Universal Host Materials for High-Efficiency Phosphorescent and

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Universal Host Materials for High-Efficiency Phosphorescent and Delayed-Fluorescence OLEDs Wei Li, Jiuyan Li, Fang Wang, Zhuo Gao, and Shufen Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08291 • Publication Date (Web): 06 Nov 2015 Downloaded from http://pubs.acs.org on November 9, 2015

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ACS Applied Materials & Interfaces

Universal Host Materials for High-Efficiency Phosphorescent and Delayed-Fluorescence OLEDs

Wei Li, Jiuyan Li,* Fang Wang, Zhuo Gao, Shufen Zhang State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian, 116024, China. E-mail: [email protected]

ABSTRACT A series of bipolar hosts, namely 5-(2-(9H-carbazol-9-yl)-phenyl)-1,3-dipyrazolbenzene (o-CzDPz), 5-(3-(9H-carbazol-9-yl)-phenyl)-1,3-dipyrazolbenzene 5-(9-phenyl-9H-carbazol-3-yl)-1,3-dipyrazolbenzene

(m-CzDPz), (3-CzDPz)

and

5-(3,5-di(9H-carbazol-9-yl)-phenyl)-1,3-dipyrazolbenzene (mCPDPz) are developed for phosphorescent and thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). They are designed by selecting pyrazole as n-type unit and carbazole as p-type one. The triplet energy (ET), the frontier molecular orbital level, and charge transporting abilities, are adjusted by varying the molar ratio of pyrazole to carbazole and the linking mode between them. They have high ET values of 2.76-3.02 eV. Their electroluminescence performance is evaluated by fabricating both phosphorescent and TADF devices with blue or green emitters. The m-CzDPz hosted blue phosphorescent OLEDs achieves high efficiency of 48.3 cd A-1 (26.8%), the 3-CzDPz hosted green phosphorescent device exhibits 91.2 cd A-1 (29.0%). The blue and green TADF devices with 3-CzDPz host also reach high efficiencies of 26.2 cd A-1 (15.8%) and 41.1 cd A-1 (13.3%), respectively. The excellent performance of all these OLEDs verifies that these pyrazole-based bipolar compounds are capable of being universal host materials for OLED application. The influence of molar ratio of n-type unit to p-type one and the molecular

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conformation of these hosts on their device performance is discussed and interpreted. Keywords: phosphorescent organic light-emitting diodes (OLEDs), thermally activated delayed fluorescence (TADF), bipolar host, universal host materials, pyrazole

INTRODUCTION Recently, phosphorescence organic light emitting diodes (PhOLEDs) and thermally activated delayed fluorescence (TADF) OLEDs have attracted extensive research attention.1-7 since they can reach a theoretical maximum internal quantum efficiency of unity by utilizing both singlet and triplet excitons for light emission.6,8 It is well established that the singlet and triplet excitons generated by charge recombination in OLEDs have the molar ratio of 1:3. For phosphorescent emitters that are typically transition metal complexes, the singlet excitons can be easily converted via intersystem crossing into triplet excitons, and then all the triplet excitons, despite the formation pathway, are possible to emit phosphorescence due to the strong spin-orbital coupling effect of the central metal.8 While for the TADF emitters that are usually noble-metal-free organic molecules, the intrinsically non-emissive triplet excitons can be readily converted into singlet ones via reverse intersystem crossing due to extremely small singlet-triplet energy difference (∆EST), then all the singlet excitons are possible to emit either prompt fluorescence or delayed fluorescence. In comparison with the traditional fluorescent emitters that can only utilize the singlet excitons emitting fluorescence to reach a maximum internal quantum efficiency of 25%, the phosphorescent and TADF emitters are absolutely advantageous in terms of the principal internal quantum efficiencies of 100% and the resultant external quantum efficiencies (EQE) of 20-30%.9,10 Evidently, triplet excitons are essentially important for both phosphorescent and TADF emitters during the light emission process. In order to avoid any unwanted quenching pathway for triplet excitons in neat films, the phosphors and TADF emitters are usually doped in certain host matrixs at a certain concentration to fabricate OLEDs.11,12 Therefore, the host materials are quite important as well to determine the overall performance of the whole devices since the host materials usually occupy the


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major part of the emitting layer. In principle, an ideal host material should possess significantly high triplet energy, appropriate highest occupied orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels, bipolar charge transporting feature, sufficient spectral overlap with the doped emitter, and so on.13 Following these requirements, a lot of excellent host materials have been developed and many high-performance phosphorescent and TADF devices have been obtained.14-16 However, there have been rare reports on the universal host materials that are not only suitable for both phosphorescent and TADF emitters, but also capable of acting as hosts for wide color range of emitters.12,17,18 The universal host materials would be most beneficial especially when both phosphorescent and TADF dopants of various emitting colors are incorporated into one device to achieve white light emission.

N N B(OH)2

N

ii

N

N N

o-CzDPz

A1 B(OH)2

N N

N

ii N

Br i 2 N

N H

m-CzDPz

A2

N N

+ Br

Br Br

N N

B(OH)2 N N

ii

N N

N

1 N

N N

3-CzDPz

A3

B(OH)2 N

N

i: CuI / 1,10-phenanthroline, K2CO3/DMF/refluex ii: Pd(PPh3)4, K2CO3, Toluene/Ethanol

N N

N

N N

N

ii

A4

mCPDPz

Scheme 1 Molecular structures and syntheses of o-CzDPz, m-CzDPz, 3-CzDPz and mCPDPz. It has been well established that the balanced charge transportation in the emitting layer is one of the


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most important factors to contribute to a high emission efficiency of OLEDs. In general, bipolar host materials that contain both hole transporting and electron transporting moieties are utilized to facilitate charge balance and to enhance emission efficiency and to reduce efficiency roll-off.19,20 However, it is well known that the electron transporting mobilities of most of the concurrent organic semiconductors are lower by orders of magnitude than the hole mobilities of many hole transporting molecules.13 Therefore, to find a suitable n-type functional unit to match the present p-type groups is always a challenging work in the process of molecular design of bipolar host materials.5 In this report, we design novel bipolar host materials with the emphasis to facilitate the charge balance ability from the following two aspects. One is to select parazole as n-type unit to match the p-type carbazole to form the bipolar molecules. Recently, our group reported using pyrazole as n-type moiety to construct bipolar phosphorescent host material named CPMP for OLEDs application.21 Compared with the unipolar (hole-type) analogue, the pyrazole-containting CPMP show bipolar charge transporting character and finally led to much improved EL performance. The other is to adjust the molar ratio of n-type unit to p-type group with expectation to further tune the positive and negative charges balance state of the resultant host mterials. It is reasonable to expect that the electron mobility and thus the bipolar feature of host materials can be enhanced by introducing more n-type units.22-24 In this way, three novel pyrazole-containing isomers with the pyrazole to carbazole molar ratio of 2:1, named o-CzDPz, m-CzDPz and 3-CzDPz, were designed and prepared. The linking mode between pyrazole and carbazole groups in these molecules are varied to tune the steric conformation and conjugation state and thus to tune the physical properties including the triplet energy and charge transporting ability of the neat films. In order to verify the influence of increased molar ratio of n-type to p-type units on the EL performance of the bipolar hosts, a control compound with equal numbers of pyrazole and carbazole groups, namely mCPDPz, was designed and synthesized (Scheme 1). All these compounds exhibit high triplet energies of 2.76∼3.02 eV, which are high enough to guarantee their capability to act as hosts for all RGB emitters. They were used as hosts to fabricate phosphorescent and TADF OLEDs with the traditional


