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New Benzimidazole-Based Bipolar Hosts: Highly Efficient Phosphorescent and Thermally Activated Delayed Fluorescent OLEDs Employing the Same Device Structure Yali Zhao, Chao Wu, Panlong Qiu, Xiaoping Li, Qiang Wang, Jiangshan Chen, and Dongge Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10464 • Publication Date (Web): 05 Jan 2016 Downloaded from http://pubs.acs.org on January 10, 2016

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ACS Applied Materials & Interfaces

New Benzimidazole-Based Bipolar Hosts: Highly Efficient Phosphorescent and Thermally Activated Delayed Fluorescent OLEDs Employing the Same Device Structure Yali Zhao,† Chao Wu,† Panlong Qiu,† Xiaoping Li,‡ Qiang Wang,*,† Jiangshan Chen,§ and Dongge Ma*,§ †

School of Materials Science and Engineering, Shaanxi Normal University, Xi'an 710062, P. R.

China ‡

College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, P. R.

China §

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied

Chemistry, University of Chinese Academy of Sciences, Changchun 130022, P. R. China KEYWORDS: organic light-emitting diode, bipolar host, benzimidazole, phosphorescence, thermally activated delayed fluorescence

ACS Paragon Plus Environment

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ABSTRACT: The rapid development of the thermally activated delayed fluorescence (TADF) emitters makes it necessary to produce new versatile host materials for both phosphorescent and TADF emitters. Three new bipolar host materials, 9,9'-(2'-(1H-benzimidazol-1-yl)-[1,1'biphenyl]-3,5-diyl)bis(9H-carbazole)

(o-mCPBI),

9,9'-(3'-(1H-benzimidazol-1-yl)-[1,1'-

biphenyl]-3,5-diyl)bis(9H-carbazole) (m-mCPBI), and 9,9'-(4'-(1H-benzimidazol-1-yl)-[1,1'biphenyl]-3,5-diyl)bis(9H-carbazole) (p-mCPBI), are designed and synthesized by integrating mCP with benzimidazole moiety via ortho-, meta-, and para-position of N-phenyl. The influence of different linking modes on the thermal, photophysical, electrochemical, and charge transport properties of the compounds is studied. By employing the same device structure except the emitting layer, the device performances of the blue, green, yellow, red, white phosphorescent and blue, green TADF organic light-emitting diodes (OLEDs) based on the three hosts are investigated. Among these three hosts, o-mCPBI exhibits the best device performance with external quantum efficiencies of over 20% for phosphorescent OLEDs and enhanced efficiencies for TADF devices. All these devices show relatively low efficiency roll-offs at high brightness. The versatility of the benzimidazole-based bipolar host o-mCPBI, such as an extremely high triplet energy level, suitable molecular orbital energy levels, improved thermal and charge transport properties, and excellent device performances for various colors, makes it a universal host material for highly efficient both phosphorescent and TADF OLEDs.

INTRODUCTION Since the first ultra-thin multi-layered electroluminescent (EL) device was reported by Tang et al. in 1987, organic light-emitting diodes (OLEDs) have been attracting great interest over the


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past few decades for their potential applications in full color displays and solid state lighting sources due to their superior characteristics.1 To achieve high-efficiency OLEDs, phosphorescent organometallic materials that can utilize radiative transitions from the triplet state are employed because phosphorescent OLEDs using triplet excitons for light emission can reach a theoretical maximum internal quantum efficiency (IQE) of 100%.2,3 Other strategies such as generating a singlet excited (S1) state by triplet–triplet annihilation (TTA) and hybridized local and charge transfer (HLCT) have also been proposed to harvest the triplet excitons for luminescence.4,5 However, the efficiency of generating singlet excitons by TTA is only 37.5% at maximum, which decreases the EL quantum efficiency of OLEDs using phosphorescent materials. Recently, Adachi’s group has proposed a method of achieving a 100% IQE through up-conversion from the triplet excited (T1) state to S1 state, which is known as the thermally activated delayed fluorescence (TADF) process.6,7 The rare-metal-free TADF materials have attracted great attention as next generation organic EL materials because of high quantum efficiency of the TADF devices achieved without using noble metals such as Ir and Pt as emitting materials.8–11 Phosphorescent and TADF OLEDs represent active areas of recent research in organoelectronics with external quantum efficiency (EQE) over 20%. As the phosphorescent or TADF emitters are preferably dispersed at the molecular level into a host to prevent concentration quenching,12 host materials with sufficiently high triplet energies (ETs) are imperative to emitters for excitons to remain on dopant molecules. The lagging of the development of blue phosphorescent OLEDs behind that of green and red ones in terms of efficiency and stability raises higher requirements to the host materials. Carbazole (Cz) derivatives have been widely employed as host materials due to their sufficiently high ETs and good hole transport ability. 1,3-Bis(carbazol-9-yl)benzene (mCP) is one of the most extensively


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used hosts for triplet emitters.13 Although mCP possesses a suitable ET of 2.9 eV, its relatively poor thermal and morphological stability due to the low glass transition temperature (Tg) below 60 °C may hinder its application as a host material in OLEDs.14 Moreover, the hole transport property of mCP makes charge recombination occur near the interface between the emitting layer (EML) and the electron transport layer (ETL), which is detrimental to device efficiency and lifetime due to TTA and triplet–polaron quenching (TPQ) at high current densities pertaining to practical applications.15–17 One alternative design strategy of host materials to achieve high and balanced electric flux is to bond electron-donating (D) and electron-accepting (A) moieties chemically into a single bipolar molecule. The capability of the bipolar hybrid hosts has been demonstrated for facilitating injection and transport of holes and electrons, broadening the recombination zone, and consequentially alleviating efficiency roll-off.18–22 The D moieties for most bipolar hosts are limited to Cz and triphenylamine/diphenylamine. The A moieties, by contrast, vary substantially from heteroaromatic rings, such as oxadiazole, triazole, triazine, benzimidazole, pyridine, phenanthroline, etc, to phosphine oxide, phosphine sulfide, and sulfonyl group, etc. Among these acceptors, the benzimidazole derivatives exhibit good electron mobility and the trimer of N-arylbenzimidazoles, TPBI, has been used frequently as the ETL in both fluorescent and phosphorescent OLEDs.23–27 On the other hand, N-arylbenzimidazole moieties have been widely reported to connect with donors such as arylamine to form bipolar emitters or hosts.28–34 However, the commonly used C-connectivity of the benzimidazole unit in most of these bipolar hosts has suppressed the ET values due to the extended π-conjugation. Methods to limit the degree of π-conjugation between benzimidazole and arylamine groups include introducing a methyl steric group or non-conjugated tetraphenylsilicon skeleton and connecting the two units through meta-linkage. By changing the linking topology from a C- into a sp3


