Extension of Molecular Structure toward Solution-Processable Hosts

Dec 24, 2012 - Two new host materials, namely DBISiPA and DOXDSiPA, were designed and synthesized by incorporating an electron-donating ...
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Extension of Molecular Structure toward Solution-Processable Hosts for Efficient Blue Phosphorescent Organic Light-Emitting Diodes Shaolong Gong,†,§ Cheng Zhong,†,§ Qiang Fu,‡ Dongge Ma,*,‡ Jingui Qin,† and Chuluo Yang*,† †

Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan 430072, People’s Republic of China ‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ABSTRACT: Two new host materials, namely DBISiPA and DOXDSiPA, were designed and synthesized by incorporating an electron-donating diphenylamine unit and an electronaccepting benzimidazole or oxadiazole moiety into one molecule via double-silicon-bridged linkage. This extension of molecular structure endows these compounds with good solution processability and high thermal stability without lowering their triplet energies. DBISiPA and DOXDSiPA exhibit similar energy levels and significantly higher glass transition temperatures (146 and 149 °C) relative to the corresponding single-silicon-bridged congeners. As a result, solution-processed blue phosphorescent devices employing bis(4,6(difluorophenyl)pyridine-N,C2′)picolinate (FIrpic) as guest and the two compounds as hosts exhibit high efficiencies with the maximum current efficiency of 16.2 cd A−1 for DBISiPA and 15.2 cd A−1 for DOXDSiPA. These efficiencies are significantly higher than those of the control blue device employing typical nonconjugated polymer, poly(9-vinylcarbazole) (PVK), as the host, and even comparable to those of the complicated control device employing PVK doped with 30 wt % 1,3-bis[(4-tertbutylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD-7) as the mixed host.

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device performance.11,12 Since higher molecular precision of small-molecule host materials can overcome the abovementioned discrepancies, the development of solution-processable small-molecule host materials suitable for blue PhOLEDs is highly desirable to realize this goal. As an effective host for solution-processed blue PhOLEDs, it should possess a high enough triplet energy level to confine the triplet excitons on the phosphor,13 good charge transporting characteristic to balance the carrier flux in the emitting layer,14 and high-lying highest occupied molecular orbital (HOMO) level to match the Fermi level of the commonly used holeinjection material poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, −5.1 eV).15 Besides these electrical properties, the hosts should have good film-forming ability and morphological stability during device fabrication and operation. In general, host materials with low molecular weight usually suffer from the damaged film integrity during the device operation due to their inherently low glass-transition temperature.16 As a result, the ideal host materials for solutionprocessed PhOLEDs should have relatively large molecular size and high molecular weight. Given all of the requirements needed for the host materials, it becomes a significant challenge

INTRODUCTION Phosphorescent organic light-emitting diodes (PhOLEDs) have generated interest, driven by their bright future for nextgeneration flat-panel displays and lighting sources. Through utilizing both singlet and triplet exitons for emission, PhOLEDs can reach a nearly 100% internal quantum efficiency in theory.1−3 To realize high-efficiency PhOLEDs, phosphorescent emitters of heavy-metal complexes are usually doped into a suitable host material to suppress concentration quenching and triplet−triplet annihilation.4 To date, green and red PhOLEDs with 100% internal quantum efficiency have been achieved,5−8 but highly efficient, stable blue PhOLEDs remain to be further developed because of the lack of suitable host materials. Among these, Kido et al. reported a highly efficient blue phosphorescent device via thermal high-vacuum evaporation technology, with the external quantum efficiencies (EQE) of 26% and 25% at a practical luminance of 100 and 1000 cd m−2, respectively.9 Despite the efficiency improvement for blue PhOLEDs, their high fabrication cost, device complexity, and control difficulties still hamper the practical applicability of blue PhOLEDs. Solution processes, such as spin-coating, inkjet printing and roll-to-roll fabrication, offer an attractive alternative approach in terms of their low-cost and large-area manufacturability that is more amenable to commercial interests.10 Although polymeric host materials can be easily fabricated by a solution process, their batch-to-batch variations in solubility, molecular weight and purity can result in different processing properties and © 2012 American Chemical Society

