Highly twisted organic molecules with ortho linkage as the efficient bipolar hosts for sky-blue thermally activated delayed fluorescence emitter in OLEDs

Accepted Manuscript Highly twisted organic molecules with ortho linkage as the efficient bipolar hosts for sky-blue thermally activated delayed fluorescence emitter in OLEDs Jingfang Pei, Xiaoyang Du, Chao Li, Chuan Wang, Cong Fan, Haochen Tan, Bei Cao, Fangyi Huang, Silu Tao, Jingze Li PII:

S1566-1199(17)30370-1

DOI:

10.1016/j.orgel.2017.07.038

Reference:

ORGELE 4230

To appear in:

Organic Electronics

Received Date: 17 May 2017 Revised Date:

22 June 2017

Accepted Date: 27 July 2017

Please cite this article as: J. Pei, X. Du, C. Li, C. Wang, C. Fan, H. Tan, B. Cao, F. Huang, S. Tao, J. Li, Highly twisted organic molecules with ortho linkage as the efficient bipolar hosts for sky-blue thermally activated delayed fluorescence emitter in OLEDs, Organic Electronics (2017), doi: 10.1016/ j.orgel.2017.07.038. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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GRAPHIC ABSTRACT

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The new host materials based on the ortho-linked carbazole/diphenylpyridine hybrids could realize max. CE of 34.8 cd A-1, PE of 33.1 lm W-1 and EQE of 16.0 % in the sky-blue TADF OLEDs.

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Highly Twisted Organic Molecules with Ortho Linkage as the Efficient Bipolar Hosts for Sky-blue Thermally Activated Delayed Fluorescence Emitter in OLEDs Jingfang Pei,1+ Xiaoyang Du,2+ Chao Li,1+ Chuan Wang,1 Cong Fan,1* Haochen Tan,1 Bei Cao,3*

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Fangyi Huang,1 Silu Tao,2* and Jingze Li1*

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1. State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Microelectronics and Solid-state Electronics, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, PR China E-mail: [email protected]; [email protected] 2. School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, PR China E-mail: [email protected] 3. State Key Laboratory of Synthetic Chemistry, Institute of Molecular Functional Materials, Department of Chemistry, The University of Hong Kong, Hong Kong, PR China E-mail: [email protected] [+] These authors contributed equally to this work

KEYWORDS: twisted conformation; host materials; triplet energy; delayed fluorescence; OLEDs

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ABSTRACT: Two new host materials, CzDPPy and tCzDPPy, were designed and synthesized through the Ullmann-coupling reaction between carbazole/3,6-di-tert-butyl-carbazole and 2,6-bis(2-bromophenyl)pyridine. Their single-crystal structure, thermal, electrochemical, opt-electronic and bipolar carrier-transporting properties were fully investigated. Due to the

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steric hindrance of carbazole at the ortho positions of diphenylpyridine, both CzDPPy and tCzDPPy adapted highly twisted molecular conformation, which could effectively minimize their π conjugation and endow them the high triplet energies of 2.67 and 2.64 eV, respectively.

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Organic light-emitting devices (OLEDs) were fabricated by using CzDPPy and tCzDPPy as the host materials and 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene (2CzPN) as the sky-blue TADF emitter. The peak current efficiency of 34.8 cd A-1, power efficiency of 33.1 lm W-1 and external quantum efficiency of 16.0% were realized for the CzDPPy-based TADF OLED, along with the satisfactory CIE coordinate of (0.18, 0.34) at 100 cd m-2.

1. INTRODUCTION Currently, the development of organic light-emitting diodes (OLEDs) has reached a new stage by using the third-generation emitting materials possessing the unique property of thermally

ACCEPTED MANUSCRIPT activated

delayed

fluorescence

(TADF)

[1-4].

