Efficient Solution-Processed Deep-Blue Organic Light-Emitting Diodes

Article pubs.acs.org/cm

Efficient Solution-Processed Deep-Blue Organic Light-Emitting Diodes Based on Multibranched Oligofluorenes with a Phosphine Oxide Center Cui Liu,† Yanhu Li,‡ Yifan Li,† Chuluo Yang,*,† Hongbin Wu,*,‡ Jingui Qin,† and Yong Cao‡ †

Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan 430072, People’s Republic of China ‡ Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, People’s Republic of China ABSTRACT: A series of multibranched oligofluorenes with a phosphine oxide center were designed and synthesized through Suzuki cross-coupling reaction. Their thermal, photophysical, and electrochemical properties were investigated. The phosphine oxide linkage can disrupt the conjugation, and then allow the molecule system to extend to improve the solution processability and photoluminescent quantum yields without depreciating the deep blue emission. The noncoplanar molecular structures resulting from the phosphine oxide triangular pyramidal configuration can suppress the intermolecular interaction. All compounds display strong deep-blue emission both in solution and the solid state. Solution-processed devices based on these oligofluorenes exhibit highly efficient deep-blue electroluminescence, and the device performances are significantly enhanced with the extension of the oligofluorene branches. The double-layered device featuring PPO-TF3 as emitter shows a maximum current efficiency of 1.88 cd A−1 and a maximum external quantum efficiency of 3.39% with Commission Internationale de l′Eclairage (CIE) coordinates of (0.16, 0.09) that are very close to the National Television Standards Committee’s blue standard. KEYWORDS: deep-blue fluorescent OLEDs, solution process, oligofluorene, phosphine oxide

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INTRODUCTION Organic light-emitting diodes (OLEDs) have attracted considerable scientific and industrial attention because of their applications in flat-panel displays and solid-state lighting resources.1−16 Full-color displays require primary RGB emission of relatively equal stability, efficiency, and color purity. The development of deep-blue emission, which is defined as having blue electroluminescence (EL) emission with a Commission Internationale de l′Eclairage (CIE) y coordinate value of less than 0.15, is of special significance because such emitters can not only effectively reduce the power consumption of a full-color OLED but also be utilized to generate light of other colors by energy cascade to a lower-energy fluorescent or phosphorescent dopant.17−19 However, the performance of blue light emitting devices is often inferior to that of green and red counterparts for the intrinsic wide band gap of blue-emitting materials which makes it hard to inject charges into emitting layer. Polyfluorene and its derivatives are considered to be the most promising blue light emitting materials due to their high photoluminescence (PL) efficiencies, wide band-gaps, good thermal stabilities, interesting morphological properties, and easy tunability of properties by substitution and/or copolymerization.20−26 However, the application of polyfluorenes as blue emitters in OLEDs is hampered by their undesirable long© XXXX American Chemical Society

wavelength emission in the process of photoirradiation, heat treatment, or device operation, which has been attributed to physical (excimers or aggregate formation)27−29 or chemical (keto defects)30,31 degradation processes. Recently, monodisperse oligofluorenes that can be deposited via solution process have been of great interest for use in OLEDs.32−41 In comparison with polymers, monodisperse oligofluorenes are characterized by well-defined and uniform molecular structures as well as high chemical purity. From a practical standpoint, high chemical purity and structural uniformity are crucial factors that could influence the performance of OLEDs. It has been demonstrated that the performance and stability of monodisperse oligofluorenes in OLEDs are higher than that of polyfluorenes.42−44 Monodisperse star-shaped oligofluorenes with different cores, such as benzene,40 triazatruxene,32,34 pyrene,35 4,4′,4″-tris(carbazol-9-yl)-triphenylamine (TCTA),33 1,3,5-tri(anthracen10-yl)benzene,39 fully bridged triphenylamine,41 truxene,45 and spirofluorene,46 have been reported. However, there is still no multibranched oligofluorene reported so far.41 Received: May 21, 2013 Revised: July 28, 2013