Received: September 13, 2012 Revised: December 3, 2012 Published: December 24, 2012 549

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Scheme 1. Extention of Molecular Structure from Single- to Double-Silicon-Bridged Molecules

Cyclic voltammetry (CV) was carried out in nitrogen-purged dichloromethane (oxidation scan) or tetrahydrofuran (THF, reduction scan) at room temperature 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. The onset potential was determined from the intersection of two tangents drawn at the rising and background current of the cyclic voltammogram. Computational Details. The geometrical and electronic properties were performed with the Gaussian 09 program package.21 The optimized calculation was obtained by means of the B3LYP (Becke three parameters hybrid functional with Lee−Yang−Parr correlation functionals) with the 6-31G(d) atomic basis set.22,23 Then the electronic structures were calculated at τHCTHhyb/6-311++G(d, p) level.24 Molecular orbitals were visualized using Gaussview. Device Fabrication and Measurement. The holeinjection material PEDOT:PSS, hole/exciton-blocking material 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (Tm3PyPB),25 electrontransporting materials 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI)26 and OXD-7,27 the blue phorsphor FIrpic and the polymeric host material PVK15 were commercially available. Commercial ITO (indium tin oxide) coated glass with sheet resistance of 10 Ω per square was used as the starting substrates. Before device fabrication, the ITO glass substrates were precleaned carefully and treated by oxygen plasma for 2 min. PEDOT:PSS (40 nm) was first spin-coated to smooth the ITO surface and dried at 120 °C for 30 min under vacuum. Then the emissive layer of DBISiPA or DOXDSiPA doped with 15 wt % FIrpic (40−50 nm) was spin-coated from chlorobenzene solution onto the PEDOT:PSS layer and dried at 80 °C for 30 min to remove residual solvent. Finally, Tm3PyPB (5 nm), TPBI (30 nm), and a cathode composed of lithium fluoride (LiF, 1 nm) and aluminum (Al, 100 nm) were sequentially deposited onto the substrate by vacuum deposition in the vacuum of 10−6 Torr. J-V-L of the devices was measured with a Keithley 2400 Source meter and a Keithley 2000 Source

for the search of the host materials suitable for solutionprocessed blue PhOLEDs. In our recent work, we developed a series of single-siliconbridged molecules with high triplet energies and bipolar transporting properties as host materials for blue PhOLEDs, which achieved good device performance via thermal highvacuum evaporation technology.17−19 However, their low molecular weights are not suitable for the realization of PhOLEDs via a low-cost solution process. To develop the solution-processable host materials, we further extend the molecular structure by integrating a double silicon-bridge between the electron-donating arylamine group and electronaccepting benzimidazole or oxadiazole moiety (Scheme 1). We anticipate that the extended molecular structure would improve their solution processability to form high-quality films, and enhance their thermal stability, without lowering their triplet energies.20 We will present a comprehensive investigation that encompasses the thermal, photophysical, and electrochemical properties of the compounds as well as the theoretical modeling and demonstrate the applicability of these compounds as host materials for solution-processed blue PhOLEDs.

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EXPERIMENTAL SECTION General Information. 1H NMR and 13C NMR spectra were measured on a MERCURY-VX300 spectrometer. Elemental analyses of carbon, hydrogen, and nitrogen were performed on a Vario EL III microanalyzer. GC−mass spectra were measured on a Thermo Trace DSQ II GC/MS. MALDITOF 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 (PL) spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer. Differential scanning calorimetry (DSC) was performed on a NETZSCH DSC 200 PC unit at a heating rate of 10 °C min−1 from 20 to 300 °C under argon. The glass transition temperature (Tg) was determined from the second heating scan. Thermogravimetric analysis (TGA) 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 10 °C min−1 from 25 to 800 °C. 550

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Scheme 2. Synthesis and Chemical Structures of DBISiPA and DOXDSiPA