Compared

to

the

second-generation

phosphorescent materials [5-11], the TADF emitters can harvest all the electro-generated singlet excitons for light emission, since parts of the singlet excitons can come from the underlying

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triplet excitons through the reverse intersystem crossing (RISC) process for the very small singlet-triplet (S1-T1) energy gap. Consequently, the TADF emitters can realize the maximum internal quantum efficiency of 100%, which is the same as the phosphors in terms of device

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efficiency. Much more importantly, under the sustainability concerns for the high-price and

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low-abundance noble metals (e.g., Ir and Pt) involved in the phosphors [12-14], the TADF emitters are gradually taking advantages and become more attractive for long-term and large-scale practical applications, since most TADF emitters are absolutely pure organic molecules composed by abundant elements [15]. Recently, very impressive and encouraging

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efficiencies have been reported for the OLEDs based on TADF emitters [16-20]. However, of the three primary colors (blue, green and red), the efficiency and operational lifetime for blue TADF OLEDs are still unsatisfactory [21, 22].

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Similar in the cases of phosphorescent OLEDs (PhOLEDs), the TADF emitters are still necessary

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to be dispersed into a host matrix at low doping level to avoid the competitive de-excitation pathways (e.g., triplet-triplet annihilation and/or concentration quenching) for emissive excitons. Meanwhile, since their singlet excitons are partially originated from the triplet state, microsecond and/or millisecond lifetimes of singlet excitons are expectedly observed, which usually could result in the fast efficiency decay of device at high current density [23]. Therefore, it is always urgent to develop highly efficient host materials for TADF emitters, particularly for blue-emitting OLEDs.

ACCEPTED MANUSCRIPT On the other hand, since the emissive excitons of the blue TADF emitter are all from its singlet state, both the singlet and triplet states of the host material should be higher than the singlet state of the blue TADF emitter doped [24, 25], even the energy gap (ΔEST) between S1 and T1 for

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the blue TADF emitter is very small. This situation is a little different from the host design for PhOLEDs. Otherwise the energy of the excitons located on the blue TADF emitter could exothermically transfer to the lower singlet or triplet state of the host through Förster or Dexter

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pathway [26]. At the same time, the host material for blue TADF emitter also should possess

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bipolar carrier-transporting ability and suitable HOMO (highest occupied molecular orbital)/LUMO (lowest unoccupied molecular orbital) levels. As the energy gap (ΔEST) between S1 and T1 for host material is usually large, most attentions are still focusing on the triplet energy of host material. To guarantee the high triplet energy, minimizing the π conjugation (e.g.,

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meta-/ortho-linkage and spiro carbon [27-30]) and/or employing insulating linkages (e.g., adamantane and tetraaryl silane [31, 32]) to interrupt the π conjugation are widely adapted. Nevertheless, the efficient host materials suitable for blue/sky-blue TADF emitters are still scarce

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[33, 34]. For example, Wang et al. manifested that the host material of o-mCPBI could

realize

the

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[9,9’-(2’-(1H-benzimidazol-1-yl)-[1,1’-biphenyl]-3,5-diyl)bis(9H-carbazole)]

maximum (max.) external quantum efficiency (EQE) of 10.2% for the sky-blue TADF emitter of 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene (2CzPN) [35]; while Li et al. reported the host of o-CzCN could achieve the max. current efficiency (CE) of 29.23 cd A-1 and EQE of 14.98% for the 2CzPN emitter [36]. In this article, two new host materials, namely 2,6-bis(2-(9H-carbazol-9-yl)phenyl)pyridine (CzDPPy) and 2,6-bis(2-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)pyridine (tCzDPPy), were

ACCEPTED MANUSCRIPT designed by adapting the effective strategy of ortho linkage, which could bring in the highly twisted molecular conformation for the steric hindrance of carbazole moieties and ultimately endow them the high triplet energies. At the same time, under the consideration of synthetic

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steps and molecular symmetry, the electron-transporting moiety was initially selected to be the electron-deficient pyridine ring due to its good electron affinity and relatively-low LUMO level when compared to benzene ring [37, 38]. Indeed, both CzDPPy and tCzDPPy exhibited high

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triplet energies above 2.6 eV and simultaneously possessed bipolar carrier-transporting

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properties, which were suitable to host sky-blue TADF emitters. By using 2CzPN as the sky-blue TADF emitter, the fabricated OLED hosted by CzDPPy could realize the max. CE of 34.8 cd A-1, power efficiency (PE) of 33.1 lm W-1 and EQE of 16.0%, along with the satisfactory Commission International de I’Eclairage coordinate (CIE) of (0.18, 0.34) at 100 cd m-2.

