- Email: [email protected]
A multifunctional bipolar host material based on phenanthroimidazole for efficient green and red PhOLEDs with low turn-on voltage
Accepted Manuscript A multifunctional bipolar host material based on phenanthroimidazole for efficient green and red PhOLEDs with low turn-on voltage Xiang Chen, Xu-Ming Zhuang, Zhong-Yi Wang, Jie-Ji Zhu, Shan-Shun Tang, Xu-Hui Zheng, Yu Liu, Qing-Xiao Tong PII:
S1566-1199(19)30114-4
DOI:
https://doi.org/10.1016/j.orgel.2019.03.013
Reference:
ORGELE 5151
To appear in:
Organic Electronics
Received Date: 7 January 2019 Revised Date:
20 February 2019
Accepted Date: 7 March 2019
Please cite this article as: X. Chen, X.-M. Zhuang, Z.-Y. Wang, J.-J. Zhu, S.-S. Tang, X.-H. Zheng, Y. Liu, Q.-X. Tong, A multifunctional bipolar host material based on phenanthroimidazole for efficient green and red PhOLEDs with low turn-on voltage, Organic Electronics (2019), doi: https://doi.org/10.1016/ j.orgel.2019.03.013. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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A
multifunctional
bipolar
host
material
based
on
phenanthroimidazole for efficient green and red PhOLEDs with
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low turn-on voltage Xiang Chena, Xu-Ming Zhuangb, Zhong-Yi Wanga, Jie-Ji Zhua, Shan-Shun Tanga, Xu-Hui Zhenga, Yu Liub*, Qing-Xiao Tonga* a
Department of Chemistry and Key Laboratory for Preparation and Application of Ordered
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Structural Material of Guangdong Province, Shantou University, Guangdong, 515063, P. R. China. E-mail: [email protected]
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and
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Engineering, Jilin University, Changchun, Jilin 130012, P. R. China Email: [email protected]
Bipolar blue or deep-blue emitting materials are particularly effective to achieve
fluorophore,
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high-performance full color OLEDs. In this study, a novel deep-blue bipolar PPI-F-Cz
was
designed
and
synthesized
by
attaching
an
electron-withdrawing chromophore phenanthro[9,10-d] imidazole (PI) and an
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electron-donating group carbazole to the sp3-hybridized C9 atom of the fluorene. Profiting from balanced carrier mobility, the molecule shows high photoluminescence
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(PL) and electroluminescence (EL) efficiencies. Using PPI-F-Cz as a non-doped emitting layer (EML), an organic light-emitting device exhibits high performance with CIE color coordinates of (0.155, 0.055), a maximum external quantum efficiency (EQE) of 4.64%, current efficiency (CE) of 7.68 cd/A, and power efficiency (PE) of 8.04 lm/W. Additionally, equipped with the bipolar transport properties and high triplet energy (2.40 eV), PPI-F-Cz was allowed to be used as an efficient host for green and red PhOLEDs with the maximum EQEs of 16.83% and 20.98%, CEmax of 58.47 cd/A and 25.19 cd/A, PEmax of 53.66 lm/W and 23.28 lm/W respectively.
ACCEPTED MANUSCRIPT 1. Introduction Organic light emitting diodes (OLEDs) have received great attention from both the research community and industry for their excellent potential in full color flat-panel displays and solid-state lighting [1, 2]. Traditional OLEDs were fabricated
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using fluorescent materials, which can only utilize excitons in the singlet state, corresponding to 25% of all the electronically generated excitons [3]. Thanks to the exploration of many researchers, phosphorescence organic light-emitting devices (PhOLEDs) breaks through the 25% limit by using triplet excitons, achieving 100%
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exciton utilization [4]. However, owing to the long lifetime of triplet excited states, the phosphorescent emitters are typically doped into host materials at low
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concentration to effectively suppress the triplet-triplet annihilation (TTA) and/or concentration quenching effects in the emission layer (EML) of PhOLEDs [5, 6]. Bipolar host materials, possessing balanced injecting/transporting properties to broaden the hole-electron recombination zone have drawn increasing attention due to their successful application in high performance PhOLEDs [7-10].
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Achieving high-performance full color OLEDs by introducing bipolar blue or deep-blue emitting materials is a particularly effective strategy, these materials can not only provide a more balanced carrier transporting ability along with suitable
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highest occupied molecular orbit (HOMO) and lowest unoccupied molecular orbit (LUMO) energy levels to broaden the hole-electron recombination zone, but also can
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reduce production costs and simplify the manufacturing process [11-13]. Phenanthroimidazole (PI) is widely used in the design of bipolar host materials
due to its bipolar character, which can balance carrier transporting property leading to better luminance efficiency [14, 15]. carbazole-based groups containing nitrogen atoms with a sp3 hybrid mode tend to donate electrons, thus exhibiting good hole injecting/transporting abilities, Different from triphenylamine moieties, carbazole units show a more rigid molecular structure and poorer π-electron-donating ability [16-22]. Here, 9,9-diphenyl-fluorene is used as a bridging group to link PI and carbazole, because fluorene itself also has good carrier transport properties and the
ACCEPTED MANUSCRIPT sp3-hybridized C9 atom of the fluorene can interrupt conjugation between PI and carbazole moieties which is expected to obtain bipolar host materials with high triplet energy. As a result, a non-doped device based on PPI-F-Cz was achieved with EQEmax of 4.64%, CIE coordinate of (0.155, 0.055), CEmax of 7.68 cd/A, and PEmax of
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8.04 lm/W. Moreover, due to its proper triplet energy, a series of highly efficient green (EQEmax: 16.83%、CEmax: 58.47 cd/A and PEmax: 53.66 lm/W) and red (EQEmax: 20.98%、CEmax: 25.19 cd/A and PEmax: 23.28 lm/W) PhOLEDs were obtained by
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using PPI-F-Cz as a universal host material. It is worth noting that all the devices
2.Experimental Section 2.1 Synthesis
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exhibit a low turn-on voltage mainly due to balanced carrier transporting ability.
Scheme 1 shows the synthetic route of PPI-F-Cz. All the reagents and solvents for the synthesis were purchased from commercial sources and used directly without
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further purification. Compound 1 was synthesized as reported [23]. Suzuki cross-coupling reaction was used twice to synthesize the compound 2 and target product PPI-F-Cz. The crude product was purified by silica column chromatography
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analysis.