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iridium phosphors and TADF molecules as doped emitters, respectively, and excellent performance was obtained. For example, the m-CzDPz hosted blue PhOLED and 3-CzDPz hosted green PhOLED exhibited high external quantum efficiencies of 26.8% and 29.0%, the 3-CzDPz hosted blue and green TADF devices also achieved high efficiencies of 15.8% and 13.3 %, indicating these pyrazole-containing compounds are capable of acting as universal hosts to fabricate high-performance OLEDs.

EXPERIMENTAL SECTION General information. A 500 MHz and 126 MHz Varian Unity Inova spectrophotometer was used to record the 1H NMR and13C NMR spectra, and a HP1100LC/MSD MS spectrometer was used to record mass spectra. Thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) studies were performed using Perkin-Elmer thermogravimeter (Model TGA7) and a Netzsch DSC 201, at a heating rate of 10 C min-1 under a nitrogen atmosphere, respectively. The fluorescence and UV-vis absorption spectra of both solutions and thin films were measured on a Perkin-Elmer LS55 spectrometer and a Perkin-Elmer Lambda 35 spectrophotometer at room temperature, respectively, while the phosphorescence spectra were obtained on an Edinburgh FLS920 Spectrometer at 77 K in 2-MeTHF. The cyclic voltammetry properties were studied on an electrochemical workstation (BAS100B, USA). A conventional three electrode configuration including a glass carbon working electrode, a Pt-wire counter electrode, and a saturated calomel electrode (SCE) reference electrode were used. Upon fully deoxygenated with argon, the sample solution in dichloromethane containing 0.1 M [Bu4N]PF6 as electrolyte was scanned at a rate of 100 mV s-1 at room temperature. Density functional theory (DFT) calculations using B3LYP functional were performed with Gaussian 03. The basis set used for the C, H, N atoms was 6-31G. There are no imaginary frequencies for both the optimized structures. OLED fabrication and measurements. The ITO glass substrates with a sheet resistance of 30 Ω per square were first cleaned, dried, and treated by UV-ozone for 20 min, on which a 40 nm thick


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PEDOT:PSS film was deposited and baked at 120 ℃ for 30 min in air. Subsequently, the substrate was transferred into a vacuum chamber to deposit the organic layers with a base pressure less than 10-6 Torr (1 Torr = 133.32 Pa). Finally, a thin layer of LiF (1 nm) was thermally deposited onto organic layers to facilitate electron injection and then an Al film (200 nm) was deposited as the cathode. The emitting area of each pixel was determined by effective overlapping of two electrodes as 9 mm2. The performance parameters including EL spectra, CIE coordinates and J-V-B curves of the devices were measured using a program-controlled PR705 photometer and a source-measure-unit Keithley 236 under ambient conditions at room temperature. The forward viewing external quantum efficiency (ηext) was calculated by using the current efficiency, EL spectra and human photopic sensitivity. Syntheses. The starting material, arylboronic acid including (2-(9H-carbazol-9-yl) phenyl)boronic acid (A1), (3-(9H-carbazol-9-yl)phenyl)boronic acid (A2), and (3,6-di(9H-carbazol-9-yl)phenyl)boronic acid (A4), were synthesized according to the literature method.25 9-phenyl-9H-carbazol-3-yl-3-boronic acid (A3) was purchased from J&K chemical and was used without further purification. Synthesis of 3,5-dipyrazolyl-1-bromobenzene (1): A mixture of CuI (191 mg, 1 mmol), 1,10-phenanthroline (400 mg, 2 mmol), and DMF (20 mL) was refluxed for 5 min at 120 ℃ and then cooled to room temperature. Then pyrazole (748 mg, 11 mmol), 1,3,5-tribromobenzene (1.57 g, 5 mmol), and K2CO3 (2.76 g, 20 mmol) was added to the above system and the mixture was refluxed for 24 h. Upon removal of the inorganic salts by filtering and the solvent by evaporation under reduced pressure, the residue was isolated by column chromatography on silica gel with petroleum ether/ethyl acetate (4:1) as the eluent to give compound 1 (720 mg, 2.5 mmol, 50 % yield) as a white solid. 1H NMR (500 MHz, DMSO) δ 8.93 (d, J = 7.3 Hz, 2H), 8.35 (d, J = 7.3 Hz, 1H), 8.14 (s, 1H), 8.00 (d, J = 7.3 Hz, 1H), 7.91 (d, J = 7.3 Hz, 2H), 6.61 (s, 2H). TOF-EI-MS (m/z): 288.0012 [M]+. General procedure for synthesis of 5-(2-(9H-carbazol-9-yl)-phenyl)-1,3-dipyrazol benzene (o-CzDPz) and other final products: The deoxygenrated reaction mixture containing intermediate 1 (289 mg, 1 mmol), the corresponding boronic acid (2-(9H-carbazol-9-yl)phenylboronic acid for o-CzDPz,


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3-(9H-carbazol-9-yl)phenylboronic acid for m-CzDPz, 9-phenyl-9H-carbazol-3-yl-3-boronic acid for 3-CzDPz, and 3,6-di(9H-carbazol-9-yl)phenylboronic acid for mCPDPz) (1.1 mmol), toluene (10 mL), ethanol

(2

mL),

aqueous

sodium

carbonate

(2

M,

2.5

mL,

5

mmol),

and

tetrakis(triphenylphosphino)palladium(0) (57 mg, 0.05 mmol) was refluxed under nitrogen atmosphere overnight. Upon cooled to room temperature and diluted by water (20 mL), the reaction mixture was separated into organic layer and aqueous layer, the later of which was extracted with dichloromethane (3 × 20 mL). After the combined organic layers were washed with brine (50 mL), dried over anhydrous magnesium sulfate and filtered, the solvent was removed under reduced pressure. Then the residue was isolated by column chromatography over silica using petroleum ether/ethyl acetate (1:1) as eluent, and further purified by repeated recrystallization in methanol/chloroform to give pure products as white solids. o-CzDPz: Yield: 73%. 1H NMR (500 MHz, CDCl3) δ 8.06 (d, J = 7.7 Hz 2H), 7.84 (s, 1H), 7.77 (s, 1H), 7.70-7.64 (m, 2H), 7.62 (d, 2H), 7.61 (s, 1H), 7.34 (t, J = 7.7 Hz, 2H), 7.22 (t, J = 7.7 Hz, 4H), 7.12 (d, J = 7.7 Hz, 2H), 7.03 (s, 2H), 6.27 (s, 2H).