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hybridized N-bridge, Wong and coworkers reported benzimidazole/Cz hybrid hosts with higher ET values and weak orbital interactions between the D and A units.35,36 The ET value of a new benzimidazole/Cz host material incorporating both meta-linkage and N-bridge reached as high as 2.78 eV, as Wang et al. demonstrated, despite the relatively low Tg at 86 °C.37 To sum up, benzimidazole unit has great potential as a component for bipolar hybrid host materials and there is still much room for improvement of the comprehensive performance for benzimidazole-based bipolar hosts. Moreover, the rapid development of TADF emitters makes it necessary to produce new versatile host materials for both phosphorescent and TADF emitters.38,39 In this work, we designed and synthesized a series of bipolar host materials, o-mCPBI, mmCPBI, and p-mCPBI by integrating mCP with benzimidazole moiety via ortho-, meta-, and para-position of N-phenyl. The influence of different linking modes on the thermal, photophysical, electrochemical, and charge transport properties of the compounds was studied. By employing the same device structure except the EML, the device performances of the blue, green, yellow, red, white phosphorescent and blue, green TADF OLEDs based on the three hosts were investigated. With the highest ET value of 3.00 eV among these three hosts, o-mCPBI exhibited the best device performance with EQEs of over 20% for phosphorescent OLEDs and enhanced efficiencies for TADF devices. All these devices showed relatively low efficiency rolloffs at high brightness. The versatility of the benzimidazole-based bipolar host o-mCPBI, such as an extremely high ET, suitable molecular orbital energy levels, improved thermal and charge transport properties compared to mCP, and excellent device performances for various colors without changing the main device structure, makes it a universal host material for highly efficient both phosphorescent and TADF OLEDs. EXPERIMENTAL SECTION


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General Information. All chemicals, reagents, and solvents were used as received from commercial sources without further purification. All reactions were carried out under nitrogen and anhydrous conditions unless noted otherwise. The intermediates 1-(2-bromophenyl)-1Hbenzimidazole,

1-(3-bromophenyl)-1H-benzimidazole,

and

1-(4-bromophenyl)-1H-

benzimidazole were synthesized according to Scheme S1 by a catalyst-free method. 1H NMR (600 MHz) and

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C NMR (75 MHz) spectra were measured on Bruker Avance spectrometers

using CDCl3 as a solvent and the spectral data were reported in ppm relative to tetramethylsilane (TMS) as an internal standard. LC-Mass spectra were measured on a Bruker Avance Mass Spectrometer (maXis, ESI). Elemental analyses of carbon, hydrogen, and nitrogen were performed on a Vario EL III microanalyzer. UV-Vis absorption spectra were recorded on a PG TU-1901 recording spectrophotometer. photoluminescence (PL) spectra were measured on a Hitachi F-7000 fluorescence spectrophotometer and phosphorescence spectra at 77 K were recorded on a Perkin-Elmer LS 50B spectrofluorometer. Differential scanning calorimetry (DSC) was performed on a Mettler Toledo DSC1 STARe system at a heating rate of 10 °C/min from 20 to 300 °C under nitrogen. The Tg was determined from the second heating scan. Thermogravimetric analysis (TGA) was undertaken with a TA instrument Q600 at a scanning rate of 10 °C/min under nitrogen. Cyclic voltammetry (CV) was carried out using nitrogenpurged anhydrous tetrahydrofuran (THF) for the reduction and dichloromethane for the oxidation 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 pseudo-reference electrode with ferrocenium–ferrocene (Fc+/Fc) as the internal standard. Cyclic voltammograms were obtained at a scan rate of 100 mV/s. Formal


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potentials are calculated as the average of cyclic voltammetric anodic and cathodic peaks. The onset potential was determined from the intersection of two tangents drawn at the rising and background current of the cyclic voltammogram. The geometrical and electronic properties of the three hosts were performed with the Gaussian 09 program package. The calculation was optimized by means of B3LYP with the 6-31G(d) atomic basis set. The molecular orbitals were visualized using Gaussview. Synthesis of 1-(2-bromophenyl)-1H-benzimidazole. A mixture of benzimidazole (1.18 g, 10 mmol), 2-bromofluorobenzene (3.50 g, 20 mmol), K3PO4 (10.60 g, 50 mmol), and N,Ndimethylformamide (DMF, 100 mL) was refluxed for 16 h. After cooling, the mixture was treated with water, extracted with chloroform, and the organic layer was dried over anhydrous MgSO4. After removal of the solvent, the residue was purified by column chromatography on silica gel using ethyl acetate/petroleum at 2:5 by volume as the eluent to give a white powder. Yield: 79%. 1H NMR (600 MHz, CDCl3, δ): 8.07 (s, 1H), 7.90 (d, J = 8.0 Hz, 1H), 7.83 (dd, J = 8.1, 1.2 Hz, 1H), 7.54-7.50 (m, 1H), 7.46 (dd, J = 7.8, 1.6 Hz, 1H), 7.42 (dt, J = 8.0, 1.6 Hz, 1H), 7.38-7.30 (m, 2H), 7.20 (d, J = 7.9 Hz, 1H). Synthesis of 1-(3-bromophenyl)-1H-benzimidazole. 1-(3-bromophenyl)-1H-benzimidazole was synthesized and purified according to the same procedure as for 1-(2-bromophenyl)-1Hbenzimidazole but from 3-bromofluorobenzene. 1-(3-bromophenyl)-1H-benzimidazole was afforded as a white powder. Yield: 60%. 1H NMR (600 MHz, CDCl3, δ): 8.17 (s, 1H), 7.90 (dd, J = 6.4, 2.7 Hz, 1H), 7.71 (t, J = 1.6 Hz, 1H), 7.63 (dd, J = 6.2, 4.5 Hz, 1H), 7.57-7.53 (m, 1H), 7.51-7.45 (m, 2H), 7.40-7.36 (m, 2H).