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and characterized with 1H NMR spectroscopy, mass spectrometry and elemental
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Scheme 1. Synthetic routes and molecular structure of PPI-F-Cz Synthesis of Compound 2. A mixture of compound 1 (1.45 g, 2.92 mmol), 9,9-bis(4-bromophenyl)-9H-fluorene (1.15 g, 2.92 mmol), 160 mg (0.137 mmol)
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tetrakis-(triphenylphosphine) palladium(0) (Pd(PPh3)4), 8 mL aqueous K2CO3 (2 M) with 8 mL ethanol and 30 mL toluene were added into a 150 mL degassed three-necked flask and refluxed under an argon atmosphere. After 24 h, the mixture
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was cooled to room temperature, the organic phase was washed with 20 mL water and extracted with dichloromethane and dried over anhydrous MgSO4 before removing the solvent. Finally, the residue was purified by column chromatography (petroleum
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ether: CH2Cl2 = 1:4) to give a pure white powder. Yield: 0.95 g (47.5%). Synthesis of PPI-F-Cz. The synthesis process is similar to compound 2. Final
product is a pure white powder. Yield: 0.56 g (53%). 1H NMR (400 MHz, CD2Cl2) δ 8.82 (dd, J = 22.1, 8.4 Hz, 2H), 8.19 (d, J = 7.6 Hz, 2H), 7.87 (dd, J = 19.0, 8.0 Hz, 6H), 7.72 (d, J = 7.2 Hz, 5H), 7.69-7.59 (m, 9H), 7.59-7.53 (m, 4H), 7.53-7.43 (m, 6H), 7.39 (t, J = 8.2 Hz, 6H), 7.35-7.24 (m, 4H), 7.20 (d, J = 8.5 Hz, 1H). 13C NMR (126 MHz, CDCl3) δ 150.96 (d, J = 11.0 Hz), 150.60 (s), 145.86-145.18 (m), 140.76 (d, J = 6.8 Hz), 140.17 (d, J = 6.4 Hz), 139.74 (s), 138.59 (dd, J = 25.5, 20.5 Hz), 137.51 (s), 136.75 (s), 130.18 (s), 129.96-129.53 (m), 129.46-128.98 (m),
ACCEPTED MANUSCRIPT 128.84-128.51 (m), 128.22 (dd, J = 13.5, 7.9 Hz), 127.73 (dd, J = 25.7, 16.1 Hz), 127.23 (d, J = 7.7 Hz), 127.09-126.51 (m), 126.20 (d, J = 12.6 Hz), 125.94 (s), 125.60 (s), 124.86 (s), 124.10 (s), 123.06 (dd, J = 45.0, 35.9 Hz), 120.84 (s), 120.42-120.14 (m), 119.95 (s), 109.82 (s). MS (ESI) (m/z): Calculated for C70H45N3: 927.36 Found
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[M + H]+: 928.3647. 2.2 Characterization 1
H and
13
C NMR measurement was recorded with a Varian Gemin-400
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spectrometer. Electrospray ionization mass spectrometry (ESI/MS) was performed on a PE SCIEX APIMS spectrometer. Decomposition temperature (Td) was measured
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with a TA Instrument TGAQ50 at a heating rate of 10 K/min under nitrogen atmosphere. Glass transition temperature (Tg) was determined on a TA Instrument DSC2910. Absorption and photoluminescence spectra were measured with a Perkin-Elmer Lambda 2S UV-Vis spectrophotometer and a Perkin-Elmer LS50B Luminescence spectrophotometer. Cyclic voltammetry was performed on a BAS
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100B/W electrochemical analyzer with a three-electrode system (a platinum disk as working electrode, a platinum wire as the auxiliary electrode, a silver wire as the pseudo-reference electrode with Fc/Fc+ as internal standard, which has an absolute
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highest occupied molecular orbital (HOMO) level of 4.80 eV). Nitrogen saturated DCM was used as solvent with 0.1 mol/L tetrabutylammonium hexafluorophosphate
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as the supporting electrolyte. The geometric and electronic structure of them were calculated via Density functional theory (DFT) with the three parameter Becke-style hybrid functional (B3LYP). HOMO and LUMO were predicated by 6-31G (d, p) basis set with the Gaussian 03 program package. PL lifetime was measured on a Horiba Jobin Yvon FL-TCSPC fluorescence spectrophotometer. 2.3 Device Fabrication and Measurement Pre-patterned indium tin oxide (ITO) glass substrates with a sheet resistance of 15 Ω/square were routinely cleaned organic solvents, deionized water, dried under N2
ACCEPTED MANUSCRIPT flow and then stored in an oven at 120 °C before using. After a 15 min UV-ozone treatment, the ITO substrates were immediately transferred into a deposition chamber with a base pressure of 5 × 10 -7 Torr for organic and cathode depositions. Deposition of the organic layers were monitored with a quartz oscillating crystal and controlled at
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1 Å/s. The cathodes were prepared by deposition of LiF at a rate of 0.1-0.2 Å s-1 and then Al (5 Å/s, 150 nm) via thermal evaporation through a shadow mask. Electroluminescent spectra and the corresponding CIE coordinates were measured with a Spectra scan PR650 photometer. Current-voltage-luminance (J-V-L)
3. Results and Discussion 3.1 Thermal Properties
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atmosphere without device encapsulation.
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characteristics were recorded with a Keithley 2400 Source meter under ambient
Thermal properties of PPI-F-Cz were investigated by thermogravimetric analysis
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(TGA) and differential scanning calorimetry (DSC) under N2 protection at a heating rate of 10 K/min. It has a high decomposition temperature (Td, corresponding to 5% weight loss) of 502 oC and a glass transition temperature (Tg) of 217 oC (Figure 1 and Table 1). Such good thermal stability should be attributed to the bulky and rigid
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skeleton of the PI unit [24, 25]. Different from triphenylamine moiety in PPI-F-TPA, carbazole units show a more rigid molecular structure, which makes PPI-F-Cz show
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3.2 Photophysical Properties
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Figure 1. TGA and DSC curves of PPI-F-Cz
UV-vis absorption and PL spectra of PPI-F-Cz in diluted dichloromethane solution (~10-6 mol/L) and film are shown in Figure 2 (see also Table 1). The lowest absorption band at ~265 nm may be originated from the π-π* transition of isolated
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benzene ring connected with imidazole [26]. Obvious absorption peaks at ~310 nm and ~ 330 nm were observed, which should be assigned to the π-π* transition of the PI group [27]. PPI-F-Cz exhibits a strong deep blue emission at 415 nm in DCM with
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a high photoluminescent quantum yield (PLQY) of 77.5%. Moreover, there is only 13 nm redshift observed in the solid thin film comparing with that in solution, indicating
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the π-π interaction in solid state is effectively compressed by the rigidly twisted molecular structure. Through the phosphorescence spectrum, the triplet level of PPI-F-Cz was measured to be 2.40 eV, which is sufficient for sensitizing phosphorescent dopants with ETs below 2.40 eV [28]. Therefore, PPI-F-Cz might be a promising candidate as both a blue emitter and a host for green and red PhOLEDs. To further investigate intramolecular charge transfer (ICT) characteristics of PPI-F-Cz, we also measured the emission spectra in toluene, DCM, THF and acetonitrile solvents with different polarity. As shown in Figure 2, with increasing solvent polarity, there is no obvious red-shift in emission spectra, indicating a weak
ACCEPTED MANUSCRIPT ICT interaction in PPI-F-Cz. We speculate that this is mainly because sp3-hybridized C9 atom of the fluorene makes the molecule highly twisted, which effectively
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interrupts the conjugation.
Figure 2. Absorption in DCM and PL spectra of PPI-F-Cz in different solvents and solid thin film. 3.3 Electrochemical Properties
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We used cyclic voltammetry to investigate the electrochemical properties of PPI-F-Cz and calculate its energy level. As shown in Figure 3, PPI-F-Cz shows quasi-reversible oxidation and reduction processes with an oxidation onset potential
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(Eonset) of 1.05 V. According to the equation: HOMO = - (Eonset + 4.8 – EFc) eV and Eg = 3.13 eV, its HOMO and LUMO are calculated to be -5.45 and -2.32 eV, respectively.
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The HOMO of PPI-F-Cz is much higher than that of the classic host material 4,4-N,N’- dicarbazolylbiphenyl (CBP: 6.0 eV) and matches well with the common hole transporting material 4,4’-bis(N-(1-naphthyl)-N-phenylamine)-biphenyl (NPB: 5.4 eV), implying the negligible hole-injection barrier in devices. Besides, the LUMO of PPI-F-Cz is deeper than PPI-F-TPA (LUMO: -2.16 eV), which is more conducive to electron injection.
