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Efficient non-doped blue fluorescent OLEDs based on bipolar phenanthroimidazole-triphenylamine derivatives
Optical Materials 101 (2020) 109726
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Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Efficient non-doped blue fluorescent OLEDs based on bipolar phenanthroimidazole-triphenylamine derivatives Huanan Peng a, *, 1, Zhehan Wei b, 1, Lidan Wu a, Xiaomeng Li b, ** a b
College of Chemistry and Environmental Sciences, Shangrao Normal University, Shangrao, 334001, PR China School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen, Shenzhen, Guangdong, 518172, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Fluorescent materials Phenanthroimidazole Triphenylamine Non-doped OLEDs Bipolar characteristic
Blue organic emissive materials are still the most important bottlenecks for the development of organic lightemitting diodes (OLEDs). To enrich the material library, herin, three bipolar phenanthroimidazole derivatives, namely MePPIM-TPA, ClPPIM- TPA and BuPPIM-TPA, are synthesized using triphenylamine as electron-donor and phenanthroimidazole as the electron-acceptor. The photophysical, thermal and electrochemical properties of three compounds are investigated with high decomposition temperature up to 350 � C, and strong blue emission. Single-carrier devices are fabricated to show that three compounds have good bipolar carrier transport characteristic. The non-doped fluorescent OLEDs devices using three compounds as emitting layers are fabricated among which the devices based on MePPIM-TPA achieved the maximum luminance of 1743 cd/m2, the maximum external quantum efficiency (EQE) of 2.99% which is relatively comparable to commonly used blue emitters.
1. Introduction
system to improve electroluminescence efficiency, and device fabrica tion turn out to be complicated and expensive [14,15]. Moreover, phase separation and energy transfer cannot be ignored in host-dopant systems [16,17]. In view of these issues, the development of blue fluorescent emitters for non-doped OLEDs still remains a subject of much current interest. Up to now, various organic molecules based on anthracene [18], fluorene [19], pyrene [20], fluoranthene [21], oxadiazoles [22], phos phineoxide [21], carbazole [23], triphenylamine [24] and phenan throimidazole [25,26] have been synthesized and employed as deep-blue emitters for efficient non-doped OLEDs. However, phenan throimidazole have been widely used an excellent building block for blue emitters due to its advantage, such as satisfactory colour purity, high carrier mobilities, excellent thermal stability, high fluorescence quantum, good film forming ability and simple synthesis method, as well as the ease of modification at the N1 and C2 positions [25]. Further more, phenanthroimidazole-based fluorophores usually have high triplet energy, and thus can be used as the host materials to structure blue, green and red PhOLEDs [25,27,28]. In this work, three bipolar phenanthroimidazole derivatives, namely
Since pioneering work carried out by Tang et al., in 1987, organic light-emitting diodes (OLEDs) have attracted much scientific and com mercial attention due to their potential application in the new genera tion display and lighting technologies [1–3]. As one of three typical emitting materials (red, green, and blue), blue emitting materials play a key role in full-color display and white lighting. They can not only cut down power consumptions and increase the color gamut, but also can serve as the host materials to produce other color light through energy transfer to low energy dopants [4–7]. However, the performance of blue emitting materials still inferior to red and green emitting materials owing to the intrinsic wide band-gap with relatively lower highest occupied molecular orbital (HOMO) energy levels and higher lowest unoccupied molecular orbital (LUMO) energy levels [8,9]. In recent years, more effort has been devoted to develop the blue phosphorescent materials and blue thermally activated delayed fluorescence (TADF) materials, which can reach 100% internal quantum efficiency in theory by utilizing 25% singlet and 75% triplet excitons [10–13]. Nevertheless, blue phosphorescent and TADF devices usually adopt a host-dopant
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Peng), [email protected] (X. Li). 1 These authors contributed equally to this paper. https://doi.org/10.1016/j.optmat.2020.109726 Received 8 November 2019; Received in revised form 21 January 2020; Accepted 27 January 2020 0925-3467/© 2020 Elsevier B.V. All rights reserved.
