Constructing New n-Type, Ambipolar, and p-Type Aggregation

Article pubs.acs.org/JPCC

Constructing New n‑Type, Ambipolar, and p‑Type AggregationInduced Blue Luminogens by Gradually Tuning the Proportion of Tetrahphenylethene and Diphenylphophine Oxide Guangyuan Mu,† Wenzhi Zhang,† Peng Xu,† Hongfeng Wang,† Yixing Wang,† Lei Wang,*,† Shaoqing Zhuang,†,‡ and Xunjin Zhu*,‡ †

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, P. R. China Centre for Advanced Luminescence Material, Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Hong Kong, P. R. China

‡

S Supporting Information *

ABSTRACT: By gradually adjusting the proportion of tetraphenylethene (TPE) and phosphine oxide (PO), the series of n-type, ambipolar, and p-type aggregationinduced emission (AIE) luminogens (E,Z)-((1,2-diphenylethene-1,2-diyl)bis(4,1phenylene))bis(diphenylphosphine oxide) (TPEDPO), diphenyl(4-(1,2,2triphenylvinyl)phenyl)phosphine oxide (TPEPO), phenylbis(4-(1,2,2triphenylvinyl)phenyl)phosphine oxide (DTPEPO), and tris(4-(1,2,2-triphenylvinyl)phenyl)phosphine oxide (TTPEPO) were developed for the first time. Their optical and electronic properties and AIE characteristics were investigated carefully. Timeresolved spectra indicate that the transient fluorescence decay lifetimes of aggregation-induced blue luminogens change regularly with the aggregation degree in THF/water solution. Particularly, identical electron and hole mobilities (∼2.3 × 10 −4 cm2 V−1 s−1) can be realized when the TPE/PO ratio is 1:1. In electroluminesent (EL) devices, the sky-blue-doped device based on TPEPO was found to exhibit a low turn-on voltage of 2.8 V, a maximum luminance (Lmax) of 8656 cd m−2, a maximum current efficiency (ηC,max) of 9.32 cd A−1, and a maximum power efficiency (ηP,max) of 10.23 lm W−1.

1. INTRODUCTION Aggregation-induced emission (AIE) has been intensively studied1−6 since it was first reported by Tang et al.7 As a result, numerous AIE-active luminescent molecules involving siloles,8 cyanostilbenes,9 and tetraphenylethene derivatives10,11 have been reported. Among them, AIE-active tetraphenylethene (TPE) derivatives have been widely explored as emitters because of their facile synthesis and thermal stability.12 Because TPE is a hole-dominated unit, its hole mobility (∼10−4 cm2 V−1 s−1) is 1 order of magnitude higher than its electron mobility (∼10−5 cm2 V−1 s−1),13 and consequently, previous studies of TPE derivatives have mainly focused on hole transport and ptype host materials. For example, Kim et al.14 synthesized ptype host materials PDA-TPE and TPA-TPE whose hole mobilities even exceed ∼10−2 cm2 V−1 s−1, and Liu et al.15 recently synthesized p-type host materials TPETPA and TPEDTPA, which they used to fabricate high-efficiency electroluminesent (EL) devices. On the other hand, the development of molecules with tunable charge mobilities for device fabrication has been an ongoing challenge in the field of molecular design.16−19 Nevertheless, to the best of our knowledge, this has not yet been realized in TPE-based materials. Recently, Tang and co-workers20,21 synthesized TPE-based bipolar emitters by conjugating electron donors and acceptors © 2014 American Chemical Society

through an aromatic ring. Moreover, our group has fabricated organic light-emitting diodes (OLEDs) utilizing bipolar TPEbased host materials with oxadizole as a functional group.13 The obtained devices exhibit high efficiencies with sky-blue doping. All of these recent developments indicate the possibility of systematically tuning electron/hole charge mobilities through purely chemical modifications of the AIE-active TPE molecule. To achieve this objective, we rationally selected the versatile phosphine oxide (PO) moiety as an electron-transport unit, which exhibits excellent electron-injection and -transport properties.22 In this contribution, we report the synthesis of four newly designed blue luminogens that can be tuned from ntype to ambipolar and p-type by adjusting the proportion of TPE and PO moieties. Furthermore, to investigate their performance in devices, OLEDs utilizing the four obtained AIE luminogens as emitters were fabricated, and their EL characteristics were investigated.