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C NMR (126 MHz, CDCl3) δ 141.48, 141.11,

140.66, 140.48, 139.76, 134.65, 131.38, 130.46, 129.81, 129.43, 126.75, 126.22, 122.85, 120.27, 119.85, 115.57, 109.82, 108.81, 107.70. TOF-EI-MS (m/z): 451.1799 [M]+. Anal.calcd for C30H21N5: C, 79.80; H, 4.69; N, 15.51; Found: C, 79.69; H, 4.67; N, 15.48. 5-(3-(9H-carbazol-9-yl)-phenyl)-1,3-dipyrazolbenzene (m-CzDPz): Yield: 77%. 1H NMR (500 MHz, CDCl3) 8.17 (d, J = 7.8 Hz, 2H), 8.09 (s, 1H), 8.06 (s, 2H), 7.90 (d, J = 7.8 Hz, 3H), 7.82 (d, J = 7.8 Hz 1H), 7.76 (s, 1H), 7.73 (t, J = 7.8 Hz, 2H), 7.63 (t, J = 7.8 Hz, 1H). 7.43 (q, J = 7.8 Hz, 4H) 7.31 (t, J = 7.8 Hz, 2H), 6.51 (s, 2H).

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C NMR (126 MHz, CDCl3) δ 142.98, 141.64, 141.62, 141.59, 140.90,

138.49, 130.57, 128.17, 127.04, 126.43, 126.08, 125.95, 123.46, 120.42, 120.10, 115.46, 109.72, 109.00, 108.24. TOF-EI-MS (m/z): 451.1800 [M]+. Anal.calcd for C30H21N5: C, 79.80; H, 4.69; N, 15.51; Found: C, 79.74; H, 4.71; N, 15.48. 5-(9-phenyl-9H-carbazol-3-yl)-1,3-dipyrazolbenzene (3-CzDPz): Yield: 78%. 1H NMR (500 MHz,


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CDCl3) δ 8.48 (s, 1H), 8.23 (d, J = 7.7 Hz, 1H), 8.12 (s, 2H), 8.03 (s, 1H), 8.00 (s, 2H), 7.81 (s, 2H), 7.75 (d, J = 8.1 Hz, 1H), 7.64 (t, J = 7.7 Hz, 2H). 7.60 (d, J = 7.7 Hz, 2H), 7.53-7.46 (m, 4H), 7.34 (t, J = 7.7 Hz, 1H), 6.54(s, 2H).

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C NMR (126 MHz, CDCl3) δ 144.87, 141.47, 141.44, 141.40, 140.88,

137.50, 131.70, 129.97, 127.66, 127.12, 127.08, 126.36, 125.41, 123.98, 123.34, 120.54, 120.30, 119.12, 115.67, 110.15, 110.01, 108.01, 107.96. TOF-EI-MS (m/z): 451.1802 [M]+. Anal. calcd for C30H21N5: C, 79.80; H, 4.69; N, 15.51; Found: C, 79.76; H, 4.68; N, 15.53. 5-(3,5-di(9H-carbazol-9-yl)-phenyl)-1,3-dipyrazolbenzene (mCPDPz): Yield: 66%. 1H NMR (500 MHz, CDCl3) 8.18 (d, J = 8.2 Hz,4H), 8.14 (s, 1H), 8.10-8.01 (m , 4H), 7.97 (s, 2H), 7.75 (s, 2H), 7.97 (s, 2H), 7.89 (s, 1H), 7.59 (d, J = 8.2 Hz, 4H), 7.47 (t, J = 7.4 Hz, 4H), 7.34 (t, J = 7.4 Hz, 4H),6.51(s, 2H). 13C NMR (126 MHz, CDCl3) δ 143.35, 142.07, 141.80, 141.67, 140.62, 140.13, 126.99, 126.32, 125.06, 124.66, 123.73, 120.56, 120.53, 115.39, 109.66, 109.41, 108.33. TOF-EI-MS (m/z): 616.2374 [M]+. Anal. calcd for C42H28N6: C, 81.80; H, 4.58; N, 13.63; Found: C, 81.85; H, 4.57; N, 13.58.

RESULTS AND DISCUSSION Synthesis and thermal properties. The synthetic procedure of these materials are shown in Scheme 1. All these four compounds are synthesized through a Suzuki cross-coupling reaction between 3,5-dipyrazolyl-1-bromobenzene (1) and the corresponding carbazole-containing boronic acid. The intermediate 1 was prepared by coupling of 1,3,5-tribromobenzene with pyrazole in the presence of cuprous iodide under the Ullmann reaction conditions. Except the 9-phenyl-9H-carbazol-3-yl-3-boronic acid (A3) that was commercial available and used as obtained, the carbazole-containing boronic acid intermediates A1, A2, and A4 were synthesized according to literature methods,5,25 as described in Scheme 1. The target compounds, o-CzDPz, m-CzDPz, 3-CzDPz and mCPDPz, were then prepared at yields of 66-77% through a Suzuki cross-coupling reaction between intermediates 1 and the corresponding boronic acid. All these compounds could be easily purified by column chromatography and recrystallization to reach a high purity for OLED applications. The thermal stability and morphological stability of these compounds were studied by ACS Paragon Plus Environment

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thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). As shown by the TGA thermograms (Figure S1 in supporting information), all these compounds exhibit high thermal decomposition temperatures (Td, corresponding to a 5% weight loss) in the range of 333∼434 ℃, indicating good thermal stabilities. As illustrated by the DSC traces in Figure 1, the glass transition was detected when the super-cooled isotropic sample of each compound was reheated for the second run and the glass transition temperature (Tg) regularly increases from 69.5, via 82.8, 89.4 to 139.5 ℃ for o-CzDPz, m-CzDPz, 3-CzDPz and mCPDPz (Table 1), in the order of reducing torsion resistance and increasing molecular weight. All these hosts exhibit higher Tg than the widely used host material 1,3-bis(9H-carbazol-9-yl)benzene (mCP) (Tg = 60 ℃),13 indicating improved amorphous stabilities.