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dx.doi.org/10.1021/cm401640v | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

nm) and Al (120 nm) layer were evaporated with a shadow mask at a base pressure of 3 × 10−4 Pa. The thickness of the evaporated cathode was monitored by using a quartz crystal thickness/ratio monitor (model STM-100/MF, Sycon), and the thickness of spin-coated PEDOT:PSS and EML was measured by using the terrace detector. The overlapping area between the cathode and anode defined a pixel size of 19 mm2. Except for the deposition of the PEDOT layers, all the fabrication processes were carried out inside a controlled atmosphere of nitrogen drybox (Vacuum Atmosphere Co.) containing less than 10 ppm oxygen and moisture. The current density−luminance−voltage characteristic was measured using a Keithley 236 source measurement unit and a calibrated silicon photodiode. The forward-viewing luminance was calibrated by using a spectrophotometer (SpectraScan PR-705, Photo Research), and the forward-viewing LE was calculated accordingly. Throughout the whole paper, reported values of luminance and LE are for forward-viewing direction only. The external quantum efficiency of EL was collected by measuring the total light output in all directions in an integrating sphere (IS-080, Labsphere). The EL spectra were collected via a PR-705 photometer. Synthesis. Oligofluorene boronic acids of different chain lengths (F1−F3) were prepared according to a reported procedure.45,46 The Suzuki coupling reaction was conducted under a nitrogen atmosphere and by avoiding light exposure. Bis(4-bromophenyl)(phenyl)phosphine Oxide. To a solution of 1,4dibromobenzene (4.72 g, 20 mmol) in anhydrous THF (160 mL), nbutyllithium (2.26 M in hexane, 8.4 mL, 19 mmol) was added dropwise at −78 °C. The reaction was kept at this temperature for 2 h, and then 1.26 mL (9.3 mmol) of dichloro(phenyl)phosphine was added. After the mixture was stirred for 1 h at −78 °C, it was allowed to warm to room temperature, stirred overnight followed by quenching with 5 mL of methanol. Water was added, and the mixture was extracted with CH2Cl2, washed with water, and dried over anhydrous Na2SO4. After the solvent had been completely removed, 30% hydrogen peroxide (30 mL) and CH2Cl2 (60 mL) were added to the obtained residue and they were stirred overnight at room temperature. The organic layer was separated and washed with water and then brine. The extract was evaporated to dryness, and the residue was purified by column chromatography on silica gel using dichloromethane/methanol (30:1 by vol) as the eluent to give a white solid (3.77 g). Yield: 93%. 1H NMR (300 MHz, CDCl3, δ): 7.68−7.58 (m, 6H), 7.58−7.45 (m, 7H). 13C NMR (75 MHz, CDCl3, δ): 133.38, 133.31, 132.35, 131.80, 131.77, 131.47, 131.12, 130.77, 130.43, 128.69, 128.60, 127.38, 127.36. Anal. Calcd for C18H13Br2OP (%): C 49.58, H 3.00; found: C 49.50, H 3.33. MS (EI) m/z calcd for C18H13Br2OP: 435.91; found: 435.83. Tris(4-bromophenyl)phosphine Oxide. Tris(4-bromophenyl)phosphine oxide was prepared according to the similar procedure to bis(4-bromophenyl)(phenyl)phosphine oxide by using phosphorus trichloride to replace dichloro(phenyl)phosphine. Yield: 30%. 1H NMR (300 MHz, CDCl3, δ): 7.68−7.60 (m, 6H), 7.54−7.44 (m, 6H). 13C NMR (75 MHz, CDCl3, δ): 133.60, 133.51, 132.64, 132.33, 132.20, 131.14, 130.09, 129.00, 127.97. Anal. Calcd for C18H12Br3OP (%): C 41.98, H 2.35; found: C 42.16, H 2.54. MS (EI) m/z calcd for C18H12Br3OP: 513.82; found: 514.54. Bis(4-(9,9-dihexyl-9H-fluoren-2-yl)phenyl)(phenyl)phosphine Oxide (PPO-BF1). To a mixture of bis(4-bromophenyl)(phenyl)phosphine oxide (0.44 g, 1.00 mmol), fluorene boronic acid (F1) (0.98 g, 2.60 mmol), Pd(PPh3)4 (0.069 g, 0.06 mmol), and 2 M K2CO3 (5.00 mL, 10.00 mmol) was added degassed toluene (20 mL) and ethanol (5 mL). The solution was heated to reflux for 48 h under argon. After cooling to room temperature, the solution was extracted with CH2Cl2 and the organic layer was washed with brine and H2O, and then dried over anhydrous Na2SO4. After the solvent had been removed, the residue was purified by column chromatography on silica gel using petroleum/ethyl acetate (2:1 by vol) as the eluent to give a white powder (0.75g). Yield: 79%. 1H NMR (300 MHz, CDCl3, δ): 7.88−7.70 (m, 14H), 7.63−7.55 (m, 5H), 7.55−7.47 (m, 2H), 7.38−7.30 (m, 6H), 2.00 (t, J = 7.2 8H), 1.15−0.95 (m, 24H), 0.75 (t, J = 6.6 12H), 0.71− 0.60 (m, 8H). 13C NMR (75 MHz, CDCl3, δ): 151.92, 151.35, 145.55, 141.71, 140.75, 138.90, 133.02, 132.88, 132.52, 132.39, 131.93, 130.54, 128.96, 128.80, 127.58, 127.42, 127.18, 126.48, 123.23, 121.85, 120.41,