2. EXPERIMENTAL SECTION 2.1. General Information. All solvents and chemicals were purchased from commercial suppliers and used without further Received: February 19, 2014 Revised: March 28, 2014 Published: March 31, 2014 8610

dx.doi.org/10.1021/jp501752a | J. Phys. Chem. C 2014, 118, 8610−8616

The Journal of Physical Chemistry C

Article

TTPEPO. Yield: 40%. 1H NMR (400 MHz, CDCl3, δ): 7.62−7.60 (m, 4H), 7.54−7.50 (m, 16H), 7.53−7.48 (m, 2H), 7.39−7.34 (m, 4H),7.16−7.02 (m, 21H), 7.01−6.95 (m, 12H). 13 C NMR (CDCl3, 600 MHz, δ): 147.66, 147.64, 143.13, 142.88, 142.56, 139.93, 131.50, 131.44, 131.28, 131.22, 131.20, 131.12, 130.49, 130.37,129.67, 127.85, 127.76, 127.72, 126.83, 126.78, 126.76. MS (APCI) m/z: 1039.7 ([M+]; calcd for C78H57OP, 1040.41).

purification. The intermediates (E,Z)-1,2-bis(4-bromophenyl)1,2-diphenylethene (2BrTPE) and (2-(4-bromophenyl)ethene1,1,2-triyl)tribenzene (1BrTPE) were synthesized according to the reported procedures and confirmed by comparing their characterization data with literature data.23,24 The instruments and characterization methodologies are provided in the Supporting Information and can also be found in our previous publications.25,26 2.2. OLED Device Fabrication and Measurement. EL devices were fabricated by vacuum thermal evaporation technology according to a method modified from our previous approach.25,26 Before the deposition of the emitting layer, clear indium tin oxide (ITO) substrates with a sheet resistance of 25 Ω/square were treated with oxygen plasma for 5 min. The emitting materials were deposited on the ITO substrates under a vacuum of 5 × 10−6 Torr at a rate of 0.9−1.1 Å/s. Subsequently, a cathode composed of LiF (1 nm) and aluminum (150 nm) was deposited on the substrate under a vacuum of 10−6 Torr. The current density−voltage−brightness (J−V−B) characteristics of the devices were measured with a Keithley 2400 SourceMeter and Photo Research PR-655 spectroradiometer. 2.3. General Procedure for the Synthesis of Target Compounds. Under N2 atmosphere, n-butyllithium (2.4 M, 2.3 mL, 5.5 mmol) was slowly added to a solution of (2-(4bromophenyl)ethene-1,1,2-triyl)tribenzene (2.05 g, 5 mmol) in tetrahydrofuran (THF, 80 mL) at −78 °C. Then, chlorodiphenyl phosphine (0.9 mL, 5 mmol) was added, and the reaction mixture was kept stirring for 3 h at −78 °C. After the reaction had been quenched with 4 mL of methanol, H2O was added at room temperature, and the mixture was extracted with CH2Cl2 and dried over anhydrous Na2SO4. The residue after solvent evaporation was added to 30% hydrogen peroxide (6 mL) and CH2Cl2 (20 mL) and stirred overnight at room temperature. The separated organic layer was washed with water and brine and evaporated to dryness. Then, the crude product was purified by column chromatography on silica gel eluting with acetic ether/petroleum ether (v/v, 1:1) to give the target compounds. TPEDPO. Yield: 61% (2.32 g). 1H NMR (400 MHz, CDCl3, δ): 7.61−7.50 (m, 12H), 7.46−7.35 (m, 12H), 7.14−7.09 (m, 10H), 6.99−6.97 (m, 4H). 13C NMR (CDCl3, 600 MHz, δ): 147.17, 142.25, 141.49, 132.68, 132.12, 132.06, 131.97, 131.63, 131.56, 131.26, 131.1, 130.62, 129.92, 128.50, 128.42, 127.98, 127.22, 127.15. MS (APCI) m/z: 732.85 ([M+]; calcd for C50H38O2P2, 732.23). TPEPO. Yield: 63% (1.67 g). 1H NMR (400 MHz, CDCl3, δ): 7.53−7.64 (m, 6H), 7.37−7.49 (m, 6H), 7.10−7.16 (m, 11H), 7.01−7.06 (m, 6H). 13C NMR (CDCl3, 600 MHz, δ): 147.88, 143.14, 143.07, 142.82, 142.58, 139.88, 132.80, 132.14, 132.08, 131.92, 131.90, 131.56, 131.49, 131.39, 131.34, 131.31, 131.26, 131.22, 130.09, 129.39, 128.48, 128.40, 127.85, 127.75, 127.72, 126.82. MS (APCI) m/z: 532.20 ([M+]; calcd for C38H29OP, 532.40). DTPEPO. Yield: 41%. 1H NMR (400 MHz, CDCl3, δ): 7.51−7.62 (m, 3H), 7.38−7.42 (m, 2H), 7.22−7.27 (m, 4H), 7.12−7.16 (m, 22H), 6.95−7.061 (m, 12H). 13C NMR (CDCl3, 600 MHz, δ): 147.76, 143.13, 143.09, 142.85, 142.58, 139.91, 132.13, 132.06, 131.78, 131.30, 131.25, 131.21, 130.24, 128.37, 128.29, 127.84, 127.75, 127.71, 126.83, 126.78, 126.75. MS (APCI) m/z: 786.8 ([M+]; calcd for C58H43OP, 786.31).