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2. EXPERIMENTAL SECTION

General information: 2,6-Bis(2-bromophenyl)pyridine was synthesized according to the

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reported procedures [39]. All reagents commercially available were used as received unless otherwise stated. All reactions were carried out using Schlenk tube in the N2 atmosphere. 1H and 13

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C NMR spectra were measured on Bruker AV II 400 MHz spectrometer using CDCl3 as solvent.

Elemental analysis (C, H and N) were performed on a Euro EA 3000 analyzer (Leeman Labs Inc.). Electrospray ionization mass spectrometry (ESI-MS) measurements were performed on GCMS-QP2010 Plus Instrument (Shimadzu). Single crystal X-ray diffraction data were obtained from a Bruker Xcalibur Eos CCD diffractometer using a graphite-monochromated MoKα (λ = 0.71073 Å) radiation at room temperature.

ACCEPTED MANUSCRIPT The thermogravimetric analysis (TGA) was recorded on a NETZSCH STA 449C instrument. And the differential scanning calorimetry (DSC) was performed on a NETZSCH DSC 200 PC unit at a heating rate of 20 °C min-1 from 25 to 600 °C under argon. The glass transition temperature (Tg)

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was determined from the second heating scan at a heating rate of 10 °C min-1. UV-Vis absorption spectra were recorded on a Shimadzu UV-2700 spectrophotometer. Fluorescent and phosphorescent spectra were recorded on a Hitachi F-4600 fluorescence spectrophotometer,

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where the phosphorescent spectra were collected at 77 K by selecting the phosphorescent

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model and almost all fluorescent excitons could be filtered by this model due to the delayed recording time. Cyclic voltammetry (CV) measurements were carried out in the nitrogen-purged CH2Cl2 solution (oxidation scan) at room temperature with a CHI voltammetric analyzer. Tetrabutylammonium hexafluorophosphate ((N-butyl)4NPF6, 0.1 M) was used as the supporting

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electrolyte. The conventional three-electrode configuration consisted of a Pt working electrode, a Pt wire auxiliary electrode, and the Ag/AgCl reference electrode. The scan rate was 0.1 V s-1. The onset potentials were determined from the intersection of two tangents drawn at the rising

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and background currents in the CV curves. The HOMO levels were calculated according to the

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formula: HOMO = - (4.8 eV + (Eonsets - E1/2(Fc/Fc+))), where the E1/2 potential of ferrocene/ferrocenium ion (Fc+/Fc) redox couple was used as the external standard. The LUMO levels were calculated as follows: LUMO = HOMO + Eg. The triplet energy (T1) of 2CzPN was 2.62 eV for reference by calculating its photoluminescence peak of 473 nm at toluene, but all the emissive excitons at this peak were in principle singlet (S1) [3]. This calculation was rational only when the energy gap (ΔEST) between S1 and T1 was very small (<100 meV).

ACCEPTED MANUSCRIPT Synthesis

of

2,6-bis(2-(9H-carbazol-9-yl)phenyl)pyridine

(CzDPPy):

A

mixture

of

2,6-bis(2-bromophenyl)pyridine (200 mg, 0.52 mmol), carbazole (220 mg, 1.32 mmol), CuI (50 mg, 0.25 mmol), K2CO3 (690 mg, 5 mmol) and 18-crown-6-ether (66 mg, 0.25mmol) in 5 ml

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1,2-dichlorobenzene (o-DCB) was heated at 160 °C under N2 atmosphere for 2 days. After cooled to room temperature, the reaction was quenched with ammonium hydroxide and water, extracted with CH2Cl2 and dried over anhydrous Na2SO4. The crude product was purified by

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column chromatography on silica gel using 3:2 (v:v) petroleum ether/CH2Cl2 as eluent to afford

