A host material consisting of phosphinic amide for efficient sky-blue phosphorescent organic light-emitting diodes

Synthetic Metals 205 (2015) 11–17

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A host material consisting of phosphinic amide for efficient sky-blue phosphorescent organic light-emitting diodes Fu-Peng Wu, Yue-Min Xie, Lin-Song Cui, Xiang-Yang Liu, Qian Li, Zuo-Quan Jiang * , Liang-Sheng Liao * Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM) & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 5 February 2015 Received in revised form 18 March 2015 Accepted 23 March 2015 Available online xxx

A new bipolar host material (PBCz-PO) consisting a phosphinic amide is developed to show a resonance effect. It possesses quite high triplet energy (2.76 eV), and this high triplet energy make it suitable to be blue host in phosphorescent organic light-emitting diodes (PHOLEDs). The device with PBCz-PO as a host and FIrpic as a dopant achieved a maximum current efficiency (hc) of 31.5 cd/A, a maximum power efficiency (hp) of 31.0 lm/W and a maximum external quantum efficiency (EQE) of 13.4% as well as low driving voltage. ã 2015 Elsevier B.V. All rights reserved.

Keywords: Bipolar host material Phosphinic amide Resonance effect Phosphorescence Organic light-emitting diodes

1. Introduction Phosphorescent organic light-emitting diodes (PHOLEDs) are considered as the most promising candidates for the nextgeneration display and lighting technology due to the ideal characteristics of electrophosphorescence, such as the 100% internal quantum efficiency which result from the highly efficient utilization of triplet excitons [1–8]. Although tremendous efforts have been paid to increase the luminous efficiency of PHOLEDs, the development of a stable and efficient blue PHOLED remains a great challenge. One of the most important problem is concentration quenching and triplet–triplet annihilation, which result in significant efficiency roll-offs in PHOLEDs [9–23]. A basic strategy widely used to restrain these effects is to dope the phosphorescent units into a suitable host materials to reduce their intermolecular interaction. It is desirable that the host materials ought to have a large enough triplet energy gap (T1) for efficient energy transfer to the guest, good carrier transport properties for a balanced recombination of holes and electrons in the emitting layer, and energy-level matching with neighboring layers for effective charge injection [24–32]. These inevitable demands of host materials provide organic chemists with

* Corresponding authors. Tel.: +86 6 512 65880093; fax: +86 512 65880820. E-mail addresses: [email protected] (Z.-Q. Jiang), [email protected] (L.-S. Liao). http://dx.doi.org/10.1016/j.synthmet.2015.03.023 0379-6779/ ã 2015 Elsevier B.V. All rights reserved.

tremendous opportunities to contribute their expertise to this technology. The design of bipolar host materials configured with electrondonating groups as donors (D) and electron-withdrawing groups as acceptors (A) seems to be the most appealing strategy because they can provide more balance in electron and hole transport [33–45]. However, these strong D–A interaction will suppress the exciton recombination due to the not high enough T1 [46–48]. So, Yuan et al. adopted the twisted p-conjugated bridge to partially reduce the electronic coupling of the donor and acceptor [49]. However, it is very difficult for this strategy to achieve hosts both with relative high T1 values and with strong electrical activities. Recently, a special insulating linkage based on diphenylphospine oxide (DPPO) has been utilized and realized excellent device performance, such as ultralow driving voltages (less than 3 V for onset) and high and stable efficiencies [50]. The P¼O moieties not only serve as insulating linkages but also modify the electrical properties of the molecules as a kind of electron-withdrawing group. In addition, a special effect of N P¼O on charge redistribution by resonance variation was also observed [51]. Our group recently reported a series of high-performance PO hosts with high T1 values and relatively balanced carrier injection/ transporting capabilities. The device based on FIr6 as dopant can achieve power efficiency of 23.7 lm W 1, EQE of 13.6% [17]. Here, in this work, we report another efficient bipolar host for sky-blue electrophosphorescence, based on the structure of the