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Synthesis of 1-(4-bromophenyl)-1H-benzimidazole. 1-(3-bromophenyl)-1H-benzimidazole was synthesized and purified according to the same procedure as for 1-(2-bromophenyl)-1Hbenzimidazole but from 4-bromofluorobenzene. 1-(4-bromophenyl)-1H-benzimidazole was afforded as a white powder. Yield: 69%. 1H NMR (600 MHz, CDCl3, δ): 8.14 (s, 1H), 7.90 (dd, J = 5.9, 3.1 Hz, 1H), 7.73-7.70 (m, 2H), 7.51 (dd, J = 5.8, 3.2 Hz, 1H), 7.43-7.40 (m, 2H), 7.387.35 (m, 2H). Synthesis of 9,9'-(5-bromo-1,3-phenylene)bis(9H-carbazole). A mixture of 9H-carbazole (3.34 g, 20 mmol), t-BuOK (2.36 g, 20 mmol), and dimethyl sulfoxide (DMSO, 10 mL) was stirred at 120 °C before injecting 1-bromo-3,5-difluorobenzene (1.93 g, 10 mmol). The reaction mixture was stirred at 140 °C for 2 h. After cooling, the mixture was extracted with chloroform and the organic extracts were combined, washed with water, and the organic layer was dried over anhydrous MgSO4. Upon evaporating off the solvent, the crude product was purified by purified by column chromatography on silica gel with hexane/chloroform at 8:1 by volume as the eluent to give a white powder. Yield: 75%. 1H NMR (600 MHz, CDCl3, δ): 8.15 (d, J = 7.7 Hz, 4H), 7.86 (d, J = 1.8 Hz, 2H), 7.79 (t, J = 1.8 Hz, 1H), 7.54 (d, J = 8.2 Hz, 4H), 7.46 (t, J = 7.7 Hz, 4H), 7.33 (dd, J = 11.4, 4.2 Hz, 4H). Synthesis of 9,9'-(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-phenylene)bis(9Hcarbazole). A mixture of 9,9'-(5-bromo-1,3-phenylene)bis(9H-carbazole) (2.00 g, 4 mmol), bis(pinacolato)diboron (1.30 g, 5 mmol), [1,10-bis(diphenylphophino)ferrocene]dichloropalladium(II) (0.10 g, 0.12 mmol), potassium acetate (1.30g, 12 mmol), and anhydrous 1,4dioxane (30 mL) was refluxed under nitrogen for 24 h. After cooling, the mixture was evaporated under reduced pressure, treated with saturated sodium chloride solution, extracted with chloroform, and the organic layer was dried over anhydrous MgSO4. After removal of the


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solvent, the residue was purified by column chromatography on silica gel using ethyl acetate/petroleum at 1:10 by volume as the eluent to give a white powder. Yield: 71%. 1H NMR (600 MHz, CDCl3, δ): 8.15 (d, J = 7.7 Hz, 4H), 8.11 (d, J = 1.9 Hz, 2H), 7.87 (s, 1H), 7.51 (d, J = 8.2 Hz, 4H), 7.44 (t, J = 7.7 Hz, 4H), 7.30 (t, J = 7.4 Hz, 4H), 1.36 (s, 12H). Synthesis of 9,9'-(2'-(1H-benzimidazol-1-yl)-[1,1'-biphenyl]-3,5-diyl)bis(9H-carbazole) (omCPBI). A mixture of 1-(2-bromophenyl)-1H-benzimidazole (0.30 g, 1.1 mmol), 9,9'-(5(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-phenylene)bis(9H-carbazole)

(0.65

g,

1.2

mmol), tetrakis(triphenylphosphine)palladium(0) (0.03 g, 0.03 mmol), and 1 M sodium carbonate (2.7 mL) in 40 mL of 1,4-dioxane was stirred at 120 °C for 48 h under nitrogen atmosphere, cooled down to room temperature,

treated with water, and extracted with

chloroform. The organic extracts were combined and dried over anhydrous MgSO4. After evaporating off the solvent, the crude product was purified by gradient column chromatography on silica gel using ethyl acetate/petroleum at 2:1 to 1:1 by volume as the eluent to give a white powder. Yield: 60%. 1H NMR (600 MHz, CDCl3, δ): 8.14 (s, 1H), 8.08 (d, J = 7.7 Hz, 4H), 8.01 (d, J = 8.1 Hz, 1H), 7.76 (dd, J = 7.7, 1.4 Hz, 1H), 7.68-7.60 (m, 4H), 7.55 (d, J = 7.6 Hz, 1H), 7.47 (d, J = 1.9 Hz, 2H), 7.41-7.36 (m, 5H), 7.30-7.27 (m, 5H), 7.10 (d, J = 8.2 Hz, 4H).

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C

NMR (75 MHz, CDCl3, δ): 143.38, 143.02, 141.77, 140.48, 139.73, 137.03, 134.15, 133.59, 131.84, 129.86, 129.50, 127.84, 126.27, 125.89, 125.03, 123.99, 123.56, 123.12, 120.72, 120.44, 120.38, 110.76, 109.43. MS (ESI) m/z: 601 [M + H]+. Anal. calcd for C43H28N4 (%): C 85.98, H 4.70, N 9.33; found: C 86.36, H 4.76, N 9.35. Synthesis of 9,9'-(3'-(1H-benzimidazol-1-yl)-[1,1'-biphenyl]-3,5-diyl)bis(9H-carbazole) (m-mCPBI). m-mCPBI was synthesized and purified according to the same procedure as for omCPBI but from 1-(3-bromophenyl)-1H-benzimidazole. m-mCPBI was afforded as a white


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powder. Yield: 62%. 1H NMR (600 MHz, CDCl3, δ): 8.35 (s, 1H), 8.17 (d, J = 7.7 Hz, 4H), 7.977.83 (m, 6H), 7.74 (t, J = 7.8 Hz, 1H), 7.60 (d, J = 8.2 Hz, 6H), 7.46 (t, J = 7.6 Hz, 4H), 7.36 (dt, J = 31.1, 7.4 Hz, 6H).

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C NMR (75 MHz, CDCl3, δ): 143.33, 142.12, 141.62, 140.57, 140.24,

137.30, 133.63, 130.97, 126.92, 126.34, 124.73, 124.44, 124.06, 123.90, 123.78, 123.11, 122.79, 120.74, 120.61, 110.42, 109.65. MS (ESI) m/z: 601 [M + H]+. Anal. calcd for C43H28N4 (%): C 85.98, H 4.70, N 9.33; found: C 85.65, H 4.47, N 9.19. Synthesis of 9,9'-(4'-(1H-benzimidazol-1-yl)-[1,1'-biphenyl]-3,5-diyl)bis(9H-carbazole) (pmCPBI). p-mCPBI was synthesized and purified according to the same procedure as for omCPBI but from 1-(4-bromophenyl)-1H-benzimidazole. p-mCPBI was afforded as a white powder. Yield: 63%. 1H NMR (600 MHz, CDCl3, δ): 8.47 (s, 1H), 8.19 (d, J = 7.8 Hz, 4H), 7.97 (dd, J = 23.4, 5.1 Hz, 5H), 7.90 (t, J = 1.8 Hz, 1H), 7.70 (d, J = 8.4 Hz, 2H), 7.63 (d, J = 8.2 Hz, 5H), 7.48 (t, J = 7.7 Hz, 4H), 7.44 (t, J = 6.2 Hz, 2H), 7.35 (t, J = 7.4 Hz, 4H).

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C NMR (75

MHz, CDCl3, δ): 144.17, 143.30, 142.10, 140.62, 140.17, 138.94, 136.55, 128.87, 126.35, 124.53, 124.43, 124.29, 123.98, 123.79, 123.09, 120.81, 120.63, 120.62, 110.49, 109.70. MS (ESI) m/z: 601 [M + H]+. Anal. calcd for C43H28N4 (%): C 85.98, H 4.70, N 9.33; found: C 85.66, H 4.78, N 9.18. Device Fabrication and Measurements. Devices were grown on ITO-coated glass substrates by thermal evaporation of the materials without breaking vacuum. Current–brightness–voltage characteristics were measured using Keithley source measurement units (Keithley 2400 and Keithley 2000) with a calibrated silicon photodiode. The EL spectra were measured using a SpectraScan PR650 spectrophotometer. All the measurements were carried out in an ambient atmosphere.


