[1,2,4]Triazolo[1,5-a]pyridine as Building Blocks for Universal Host

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[1,2,4]Triazolo[1,5-a]pyridine as a building block for universal host materials for high-performance RGB and White PhOLEDs Wenxuan Song, Lijiang Shi, Lei Gao, Peijun Hu, Haichuan Mu, Zhenyuan Xia, Jinhai Huang, and Jianhua Su ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18202 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 25, 2018

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ACS Applied Materials & Interfaces

[1,2,4]Triazolo[1,5-a]pyridine as a building block for universal host materials for highperformance RGB and White PhOLEDs

Wenxuan Song,a Lijiang Shi,a Lei Gao,b Peijun Hu,b Haichuan Mu,c Zhenyuan Xia,d Jinhai Huang *d and Jianhua Su*a a

Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China

University of Science & Technology, Shanghai 200237, PR China. b

c

School of Chemistry and Chemical Engineering, Queen's University, Belfast.

Department of Physics, School of Science, East China University of Science and

Technology, Shanghai 200237, PR China. d

Shanghai Taoe Chemical Technology Co., Ltd, Shanghai, PR China.

KEYWORDS: [1,2,4]triazolo[1,5-a]pyridine, universal host materials, white organic light-emitting diode, high triplet energy, phosphorescent organic light-emitting diodes (PhOLEDs), high efficiency.

ABSTRACT

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The electron-accepting [1,2,4]triazolo[1,5-a]pyridine (TP) moiety was introduced to build bipolar host materials for the first time, and two host materials based on this TP acceptor and carbazole donor, namely o-CzTP, m-CzTP, were designed and synthesized. These two TP-based host materials possess high triplet energy (>2.9 eV) and appropriate HOMO/LUMO levels as well as the bipolar transporting feature, which permit their applicability as universal host materials in multicolor phosphorescent organic light-emitting devices (PhOLEDs). Blue, green and red PhOELDs based on oCzTP and m-CzTP with the same device configuration all show high efficiencies and low efficiency roll-off. The devices hosted by o-CzTP exhibit maximum external quantum efficiencies (ηext) of 27.1%, 25.0% and 15.8% for blue, green and red light emitting, respectively, which are comparable with the best electroluminescene (EL) performance reported for FIrpic-based blue, Ir(ppy)3-based green and Ir(pq)2(acac)based red PhOLEDs equipped with single-component host. The white PhOLEDs based on o-CzTP host and red, green, blue three lumophor were fabricated with the same device structure, which exhibit the maximum current efficiency and ηc of 40.4 cd/A and 17.8%, respectively, with the color rendering index (CRI) value of 75.

INTRODUCTION

Organic light-emitting devices (OLEDs) have received extensive research attention with their promising application in energy-saving flat-panel displays and general lighting over the last three decades. Conventional OLEDs based on fluorescent materials have been widely used in commercial devices due to their


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high colour purity and long operational lifetimes.1-8 However, the traditional fluorescent molecules are limited by their 25% internal quantum efficiencies (IQEs) due to the deactivation of triplet excitations. Compared with traditional fluorescent OLEDs, phosphorescent organic light-emitting devices (PhOLEDs) are more attractive because they can achieve ideally 100% IQEs by harvesting both the singlet and the triplet excitons.9-15 In recent years, the thermally activated delayed fluorescence (TADF) materials have drawn continuous attentions owing to their 100% internal quantum efficiencies result from the upconversion of the triplet exciton from T1 to S1.16-18 TADF materials without rare metal have many advantages over phosphorescent materials, including low cost and rich resources. However, TADF OLEDs often suffer from severe efficiency roll-off and relatively short lifetime due to long excitons lifetime of the TADF materials, which limits the applications of TADF OLEDs to some degree. 18 It is therefore necessary to develop high efficiency PhOLEDs for practical applications. The PhOLEDs usually adopt a host-dopant emitter system, in which dopant molecules are well dispersed in the host matrix inside the emitting layer (EML). This structure can effectively suppress the detrimental triplet-triplet annihilation (TTA) from phosphors. Therefore, host materials, not only the phosphors dopant, play a significant role in the electroluminescence (EL) performance of the whole device.19 In general, to improve the EL performance of PhOLEDs, an ideal host material must meet some fundamental requirements: (1) high triplet energy (ET) for efficient energy transfer to the guest; (2)



appropriate highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels for effective charge injection; (3) high glass transition temperature (Tg) to increase the thermal and morphological stability of the device; (4) balanced charge-transport property for a broadening recombination zone of holes and electrons in the emitting layer.20-24

