Modulation of n-Type Units in Bipolar Host Materials toward

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Modulation of N-type Units in Bipolar Host Materials towards High-Performance Phosphorescent OLEDs Fang Wang, Di Liu, Jiuyan Li, and Mengyao Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11667 • Publication Date (Web): 11 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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ACS Applied Materials & Interfaces

Modulation of N-type Units in Bipolar Host Materials towards High-Performance Phosphorescent OLEDs Fang Wang, Di Liu, Jiuyan Li,* Mengyao Ma State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China. E-mail: [email protected]

ABSTRACT: 9′-Pyridinyl-9′H-9,3′:6′,9″-tercarbazole (PyCz) is a bipolar host material in phosphorescent organic light-emitting diodes (OLEDs). A second n-type unit, either pyridine or diphenylphosphine dioxide (DPPO), is introduced onto pyridine ring of PyCz at para- or meta-site to design and prepare four novel “dual n-type unit bipolar host” materials m-BPyCz, p-BPyCz, m-POPyCz and p-POPyCz. The incorporation of the second n-type unit pulls down the lowest unoccupied molecular orbitals (LUMO) and facilitates electron injection and transportation, resulting in better charge balancing ability. As a result, these dual n-type unit bipolar hosts exhibit higher efficiencies and slower efficiency roll-off in their blue and green phosphorescent OLEDs. In particular, m-POPyCz containing a bulky DPPO as the second n-type unit with a meta-linking possesses the best charge balancing state and generates a maximum external quantum efficiency (ηext) of 27.0% (corresponding to a current efficiency of 51.9 cd A−1 and a power efficiency of 46.5 lm W−1) in its sky-blue device, and still remained a high ηext of 23.6% even at the practical

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ACS Paragon Plus Environment


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brightness of 1000 cd m−2. These results clearly demonstrate that the “dual n-type unit bipolar hosts” with optimized substitution position and steric effect is a new and effective type of host materials for high-performance OLEDs. Keywords: phosphorescent organic light-emitting devices (PhOLEDs), dual n-type unit bipolar host, diphenylphosphine dioxide (DPPO), charge balance, meta-linking

INTRODUCTION Over the past two decades, organic light-emitting devices (OLEDs) have progressed rapidly in terms of both materials and device technics.1-3 By harvesting all the electrically generated singlet and triplet excitons for light emission, the transition metal complex phosphors can realize the theoretical internal quantum efficiency up to 100%,4,5 and thus the phosphorescent OLEDs (PhOLEDs) have drawn considerable attention from the fields of practical application and science. PhOLEDs are usually fabricated by host-guest doping structure in the emitting layer to suppress the severe triplet-triplet annihilation (TTA), concentration quenching, and triplet exciton-polaron quenching (TPQ), etc.6,7 In general, the most successful phosphors are those iridium complexes that are characterized by short lifetimes and wide color tunability, such as blue

emitting

iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate

(FIrpic)8 and green tris[2-phenylpyridinato-C2,N] iridium(III) (Ir(ppy)3)9. More importantly, the efficiencies of FIrpic and Ir(ppy)3-based PhOLEDs can have enormous variation by using different hosts.4-7 The host matrix occupying the majority of the emitting layer controls the charge injection at interfaces, charge 2


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transportation in emitting layer, distribution of the charge recombination zone and even the fabrication method of the device, consequently playing the same important role as the dopant to determine the overall performance of the PhOLEDs. Thus developing efficient host materials to achieve high external quantum efficiency (EQE) with a low driving voltage and a low efficiency roll-off is especially crucial.5 It has been established that the bipolar host materials that contain both electron-transporting n-type unit and hole-transporting p-type group are most ideal to improve the electroluminescence (EL) efficiency and reduce efficiency roll-off due to remarkable charge balance ability in comparison with the uni-polar host materials.10 In principle, a good bipolar host material should possess suitably high triplet energy and sufficient spectral overlap with dopant to guarantee efficient forward energy transfer, energy-favorably-aligned highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels to facilitate positive and negative charge injections into emitting layer at interfaces, high thermal and amorphous stabilities to lead to a long device lifetime. At the same time, the combination of n-type and p-type units and the molecular configuration are also crucial to determine the charge transportation ability and thus the charge balance state of the bipolar host materials. Only when the positive and negative charges are best balanced in the emitting layer, can the charge recombination and light emission be possible to reach the high quantum efficiencies of unity. Various electron-deficient moieties capable of electron transporting, such as pyridine11, phosphine oxide12,13, triazole14, pyrazole4, triazine1, cyano15,16, benzimidazole17, oxadiazole18, are used to design bipolar host 3



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materials with the hole-transporting carbazole derivatives. Nevertheless, it was observed that the electron-transporting mobilities in many n-type organic units are several orders of magnitude lower than the hole-transporting mobilities in p-type units, and selecting or developing suitable n-type functional units or skeletons to match the p-type groups always remains a rather challenging task. In our recent publications,7 we have raised up the concept of “dual n-type units bipolar hosts”, in which two different n-type units are combined by direct linking to improve the electron injection and transportation so as to facilitate the charge balance ability of the bipolar host materials and dramatically improve the OLEDs performance. In the present work, we further report the strategy of dual n-type units and optimization of the molecular configuration by tuning substitution position and steric effect combine to generate high-performance bipolar host materials. Herein,

a

typical

bipolar

host

material

9′-(pyridin-3-yl)-9′H-9,3′:6′,9″-tercarbazole (PyCz)19 that contains pyridine as n-type unit and the oligo-carbazole 9′H-9,3′:6′,9″-tercarbazole (TCz) as p-type group was prepared first. Enlightened by the success of our previous explorations, 7 a second electron-deficient group that is either pyridine or diphenylphosphine oxide (DPPO) was introduced onto the pyridine ring of PyCz as the second n-type unit to form novel “dual n-type units bipolar host”. At the same time, the substitution position of the second n-type unit was adjusted as para- or meta- relative to the p-type carbazole. In this way, four novel bipolar host materials, namely 9’-(3-(pyridin-3-yl)pyridin-5-yl)-9′ H-9,3’:6’,9’’-tercarbazole

(m-BPyCz), 4


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9’-(2-(pyridin-3-yl)pyridin-5-yl)-9’H-9,3’:6’,9’’-tercarbazole

(p-BPyCz),

9’-(3-(diphenyl-phosphinoyl)pyridin-5-yl)-9’H-9,3’:6’,9’’-tercarbazole (m-POPyCz), and

9’-(5-(diphenyl-phosphinoyl)pyridin-2-yl)-9’H-9,3’:6’,9’’-tercarbazole

(p-POPyCz), were designed and prepared. It was observed that the incorporation of the second n-type unit, no matter whether it is same as or different from the original one, always pulls down the LUMO level of the hosts and improve electron injection, the meta-linkage style is more favorable than the para-style for an limited π-conjugation and high triplet energy of the host molecule, the bulky DPPO with large steric hindrance effect seems more capable to balance positive and negative transportation for host materials. Finally, the novel dual n-type unit bipolar host m-POPyCz with meta-substitution and bulky DPPO group realized the best charge balance ability and the best device performance. In particular, the m-POPyCz hosted sky-blue PhOLED exhibited high efficiencies of 27.0% (corresponding to 51.9 cd A−1 and 46.5 lm W−1), and at the brightness level of 100 and 1000 cd m−2, they still remained at 26.5% and 23.6%.