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RESULTS AND DISCUSSION

Scheme 1. Synthesis of o-mCPBI, m-mCPBI, and p-mCPBI The synthetic routes and chemical structures of the host materials are depicted in Scheme 1. The electron-withdrawing benzimidazole moiety was connected to the widely used hole transport host mCP at different positions of N-phenyl by coupling reaction. The intermediates 1-(2bromophenyl)-1H-benzimidazole,


and

1-(4-

bromophenyl)-1H-benzimidazole were prepared according to a catalyst-free N-arylation method.40 The chemical structures of all three host materials were confirmed by 1H NMR and 13

C NMR spectra, mass spectra, and elemental analysis (see Experimental Section). The rigid molecular configuration of the three hosts renders them excellent thermal and

morphological stabilities, as determined by TGA and DSC. The decomposition temperatures (Tds, corresponding to 5% weight loss upon heating) were measured to be 400, 394, and 429 °C for o-mCPBI, m-mCPBI, and p-mCPBI, respectively, showing higher heat tolerance than mCP


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with Td at 312 °C (Figure S1, Supporting Information).14 In addition, according to the second heating scan using DSC, distinct Tgs were observed at 130, 124, and 141 °C for o-mCPBI, mmCPBI, and p-mCPBI, respectively, all of which are substantially higher than the reported values between 50 and 60 °C for mCP (Figure S2, Supporting Information).14,41 The improvement of their morphological stability is the outcome of the introduction of benzimidazole moiety. These morphological and thermal stabilities are desirable for the host material as they can suppress aggregation and phase separation and ensure amorphous characteristics in the solid film state.42,43

o-mCPBI UV-Vis PL Phosphorescence

Normalized Intensity, a.u.

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m-mCPBI

p-mCPBI

250

300

350

400

450

500

550

600

Wavelength, nm Figure 1. UV–Vis absorption, fluorescence, and phosphorescence spectra of o-mCPBI, mmCPBI, and p-mCPBI in toluene.


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Figure 2. Calculated spatial distributions of the HOMO and LUMO levels. The values of optimized dihedral angles at the biphenyl linkages are indicated. The different linking topologies between mCP and benzimidazole units may significantly affect the photophysical properties. UV–Vis absorption and PL spectra of the three hosts in dilute toluene solution, as well as their phosphorescence spectra in toluene at 77 K, are presented in Figure 1. The three compounds have analogous absorption bands with peaks at 324 and 338 nm, which could be attributed to the n–π* transitions of extended conjugation of the Cz moiety.38,44 The strong absorption at shorter wavelengths around 286–292 nm can be assigned to the π–π* transitions and the suppression of π-conjugation from p-mCPBI to o-mCPBI results in a blueshift of the peak. This suppression also allows o-mCPBI to present a shoulder peak at 349 nm in its emission spectrum, which originates from the fluorescence of mCP.41,44 The


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structureless emission spectrum of p-mCPBI suggests that its first excited state possesses some charge-transfer characteristic due to the strong π-conjugation between D and A moieties. The phosphorescence spectra of the three hosts, as shown in Figure 1, differ widely in the shape and peak positions. By taking the highest energy peak of phosphorescence as the transition energy of T1→S0, the ET values are estimated to be 3.00, 2.80, and 2.71 eV for o-mCPBI, m-mCPBI, and pmCPBI, respectively. The ET of o-mCPBI is very close to 3.02 eV of the unsubstituted Cz,45 which results from the enhanced non-planarity at the biphenyl linkage in the middle and the largely limited π-conjugation through the twisted biphenyl core, as illustrated in Figure 2. The dihedral angles increase from 39° for p-mCPBI to 43° for m-mCPBI and 57° for o-mCPBI according to density functional theory (DFT) quantum-chemical calculations, which leads to a decrease in coupling between mCP and benzimidazole. Figure 2 also presents the distributions of the frontier molecular orbitals, i.e., the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), calculated using the Gaussian 09 software package. In particular, molecular geometries were optimized with B3LYP/6-31G(d). For all the three molecules, the HOMOs are situated on the Cz or dicarbazolylphenyl unit, while the LUMOs are mainly localized on the phenylbenzimidazole segment. It is noted that with a mosttwisted conformation due to the ortho-linkage, o-mCPBI has a most-separated HOMO and LUMO level distribution localized on the D and A moieties, respectively. The HOMO levels of the hosts determined by the oxidation scans of CV are placed at 5.44, 5.40, and 5.44 eV for omCPBI, m-mCPBI, and p-mCPBI, respectively, which are close to those of common hole transport materials (Figure S3, Supporting Information). The LUMO levels of o-mCPBI, mmCPBI, and p-mCPBI were estimated at 1.86, 1.84, and 1.89 eV, respectively, by deducting the optical band gap from HOMO energy.


14

Page 15 of 32

Hole Electron mCP o-mCPBI m-mCPBI p-mCPBI

2

300 Current Density, mA/cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


250 200 150 100 50 0

0

5

10

15 20 Voltage, V

25

Figure 3. Current density versus voltage characteristics of the hole-only and electron-only devices. The introduction of benzimidazole moiety could facilitate efficient charge carrier transport and charge injection from adjacent charge transport layers. To confirm this, the hole and electron transport properties of the hosts were investigated by using the hole- and electron-only devices with the structure of: ITO/MoO3 (5 nm)/TAPC (30 nm)/Host (30 nm)/TAPC (30 nm)/Al (100 nm) and ITO/TmPyPB (30 nm)/Host (30 nm)/TmPyPB (30 nm)/LiF (1 nm)/Al (100 nm). For comparison, the carrier-only devices for mCP were also fabricated and the current density versus voltage curves of the devices are compiled in Figure 3. Obviously, mCP has the highest hole current density and the lowest electron current density. The three benzimidazole-based hosts exhibit lower hole current density but drastically enhanced electron current density, showing more bipolar transport property than mCP. This should broaden the recombination zone of the EML to alleviate efficiency roll-off.


15


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Page 16 of 32

Figure 4. Schematic energy level diagram and molecular structures of the used materials, including 1,1-bis((di-4-tolylamino)phenyl)cyclohexane (TAPC), mCP, o-mCPBI, m-mCPBI, pmCPBI, 1,3,5-tri(m-pyrid-3-ylphenyl)benzene (TmPyPB), bis(4,6-(difluorophenyl)pyridinatoN,C2′)iridium(picolinate)

(FIrpic),

bis(2-phenylpyridinato-N,C2′)iridium(acetylacetonate)

((ppy)2Ir(acac)), bis(4-phenylthieno[3,2-c]pyridinato-N,C2′) iridium(acetylacetonate) (PO-01), bis(2,4-diphenylquinolyl-N,C2′)iridium(acetylacetonate) ((PPQ)2Ir(acac)), 1,2-bis(carbazol-9-yl)4,5-dicyanobenzene (2CzPN), and 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN).