In comparison with the traditional unipolar hosts, bipolar host materials containing both electron-donating and electron-accepting aromatic moieties can facilitate the charge balance in the EML and achieve much improved EL performance and efficiency roll-off.25 However, the strong intramolecular charge transfer interactions between donor and acceptor groups in bipolar hosts may result in lower triplet energies and the subsequent back-transfer of energy from the guest to the host, thus reducing the device efficiency. To solve this problem, a rational designed non-conjugation linkers, such as sp3-C and Si, or highly twisted π-conjugation spacers, should be introduced in the host molecules to disrupt the π-conjugation.

Numerous bipolar host materials have been reported based on the above strategies, but the research on universal host materials for blue, green and red PhOLEDs is relatively limited. Universal host materials for full-colour PhOLEDs can greatly simplify the fabrication processes and reduce the unfavourable energy loss from the multiple emitting layers.26-27 Ma and coworkers designed and synthesized bipolar host materials p-BISiTPA by


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introducing

hole-transporting

triphenylamine

and

electron-

transporting

benzimidazole into one molecule via a silicon-bridge.28 The p-BISiTPA based devices can achieve the external quantum efficiencies (ηext) of 20.5% for orange red, 22.7% for green and 16.1% for blue. Besides, the two-colour-white device hosted by p-BISiTPA exhibited maximum efficiencies of 19.1% and 51.8 cd A−1. Recently, Wong et al. reported two bipolar host materials, namely o-CPhBzIm and m-CPhBzIm, utilizing a carbazole as hole-transporting unit and two benzimidazoles as electron-transporting units via phenylene-bridge.29 Low efficiency roll-off PhOLEDs based on o-CPhBzIm exhibit excellent ηext of 19.4% for red, 18.9% for green, 15.6% for blue, and 18.5% for white (three-color-based) with the same device structure.

Herein, we report a novel electron-deficient unit, [1,2,4]triazolo[1,5a]pyridine (TP), as a building block for bipolar PhOLED host materials. This Nheterocyclic moiety has been widely used in medicinal chemistry.30-32 Compared with classical electron-transport unit 1,2,4-triazole, fused triazole with a benzene core structure, TP has a larger planar rigid structure, which can be conducive to improving the morphological stability and thermal durability of host materials. 3335

Furthermore, the large and rigid unit helps to realize highly twisted π-

conjugation structure, thus maintain the high triplet energies. We select carbazole as the hole-transporting units to synthesize two TP derivatives, 9,9'-(2([1,2,4]triazolo[1,5-a]pyridin-2-yl)-1,3-phenylene)bis(9H-carbazole) (o-CzTP) and 9,9'-(5-([1,2,4]triazolo[1,5-a]pyridin-2-yl)-1,3-phenylene)bis(9H-carbazole)



(m-CzTP) as universal host materials for PhOLEDs. As excepted, high ET values of 2.93 eV and 2.92 eV were obtained for o-CzTP and m-CzTP due to the highly twisted structure resulted from steric conformation between the substituents and core benzene ring. Blue, green and red PhOLEDs were fabricated by these two universal hosts, and excellent EL performance are obtained. The PhOLEDs hosted by o-CzTP exhibit maximum current efficiency (ηc) of 52.3 cd/A, 82.4 cd/A and 29.9 cd/A (ηext of 27.1%, 25.0% and 15.8%) for FIrpic-based blue, Ir(ppy)3-based green and Ir(pq)2(acac)-based red light emitting, respectively, which are comparable with the best EL performance reported for the universal and individual hosts. Furthermore, o-CzTP was applied as host to fabricate WOLED with three lumophor and the maximum ηc and ηext up to respective 40.4 cd/A and 17.8% were obtained. The chromaticity coordinates (0.44, 0.42) and the correlated colour temperature (CCT) of 3090 K of this WOLED agree well with the warm-white standard illumination, namely the CIE coordinate (0.448, 0.408) with a CCT of 2856 K.