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3,30 -bicarbazole (BCz) [52–55] as the donor and phosphine oxide as the acceptor named diphenyl(90 -phenyl-[3,30 -bicarbazol]-9-yi) phosphine oxide (PBCz-PO). The greater contribution of enantiotropic N+¼P O resonances to PBCz-PO results in a much enhanced electron transportation from the polarized carbazolyl. As a result, PBCZ-PO endowed its sky-blue PHOLED with impressively low driving voltages: 3.3 V for 100 cd/m2 and under 4.1 V for 1000 cd/m2, as well as favorable electroluminescence (EL) efficiencies, such as the maximum external quantum efficiency (EQE) of up to 13.4%, the current efficiency (hc) of 31.5 cd/A, and the power efficiency (hp, max) of 31.0 lm/W. 2. Experimental 2.1. Materials and methods All of the chemicals, i.e., (9-phenyl-9H-carbazol-3-yl) boronic acid, 3-bromo-9H-carbazole, chlorodiphenylphosphine and nbutyl lithium were purchased from Bepharm Limited and Alfa Aesar. All of these materials were used without further purification. THF was purified using the PURE SOLV (Innovative Technology) purification system. Chromatographic separations were carried out using silica gel (200–300 nm). All the other reagents were used as received from commercial sources unless otherwise stated. 1H NMR and 13C NMR spectra were recorded on a Varian Unity Inova 400 spectrometer at room temperature. Mass spectra were recorded on a Thermo ISQ mass spectrometer using a direct exposure probe. UV–vis absorption spectra were recorded on a PerkinElmer Lambda 750 spectrophotometer. PL spectra and phosphorescent spectra were recorded on a Hitachi F-4600 fluorescence spectrophotometer. Differential scanning calorimetry (DSC) was performed on a TA DSC 2010 unit at a heating rate of 10  C/min under nitrogen. The glass transition temperatures (Tg) was determined from the second heating scan. Thermogravimetric analysis (TGA) was performed on a TA SDT 2960 instrument at a heating rate of 10  C/min under nitrogen. The temperature at 5% weight loss was used as the decomposition temperature (Td). Cyclic voltammetry (CV) was carried out on a CHI600 voltammetric analyzer at room temperature with a conventional three-electrode configuration consisting of a platinum disk working electrode, a platinum wire auxiliary electrode, and an Ag wire pseudo-

reference electrode with ferrocenium–ferrocene (Fc+/Fc) as the internal standard. Nitrogen-purged DCM was used as the solvent for the oxidation scan and DMF was used for the reduction scan, with tetrabutylammonium hexafluorophosphate (TBAPF6) (0.1 M) as the supporting electrolyte. The cyclic voltammo gram was obtained at a scan rate of 100 mV/s. The ultraviolet photoemission spectroscopy (UPS) characterization was performed in a Kratos AXIS UltraDLD ultrahigh vacuum (UHV) analysis system. 2.2. Fabrication of the OLED The OLED was fabricated through vacuum deposition of the materials at ca. 2  106 Torr onto ITO-coated glass substrates having a sheet resistance of ca. 30 V per square. The ITO surface was cleaned ultrasonically and sequentially with acetone, ethanol, and deionized water, in an ultrasonically bath, dried in an oven, and finally exposed to UV-ozone for about 30 min. Organic layers were deposited at a rate of 2–3 Å/s, and subsequently the HAT-CN and Liq were deposited at 0.2 Å/s and then capped with Al (ca. 4 Å/ s) through a shadow mask without breaking the vacuum. For the device, the emitting area was determined by the overlap of two electrodes as 0.09 cm2. The EL spectra, CIE coordinates and J–V–L curves of the device were measured with a PHOTO RESEARCH SpectraScan PR 655 photometer and a KEITHLEY 2400 SourceMeter constant current source at room temperature. The EQE values were calculated according to the previously reported methods. 2.3. Synthesis of 9-phenyl-9H, 90 H-3, 30 -bicarbazole (PBCz) 3-bromo-9H-carbazole (2.56 g, 10.4 mmol), (9-phenyl-9H-carbazol-3-yl) boronic acid (3.28 g, 11.4 mmol) and tetrakis-(triphenylphosphine) palladium (0) (0.6 g, 0.52 mmol) were dissolved in THF/2 M K2CO3 (3:1, v/v). The reaction mixture was heated to 60  C for 8 h under an argon atmosphere. After cooling to room temperature, the organic layer was separated and evaporated to remove the solvent. The residue was purified by column chromatography with 1:3 (v/v) dichloromethane–petroleum ether as the eluent and recrystallized from dichloromethane–petroleum to give a white crystalline powder (3.02 g, 71%). 1H NMR (400 MHz, CDCl3) d 8.40 (s, 1H), 8.25 (d, J = 7.7 Hz, 1H), 8.19 (d, J = 7.8 Hz, 1H),