16

Page 17 of 32

The high thermal and morphological stabilities, suitable energy levels, bipolar charge transport capabilities, and excellent photophysical properties, especially the extremely high ET of omCPBI, allowed us to be optimistic about the potential of the benzimidazole-based materials as the hosts. Next, a comprehensive study on both phosphorescent and TADF OLEDs hosted by the three materials was performed by employing a uniform simple device structure of ITO/MoO3 (5 nm)/TAPC (65 nm)/Host: Guest (15 nm)/TmPyPB (35 nm)/LiF (1 nm)/Al (100 nm) for various colors, including blue, green, yellow, red, and white emission. The energy level diagrams and

10 1

10 B0, mCP B1, o-mCPBI B2, m-mCPBI B3, p-mCPBI

0.1 0.01

0.01

0.1

1

EQE, %

Power Efficiency, lm/W

molecular structures of the materials used in these devices are depicted in Figure 4.

(a) 10

100

1

2

Current Density, mA/cm 100 10

10

1 W1, o-mCPBI W2, m-mCPBI W3, o-mCPBI

0.1

EQE, %


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b) 0.01 1E-3

0.01

0.1

1

10

100

1

2

Current Density, mA/cm

Figure 5. Power efficiency and EQE versus current density characteristics of a) FIrpic-based blue and b) white phosphorescent OLEDs.


17


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Page 18 of 32

Table 1. EL data of the phosphorescent and TADF OLEDs.

Device

a

Host

Vonb

EQEmaxc

EQE1000d

EQE5000e

CEmaxf

PEmaxg

[V]

[%]

[%]

[%]

[cd A−1]

[lm W−1]

Guest

CIEh

B0

mCP

FIrpic

3.0

16.7

11.8

9.5

40.0

38.0

(0.16, 0.40)

B1

o-mCPBI

FIrpic

3.1

20.2

17.0

14.8

48.7

47.8

(0.15, 0.40)

B2

m-mCPBI

FIrpic

3.1

15.0

11.7

8.5

35.1

34.4

(0.15, 0.38)

B3

p-mCPBI

FIrpic

3.0

7.6

5.8

5.3

17.5

18.3

(0.15, 0.37)

B4

o-mCPBI

2CzPN

3.6

10.2

6.5

5.6

21.4

17.7

(0.16, 0.30)

G1

o-mCPBI

(ppy)2Ir(acac)

3.0

22.7

22.6

19.6

85.6

70.5

(0.31, 0.64)

G2

m-mCPBI

(ppy)2Ir(acac)

2.8

18.0

18.0

15.3

66.5

60.1

(0.31, 0.64)

G3

p-mCPBI

(ppy)2Ir(acac)

2.8

6.2

6.2

4.9

21.5

21.2

(0.31, 0.62)

G4

o-mCPBI

4CzIPN

3.4

18.7

18.5

17.0

60.4

42.0

(0.27, 0.58)

Y1

o-mCPBI

PO-01

3.0

24.7

22.4

19.5

79.2

78.8

(0.49, 0.51)

Y2

m-mCPBI

PO-01

2.8

23.6

21.2

18.9

74.9

78.4

(0.49, 0.51)

R1

o-mCPBI

(PPQ)2Ir(acac)

3.6

19.8

10.6

8.9

24.5

21.4

(0.64, 0.36)

R2

m-mCPBI

(PPQ)2Ir(acac)

3.4

18.7

8.6

7.2

23.6

21.8

(0.64, 0.36)

R3

p-mCPBI

(PPQ)2Ir(acac)

3.0

12.8

6.1

4.6

15.1

15.8

(0.65, 0.35)

W1

o-mCPBI

FIrpic, PO-01

3.0

23.6

17.8

15.6

70.7

71.3

(0.34, 0.49)

W2

m-mCPBI

FIrpic, PO-01

3.0

23.2

16.1

13.8

65.5

68.6

(0.36, 0.46)

W3

o-mCPBI

FIrpic, (ppy)2Ir(acac), (PPQ)2Ir(acac)

3.0

20.9

18.1

14.6

40.2

39.0

(0.44, 0.44)

a

The uniform device structure: ITO/MoO3 (5 nm)/TAPC (65 nm)/Host: Guest (15 nm)/TmPyPB (35 nm)/LiF (1 nm)/Al (100 nm). Doping concentrations were fixed at 8 wt% for monochromatic OLEDs, 8 wt% FIrpic: 1 wt% PO-01 for device W1, and 8 wt% FIrpic: 0.75 wt% PO-01 for W2. The EML of device W3 was o-mCPBI: 8 wt% (PPQ)2Ir(acac) (7.5 nm)/omCPBI: 8 wt% (ppy)2Ir(acac) (3.5 nm)/o-mCPBI: 8 wt% FIrpic (4 nm); bThe applied voltage required for 1 cd/m2; cMaximum external quantum efficiency (EQE); dEQE at 1000 cd/m2; eEQE at 5000 cd/m2; fMaximum current efficiency (CE); gMaximum power efficiency (PE); hEL recorded at 8 V.


18

Page 19 of 32

2CzPN FIrpic 4CzIPN (ppy)2Ir(acac) PO-01 (PPQ)2Ir(acac)

Normalized Intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5 V (0.34, 0.49) 6 V (0.34, 0.49) 7 V (0.34, 0.49) 8 V (0.34, 0.49) 9 V (0.34, 0.49) 10 V (0.33, 0.48) 11 V (0.33, 0.48) 12 V (0.33, 0.48)

CRI: 56

5 V (0.45, 0.45) 6 V (0.45, 0.44) 7 V (0.45, 0.44) 8 V (0.44, 0.44) 9 V (0.44, 0.44) 10 V (0.44, 0.44) 11 V (0.44, 0.44) 12 V (0.43, 0.44)

CRI: 82

400

500

600

700

Wavelength, nm Figure 6. EL spectra of the monochromatic OLEDs (top) and the white OLEDs based on two complementary (middle) and three primary colors (bottom) hosted by o-mCPBI. We first fabricated the blue devices B0–B3 by utilizing FIrpic as the dopant. The device B0 hosted by mCP was prepared for comparison. The key device data are summarized in Table 1. As shown in Table 1 and Figure 5a, the maximum EQEs for devices B1–B3 were 20.2%, 15.0%, and 7.6%, respectively. Device B1 hosted by o-mCPBI achieved near unity IQE with a peak current efficiency (CE) of 48.7 cd/A and peak power efficiency (PE) of 47.8 lm/W. Despite the highest current at a certain driving voltage (Figure S4, Supporting Information), device B0 using mCP as the host, by contrast, exhibited a lower maximum EQE of 16.7%, which is close to the