EXPERIMENTAL SECTION

Synthetic routes of the two compounds are outlined in Scheme 1. The intermediates 2,6-2CzBN and 3,5-2CzBN were synthesised according to the literature. 36

Synthesis

of

9,9'-(2-([1,2,4]triazolo[1,5-a]pyridin-2-yl)-1,3-

phenylene)bis(9H-carbazole) (o-CzTP) A mixture of 2,6-2CzBN (4.30g,


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10.0mmol), pyridin-2-amine (1.13g, 12.0mmol), CuBr (76 mg, 0.54 mmol), 1,10-phenanthroline (97mg, 0.54mmol) and ZnI2 (0.34g, 1.08mmol) were added to 30mL of 1,2-Dichlorobenzene. The reaction mixture was stirred at 130 °C for 24 h under atmospheric air. After cooling to room temperature, the reaction was diluted with ethyl acetate and filtered over glass filter. The filtrate was concentrated and purified by silica gel column chromatography using a petroleum and ethyl acetate mixture (v:v=3:1) to afford white solid o-CzTP (2.41g, 45.8%).1H NMR (400 MHz, CDCl3) δ = 8.02 (d, J=7.7 Hz, 4H), 7.92 (dd, J=8.5 Hz, 7.3 Hz, 1H), 7.80 (d, J=7.7 Hz, 1H), 7.75 (d, J=6.9 Hz, 1H), 7.37 -7.30 (m, 8H), 7.23 – 7.12 (m, 4H), 7.02 – 6.88 (m, 2H), 6.45 – 6.41 (m, 1H). 13C NMR (101 MHz, CDCl3) δ 158.78, 149.93, 141.54, 139.27, 131.68, 131.33, 129.79, 128.35, 127.55, 125.85, 123.30, 119.83, 119.68, 116.08, 112.84, 110.04. HRMS (ESI, m/z): [M+H]+ calcd for: C36H24N5, 526.2032, found, 526.2025. Anal. Calcd for C36H23N5: C, 82.26; H, 4.41; N,13.32. Found: C, 82.63;H, 3.82; N, 13.29.

Synthesis

of

9,9'-(5-([1,2,4]triazolo[1,5-a]pyridin-2-yl)-1,3-

phenylene)bis(9H-carbazole) (m-CzTP) The m-CzTP was prepared from 3,52CzBN following the same procedure for synthesis of o-CzTP. White solid. Yield: 68.4%.1H NMR (400 MHz, CDCl3) δ = 8.56 (d, J=1.9 Hz, 2H), 8.48 (d, J=6.8 Hz, 1H), 8.07 (d, J=7.7 Hz, 4H), 7.82 (t, J=1.9 Hz, 1H), 7.66 (d, J=9.0 Hz, 1H), 7.53 (d, J=8.2 Hz, 4H), 7.42 (d, J=8.2 Hz, 1H), 7.36 (t, J=7.3 Hz, 4H), 7.23 (t, J=7.5 Hz, 4H), 6.92 (dd, J=9.9 Hz, 3.8 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 162.74, 151.85, 140.64, 139.87, 134.69, 129.93, 128.53, 126.50, 126.29,



124.66, 123.71, 120.49, 120.45, 116.72, 114.15, 109.89. HRMS (ESI, m/z): [M+H]+ calcd for: C36H24N5, 526.2032, found, 526.2026. Anal. Calcd for C36H23N5: C, 82.26; H, 4.41; N,13.32. Found: C, 82.33;H, 3.86; N, 13.28.

Scheme 1. Synthetic routes of o-CzTP and m-CzTP.

RESULTS AND DISCUSSION

Synthesis and characterization. Two different TP based host materials (o-CzTP and m-CzTP), in which TP core is connected to the central benzene ring of N,Ndicarbazolyl-3,5-benzene (mCP) by ortho and meta-position, were synthesized. The key synthetic step in the preparation of the new materials is shown in Scheme 1. The intermediates 2,6-2CzBN and 3,5-2CzBN were prepared by previous literature.36 The TP core can be obtain by transition-metal-catalysed reaction of 2-aminopyridines and nitriles.37 It is noteworthy that the compound o-CzTP was obtained with the yield of 35.8% while the yield of m-CzTP was 68.5%, which can be attributed to the high steric demand resulting from the ortho


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linkage mode. Both compounds were characterized by 1H and 13C NMR spectra, high-resolution mass spectrometry (HRMS) and the elemental analysis. The detailed synthetic procedures and characterization of o-CzTP and m-CzTP are described in the Experimental Section. To further determine the molecular structure, single-crystal X-ray crystallographic analysis was carried out and the result is shown in Figure 1. This molecule shows a highly twisty structure with the dihedral angles θ1 = 62.9°, θ2 = 68.7° and θ3 = 51.41°, which will be beneficial to achieve high ET.

Figure 1. Molecular structure of o-CzTP.