10H, Ar), 7.42−7.32 (m, 14H, Ar), 7.11−7.03 (m, 4H, Ar), 6.97 (d, J = 8.1 Hz, 2H, Ar), 5.73 (s, 1H, NH), 2.30 (s, 3H, CH3). 13C NMR (75 MHz, CDCl3) δ [ppm]: 151.85, 145.41, 142.38, 138.81, 137.35, 136.92, 136.74, 136.52, 136.03, 135.94, 134.05, 131.43, 130.27, 129.59, 129.53, 129.26, 128.40, 128.19, 127.55, 127.09, 123.21, 122.85, 122.30, 119.70, 119.46, 114.91, 110.21 (Ar), 20.44 (CH3). MS (EI): m/z 633.0 (M+). Anal. Calcd for C44H35N3Si (%): C 83.37, H 5.57, N 6.63. Found: C 83.57, H 5.79, N 6.31. Bis(4-{[4-(1-phenyl-1H-benzimidazol-2-yl)phenyl](diphenyl)silyl}phenyl)(4-methylphenyl)amine (DBISiPA). A mixture of p-BISiN-H (1.45 g, 2.29 mmol), p-BISi-Br (1.52 g, 2.50 mmol), Pd(OAc)2 (11 mg, 0.05 mmol), tBuONa (0.288 g, 3.00 mmol), (tBu)3PHBF4 (44 mg, 0.15 mmol), and toluene (20 mL) was refluxed under argon for 18 h. After cooling, the mixture was extracted with brine and CH2Cl2, and dried over anhydrous Na2SO4. After removal of the solvent, the residue was purified by column chromatography on silica gel using ethyl acetate/petroleum ether (1:3 by vol.) as the eluent to give a white powder. Yield: 60%. 1H NMR (300 MHz, CDCl3) δ [ppm]: 7.89 (d, J = 8.1 Hz, 2H, Ar), 7.59−7.45 (m,

multimeter equipped with a calibrated silicon photodiode. Electroluminescence (EL) spectra were measured by JY SPEX CCD3000 spectrometer. All measurements were carried out at room temperature under ambient conditions. Synthesis of Materials. 2-{4-[(4-Bromophenyl)(diphenyl)silyl]phenyl}-1-phenyl-1H-benzimidazole (p-BISiBr) and 2-{4-[(4-bromophenyl)(diphenyl)silyl]phenyl}-5-(4tert-butylphenyl)-1,3,4-oxadiazole (p-OXDSi-Br) were prepared according to the reported procedures.18,28,29 4-{[4-(1-Phenyl-1H-benzimidazol-2-yl)phenyl](diphenyl)silyl}-N-(4-methylphenyl)aniline (p-BISiN-H). A mixture of p-BISi-Br (2.00 g, 3.30 mmol), p-toluidine (0.536 g, 5.00 mmol), Pd(OAc)2 (15 mg, 0.066 mmol), tBuONa (0.38 g, 3.96 mmol), (tBu)3PHBF4 (57 mg, 0.20 mmol), and toluene (20 mL) was refluxed under argon for 18 h. After cooling, the mixture was extracted with brine and CH2Cl2, and dried over anhydrous Na2SO4. After removal of the solvent, the residue was purified by column chromatography on silica gel using ethyl acetate/petroleum ether (1:5 by vol.) as the eluent to give a light yellow powder. Yield: 75%. 1H NMR (300 MHz, CDCl3) δ [ppm]: 7.88 (d, J = 7.8 Hz, 1H, Ar), 7.59−7.48 (m, 551

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Thermal Properties. The thermal properties of the compounds were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA measurement reveals their high thermal-decomposition temperatures (Td, corresponding to 5% weight loss) of 495 °C (DBISiPA) and 475 °C (DOXDSiPA) (Figure 1 and Table 1).