19


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Page 20 of 32

reported value achieved by using a similar device structure.46 Moreover, the high quantum efficiency of the o-mCPBI device was maintained even at high luminance, and the EQE at 1000 cd/m2 was as high as 17.0%. The high efficiencies and low efficiency roll-off of the device based on o-mCPBI can be attributed to high ET, balanced charge density, and effective exciton confinement in the EML. The relatively low ET of p-mCPBI is responsible for the lowest EQE of device B3. As the three hosts possess similar HOMO/LUMO levels (see Figure 4), the lower efficiency of device B2 than that of device B1 can also be attributed to the lower ET value of mmCPBI. Small perturbation could help excitons to overcome the triplet energy of m-mCPBI because of the relatively poor exciton confinement in device B2. Next, green, yellow, and red phosphorescent devices using the same device structure but different dopants, such as (ppy)2Ir(acac), PO-01, and (PPQ)2Ir(acac), were fabricated to evaluate the applicability of the hosts for low energy triplet emitters. As Table 1 shows, the EQEs of devices based on o-mCPBI and m-mCPBI achieved near or over 20% for all three emitters with their intrinsic emission spectra, as shown in Figure 6. More importantly, all the devices also showed small efficiency roll-offs. For instance, the EQE of device G1 remained almost unchanged at 22.6% when the luminance reached 1000 cd/m2, compared to the peak EQE of 22.7%. The poor performance of devices hosted by p-mCPBI, especially device G3, is likely due to a relatively large leakage current as indicated by the highest current density but lowest luminance compared with that of devices using the other two hosts (Figure S5, Supporting Information). With an ET high enough to host green to red phosphorescent emitters, the efficiency of p-mCPBI based devices could be improved by optimizing the device structure.


20

Page 21 of 32

2CzPN 4CzIPN

4

2 2

10

100

1

10

Brightness, cd/m

3

10

10

1

10

2CzPN 4CzIPN

0.1

(a) 0

(b) 0

2

4

6

8

10

12

EQE, %

10

200


2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

0.01

0.01

0.1

1

10

2

100

1


Voltage, V

Figure 7. (a) Current density-voltage-luminance and (b) power efficiency and EQE versus current density characteristics for devices B4 and G4. Then, two TADF emitters, the blue emitter 2CzPN and the green emitter 4CzIPN, were employed to examine the universality of the best performing host o-mCPBI without changing the device configuration (devices B4 and G4 in Table 1). Device G4 afforded a peak EQE of 18.7% with hardly any efficiency roll-off until the luminance of 1000 cd/m2, as shown in Figure 7. The maximum EQE of device B4 was 10.2%, which is higher than the literature value of 8%.6 These results suggest that the new benzimidazole-based material is also a promising host for TADF emitters, regardless of the color. Finally, three single-host white phosphorescent OLEDs, W1–W3, were investigated to excavate the potential of the host materials. Devices W1 and W2 were based on two complementary colors by co-doping FIrpic and PO-01 into o-mCPBI and m-mCPBI, respectively, to form the EML. As shown in Figure 5b and Table 1, the maximum EQEs and PEs of device W1 reached 23.6% and 71.3 lm/W, respectively, while those of device W2 were also as high as 23.2% and 68.6 lm/W. Moreover, the EL spectra of devices W1 and W2 depicted in Figure 6 and S6b indicated that both devices exhibited very high chromatic stability over a wide


21


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Page 22 of 32

range of voltages. When the applied voltage changed from 5 to 12 V (corresponding luminance from 205 to 33361 cd/m2, Figure S6a in the Supporting Information), the CIE coordinates of device W1 varied slightly from (0.34, 0.49) to (0.33, 0.48). The performance of the threeprimary-color white device W3 based on o-mCPBI was also impressive. Device W3 obtained a peak EQE of 20.9% and a slight decline to 18.1% at 1000 cd/m2. Besides a small color variation, the spectrum of device W3 showed a rather high color-rendering index (CRI) keeping at 82 over the entire operational voltages (see Figure 6). The excellent performance of device W1–W3 can be attributed to the use of the new bipolar benzimidazole-based hosts, which possess high ET values and capabilities to generate balanced charge fluxes and a broad and stable distribution of recombination region within the EML. CONCLUSIONS In summary, three novel benzimidazole-based hosts, o-mCPBI, m-mCPBI, and p-mCPBI have been developed by incorporating benzimidazole unit into mCP via ortho-, meta-, and paraposition of N-phenyl. Different linking modes resulted in pronounced differences in various properties of the three hosts. Compared with the widely used host mCP, the introduction of benzimidazole moiety in the new hosts produced higher Tgs and more balanced charge fluxes without lowering their triplet energy levels when using ortho- or meta-linkage. Among them, mmCPBI can achieve highly efficient phosphorescent monochromatic and white OLEDs, while omCPBI has been demonstrated to be an universal host for high-performance both phosphorescent and TADF OLEDs, owing to their excellent thermal and morphological stabilities, high ET values, proper energy levels, and bipolar transport capabilities. The blue, green, yellow, red, and white phosphorescent OLEDs with EQEs of 20% or higher have been realized by using omCPBI as the host. The single-host white OLEDs exhibited impressive color stability and


22

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


superior color rendition. The TADF OLEDs hosted by o-mCPBI achieved comparable or higher efficiencies in comparison with the reported values. Note that all these devices employed the same device structure except the EML and showed low efficiency roll-offs at high luminance. These findings demonstrate the universality of the new benzimidazole-based hosts, especially omCPBI. Our study may promote the development of the universal host materials for both phosphorescent and TADF emitters. ASSOCIATED CONTENT Supporting Information Synthetic

scheme

of


1-(3-bromophenyl)-1H-

benzimidazole, and 1-(4-bromophenyl)-1H-benzimidazole, TGA thermograms, DSC traces, and CV scans of o-mCPBI, m-mCPBI, and p-mCPBI, current density-voltage-luminance characteristics for devices B0–B3, G1–G3, W1, and W2, and EL spectra of device W2 at various voltages. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors * (Q.W) E-mail: [email protected]. * (D.M) E-mail: [email protected]. ACKNOWLEDGMENTS The authors acknowledge the financial support by the National Natural Science Foundation of China (51303099), the Natural Science Basic Research Plan in Shaanxi Province of China


23


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Page 24 of 32

(2014JQ6224), the Fundamental Research Funds for the Central Universities (GK201302035, GK201301002, and GK201501002), and the Natural Science Foundation of Shandong Province (ZR2014BP017). REFERENCES (1) Tang, C. W.; VanSlyke, S. A. Organic Electroluminescent Diodes. Appl. Phys. Lett. 1987, 51, 913–915. (2) Ma, Y.; Zhang, H.; Shen, J.; Che, C. Electroluminescence from Triplet Metal-Ligand Charge-Transfer Excited State of Transition Metal Complexes. Synth. Met. 1998, 94, 245–248. (3) Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices. Nature 1998, 395, 151–154. (4) Kondakov, D. Y.; Pawlik, T. D.; Hatwar, T. K.; Spindler, J. P. Triplet Annihilation Exceeding Spin Statistical Limit in Highly Efficient Fluorescent Organic Light-Emitting Diodes. J. Appl. Phys. 2009, 106, 124510. (5) Yao, L.; Zhang, S.; Wang, R.; Li, W.; Shen, F.; Yang, B.; Ma, Y. Highly Efficient nearInfrared Organic Light-Emitting Diode Based on a Butterfly-Shaped Donor-Acceptor Chromophore with Strong Solid-State Fluorescence and a Large Proportion of Radiative Excitons. Angew Chem. Int. Ed. 2014, 53, 2119–2123. (6) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient Organic Light-Emitting Diodes from Delayed Fluorescence. Nature 2012, 492, 234–238.