Scheme 1. Molecular structure and synthesis of PBCz-PO.

F.-P. Wu et al. / Synthetic Metals 205 (2015) 11–17

1.0

100

o Td = 386 C

Intensity (a.u.)

80

Weight (%)

13

60

40

20

0.8

0.6

0.4

0.2

16.69 eV

1.18 eV

0

0.0 200

300

400

500

600

22

20

18

Temperture(oC)

16

14

12

10

8

6

4

2

0

Binding Energy(eV)

Fig. 1. TGA thermograms recorded at a heating rate of 10  C/min.

Fig. 5. UPS spectra of PBCz-PO.

C30H20N2 (%): C 88.21, H 4.93, and N 6.86; found: C 88.16, H 5.08, N 6.74. 2.4. Synthesis of diphenyl (90 -phenyl-9H, 90 H-[3,30 -bicarbazol]-9-yl) phosphine oxide (PBCz-PO)

Endothermic

Tg = 116 oC

80

90

100

110

120

130

140

150

o

Temperature( C) Fig. 2. DSC traces recorded at a heating rate of 10  C/min.

7.81 (dd, J = 8.4, 1.8 Hz, 2H), 7.69–7.61 (m, 5H), 7.56–7.41 (m, 8H), 7.30 (d, J = 19.3 Hz, 2H). 13C NMR (100 MHz, CDCl3) d 139.70–139.33, 137.67–137.08, 129.41, 126.94, 126.58–126.51, 125.52, 123.54– 123.34, 123.08, 119.93, 119.47, 119.05–118.95, 118.51–118.32, 110.38–110.11, 109.39. MS (EI): m/z 408.5 [M+]. Anal. calcd for

UV

PL

Phos PBCz-PO

Current (A)

Normalized Intensity (arb.unit)

1.0

The critical intermediate (PBCz) was synthesized by the classic Suzuki–Miyaura cross coupling reaction. Then PBCz (2.05 g, 4.9 mmol) was dissolved in anhydrous THF (60 mL) under nitrogen and was cooled to 78  C. n-BuLi (2.4 M in hexane, 1.1 mL) was then slowly added dropwise. After stirring for 30 min at 78  C dichlorophenylphosphine (1.19 g, 5.4 mmol) in THF (20 mL) was added slowly. The resulting mixture was stirred for 1 h at 78  C, and allowed to warm to room temperature. The mixture was quenched with 10 mL of water and the organic layer was extracted with ethyl acetate, washed with water and brine, dried over anhydrous Na2SO4, filtered and evaporated under reduced pressure. The crude material was purified by column chromatography with 2:3 (v/v) dichloromethane–petroleum ether as the eluent. The obtained white powdery product was dissolved in 1,2dichloromethane (50 mL) and to the solution was added 30% aqueous H2O2 (10 mL). The mixed solution was stirred overnight at ambient temperature. The organic layer was separated and washed with dichloromethane and water, dried over anhydrous Na2SO4, filtered and evaporated under reduced pressure. The crude material was purified by column chromatography with 2:3 (v/v).