In this context, we designed and synthesized a series of multibranched oligofluorenes with a phosphine oxide center. We wish that the deep-blue emission of these materials could be wellpreserved with the extension of molecules due to the disrupted conjugation via phosphine oxide center. The introduction of the electron-withdrawing phosphine oxide can also improve the electron transporting ability of the materials. Moreover, the existence of phosphine oxide group results in a triangular pyramidal structure, which could effectively hinder the close molecular packing in the solid state and prevent excimer formation and fluorescence quenching. The thermal, photophysical, and electrochemical properties of oligofluorenes as well as the characteristics of devices incorporating these molecules were investigated. All these compounds show deep-blue emission with high fluorescence quantum yields in film. Solution-processed devices based on these oligomers exhibit efficient deep-blue electroluminescence. Furthermore, the device performance is significantly enhanced with the extension of the oligofluorene branches. The double-layered device with PPOTF3 as emitter shows a maximum current efficiency of 1.88 cd A−1 and a maximum external quantum efficiency of 3.39% with CIE coordinates of (0.16, 0.09).

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EXPERIMENTAL SECTION

General Information. 1H NMR and 13C NMR spectra were measured on a MECUYR-VX300 spectrometer. Elemental analyses of carbon, hydrogen, and nitrogen were performed on a Vario EL III microanalyzer. Mass spectra were measured on a ZAB 3F-HF mass spectrometer. MALDI-TOF (matrix-assisted laser-desorption/ionization time-of-flight) mass spectra were performed on a Bruker BIFLEX III TOF mass spectrometer. UV−vis absorption spectra were recorded on a Shimadzu UV-2500 recording spectrophotometer. Photoluminescence spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer. The PL quantum yields of solid state films were measured by an absolute method using the Edinburgh Instruments (FLS920) integrating sphere excited with Xe lamp. Differential scanning calorimetry (DSC) was performed on a NETZSCH DSC 200 PC unit at a heating rate of 10 °C min−1 from room temperature to 300 °C under argon. The glass transition temperature was determined from the second heating scan. Thermogravimetric analysis was undertaken with a NETZSCH STA 449C instrument. The thermal stability of the samples under a nitrogen atmosphere was determined by measuring their weight loss while heating at a rate of 15 °C min−1 from 25 to 800 °C. Cyclic voltammetry was carried out in nitrogen-purged THF (reduction scan) and dichloromethane (oxidation scan), respectively, with a CHI voltammetric analyzer. Tetrabutylammonium hexafluorophosphate (TBAPF6) (0.1 M) was used as the supporting electrolyte. The conventional three-electrode configuration consists of a platinum working electrode, a platinum wire auxiliary electrode, and an Ag wire pseudoreference electrode with ferrocenium-ferrocene (Fc+/Fc) as the internal standard. Cyclic voltammograms were obtained at scan rate of 100 mV s−1. Formal potentials are calculated as the average of cyclic voltammetric anodic and cathodic peaks. The onset potential was determined from half-wave potential of the oxidation. Device Fabrication and Measurement. Patterned ITO coated glass with a sheet resistance of 15−20 Ω square−1 were cleaned by a surfactant scrub, then underwent a wet-cleaning process inside an ultrasonic bath, beginning with deionized water, followed by acetone and isopropanol. After oxygen plasma cleaning for 4 min, 40 nm of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) (Bayer Baytron P 4083) used as a hole-injection layer at the anode interface was spin-coated on the ITO substrate and then dried in a vacuum oven at 80 °C overnight. The emissive layer (EML) was coated onto the anode by spin-coating from chlorobenzene solution, and then annealed at 80 °C for 10 min to remove the solvent residue. The thickness of the EML was about 50 nm. Finally, an electrontransporting layer of TPBI (30 nm) and a cathode composed of CsF (1.5 B