Photophysical properties. The UV–vis absorption, photoluminescence (PL) in toluene (1×10−5 M) and the vacuum evaporated thin films of o-CzTP and m-CzTP are shown in Figure 2 and summarized in Table 1. The absorption peaks of oCzTP and m-CzTP are similar, indicating that these absorptions are mainly stemmed from the Bis(N- carbazolyl)benzene. The strong absorption peak at 290 nm can be attributed to the π−π* transition of the carbazole-centered unit, while the weaker peaks at approximately 325 nm and 340 nm can be assigned to n-π*



transition of long-pair electrons from the carbazole units to the entire aromatic compound. In addition, The Eg calculated from the absorbance edge of the absorption spectrum of o-CzTP and m-CzTP is 3.60 eV and 3.56 eV, respectively. Both o-CzTP and m-CzTP in toluene solution exhibit purple-blue emission with the peak at 397 and 373 nm. To further investigate their optical properties, the emission spectra of o-CzTP and m-CzTP were studied in various solutions and vacuum evaporated solid-state film (Figure S5). According to Figure S5, the emission maxima show solvatochromic shifts from toluene to DMF, and the fluorescent peaks are shifted up to 28 nm for o-CzTP and 42 nm for m-CzTP. This result indicates that m-CzTP has more serious intramolecular charge transfer (ICT) character than that of o-CzTP. As compared to o-CzTP, m-CzTP in the film state exhibits about 40 nm more red shifting with respect to that in the solution. This reflects relatively strong π-π stacking interactions existing in vacuum evaporated film of m-CzTP, which can be unfavourable for producing high carrier-transportation ability. According to the onset of the fluorescence spectra in toluene, the singlet states (S1) are estimated to be 3.39 eV and 3.58 eV for oCzTP and m-CzTP, respectively.38

The triplet energies (ET) of o-CzTP and m-CzTP were calculated to be 2.93 eV and 2.92 eV from the highest-energy vibronic sub-band in 77 K phosphorescence spectra (Figure 2b). The triplet energies of these two mCP derivatives are closed to that of parent mCP (ET = 2.9 eV), indicating that such molecular design strategy, by interrupting π-conjugation via the highly twisted link mode (ortho or


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meta-linked), does not significantly reduce the triplet energy. Both compounds exhibit high enough triplet energies to act as host materials for all RGB (red, green and blue) emitters. Meanwhile, relatively small singlet–triplet energy splitting (∆EST = 0.46 eV) can be achieved in o-CzTP. In contrast, the ∆EST of most hosts reported are in the range of 0.5 – 1.0 eV.16, 36 It is well known that an ideal host material should have not only a high ET, but also a lower S1 to improve carrier injecting into the emitting layers (EMLs) and reduce the driving voltage.39-41 Furthermore, this also suggests that TP core can be used as the electron-acceptor to build TADF materials.

Figure 2. (a) Absorption, fluorescence spectra at room temperature and (b)



phosphorescent spectra in 2-methyl-tetrahydrofuran at 77 K of two compounds.

Thermal and morphology properties. To examine the thermal stabilities of oCzTP and m-CzTP, the thermal gravimetric analyses (TGA) and differential scanning calorimetry (DSC) measurements were performed. The results are shown in Figure 3, Figure S6 and summarized in Table 1. Both two compounds exhibit good thermal stability with the thermal decomposition temperatures (Td, corresponding to 5% weight loss) of 417 oC and 435 oC, respectively. A high glass-transition temperature (Tg = 139 oC) of m-CzTP was obtained while the Tg of o-CzTP was not observed even in the second DSC scan. Notably, the Tg values of m-CzTP is 70 oC higher than that of mCP,42 suggesting that the introduction of TP core improves the their thermal stability tremendously. Since the morphology stability is a critical factor for achieving high-efficiency PhOLEDs, atomic force microscopy (AFM) was employed to further study thermal stabilities of the vacuum evaporated thin films of o-CzTP and m-CzTP. As shown in Figure 3b, the respective root-mean-square (RMS) roughness of o-CzTP and m-CzTP were obtained to 0.525 nm and 0.454 nm. The RMS roughness of oCzTP and m-CzTP still remain at respective 0.541 nm and 0.402 nm, after thermal annealing at 80 oC for 3 hours under air atmosphere. The AFM results show that o-CzTP and m-CzTP possess great thermal and morphological stability in the neat film, which will be beneficial to improve the EL performance of PhOLEDs.