0.5

0.0 300

350

400

450

500

550

600

650

Wavelength (nm) Fig. 3. Room-temperature UV–vis absorption and fluorescence (PL) spectra of PBCz-PO in CH2Cl2 solution and phosphorescence (Phos) spectrum of the compound measured in a frozen 2-methyltetrahydrofuran matrix at 77 K.

-1.6

-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

1.6

2.0

Potential vs. Fc/Fc+ Fig. 4. Cyclic voltammograms of PBCz-PO in DCM and DMF for oxidation and reduction scans, with 0.1 M TBAPF6 as the supporting electrolyte.

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Table 1 Key physical properties of the PBCz-PO. Tg/Td ( C)

EHOMO (eV)

Eg (eV)

ELUMO (eV)

labs,max (nm)

lPL,max (nm)

ET (eV)

116/386

5.71

3.37

2.34

310

404

2.76

Fig. 6. HOMO and LUMO distribution of PBCz-PO.

3. Results and discussion 3.1. Synthesis and characterization Asdepicted inScheme 1,the novelbipolarhost material(PBCz-PO) was designed and synthesized. The critical intermediate (PBCz) was synthesized by the classic Suzuki–Miyaura cross coupling reaction of (9-phenyl-9H-carbazol-3-yl) boronic acid and 3-bromo-9H-carbazole. PBCz-PO was synthesized by treating the intermediate with n-BuLi, then quenching the reactionwith chlorodiphenylphosphine. Detailed syntheticproceduresaredemonstratedin Section 2.Finally, the target material was further purified by repeated temperature gradient vacuum sublimation. Then the chemical structure of the target compound was fully characterized by 1H NMR and 13C NMR spectroscopy, mass spectrometry and elemental analysis. 3.2. Thermal properties The thermal properties of PBCz-PO were investigated through thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The thermal decomposition temperatures (Td, corresponding to 5% weight loss, see Fig.1) was 386  C. Its glass-transition temperature was observed at 116  C (see Fig. 2), which reflects this host has good thermal stability. The good thermal and amorphous

stability enable it to form morphologically stable and uniform amorphous film by vacuum deposition for OLED fabrication. 3.3. Photophysical properties Fig. 3 shows the UV–vis absorption and fluorescence (PL) spectra of PBCz-PO in dilute CH2Cl2 solution, as well as the phosphorescence (Phos) spectra in a frozen 2-methyltetrahydrofuran matrix at 77 K. At around 260–310 nm, the compound has the strongest absorption that is mostly contributed by p–p* transition of the carbazole unite while the weaker absorption in the range of 340–360 nm is associated with the n–p* electron transition of the entire conjugated backbone. The band gap energy (Eg) was calculated as 3.37 eV according to the edge of the UV–vis absorption peak. Besides, the triplet energy of PBCz-PO

10

Current Density mA/cm2

Dichloromethane–petroleum ether as the eluent to give a white solid (1.43 g, 48%).). 1H NMR (400 MHz, CDCl3) d 8.41 (s, 1H), 8.35 (s, 1H), 8.22 (d, J = 4.0 Hz, 1H), 8.15 (d, J = 4.0 Hz, 1H), 7.83 (d, J = 8.0 Hz, 2H), 7.79 (d, J = 12.0 Hz, 2H), 7.72 (dd, J = 8.5, 1.7 Hz, 1H), 7.69–7.57 (m, 7H), 7.57–7.46 (m, 7H), 7.44 (dd, J = 6.0, 4.1 Hz, 3H), 7.36–7.29 (m, 3H). 13C NMR (100 MHz, CDCl3) d 141.78, 140.87, 140.21, 139.72, 137.22, 135.68, 133.06, 132.71, 131.69, 131.09, 129.83, 129.45, 128.62, 127.17–124.90, 123.46, 123.01, 121.53, 120.12– 119.34, 118.40, 117.84, 114.67, 109.50. MS (EI): m/z 608.05 [M+]. Anal. calcd for C42H29N2OP (%): C 82.88, H 4.80, N 4.60; found: C 82.76, H 4.92, N 4.53.