20H, Ar), 7.42−7.35 (m, 28H, Ar), 7.10−7.04 (m, 8H, Ar), 2.33 (s, 3H, CH3). 13C NMR (75 MHz, CDCl3) δ [ppm]: 152.06, 148.66, 144.03, 142.83, 137.17, 136.77, 136.43, 136.23, 136.14, 134.11, 133.92, 130.75, 130.10, 129.80, 129.54, 128.55, 128.41, 127.78, 127.27, 126.36, 125.85, 123.36, 122.96, 122.07, 119.74, 110.39 (Ar), 20.84 (CH3). MALDI-TOF: m/z 1161.3 (M+). Anal. Calcd for C81H61N5Si2 (%): C 83.83, H 5.30, N 6.03. Found: C 83.81, H 4.99, N 5.65. 4-[{4-[5-(4-Tert-butylphenyl)-1,3,4-oxadiazol-2-yl]phenyl}(diphenyl)silyl]-N-(4-methylphenyl)aniline (pOXDSiN-H). Prepared as a light yellow solid according to a similar procedure to p-BISiN-H, from p-OXDSi-Br and ptoluidine. Yield: 63%. 1H NMR (300 MHz, CDCl3) δ [ppm]: 8.12 (d, J = 7.8 Hz, 2H, Ar), 8.06 (d, J = 8.1 Hz, 2H, Ar), 7.75 (d, J = 7.8 Hz, 2H, Ar), 7.60−7.54 (m, 6H, Ar), 7.46−7.37 (m, 8H, Ar), 7.22−7.00 (m, 6H, Ar), 4.76 (s, 1H, NH), 2.31 (s, 3H, CH3), 1.37 (s, 9H, C(CH3)3). 13C NMR (75 MHz, CDCl3) δ [ppm]: 164.64, 164.35, 155.28, 145.80, 139.99, 138.98, 137.56, 136.83, 136.23, 133.97, 131.72, 129.78, 129.64, 127.88, 126.71, 125.99, 125.82, 124.59, 122.05, 120.97, 120.01, 115.17 (Ar), 34.98, 31.03 (C(CH3)3), 20.67 (CH3). MS (EI): m/z 641.0 (M+). Anal. Calcd for C43H39N3OSi (%): C 80.46, H 6.12, N 6.55. Found: C 80.63, H 5.74, N 6.42. Bis{4-[{4-[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]phenyl}(diphenyl)silyl]phenyl}(4-methylphenyl)amine (DOXDSiPA). Prepared as a white solid according to a similar procedure to DBISiPA, from p-OXDSiN-H and p-OXDSi-Br. Yield: 62%. 1H NMR (300 MHz, CDCl3) δ [ppm]: 8.12 (d, J = 8.1 Hz, 4H, Ar), 8.06 (d, J = 8.7 Hz, 4H, Ar), 7.74 (d, J = 7.8 Hz, 4H, Ar), 7.59−7.54 (m, 12H, Ar), 7.46−7.40 (m, 15H, Ar), 7.12−7.09 (m, 9H, Ar), 2.33 (s, 3H, CH3), 1.37 (s, 18H, C(CH3)3). 13C NMR (75 MHz, CDCl3) δ [ppm]: 164.02, 163.66, 154.64, 148.24, 143.39, 139.01, 136.64, 136.25, 135.64, 133.65, 133.03, 129.61, 129.17, 127.35, 126.10, 125.92, 125.37, 125.29, 125.00, 124.13, 121.61, 120.39 (Ar), 34.36, 30.44 (C(CH3)3), 20.26 (CH3). MALDI-TOF: m/z 1176.4 (M+). Anal. Calcd for C79H69N5O2Si2 (%): C 80.69, H 5.91, N 5.95. Found: C 80.71, H 5.83, N 5.58.

Figure 1. TGA traces of DBISiPA and DOXDSiPA recorded at a heating rate of 10 °C min−1. Inset: DSC traces of DBISiPA and DOXDSiPA recorded at a heating rate of 10 °C min−1.