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Table 1. Physical properties of o-CzTP and m-CzTP. λabs

λema

Egb

HOMO/LUMOc

HOMO/LUMOd

E Se

ET f

Tdg

Tgg

(nm)

(sol/film, nm)

(eV)

(eV)

(eV)

(eV)

(eV)

(oC)

(oC)

o-CzTP

292, 325, 338

397/405

3.60

-5.38/-1.78

-5.51/-1.57

3.39

2.93

417

130

m-CzTP

292, 325, 338

373/414

3.56

-5.34/-1.78

-5.71/-2.01

3.58

2.92

435

NA

a

-5

b

Determined with toluene solution (10 M) and thin film (20 nm); Estimated from onset of the absorption spectra (Eg = 1241/λonset); c Obtained from the onset of the oxidation voltages in CH2Cl2 and the equation: ELUMO = EHOMO + Eg; d Obtained using Gaussian 09 at the B3LYP/631G(d) level; e Obtained by the onset of the fluorescence spectra in toluene; f Estimated from the phosphorescence spectrum in 2-methyl-tetrahydrofuran at 77 K; g Measured from the DSC and TGA instruments, respectively.

Figure 3. (a) TGA curves of o-CzTP and m-CzTP and (b) AFM images of the thermally evaporated thin films of o-CzTP and m-CzTP. (ⅰ), (ⅱ) without thermal



treatment and (ⅲ), (ⅳ) after thermal treatment at 80 oC for 3 h in ambient air. Electrochemical properties and DFT calculations. To measure the frontier energy levels of these two materials, cyclic voltammetric (CV) tests are carried out, and the data are collected in Table 1. As showed in Figure 4, these two compounds have a quasi-reversible oxidation wave at around 1 V. According to the equation of EHOMO = -(Eonset + 4.4), the HOMO energy levels of o-CzTP and m-CzTP can ox be obtained to be -5.38 eV and -5.34 eV.43-44 And the LUMO energy levels of 1.78 eV for both materials can be determined by the EHOMO and Eg. These appropriate frontier energies can facilitate hole injection from the TCTA (−5.3/−2.36 eV) and electron injection from the TmPyPB (−6.68/−2.73 eV) to the EML.

Figure 4. Cyclic voltammograms of o-CzTP and m-CzTP in CH2Cl2


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To further understand the molecular orbitals (MOs) of these two compounds, density functional theory (DFT) calculation was carried out. 6-31G+(d) basic st and B3LYP functional with exchange correlation was employed. The results for theoretical computation are shown in Figure 5. The LUMO orbital energy levels of these two compounds are mainly distributed on the electron-transport core phenyl-[1,2,4]triazolo[1,5-a]pyridine, while the HOMO levels are mainly located at the hole-transport carbazole moieties. The spatially separated HOMO and LUMO levels of those hosts can be conducive to reduce ∆EST, thus improving the EL performance of PhOLEDs.



Figure 5. Spatial distributions of the HOMO and LUMO levels for o-CzTP and mCzTP

Carrier transport properties. Bipolar charge transport property of host material plays a significant role in achieving excellent PhOLEDs EL performance. Hole and electron-only devices with the structures of ITO/PEDOT:PSS (40 nm)/TAPC (5 nm)/host (60 nm)/TAPC (5 nm)/Al (80 nm) and ITO/TmPyPB (5 nm)/host (60 nm)/TmPyPB(5 nm)/LiF (0.6 nm)/Al(80 nm) were fabricated to evaluate their bipolar charge transport property. The chemical structures and frontier energy levels of these materials of the devices are shown in Figure S8.45 In hole-only devices, holes can be injected from the anode easily in the presence of PEDOT:PSS and TAPC, and the electron injection were prevented due to the large electron barrier of TAPC/Al interface. Accordingly, in electron-only devices, LiF and TmPyPB were used to facilitate the electron injection from the cathode, and the ITO/TmPyPB interface was designed to prevent holes injection due to the large hole injection barrier.

From Figure 6, both hole-only and electron-only devices of four compounds exhibit high current densities in the voltage range typically suitable for OLEDs, indicating these compounds have both hole-transporting and electrontransporting abilities and possess bipolar carrier-transport feature. Meanwhile, the hole and electron currents of the m-CzTP are much lower than those of oCzTP, which can be explained by the difference of their molecular stacking order


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in neat film. As shown in Figure 2a, 40 nm red-shifting in PL spectrum of the neat film compared to that of solution suggests that relatively strong π-π stacking interactions exist in vacuum evaporated film of m-CzTP, which may suppress charge transportation.

Figure 6. J-V curves of hole-only device and electron-only device for two compounds.