3. RESULTS AND DISCUSSION 3.1. Synthesis. The synthetic routes for the new bipolar hosts TPEDPO, TPEPO, DTPEPO, and TTPEPO are shown in Scheme 1. The intermediates (E,Z)-1,2-bis(4-bromophenyl)Scheme 1. Synthetic Routes for TPEDPO, TPEPO, DTPEPO, and TTPEPO

1,2-diphenylethene (2BrTPE) and (2-(4-bromophenyl)ethene1,1,2-triyl)tribenzene (1BrTPE) were synthesized according to the reported procedures and confirmed by comparing their characterization data with literature data.23,24 Then, 2BrTPE and 1BrTPE were phosphorylated with the corresponding halogenated phosphine, yielding the target bipolar host materials after oxidation with hydrogen peroxide. Their structures were fully characterized by 1H NMR and 13C NMR spectroscopies and mass spectrometry. TPEDPO was found to have a low stereoregularity and to be a mixture of Z and E isomers marked with a zigzag line in the molecular structures. A Z/E ratio of ∼1/1 can be deduced from McMurry coupling reaction mechanism.15 Repeated temperature-gradient vacuum sublimation was applied for the further purification of the new bipolar host materials. 3.2. Thermal Properties. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) under a 8611



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nitrogen atmosphere were carried out to test the thermal stability of the new bipolar host luminogens, and the data are listed in Table 1. All of the materials exhibited high Table 1. Electrochemical, Photophysical, and Thermal Properties of TPE/PO Derivatives

TPEDPO TPEPO DTPEPO TTPEPO

HOMO (eV)

LUMO (eV)

Eg (eV)

Absa

PL (nm)b

Td/Tg (°C)

−6.11 −6.08 −6.12 −6.12

−2.84 −2.71 −2.79 −2.79

3.27 3.37 3.33 3.33

320 312 312 312

477 481 482 488

416/no 308/79 387/no 460/140

a

Measured in THF solvent at room temperature. bMeasured at a water fraction (f w) of 99% in THF solvent at room temperature.

Figure 2. UV−vis spectra of TPEDPO, TPEPO, DTPEPO, and TTPEPO in THF solution (1 μM).

decomposition temperatures (Td, corresponding to 5% weight loss) from 308 to 562 °C in a nitrogen atmosphere (see Figure 1), which indicates that these new luminogens are stable