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CzDPPy as a white crystalline solid. Yield: 174 mg, 60%. 1H NMR (400 MHz, CDCl3, δ): 6.76 (d, J = 8.0 Hz, 4H, carbazole), 6.31-6.18 (m, 8H), 6.00-5.89 (m, 8H), 5.80 (d, J = 8.0 Hz, 4H), 5.30 (t, J = 8.0 Hz, 1H, pyridine); 5.09 (d, J = 8.0 Hz, 2H, pyridine); 13C NMR (100 MHz, CDCl3, δ): 155.51, 141.00, 139.20, 134.77, 129.77, 128.75, 125.78, 123.11, 120.85, 120.01, 119.56, 110.07; Anal. Calcd. for

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C41H27N3 (%): C, 87.67; H, 4.85; N, 7.48. Found: C, 88.06; H, 4.57; N, 7.39; ESI m/z Calcd. [M]+: 561.2, Found: [M+1]+ 562.3.

Synthesis of 2,6-bis(2-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)pyridine (tCzDPPy): The

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procedure was similar to the synthesis of CzDPPy except that the material was

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3,6-di-tert-butyl-9H-carbazole (363 mg, 1.30 mmol) instead of carbazole. tCzDPPy was isolated as a white crystalline solid. Yield: 265 mg, 65%. 1H NMR (400 MHz, CDCl3, δ): 7.15 (s, 4H, carbazole), 6.77-6.74 (m, 2H), 6.66-6.60 (m, 4H), 6.54-6.52 (m, 2H), 6.42-6.39 (m, 4H), 6.13 (d, J = 8.0 Hz, 4H), 5.77 (t, J = 8.0 Hz, 1H, pyridine), 5.59 (d, J = 8.0 Hz, 2H, pyridine), 0.54 (s, 36H, t-butyl); 13C NMR (100 MHz, CDCl3, δ): 142.33, 139.64, 135.52, 131.78, 129.58, 129.22, 128.28, 123.36, 123.12, 120.98, 115.83, 109.65, 34.63 (C(CR)4), 32.00 (CH3); Anal. Calcd. for C57H59N3 (%): C, 87.08; H, 7.57; N, 5.35. Found: C, 87.35; H, 7.27; N, 5.80; ESI m/z Calcd. [M]+: 786.1, Found: [M+1]+ 787.0.

ACCEPTED MANUSCRIPT Quantum calculations: All the calculations were performed by Gaussian09 package at the level of density functional theory (DFT) [40]. Both CzDPPy and tCzDPPy were optimized by the B3PW91 hybrid functional [41-43], with the 6-311+G* basis set [44]. The solvent effect was with

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self-consistent reactions field (SCRF) based on the SMD model [45], in which the solvent model was set as tetrahydrofuran. The time-dependent density functional theory (TD-DFT) calculations based on the optimized ground state geometries of both CzDPPy and tCzDPPy were performed

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with the same calculation level as optimization.

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OLED fabrication and test: Indium tin oxide (ITO) coated glasses with a sheet resistance of 15 ohms per square was used as substrates. Before device fabrication, the ITO glass substrates were carefully pretreated by washing with isopropyl alcohol and deionized water, dried in an oven at 120 oC over 1 hour and then treated with ultraviolet-ozone for 25 min before loaded into a

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vacuum deposition chamber with a base vacuum greater than 10-6 Torr. Organic layers were successively deposited on the ITO glass substrates at a rate of 1-2 Å s-1. The hole-/electron-injecting layer of MoO3/LiF and the cathode of Al were deposited at the rates of

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0.1 and 10 Å s-1, respectively. The electroluminescence (EL) spectra, CIE coordinates, and

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luminance characteristics were all measured using a Spectrascan PR655 photometer under an ambient atmosphere. The current density–voltage (J–V) characteristic was measured using a computer-controlled Keithley 2400 sourcemeter.

3. RESULTS AND DISCUSSION 3.1 Synthesis and characterization As shown in Scheme 1, the two host materials of CzDPPy and tCzDPPy could be realized by the Ullmann-coupling

reaction

between

2,6-bis(2-bromophenyl)pyridine

[39]

and

ACCEPTED MANUSCRIPT carbazole/3,6-di-tert-butyl-carbazole with the satisfactory yields of 60% and 65%, respectively. Both two compounds were fully characterized by 1H and

13

C NMR spectroscopy, mass

spectrometry and elemental analysis (see details in Experimental section). The structure of

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CzDPPy was further selected to perform the single crystal X-ray diffraction analysis, which exhibited monoclinic with the space group of C12/c1 (Z=8). As shown in Figure 1, due to the steric hindrance of carbazole moieties at the ortho positions of 2,6-diphenylpyridine, the

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molecular structure of CzDPPy expectably adapted highly twisted conformation, as clearly

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confirmed by the high torsion angle (141o) of C4-C5-C24-C25.