The DSC trace exhibits distinct glass-transition temperatures (Tg) of 146 °C (DBISiPA) and 149 °C (DOXDSiPA) during the second heating scans (the inset of Figure 1), which are significantly higher than those of the corresponding singlesilicon-bridged congeners, p-BISiTPA (102 °C) and pOXDSiTPA (100 °C), respectively,18 indicating that their increased molecular sizes significantly improve their thermal stability. These results demonstrate that they could form morphologically stable and uniform amorphous films upon solution-process, which is highly important for enhancing the efficiency and lifetime of PhOLEDs. Photophysical Properties. Figure 2 shows the electronic absorption, fluorescence, and phosphorescence spectra of the compounds. The absorption peaks around 300 nm are consistent with the triphenylamine-centered π−π* transition, and almost no solvatochromism is noticed in the absorption bands of these compounds. In addition, almost no charge transfer absorption band from the electron-donating triphenylamine to the electron-withdrawing benzimidazole or oxadiazole group can be observed, suggesting the disruption of the πconjugation between the two parts owing to the silicon-bridged linkage mode. The two compounds show near-ultraviolet emissions of 366 nm (DBISiPA) and 367 nm (DOXDSiPA) in low-polarity hexane solution. In more polar dichloromethane solution, the two compounds display pronounced dual emission bands simultaneously (Figure 2):31 the emission peaks of 368 nm can be assigned to the π−π* transition; the dominating emissions at longer wavelengths of 491 nm (DBISiPA) and 506 nm (DOXDSiPA) originate from through-space donor−acceptor charge transfer transitions. The triplet energies of DBISiPA and DOXDSiPA are determined to be 2.68 and 2.69 eV, respectively, from the highest-energy vibronic sub-band of the phosphorescence spectra in frozen 2-methyltetrahydrofuran matrix at 77 K. Remarkably, these double-silicon-bridged compounds exhibit similar triplet energy levels as the correspoding single-silicon-bridged congeners, p-BISiTPA (2.69 eV) and p-OXDSiTPA (2.70 eV),18 respectively, suggesting that the electronic coupling in the whole molecular remains unvaried when extending the molecular size via the

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RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic routes and chemical structures of the compounds, bis(4-{[4-(1phenyl-1H-benzimidazol-2-yl)phenyl](diphenyl)silyl}phenyl)(4-methylphenyl)amine (DBISiPA), and bis{4-[{4-[5-(4-tertbutylphenyl)-1,3,4-oxadiazol-2-yl]phenyl}(diphenyl)silyl]phenyl}(4-methylphenyl)amine (DOXDSiPA), are depicted in Scheme 2. We initially attempted to synthesize the target compounds in the palladium-catalyzed C−N coupling reaction from direct replacement of the two N−H of p-toluidine with the bromide compound (p-BISi-Br or p-OXDSi-Br),30 but we only got the single-N−H-replaced precursor (p-BISiN-H or pOXDSiN-H). And then, the key precursor was converted into the target compound (DBISiPA or DOXDSiPA) with the corresponding bromide compound under the same reaction conditions, respectively. Detailed synthetic procedures are presented in the Experimental Section. The chemical structures of the compounds were fully characterized by 1H NMR and 13C NMR, mass spectrometry and elemental analysis. Additionally, all of the compounds exhibit good solubility in the common solvents, such as dichloromethane, tetrahydrofuran, toluene and chlorobenzene. The good solubility of these compounds is desirable for the fabrication of solution-processed PhOLEDs. 552

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Table 1. Physical Data of DBISiPA and DOXDSiPA compound

Tga/Tdb [°C]

λabsc [nm]

λem,maxc [nm]

λabsd [nm]

λem,maxd [nm]

ETe [eV]

HOMO/LUMOexpf [eV]

HOMO/LUMOcalg [eV]

DBISiPA DOXDSiPA

146/495 149/475

306 295

366 367

303 296

368, 491 368, 506

2.68 2.69

−5.25/NA −5.27/−2.35

−5.20/−1.78 −5.28/−2.19

a Obtained from DSC measurements. bObtained from TGA measurements. cMeasured in hexane. dMeasured in CH2Cl2. eMeasured in 2methyltetrahydrofuran matrix at 77 K. fDetermined from the onset of oxidation/reduction potentials. gValues from DFT calculation.