EXPERIMENTAL SECTION

General Information. characterization (1H NMR,

13

The instruments and methods for compound C NMR and mass spectra), spectra measurements

(UV-vis absorption and photoluminescence emission spectra in neat film and solvents, low-temperature phosphorescence spectra in frozen 2-MeTHF matrix at 77 K), thermal properties (thermogravimetric analyses and differential scanning calorimetry), 5



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electrochemical properties investigation (cyclic voltammetry), the theoretical calculation with Gaussian software, and the OLED fabrication and optoelectronic measurements have been depicted in previous publications of the same group.6 Syntheses. The intermediates 5-bromo-3-(diphenyl-phosphinoyl)pyridine (3), 5-bromo-2-(diphenyl-phosphinoyl)pyridine (4) and 9′H-9,3′:6′,9″-tercarbazole (5) were synthesized according to the literature procedures.7,20 General procedure for synthesis of 5-bromo-3-(pyridin-3-yl)pyridine (1) and 5-bromo-2-(pyridin-3-yl)pyridine (2). Under nitrogen atmosphere, A mixture of pyridine-3-boronic acid (246 mg, 1 mmol), the corresponding brominated pyridine (3,5-dibromopyridine for 1 and 2,5-dibromopyridine for 2) (434 mg, 1 mmol), toluene (10 mL), ethanol (2 mL), aqueous potassium carbonate (2 M, 2.5 mL, 4 mmol), and tetrakis-(triphenylphosphino)palladium(0) (57 mg, 0.05 mmol) was refluxed at 80 ℃ overnight. After cooling and filtrating and diluting by water (20 mL), the aqueous solution was separated from the organic layer and then extracted with dichloromethane (3 × 15 mL). After the combined organic layers were dried over anhydrous magnesium sulfate and filtered, the filtrate was evaporated under reduced pressure. The residue was isolated by silica gel column chromatography using ethyl acetate/petroleum ether (1:7) as eluent, and further purified by repeated recrystallization in methanol/chloroform to afford pure 1 and 2 as white powders. 5-bromo-3-(pyridin-3-yl)pyridine (1). Yield: 75%. 1H NMR (500 MHz, CDCl3) δ: 8.84 (d, J = 1.5 Hz, 1H), 8.76 (d, J = 2.0 Hz, 1H), 8.73 (d, J = 2.5 Hz, 1H), 8.70 (dd, J = 3.0, 1.5 Hz, 1H), 8.04 (t, J = 2.0 Hz, 1H), 7.88 (dt, J = 4.0, 1.5 Hz, 1H), 7.44 (qd, 6


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J = 4.0, 0.5 Hz, 1H). TOF-EI-MS (m/z): Calcd. for C10H7BrN2 233.9793, Found 233.9775 [M]+. 5-bromo-2-(pyridin-3-yl)pyridine (2). Yield: 70%. 1H NMR (500 MHz, CDCl3) δ: 9.18 (d, J = 2.0 Hz, 1H), 8.78 (d, J = 2.0 Hz, 1H), 8.68 (d, J = 4.5 Hz, 1H), 8.31 (dt, J = 8.0, 3.5 Hz, 1H), 7.93 (ddd, J =5.5, 2.0, 0.5, 1H), 7.67 (d, J = 8.5 Hz, 1H), 7.42 (dd, J = 5.0, 4.0 Hz, 1H). TOF-EI-MS (m/z): Calcd. for C10H7BrN2 233.9793, Found 233.9782 [M]+. General

procedure

for

synthesis

of

9′-(3-(pyridin-3-yl)pyridin-5-yl)-

9′H-9,3′:6′,9″-tercarbazole (m-BPyCz) and other final products. A mixture of 9′H-9,3′:6′,9″-tercarbazole (498 mg, 1 mmol), the corresponding brominated pyridine (1 for m-BPyCz, 2 for p-BPyCz, 3 for m-POPyCz, 4 for p-POPyCz and 3-bromopyridine for PyCz) (1 mmol), CuI (95 mg, 0.5 mmol), 1,10-phenanthroline (90 mg, 0.5 mmol) and potassium carbonate (555 mg, 4 mmol) in dry N,N-dimethylformamide (DMF) (15 mL) was stirred and refluxed under N2 atmosphere for 24 h. After removing the inorganic salts by filtering, evaporating the solvent under reduced pressure, the crude solid product was isolated by silica gel column chromatography using ethyl acetate/petroleum ether (1:2) as eluent, and further purified by repeated recrystallization in methanol/chloroform to afford pure target products as white solids. m-BPyCz. Yield: 58%. 1H NMR (500 MHz, CDCl3) δ: 9.13 (s, 1H), 9.08 (s, 1H), 9.03 (s, 1H), 8.76 (d, J = 5.0 Hz, 1H), 8.32 (s, 2H), 8.29 (s, 1H), 8.16 (d, J = 5.0 Hz, 4H), 8.07 (d, J = 7.5 Hz, 1H), 7.68 (s, 4H), 7.53 (t, J = 5.0 Hz, 1H), 7.40 – 7.43 (m, 7



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8H), 7.30 – 7.23 (m, 4H).