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Scheme 1. Synthetic route for CzDPPy and tCzDPPy.

Figure 1. Oak ridge thermal ellipsoidal plot (ORTEP) diagram of CzDPPy.

3.2 Thermal properties

ACCEPTED MANUSCRIPT Both CzDPPy and tCzDPPy exhibited satisfactory thermal stability, as manifested by their high thermal decomposition temperatures (Td, corresponding to 5% weight loss in the thermogravimetric analysis) of 300 °C for CzDPPy and 290 °C for tCzDPPy respectively, as

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depicted in Figure 2. Their glass transition temperatures (Tg) appeared at 103 °C for CzDPPy and 149 °C for tCzDPPy in the differential scanning calorimetry (DSC) thermograms (inset of Figure 2). The high Tg values were highly important to form morphologically stable and uniformly

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amorphous films upon thermal evaporation. 120 o

80

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100

CzDPPy tCzDPPy

60 40

DSC endo

Weight loss (%)

CzDPPy tCzDPPy

290 C o 300 C

20

o

103 C

o

149 C

0 40

80

100

100 120 140 o 160 180 200 220

Temperature ( C)

200

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0

60

300

o 400

500

Temperature ( C)

600

Figure 2. TGA and DSC (inset) thermograms for CzDPPy and tCzDPPy.

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3.3 Photophysical properties

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CzDPPy tCzDPPy

PL 1.0

CzDPPy tCzDPPy

0.8

0.6

0.4

0.2

UV-Vis

Normalized Intensity (a.u.)

Normalized Intensity (a.u.)

1.0

0.8 0.6 0.4 0.2 0.0

400

450

500

550

Wavelength (nm)

600

650

0.0 300 350 400 450 500 550 600 650 700 750 800 850

Wavelength (nm)

Figure 3. UV-Vis absorption and PL spectra of CzDPPy and tCzDPPy in CH2Cl2 solution at room temperature (10 M), and their phosphorescence spectra in 2-MeTHF solution at 77 K (inset).

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ACCEPTED MANUSCRIPT The absorption and photoluminescence (PL) spectra of the two host materials tested in CH2Cl2 solution at room temperature were shown in Figure 3. CzDPPy showed typical absorption peaks at 291, 325 and 338 nm, while tCzDPPy showed similar and red-shifted absorption peaks at 295,

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330 and 344 nm, respectively. Meanwhile, CzDPPy and tCzDPPy exhibited blue emissions at 406 nm and 423 nm in CH2Cl2 solution. The slight red shift of tCzDPPy relative to CzDPPy in both absorptions and emissions can be largely attributed to the electron-donating tert-butyl moiety

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[46]. The phosphorescence spectra of CzDPPy and tCzDPPy measured from the frozen

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2-methyltetrahydrofuran (2-MeTHF) matrix at 77 K were depicted in the inset of Figure 3, where their triplet states were probably more charge-transfer character than locally-excited character. Regardless, their triplet energies (ET) were determined to be ca. 2.67 eV for CzDPPy and 2.64 eV for tCzDPPy respectively by using the highest-energy peaks (465 and 470 nm) from the

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3.4 DFT calculations

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phosphorescence spectra.

Figure 4. The calculated energy levels, orbital spatial distributions, transition energy (λ) and oscillator strength (f) for CzDPPy and tCzDPPy, respectively.