waves, arising from the reduction of the n-type oxadiazole segments;6 while DBISiPA exhibits no reduction wave in the cathodic scan up to −3.1 V.32 The HOMO/LUMO energy levels of the compounds are determined from the onset of the oxidation potentials with regard to the energy level of ferrocene (−4.8 eV below vacuum). The estimated HOMO levels of DBISiPA and DOXDSiPA are −5.25 and −5.27 eV, which are significantly higher than the calculated value of −7.0 eV of the tetraarylsilane compounds previously reported,33 and close to that of PEDOT:PSS (−5.1 eV),15 consequently resulting in low hole-injection barriers from PEDOT:PSS to the hosts. The LUMO level of DOXDSiPA is −2.35 eV, similar to other oxadiazole derivatives.18,19,34 Theoretical Calculations. DFT calculations were performed to investigate the structure−property relationship of the compounds at the molecular level (for details, see the Experimental Section). As shown in Figure 4, their HOMO

Figure 2. Room temperature UV−vis absorption and PL spectra of in hexane and dichloromethane solution at 5 × 10−6 M, and phosphorescence spectra in frozen 2-methyltetrahydrofuran matrix at 77 K for (a) DBISiPA and (b) DOXDSiPA.

double-silicon-bridged linkage. The triplet energy levels of DBISiPA and DOXDSiPA are slightly higher than that of blue phosphor FIrpic (2.65 eV),13 which enables them as suitable hosts for FIrpic. Electrochemical Properties. Cyclic voltammetry (CV) was performed to investigate the electrochemical properties of the compounds (Figure 3). During the anodic scan in dichloromethane, the two compounds exhibit one quasireversible, one-electron oxidation process, which can be assigned to the oxidation of arylamine moiety. Upon cathodic sweeping in THF, DOXDSiPA exhibits reversible reduction

Figure 4. Calculated spatial distributions of the HOMO and LUMO levels of DBISiPA and DOXDSiPA.

and LUMO orbitals are mainly localized on the electron-rich triphenylamine and the electron-deficient benzimidazole or oxadiazole moiety, respectively. Both of the compounds have almost complete separation of the HOMO and LUMO at holeand electron-transporting moieties, respectively, which can be attributed to the incorporation of silicon-bridged linkage between electron donor and acceptor units, resulting in minimal intramolecular interactions between the two moieties. The complete separation benefits efficient hole- and electrontransporting properties, and prevents reverse energy transfer.35 The calculated HOMO/LUMO values are −5.20/−1.78 eV for DBISiPA and −5.28/−2.19 eV for DOXDSiPA, respectively, which correlate with the experimental results (Table 1). Phosphorescent OLEDs. To evaluate the utility of the two compounds as host materials for blue phosphor FIrpic, we fabricated blue devices with the configuration of ITO/ PEDOT:PSS (40 nm)/hosts: FIrpic (15 wt %, 40−50 nm)/ Tm3PyPB (5 nm)/TPBI (30 nm)/LiF (1 nm)/Al (100 nm). PEDOT:PSS and LiF served as hole- and electron-injecting layers, respectively; Tm3PyPB and TPBI were used as the hole/exciton-blocking and electron-transporting layer, respectively; FIrpic doped in host DBISiPA or DOXDSiPA was used as the emitting layer. For comparison, we also fabricated two

Figure 3. Cyclic voltammograms of DBISiPA and DOXDSiPA in CH2Cl2 for oxidation and in THF for reduction. 553

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control blue devices based on the host PVK, which is the most widely used polymeric host material for blue electrophosphorescence. 36,37 As shown in Figure 5, DBISiPA- and

Table 2. Electroluminescence Characteristics of the Devices host

Vona [V]

DBISiPA DOXDSiPA PVK PVK:OXD-7

8.6 7.6 8.8 6.8

Lmaxb [cd m−2] (V at Lmax, V) 1573, 1169, 4298, 10 158,

15.8 16.0 17.6 14.0

CEmaxc [cd A−1]