13

C NMR (126 MHz, CDCl3) δ: 150.14, 148.23, 147.92,

147.53, 141.67, 140.38, 135.13, 134.80, 134.49, 132.71, 132.32, 131.28, 126.73, 125.97, 124.55, 124.08, 123.26, 120.36, 120.06, 119.86, 110.94, 109.61. TOF-EI-MS (m/z): calcd. for C46H29N5 651.2423, found 651.2418 [M]+. Elemental analysis: C, 84.77; H, 4.48; N, 10.75; Found: C, 84.75; H, 4.49; N, 10.73. p-BPyCz. Yield: 65%. 1H NMR (500 MHz, CDCl3) δ: 9.39 (d, J = 2.0 Hz, 1H), 9.18 (d, J = 2.5 Hz, 1H), 8.76 (dd, J = 3.5, 1.5 Hz, 1H), 8.53 (dt, J = 4.0, 1.5 Hz, 1H), 8.31 (d, J = 2.0 Hz, 2H), 8.22 – 8.13 (m, 6H), 7.71 – 7.66 (m, 4H), 7.54 (dd, J = 5.0, 3.0 Hz, 1H), 7.43 – 7.39 (m, 8H), 7.31 – 7.28 (m 4H). 13C NMR (126 MHz, CDCl3) δ: 154.20, 150.35, 148.61, 148.21, 141.67, 140.35, 135.37, 134.59, 133.35, 131.17, 126.64, 125.96, 124.50, 123.24, 121.43, 120.35, 119.99, 119.84, 110.96, 109.61. TOF-EI-MS (m/z): calcd. for C46H29N5 651.2423, found 651.2426 [M]+. Elemental analysis: C, 84.77; H, 4.48; N, 10.75; Found: C, 84.80; H, 4.47; N, 10.72. m-POPyCz. Yield: 51%. 1H NMR (500 MHz, CDCl3) δ: 9.24 (t, J = 2.0 Hz, 1H), 8.91 (dd, J = 1.5, 1.0 Hz, 1H), 8.52 (d, J = 10 Hz, 1H), 8.27 (d, J = 1.5 Hz, 2H), 8.16 (d, J = 7.5 Hz, 4H), 7.82 (dd, J = 5.0, 2.0 Hz, 4H), 7.67 – 7.62 (m, 4H), 7.60 – 7.57 (m, 6H), 7.42 – 7.36 (m, 8H), 7.29 (t, J = 7.0 Hz, 4H). 13C NMR (126 MHz, CDCl3) δ: 151.09, 150.79, 141.62, 139.89, 137.68, 134.43, 132.89, 132.08, 131.99, 131.49, 130.65, 129.14, 129.04, 126.74, 125.98, 124.69, 123.26, 120.36, 120.01, 119.88, 110.77, 109.59. TOF-MALDI-MS (m/z): calcd. for C53H35N4OP 774.2548, Found 774.2 [M]+. Elemental analysis: C, 82.15; H, 4.55; N, 7.23; found: C, 82.13; H, 4.54; N, 7.25. 8


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p-POPyCz. Yield: 62%. 1H NMR (500 MHz, CDCl3) δ: 8.92 (d, J = 5.0 Hz, 1H), 8.38 (t, J = 9.0 Hz, 1H), 8.25 (d, J = 5.0 Hz, 4H), 8.16 (d, J = 5.0 Hz, 4H), 7.96 (d, J = 7.0 Hz, 1H), 7.83 (dd, J = 7.5, 5.0 Hz, 4H), 7.66 (q, J = 7.5 Hz, 4H), 7.59 (t, J = 7.45 Hz, 4H), 7.40 (d, J = 3.5 Hz, 8H), 7.30 – 7.23 (m, 4H). 13C NMR (126 MHz, CDCl3) δ: 153.97, 153.04, 152.94, 142.59, 142.52, 141.52, 138.67, 132.64, 132.62, 132.09, 132.02, 129.01, 128.91, 126.59, 125.96, 125.61, 123.25, 120.34, 119.86, 119.53, 117.84, 117.77, 113.32, 109.61. TOF-Maldi-MS (m/z): calcd. for C53H35N4OP 774.2548, found 774.2 [M]+. Elemental analysis: C, 82.15; H, 4.55; N, 7.23; Found: C, 82.16; H, 4.53; N, 7.22. PyCz. Yield: 79%. 1H NMR (500 MHz, CDCl3) δ: 9.09 (s, 1H), 8.86 (d, J = 5.0 Hz, 2H), 8.30 (s, 2H), 8.17 (d, J = 5.0 Hz, 4H), 8.13 (d, J = 5.0 Hz, 1H), 7.71 (s, 1H), 7.64 (q, J = 5.0, 4H), 7.40 (m, 8H), 7.29 (td, J = 5.0, 2.5, 4H). 13C NMR (126 MHz, CDCl3) δ: 149.18, 148.56, 141.67, 140.43, 134.78, 131.04, 126.59, 125.94, 124.36, 123.21, 120.33, 119.94, 119.80, 110.89, 109.61. TOF-Maldi-MS (m/z): calcd. for C41H26N4 574.2157, found 574.2 [M]+. Elemental analysis: C, 85.69; H, 4.56; N, 9.75; Found: C, 85.67; H, 4.57; N, 9.76.

RESULTS AND DISCUSSION Synthesis and Thermal Properties. The chemical structure of m-BPyCz, p-BPyCz, m-POPyCz, p-POPyCz and PyCz are shown in Scheme 1, their synthetic procedures are illustrated in Scheme S1 in supporting information. All these target compounds were prepared at moderate yields of 51−79% by CuI mediated Ullmann reaction from 9



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the brominated n-type unit 1-5 and 9′H-9,3′:6′,9″-tercarbazole. The two intermediates 1 and 2 were prepared through a Suzuki cross-coupling reaction using pyridine-3-boronic

acid

with

3,5-dibromopyridine

or

2,5-dibromopyridine,

respectively. Intermediates 3 and 4 were synthesized according to the literature methods21, and other reactants were commercial available and used without further purification. To reach a high purity for PhOLEDs application, all these compounds need to be thoroughly purified by silica gel column chromatography, and further purification was carried out by repeated recrystallization in methanol/chloroform based on their good solubility in common organic solvents. The chemical structures of these compounds were confirmed by 1H NMR(Figures S1-5),

13

S6-10),

analysis

EI-MS

spectroscopy,

and

elemental

C NMR(Figures data.