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Figure 3. Cyclic voltammogram of PPI-F-Cz 3.4 Theoretical Calculation
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To gain a better insight into the geometric and electric properties of PPI-F-Cz at the molecular level, DFT calculation (B3LYP/6-31G(d)) was performed. Orbitals were analyzed using Multiwfn program (version 3.5) [29]. As displayed in Figure 4, the HOMO of PPI-F-Cz is distributed over the whole PI group and part of the phenyl linker. Whereas, the electron distributions of the corresponding LUMO is dominantly
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localized on the linkage of imidazole and fluorene. Therefore, a partial HOMO-LUMO separation is realized which results in potential bipolar charger transport properties and a larger oscillator strength (f = 1.1245). Compared to
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PPI-F-TPA, in which the HOMO and LUMO are almost completely spatially separated, such frontier molecular orbital feature of PPI-F-Cz leads to effective
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electronic communication, and obvious overlap is more likely to endow it an efficient luminescence emitter in OLED [24, 30]. To evaluate the nature of excited states, natural transition orbitals (NTOs) of S0
→ Sn (n = 1-5) and T1 were calculated by using the B3LYP/6-31G(d) method (Figure S1) and analyzed by using the Multiwfn program (version 3.5)[29]. The spatial distribution of holes and particles of the S1 state are different for PPI-F-Cz and PPI-F-TPA. The holes mainly distribute on the triphenylamine, the particles mainly distribute on the linkage of imidazole and fluorene of PPI-F-TPA. The transition of S0-S1 clearly reveal the CT transition characteristic. However, the spatial distribution
ACCEPTED MANUSCRIPT of holes and particles of PPI-F-Cz has large proportion overlap, indicating the HLCT transition characteristic, which is beneficial to increase the Φf. The rest of high energy excited states are similar for PPI-F-Cz and PPI-F-TPA. Collectively, such excited state features of PPI-F-Cz further confirm that applying carbazole moiety as the donner is
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performance.
Figure 4. Calculated HOMO and LUMO distributions of PPI-F-Cz and PPI-F-TPA
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Table 1. The thermal and photophysical properties of PPI-F-Cz Td/a) Tg
λabs (solu)
[°C]
[nm]
502/217
265/310/330
λem b) [nm]
Φf c) [%]
HOMO/LUMOd)
ETe)
(solu/film/phos)
(solu/film)
[eV]
[eV]
415/428/516
77.8/63.3
-5.45/-2.32
2.40
Compound
PPI-F-Cz
5% weight loss temperature; b) The PL spectra in DCM, neat film and 2-MeTHF glass matrix at 77 K;
c)
The solution-state quantum yield measured in DCM by using 9,10-diphenylanthracene as a
standard (Φf = 0.90 in cyclohexane) and the film-state quantum yield on the quatrz plate estimated by an intergrating sphere apparatus；d) HOMO: calculated from the onset oxidation potentials of the CV cures, LUMO: estimate from the equation: ELUMO = EHOMO + Eg;
e)
Estimated from the
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3.5 Electrical Properties To evaluate electrical properties of PPI-F-Cz, hole- and electron-only devices
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were fabricated with the respective configuration of ITO/NPB(20 nm)/PPI-F-Cz(80 nm)/NPB(20 nm)/Al(150 nm) and ITO/TPBI(20 nm)/PPI-F-Cz(80 nm)/TPBI(20 nm)/LiF(1 nm)/Al(150 nm). As shown in Figure 5, PPI-F-Cz performs well in both transporting positive and negative carriers, which is attributed to the good electron
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transporting property of the carbazole group and the relatively deep LUMO level. It can be seen from the Figure 5 that as the voltage increases, a significant current flows
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through the device, indicating that the compound PPI-F-Cz has a bipolar characteristic, which is beneficial to balance the holes and electrons in the emitting layer based on PPI-F-Cz [31, 32]. Also, the electron-only device shows slightly lower current density than the corresponding hole-only device, which should be attributed to the LUMO
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energy level of the compound is not deep enough.
Figure 5. Current density-voltage characteristics of the electron- and the hole-only devices.
3.6 Electroluminescence Properties We fabricated a multilayer non-doped device (M) with a configuration of ITO/N, N’-bis-(1-naphthyl)-N,N’-diphenyl-1,10-biphenyl-4,4’-diamine
(NPB,
60
nm)/4,4’,4’’-tris(N-carbazolyl) triphenyl amine (TCTA, 5 nm)/ PPI-F-Cz (30
ACCEPTED MANUSCRIPT nm)/1,3,5-tri(phenyl-2-benzimidazolyl)benzene (TPBI, 30 nm)/ LiF (1 nm)/ Al (100 nm) (Figure 6). In this device, ITO is the anode, Al is the cathode, NPB is used as the hole-transporting layer (HTL) and TCTA is a buffer layer, TPBI is the electron-transporting layer (ETL) and hole-blocking layer, PPI-F-Cz is the emitting
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layer (EML), LiF is electron injection layer. Key device performance parameters are listed in Table 2. From Figure 6, the multilayer device (M) shows blue emission peaks at 444 nm with corresponding CIE coordinates of (0.155, 0.055). No excimer or exciplex peak can be observed, implying that hole and electron recombination is well
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confined within the EML [33]. J-V-L characteristics show a maximum brightness of 1213 cd/m2 (3 V), low starting voltage indicates that the energy gap between the
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HOMO level of the light-emitting layer and the hole transport layer is small. And the device also delivers a maximum EQE of 4.64%, a maximum CE of 8.04 cd/A and a maximum PE of 7.68 lm/W, respectively. The non-doped device’s relevant data are
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summarized in Table 2.
Figure 6. Energy-level diagram of the materials used in the devices.
Figure
7.
EL
spectra
density-voltage-luminescence
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under
different
characteristics
(b),
voltages
(a),
CE-Current
Current density-PE
light-emitting devices.
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characteristics (c) and EQE and luminance plots (d) of PPI-F-Cz-based non-doped
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In view of the high triplet energy level, we have further studied the electroluminescence properties of PPI-F-Cz as the host material of green and red
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phosphors. The corresponding device structure is ITO/MoO3 (1 nm)/(TAPC) (40 nm)/EML (30 nm)/TPBI (30 nm)/LiF (1 nm)/Al (100 nm). In this device, TAPC is a hole transport layer, LiF and TPBI are electron injection and electron transport layers, respectively, and the light-emitting layer is PPI-F-Cz: 20wt% Ir(ppy)3 or 5wt% Ir(MDQ)(acac).
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Figure 8. EL spectra of green and red PhOLEDs based on PPI-F-Cz (a); Current density-voltage-Luminescence characteristics of green and red PhOLEDs (b);
luminance plots (d)
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CE-Current density-PE characteristics of green and red emitting devices (c); EQE-
The green and red phosphors’ device performance is shown in Figure 8. Figure 8
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(a) is an electroluminescence spectrum of green and red PhOLEDs based on PPI-F-Cz, the luminescence spectra corresponding to the spectra of Ir(ppy)3 and Ir(MDQ)(acac),
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respectively, no PPI-F-Cz’s emission peak and new emission peaks appearing in fabricated PhOLEDs, indicating complete transfer of energy from host to guest materials, and the turn-on voltage is 2.75 V, such low turn-on voltage is mainly due to more balanced carrier transport performance and smaller injection energy barrier [34, 35]. According to Figure 8 (c) and (d), the green PhOLEDs, using PPI-F-Cz as the host, had a maximum CE of 58.47 cd/A, a maximum PE of 53.66 lm/W and a maximum EQE of 16.83%. The red-emitting device showed better performance with a maximum CE of 25.19 cd/A, a maximum PE of 23.28 lm/W and a maximum EQE of 20.98%, respectively. we speculate that PPI-F-Cz is more matched with
ACCEPTED MANUSCRIPT red-emitting material during the Dexter energy transfer process. Compared to the recent reports about highly efficient red devices as far as we know, PPI-F-Cz is revealed to be one of the best hosts for red PhOLEDs [36-39]. Such high efficiency may be attributed to the balanced charge flow and wide recombination zone in EML,
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resulting from the bipolar features and relatively high ET1 of PPI-F-Cz. Theirs related device performance is comparable to the material PPI-F-TPA that our research group has reported, the relevant data are summarized in Table 2.
