. X.; Chen, Z. J.; Qu, B.; Luo, J. X.; Kong, S.; Gong, Q. H.; Kido, J. Adv. Mater. 2011, 23, 926. (2) For example: (a) Kido, J.; Kimura, K.; Nagai, K. Science 1995, 267, 5202. (b) D’Andrade, B. W.; Holmes, R. J.; Forrest, S. R. Adv. Mater. 2004, 16, 624. (c) Wu, F. I.; Shih, P. I.; Tseng, Y. H.; Shu, C. F.; Tung, Y. L.; Chi, Y. J. Mater. Chem. 2007, 17, 167. (d) Wu, F. I.; Yang, X. H.; Neher, D.; Dodda, R.; Tseng, Y. H.; Shu, C. F. Adv. Funct. Mater. 2007, 17, 1085. (e) Tong, Q. X.; Lai, S. L.; Chan, M. Y.; Tang, J. X.; Kwong, H. L.; Lee, C. S.; Lee, S. T. Appl. Phys. Lett. 2007, 91, 023503. (f) Adamovich, V.; Brooks, J.; Tamayo, A.; Alexander, A. M.; Djurovich, P. I.; D’Andrade, B. W.; Adachi, C.; Forrest, S. R.; Thompson, M. E. New. J. Chem. 2002, 26, 1171. (g) Cocchi, M.; Virgili, D.; Fattori, V.; Rochester, D. L.; Williams, J. A. G. Adv. Funct. Mater. 2007, 17, 285. (h) Yang, X. H.; Wang, Z. X.; Madakuni, S.; Li, J.; Jabbour, G. E. Adv. Mater. 2008, 20, 2405. (i) Yang, X. H.; Wang, Z. X.; Madakuni, S.; Li, J.; Jabbour, G. E. Appl. Phys. Lett. 2008, 93, 193305. (j) Yang, X. H.; Froehlich, J. D.; Chae, H. S.; Li, S.; Mochizuki, A.; Jabbour, G. E. Adv. Funct. Mater. 2009, 19, 2623. (k) Yang, X. H.; Froehlich, J. D.; Chae, H. S.; Harding, B. T.; Li, S.; Mochizuki, A.; Jabbour, G. E. Chem. Mater. 14352

dx.doi.org/10.1021/jp203115c |J. Phys. Chem. C 2011, 115, 14347–14352