Scheme 1. Structures of m-BPyCz, p-BPyCz, m-POPyCz, p-POPyCz and PyCz. The thermal and morphological stabilities of these compounds were monitored by thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC), respectively, and all the pertinent data are summarized in Table 1. As shown by the TGA thermograms in Figure S11, m-BPyCz, p-BPyCz, m-POPyCz, p-POPyCz and PyCz exhibited high thermal decomposition temperatures (Tds, corresponding to 5% weight loss) at 471, 480, 485, 498 and 446 ℃ , respectively. Apparently the

10


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incorporation of the second n-type unit increases the molecular weight and thus improves the thermal stability of these dual n-type units bipolar host materials. Figure 1 illustrates the DSC traces of these compounds. All these compounds possess high glass transition temperatures (Tgs) in the range of 125−156 ℃. It is interesting that all the para-substituted compounds have higher Tgs than their meta-substituted isomers. For example, the Tg of p-BPyCz (133 ℃) is higher than that of m-BPyCz (125 ℃), and p-POPyCz (155 ℃) is higher than m-POPyCz (143 ℃). We attribute the higher amorphous stability and glass transition temperature in para-substituted isomers to the important contribution of increased molecular rigidity and increased coplanarity with reducing torsion resistance. This is in consistent with our previous observation in another series of host materials.22 In general, an amorphous film with high Tg is less vulnerable to heat, and results in more stable device performance owing to maintaining the film morphology during the operation of the device.23 Based on these results, we could expected that all above materials could form morphologically stable and uniform amorphous films by vacuum deposition for OLED fabrication and maintain stability during the operation. 125 ℃ 133 ℃

Endothermic

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143 ℃

m-BPyCz p-BPyCz m-POPyCz p-POPyCz PyCz

60

80

125 ℃

100 120 140 160 Temperature (℃ )

155 ℃

180

200

11



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Figure 1. DSC traces (at the second heating cycle) of m-BPyCz, p-BPyCz, m-POPyCz, p-POPyCz and PyCz.

Table 1. Physical data of m-BPyCz, p-BPyCz, m-POPyCz, p-POPyCz and PyCz λabs a

λem max

a

ES b

ET b

Eg c, d

HOMO/LUMO e

Td

Tg

[nm]

[nm]

[eV]

[eV]

[eV]

[eV]

[℃]

[℃]

m-BPyCz

296,326,344

407

3.31

2.94

3.21, 2.96

−5.50/−2.54

471

125

p-BPyCz

295,329,345

426

3.14

2.68

3.17, 2.92

−5.51/−2.59

480

133

m-POPyCz

296,326,343

423

3.29

2.96

3.20, 3.09

−5.63/−2.54

485

145

p-POPyCz

295,330,345

430

3.23

2.94

3.18, 3.06

−5.61/−2.55

498

156

PyCz

296,325,344

398

3.25

2.92

3.25, 3.31

−5.50/−2.19

446

125

Compound

a

Absorption and fluorescence wavelengths in neat film.

77 K.

c

b

Measured in 2-MeTHF at

The optical band gap, calculated by the absorption edge technique.

d

The

electrochemical band gap determined as the potential difference between oxidation onset and reduction onset multiplied by the electron charge (e).

e

Determined using

electrochemical potentials. Photophysical properties. Figure 2 (a) illustrates the room-temperature UV-vis absorption and photoluminescence (PL) spectra in dilute dichloromethane (DCM) solutions. Based on the same molecular skeleton and similar functional groups, these molecules possess similar absorption spectra profiles. The strong absorption bands at around 290 nm could be assigned to the π-π∗ transition of carbazole ring and the weak bands at 300–360 nm can be attributed to n-π* transitions of carbazole units18. Remarkably, the 300–360 nm absorption bands of both p-BPyCz and p-POPyCz are 12


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much stronger and appear at slightly longer wavelengths in comparison with m-BPyCz and m-POPyCz. We suppose these long-wavelength absorption bands are partially contributed by the intramolecular charge transfer (ICT) transition in para-isomers due to reduced distortion and enhanced π-conjugation. Upon photoexcitation in DCM solution, PyCz emits purple blue fluorescence with peak at 390 nm. As shown in Figure S12 (a), the PL spectrum of PyCz is almost identical in both profile and wavelength to that of its building block TCz, implying that the 390 nm peaked fluorescence for PyCz originates from the singlet local excited (LE) state of the oligocarbazole moiety. Compared with PyCz, m-BPyCz, p-BPyCz, m-POPyCz and p-POPyCz show bathchromatic shifted fluorescence with major emission peaks at 467-480 nm, which should be assigned to the fluorescence from the photoinduced ICT states.24 As a special case, in addition to the major emission at 467 nm, there is also a “shoulder” peak at 392 nm for m-BPyCz in DCM solution. It is supposed that both LE and ICT excited states coexist for m-BPyCz in such moderately polar solvent as DCM, which emit fluorescence at 392 nm and 467 nm respectively. For p-BPyCz, its ICT fluorescence (480 nm) is longer than other analogues. This should be because the least distorted p-BPyCz has the strongest charge transfer effect and lowest-energy CT state. In order to gain insight on the excited states and fluorescence behavior of these molecules, their PL spectra were measured in different solvents with various polarity. As shown in Figure S12 (b-f), m-BPyCz emitted purple fluorescence at 392 nm in non-polar hexane, which should be from its LE state like for PyCz. The fine vibronic 13



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

structure of the fluorescence spectrum in hexane confirms its LE excited state nature. While in polar solvents including DCM, acetonitrile (AN) and ethanol, m-BPyCz emitted dual fluorescence at around 392 nm and 467-500 nm, which should be assigned to the LE and CT fluorescence25, respectively. With increasing the solvent polarity in AN and ethanol, the longer-wavelength fluorescence bands further red shifted in comparison with that in DCM, confirming that this band is indeed from the CT excited state. m-POPyCz and p-BPyCz exhibited the similar spectral behavior, i.e. m-POPyCz exhibits LE fluorescence at 296 nm in hexane, and a second fluorescence band appears at long wavelengths in polar solvents (AN and ethanol), which should be assigned to the CT fluorescence25; p-BPyCz exhibits LE fluorescence at 390 nm and CT fluorescence at 498 nm in acetonitrile, and the LE fluorescence maintains at the same time. In contrast, for two para-substituted analogues p-BPyCz and p-POPyCz (Figures S12 (c) and (e)), the structureless CT emission dominated their fluorescence spectra in all solvents. For the reference compound PyCz, its fluorescence in different solvents mainly located at around 400 nm and did not show significant changes with increasing solvent polarity (Figure S12 (f)). The exclusive LE emission of PyCz in turn verifies that the incorporation of the second n-type unit in m-BPyCz, p-BPyCz, m-POPyCz and p-POPyCz indeed enhanced the charge transfer effect by increasing electron-withdrawing ability of the n-type units, especially in the para-substituted analogues.

14


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1.0


(a)

0.8 PL Intensity (a. u.)

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


1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 300

350

400 450 500 550 Wavelength (nm)

600

PL Intensity (a. u.)