changes of TPEDPO in THF and THF/water mixtures with water fractions (f w) from 0% to 99%. For dilute THF solution ( f w = 0), a weak peak emerged at 402 nm with a shoulder peak at 421 nm, owing to the fact that intramolecular rotations cannot completely deactivate the excited states of the molecule through rotational energy relaxation channels.28,29 When the water percentage was increased, the peak intensity at 402 nm decreased gradually, and an intense emission peak appeared at 481 nm for f w = 90. When the water fraction was over 90%, the PL intensity increased swiftly, indicating aggregate formation. For TPEPO and DTPEPO (Figure 3b,c), the behaviors were slightly different from that of TPEDPO. First, in dilute THF solution, the PL curves of both TPEPO and DTPEPO were essentially flat lines parallel to the abscissa; then, when larger amounts of water were added, intense emission was observed at 481 nm without obvious changes in the 402−421-nm range. Finally, for TTPEPO, the AIE behavior was similar to that of TPEDPO, with sharper changes in the peak at 402 nm with increasing water percentage (Figure 3d). Therefore, AIE-active behavior was thus confirmed in all four TPE-based dyes. The PL properties for the films of the four luminogens were also studied carefully. As shown in Figure 3, the emissions of the film state clearly shifted to the red, which could be induced by stronger molecular aggregation (Figure S15, Supporting Information). Notably, as the solvent was changed from pure THF solution to 99% aqueous mixtures, aggregate states in the normalized I/ I0 values for TPEPO increased from 0.0021 to 1. This means that, at an f w value of 99%, the PL intensity of TPEPO was 470fold higher than that in pure THF. The dramatically enhanced PL intensity can be assigned to the AIE-active properties and the strong molecular polarity of the phophine oxide unit. The lifetime of the AIE luminogens at different aggregation states were also investigated by time-resolved spectroscopy. For TPEPO (Figure 4b), the lifetime of 481 nm exhibited a doubledecay curve, and the decay times gradually increased from 1.7/ 5.1 to 2.5/6.9 ns upon addition of a poor solvent (f w from 90% to 99%). This demonstrates that poor solvent can result in the formation of bulky aggregates, which could reduce the specific surface area and increase the lifetime of the exciton. Meanwhile, the decay lifetime of TPEPO film (Figure S16, Supporting Information) was almost equal to that in 99% water solution, which means that TPEPO molecules aggregate completely in 99% water and exist in suspension. A similar phenomenon was observed for the other three compounds (Figure 4a,c,d), and the decay lifetimes of these luminogens became longer with

Figure 1. TGA thermograms of TPEDPO, TPEPO, DTPEPO, and TTPEPO recorded under nitrogen at a heating rate of 10 °C min−1.

enough to be fabricated into devices by vacuum thermal evaporation technology. TTPEPO was found to have the highest thermal stability among the four compounds and to exhibit a glass transition temperature (Tg) of 140 °C in the DSC heating cycle. TPEPO exhibited a lower Tg of 79 °C because of its lower molecular weight. For TPEDPO and DTPEPO, no obvious glass transition temperatures (Tg) were observed. 3.3. Photophysical Properties. All four luminogens TPEDPO, TPEPO, DTPEPO, and TTPEPO were found to be soluble in tetrahydrofuran (THF), chloroform, and dichloromethane, among others, but not in water. Their absorption spectra were recorded in dilute THF solution. As shown in Figure 2, TPEDPO exhibits a maximum absorption peak at about 320 nm, and the other three luminogens exhibit maximum absorption peaks at about 312 nm. The blue-shifted absorbances of TPEPO, DTPEPO, and TTPEPO can be attributed the steric hindrance of the TPE moiety. The AIE characteristics of the four luminogens were investigated through their fluorescent behaviors. As usual, THF and water were used as the solvent pair because of good miscibility. Molecules were dissolved in high-solubility solvents and in miscible mixtures of solvents in which they are soluble and insoluble. Figure 3a presents the photoluminescence (PL) 8612



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Figure 3. Photoluminescence (PL) spectra of (a) TPEDPO, (b) TPEPO, (c) DTPEPO, and (d) TTPEPO in THF/water mixtures with different water fractions (f w). Conditions: excitation wavelength, 360 nm; concentration, 1 μM.

Figure 4. Transient fluorescence decay lifetimes of TPEDPO, TPEPO, DTPEPO, and TTPEPO in THF/water mixtures (1 μM) with different water fractions after excitation with a 365-nm pulse laser.

HOMO/LUMO energy levels of TPEDPO, TPEPO, DTPEPO, and TTPEPO were calculated as 6.11/2.84, 6.08/2.71, 6.12/2.79, and 6.12/2.79 eV, respectively. 3.5. Carrier Mobility. The electron and hole mobilities of TPEDPO, TPEPO, DTPEPO, and TTPEPO were measured by the time-of-flight (TOF) transient photocurrent technique. The carrier transit time, tT, was evaluated from the intersection point of two asymptotes in the double-logarithmic representations of the TOF transits (Figures S17−S20, Supporting Information). The electron and hole mobilities versus E1/2 for the four fluorophores are shown in Figure 6. When the TPE/ PO ratio was 1:2, the electron mobility (∼3 × 10 −4 cm2 V−1 s−1) was 3 times higher than the hole mobility (∼1 × 10−4 cm2

increased water fraction. As the water fraction reached 95%, the lifetimes of these luminogens no longer increased, which demonstrates that bulky aggregation occurred completely. The time-resolved spectra here thus provide a new way of studying AIE-active properties. 3.4. Electrochemical Properties. To ensure the practicability of the four luminogens in optoelectronics, their electrochemical properties were investigated by cyclic voltammetry (Figure 5). The onset oxidation peaks (Eox) for the four compounds TPEDPO, TPEPO, DTPEPO, and TTPEPO were found to be 1.36, 1.33, 1.37, and 1.37 eV, respectively, versus the ferrocenium/ferrocene (Fc+/Fc) redox couple. From the onset of the oxidation potentials absorption spectra, the 8613