Article

Scheme 1. Synthesis of the Multibranched Oligofluorenes

CDCl3, δ): 7.91−7.70 (m, 21H), 7.62−7.57 (m, 6H), 7.38−7.30 (m, 6H), 2.00 (m, 12H), 1.11−0.96 (m, 36H), 0.74 (t, J = 6.6 18H), 0.70− 0.59 (m, 12H). 13C NMR (75 MHz, CDCl3, δ): 151.58, 151.02, 145.25, 141.38, 140.41, 138.58, 132.58, 130.27, 127.29, 126.84, 126.14, 122.88, 121.52, 119.89, 55.17, 40.35, 31.42, 29.64, 23.71, 22.51, 13.94. Anal. Calcd for C93H111OP (%): C 87.55, H 8.77; found: C 87.88, H 8.95. MS (MALDI-TOF) m/z calcd for C93H111OP: 1275.84; found: 1276.45. Tris(4-(9,9,9′,9′-tetrahexyl-9H,9′H-[2,2′-bifluoren]-7-yl)phenyl)phosphine Oxide (PPO-TF2). PPO-TF2 was prepared according to the same procedure as PPO-BF1 but using bifluorene boronic acid (F2) and tris(4-bromophenyl)phosphine oxide to give a white power. Yield: 57%. 1 H NMR (300 MHz, CDCl3, δ): 7.95−7.71 (m, 22H), 7.71−7.59 (m, 23H), 7.38−7.29 (m, 6H), 2.17−1.97 (m, 24H), 1.20−0.99 (m, 72H), 0.82−0.62 (m, 60H). 13C NMR (75 MHz, CDCl3, δ): 151.92, 151.80, 151.46, 150.97, 145.25, 141.10, 140.94, 140.72, 140.34, 139.58, 138.59, 132.75, 132.62, 127.31, 127.15, 126.78, 126.24, 126.03, 122.90, 121.62, 121.41, 120.15, 119.86, 119.71, 55.35, 55.15, 40.35, 31.43, 29.66, 23.77, 22.52, 13.96. Anal. Calcd for C168H207OP (%): C 88.76, H 9.18; found: C 88.59, H 8.91. MS (MALDI-TOF) m/z calcd for C168H207OP: 2272.59; found: 2272.81. Tris(4-(9,9,9′,9′,9″,9″-hexahexyl-9H,9′H,9″H-[2,2′:7′,2″-terfluoren]-7-yl)phenyl)phosphine Oxide (PPO-TF3). PPO-TF3 was prepared according to the same procedure as PPO-BF1 but using terfluorene boronic acid (F3) and tris(4-bromophenyl)phosphine oxide to give a pale yellow power. Yield: 69%. 1H NMR (300 MHz, CDCl3, δ): 7.98− 7.75 (m, 27H), 7.75−7.59 (m, 36H), 7.42−7.30 (m, 6H), 2.19−1.98 (m, 36H), 1.20−1.00 (m, 108H), 0.89−0.63 (m, 90H). 13C NMR (75 MHz, CDCl3, δ): 152.10, 151.74, 151.27, 145.57, 141.39, 141.22, 141.04, 140.63, 140.18, 139.90, 138.90, 133.04, 132.91, 127.60, 127.44, 127.06, 126.44, 123.18, 121.77, 120.47, 120.24, 119.98, 55.60, 55.43, 40.64, 31.72, 29.93, 24.11, 22.80, 14.26. Anal. Calcd for C243H303OP (%): C 89.23, H 9.34; found: C 88.95, H 9.03. MS (MALDI-TOF) m/z calcd for C243H303OP: 3270.35; found: 3270.15.