Page 15 of 36

0.6 0.4 0.2 0.0 350

650


(b)

400

450 500 Wavelength (nm)

550

600

Figure 2. (a) UV-vis absorption and PL spectra of m-BPyCz, p-BPyCz, m-POPyCz, p-POPyCz and PyCz in dilute DCM solutions at room temperature. (b) Phosphorescence spectra of m-BPyCz, p-BPyCz, m-POPyCz, p-POPyCz and PyCz in frozen 2-Me-THF matrix at 77 K. The phosphorescence were measured in frozen 2-methyltetrahydrofuran (2-Me-THF) matrix at 77 K (Figure 2 (b)). The triplet energies (ETs) were determined from the highest energy vibronic band of phosphorescence spectra to be 2.93 (m-BPyCz), 2.68 (p-BPyCz), 2.96 (m-POPyCz), 2.94 (p-POPyCz) and 2.91 eV (PyCz). The phosphorescence spectrum of p-BPyCz shows large red shift and substantially lower ET energy than all other analogues, because the additional pyridine with para-linkage mode leads to a more extended π-conjugation and the pyridylphenyl moiety represents the longest conjugation part within the molecule and determines the triplet energy level. While the meta-linked isomer m-BPyCz has comparable triplet energy with PyCz, indicating the meta-linking style generates largely distorted molecular configuration and limited conjugation. Different from the cases for p-BPyCz and m-BPyCz, both m-POPyCz and p-POPyCz possess similar ET energy levels with PyCz, which should be because the saturated P atom (sp3-type 15



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hybridization) in DPPO does not extend the molecular conjugation despite its substitution position. To estimate the singlet-triplet energy differences (∆EST), the low-temperature photoluminescence (LT PL) spectra were measured by using the fluorescence mode and shown in Figure S13 (a). It is clear that both the fluorescence (350-410 nm) and phosphorescence of these molecules were detected. The energy of first singlet state (ES) of each molecule was determined from the shortest wavelength of the fluorescence spectrum in Figure S3 (a) as 3.31 (m-BPyCz), 3.14 (p-BPyCz), 3.29 (m-POPyCz), 3.23 (p-POPyCz) and 3.25 eV (PyCz). And their ∆EST were estimated to be 0.29−0.46 eV, which are all smaller than most of the reported host materials (0.5−1 eV).20 As well known, smaller singlet and triplet energy difference are usually favorable for lower driving voltages in OLEDs.26 The triplet energies of all these host materials are sufficiently higher than FIrpic (2.65 eV)6 and Ir(ppy)3 (2.40 eV)6, it is expected that these compounds can function as host materials for blue and green PhOLEDs. The thin-film absorption and fluorescence spectra of these compounds are shown in Figure S13 (b), and the optical band gaps were determined by absorption edge technique22 and all the photophysical data are listed in Table 1. Electrochemical properties and theoretical calculations. The electrochemical properties of m-BPyCz, p-BPyCz, m-POPyCz, p-POPyCz and PyCz were investigated by using cyclic voltammetry (CV), and their cyclic voltammograms are shown in Figure 3. The oxidation and reduction potentials were measured in dilute DCM solution (for anodic scan) and N,N-dimethylformamide (DMF) solution (for cathodic scan) at a scan rate of 100 mV s-1 with 0.1 M of tetra(n-butyl)ammonium 16


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hexa-fluorophosphate (n-Bu4NPF6) as supporting electrolyte. All these compounds show reversible or semi-reversible oxidation and reduction behaviors, implying the simultaneous electron-donating and -withdrawing character and possible bipolar charge transporting feature. The onset potentials of the first oxidation wave (Eonset ox ) and first reduction wave (Eonset rex ) were used to determine the HOMO and LUMO levels + 4.4) and ELUMO = −e(Eonset + 4.4). according to the equations of EHOMO = −e(Eonset ox rex The HOMO levels of m-BPyCz, p-BPyCz, m-POPyCz, p-POPyCz and PyCz are calculated as −5.50, −5.51, −5.63, −5.61 and −5.50 eV, respectively, which imply a small hole-injection barrier form a widely used hole transporting material 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC, −5.50 eV)4. The tiny difference (0.13 eV) in HOMO levels among all these five compounds indicate the introduction of the second n-type unit did not disturb the HOMO distribution. LUMO levels of these dual n-type unit bipolar hosts were calculated as −2.54 (m-BPyCz), −2.59 (p-BPyCz), −2.54 (m-POPyCz), and −2.55 eV (p-POPyCz), which are remarkably deeper than PyCz (−2.19 eV), confirming that the second n-type unit can stabilized the LUMOs and must be an effective strategy to facilitate electron injection into emitting layer. Apparently, the incorporation of the second n-type unit directly on the original pyridine unit can further pull down the LUMOs to improve the electron injection but without perturbing the HOMOs. The electrochemical bandgaps (Egs) were determined as the potential difference between Eonset and Eonset multiplied by the electron charge ox rex (e), as summarized in Table 1.

17



m-BPyCz p-BPyCz

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

m-POPyCz p-POPyCz PyCz

2000

1000 0 -1000 Potential (mV vs SCE)

-2000

Figure 3. Cyclic voltammograms of m-BPyCz, p-BPyCz, m-POPyCz, p-POPyCz and PyCz measured in dilute DCM (anodic) and DMF (cathodic) solutions. In order to understand the structure-property relationship of these molecules, their HOMO/LUMO distribution and optimized molecular configurations were calculated by using density functional theory (DFT)7 at the B3LYP 6-31G level and are shown in Figure 4. For all five compounds, their HOMOs are universally localized on the TCz moiety with the major contribution from the peripheral carbazoles, interpreting their comparable electrochemical oxidation potentials and similar HOMO levels. While the distribution of LUMOs has some difference. For the parent PyCz, the LUMO mainly locates on the pyridine ring with small electron density on the nearest carbazole ring. With introduction of the second pyridine ring to form m-BPyCz and p-BPyCz, their LUMOs are equally distributed on the bipyridine moiety, indicating the important contribution of the second pyridine ring to the LUMOs. With introduction of the DPPO group in m-POPyCz and p-POPyCz, their LUMOs mainly locate on the original pyridine ring. The very tiny contribution of DPPO group to the LUMOs in spite of its strong electron-withdrawing feature should 18