ACCEPTED MANUSCRIPT The geometrical structures of CzDPPy and tCzDPPy were optimized by the calculations of DFT using B3PW91 hybrid functional (see Experimental section). Similar with the crystal information mentioned above, both two molecules were highly twisted after optimization due to the bulky

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carbazole moieties at the ortho positions, as depicted in Figure 4. Expectably, the HOMO levels of the two molecules were mainly distributed on the carbazole moiety while the LUMO levels were largely located on the diphenylpyridine moiety. The separation between HOMO and LUMO

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levels might be favorable to the efficient hole- and electron-transporting behaviors. However,

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the absorption spectra based on the TD-DFT calculations revealed that the S1 states for both CzDPPy and tCzDPPy were referred to the transitions from HOMO-1 to LUMO (Figure 4). And the limited spatial overlap in the S1 state between HOMO-1 and LUMO on one ortho-substituted benzene ring could lead to the small transition dipole moment and consequently very small

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oscillator strength (f = 0.0316 and 0.0314 for CzDPPy and tCzDPPy, respectively). Meanwhile, the HOMO-to-LUMO transitions for both CzDPPy and tCzDPPy were mainly located on the S2 and S3 states also with theoretically low intensities (f < 0.05). Therefore, the onsets of the last

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absorptions in Figure 3 may not clearly reflect the HOMO-LUMO transitions for their complete

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spatial separation and very weak oscillator strength [47]. Alternatively, the intersection points (350 and 363 nm) between their absorption and fluorescence spectra in solution were employed to estimate their optical energy gaps (HOMO-LUMO gaps, Eg) [48], where the calculated Eg values were 3.55 and 3.42 eV for CzDPPy and tCzDPPy respectively. On the other hand, among their excited states, the HOMO-1-to-LUMO+2 and HOMO-to-LUMO+3 transitions on the carbazole group possessed comparatively high oscillator strength values for both CzDPPy (f = 0.1119 and 0.0760) and tCzDPPy (f = 0.0562 and 0.0886), and these transitions represented the

ACCEPTED MANUSCRIPT locally-excited states. Due to the electron-donating effect of the tert-butyl groups, the HOMO and HOMO-1 levels of tCzDPPy were elevated compared to CzDPPy, and so the absorptions of tCzDPPy were red-shifted in comparison with CzDPPy.

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3.5 Electrochemical properties

0.0

0.2

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Current density (a.u.)

CzDPPy tCzDPPy

0.4

0.6

0.8

1.0

1.2

1.4

Potential vs. Ag/AgCl

Figure 5. Oxidation behaviours of CzDPPy and tCzDPPy in CV measurements.

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The electrochemical behaviors of CzDPPy and tCzDPPy were examined by cyclic voltammetry (CV) in CH2Cl2 solution. As shown in Figure 5, CzDPPy exhibited irreversible oxidation process

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while tCzDPPy showed quasi-reversible oxidation process, and their onsets of oxidation potentials were 1.21 and 1.09 V (vs. Ag/AgCl), respectively. The irreversible oxidation behavior of

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CzDPPy was probably ascribed to the electrochemically-active sites in the 3-positions of carbazole [49]. Consequently, their HOMO levels determined from the onset potentials were -5.52 eV for CzDPPy and -5.40 eV for tCzDPPy (relative to vacuum energy level). Compared to CzDPPy, the higher HOMO energy level of tCzDPPy was due to the electron-donating ability of tert-butyl moiety. Deduced from the HOMO energy levels and the energy gaps (Eg), their LUMO levels were -1.97 eV for CzDPPy and -1.98 eV for tCzDPPy, respectively [50]. All the data were summarized in Table 1.

ACCEPTED MANUSCRIPT Table 1. Thermal and opt-electronic properties for CzDPPy and tCzDPPy

Tg/Td ( C) λabs, max (nm) a λF, max (nm) b λP, max (nm) c HOMO (eV) d LUMO (eV) e ET (eV) f

CzDPPy

tCzDPPy

103/290 291, 325, 338 406 465 -5.52 (-5.76) -1.97 (-1.45) 2.67

149/300 295, 330, 344 423 470 -5.40 (-5.52) -1.98 (-1.37) 2.64

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o

a Absorptions measured in CH2Cl2 solution at room temperature; b Fluorescence peaks measured in CH2Cl2 solution at room temperature; c Phosphorescence peaks measured in 2-MeTHF matrix at 77 K; d Deduced from the onsets of oxidation potentials; e Deduced from HOMO and Eg; f Triplet energy values estimated from

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phosphorescence peaks. Calculation results were shown in parentheses.