24

Page 25 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(7) Adachi, C. Third-Generation Organic Electroluminescence Materials. Jpn. J. Appl. Phys. 2014, 53, 060101. (8) Tao, Y.; Yuan, K.; Chen, T.; Xu, P.; Li, H.; Chen, R.; Zheng, C.; Zhang, L.; Huang, W. Thermally Activated Delayed Fluorescence Materials Towards the Breakthrough of Organoelectronics. Adv. Mater. 2014, 26, 7931–7958. (9) Zhang, Q.; Kuwabara, H.; Potscavage, W. J., Jr.; Huang, S.; Hatae, Y.; Shibata, T.; Adachi, C. Anthraquinone-Based Intramolecular Charge-Transfer Compounds: Computational Molecular Design,

Thermally

Activated

Delayed

Fluorescence,

and

Highly

Efficient

Red

Electroluminescence. J. Am. Chem. Soc. 2014, 136, 18070–18081. (10) Lee, D. R.; Kim, B. S.; Lee, C. W.; Im, Y.; Yook, K. S.; Hwang, S. H.; Lee, J. Y. Above 30% External Quantum Efficiency in Green Delayed Fluorescent Organic Light-Emitting Diodes. ACS Appl. Mater. Interfaces 2015, 7, 9625–9629. (11) Tang, C.; Yang, T.; Cao, X.; Tao, Y.; Wang, F.; Zhong, C.; Qian, Y.; Zhang, X.; Huang, W. Tuning a Weak Emissive Blue Host to Highly Efficient Green Dopant by a CN in Tetracarbazolepyridines for Solution-Processed Thermally Activated Delayed Fluorescence Devices. Adv. Opt. Mater. 2015, 3, 786–790. (12) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Very HighEfficiency Green Organic Light-Emitting Devices Based on Electrophosphorescence. Appl. Phys. Lett. 1999, 75, 4–6.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 32

(13) Adamovich, V.; Brooks, J.; Tamayo, A.; Alexander, A. M.; Djurovich, P. I.; D'Andrade, B. W.; Adachi, C.; Forrest, S. R.; Thompson, M. E. High Efficiency Single Dopant White Electrophosphorescent Light Emitting Diodes. New J. Chem. 2002, 26, 1171–1178. (14) Lee, T. Y. H.; Wang, Q.; Wallace, J. U.; Chen, S. H. Temporal Stability of Blue Phosphorescent Organic Light-Emitting Diodes Affected by Thermal Annealing of Emitting Layers. J. Mater. Chem. 2012, 22, 23175–23180. (15) Baldo, M. A.; Adachi, C.; Forrest, S. R. Transient Analysis of Organic Electrophosphorescence. II. Transient Analysis of Triplet-Triplet Annihilation. Phys. Rev. B 2000, 62, 10967–10977. (16) Reineke, S.; Walzer, K.; Leo, K. Triplet-Exciton Quenching in Organic Phosphorescent Light-Emitting Diodes with Ir-Based Emitters. Phys. Rev. B 2007, 75. 125328. (17) Song, D.; Zhao, S.; Luo, Y.; Aziz, H. Causes of Efficiency Roll-Off in Phosphorescent Organic Light Emitting Devices: Triplet-Triplet Annihilation versus Triplet-Polaron Quenching. Appl. Phys. Lett. 2010, 97, 243304. (18) Mi, B.; Gao, Z.; Liao, Z.; Huang, W.; Chen, C. H. Molecular Hosts for Triplet Emitters in Organic Light-Emitting Diodes and the Corresponding Working Principle. Sci. China: Chem. 2010, 53, 1679–1694. (19) Tao, Y.; Yang, C.; Qin, J. Organic Host Materials for Phosphorescent Organic LightEmitting Diodes. Chem. Soc. Rev. 2011, 40, 2943–2970.


26

Page 27 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(20) Xiao, L.; Chen, Z.; Qu, B.; Luo, J.; Kong, S.; Gong, Q.; Kido, J. Recent Progresses on Materials for Electrophosphorescent Organic Light-Emitting Devices. Adv. Mater. 2011, 23, 926–952. (21) Duan, L.; Qiao, J.; Sun, Y.; Qiu, Y. Strategies to Design Bipolar Small Molecules for OLEDs: Donor-Acceptor Structure and Non-Donor-Acceptor Structure. Adv. Mater. 2011, 23, 1137–1144. (22) Chaskar, A.; Chen, H. F.; Wong, K. T. Bipolar Host Materials: A Chemical Approach for Highly Efficient Electrophosphorescent Devices. Adv. Mater. 2011, 23, 3876–3895. (23) Gao, Z.; Lee, C. S.; Bello, I.; Lee, S. T.; Chen, R.-M.; Luh, T.-Y.; Shi, J.; Tang, C. W. Bright-Blue Electroluminescence from a Silyl-Substituted Ter-Phenylene-Vinylene

Derivative.

Appl. Phys. Lett. 1999, 74, 865–867. (24) Wong, T. C.; Kovac, J.; Lee, C. S.; Hung, L. S.; Lee, S. T. Transient Electroluminescence Measurements on Electron-Mobility of N-arylbenzimidazoles. Chem. Phys. Lett. 2001, 334, 61– 64. (25) Hung, W.-Y.; Ke, T.-H.; Lin, Y.-T.; Wu, C.-C.; Hung, T.-H.; Chao, T.-C.; Wong, K.-T.; Wu, C.-I. Employing Ambipolar Oligofluorene as the Charge-Generation Layer in Time-ofFlight Mobility Measurements of Organic Thin Films. Appl. Phys. Lett. 2006, 88, 064102. (26) Shirota, Y.; Kageyama, H. Charge Carrier Transporting Molecular Materials and Their Applications in Devices. Chem. Rev. 2007, 107, 953–1010.