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be understandable if taking the non-conjugated linkage into account. The complete spatial separation of HOMOs and LUMOs indicates that all these compounds may possess bipolar charge transporting property and the relatively smaller singlet-triplet energy differences can also be expected.20 As shown by the optimized structures of all these compounds in Figure 4, the inner and the outer carbazole rings possess a large dihedral angle of 58.39–60.04°, and the inner carbazole rings also have large dihedral angles of 40.61-64.27° with the pyridine ring, which together lead to the highly twisted molecular configuration and guarantee high triplet energies for most of these compounds. In particular for isomers m-BPyCz and p-BPyCz, with the substitution position of the second pyridine changing from meta- to para-, the dihedral angles between the two pyridine rings greatly reduced from 37.08° to 14.06°, indicating the much flatter conformation in the n-type unit terminal and more effective conjugation between two pyridine rings in p-BPyCz, which should finally be responsible for its rather lower triplet energy (2.68 eV) than all other analogues (∼2.9 eV). In contrast, the two phenyl rings of DPPO group in m-POPyCz and p-POPyCz have large dihedral angles of 85.75° and 79.62° (m-POPyCz) and 79.72° and 74.53° (p-POPyCz) with the central pyridine rings. This looks like the phenyl rings of DPPO group are partically perpendicular to the central pyridine core, further enhancing the three-dimensional non-coplanar character of these two molecules in comparison with m-BPyCz, p-BPyCz and PyCz. Evidently, the molecular conformation of each material will definitely influence the molecular degree of order in bulky film, consequently determining the charge transporting mobilities and charge balancing state. 19



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Figure 4. Optimized geometry and HOMO/LUMO distribution of m-BPyCz, p-BPyCz, m-POPyCz, p-POPyCz and PyCz. Charge transporting properties. In order to evaluate the charge-transporting properties of m-BPyCz, p-BPyCz, m-POPyCz, p-POPyCz and PyCz, the hole-only devices with structure of ITO/PEDOT:PSS (40 nm)/TAPC (7 nm)/host (70 nm)/TAPC (7 nm)/Al (200 nm) and electron-only devices with structure of ITO/TmPyPB (7 nm)/host (70 nm)/TmPyPB (7 nm)/LiF (1 nm)/Al (200 nm) were fabricated, respectively.

In

these

devices,

poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and TAPC4 served as the hole-injecting layers; 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) and LiF served as the electron-injecting materials. The TAPC/Al interface was designed to prevent electron injection in hole-only devices and ITO/TmPyPB interface to prevent hole injection in electron-only devices due to large electron and 20


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hole barriers of 2.3 eV and 1.88 eV, respectively. The chemical structures of involved materials and the energy level diagrams of these single-carrier devices are illustrated in Figure S14. The thickness of host layer is decuple thicker than adjacent ancillary layers to reduce the possible influence from them and to reflect the intrinsic charge transporting ability of the host materials. The current density versus voltage curves of these single-carrier devices are shown in Figure 5. Each device exhibited significantly high current density in the typical voltage range of OLEDs, implying that all these five compounds possess sufficient hole-transporting and electron-transporting abilities, that is the bipolar charge transporting character. All hole-only devices have comparable current densities, indicating these five host materials possess comparable hole transporting abilities. This is reasonable since all of them have the same p-type unit that is responsible for the hole transportation in bulky layer. It is also implied that the second n-type unit do not influence the hole transporting moieties. On the other hand, in the electron-only devices, the current densities of m-BPyCz, p-BPyCz, m-POPyCz and p-POPyCz are significantly higher than that of PyCz. As shown in Figure S14, the electron injection barriers at the host/TmPyPB interface are around 0.18 eV in these dual n-type unit bipolar hosts devices, which are much lower than that (0.54 eV) at PyCz/TmPyPB interface. Evidently more efficient electron injection should be mainly responsible for the higher electron currents. In addition, we suppose the incorporation of the second pyridine or DPPO group in these four compounds will definitely increase the electron transporting mobilities based on the intrinsic electron-transporting nature of pyridine and DPPO. Evidently, both electron-injecting 21



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and transporting abilities are increased remarkably after incorporating the second n-type units, and it is reasonable to expect that the PhOLEDs containing these four host materials could achieve more excellent device performances. A regular trend exists that two pyridine-containing compounds exhibited higher electron currents than the DPPO-containing compounds, and the para-substituted compounds (p-BPyCz and p-POPyCz) have higher electron currents than their meta-linked isomer (m-BPyCz and m-POPyCz) (Figure 5). Considering the similar LUMO levels of these four compounds, it would be safe to assign the lower electron current of the two DPPO-containing molecules to their lower electron transporting mobilities due to severe non-coplanar conformations. The higher electron currents of two para-substituted analogues than their meta-isomers should also resulted from their less twisted conformation and better degree of order and higher charge mobility. By analyzing Figure 5, it can be observed that the electron currents of all electron-only devices are higher than the hole currents of all hole-only devices at a giving voltage. For example, m-POPyCz exhibited the hole current of 45 mA cm-2 at 18 V and electron current of 62 mA cm-2 at 15 V, which has the smallest difference among all these five materials. It was well established that the comparable hole current and electron current in OLEDs would be most favorable for high charge recombination efficiency. Evidently, m-POPyCz has the best charge balancing ability here due to the presence of dual n-type units and the most appropriate non-coplanar molecular conformation caused by the bulky DPPO group, which will definitely be favorable for good performance in OLEDs when used as host. 22


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160

m-BPyCz-hole only p-BPyCz-hole only m-POPyCz-hole only p-POPyCz-hole only PyCz-hole only m-BPyCz-electron only p-BPyCz-electron only m-POPyCz-electron only p-POPyCz-electron only PyCz-electron only

140

2

Current density (mA/cm )

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


120 100 80 60 40 20 0 0

2

4

6

8 10 12 Voltage (V)

14

16

18

Figure 5. The current density versus voltage curves of the single-carrier devices for compounds m-BPyCz, p-BPyCz, m-POPyCz, p-POPyCz and PyCz. Electroluminescent devices. These five compounds were used as hosts to fabricate blue and green PhOLEDs with structures of ITO/PEDOT:PSS(40 mn)/TAPC(20 nm)/host:(x wt%) Ir dopant (20 nm)/TmPyPB(40 nm)/LiF(1 nm)/Al(200 nm) (Figure S4), in which FIrpic and Ir(ppy)3 were used as sky-blue and green dopant with the optimized

doping

concentration

of

6wt%

and

8wt%,

respectively.