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Table 2 Key performance summary of the PPI-F-Cz- and PPI-F-TPA-based blue, green and red emitting devices
PPI-F-Cz
PPI-F-TPA
Vona)
λEL
CE max b) -1
PE max c) -1
EQE max d)
CIE
[V]
[nm]
[cd A ]
[lm W ]
[%]
[x, y]
B
3.0
444
7.68
8.04
4.64
(0.155, 0.055)
G
2.75
515
58.47
53.66
16.83
(0.327, 0.611)
R
2.75
624
25.19
23.28
20.98
(0.612, 0.379)
B
4.2
425
1.35
1.00
3.11
(0.16, 0.05)
G
2.7
510
57.0
60.0
15.6
(0.34, 0.62)
R
2.8
612
27.0
28.3
12.5
(0.58, 0.42)
Von is the voltage at 1 cd/m2.
b)
The maximum current efficiency. c) The maximum
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a)
Emitter
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Conclusions
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power efficiency. d) The maximum external quantum efficiency
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In summary, we synthesized a bipolar host luminescent material PPI-F-Cz, which
has very good thermal stability and good film formation. The rigidly twisted molecular structure based on an indirect linkage, the sp3-hybridized C9 atom of fluorene reduces the charge transfer and intermolecular interactions in the molecule. The luminescence quantum yield and the triplet energy level are high, thus ensuring that the molecule can not only perform well in a multilayer non-doped device (EQE of 4.64%, CE of 7.68 cd/A and PE of 8.04 lm/W at CIE coordinates of (0.155, 0.055)), but also serve as a host for green and red phosphors excellently, showing one of the highest EQE of 20.98%(Red). All these devices with low turn-on voltage indicate
ACCEPTED MANUSCRIPT PPI-F-Cz is a suitable material for the common device fabrication to realize high performance. The results also broaden the way of thinking for designing novel molecular structure for building multifunctional deep-blue emitters.
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Acknowledgements
This work was supported by the National Natural Science Foundation of China
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(Project No. 51673113).
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[10] C. Fan, L.P. Zhu, T.X. Liu, B. Jiang, D.G. Ma, J.G. Qin, C.L. Yang, Using an Organic Molecule with Low Triplet Energy as a Host in a Highly Efficient Blue Electrophosphorescent Device, Angew Chem Int Edit, 53 (2014) 2147-2151. [11] A. Chaskar, H.F. Chen, K.T. Wong, Bipolar Host Materials: A Chemical Approach for Highly Efficient Electrophosphorescent Devices, Adv Mater, 23 (2011) 3876-3895. [12] C.W. Lee, J.Y. Lee, Benzo[4,5]thieno[2,3-b]pyridine derivatives as host materials for high efficiency green and blue phosphorescent organic light-emitting diodes, Chem Commun, 49 (2013) 1446-1448. [13] W. Li, J. Zhao, L. Li, X. Du, C. Fan, C. Zheng, S. Tao, Efficient solution-processed blue and white OLEDs based on a high-triplet bipolar host and a blue TADF emitter, Org Electron, 58 (2018) 276-282. [14] X.H. Ouyang, X.L. Li, L. Ai, D.B. Mi, Z.Y. Ge, S.J. Su, Novel "Hot Exciton" Blue Fluorophores for High Performance Fluorescent/Phosphorescent Hybrid White Organic Light-Emitting Diodes with Superhigh Phosphorescent Dopant Concentration and Improved Efficiency Roll-Off, Acs Appl Mater Inter, 7 (2015) 7869-7877. [15] D. Liu, M.X. Du, D. Chen, K.Q. Ye, Z.L. Zhang, Y. Liu, Y. Wang, A novel tetraphenylsilane-phenanthroimidazole hybrid host material for highly efficient blue fluorescent, green and red phosphorescent OLEDs, J Mater Chem C, 3 (2015) 4394-4401. [16] S.H. Ye, Y.Q. Liu, K. Lu, W.P. Wu, C.Y. Du, Y. Liu, H.T. Liu, T. Wu, G. Yu, An Alternative Approach to Constructing Solution Processable Multifunctional Materials: Their Structure, Properties, and Application in High-Performance Organic Light-Emitting Diodes, Adv Funct Mater, 20 (2010) 3125-3135. [17] Y.X. Zhang, L. Zhang, L.S. Cui, C.H. Gao, H. Chen, Q. Li, Z.Q. Jiang, L.S. Liao, Control of Conjugation Degree via Position Engineering to Highly Efficient Phosphorescent Host Materials, Org Lett, 16 (2014) 3748-3751. [18] Z.Q. Liu, Q. Fang, D. Wang, D.X. Cao, G. Xue, W.T. Yu, H. Lei, Trivalent boron as an acceptor in donor-pi-acceptor-type compounds for single- and two-photon excited fluorescence, Chem-Eur J, 9 (2003) 5074-5084. [19] O. Kwon, S. Barlow, S.A. Odom, L. Beverina, N.J. Thompson, E. Zojer, J.L. Bredas, S.R. Marder, Aromatic amines: A comparison of electron-donor strengths, J Phys Chem A, 109 (2005) 9346-9352. [20] Z. Gao, Y. Liu, Z. Wang , F. Shen, H. Liu, G. Sun, L. Yao, Y. Lv, P. Lu, Y. Ma, High-Efficiency Violet-Light-Emitting Materials Based on Phenanthro[9,10-d]imidazole, Chemistry – A European Journal, 19 (2013) 2602-2605. [21] Y. Zhang, S.L. Lai, Q.X. Tong, M.F. Lo, T.W. Ng, M.Y. Chan, Z.C. Wen, J. He, K.S. Jeff, X.L. Tang, W.M. Liu, C.C. Ko, P.F. Wang, C.S. Lee, High Efficiency Nondoped Deep-Blue Organic Light Emitting Devices Based on Imidazole-pi-triphenylamine Derivatives, Chem Mater, 24 (2012) 61-70. [22] Z. Jiang, T. Ye, C. Yang, D. Yang, M. Zhu, C. Zhong, J. Qin, D. Ma, Star-Shaped Oligotriarylamines with Planarized Triphenylamine Core: Solution-Processable, High-Tg Hole-Injecting and Hole-Transporting Materials for Organic Light-Emitting Devices, Chem Mater, 23 (2011) 771-777.