120.22, 55.52, 40.69, 31.76, 29.97, 24.06, 22.84, 14.26. Anal. Calcd for C68H79OP (%): C 86.58, H 8.44; found: C 86.70, H 8.53. MS (MALDITOF) m/z calcd for C68H79OP: 942.59; found: 943.12. Bis(4-(9,9,9′,9′-tetrahexyl-9H,9′H-[2,2′-bifluoren]-7-yl)phenyl)(phenyl)phosphine Oxide (PPO-BF2). PPO-BF2 was prepared according to the same procedure as PPO-BF1 but using bifluorene boronic acid (F2) and bis(4-bromophenyl)(phenyl)phosphine oxide to give a white power. Yield: 80%. 1H NMR (300 MHz, CDCl3, δ): 7.89− 7.73 (m, 17H), 7.72−7.49 (m, 16H), 7.42−7.31 (m, 6H), 2.15−1.98 (m, 16H), 1.20−1.01 (m, 48H), 0.95−0.65 (m, 40H). 13C NMR (75 MHz, CDCl3, δ): 152.23, 152.12, 151.79, 151.30, 145.55, 141.41, 141.27, 141.05, 140.68, 139.91, 138.89, 133.04, 132.89, 132.53, 128.96, 127.58, 127.42, 127.09, 126.58, 126.35, 123.23, 121.81, 120.46, 120.18, 120.02, 55.67, 55.47, 40.65, 31.73, 29.96, 24.08, 22.82, 14.25. Anal. Calcd for C118H143OP (%): C 88.12, H 8.96; found: C 88.45, H 9.01. MS (MALDI-TOF) m/z calcd for C118H143OP: 1608.09; found: 1607.43. Bis(4-(9,9,9′,9′,9″,9″-hexahexyl-9H,9′H,9″H[2,2′:7′,2″- terfluoren]-7-yl)phenyl)(phenyl)phosphine Oxide (PPO-BF3). PPO-BF3 was prepared according to the same procedure as PPO-BF1 but using terfluorene boronic acid (F3) and bis(4-bromophenyl)(phenyl)phosphine oxide to give a pale yellow power. Yield: 87%. 1H NMR (300 MHz, CDCl3, δ): 7.89−7.73 (m, 22H), 7.73−7.49 (m, 23H), 7.39−7.26 (m, 6H), 2.17−1.97 (m, 24H), 1.20−1.00 (m, 72H), 0.90− 0.65 (m, 60H). 13C NMR (75 MHz, CDCl3, δ): 151.48, 151.12, 150.66, 144.88, 140.60, 140.44, 140.02, 139.58, 139.28, 138.26, 132.39, 132.25, 131.89, 128.30, 128.14, 126.92, 126.76, 126.44, 125.82, 122.57, 121.17, 119.82, 119.61, 119.35, 54.98, 54.81, 40.00, 31.09, 29.30, 23.47, 22.17, 13.62. Anal. Calcd for C168H207OP (%): C 88.76, H 9.18; found: C 88.74, H 9.15. MS (MALDI-TOF) m/z calcd for C168H207OP: 2272.59; found: 2271.97. Tris(4-(9,9-dihexyl-9H-fluoren-2-yl)phenyl)phosphine Oxide (PPOTF1). PPO-TF1 was prepared according to the same procedure as PPOBF1 but using fluorene boronic acid (F1) and tris(4-bromophenyl)phosphine oxide to give a white power. Yield: 51%. 1H NMR (300 MHz, C



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Figure 1. TGA thermograms (a, c) and DSC traces (b, d) of the materials recorded at a heating rate of 10 °C min−1.