10

10

-3

-4

Hole-only Electron-only

-5

-10

-5

0

5

10

15

20

25

30

35

40

45

50

Voltage (V) Fig. 7. Current density versus voltage characteristic of the hole-only and electrononly device for PBCz-PO compound.

F.-P. Wu et al. / Synthetic Metals 205 (2015) 11–17

15

Fig. 8. Energy level diagrams for the device.

was determined to be 2.76 eV by the highest energy 0–0 phosphorescent emission in 2-methyltetrahydrofuran at 77 K, which is match with FIrpic (ET = 2.65 eV) and sufficiently high to serve as host material for sky-blue PHOLEDs. This high triplet energy is contributed by the P¼O moieties that serve as insulating linkage then reduce the electronic coupling of the donor and acceptor. 3.4. Electrochemical properties The electrochemical properties of PBCz-PO characterized by cyclic voltammetry (CV) are shown in Fig. 4. The measurement was carried out using ferrocene as the internal reference in CH2Cl2. The HOMO energy was deduced from the onset of the oxidation potential with regard to the energy level of ferrocene (4.8 eV below the vacuum). In order to get more reliable energy levels about frontier molecular orbitals (FMOs), ultraviolet photoemission spectroscopy (UPS) was used to determine the HOMO levels (Fig. 5). The HOMO level of the compound was estimated as 5.71 eV (i.e., EHOMO = 21.22 eV + (16.69 eV – 1.18 eV)). Then the LUMO level was determined as 2.34 eV according to the equation ELUMO = EHOMO + Eg (Table 1). 3.5. Theoretical calculation To get insight understanding about the FMOs, DFT calculation was utilized to simulate its FMO spatial distributions at a B3LYP/631G (d) level. As shown in Fig. 6, the HOMO of PBCz-PO is delocalized in the long BCz backbone while the LUMO is distributed mostly on the biphenyl phosphine oxide moiety. Obviously, the overlap between HOMO and LUMO is rather small, indicating the bipolar property and high triplet energy. 3.6. Phosphorescent OLED Before testifying the function of the bipolar compound (PBCzPO) as host material of sky-blue phosphorescent OLEDs emitters, a

hole-only device and an electron-only device [56–57] with the configuration of ITO/TmPyPB (20 nm)/host (100 nm)/TmPyPB (20 nm)/Liq (2 nm)/Al (100 nm) and ITO/MoO3 (20 nm)/host (100 nm)/MoO3 (20 nm)/Al (100 nm) were fabricated to evaluate the carrier transporting characteristics in PHOLEDs, respectively. The current density–voltage characteristics of this two singlecarrier devices are shown in Fig. 7. It can be seen that both the hole current density and the electron current density are independent on voltage, though exist a gap between them. However, the difference in the current density between the hole-only and electron-only device based on PBCz-PO is much smaller compared to BCzPh [44] at the same voltage, which clearly indicates that PBCz-PO is a potential bipolar material capable of better balance carrier injection/transporting. In addition, the compound has longer HOMO according to the theoretical calculation, which is consistent with the difference of hole-only and electron-only devices. Those features lay a foundation of designing a device with high efficiencies and low efficiency roll-offs. Then sky-blue phosphorescent OLED with typical sandwiched structure by sequential vapor deposition of materials onto a glass substrate coated with ITO was fabricated. Besides, the sky-blue Irbased FIrpic [58] complex was doped into the host as the emitting layer to investigate the performance of this new bipolar host material. We utilized strongly electron-deficient 1,4,5,8,9, 11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) and 8-Hydroxyquinolinolato-lithium (Liq) as the hole-injection layer (HIL) and the electron-injection layer, 1,1-bis[4-[N,N-di(p-tolyl) amino]phenyl] cyclohexane (TAPC) as hole-transporting layer (HTL) and electron-blocking layer. 1,3,5-tri [(3-pyridyl)-phen-3yl]benzene (TmPyPB) was used as electron-transporting as well as hole-blocking layer, and Al acted as the cathode. The device structure was ITO/HAT-CN (10 nm)/TAPC (40 nm)/PBCz-PO: FIrpic (5 wt%, 20 nm)/TmPyPB (45 nm)/Liq (2 nm)/Al (120 nm). Fig. 8 depicts the relative energy levels of the materials employed in the device. Fig. 9 shows current density–voltage–luminance characteristics, current efficiency and power efficiency versus luminance curves, external quantum efficiency versus luminance curves and