69.5℃

Endothermic

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


82.8℃

o-CzDPz

89.4℃

m-CzDPz 3-CzDPz

139.5℃ mCPDPz

50

100 150 o Temperature ( C)

200

Figure 1. DSC traces (at the second heating cycle) of o-CzDPz, m-CzDPz, 3-CzDPz and mCPDPz. Photophysical Properties. Figure 2a presents the electronic absorption and photoluminescence (PL) spectra of these compounds in dilute CH2Cl2 solutions at room temperature. The absorption and PL spectra of their thin films are provided in Figure S2 in supporting information. The absorption maxima of o-CzDPz, m-CzDPz, 3-CzDPz and mCPDPz in dilute CH2Cl2 solutions were observed at 293, 293,288 and 292 nm, respectively, which are assigned to the π-π* absorption of the carbazole-centered units. The weak absorption in the range of 326 nm to 348 nm for o-CzDPz, m-CzDPz, and mCPDPz, which are typically observed in carbazole-containing molecules,4,5 can be attributed to n-π* transitions ACS Paragon Plus Environment

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of the lone-pair electrons to the entire conjugated backbone surrounding the carbazole unit. In contrast to these three molecules, the absorption of 3-CzDPz covering the range of 280 nm to 350 nm is much stronger. We suggest that this should be because the π-conjugation along the phenyl ring and the benzene ring of the carbazole moiety is enhanced in this molecule so that the π-π* transition energy is reduced to lower than that of n-π* transition energy and dominates the absorption spectrum. This deduction can be proved by the theoretical calculation result in following section. For these four compounds, the absorption spectra profile and position in thin films (Figure S2) are similar to those in dilute solutions, indicating the absence of significant intermolecular interactions in the ground state in solid state. Their optical energy bandgaps are determined by absorption edge technique as 3.53, 3.50, 3.40 and 3.52 eV, respectively.26 Upon optical excitation at the absorption maxima, o-CzDPz, m-CzDPz, 3-CzDPz and mCPDPz emit purple-blue fluorescence with emission peaks at 397, 403, 380 and 407 nm, respectively. The small red shift of 6 nm from o-CzDPz to m-CzDPz is understandable since the reduced steric hindrance and less twisted molecular backbone for m-CzDPz will definitely lead to lower-energy S1 excited state. It should be noted that the emission maximum of 3-CzDPz shows relatively large blue shift of about 20 nm in comparison with the other three molecules. (a)

0.75 0.50

0.50

0.25

0.25

0.00

0.00

Intensity (a.u.)

o-CzDPz 1.00 m-CzDPz 3-CzDPz mCPDPz 0.75

PL Intensity (a.u.)

(b)

1.00

Absorbance (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

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o-CzDPz m-CzDPz 3-CzDPz mCPDPz

1.0 0.8 0.6 0.4 0.2 0.0

250

300

350 400 450 Wavelength (nm)

500

550

300

350

400

450

500

550

Wavelength (nm)

Figure 2 (a) UV-vis absorption and PL spectra of o-CzDPz, m-CzDPz, 3-CzDPz and mCPDPz in dilute CH2Cl2 solutions at 293 K. (b) LT PL spectra of o-CzDPz, m-CzDPz, 3-CzDPz and mCPDPz in frozen 2-methyltetrahydrofuran matrix at 77 K.


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The low-temperature (LT) PL spectra were measured in a frozen 2-methyltetrahydrofuran matrix at 77 K, as shown in Figure 2b. The LT fluorescence spectra below 400 nm for these compounds show well-dissolved vibronic structures and hypsochromic effect relative to the room temperature fluorescence in Figure 2a. The emission spanning from 410 nm to about 550 nm were assigned to the phosphorescence of these compounds. The triplet energies (ET) were estimated from the highest-energy vibronic sub-band of the phosphorescence spectra as 3.02 eV for o-CzDPz, 2.83 eV for m-CzDPz, 2.78 eV for 3-CzDPz and 2.76 eV for mCPDPz, respectively. It is obvious that the ET of o-CzDPz is larger than those of other three compounds. This should be because the strong steric hindrance and consequently the severe twisted configuration for o-CzDPz make carbazole, rather than the central biphenyl, stands for the longest conjugation of the molecule and the lowest triplet excited state (T1) is located on carbazole ring. This deduction can be verified by the experimental fact that o-CzDPz has similar phosphorescence bands with the reference molecules N-phenylcarbazole and carbazole (Figure S3). It is also observed from density functional theory (DFT) calculation23,24 that the spin density distribution of T1 state for o-CzDPz is mainly on carbazole ring (Figure S4), further confirming that carbazole ring controls its lowest triplet excited state. While for m-CzDPz, the spin density distribution of T1 state is mainly on carbazole ring, but with small contribution from the adjacent phenyl ring due to less twisted configuration, which in principle stabilizes the T1 state and leads to lower T1 energy. The spin density distribution of T1 state for 3-CzDPz is on the central phenyl ring and the benzene ring of the carbazole moiety due to their possible π-conjugation, and that of mCPDPz is on the dicarbazolephenyl part, leading to the similar T1 energies to m-CzDPz.23,24 The ET values of all these four compounds are higher than the typical excited state energy (2.75 eV for 450 nm) of blue emitters, so that efficient forward energy transfer from host to dopant can be expected and they are capable of acting as hosts for all RGB emitters. Electrochemical properties and theoretical calculations. Cyclic voltammetry measurements were used to study the redox activity and thus the frontier molecular orbital levels for these compounds. The


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cyclic voltammograms are shown in Figure 3. In the anodic scan, all these compounds underwent a reversible oxidation process, which can be assigned to the oxidation of their common carbazole groups. The HOMO energies were determined from the onset potential of the first oxidation wave ( E oxonset ) according to the equation of EHOMO = -e( E oxonset + 4.4)4,5,27 as ca. -5.69 to -5.61 eV, respectively. The HOMO levels of these compounds are close to that of the widely used hole transport material 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC, 5.50 eV),5 indicating small hole injection barrier from TAPC to these material layer. One additional reduction peak was observed, e.g. at 0.98 V for o-CzDPz, 0.80 V for m-CzDPz, 0.85 V for 3-CzDPz and 0.91 V for mCPDPz, in the reduction process. For non-protected carbazole derivatives at 3,6-sites, this additional reduction peak was frequently observed in previous reports due to the instability of radical cations of carbazole moiety.5,28,29 No reduction wave was detected for all these compounds during the cathodic scan to -2.0 V. The LUMO energies were calculated from the HOMO energies and the optical band gaps (Eg), which are about -2.13 eV to -2.21 eV. o-CzDPz m-CzDPz 3-CzDPz mCPDPz

Current (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

Page 12 of 31

2000

1500

1000

500

0

Potential ( mV vs SCE)

Figure 3. Cyclic voltammograms of o-CzDPz, m-CzDPz, 3-CzDPz and mCPDPz in CH2Cl2 solutions.


12

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Figure 4. HOMO and LUMO distribution and geometry optimized structures of o-CzDPz, m-CzDPz, 3-CzDPz and mCPDPz.