3.6 Sky-blue TADF OLEDs hole for CzDPPy hole for tCzDPPy electron for CzDPPy electron for tCzDPPy

500 400 300 200 100 0

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Current density (mA/cm

2

)

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600

0

2

4

6

8 10 12 Voltage (V)

14

16

18

respectively.

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Figure 6. The current density versus voltage curves of the hole- and electron-only devices for CzDPPy and tCzDPPy,

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Before the fabrication of TADF OLEDs, the bipolar carrier-transporting properties of CzDPPy and tCzDPPy were initially investigated by fabricating the hole-only devices with the structure of ITO/MoO3 (10 nm)/TAPC (10 nm)/Hosts (30 nm)/TAPC (10 nm)/MoO3 (10 nm)/Al, and the electron-only devices with the structure of ITO/LiF (1 nm)/TmPyPB (10 nm)/Hosts (30 nm)/TmPyPB (10 nm)/LiF (1 nm)/Al. In these devices, MoO3 and LiF served as the hole- and electron-injecting materials; TAPC (1,1-bis[(di-4-tolylamino)phenyl]cyclohexane) and TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene)) were used to prevent electron or hole injecting from the

ACCEPTED MANUSCRIPT cathode and anode, respectively. The current density versus voltage curves of the hole- and electron-only devices for CzDPPy and tCzDPPy were depicted in Figure 6. Encouragingly, both two host materials exhibited bipolar carrier-transporting abilities and could be confirmed by the

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distinctive observations of hole-transporting and electron-transporting currents, although their hole-transporting abilities were expectably dominating. Notably, it seemed that tCzDPPy possessed more balanced bipolar carrier-transporting property than CzDPPy.

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Subsequently, to evaluate the applicability of CzDPPy and tCzDPPy as the host materials for

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sky-blue TADF emitters, we fabricated the resulting TADF OLEDs with the following configuration: ITO/MoO3 (1 nm)/TAPC (40 nm)/TCTA (5 nm)/CzDPPy (Device A) or tCzDPPy (Device B): 2CzPN (5%, 20 nm)/DPEPO (5 nm)/TmPyPB (35 nm)/LiF (1 nm)/Al (100nm). In these devices, TAPC was used as the hole-transporting material; 4,4’,4’’-tri(N-carbazolyl)triphenylamine (TCTA) and

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bis(2-(diphenylphosphino)phenyl)ether oxide (DPEPO) were used to confine excitons inside the emitting layer (EML); TmPyPB was used as electron-transporting material as well as hole-blocking material; the sky-blue TADF emitter of 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene (2CzPN) was

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used as the emitting material with the optimized doping level of 5%. MoO3 and LiF served as the

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hole- and electron-injecting materials, respectively. The chemical structures and energy levels of the materials used in these devices were illustrated in Figure 7.

Figure 7. The TADF OLED configuration and the chemical structures of the used organic materials.

ACCEPTED MANUSCRIPT The current density-voltage-luminance (J-V-L) characteristics, current efficiency/power efficiency/external quantum efficiency versus luminance, and electroluminace spectra (EL) of the devices were shown in Figure 8. All the device data were summarized in Table 2. Both devices A

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and B exhibited turn-on voltages at 3.4 and 3.9 V respectively. Meanwhile, devices A and B showed typical EL emissions from the sky-blue emitter of 2CzPN with the peaks at 480/488 nm and the Commission Internationale de l’Eclairage (CIE) coordinates of (0.18, 0.34)/(0.22, 0.38),

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respectively, which could indicate the emissive excitons were well confined on the 2CzPN emitter.

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The slight difference for the EL spectra of CzDPPy and tCzDPPy under the same device configuration was probably caused by their different exciton recombination zone within the emitting layer, which was affected by the wide-angle optical interference of the metal cathode [51]. On the other hand, it was notable that both devices A and B exhibited relatively low

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luminance (less than 2000 cd m-2), and this situation was very similar to other 2CzPN-based OLEDs reported [36]. There may be two reasons for this low luminance of 2CzPN: i) The relatively-imbalanced electron transportation usually would lead to the less formation of the

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emissive excitons [37]; ii) The relatively-long lifetime of the singlet exciton for 2CzPN resulted

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from its low reverse intersystem crossing rate probably would cause the emission quenching at high current density [52].