The

hole-transporting TAPC (ET = 2.9 eV)4 and electron-transporting TmPyPB (ET = 2.78 eV)4 also served to confine triplet excitons in the emitting layer. The current density-voltage-brightness (J–V–B) characteristics and efficiency curves of the blue devices B1 (m-BPyCz), B2 (p-BPyCz), B3 (m-POPyCz), B4 (p-POPyCz) and B5 (PyCz) are depicted in Figure 6, and key performances are summarized in Table 2. The pure emission from FIrpic in EL spectra (Figure S15 (a)) confirmed the complete energy transfer from host to dopant and efficient confinement of excitons in the emitting layers. Among these five sky-blue devices, the m-POPyCz hosted B3 achieved the best performance with a turn-on voltage (Von, to deliver a brightness of 1 cd m−2) of 3.0 V, a maximum external quantum efficiency (EQE, ηext,max) of 27.0%, a 23



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

peak current efficiency (CE, ηc,max) of 51.9 cd A−1, and a maximum power efficiency (PE, ηp,max) of 46.5 lm W−1. Inspiringly, excellent performances were also realized in m-BPyCz hosted B1 (Von =2.9 V, ηext,max = 25.8%, ηc,max = 45.6 cd A−1, and ηp,max = 42.4 lm W−1) and p-POPyCz hosted B4 (3.0 V, 23.4%, 50.4 cd A−1, and 45.2 lm W−1). In recent years, FIrpic-based PhOLEDs using a single host were reported with high efficiencies, such as Ma24, Lee27, Li7 and Wong28 individually reported the efficiencies of 49.4 cd A−1 (27.5%), 53.1 cd A−1 (31.4%), 55.6 cd A−1 (25.3%), and 57.6 cd A−1 (26.4%), which stand for the typical data for blue devices so far. It is obvious that the efficiencies (51.9 cd A−1 and 27.0%) of our present FIrpic-based OLED with m-POPyCz host are among the best values ever reported for FIrpic-based PhOLEDs. In comparison, the PyCz based device B5 showed much inferior performance (Von = 3.5 V, ηext,max = 21.6%, ηc,max = 41.4 cd A−1, and ηp,max = 31.2 lm W−1) than all these dual n-type unit bipolar hosts. Apparently, the much lower turn-on voltages for these dual n-type units bipolar hosts should result from their lower LUMO levels and more efficient electron injection relative to the parent PyCz. Similar to the case in electron-only devices, devices B1-B4 all have higher current densities than PyCz based B5 (Figure 6 (a)). At the same time, devices B1-B4 also exhibited higher brightness over the whole driving voltage range and thus the greatly higher efficiencies (Figure 6 (b)) than B5. The superior performance should be due to the presence of the second n-type unit and thus the improved electron injection and transportation and charge balancing abilities of these dual n-type unit bipolar hosts. Therefore, we can deduce that incorporating a second n-type unit (pyridine or DPPO) 24


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to form the dual n-type units bipolar hosts is an effective design strategy of host materials to enhance PhOLEDs performances and improve the stability. Furthermore, among these dual n-type units bipolar hosts, there is also a regular trend in the efficiency orders, i.e., two DPPO-containing hosts led to higher efficiencies and slower efficiency roll-off than two pyridine-containing hosts, and the meta-substituted hosts exhibited higher efficiencies than their para-substituted isomers (Figure 6 (b) and Table 2). This order trend is valid not only for the maximum efficiencies but also for the efficiencies at high brightness region (Figure 6 (b)). For example, at a practical brightness of 1000 cd m−2, devices B1 and B3 containing the meta-linked hosts exhibited high efficiencies of 21.1% and 23.6% (versus 16.1% and 18.7% for B2 and B4), and the efficiency roll-offs related to the maximum values were only 18% and 12% (Figure S16). Even at the extremely high brightness of 10000 cd m−2, the m-POPyCz hosted B3 still remained high efficiencies of 36.7 cd A−1 and 19.1%. The m-POPyCz hosted B3 exhibited the best performance with highest efficiencies and slowest efficiency roll-off among all these blue devices. The following factors should be responsible for these performance. First, the introduction the second n-type unit (DPPO) pulls down the LUMO and improves the electron injection relative to the parent PyCz, which finally reduce the driving voltages and increase the device current and brightness and thus the efficiencies. Secondly, the introduction of DPPO can also facilitate electron transportation based on its n-type nature, which is usually essential to improve OLED performance. However, its bulky feature and the resultant non-coplanar molecular conformation impair the over-large 25



electron current in present device configuration to some extent, which makes the electron transportation match the hole transportation to achieve comparable currents, as verified by the above-mentioned single-carrier devices results (Figure 5). In this way, m-POPyCz gained the best charge balancing ability in its bulky film among these four new hosts, which is the most important factor to contribute to a high charge recombination efficiency and to prevent any possible exciton quenching. In addition, there have some reports that the twisted steric structures can greatly alleviate TTA and TPQ, disrupt host-host aggregation and excimer formation13, as well as confine the triplet exciton on FIrpic, which should account for the good performance of these meta-linked hosts. However, the p-BPyCz hosted blue devices B2 exhibited the lowest efficiencies among B1–B4, with a Von of 2.8 V, a ηext,max of 21.6%, a ηc,max of 45.0 cd A−1, and a ηp,max of 40.4 lm W−1. In addition, the most rapid efficiency roll-off can be observed for B2 (Figure 6 (b)). Although obvious improvement was gained relative to the parent PyCz, we assign the inferior performance of p-BPyCz in comparison with other dual n-type units hosts to its para-linking style and the consequent non-sufficiently high triplet energy of 2.68 eV (FIrpic 2.63 eV) and the less balanced charge transportations.