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[23] X.Y. Tang, Q. Bai, T. Shan, J.Y. Li, Y. Gao, F.T. Liu, H. Liu, Q.M. Peng, B. Yang, F. Li, P. Lu, Efficient Nondoped Blue Fluorescent Organic Light-Emitting Diodes (OLEDs) with a High External Quantum Efficiency of 9.4% @ 1000 cd m(-2) Based on Phenanthroimidazole-Anthracene Derivative, Adv Funct Mater, 28 (2018). [24] B. Liu, J.W. Zhao, C.Y. Luo, F. Lu, S.L. Tao, Q.X. Tong, A novel bipolar phenanthroimidazole derivative host material for highly efficient green and orange-red phosphorescent OLEDs with low efficiency roll-off at high brightness, J Mater Chem C, 4 (2016) 2003-2010. [25] G. Li, J.W. Zhao, D. Zhang, Z.C. Shi, Z.L. Zhu, H.Q. Song, J.J. Zhu, S.L. Tao, F. Lu, Q.X. Tong, Mechanochromic asymmetric sulfone derivatives for use in efficient blue organic light-emitting diodes, J Mater Chem C, 4 (2016) 8787-8794. [26] W.-C. Chen, C.-S. Lee, Q.-X. Tong, Blue-emitting organic electrofluorescence materials: progress and prospective, J Mater Chem C, 3 (2015) 10957-10963. [27] X. Tang, Q. Bai, Q. Peng, Y. Gao, J. Li, Y. Liu, L. Yao, P. Lu, B. Yang, Y. Ma, Efficient Deep Blue Electroluminescence with an External Quantum Efficiency of 6.8% and CIEy < 0.08 Based on a Phenanthroimidazole–Sulfone Hybrid Donor–Acceptor Molecule, Chem Mater, 27 (2015) 7050-7057. [28] X.L. Yang, G.J. Zhou, W.Y. Wong, Functionalization of phosphorescent emitters and their host materials by main-group elements for phosphorescent organic light-emitting devices, Chem. Soc. Rev., 44 (2015) 8484-8575. [29] T. Lu, F.W. Chen, Multiwfn: A multifunctional wavefunction analyzer, J. Comput. Chem., 33 (2012) 580-592. [30] L.-S. Cui, Y.-M. Xie, Y.-K. Wang, C. Zhong, Y.-L. Deng, X.-Y. Liu, Z.-Q. Jiang, L.-S. Liao, Pure Hydrocarbon Hosts for ≈100% Exciton Harvesting in Both Phosphorescent and Fluorescent Light-Emitting Devices, Adv Mater, 27 (2015) 4213-4217. [31] W.-C. Chen, Q.-X. Tong, C.-S. Lee, The Development of Phenanthroimidazole Derivatives in Blue-Emitting Organic Electroluminescence, Science of Advanced Materials, 7 (2015) 2193-2205. [32] Y.-J. Pu, G. Nakata, F. Satoh, H. Sasabe, D. Yokoyama, J. Kido, Optimizing the Charge Balance of Fluorescent Organic Light-Emitting Devices to Achieve High External Quantum Efficiency Beyond the Conventional Upper Limit, Adv Mater, 24 (2012) 1765-1770. [33] H. Wang, L. Meng, X. Shen, X. Wei, X. Zheng, X. Lv, Y. Yi, Y. Wang, P. Wang, Highly Efficient Orange and Red Phosphorescent Organic Light-Emitting Diodes with Low Roll-Off of Efficiency using a Novel Thermally Activated Delayed Fluorescence Material as Host, Adv Mater, 27 (2015) 4041-4047. [34] Z. Chen, X.-K. Liu, C.-J. Zheng, J. Ye, X.-Y. Li, F. Li, X.-M. Ou, X.-H. Zhang, A high-efficiency hybrid white organic light-emitting diode enabled by a new blue fluorophor, J Mater Chem C, 3 (2015) 4283-4289. [35] M.J. Cho, S.J. Kim, S.H. Yoon, J. Shin, T.R. Hong, H.J. Kim, Y.H. Son, J.S. Kang, H.A. Um, T.W. Lee, J.-K. Bin, B.S. Lee, J.H. Yang, G.S. Chae, J.H. Kwon, D.H. Choi, New Bipolar Host Materials for Realizing Blue Phosphorescent Organic Light-Emitting Diodes with High Efficiency at 1000 cd/m2, Acs Appl Mater Inter, 6
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(2014) 19808-19815. [36] W.-C. Chen, Y. Yuan, Z.-L. Zhu, Z.-Q. Jiang, S.-J. Su, L.-S. Liao, C.-S. Lee, De novo design of D–σ–A molecules as universal hosts for monochrome and white phosphorescent organic light-emitting diodes, Chem Sci, 9 (2018) 4062-4070. [37] K. Gao, K. Liu, X.-L. Li, X. Cai, D. Chen, Z. Xu, Z. He, B. Li, Z. Qiao, D. Chen, Y. Cao, S.-J. Su, An ideal universal host for highly efficient full-color, white phosphorescent and TADF OLEDs with a simple and unified structure, J. Mater. Chem. C, 5 (2017) 10406-10416. [38] F.-M. Hsu, C.-H. Chien, Y.-J. Hsieh, C.-H. Wu, C.-F. Shu, S.-W. Liu, C.-T. Chen, Highly efficient red electrophosphorescent device incorporating a bipolar triphenylamine/bisphenylsulfonyl-substituted fluorene hybrid as the host, J Mater Chem, 19 (2009). [39] A.m. Thangthong, N. Prachumrak, R. Tarsang, T. Keawin, S. Jungsuttiwong, T. Sudyoadsuk, V. Promarak, Blue light-emitting and hole-transporting materials based on 9,9-bis(4-diphenylaminophenyl)fluorenes for efficient electroluminescent devices, J Mater Chem, 22 (2012).
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Highlights: 1) PPI-F-Cz performs well in a multilayer non-doped device (EQE of 4.64%, CE of 7.68 cd/A and PE of 8.04 lm/W at CIE coordinates of
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(0.155, 0.055)) 2) PPI-F-Cz is as an efficient host for green and red PhOLEDs with the maximum EQEs of 16.83% and 20.98%.
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3) All the devices exhibit a low turn-on voltage (<3.0 eV).
S1566-1199(19)30114-4
DOI:
https://doi.org/10.1016/j.orgel.2019.03.013
Reference:
ORGELE 5151
To appear in:
Organic Electronics
Received Date: 7 January 2019 Revised Date:
20 February 2019
Accepted Date: 7 March 2019
Please cite this article as: X. Chen, X.-M. Zhuang, Z.-Y. Wang, J.-J. Zhu, S.-S. Tang, X.-H. Zheng, Y. Liu, Q.-X. Tong, A multifunctional bipolar host material based on phenanthroimidazole for efficient green and red PhOLEDs with low turn-on voltage, Organic Electronics (2019), doi: https://doi.org/10.1016/ j.orgel.2019.03.013. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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A
multifunctional
bipolar
host
material
based
on
phenanthroimidazole for efficient green and red PhOLEDs with
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low turn-on voltage Xiang Chena, Xu-Ming Zhuangb, Zhong-Yi Wanga, Jie-Ji Zhua, Shan-Shun Tanga, Xu-Hui Zhenga, Yu Liub*, Qing-Xiao Tonga* a
Department of Chemistry and Key Laboratory for Preparation and Application of Ordered
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Structural Material of Guangdong Province, Shantou University, Guangdong, 515063, P. R. China. E-mail: [email protected]
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and
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b
Engineering, Jilin University, Changchun, Jilin 130012, P. R. China Email: [email protected]
Bipolar blue or deep-blue emitting materials are particularly effective to achieve
fluorophore,
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high-performance full color OLEDs. In this study, a novel deep-blue bipolar PPI-F-Cz
was
designed
and
synthesized
by
attaching
an
electron-withdrawing chromophore phenanthro[9,10-d] imidazole (PI) and an
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electron-donating group carbazole to the sp3-hybridized C9 atom of the fluorene. Profiting from balanced carrier mobility, the molecule shows high photoluminescence
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(PL) and electroluminescence (EL) efficiencies. Using PPI-F-Cz as a non-doped emitting layer (EML), an organic light-emitting device exhibits high performance with CIE color coordinates of (0.155, 0.055), a maximum external quantum efficiency (EQE) of 4.64%, current efficiency (CE) of 7.68 cd/A, and power efficiency (PE) of 8.04 lm/W. Additionally, equipped with the bipolar transport properties and high triplet energy (2.40 eV), PPI-F-Cz was allowed to be used as an efficient host for green and red PhOLEDs with the maximum EQEs of 16.83% and 20.98%, CEmax of 58.47 cd/A and 25.19 cd/A, PEmax of 53.66 lm/W and 23.28 lm/W respectively.