Figure 4 depicts the optimized geometries and HOMO and LUMO distribution of these molecules, which were obtained by density functional theory (DFT) calculations using B3LYP hybrid functional theory with Gaussian 03.30 The HOMO orbitals are mainly located on carbazole moiety, with a small contribution from the adjacent phenyl ring, while the LUMO orbitals are mainly distributed in the electron-accepting dipyrazolephenyl moiety. Therefore, the complete spatial separation of HOMO and LUMO on p-type and n-type units indicates these dipyrazolphenyl-containing hosts are characterized by bipolar charge transporting properties.13,19,20 In addition, analyzing the steric molecular geometry helps to understand the difference in physical parameters for these molecules. The large steric hindrance in ortho-linked o-CzDPz definitely leads to severe twisted configuration with large dihedral angles of 86.4° between carbazole and the neighbouring phenyl ring and 71.5° between the two adjacent phenyl ACS Paragon Plus Environment

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Page 14 of 31

rings (Figure 4). Each aromatic ring is almost perpendicular to its neighbouring group in o-CzDPz, excluding the possibility of π-conjugation between any two adjacent groups. As a result, carbazole stands for the longest conjugated part in the whole o-CzDPz molecule and o-CzDPz has the same ET as the reference compound N-phenylcarbazole, as proved by Figure S3 and S4. For the meta-linked m-CzDPz, the steric hindrance is reduced in comparison with the isomer o-CzDPz. The molecule is less twisted with two dihedral angles of 55.2° and 35.9° (Figure 4) and the π-conjugation effect between any two neighbouring groups is enhanced. Although T1 state of m-CzDPz is controlled by the carbazolyl group (Figures S4), the ET is reduced to 2.83 eV due to small contribution form the adjacent phenyl ring due to improved π-conjugation. When carbazole is linked through its 3-site C atom to the dipyrazolephenyl moiety, the 3-CzDPz molecule seems to have the biphenyl backbone like in isomer m-CzDPz, with the dihedral angle of 35.2° between two neighbouring phenyl rings and similar ET of 2.78 eV. It is interesting to find that the m-CzDPz, 3-CzDPz, and mCPDPz have similar dihedral angles (Figure 4) to each other so that they have similar ET energies. Table 1. Physical data of o-CzDPz, m-CzDPz, 3-CzDPz and mCPDPz.

Compound

λ max em

λabs [nm]

a

ET

[nm] a

(eV)

HOMO

Eg b

(eV)

c

/LUMO (eV) d

Td (℃)

Tg e

(℃)

o-CzDPz

293,327,340

397

3.02

3.53

-5.69/-2.16

333

69.5

m-CzDPz

293,326,341

403

2.83

3.50

-5.63/-2.13

350

82.8

3-CzDPz

288,348

380

2.78

3.40

-5.61/-2.21

378

89.4

mCPDPz

292,326,340

407

2.76

3.52

-5.67/-2.15

434

139.5

a

Absorption and fluorescence wavelengths in dilute dichloromethane solutions.

b

Measured in

2-Me-THF at 77 K; c The optical band gap, calculated by the absorption edge technique. d Determined using electrochemical potentials and optical band gaps. e Td is the thermal decomposition temperature corresponding to 5% weight loss. Carrier-transport properties. In order to understand the contribution of pyrazole blocks to the charge transporting activity, single carrier devices were fabricated using o-CzDPz, m-CzDPz, 3-CzDPz and mCPDPz as the major functional layer. The hole-only devices have a configuration of


14

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ITO/PEDOT:PSS (40 nm)/TAPC (5 nm)/host (100 nm)/TAPC (10 nm)/Al (100 nm), and the electron-only devices have the structure of ITO/TmPyPB (10 nm)/host (100 nm)/TmPyPB (5 nm)/LiF (1 nm)/Al (100 nm). Figure S5 illustrates the chemical structures of all related materials and the energy level diagram of devices.5,12,13 In hole-only devices, PEDOT:PSS and TAPC were used to facilitate hole injection from anode and the TAPC/Al interface was designed to prevent electron injection due to too large electron barrier of 2.3 eV. In electron-only devices, a thin layer of TmPyPB was inserted between ITO anode and the host layer to prevent hole injection due to too deep HOMO level (-6.68 eV) of TmPyPB and thus the large hole barrier (1.88 eV). As shown by the current density-voltage curves of hole-only devices in Figure 5a, 3-CzDPz exhibited higher hole current than its two isomers o-CzDPz and m-CzDPz under same driving voltage. At the same time, the electron current of the electron-only device of 3-CzDPz is also higher than those of two isomers o-CzDPz and m-CzDPz (Figure 5b). The higher current in both hole-only and electron-only devices for 3-CzDPz can get interpretation if its less twisted molecular conformation and possibly better molecular stacking order in neat film are taken into account. As can be seen in Figure 4, the dihedral angles of 3-CzDPz are only 55.3° and 35.2°, compared with dihedral angles of 55.2° and 35.9 for m-CzDPz, and 86.4° and 71.5° for o-CzDPz. Smaller dihedral angles means more coplanar configuration and more ordered molecular stacking, which definitely contribute to smooth and rapid charge transportation. Another possible reason for the higher currents of 3-CzDPz is that the carbazole unit linked at its 3-site C atom is more favorable for hole transportation than the case when it is linked at 9-site N atom. The similar phenomenum was observed in our previous report.5 With increasing the molar ratio of p-type carbazole to n-type pyrazole to 1:1 in mCPDPz, the mCPDPz based hole-only device exhibited much higher hole current than all three molecules o-CzDPz, m-CzDPz and 3-CzDPz. This implies that the hole current of the single carrier devices is directly correlated with the p-type group number in these host molecules. It is reasonable that the mCP based hole-only device gave much higher hole current than all these pyrazole-containing hosts since there is only p-type unit in this molecule. On the other hand, in the electron-only devices, with increasing the


15


molar ratio of n-type pyrazole to p-type carbazole, the o-CzDPz, m-CzDPz and 3-CzDPz devices exhibited much higher electron current than the mCPDPz based device (Figure 5b). When the n-type unit is absent in uni-polar mCP, its electron current is almost zero. Both the significant hole current and electron current in single carrier devices of these pyrazole-containing materials confirmed their bipolar charge-transporting characteristics. The single carrier device results combine to draw a conclusion that increasing the molar ratio of the n-type unit in the host molecule is an effective strategy to improve the electron transporting ability and to finally facilitate the positive and negative charge balance state of the host materials.