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Current Density Luminance

35 30

10

25

3

20 15 10

10

2

5 0 3

4

5

6

7

10

1

35

(b)

4

Currient Efficiency (cd/A)

10

Luminance ( cd/cm2)

Current Density(mA/cm2)

(a)

40

Current Efficiency Power Efficiency

35 30

25

25 20 20 15

15

10

10 5 10

2

10

Normalized EL Intensity (a.u.)

20

(c)

EQE (%)

15

10

5

2

10

3

10

4

5

Luminance (cd/m2 )

Voltage (V)

10

30

Power Efficiency (lm/W)

40

45

3

10

4

(d)

1.0

0.8

@10 mA/cm2

500 nm

476 nm

CIE = (0.16, 0.40)

0.6

0.4

0.2

0.0 400

450

500

550

600

650

700

750

Wavelength (nm)

Luminance (cd/m2 )

Fig. 9. (a) Current density–voltage–luminance characteristics, (b) current efficiency and power efficiency versus luminance curves, (c) external quantum efficiency versus luminance curves, (d) the EL spectrum of device PBCZ-PO at 10 mA/cm2.

4. Conclusion Table 2 EL Performance of the PBCz-PO. Von (V)

Lmax (cd/m2)

hp, max (lm/W)

hc, max (cd/A)

hext, max (%) lEL (nm)

CIE (x,y)

3.1

9579

31.0

31.5

13.4

(0.16, 0.40)

476; 500

the EL spectrum of device PBCz-PO at 10 mA/cm2. The EL spectra of the sky-blue device implies that the emission of device come mostly from the emitter (FIrpic), indicating thorough energy transfer from host material to FIrpic due to the new bipolar host material (PBCz-PO) has matched HOMO/LUMO and the excitons can be confined in the emitting layer more effectively. Note that the driving voltages of device were very low, as only 3.3 V for 100 cd/ m2 and 4.1 V for 1000 cd/m2 were detected. This merit of device could be attributed to the bipolar transportability and contribution of enantiotropic N+¼P O resonances of PBCz-PO. Finally, the sky-blue phosphorescent OLED based on PBCz-PO has a maximum current efficiency (hc, max) 31.5 cd/A, a maximum power efficiency (hp, max) 31.0 lm/W and the maximum external quantum efficiency 13.4% (Table 2)

In summary, we have designed a new bipolar host material (PBCz-PO) based on BCz and diphenylphosphine oxide moiety showing a resonance effect. Analysis of the compound in thermal properties, photophysical properties, electrochemical properties and electroluminescence properties indicates that this compound is feasible to be a host for sky-blue phosphorescent OLEDs emitters. The sky-blue phosphorescent OLED based on PBCz-PO reached a maximum efficiency 31.5 cd/A for current efficiency (hc, max), 31.0 lm/W for power efficiency (hp, max) and 13.4% for external quantum efficiency as well as low driving voltages. We expect resonances effect like N+¼P O could be further applied in future design of host materials. Acknowledgments We sincerely gratitude to the financial support from the Natural Science Foundation of China (Nos. 21202114 and 61177016). This project is also funded by Collaborative Innovation Center (CIC) of Suzhou Nano Science and Technology, Soochow University, and by the Priority Academic Program Development of the Jiangsu Higher Education Institutions (PAPD).

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