ACCEPTED MANUSCRIPT 1. Introduction Organic light emitting diodes (OLEDs) have received great attention from both the research community and industry for their excellent potential in full color flat-panel displays and solid-state lighting [1, 2]. Traditional OLEDs were fabricated
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using fluorescent materials, which can only utilize excitons in the singlet state, corresponding to 25% of all the electronically generated excitons [3]. Thanks to the exploration of many researchers, phosphorescence organic light-emitting devices (PhOLEDs) breaks through the 25% limit by using triplet excitons, achieving 100%
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exciton utilization [4]. However, owing to the long lifetime of triplet excited states, the phosphorescent emitters are typically doped into host materials at low
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concentration to effectively suppress the triplet-triplet annihilation (TTA) and/or concentration quenching effects in the emission layer (EML) of PhOLEDs [5, 6]. Bipolar host materials, possessing balanced injecting/transporting properties to broaden the hole-electron recombination zone have drawn increasing attention due to their successful application in high performance PhOLEDs [7-10].
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Achieving high-performance full color OLEDs by introducing bipolar blue or deep-blue emitting materials is a particularly effective strategy, these materials can not only provide a more balanced carrier transporting ability along with suitable
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highest occupied molecular orbit (HOMO) and lowest unoccupied molecular orbit (LUMO) energy levels to broaden the hole-electron recombination zone, but also can
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reduce production costs and simplify the manufacturing process [11-13]. Phenanthroimidazole (PI) is widely used in the design of bipolar host materials
due to its bipolar character, which can balance carrier transporting property leading to better luminance efficiency [14, 15]. carbazole-based groups containing nitrogen atoms with a sp3 hybrid mode tend to donate electrons, thus exhibiting good hole injecting/transporting abilities, Different from triphenylamine moieties, carbazole units show a more rigid molecular structure and poorer π-electron-donating ability [16-22]. Here, 9,9-diphenyl-fluorene is used as a bridging group to link PI and carbazole, because fluorene itself also has good carrier transport properties and the
ACCEPTED MANUSCRIPT sp3-hybridized C9 atom of the fluorene can interrupt conjugation between PI and carbazole moieties which is expected to obtain bipolar host materials with high triplet energy. As a result, a non-doped device based on PPI-F-Cz was achieved with EQEmax of 4.64%, CIE coordinate of (0.155, 0.055), CEmax of 7.68 cd/A, and PEmax of
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8.04 lm/W. Moreover, due to its proper triplet energy, a series of highly efficient green (EQEmax: 16.83%、CEmax: 58.47 cd/A and PEmax: 53.66 lm/W) and red (EQEmax: 20.98%、CEmax: 25.19 cd/A and PEmax: 23.28 lm/W) PhOLEDs were obtained by
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using PPI-F-Cz as a universal host material. It is worth noting that all the devices
2.Experimental Section 2.1 Synthesis
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exhibit a low turn-on voltage mainly due to balanced carrier transporting ability.
Scheme 1 shows the synthetic route of PPI-F-Cz. All the reagents and solvents for the synthesis were purchased from commercial sources and used directly without
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further purification. Compound 1 was synthesized as reported [23]. Suzuki cross-coupling reaction was used twice to synthesize the compound 2 and target product PPI-F-Cz. The crude product was purified by silica column chromatography
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analysis.
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and characterized with 1H NMR spectroscopy, mass spectrometry and elemental
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Scheme 1. Synthetic routes and molecular structure of PPI-F-Cz Synthesis of Compound 2. A mixture of compound 1 (1.45 g, 2.92 mmol), 9,9-bis(4-bromophenyl)-9H-fluorene (1.15 g, 2.92 mmol), 160 mg (0.137 mmol)
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tetrakis-(triphenylphosphine) palladium(0) (Pd(PPh3)4), 8 mL aqueous K2CO3 (2 M) with 8 mL ethanol and 30 mL toluene were added into a 150 mL degassed three-necked flask and refluxed under an argon atmosphere. After 24 h, the mixture
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was cooled to room temperature, the organic phase was washed with 20 mL water and extracted with dichloromethane and dried over anhydrous MgSO4 before removing the solvent. Finally, the residue was purified by column chromatography (petroleum
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ether: CH2Cl2 = 1:4) to give a pure white powder. Yield: 0.95 g (47.5%). Synthesis of PPI-F-Cz. The synthesis process is similar to compound 2. Final
product is a pure white powder. Yield: 0.56 g (53%). 1H NMR (400 MHz, CD2Cl2) δ 8.82 (dd, J = 22.1, 8.4 Hz, 2H), 8.19 (d, J = 7.6 Hz, 2H), 7.87 (dd, J = 19.0, 8.0 Hz, 6H), 7.72 (d, J = 7.2 Hz, 5H), 7.69-7.59 (m, 9H), 7.59-7.53 (m, 4H), 7.53-7.43 (m, 6H), 7.39 (t, J = 8.2 Hz, 6H), 7.35-7.24 (m, 4H), 7.20 (d, J = 8.5 Hz, 1H). 13C NMR (126 MHz, CDCl3) δ 150.96 (d, J = 11.0 Hz), 150.60 (s), 145.86-145.18 (m), 140.76 (d, J = 6.8 Hz), 140.17 (d, J = 6.4 Hz), 139.74 (s), 138.59 (dd, J = 25.5, 20.5 Hz), 137.51 (s), 136.75 (s), 130.18 (s), 129.96-129.53 (m), 129.46-128.98 (m),
ACCEPTED MANUSCRIPT 128.84-128.51 (m), 128.22 (dd, J = 13.5, 7.9 Hz), 127.73 (dd, J = 25.7, 16.1 Hz), 127.23 (d, J = 7.7 Hz), 127.09-126.51 (m), 126.20 (d, J = 12.6 Hz), 125.94 (s), 125.60 (s), 124.86 (s), 124.10 (s), 123.06 (dd, J = 45.0, 35.9 Hz), 120.84 (s), 120.42-120.14 (m), 119.95 (s), 109.82 (s). MS (ESI) (m/z): Calculated for C70H45N3: 927.36 Found
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[M + H]+: 928.3647. 2.2 Characterization 1
H and
13
C NMR measurement was recorded with a Varian Gemin-400
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spectrometer. Electrospray ionization mass spectrometry (ESI/MS) was performed on a PE SCIEX APIMS spectrometer. Decomposition temperature (Td) was measured
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with a TA Instrument TGAQ50 at a heating rate of 10 K/min under nitrogen atmosphere. Glass transition temperature (Tg) was determined on a TA Instrument DSC2910. Absorption and photoluminescence spectra were measured with a Perkin-Elmer Lambda 2S UV-Vis spectrophotometer and a Perkin-Elmer LS50B Luminescence spectrophotometer. Cyclic voltammetry was performed on a BAS
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100B/W electrochemical analyzer with a three-electrode system (a platinum disk as working electrode, a platinum wire as the auxiliary electrode, a silver wire as the pseudo-reference electrode with Fc/Fc+ as internal standard, which has an absolute
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highest occupied molecular orbital (HOMO) level of 4.80 eV). Nitrogen saturated DCM was used as solvent with 0.1 mol/L tetrabutylammonium hexafluorophosphate
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as the supporting electrolyte. The geometric and electronic structure of them were calculated via Density functional theory (DFT) with the three parameter Becke-style hybrid functional (B3LYP). HOMO and LUMO were predicated by 6-31G (d, p) basis set with the Gaussian 03 program package. PL lifetime was measured on a Horiba Jobin Yvon FL-TCSPC fluorescence spectrophotometer. 2.3 Device Fabrication and Measurement Pre-patterned indium tin oxide (ITO) glass substrates with a sheet resistance of 15 Ω/square were routinely cleaned organic solvents, deionized water, dried under N2
ACCEPTED MANUSCRIPT flow and then stored in an oven at 120 °C before using. After a 15 min UV-ozone treatment, the ITO substrates were immediately transferred into a deposition chamber with a base pressure of 5 × 10 -7 Torr for organic and cathode depositions. Deposition of the organic layers were monitored with a quartz oscillating crystal and controlled at
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1 Å/s. The cathodes were prepared by deposition of LiF at a rate of 0.1-0.2 Å s-1 and then Al (5 Å/s, 150 nm) via thermal evaporation through a shadow mask. Electroluminescent spectra and the corresponding CIE coordinates were measured with a Spectra scan PR650 photometer. Current-voltage-luminance (J-V-L)
3. Results and Discussion 3.1 Thermal Properties
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atmosphere without device encapsulation.