PEmaxd [lm W−1]

16.2 15.2 8.7 14.7

4.7 5.5 2.6 5.5

CIE [x,y]f 0.16, 0.16, 0.15, 0.19,

0.32 0.33 0.30 0.35

Turn-on voltages at 1 cd m−2. bMaximum luminance. cMaximum current efficiency. dMaximum power efficiency. eCurrent efficiency at 100 cd m−2. fCommission International de I’Eclairage coordinates. a

processed FIrpic-based PhOLEDs,38−40 their efficiencies are significantly higher than those of the control blue device employing the typical nonconjugated polymer, PVK, as the host, which achieves CEmax of 8.7 cd A−1 and PEmax of 2.6 lm W−1 (Figure 7). Since PVK only transports holes, the electron-

Figure 5. EL spectra and CIE coordinates of DBISiPA- and DOXDSiPA-based blue PhOLEDs.

DOXDSiPA-based devices show the same main peak at 475 nm with a slight shoulder peak at 500 nm, arising from the typical emission of the phosphor FIrpic. Furthermore, no additional emission coming from host material DBISiPA or DOXDSiPA is observed, indicative of efficient energy transfer from these hosts to FIrpic. Figure 6 shows the current density−voltage−luminance (J-VL) characteristics and efficiency versus luminance curves for

Figure 7. Current efficiency and power efficiency versus luminance curves for the control devices.

transporting material OXD-7 is always mixed into PVK to facilitate the electron transport in the PhOLEDs. For comparison, we also fabricated another control device employing PVK doped with 30 wt % OXD-7 as the mixed host under the same conditions, which achieves CEmax of 14.7 cd A−1 and PEmax of 5.5 lm W−1 (Figure 7 and Table 2). Apparently, the EL performance of DBISiPA- and DOXDSiPA-based devices are comparable to those of the PVK:OXD-7-based device, suggesting that DBISiPA and DOXDSiPA have more balanced carrier transporting ability compared with PVK. These results further conform the advantage of our newly desiged host materials and render them promising solution-processable host materials in replacement of PVK for efficient blue PhOLEDs. In addition, the results also provide some useful guidelines for further design of solution-processable small-molecule host materials.

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CONCLUSIONS In summary, we have developed two double-silicon-bridged molecules as solution-processable hosts for blue phosphor. The extended molecular size improves the solution processability and thermal stability in comparison with the corresponding single-silicon-bridged compounds, without lowering their triplet energies and affecting their electrical properties. Consequently, the EL performance of DBISiPA- and DOXDSiPA-based devices are significantly higher than those of the control blue device employing PVK as the host and comparable to those of the PVK:OXD-7-based device. These results demonstrate that the extension of molecular structure via double-silicon-bridged linkage is an effective approach to design small-molecule

Figure 6. (a) Current density−voltage−luminance characteristics, (b) current efficiency and power efficiency versus luminance curves for DBISiPA- and DOXDSiPA-based blue PhOLEDs.

DBISiPA- and DOXDSiPA-based devices, and the EL data is summarized in Table 2. The DBISiPA-based device achieves a maximum current efficiency (CEmax) of 16.2 cd A−1, and a maximum power efficiency (PEmax) of 4.7 lm W−1 (Figure 6 and Table 2); the DOXDSiPA-based device shows CEmax of 15.2 cd A−1 and PEmax of 5.5 lm W−1. Although these values are moderate relative to the highest values reported for solution554

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The Journal of Physical Chemistry C

Article

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solution-processable host materials for highly efficient blue PhOLEDs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.Y.); [email protected] (D.M.). Author Contributions §

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Science Fund for Distinguished Young Scholars of China (No. 51125013), the National Basic Research Program of China (973 Program 2009CB623602 and 2009CB930603), and the National Natural Science Foundation of China (No. 90922020) for financial support.

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REFERENCES

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dx.doi.org/10.1021/jp309100e | J. Phys. Chem. C 2013, 117, 549−555