1000 100

80

10

40 0

1

10

1

B1 B2 B3 B4 B5

10

1 0

2

4 6 Voltage (V)

8

10

100

1000 -2 Brightness (cd m )

26


10000

-1

Power efficiency (lm W )

100

-1

Current efficiency (cd A )

120

100 (b)

10000 -2

-2

160

B1 B2 B3 B4 B5

Brightness (cd m )

200 (a) Current density (mA cm )

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

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Figure 6. (a) The J–V–B characteristics and (b) the efficiency curves for m-BPyCz, p-BPyCz, m-POPyCz, p-POPyCz and PyCz-hosted blue PhOLEDs B1, B2, B3, B4 and B5. The excellent performance of blue PhOLEDs inspired us to further investigate the potential of m-BPyCz, p-BPyCz, m-POPyCz, p-POPyCz and PyCz as hosts for green PhOLEDs. These green PhOLEDs have identical device structures as above blue ones but with 8wt% Ir(ppy)3 as the doped emitter. Their J–V–B characteristics and efficiency curves are shown in Figure 7, and all relevant key performances are summarized in Table 2. Devices G1-G5 displayed the typical green EL (Figure S15 (b)) from Ir(ppy)3. These green devices showed the similar rule as in blue devices. The four new hosts based device G1-G4 all exhibited better performance than PyCz based G5, with reduced turn-on voltages, higher current densities over the whole voltage range, higher brightness, and higher efficiencies, once again confirming the advantage and important contribution of the dual n-type units in these four host materials. Similar to the cases in single-carrier devices and blue devices, the two bipyridine-containing hosts based devices (G1 and G2) have higher current densities than DPPO-containing hosts (G3 and G4) at any giving voltage. And the meta-linked hosts exhibited higher efficiencies and slower efficiency roll-off than their para-isomers, the DPPO-containging hosts always realized higher efficiencies than the bipyridine-containing analogues. In particular, the m-POPyCz hosted device G3 exhibited the best performance with a Von of 2.8 V, a ηext,max of 24.5%, a ηc,max of 85.8 cd A−1, and a ηp,max of 89.8 lm W−1. The same rule observe in these green PhOLEDs 27



as in blue devices further confirmed the important contribution of the dual n-type units in these new bipolar host materials and the improved electron injection and the more balanced charge transportation for high-performance PhOLEDs. 400

100000

100 100

10

0

1 0

2

4 6 Voltage (V)

8

10 G1 G2 G3 G4 G5

10

1

10

-1

-1

Current efficiency (cd A )

1000

200

100

Power efficiency (lm W )

100 (b)

10000 -2

-2

300

G1 G2 G3 G4 G5

Brightness (cd m )

(a) Current density (mA cm )

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

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1

100

1000 -2 Brightness (cd m )

10000

Figure 7. (a) The J–V–B characteristics and (b) the efficiency curves for m-BPyCz, p-BPyCz, m-POPyCz, p-POPyCz and PyCz-hosted green PhOLEDs G1, G2, G3, G4 and G5. Table 2: EL data summary of the blue and green PhOLED.

Devices

Host

Von

Lmax

ηc a

ηp b

ηext a

CIE c

(V)

(cd m−2)

(cd A−1)

(lm W−1)

(%)

(x, y)

Dopant

B1

m-BPyCz

2.9

18942

45.6, 37.1

42.4

25.8, 21.1

0.14, 0.30

B2

p- BPyCz

2.8

15709

45.0, 33.4

40.4

21.6, 16.1

0.14,0.37

B3

m-POPyCz

3

28500

51.9, 44.0

46.5

27.0, 23.6

0.14,0.31

B4

p- POPyCz

3

16100

50.4, 40.7

45.2

23.4, 18.7

0.15,0.38

B5

PyCz

3.5

16633

41.4, 24.1

31.2

21.6, 11.8

0.14,0.33

G1

m-BPyCz

2.8

56957

79.7 59.9

83.4

23.2, 17.3

0.30,0.63

G2

p- BPyCz

2.7

32750

78.1, 67.6

70.1

21.8, 18.0

0.30,0.64

G3

m-POPyCz

2.8

71570

85.8, 66.4

89.8

24.5, 19.0

0.30,0.63

G4

p- POPyCz

2.9

71960

83.0, 67.2

74.5

23.5, 19.0

0.31,0.63

G5

PyCz

3.1

22951

72.1, 57.6

56.6

20.4, 16.2

0.31,0.63

FIrpic

Ir(ppy)3

28


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a

Order of measured values: maximum, then at 1000 cd m−2. b maximum values.

c

CIE: Commission International de I’Eclairage coordinates, measured at 7V.

CONCLUSION A group of “dual n-type unit bipolar host” materials have been developed for application in PhOLEDs. A second n-type unit, either pyridine or diphenylphosphine oxide (DPPO), was incorporated into the original pyridine ring to form the dual n-type unit (bipyridine or DPPO-pyridine) of the novel bipolar host materials in combination with the oligocarbazole as p-type group. The structures of these hosts were tuned by modulating the n-type units. It was demonstrated that three factors are most essential to determine the property and performance of these hosts: (1) The incorporation of the second n-type unit pulls down the LUMO levels and facilitated the electron injection and transportation, universally resulting in lower driving voltages and higher efficiencies in sky-blue and green PhOLEDs in comparison with the reference host without the second n-type unit. (2) Meta-linking style and thus the limited

π-conjugation led to relatively high triplet energies than the para-linkage. (3) The bulky DPPO as the second n-type unit increased the three-dimensional non-coplanar feature of the hosts and retarded electron transportation to certain extent, leading to more balanced positive and negative charge transportations. The enhanced charge balancing ability caused by bulky DPPO group finally resulted in higher efficiency and slow efficiency roll-off in OLEDs. Only when the above three factors take effect simultaneously in one host molecule, the overall parameters of the host are optimized 29



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Page 30 of 36

as much as possible and ideal performance are gained. The m-POPyCz designed in this way, i.e. with meta-linked bulky DPPO as the second n-type unit, exhibited excellent performance with maximum EQE of 27.0% and current efficiency of 51.9 cd A−1 in sky-blue PhOLEDs. Even at high brightness of 1000 and 10000 cd m-2, the efficiencies still remained at 23.6% and 19.1%. The present study demonstrated that the enhancement of electron injection and transportation in dual n-type unit bipolar hosts is necessary to be controlled to a suitable degree to reach an excellent charge balancing state by appropriate molecule design for high-performance PhOLEDs.

Acknowledgements We acknowledge support from the National Natural Science Foundation of China (21374013 and 21421005), the Liaoning Natural Science Foundation (20170540152), the Fundamental Research Funds for the Central Universities (DUT16ZD221), and Program for DUT Innovative Research Team (DUT2016TB12). Supporting Information. The supporting information is available free of charge via the internet at … or from the author. 1H NMR and

13

C NMR spectra,

Synthetic Routes, TGA thermograms, absorption and fluorescence spectra, energy level diagram of PhOLED, EL spectra and external quantum efficiency-brightness curves. REFERENCES (1) Guo, K.; Wang, H.; Wang, Z.; Si, C.; Peng, C.; Chen, G.; Zhang, J.; Wang, G.;

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Kim,

M.;

Lee,

J.-Y.

Engineering

the

Substitution

34


Position

of

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35



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TOC Graphic Abstract

36


Page 36 of 36

Modulation of n-Type Units in Bipolar Host Materials toward

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