(a)

o-CzDPz m-CzDPz 3-CzDPz mCPDPz mCP

90 60 30 0 0

5

10 15 Voltage (V)

o-CzDPz m-CzDPz 3-CzDPz mCPDPz mCP

(b)

150

2

Current density (mA/cm )

120

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

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20

25

120 90 60 30 0 0

5

10 15 Voltage (V)

20

25

Figure 5 The current density versus voltage curves of the (a) hole-only and (b) electron-only devices for the compounds o-CzDPz, m-CzDPz, 3-CzDPz, mCPDPz and mCP. Electroluminescent devices. As these four novel host materials have sufficiently high ET, sky-blue PhOLEDs were first fabricated using the traditional sky-blue phosphor FIrpic as the doped emitter in these pyrazole-containing host materials. These blue phosphorescent devices (A1-A4) have the structures of ITO/PEDOT:PSS(40 mn)/TAPC(20 nm)/host:5wt% Firpic (20 nm)/TmPyPB(40 nm)/LiF(1 nm)/Al (200 nm). TAPC was used as the hole-transporting layer and TmPyPB as electron-transporting and hole-blocking layer (Figure S5), PEDOT:PSS and LiF as hole- and electron-injecting layers, respectively. In addition, TAPC and TmPyPB are deliberately inserted next to


16

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the emitting layer to suppress triplet excitons quenching of the doped iridium phosphor since the triplet energies of TAPC (2.87 eV)14 and TmPyPB (2.78 eV)14 are significantly higher than Firpic (2.65 eV).5 The current density-voltage-brightness (J-V-L) characteristics and efficiency curves are shown in Figure 6, and the EL data are summarized in Table 2. Among these four sky-blue phosphorescent devices, the m-CzDPz hosted device A2 achieved the best electroluminescence performance with a turn-on voltage (to deliver a brightness of 1 cd m-2) of 3.8 V, a maximum current efficiency (ηc, max) of 48.3 cd A-1, a maximum power efficiency (ηp, max) of 30.3 lm W-1, and a peak forward viewing external quantum efficiency (ηext, max) of 26.8%. Inspiringly, excellent performance was also realized in the o-CzDPz and 3-CzDPz hosted devices A1 and A3, with the maximum efficiencies of 46.8 cd A-1 (corresponding to 29.4 lm W-1 and 25.6%) and 40.8 cd A-1 (28.9 lm W-1 and 19.54%), respectively. However, with reducing the molar ratio of the n-type pyrazole to p-type carbazole unit of the host material, the mCPDPz hosted device A4 exhibited much lower performance, with maximum efficiencies of 35.7 cd A-1 (22.4 lm W-1 and 16.8%). We suggest that the superior performance of o-CzDPz, m-CzDPz and 3-CzDPz devices to that of mCPDPz should benefit from their higher molar ratio of n-type pyrazole units and thus more balanced positive and negative charge transportation in the emitting layer. Besides, the high PLQY of the doped o-CzDPz (97.9%), m-CzDPz (99.8%) and 3-CzDPz (85.1%) films with Firpic is also responsible for the remarkable performance of these blue devices (Table 2). Hong,31 Kido,32 Ma,22 Lee,9,33 Wong,34 and Kim35 individually reported high efficiencies of 46.3 cd A-1 (corresponding to ηext of 27.2%), 48.6 (21.8%), 49.4 cd A-1 (27.5%), 53.6 cd A-1 (30.1%), 53.1 lm W-1 (31.4%), 58.7 cd A-1 (26.4%), and 62.2 cd A-1 (29.5%, using exciplex-forming cohost) for Firpic-based blue PhOLEDs in recent years, which stand for the typical data for blue PhOLEDs so far. It is clear that the efficiencies (48.3 cd A-1 and 26.8%) of our present blue OLED with m-CzDPz host are close to the best data ever reported for Firpic-based devices so far, especially among those with single bipolar host. According to the equation:36 EQE = ηop ×ηPLQY ×ηr ×γ, where ηop is the optical out-coupling factor, ηr is the ratio of radiative excitons, ηPLQY is photoluminence quantum yield and γ is the carrier balance factor.


17


Given that ηop is 20%-30% and γ is 1, the theoretical EQE for these new hosts based blue devices agree well with the experimental data (Table 2). The Commission International de I’Eclairage (CIE) coordinates of each blue phosphorescent device is independent of the driving voltage, indicating the stable emission color of these devices. However, with changing the host, the CIE coordinates of these devices, i.e. (0.14, 0.31) for A1, (0.14, 0.29) for A2, (0.15, 0.36) for A3, and (0.15, 0.37) for A4, differ significantly from each other (Figure S6 a). The long-wavelength vibrational peak at 499 nm was intensified in the 3-CzDPz and mCPDPz devices in comparison with other two devices. We assign this bathochromic spectral

change to

the

charge recombination

zone shift

to

the emitting

layer/electron-transport-layer interface with increasing hole transporting mobility in 3-CzDPz and mCPDPz devices, inducing an optical effect in these device accompanied by an increase of the vibrational peak.37,38

2

10

60

1

30

10

0

0

10 3

6 9 Voltage (V)

2 3

10

120 2

10

80

1

40

10

0

0

0

3

6 9 Voltage (V)

12

10

External quantum efficiency (%)

160

4

10

Brightness (cd/m )

2

B1 B2 B3 B4

200

10 A1 A2 A3 A4

0

0

10

12

(d)

(c) 240

1

1

10

-2

10

-1

0

1

10 10 10 2 Current density (mA/cm )

2

10

10

2

10

2

1

10

10

1

10

B1 B2 B3 B4 0

10

Current efficiency (cd/A)

2 3

10


90

EL brightness (cd/m )

120


(b)

4

10

A1 A2 A3 A4

2


(a) 150


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 18 of 31

0

-2

10

-1

0

1


2

10

10

Figure 6 (a) Current density-voltage-brightness (J-V-L) characteristics and (b) efficiency curves for FIrpic-based blue PhOLEDs A1-A4, (c) J-V-L characteristics and (d) efficiency curves for


18

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Ir(ppy)3-based green PhOLEDs B1-B4. To test the applicability of the hosts to the green phosphorescent emitter, we fabricated green phosphorescent devices B1-B4 by doping 6 wt% tris[2-phenylpyridinato-C2,N] iridium(III) (Ir(ppy)3) in the o-CzDPz, m-CzDPz, 3-CzDPz and mCPDPz as the emitting layers, with the identical device structure as devices A1-A4. Figure 6c and 6d show the J-V-L characteristics and efficiency curves of these green phosphorescent devices and excellent performance was also achieved (Table 2). For example, the m-CzDPz and 3-CzDPz hosted devices B2 and B3 exhibited the maximum efficiencies of 87.3 cd A-1 (54.8 lm W-1 and 26.3%) and 91.2 cd A-1 (57.6 lm W-1 and 29.0%), respectively. The PLQY of 6 wt% Ir(ppy)3-doped these new hosts films were ranging from 84.1%-99.6% (Table 2), which coincided with the EQE of green phosphorescent devices (18.6%-29.0%), assuming the outcoupling factor of 20%-30%.36 More importantly, these green devices are also characterized by slow efficiency decay. For example, at a practical brightness of 1000 cd m-2, the efficiencies of 3-CzDPz based device B3 still remain at 84.4 cd A-1 and 26.6%, which corresponds to a reduce of 7.4% from the maximum values. Even at an extremely high brightness of 10000 cd m-2, the efficiencies still remained as high as 55.3 cd A-1 and 17.6%. The excellent performance of these pyrazole-containing bipolar host materials in phosphorescent devices

enlightened

us

to

explore

1,2-Bis(carbazol-9-yl)-4,5-dicyanobenzene

the

possibility

to

use

them

(2CzPN)

for

TADF

emitters. and

1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) are two famous TADF materials that were first reported by Adachi group6 and are widely used in OLEDs nowadays.10,12 2CzPN is a sky-blue TADF material with the emission peak of 473 nm and 4CzIPN is green one with the emission peak of 507 nm in toluene. In this work, we fabricated sky-blue TADF devices C1-C4 and green TADF devices D1-D4 by doping 3 wt% 2CzPN and 6 wt% 4CzIPN in o-CzDPz, m-CzDPz, 3-CzDPz and mCPDPz hosts, respectively. The device structures are identical to those of the above phosphorescent ones. Figure 7 illustrates the J-V-L characteristics and efficiency curves for both the blue and green TADF devices ACS Paragon Plus Environment