Figure 2. UV−vis absorption and PL spectra of the compounds in toluene solution at 5 × 10−6 M (a, c) and film state (b, d).

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were prepared by reported procedure.45,46 The key intermediates bis(4-bromophenyl)(phenyl)phosphine oxide and tris(4bromophenyl)phosphine oxide were prepared from 1,4dibromobenzene, sequentially through a lithium-halogen ex-

RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic routes and chemical structures of the materials are depicted in Scheme 1. Oligofluorene boronic acids of different chain lengths (F1−F3) D



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Table 1. Thermal, Photophysical, and Electrochemical Data of the Materials PPO-BF1 PPO-BF2 PPO-BF3 PPO-TF1 PPO-TF2 PPO-TF3

Td/Tg (°C)

λabs (nm)a

λabs (nm)b

λem, max (nm)a

λem, max (nm)b

HOMO/LUMO (eV)cd

ΦPL (%)e

417/66 420/85 420/94 415/77 420/92 423/103

321 347 361 322 349 363

321 352 367 326 355 367

355 (sh), 371 394, 413 (sh) 408, 429 (sh) 357 (sh), 372 395, 414 (sh) 409, 430 (sh)

379 404 (sh), 421 415, 437 (sh) 401 408 (sh), 419 416, 437 (sh)

−5.53/−2.10 −5.52/−2.40 −5.45/−2.47 −5.58/−2.18 −5.56/−2.46 −5.48/−2.50

31 58 90 33 71 99

a Measured in toluene. bMeasured in film. cDetermined from the onset of oxidation potentials. dDeduced from HOMO and Eg estimated from the red edge of the longest absorption wavelength for the solid-film sample. eFluorescence quantum yields in solid state films, measured on the quartz plate using an integrating sphere.

change reaction with n-butyllithium (n-BuLi), a coupling reaction with dichlorophenylphosphine and phosphorus trichloride, respectively, and an oxidation reaction with hydrogen peroxide. The target compounds were successfully prepared through Suzuki cross-coupling reactions of the phosphine oxide intermediates and the oligofluorene boronic acids. All the compounds were fully characterized by 1H NMR, 13C NMR, MALDI-TOF, and elemental analysis. Thermal Properties. The good thermal stability of the compounds is indicated by their high decomposition temperatures (Td, corresponding to 5% weight loss) in the range of 415−423 °C in the thermogravimetric analysis (Figure 1). The reason for the first weight loss to around 65% of the original weight is the cleavage of alkyl side chains. Their glass transition temperatures (Tg) determined through differential scanning calorimetry are between 66 and 103 °C, and enhanced with increaing oligofluorene branch length. Moreover, when the oligofluorene branches of these molecules extended from two to three, Tg of the corresponding compound rises by about 10 °C, which can be attributed to the increased molecular size and molecular weight. The good thermal and morphological stability of these materials enables the preparation of homogeneous and stable amorphous thin films through solution processing, which is crucial for the operation of OLEDs. Photophysical Properties. Figure 2 shows the absorption and fluorescence spectra of these compounds in both toluene solution and solid state films. The photophysical data of the compounds are presented in Table 1. For two-branched molecules, the absorption spectra of three compounds in toluene solution show intense π−π* absorption bands, which progressively red-shift from 321 nm for PPO-BF1 to 361 nm for PPO-BF3 with increasing fluorene units. The emission spectrum of PPO-BF1 locates in near-ultraviolet region, while PPO-BF2 and PPO-BF3 display deep-blue emission. The emission maxima of three compounds are 371, 394, and 408 nm in solution, respectively. The absorption and emission spectra of the films are similar to those of the solution, and only exhibit a small red-shift in the emission maximum (8−10 nm), indicating the absence of intermolecular interactions in thin films. This can be attributed to the noncoplanar configuration resulted by phosphine oxide group that have effectively prevented close-packing of constituent molecules and restrained intermolecular aggregation. Fluorescence quantum yields (ΦPL) of two-branched materials in solid state measured on the quartz plate using an integrating sphere are 0.31 for PPO-BF1, 0.58 for PPO-BF2, and 0.90 for PPO-BF3, significantly enhanced with the extension from one fluorene unit to two and three. The three-branched oligofluorenes show similar absorption and emission spectra to their twobranched analogues with slight red-shift, indicating that the conjugation has not been expanded when the oligofluorene