Electroluminescent properties. To investigate the performance of TP based bipolar hosts o-CzTP and m-CzTP, we initially fabricated blue devices by utilizing FIrpic as the emitter with the structures of ITO/PEDOT:PSS (40 nm)/TAPC (45 nm)/TCTA (5 nm)/FIrpic:hosts (X %wt, 20 nm)/TmPyPB (X nm)/LiF (0.6 nm)/Al (80 nm)., in which PEDOT:PSS, TAPC, TCTA, FIrpic doped in o-CzTP and m-CzTP, TmPyPB and LiF are employed as the hole injection layer (HIL), hole transporting layer (HTL), electron-blocking layer



(EBL), emission layer (EML), electron-transporting layer (ETL)/hole-blocking layer (HBL) and electron injection layer (EIL), respectively. Figure S8 shows the frontier energy levels and the molecular structures of each layer in the device. To optimize the EL performance, a series of o-CzTP based devices with various dopant concentrations in the EML and ETL thickness were fabricated. The EL performance of the blue PhOLEDs are shown in Figure S9 and Figure S10, from which the optimal configuration of blue PhOLED was obtained as following: ITO/PEDOT:PSS (40 nm)/TAPC (45 nm)/TCTA (5 nm)/FIrpic:hosts (10%wt, 20 nm)/TmPyPB (55 nm)/LiF (0.6 nm)/Al (80 nm). The EL performance of devices B1 and B3 (B stands for blue PhOLED) based on o-CzTP and m-CzTP are displayed in Figure 7 and summarized in Table 2. Both devices B1 and B3 exhibit a main peak at 472 nm and a shoulder peak at 495 nm from FIrpic. Compared with device B3, device B1 exhibits an enlarged shoulder peak, which can be attributed to the recombination zone shift resulted from different charge transport abilities of o-CzTP and m-CzTP.43 Device B3 hosted by m-CzTP exhibits the maximum current efficiency (ηc,max), maximum power efficiency (ηp,max) and maximum external quantum efficiency (ηext,max) of 44.8 cd A-1, 27.4 lm W-1, and 23.9%, respectively. In comparison, device B1 based on o-CzTP demonstrats better EL performance with ηc,max, ηp,max and ηext,max of 52.3 cd A-1, 40.5 lm W-1 and 27.1%, respectively. The obviously improved EL performance and lower turn on-voltage of B1 may be attributed to the following reasons. The first and the one mentioned above, compared with m-CzTP, o-CzTP has a small


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∆EST, which means narrow energy gaps. The narrow energy gaps can improve carrier injecting into the emitting layer and reduce the driving voltage, thus improve the EL performance.39-41, 46 Secondly, as shown in Figure 6, o-CzTP has higher hole and electron mobility, which can facilitate both hole and electron transport in the EML, thus improving the EL performance of PhOLEDs.36, 47 This can be confirmed by the EL spectra of devices B1 and B3 as noted above. It is noteworthy that both devices B1 and B3 show low efficiency roll off. At a practical application brightness of 1000 cd m-2, ηext still remains at 24.3% with the efficiency roll-off value of 10.3% for device B1 and 21.3% with the efficiency roll-off value of 10.9% for device B3. It is worth mentioning that the efficiencies of our work (27.1%, 52.3 cd A-1) are among the best values ever reported for blue single-host devices, such as Kido’s work21 (21.8%, 48.6 cd A1

), Ma’s work48 (27.5%, 49.4 cd A-1), Lee’s work3 (30.1%, 53.6 cd A-1), Li’s

work49 (25.3%, 55.6 cd A-1) and Leung’s work50 (27%, 57.5 cd A-1).

Next, o-CzTP and m-CzTP were used as hosts to fabricated green devices G1 and G4 (G stands for the green PhOLED) utilizing 5wt% Ir(ppy) 3 as the dopant with the same device structure as the device B1. The current-voltageluminance (J–V–L) characteristics and EL efficiency as well as the EL spectra are shown in Figure 7 and summarized in Table 2. Similar to the case of blue PhOLEDs, the turn-on voltages of these green devices also decrease from 4.6 V to 3.4 V by replacing the host of o-CzTP with m-CzTP. The o-CzTP hosted device G1 and m-CzTP hosted device G4 display good EL performance with the



respective maximum ηext of 83.4 cd A-1 (62.1 lm W-1 and 25.0%) and 76.5 cd A1

(42.8 lm W-1 and 22.3%). Furthermore, devices G1 and G4 exhibit low

efficiency roll-off again. At the luminance of 1000 cd m-2, the ηc of devices G1 and G4 still remain at 77.2 cd A-1 and 67.6 cd A-1, respectively. These improved EL performance and lower turn-on voltages of G1 reinforces the significant contributions of high carrier mobility and much-improved carrier injection again. Just like the case of blue PhOLEDs, the ηext and ηc of green PhOLEDs (25.0%, 83.4 cd A-1) can be compared with the best data reported for Ir(ppy) 3-based devices, such Lee’s work20 (30.4%, 93.6 cd A-1), Li’s work49 (28.2%, 98.2 cd A1