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Page 20 of 31

C1-C4 and D1-D4, respectively. The EL spectra of these devices are shown in Figure S6. The EL data are summarized in Table 2. For the sky-blue TADF devices C1-C4, device C3 with 3-CzDPz host exhibited much better electroluminescent performance with maximum efficiencies of 26.2 cd A-1, 20.6 lm W-1 and 15.8%, which were doubled relative to the initially reported value (8%) by Adachi6 and are higher than the highest data (13.6%) achieved for 2CzPN-based TADF devices reported so far.39 The enhanced EL efficiencies for 2CzPN devices should benefit from the utilization of these pyrazole-containing bipolar host materials and thus the more balanced charge transportation in the emitting layer. The effectives of a host-to-guest energy transfer can be measured according to the size of the overlapping area between the host photoluminescence (PL) and guest ultraviolet-visible (UV-Vis) spectra in thin films. As shown in the UV-Via absorption and PL emission spectra of the hosts and 2CzPN (Figure S7), extensive overlap of the PL emission of o-CzDPz, m-CzDPz, 3-CzDPz and mCPDPz withUV-Vis absorption of 2CzPN induced efficient energy transfer. Besides, o-CzDPz, m-CzDPz, and mCPDPz based blue TADF devices C1, C2, and C4 also realized good performance data (Table 2). In particular, the o-CzDPz hosted device C1 revealed the ηc, max of 26.2 cd A-1, ηp, max of 15.6 lm W-1 and ηext, max of 14.5%. The PLQY of 3 wt% 2CzPN-doped these new hosts films were ranging from 46.7%-70.3% (Table 2), which coincided with the EQE of blue TADF devices (10.9%-15.8%), assuming the outcoupling factor of 20%-30%.36 For the green TADF devices D1-D4, all these green TADF devices are characterized by high brightness, high efficiencies, and slow efficiency roll off. For example, the m-CzDPz hosted device D2 exhibited a maximum brightness of 24050 cd m-2. The maximum efficiencies of 41.1 cd A-1, 32.2 lm W-1, and 13.3 % were achieved by 3-CzDPz based device D3. More importantly, at a practical brightness of 1000 cd m-2, the efficiencies of device D3 still remain at 39.1 cd A-1 and 12.7%, which are reduced by only 4.9% relative to the maximum values (Figure S8). It is well-known that the emission efficiency of TADF devices usually decays rapidly with increasing driving current due to severe triplet-triplet annihilation.16,17,40-43 The slow efficiency roll-off of the green TADF devices in present study is really a valuable merit that will definitely favorable for application


20

Page 21 of 31

under high brightness. The high emission efficiencies observed in present blue and green TADF devices and the slow efficiency roll-off especially in present green TADF devices D1-D4 can be attributed to the use of the bipolar host that may result in balanced charge fluxes and a broad distribution of recombination regions within the emitting layer. However, the EQE of these hosts based green TADF devices are still not comparable with the highest values in literature.12,44,45 We suppose the less matched energy levels between the host and the TADF emitter should be one possible reason. As shown by the energy diagram (Figure S5), both the HOMO and LUMO of 4CzIPN are located at deeper levels than all these hosts, making the hole injection and electron injection preferably into the dopant and the host molecules in the emitting layer, respectively. This definitely caused imbalanced carrier injections and transportation and is not favorable for a high EQE. In addition, the PLQY of 6 wt% 4CzIPN-doped hosts films range from 57.6%-66.6% (Table 2), which coincide with the EQE of green TADF devices (12.1%-13.7%), if assuming the outcoupling factor of 20%-30%.36

10

2

80 40

10

1

0 6 9 Voltage (V)

12

10

0

1

10

10

10

1

C1 C2 C3 C4

0

-2

-1

10

0

10 0

1



3

120

3

2

10

4

10

160

2

2

D1 D2 D3 D4

3

10

120 2

80

10

40

10

1

0 0

0

3

6

Voltage (V)

9

12

10

10

1

10

1

D1 D2 D3 D4

10

0


200


(d) 102

Brightness (cd/m )

(c)

10

(b) 102 External quantum efficiency (%)

160

4

2

C1 C2 C3 C4

10

Brightness (cd/m )

200

2


(a)


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


0

10

-2

-1

0

1


10

2

10

Figure 7 (a) J-V-L characteristics and (b) efficiency curves for 2CzPN-based blue TADF devices ACS Paragon Plus Environment

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Page 22 of 31

C1-C4, (c) J-V-L characteristics and (d) efficiency curves for 4CzIPN-based green TADF devices D1-D4. It should be noted that in comparison with the mCPDPz host that contains equal number of n-type and p-type groups, the three hosts o-CzDPz, m-CzDPz, 3-CzDPz always lead to superior device performance with higher EL efficiencies and slower efficiency roll-off, especially in the blue and green phosphorescent devices. This enhanced device performance should benefit from the increased molar ratio of n-type pyrazole unit to p-type carbazole group in these molecules that is then helpful to improve electron transportation and to facilitate the positive and negative charge balance in the emitting layer. The always superior device performance in o-CzDPz, m-CzDPz, 3-CzDPz hosted devices observed in present study once again proves experimentally that the positive and negative charge balance state in the emitting layer of OLEDs is one of the most important factors to determine the overall performance of both phosphorescent and TADF OLEDs. In addition, it is interesting to find that in all four types of OLEDs in present study, the mCPDPz hosted devices almost always show the broadened and even red-shifted EL spectra except in the D series of devices (Figure S6) in comparison with other three hosts. The hypsochromic effect caused by o-CzDPz, m-CzDPz, 3-CzDPz hosts that containing higher molar ratio of n-type units would be especially beneficial for these blue phosphorescent and TADF devices to approach real blue light emission. We suppose that the hypsochromic shift is also correlated with the different charge balance ability of the host materials and the recombination zone shift away from the emitting layer/electron transporting layer interface with increasing the electron transporting ability of the hosts. This difference in spectra profile and emission color further confirms the important contribution of the charge balance to the overall performance of OLEDs.