branches increase from two to three. Fluorescence quantum yields of three-branched oligofluorenes are 0.33, 0.71 and 0.99, respectively. Electrochemical Properties. The electrochemical properties of the compounds were probed by cyclic voltammetry (CV, Figure 3). All compounds exhibit overlapped multireversible

Figure 3. Cyclic voltammograms of the materials in CH2Cl2 for oxidation.

oxidation waves, attributed to the oxidation of oligofluorene branches. With the extension of fluorene units in each branch, the onset of the oxidation potentials decreases from 0.73 (PPO-BF1) to 0.65 eV (PPO-BF3) for two-branched oligofluorenes, and 0.78 (PPO-TF1) to 0.68 eV (PPO-TF3) for three-branched oligofluorenes. The highest occupied molecular orbital (HOMO) energy levels estimated from the onsets of the oxidation potentials are around −5.50 eV with regard to the energy level of ferrocene (4.8 eV below vacuum). The band gaps E



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(Eg) of these compounds, on the basis of the red edge of the longest absorption wavelength for the solid-film sample, were estimated to be 2.98−3.43 eV. The lowest unoccupied molecular orbital (LUMO) levels range from 2.10 to 2.50 eV, which are deduced from HOMO and Eg (Table 1). Electroluminescence. Compounds PPO-BF2, PPO-BF3, PPO-TF2, and PPO-TF3 exhibit deep-blue emission, high fluorescence quantum yield and good solubility in common solvents. To evaluate EL properties of these materials, simple double-layer devices, featuring PPO-BF2, PPO-BF3, PPO-TF2, and PPO-TF3 as nondoped blue emitters, were fabricated with the configurations of indium tin oxide (ITO)/PEDOT:PSS (40 nm)/emission layer (EML, 50 nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) (30 nm)/CsF (1.5 nm)/Al (120 nm) (emitter: PPO-BF2, device A; PPO-BF3, device B; PPOTF2, device C; PPO-TF3, device D), in which the emission layer was spin-coated from chlorobenzene solution. PEDOT:PSS and CsF served as hole- and electron-injecting layers, respectively. TPBI was inserted between the EML and LiF as the electrontransporting layer. The devices based on these emitters exhibit deep-blue emission in their EL spectra with emission maxima of 424 nm for devices A and C, and 438 nm for devices B and D (Figure 4).

Figure 5. (a) Current efficiency and external quantum efficiency versus current density curves and (b) current density−voltage−brightness (J− V−L) characteristics for devices A−D.

performances are comparable with those of the star-shaped oligofluorenes we have reported before41 and among the highest for nondoped deep-blue OLEDs based on solution-processable starburst oligofluorenes until now.32−41 We noted that the EL performance is significantly enhanced with the extension of fluorene units from two to three in each branch. The improvements can be mainly elucidated from the substantially higher photoluminescence quantum yields of PPOBF3 over PPO-BF2, PPO-TF3 over PPO-TF2. In addition, the HOMO and LUMO energy levels of PPO-BF3 and PPO-TF3 are more matched with adjacent PEDOT and TPBI layers than those of their analogues PPO-BF2 and PPO-TF2, respectively. Therefore, more balanced charge flux can be anticipated in device B and D. The operating stability test under continuous stress was also carried out with an initial luminance of 100 cd m−2 for device D. The lifetime (defined as the time it takes for the display to reach 50% of the original luminance) of the device is found to be ∼10 h, indicating that the lifetime of the devices still remains as the major hindrance to practical applications. We note that the operating lifetime of the real device is closely related to some of extrinsic factors (such as the interface properties of the electrode, the reliability of encapsulation, etc.); the observed short lifetime of the devices should not be solely attributed to the intrinsic properties of the compound reported here. Since the lifetime of the device can be significantly improved through rational materials design by improving the charge transporting properties and thermal properties, our forthcoming work will pay more attention to this issue.