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cally tuned from n-type to ambipolar and p-type simply by introducing PO functional groups. 3.6. Electroluminescence. Using TPEDPO, TPEPO, DTPEPO, and TTPEPO as blue emitters, we fabricated electroluminesent (EL) devices I−IV with the following structure: ITO/MoO3 (10 nm)/NPB (40 nm)/blue emitters (20 nm)/TPBi (30 nm)/LiF(1 nm)/Al (100 nm). MoO3 was used as the electron-injecting layer. 4,4-Bis[N-(1-naphthyl)-Nphenyl-1-amino]biphenyl (NPB) was selected as the holetransporting layer. TPBi acted as the electron-transporting layer, and LiF and Al were applied as the electron-injecting layer and cathode, respectively. The current density versus voltage versus brightness (J−V−B) and current density versus current efficiency characteristics are shown in panels a and b, respectively, of Figure 7. Devices I−IV based on TPEDPO, TPEPO, DTPEPO, and TTPEPO, respectively, exhibited turnon voltages of 3.9, 4.2, 3.6, and 4.0 V, respectively (see Table 2). Devices I−IV exhibited maximum luminance (Lmax) values

Figure 5. Cyclic voltammograms of DTPEPO (blue), TPEPO (green), TPEDPO (red), and TTPEPO (yellow). Measurements were performed in dry DCM with 0.1 M TMAPF6 at a scan rate of 100 mV s−1.

Table 2. EL Performances of the Devicesa device

Figure 6. Electron and hole mobilities versus E1/2 for TPEDPO, TPEPO, DTPEPO, and TTPEPO.

Von (V)

Lmax (cd/m2)

ηC,max (cd/A)

ηP,max (lm/W)

I

TPEDPO

3.9

396

0.92

0.45

II

TPEPO

4.2

443

0.98

0.69

III

DTPEPO

3.6

834

0.50

0.33

IV

TTPEPO

4.0

541

0.30

0.15

V

TPEDPO

2.8

6776

5.59

6.8

VI

TPEPO

2.8

8656

9.7

10.23

VII

DTPEPO

2.8

14090

6.3

7.07

VIII

TTPEPO

2.8

6254

4.5

5.59

CIE (x, y)b (0.15, 0.19) (0.16, 0.19) (0.17, 0.20) (0.17, 0.24) (0.16, 0.36) (0.15, 0.35) (0.16, 0.38) (0.15, 0.32)

a Von, voltage of 1 cd/m2; Lmax, maximum luminance; ηC,max, maximum current efficiency; ηP,max, maximum power efficiency. bMeasured at 100 cd/m2.

V−1 s−1), which demonstrates that TPEDPO is electrondominated. As the proportion of TPE was increased, the electron mobility of the new fluorophores decreased, whereas the hole mobility increased. As the TPE/PO ratio reached 1:1, the electron mobility was almost equal to the hole mobility (∼2.3 × 10 −4 cm2 V−1 s−1), which indicates that TPEPO is a perfect ambipolar material. Moreover, TTPEPO is holedominated because of its higher proportion of TPE. It is therefore illustrated that, by pure chemical modification, the electronic nature of TPE-based AIE materials can be systemi-

of 396, 1058, 952, and 834 cd m−2, respectively; maximum current efficiencies (ηC,max) of 0.92, 0.98, 0.50, and 0.30 cd A−1, respectively; and maximum power efficiencies (ηP,max) of 0.45, 0.69, 0.33, and 0.15 lm W−1, respectively. Among the four blue emitters, TPEPO exhibited the best performance, which can be attributed to its more balanced electron- and hole-transport

Figure 7. (a) J−V−B characteristics of TPEDPO, TPEPO, DTPEPO, and TTPEPO. (b) Current efficiency as a function of current density in multilayer EL devices of TPE/PO derivatives. Inset: EL spectra of devices I−IV. 8614