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characteristics were recorded with a Keithley 2400 Source meter under ambient
Thermal properties of PPI-F-Cz were investigated by thermogravimetric analysis
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(TGA) and differential scanning calorimetry (DSC) under N2 protection at a heating rate of 10 K/min. It has a high decomposition temperature (Td, corresponding to 5% weight loss) of 502 oC and a glass transition temperature (Tg) of 217 oC (Figure 1 and Table 1). Such good thermal stability should be attributed to the bulky and rigid
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skeleton of the PI unit [24, 25]. Different from triphenylamine moiety in PPI-F-TPA, carbazole units show a more rigid molecular structure, which makes PPI-F-Cz show
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3.2 Photophysical Properties
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Figure 1. TGA and DSC curves of PPI-F-Cz
UV-vis absorption and PL spectra of PPI-F-Cz in diluted dichloromethane solution (~10-6 mol/L) and film are shown in Figure 2 (see also Table 1). The lowest absorption band at ~265 nm may be originated from the π-π* transition of isolated
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benzene ring connected with imidazole [26]. Obvious absorption peaks at ~310 nm and ~ 330 nm were observed, which should be assigned to the π-π* transition of the PI group [27]. PPI-F-Cz exhibits a strong deep blue emission at 415 nm in DCM with
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a high photoluminescent quantum yield (PLQY) of 77.5%. Moreover, there is only 13 nm redshift observed in the solid thin film comparing with that in solution, indicating
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the π-π interaction in solid state is effectively compressed by the rigidly twisted molecular structure. Through the phosphorescence spectrum, the triplet level of PPI-F-Cz was measured to be 2.40 eV, which is sufficient for sensitizing phosphorescent dopants with ETs below 2.40 eV [28]. Therefore, PPI-F-Cz might be a promising candidate as both a blue emitter and a host for green and red PhOLEDs. To further investigate intramolecular charge transfer (ICT) characteristics of PPI-F-Cz, we also measured the emission spectra in toluene, DCM, THF and acetonitrile solvents with different polarity. As shown in Figure 2, with increasing solvent polarity, there is no obvious red-shift in emission spectra, indicating a weak
ACCEPTED MANUSCRIPT ICT interaction in PPI-F-Cz. We speculate that this is mainly because sp3-hybridized C9 atom of the fluorene makes the molecule highly twisted, which effectively
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interrupts the conjugation.
Figure 2. Absorption in DCM and PL spectra of PPI-F-Cz in different solvents and solid thin film. 3.3 Electrochemical Properties
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We used cyclic voltammetry to investigate the electrochemical properties of PPI-F-Cz and calculate its energy level. As shown in Figure 3, PPI-F-Cz shows quasi-reversible oxidation and reduction processes with an oxidation onset potential
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(Eonset) of 1.05 V. According to the equation: HOMO = - (Eonset + 4.8 – EFc) eV and Eg = 3.13 eV, its HOMO and LUMO are calculated to be -5.45 and -2.32 eV, respectively.
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The HOMO of PPI-F-Cz is much higher than that of the classic host material 4,4-N,N’- dicarbazolylbiphenyl (CBP: 6.0 eV) and matches well with the common hole transporting material 4,4’-bis(N-(1-naphthyl)-N-phenylamine)-biphenyl (NPB: 5.4 eV), implying the negligible hole-injection barrier in devices. Besides, the LUMO of PPI-F-Cz is deeper than PPI-F-TPA (LUMO: -2.16 eV), which is more conducive to electron injection.
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Figure 3. Cyclic voltammogram of PPI-F-Cz 3.4 Theoretical Calculation
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To gain a better insight into the geometric and electric properties of PPI-F-Cz at the molecular level, DFT calculation (B3LYP/6-31G(d)) was performed. Orbitals were analyzed using Multiwfn program (version 3.5) [29]. As displayed in Figure 4, the HOMO of PPI-F-Cz is distributed over the whole PI group and part of the phenyl linker. Whereas, the electron distributions of the corresponding LUMO is dominantly
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localized on the linkage of imidazole and fluorene. Therefore, a partial HOMO-LUMO separation is realized which results in potential bipolar charger transport properties and a larger oscillator strength (f = 1.1245). Compared to
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PPI-F-TPA, in which the HOMO and LUMO are almost completely spatially separated, such frontier molecular orbital feature of PPI-F-Cz leads to effective
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electronic communication, and obvious overlap is more likely to endow it an efficient luminescence emitter in OLED [24, 30]. To evaluate the nature of excited states, natural transition orbitals (NTOs) of S0
→ Sn (n = 1-5) and T1 were calculated by using the B3LYP/6-31G(d) method (Figure S1) and analyzed by using the Multiwfn program (version 3.5)[29]. The spatial distribution of holes and particles of the S1 state are different for PPI-F-Cz and PPI-F-TPA. The holes mainly distribute on the triphenylamine, the particles mainly distribute on the linkage of imidazole and fluorene of PPI-F-TPA. The transition of S0-S1 clearly reveal the CT transition characteristic. However, the spatial distribution
ACCEPTED MANUSCRIPT of holes and particles of PPI-F-Cz has large proportion overlap, indicating the HLCT transition characteristic, which is beneficial to increase the Φf. The rest of high energy excited states are similar for PPI-F-Cz and PPI-F-TPA. Collectively, such excited state features of PPI-F-Cz further confirm that applying carbazole moiety as the donner is
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performance.