Figure 4. Normalized EL spectra of devices A−D.

The EL spectras of all materials are similar to their PL spectra from the thin film samples, indicating that the excimer or exciplex have been effectively suppressed during the EL process. Figure 5 shows the current density−voltage−brightness (J− V−L) characteristics, and efficiency versus current density curves of the devices. All the device data are summarized in Table 2. The turn-on voltage of these devices is in the following order: A > B, C > D, which could be attributed to the matched HOMO and LUMO energy level for device B and D (Figure 5), resulting in lower hole and electron injection barrier than device A and C. Device A exhibit a turn-on voltage of 5.4 V, a maximum current efficiency (ηc, max) of 1.08 cd A−1, and a maximum external quantum efficiency (ηext, max) of 1.74%, with CIE coordinates of (0.17, 0.11). The performances of device C are comparable to those of device A. The device based on PPO-BF3 shows significantly improved performances with a low turn-on voltage of 4.4 V, ηc, max of 1.76 cd A−1, ηext, max of 3.17%, and CIE coordinates of (0.16, 0.08), which is very close to the National Television Standards Committee’s blue standard (0.14, 0.08). PPO-TF3-based device D exhibits better performance compared to device B, with turn-on voltage of 4.2 V, ηc, max of 1.88 cd A−1, ηext, max of 3.39%, and CIE coordinates of (0.16, 0.09). These F



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Table 2. Electroluminescence Characteristics of the Devices device

EML

Von (V)a

Lmax (cd m−2)b, V (V)

ηc, max (cd A−1)c

ηext, max (%)d

CIE (x, y)e

A

PPO-BF2

5.4

564, 10.4

PPO-BF3

4.4

672, 9.2

C

PPO-TF2

5.0

257, 16.0

D

PPO-TF3

4.2

944, 9.8

1.74 1.72 ± 0.04 3.17 3.07 ± 0.14 1.82 1.80 ± 0.03 3.39 3.41 ± 0.03

(0.17, 0.11)

B

1.08 1.06 ± 0.02 1.76 1.71 ± 0.07 1.13 1.12 ± 0.01 1.88 1.90 ± 0.02

(0.16, 0.08) (0.17, 0.10) (0.16, 0.09)

Turn-on voltage, recorded at the brightness of 1 cd m−2. bMaximum luminance. cMaximum current efficiency; statistics (italic) based on 10 cells of each type. dMaximum external quantum efficiency; statistics (italic) based on 10 cells of each type. eMeasured at 10 mA m−2. a

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CONCLUSION In summary, we have designed and synthesized a series of multibranched oligofluorenes with a phosphine oxide center. All of the compounds show excellent thermal stabilities, pronounced PL efficiencies, and good solution processability. Double-layered nondoped OLEDs based on these materials by solution process exhibit highly efficient deep-blue electroluminescence. The PPOTF3-based device achieves a maximum current efficiency of 1.88 cd A−1 and maximum external quantum efficiency of 3.39% with CIE coordinates of (0.16, 0.09). Our findings provide a valuable strategy to the rational design and development of efficient blue EL oligofluorenes and other related materials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.Y.); [email protected] (H.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

C.Y. thanks the National Basic Research Program of China (973 Program 2013CB834805 and 2009CB623602) and the National Science Fund for Distinguished Young Scholars of China (No. 51125013); H.W. thanks the National Natural Science Foundation of China (61177022).

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