22

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1 2 3 4 5 6 Device 7 8 9 A1 10 11 12 A2 13 14 A3 15 16 A4 17 18 B1 19 20 B2 21 22 B3 23 24 25 B4 26 27 C1 28 29 C2 30 31 C3 32 33 34 C4 35 36 D1 37 38 D2 39 40 D3 41 42 D4 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Electroluminescence characteristics of the devices.a Lmax

ηc b

ηp b

ηext b

ηext e

PLQY d

CIE

(V)

(cd m-2)

(cd A-1)

(lm W-1)

(%)

(%)

(%)

(x,y) c

4.2

11630

46.8,43.5,33.2

29.4,22.6.10.4

25.6,23.8,18.2

19.4-29.2

97.9

(0.14,0.31)

3.8

13390

48.3,40.9,30.9

30.3,20.9,12.1

26.6,22.5,17.0

99.8

(0.14,0.29)

3-CzDPz

3.6

16744

40.8,40.0,30.6

28.9,25.7,17.0

19.54,19.1,14.6

17.0-25.5

85.1

(0.15,0.36)

mCPDPz

3.7

13539

35.7,34.8,27.3

22.4,19.9,13.3

16.8,16.4,12.8

16.1-24.2

80.5

(0.15,0.37)

o-CzDPz

3.9

38700

86.2,85.8,82.9

54.1,52.6,41.1

25.5,25.4,24.6

19.1-28.5

95.3

(0.27,0.63)

3.8

23260

87.3,86.9,67.8

54.8,54.0,35.3

26.3,26.2,20.4

19.3-29.0

96.7

(0.28,0.63)

3-CzDPz

3.6

29240

91.2,60.8,84.4

57.6,42.1,49.1,

29.0,19.3,26.6

20.0-29.9

99.7

(0.27,0.63)

mCPDPz

4.4

43550

65.3,64.9,59.3

41.0,37.9,29.6

18.6,18.5,16.9

16.8-22.3

84.1

(0.30,0.63)

o-CzDPz

4.5

4248

26.2,14.9,5.4

16.5,7.0,2.6

14.5,8.2,3.0

12.6-18.9

63.1

(0.16,0.28)

4.7

4157

19.1,11.7,5.6

12.0,5.6,2.1

10.5,6.4,3.1

10.4-15.6

52.1

(0.16,0.30)

3-CzDPz

3.8

4753

26.2,15.0,6.8

20.6,8.6,2.9

15.8,9.0,4.1

14.1-21.1

70.3

(0.16,0.29)

mCPDPz

4.8

4615

23.3,13.3,7.4

14.6,6.5,2.6

10.9,6.2,3.5

9.34-14.0

46.7

(0.17,0.34)

o-CzDPz

4.8

12780

39.6,39.3,33.1

23.7,19.7,13.9

13.7,13.6,11.4

12.4-18.5

61.8

(0.26,0.54)

4.5

24050

37.5,36.6,33.7

22.0,20.5,13.3

12.1,11.8,10.9

11.5-17.3

57.6

(0.30,0.57)

3-CzDPz

3.8

19890

41.1,40.6,39.1

32.2,27.2,17.8

13.3,13.2,12.7

11.8-17.7

58.9

(0.31,0.58)

mCPDPz

4.9

12130

37.9,36.2,28.3

19.8,17.8,9.8

13.1,12.5,9.8

13.3-20.0

66.6

(0.22,0.49)

Host

Dopant

o-CzDPz m-CzDPz

Von

20.0-30.0

FIrpic

m-CzDPz Ir(ppy)3

m-CzDPz 2CzPN

m-CzDPz 4CzIPN

a

Abbreviations: Von, turn-on voltage. Lmax, maximum luminance. V, voltage. ηext, external quantum

efficiency. ηc, current efficiency. ηp, power efficiency. CIE (x, y), Commission International de I’Eclairage coordinates. b Order of measured values, maximum, then at 100 and 1000 cd m-2. c Measured at 7 V. d PLQY, Photoluminescence Quantum Yield of doped films measured by intergrating sphere.

e

The theoretical external quantum efficiency calculated from the PLQY assuming the light outcoupling efficiency in the range of 20-30%.


23


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Page 24 of 31

CONCLUSION A series of high-triplet-energy bipolar materials, o-CzDPz), m-CzDPz, 3-CzDPz, and mCPDPz were developed as universal hosts for both phosphorescent and delayed fluorescence OLEDs of various emitting colors. The triplet energy levels (ET) and the charge transporting mobilities were well tuned by varying the linking mode between the n-type pyrazole and the p-type carbazole groups. By carefully tuning the physical parameters of these hosts, the maximum efficiencies of 26.8% and 29.0% were achieved for m-CzDPz hosted blue and 3-CzDPz hosted green phosphorescent OLEDs, respectively. Furthermore, the 3-CzDPz hosted blue and green TADF devices exhibited high efficiencies of 15.8% and 13.3 %, respectively. More importantly, the quantum efficiency of present blue TADF device is higher than the highest efficiency of the same TADF emitter (2CzPN) reported so far, and the blue and green TADF devices using these pyrazole-containing hosts are characterized by rather slow efficiency roll-off, which is really a valuable merit for TADF devices. The present study has proved experimentally that the charge balance ability of the hosts, which is in turn achieved by adjusting both the molar ratio of n-type to p-type units and the steric molecular conformation, is the essentially important

factor

to

determine

the

overall

performance

for

both

phosphorescent

and

delayed-fluorescence OLEDs. It has been demonstrated that to increase the molar ratio of n-type pyrazole unit to p-type carbazole group in host molecules is an effective strategy to enhance electron transportation and thus facilitate the positive and negative charge balance ability of the host materials. As far as we know, this is the first report to show certain host materials are not only suitable for both phosphorescent and TADF emitters, but also apply for wide range of emitting color emitters, so that they are universal host materials for OLEDs. The excellent performance of these OLEDs implies that these hosts may find potential application for more emitting color emitters so as to realize real universal function for all primary RGB and even white OLEDs.

ACKNOWLEDGEMENTS


24

Page 25 of 31

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


We thank the National Natural Science Foundation of China (21274016, 21374013, and 21421005), the Program for Changjiang Scholars and Innovative Research Team in University (IRT-13R06) and Program for DUT Innovative Research Team (DUT2013TB07), and the Fundamental Research Funds for the Central Universities (DUT15YQ101 and DUT13LK06) for financial support of this work. Supporting Information. The supporting information is available free of charge via the internet at … or from the author. TGA thermograms, absorption spectra and RT and LT PL spectra, spin density distribution figure, energy level diagram, and EL spectra.

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Page 31 of 31

TOC Graphical Abstract Novel pyrazole-containing bipolar compounds are developed for use as universal host materials for both phosphorescent and thermally activated delayed-fluorescence organic light-emitting diodes of various emitting colors, which exhibit excellent performance with high efficiencies (26.8% and 29.0% for blue and green phosphorescent, 15.8% and 13.3% for blue and green TADF devices) and slow efficiency roll-off.


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

29.0%

26.8%

30

15.8%

Ph 13.3%

10 N

N N

TADF

N

Ar = N N N

N N

Universal Bipolar Hosts

1 0.01

0.1

1

10

100 2



31

Universal Host Materials for High-Efficiency Phosphorescent and

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