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(27) Wang, F.; Hu, J.; Cao, X.; Yang, T.; Tao, Y.; Mei, L.; Zhang, X.; Huang, W. A Low-Cost Phenylbenzoimidazole

Containing

Electron

Transport

Material

for

Efficient

Green

Phosphorescent and Thermally Activated Delayed Fluorescent OLEDs. J. Mater. Chem. C 2015, 3, 5533–5540. (28) Lai, M. Y.; Chen, C. H.; Huang, W. S.; Lin, J. T.; Ke, T. H.; Chen, L. Y.; Tsai, M. H.; Wu, C. C. Benzimidazole/Amine-Based Compounds Capable of Ambipolar Transport for Application in Single-Layer Blue-Emitting OLEDs and as Hosts for Phosphorescent Emitters. Angew. Chem. Int. Ed. 2008, 47, 581–585. (29) Ge, Z.; Hayakawa, T.; Ando, S.; Ueda, M.; Akiike, T.; Miyamoto, H.; Kajita, T.; Kakimoto, M.-a. Solution-Processible Bipolar Triphenylamine-Benzimidazole Derivatives for Highly Efficient Single-Layer Organic Light-Emitting Diodes. Chem. Mater. 2008, 20, 2532– 2537. (30) Chen, C.-H.; Huang, W.-S.; Lai, M.-Y.; Tsao, W.-C.; Lin, J. T.; Wu, Y.-H.; Ke, T.-H.; Chen, L.-Y.; Wu, C.-C. Versatile, Benzimidazole/Amine-Based Ambipolar Compounds for Electroluminescent Applications: Single-Layer, Blue, Fluorescent OLEDs, Hosts for SingleLayer, Phosphorescent OLEDs. Adv. Funct. Mater. 2009, 19, 2661–2670. (31) Takizawa, S.-y.; Montes, V. A.; Anzenbacher, Jr., P. Phenylbenzimidazole-Based New Bipolar Host Materials for Efficient Phosphorescent Organic Light-Emitting Diodes. Chem. Mater. 2009, 21, 2452–2458. (32) Gong, S.; Zhao, Y.; Yang, C.; Zhong, C.; Qin, J.; Ma, D. Tuning the Photophysical Properties and Energy Levels by Linking Spacer and Topology between the Benzimidazole and


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Page 29 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Carbazole Units: Bipolar Host for Highly Efficient Phosphorescent OLEDs. J. Phys. Chem. C 2010, 114, 5193–5198. (33) Hung, W.-Y.; Chi, L.-C.; Chen, W.-J.; Chen, Y.-M.; Chou, S.-H.; Wong, K.-T. A New Benzimidazole/Carbazole

Hybrid

Bipolar

Material

for

Highly

Efficient

Deep-Blue

Electrofluorescence, Yellow–Green Electrophosphorescence, and Two-Color-Based White OLEDs. J. Mater. Chem. 2010, 20, 10113–10119. (34) Huang, H.; Yang, X.; Pan, B.; Wang, L.; Chen, J.; Ma, D.; Yang, C. Benzimidazole– Carbazole-Based Bipolar Hosts for High Efficiency Blue and White Electrophosphorescence Applications. J. Mater. Chem. 2012, 22, 13223–13230. (35) Chen, Y.-M.; Hung, W.-Y.; You, H.-W.; Chaskar, A.; Ting, H.-C.; Chen, H.-F.; Wong, K.-T.; Liu, Y.-H. Carbazole–Benzimidazole Hybrid Bipolar Host Materials for Highly Efficient Green and Blue Phosphorescent OLEDs. J. Mater. Chem. 2011, 21, 14971–14978. (36) Mondal, E.; Hung, W. Y.; Chen, Y. H.; Cheng, M. H.; Wong, K. T. Molecular Topology Tuning of Bipolar Host Materials Composed of Fluorene-Bridged Benzimidazole and Carbazole for Highly Efficient Electrophosphorescence. Chem. Eur. J. 2013, 19, 10563–10572. (37) Pan, B.; Wang, B.; Wang, Y.; Xu, P.; Wang, L.; Chen, J.; Ma, D. A Simple Carbazole-NBenzimidazole Bipolar Host Material for Highly Efficient Blue and Single Layer White Phosphorescent Organic Light-Emitting Diodes. J. Mater. Chem. C 2014, 2, 2466–2469. (38) Cho, Y. J.; Yook, K. S.; Lee, J. Y. A Universal Host Material for High External Quantum Efficiency Close to 25% and Long Lifetime in Green Fluorescent and Phosphorescent OLEDs. Adv. Mater. 2014, 26, 4050–4055.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 32

(39) Cui, L. S.; Xie, Y. M.; Wang, Y. K.; Zhong, C.; Deng, Y. L.; Liu, X. Y.; Jiang, Z. Q.; Liao, L. S. Pure Hydrocarbon Hosts for Approximately 100% Exciton Harvesting in Both Phosphorescent and Fluorescent Light-Emitting Devices. Adv. Mater. 2015, 27, 4213–4217. (40) Diness, F.; Fairlie, D. P. Catalyst-Free N-Arylation Using Unactivated Fluorobenzenes. Angew. Chem. Int. Ed. 2012, 51, 8012–8016. (41) Tsai, M. H.; Hong, Y. H.; Chang, C. H.; Su, H. C.; Wu, C. C.; Matoliukstyte, A.; Simokaitiene, J.; Grigalevicius, S.; Grazulevicius, J. V.; Hsu, C. P. 3-(9-Carbazolyl)Carbazoles and 3,6-Di(9-Carbazolyl)Carbazoles as Effective Host Materials for Efficient Blue Organic Electrophosphorescence. Adv. Mater. 2007, 19, 862–866. (42) So, F.; Kondakov, D. Degradation Mechanisms in Small-Molecule and Polymer Organic Light-Emitting Diodes. Adv. Mater. 2010, 22, 3762–3777. (43) Scholz, S.; Kondakov, D.; Lüssem, B.; Leo, K. Degradation Mechanisms and Reactions in Organic Light-Emitting Devices. Chem. Rev. 2015, 115, 8449–8503. (44) Lin, M.-S.; Yang, S.-J.; Chang, H.-W.; Huang, Y.-H.; Tsai, Y.-T.; Wu, C.-C.; Chou, S.H.; Mondal, E.; Wong, K.-T. Incorporation of a CN Group into mCP: A New Bipolar Host Material for Highly Efficient Blue and White Electrophosphorescent Devices. J. Mater. Chem. 2012, 22, 16114–16120. (45) Tsai, M. H.; Lin, H. W.; Su, H. C.; Ke, T. H.; Wu, C. c.; Fang, F. C.; Liao, Y. L.; Wong, K. T.; Wu, C. I. Highly Efficient Organic Blue Electrophosphorescent Devices Based on 3,6Bis(Triphenylsilyl)Carbazole as the Host Material. Adv. Mater. 2006, 18, 1216–1220.


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(46) Xie, Y.-M.; Cui, L.-S.; Liu, Y.; Zu, F.-S.; Li, Q.; Jiang, Z.-Q.; Liao, L.-S. Efficient Blue/White Phosphorescent Organic Light-Emitting Diodes Based on a Silicon-Based Host Material via a Direct Carbon–Nitrogen Bond. J. Mater. Chem. C 2015, 3, 5347–5353.


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