Nevertheless, the satisfactory results were obtained for the device B hosted by tCzDPPy, with

max. CE of 28.9 cd A-1, PE of 23.3 lm W-1 and EQE of 11.7%; whereas the device A hosted by CzDPPy exhibited much better efficiencies of max. CE of 34.8 cd A-1, PE of 33.1 lm W-1 and EQE of 16.0%. Compared to CzDPPy, the inferior efficiencies for tCzDPPy were probably ascribed to its overmuch t-butyl groups, which may bring in its different molecular orientation in the emitting

ACCEPTED MANUSCRIPT film and the inefficient energy transfer to 2CzPN emitter [53], even tCzDPPy exhibited much better bipolar carrier-transporting ability. In addition, it was remarkable that the maximum efficiency values were obtained at low current density for both devices A and B. And they

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exhibited obvious efficiency decay at higher luminance. For instance, at practical brightness of 100 cd m-2, the CE/PE values were reduced to 13.5 cd A-1/9.9 lm W-1 for device A and 7.9 cd A-1/4.6 lm W-1 for device B. The fast efficiency decay may also be resulted from the inherent

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characteristics of the sky-blue emitter 2CzPN (such as low reverse intersystem crossing rate)

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and/or the imbalanced charge transportation of this device configuration [52]. Therefore, the further optimizations such as developing more efficient sky-blue TADF emitters and designing new device architectures are currently ongoing.

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Figure 8. (a) J-V-L characteristics of devices A and B; (b) Current efficiency and power efficiency versus luminance of devices A and B; (c) External quantum efficiency versus luminance of devices A and B; (d) Electroluminance spectra of devices A and B at 6 V.

ACCEPTED MANUSCRIPT Table 2. Summary of device performance for CzDPPy and tCzDPPy.

tCzDPPy

2CzPN A 3.4 1103 16.0 34.8 33.1 (0.18, 0.34)

2CzPN B 3.9 742 11.7 28.9 23.3 (0.22, 0.38)

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Emitter Device Vturn-on (V) Lmax (cd m-2) a EQEmax (%) b CEmax (cd A-1) c PEmax (lm W-1) d CIE (x, y) e

CzDPPy

a Maximum luminance; b Maximum external quantum efficiency; c Maximum current efficiency; d Maximum -2

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power efficiency; e Commission International de I’Eclairage coordinate at 100 cd m .

4. CONCLUSIONS

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In summary, two new host materials of CzDPPy and tCzDPPy were designed and synthesized for the sky-blue thermally activated delayed fluorescence emitters. The ortho linkage between carbazole and diphenylpyridine could bring in the highly twisted molecular conformation,

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ultimately leading to the minimized π conjugation and the high triplet energies of 2.67 eV for CzDPPy and 2.64 eV for tCzDPPy, respectively. In the fabricated TADF OLEDs by using 2CzPN as the sky-blue emitter, the device hosted by CzDPPy could realize the maximum current efficiency

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of 34.8 cd A-1, power efficiency of 33.1 lm W-1 and external quantum efficiency of 16.0%.

ACKNOWLEDGMENTS

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This work is supported by the National Science Foundation of China (No. 11234013, 21473022, 21673033, 51603028); the Startup Grant of UESTC (No. ZYGX2015KYQD058); the Science Fund for Distinguished Young Scholars of Sichuan Province (No. 2015JQ0006); the Fundamental Research Funds for the Central Universities (ZYGX2016Z010, ZYGX2015J048); and the University Grants Committee of HKSAR Area of Excellence Scheme (AoE/P-03/08).

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HIGHLIGHTS 1. Two new hosts were reported for sky-blue TADF OLEDs 2. The peak CE of 34.8 cd A-1, PE of 33.1 lm W-1 and EQE of 16.0% were

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realized 3. The satisfactory CIE coordinate of (0.18, 0.34) was obtained at 100 cd

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m-2