Figure 4. Calculated HOMO and LUMO distributions of PPI-F-Cz and PPI-F-TPA
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Table 1. The thermal and photophysical properties of PPI-F-Cz Td/a) Tg
λabs (solu)
[°C]
[nm]
502/217
265/310/330
λem b) [nm]
Φf c) [%]
HOMO/LUMOd)
ETe)
(solu/film/phos)
(solu/film)
[eV]
[eV]
415/428/516
77.8/63.3
-5.45/-2.32
2.40
Compound
PPI-F-Cz
5% weight loss temperature; b) The PL spectra in DCM, neat film and 2-MeTHF glass matrix at 77 K;
c)
The solution-state quantum yield measured in DCM by using 9,10-diphenylanthracene as a
standard (Φf = 0.90 in cyclohexane) and the film-state quantum yield on the quatrz plate estimated by an intergrating sphere apparatus；d) HOMO: calculated from the onset oxidation potentials of the CV cures, LUMO: estimate from the equation: ELUMO = EHOMO + Eg;
e)
Estimated from the
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3.5 Electrical Properties To evaluate electrical properties of PPI-F-Cz, hole- and electron-only devices
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were fabricated with the respective configuration of ITO/NPB(20 nm)/PPI-F-Cz(80 nm)/NPB(20 nm)/Al(150 nm) and ITO/TPBI(20 nm)/PPI-F-Cz(80 nm)/TPBI(20 nm)/LiF(1 nm)/Al(150 nm). As shown in Figure 5, PPI-F-Cz performs well in both transporting positive and negative carriers, which is attributed to the good electron
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transporting property of the carbazole group and the relatively deep LUMO level. It can be seen from the Figure 5 that as the voltage increases, a significant current flows
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through the device, indicating that the compound PPI-F-Cz has a bipolar characteristic, which is beneficial to balance the holes and electrons in the emitting layer based on PPI-F-Cz [31, 32]. Also, the electron-only device shows slightly lower current density than the corresponding hole-only device, which should be attributed to the LUMO
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energy level of the compound is not deep enough.
Figure 5. Current density-voltage characteristics of the electron- and the hole-only devices.
3.6 Electroluminescence Properties We fabricated a multilayer non-doped device (M) with a configuration of ITO/N, N’-bis-(1-naphthyl)-N,N’-diphenyl-1,10-biphenyl-4,4’-diamine
(NPB,
60
nm)/4,4’,4’’-tris(N-carbazolyl) triphenyl amine (TCTA, 5 nm)/ PPI-F-Cz (30
ACCEPTED MANUSCRIPT nm)/1,3,5-tri(phenyl-2-benzimidazolyl)benzene (TPBI, 30 nm)/ LiF (1 nm)/ Al (100 nm) (Figure 6). In this device, ITO is the anode, Al is the cathode, NPB is used as the hole-transporting layer (HTL) and TCTA is a buffer layer, TPBI is the electron-transporting layer (ETL) and hole-blocking layer, PPI-F-Cz is the emitting
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layer (EML), LiF is electron injection layer. Key device performance parameters are listed in Table 2. From Figure 6, the multilayer device (M) shows blue emission peaks at 444 nm with corresponding CIE coordinates of (0.155, 0.055). No excimer or exciplex peak can be observed, implying that hole and electron recombination is well
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confined within the EML [33]. J-V-L characteristics show a maximum brightness of 1213 cd/m2 (3 V), low starting voltage indicates that the energy gap between the
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HOMO level of the light-emitting layer and the hole transport layer is small. And the device also delivers a maximum EQE of 4.64%, a maximum CE of 8.04 cd/A and a maximum PE of 7.68 lm/W, respectively. The non-doped device’s relevant data are
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summarized in Table 2.
Figure 6. Energy-level diagram of the materials used in the devices.
Figure
7.
EL
spectra
density-voltage-luminescence
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under
different
characteristics
(b),
voltages
(a),
CE-Current
Current density-PE
light-emitting devices.
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characteristics (c) and EQE and luminance plots (d) of PPI-F-Cz-based non-doped
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In view of the high triplet energy level, we have further studied the electroluminescence properties of PPI-F-Cz as the host material of green and red
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phosphors. The corresponding device structure is ITO/MoO3 (1 nm)/(TAPC) (40 nm)/EML (30 nm)/TPBI (30 nm)/LiF (1 nm)/Al (100 nm). In this device, TAPC is a hole transport layer, LiF and TPBI are electron injection and electron transport layers, respectively, and the light-emitting layer is PPI-F-Cz: 20wt% Ir(ppy)3 or 5wt% Ir(MDQ)(acac).
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Figure 8. EL spectra of green and red PhOLEDs based on PPI-F-Cz (a); Current density-voltage-Luminescence characteristics of green and red PhOLEDs (b);
luminance plots (d)
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CE-Current density-PE characteristics of green and red emitting devices (c); EQE-
The green and red phosphors’ device performance is shown in Figure 8. Figure 8
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(a) is an electroluminescence spectrum of green and red PhOLEDs based on PPI-F-Cz, the luminescence spectra corresponding to the spectra of Ir(ppy)3 and Ir(MDQ)(acac),
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respectively, no PPI-F-Cz’s emission peak and new emission peaks appearing in fabricated PhOLEDs, indicating complete transfer of energy from host to guest materials, and the turn-on voltage is 2.75 V, such low turn-on voltage is mainly due to more balanced carrier transport performance and smaller injection energy barrier [34, 35]. According to Figure 8 (c) and (d), the green PhOLEDs, using PPI-F-Cz as the host, had a maximum CE of 58.47 cd/A, a maximum PE of 53.66 lm/W and a maximum EQE of 16.83%. The red-emitting device showed better performance with a maximum CE of 25.19 cd/A, a maximum PE of 23.28 lm/W and a maximum EQE of 20.98%, respectively. we speculate that PPI-F-Cz is more matched with
ACCEPTED MANUSCRIPT red-emitting material during the Dexter energy transfer process. Compared to the recent reports about highly efficient red devices as far as we know, PPI-F-Cz is revealed to be one of the best hosts for red PhOLEDs [36-39]. Such high efficiency may be attributed to the balanced charge flow and wide recombination zone in EML,
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resulting from the bipolar features and relatively high ET1 of PPI-F-Cz. Theirs related device performance is comparable to the material PPI-F-TPA that our research group has reported, the relevant data are summarized in Table 2.
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Table 2 Key performance summary of the PPI-F-Cz- and PPI-F-TPA-based blue, green and red emitting devices
PPI-F-Cz
PPI-F-TPA
Vona)
λEL
CE max b) -1
PE max c) -1
EQE max d)
CIE
[V]
[nm]
[cd A ]
[lm W ]
[%]
[x, y]
B
3.0
444
7.68
8.04
4.64
(0.155, 0.055)
G
2.75
515
58.47
53.66
16.83
(0.327, 0.611)
R
2.75
624
25.19
23.28
20.98
(0.612, 0.379)
B
4.2
425
1.35
1.00
3.11
(0.16, 0.05)
G
2.7
510
57.0
60.0
15.6
(0.34, 0.62)
R
2.8
612
27.0
28.3
12.5
(0.58, 0.42)
Von is the voltage at 1 cd/m2.
b)
The maximum current efficiency. c) The maximum
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a)
Emitter
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power efficiency. d) The maximum external quantum efficiency
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In summary, we synthesized a bipolar host luminescent material PPI-F-Cz, which
has very good thermal stability and good film formation. The rigidly twisted molecular structure based on an indirect linkage, the sp3-hybridized C9 atom of fluorene reduces the charge transfer and intermolecular interactions in the molecule. The luminescence quantum yield and the triplet energy level are high, thus ensuring that the molecule can not only perform well in a multilayer non-doped device (EQE of 4.64%, CE of 7.68 cd/A and PE of 8.04 lm/W at CIE coordinates of (0.155, 0.055)), but also serve as a host for green and red phosphors excellently, showing one of the highest EQE of 20.98%(Red). All these devices with low turn-on voltage indicate
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Acknowledgements
This work was supported by the National Natural Science Foundation of China
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(Project No. 51673113).
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Highlights: 1) PPI-F-Cz performs well in a multilayer non-doped device (EQE of 4.64%, CE of 7.68 cd/A and PE of 8.04 lm/W at CIE coordinates of
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(0.155, 0.055)) 2) PPI-F-Cz is as an efficient host for green and red PhOLEDs with the maximum EQEs of 16.83% and 20.98%.
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3) All the devices exhibit a low turn-on voltage (<3.0 eV).