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of 6.8, 7.07, and 5.59 lm W−1, respectively. However, the device based on TPEPO performed better, with a maximum luminance (Lmax) of 8656 cd m−2, a maximum current efficiency (ηC,max) of 9.32 cd A−1, and a maximum power efficiency (ηP,max) of 10.23 lm W−1. The detailed EL data are listed in Table 2. Interestingly, the device based on TPEPO exhibited the lowest current densities, but it showed the best efficiency (Figure 8a), which can be attributed to its more balanced carrier injection and transport ability.29 Concerning the trends of the electron/hole mobilities of these luminogens, the preliminary results reveal that electron/hole mobilities can be quantitatively controlled through fine structural modifications. Without doubt, TPEPO demonstrates the best EL efficiencies among the four molecules because of its balanced electron/hole mobilities. In contrast, the disrupted balances of the electron/hole mobilities in both TPEDPO and TTPEPO result in poor EL performance.

abilities. The EL spectra of I−IV show blue light emission with peaks centered at 448, 468, 468, and 462 nm, respectively (inset of Figure 7b). Compared with their film fluorescence spectra (Figure 3a), the EL peaks of TPEDPO, TPEPO, DTPEPO, and TTPEPO are blue-shifted, which might be caused by changes in intermolecular interactions arising from molecular arrangement. The detailed EL performances of devices I−IV are listed in Table 2. Although poor performances were obtained using nondoped OLEDs based solely on the blue emitters, better performances might be expected using these blue luminogens as hosts in host/dopant hybrid devices. 13 Herein, devices V−VIII fabricated with the structure ITO/NPB (10 nm)/blue emitter: 5% BUBD-1 (40 nm)/TPBi (10 nm)/LiF(1 nm)/Al (100 nm), in which the four luminogens were used as hosts and a styrylamine-type sky-blue fluorescent material, 2-ethyl-N-(4((E)-4-((E)-4-((2-ethyl-6-methylphenyl)(phenyl)amino)styryl)styryl)phenyl)-5-methyl-N-phenylaniline (BUBD-1) acted as a dopant. The optimal BUBD-1 concentration of 5% in these hosts was determined through systematic variation of the ratio of blue emitter to BUBU-1. According to the energy level diagram in Scheme 2, the dopant material BUBD-1 with a

4. CONCLUSIONS In summary, the series of n-type, ambipolar, and p-type AIE luminogens TPEDPO, TPEPO, DTPEPO, and TTPEPO were developed by gradually changing the TPE/PO ratio from 2:1 to 1:3. Their optical and electronic properties and AIE characteristics were investigated. When the TPE/PO ratio was 1:1, the electron mobility was almost equal to the hole mobility. The blue-doped devices based on TPEPO showed remarkable efficiencies with Von, Lmax, ηC,max, and ηP,max values of 2.8 V, 8656 cd m−2, 9.32 cd A−1, and 10.23 lm W−1, respectively. This research provides an effective method for designing AIE molecules with tunable mobility.

Scheme 2. Energy Level Diagram of the Devices Based on TPE/PO Derivatives

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ASSOCIATED CONTENT

S Supporting Information *

Characterization methods and additional NMR data. This material is available free of charge via the Internet at http:// pubs.acs.org.

lowest unoccupied molecular orbital/highest occupied molecular orbital level of 2.6/5.1 eV is well confined in TPEDPO, TPEPO, DTPEPO, and TTPEPO. In addition, the LUMO of TPEPO is much closer to electron-transport material TPBi, which is beneficial for electron injection. As shown in Figure 8, all of the devices worked at a low turnon voltage of 2.8 V. The doped devices based on TPEDPO, DTPEPO, and TTPEPO exhibited maximum luminance (Lmax) values of 6776, 14090, and 6254 cd m−2, respectively; maximum current efficiencies (ηC,max) of 5.99, 6.3, and 4.45 cd A−1, respectively; and maximum power efficiencies (ηP,max)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: 86-27-87793032.. *E-mail: [email protected]. Tel.: 852-34115094 Notes

The authors declare no competing financial interest.

Figure 8. (a) J−V−B characteristics of devices V−VIII based on TPEDPO, TPEPO, DTPEPO, and TTPEPO, respectively. (b) Current efficiency as a function of current density in BUBD-1-doped multilayer EL devices of TPEO derivatives. Inset in panel b: EL spectra of devices V−VIII. 8615



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ACKNOWLEDGMENTS This research work was supported by the NSFC/China (21161160442, 51203056) and the National Basic Research Program of China (973 Program 2013CB922104). The Analytical and Testing Centre at Huazhong University of Science and Technology is acknowledged for characterization of new compounds. X.Z. thanks Hong Kong Baptist University (Grant FRG1/13-14/022) for financial support.

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