) and Ma’s work 45 (26.1%, 92.5 cd A-1).

To investigate the performance of o-CzTP and m-CzTP as host materials for lower triplet energy iridium dopant, red devices R1 (hosted by o-CzTP) and R4 (hosted by m-CzTP) (R stands for red PhOLED) are fabricated using the identical structures. As shown in Figure 7c and Figure 7f, devices R1 and R4 utilizing 5wt% Ir(pq)2acac as the dopant exhibit pure red emission with the only emission peak of 600 nm, which indicates full energy transfer from these two host materials to low triplet energy emitter. Device R1 shows the maximum ηc of 29.9 cd A-1 and maximum ηext of 15.8%. Device R4 achieves similar performance with ηc of 29.6 cd A-1 and ηext of 17.2 %. Low efficiency roll-off is achieved for devices R1 and R4 once again. For example, device R4 shows ηc of 26.1 cd A-1 and ηext of 15.3% at the brightness of 1000 cd m-2. The efficiencies for our devices are among the best efficiencies of Ir(pq)2(acac)-based red-emitting PhOLEDs so


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far reported, such as Lin’s work51 (15.4%, 25.6 cd A-1), Lee’s work52(16.4%, 26.4 cd A-1) and Lee’s work53 (20.0%).

Figure 7. The Current density-voltage-luminance (J-V-L) characteristics of devices (a) B1, G1, R1, W2 and (d) devices B3, G4, R4; the current and external quantum efficiency (EQE) versus current density of (b) devices B1,G1,R1, W2 and (e) devices B3, G4, R4; and the EL spectra at 6 V of (c) devices B1,G1,R1 and (f) devices B3, G4, R4.



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Table 2. Electroluminescence Characteristics of PhOLEDs devices

hosts

Von(V)a

Lmax(cd m-2)b

ηc (cd A-1)c

ηp (lm W-1)d

ηext(%)c

CIE(x, y)e

B1

o-CzTP

3.3

12678

52.3 51.8 46.5

40.5

27.1 26.8 24.3

0.15, 0.34

B3

m-CzTP

4.5

10363

44.8 43.4 40.1

27.4

23.9 23.3 21.3

0.15, 0.33

G1

o-CzTP

3.4

43124

83.4 82.2 77.2

62.1

25.0 24.5 23.1

0.25, 0.64

G4

m-CzTP

4.6

36028

76.5 70.9 67.6

42.8

22.3 20.5 19.5

0.26, 0.64

R1

o-CzTP

3.4

21538

29.9 27.1 20.9

24.9

15.8 14.2 10.9

0.60, 0.39

R4

m-CzTP

4.8

18568

29.6 29.5 26.1

16.9

17.2 17.2 15.3

0.61, 0.38

W1

o-CzTP

4.2

26663

41.8 40.5 32.1

25.1

16.9 16.0 12.7

0.34, 0.47

W2

o-CzTP

4.2

26276

44.0 42.5 32.3

26.3

17.8 17.1 12.9

0.44, 0.42

aV

m-2; b

on,

c Order

turn-on voltage, at 1 cd Lmax, maximum luminance; of measured values: -2 -2 d e maximum, at 100 cd m , at 1 000 cd m ; Maximum value, Measured at 6 V.

Inspired by the excellent performance of o-CzTP in blue, green and red PhOLEDs, white OLEDs (WOLEDs) with the structure of ITO/PEDOT:PSS (45 nm)/TAPC

(45

nm)/TCTA

(5

nm)/FIrpic:

host

(10

wt

%,

10

nm)/Ir(pq)2acac:Ir(ppy)3: o-CzTP (X wt%, 5 wt %, 10 nm)/TmPyPB (55 nm)/LiF (0.6 nm)/Al （X = 0.5 and 1 for W1 and W2）were fabricated. The WOLEDs with three lumophor contain sky-blue and green-red EML. This strategy is reported to be capable to realize the balance between efficiency and colour quality.54-55 The J−V−L characteristics and EL efficiency of the device W2 (W stands for the white PhOLED) is shown in Figure 7 and summarized in Table 2 while the characterization results for W1 were shown in Figure S13. Figure 7 shows the EL spectra of these two devices, the peaks at 472 nm can be assigned to the blue phosphor FIrpic from the blue EML while the green and red peaks belong to the Ir(ppy)3 and Ir(pq)2(acac) from the green-red EML. By tuning the dopant concentration of red phosphor, the device W2 (the concentration of Ir(pq)2(acac) is at 1%) achieves a balanced blue, green and red emission as shown


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in Figure 8b. Consistently excellent chromaticity were achieved for device W2 with minor change of the CIE coordinates from (0.44, 0.42) to (0.42, 0.43) as the luminance increases from 100 to 20 000 cd m-2. Besides, a comparatively high CRI (colour rendering index) value of 75 is achieved for the device W2, which can be attributed to the three-colour system we employed. It's worth mentioning that the chromaticity coordinates (0.44, 0.42) and the CCT (3090 K) of the device W2 agree well with the warm-white standard illumination (CIE coordinates of (0.448, 0.408) and CCT of 2856 K). Compared with the standard white light with CIE coordinates (0.33, 0.33), warm white light can offer more comfortable and relaxed sensation during the night.56 Finally, the device W2 exhibits a slightly high turn on voltage of 4.1 V due to double EMLs. Similar to RGB devices, excellent efficiency (ηext,max = 17.8% and ηc,max = 44.0 cd A-1) and low roll-off for white device W2 are achieved. For example, the ηext remains at 16.7% and 14.2% at a brightness of 1000 and 5000 cd/m2, respectively.



Figure 8. EL spectra of white device W1 (a) and W2 (b) at different brightness.

CONCLUSIONS

In summary, a novel electron-accepting unit [1,2,4]triazolo[1,5-a]pyridine (TP) was developed to build bipolar host materials for PhOLEDs. Two TP based compounds were synthesized for their application as universal hosts in blue, green and red PhOLEDs and white OLEDs. The introduction of TP group into the mCP framework can improve the stability of thermal and morphology while avoid reducing their ETs. The blue, green and red PhOLEDs with a common device structure utilizing o-CzTP as the host can achieve the optimal ηext of 27.1%, 25.0% and 15.8%, which are comparable with the best values reported for


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PhOLEDs with universal and individual hosts. The white PhOLEDs with three lumophor, double emission layers and single host of o-CzTP exhibit high ηc of 44.0 cd/A and ηext of 17.8% as well as the superior colour stability. Thus, the novel TP unit can be an ideal electron-accepting building block to design bipolar host materials for high-performance full colour PhOLEDs. ASSOCIATED CONTENT Supporting Information.

Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000

Experimental details for physical measurements; characterization of o-CzTP and mCzTP; device fabrication; PL spectra in various solutions; thermal properties; energy diagram; device performance

Single-crystal structures for o-CzTP (CD17081.cif).

AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected] (J.S.).

* E-mail: [email protected] (J.H.).

ORCID



Jianhua Su: 0000-0002-4746-6022 Peijun Hu: 0000-0002-6318-1051 Notes

The authors declare no competing financial interest.

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(54) Ding, L.; Dong, S.-C.; Jiang, Z.-Q.; Chen, H.; Liao, L.-S., Orthogonal Molecular Structure for Better Host Material in Blue Phosphorescence and Larger OLED White Lighting Panel. Adv. Funct. Mater. 2015, 25 (4), 645-650. (55) Pan, B.; Wang, B.; Wang, Y.; Xu, P.; Wang, L.; Chen, J.; Ma, D., A simple carbazole-N-benzimidazole bipolar host material for highly efficient blue and single layer white phosphorescent organic light-emitting diodes. J. Mater. Chem. C 2014, 2 (14), 2466-2469. (55) Wang, Y.-K.; Sun, Q.; Wu, S.-F.; Yuan, Y.; Li, Q.; Jiang, Z.-Q.; Fung, M.-K.; Liao, L.-S., Thermally Activated Delayed Fluorescence Material as Host with Novel Spiro-Based Skeleton for High Power Efficiency and Low Roll-Off Blue and White Phosphorescent Devices. Adv. Funct. Mater. 2016, 26 (43), 7929-7936.



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[1,2,4]Triazolo[1,5-a]pyridine as Building Blocks for Universal Host

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