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Optical Materials 101 (2020) 109726
MePPIM-TPA: Light yellow solid, yield 63%, m.p. 243–245 � C; 1H NMR(400 MHz, CDCl3): 8.85 (d, J ¼ 8.0 Hz, 1H), 8.75 (d, J ¼ 8.4 Hz, 1H), 8.69 (d, J ¼ 8.4 Hz, 1H), 7.72 (t, J ¼ 7.6 Hz, 1H), 7.62 (t, J ¼ 8.0 Hz, 1H), 7.50–7.27 (m, 9H), 7.24–7.02 (m, 10H), 6.94 (d, J ¼ 8.8 Hz, 2H), 2.53 (s, 3H, CH3); 13C NMR(100 MHz, CDCl3): 150.8, 148.3, 147.2, 139.9, 136.3, 130.8, 130.0, 129.3, 129.1, 128.8, 128.2, 128.1, 127.2, 126.2, 125.5, 125.0, 124.6, 124.1, 123.9, 123.4, 123.2, 123.1, 122.7, 122.1, 120.8, 21.5. FT-IR(KBr), ν(cm 1): 3058, 3030, 2900, 1590, 1515, 1465, 1379. HRMS: calcd for C40H29N3 [MþH]þ: 552.2440, found 552.2458. ClPPIM-TPA: Yellow solid, yield 51%, m.p. 240–242 � C; 1H NMR (400 MHz, CDCl3): 8.84 (d, J ¼ 8.0 Hz, 1H), 8.76 (d, J ¼ 8.4 Hz, 1H), 8.69 (d, J ¼ 8.4 Hz, 1H), 7.72 (t, J ¼ 8.0 Hz, 1H), 7.64 (t, J ¼ 8.4 Hz, 1H), 7.59–7.39 (m, 7H), 7.30–7.27 (m, 4H), 7.25–7.04 (m, 8H), 6.96 (d, J ¼ 8.8 Hz, 2H). 13C NMR (100 MHz, CDCl3): 150.9, 148.6, 147.1, 137.5, 135.8, 130.5, 130.4, 130.1, 129.4, 129.2, 128.2, 127.8, 127.3, 127.1, 126.4, 125.7, 125.2, 124.9, 124.2, 123.6, 123.2, 123.1, 122.9, 122.8, 121.8, 120.6. FT-IR(KBr), ν(cm 1): 3059, 3034, 1590, 1518. HRMS: calcd for C39H26ClN3 [MþH]þ: 572.1894, found 572.1915. BuPPIM-TPA: Light yellow solid, yield 60%, m.p. 258–261 � C; 1H NMR (400 MHz, CDCl3): 8.79 (d, J ¼ 8.0 Hz, 1H), 8.67 (d, J ¼ 8.4 Hz, 1H), 8.61 (d, J ¼ 8.4 Hz, 1H), 7.64 (t, J ¼ 7.2 Hz, 1H), 7.55 (t, J ¼ 8.0 Hz, 1H), 7.51 (d, J ¼ 8.4 Hz, 2H), 7.42–7.34 (m, 5H), 7.18–7.08 (m, 7H), 7.00–6.93 (m, 5H), 6.85 (d, J ¼ 8.8 Hz, 2H), 1.36 (s, 9H). 13C NMR (100 MHz, CDCl3): 153.2, 150.9, 148.3, 147.3, 136.1, 130.1, 129.6, 129.3, 129.1, 128.6, 128.3, 128.2, 128.1, 127.2, 126.9, 126.2, 125.5, 124.9, 124.6, 124.0, 123.4, 123.1, 123.0, 122.7, 121.6, 120.8, 35.0, 31.4. FT-IR (KBr), ν(cm 1): 3056, 3036, 2900, 2867, 1590, 1507, 1471, 1385. HRMS: calcd for C43H35N3 [MþH]þ: 594.2909, found 594.2932.
MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA in Scheme 1, were designed and synthesized employing triphenylamine as electron-donor and phenanthroimidazole as the electron-acceptor. Three compounds all exhibited good thermal stability with glass transition temperature (Tg) of 101 � C, 118 � C and 122 � C, decomposition temperature (Td) of 356 � C, 406 � C and 433 � C, respectively. They also showed blue emission in solution and thin films. The single-carrier devices based these com pounds presented bipolar properties. Moreover, three compounds also were investigated for potential application as non-doped emitting ma terials in blue OLEDs. The present results should provide valuable in formation for designing highly efficient blue fluorescent emitters. 2. Experimental 2.1. Materials and instruments All reagents and solvents were purchased from commercial sources and used without further purification. 4-(Diphenylamino)benzaldehyde was synthesized according to previous reports [29]. Melting points were obtained with a Yanaco micro melting point apparatus and are uncorrected. Infrared (IR) spectra were recorded on a Nicolet 6700 infrared spectrometer. 1H and 13C NMR spectra were determined on a Bruker 400 MHz spectrometer utilizing CDCl3 as sol vents and tetramethylsilane (TMS) as internal standard. Mass spectra determinations were performed on an Agilent Technologies 6545 Q-TOF LCMS spectrometer. UV–Vis absorption spectra and fluorescence emis sion spectra in solution were recorded on a Varian Cary 60 spectro photometer and a Hitachi F-7000 fluorescence spectrophotometer, respectively. Thermogravimetric analysis (TGA) were performed on a Mettler Toledo TGA/DSC1 thermal analysis system under a nitrogen atmosphere at a heating rate of 10 � C/min. Differential scanning calo rimetery (DSC) measurement were carried out on a Mettler Toledo DSC3 instrument with a heating rate of 20 � C/min. Cyclic voltammetry were recorded on a CHI660C Electrochemical Workstation in CH2Cl2 con taining 0.1 M tetrabutylammounium hexafluorophosphate (Bu4NPF6) as the supporting electrolyte. The electrodes consist of a glassy carbon working electrode, a Pt wise auxiliary electrode, and a non-aqueous Ag/ Agþ reference electrode.
2.3. OLEDs fabrication and measurement The OLED devices were fabricated on the pre-patterned ITO glass substrate with sheet resistance of 15 square 1. Before device fabrication, the glass substrates were cleaned with acetone, isopropyl alcohol and deionized water, and then treated with O2 plasma for 5 min. Then, the organic layers were deposited in a thermal evaporation under a precalibrated rate of 0.1 nm/s. Finally, a cathode composed of 0.5 nm LiF and 100 nm Al were deposited through a shadow mask. The emission area of the OLEDs was 9 mm2, which defined the overlapping area be tween ITO and Al electrodes. The electroluminescence (EL) spectra and current density–voltage (J–V) characteristics of the OLEDs were measured by using an Ocean Optics fiber optic spectrometer and an Agilent 4156C semiconductor parameter analyzer, respectively. The luminance data were obtained with an SRI-RL-5000 spectral luminance meter. To calculate the external quantum efficiency (EQE), the OLEDs were placed directly onto the surface of a large integrating sphere and all emitted photons from the glass side were captured.
2.2. Synthesis and characterization The synthetic routes of MePPIM-TPA, ClPPIM-TPA and BuPPIMTPA were shown in Scheme 1. A mixture solution of 9, 10-phenanthre nequinone (1 mmol), 4-(diphenylamino)benzaldehyde (1 mmol), substituted benzenamine (1 mmol) and ammonium acetate (1 mmol) in ethanol (10 mL) was stirred and heated at 100 � C for 8 h. After completion of the reaction, the reaction mixture was cooled to room temperature. The precipitated solid was filtered, washed with ethanol, and dried by vacuum to afford the products MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA.
Scheme 1. Synthetic routes of MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA. 2
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Optical Materials 101 (2020) 109726
3. Results and discussion
in Fig. 3, they all exhibited reversible oxidation process. The oxidation onset potentials were observed at 0.56eV, 0.63eV and 0.61eV for MePPIM-TPA, for ClPPIM-TPA and for BuPPIM-TPA, respectively. According to the oxidation onset potentials, the HOMO energy levels of three compounds were calculated to be 4.96eV, 5.03eV and 5.01eV, and then the LUMO energy levels were determined to be 1.93eV, 1.96eV and 1.92eV from the calculated HOMOs and the energy gaps estimated from the onset of UV–vis absorption spectra. The HOMO and LUMO values of three compounds were summarized in Table 1.
3.1. Optical properties The photophysical properties of three compounds were studied in CH3CN by the UV–vis absorption and fluorescence emission spectra, as shown in Fig. 1a. The key optical parameters of them are summarized in Table 1. It was clearly observed that three compounds exhibit two similar absorption bands. The strong absorption peaks at around 255 nm were attributed to π-π* electronic transition of the benzene ring, whereas the longer wavelength absorption bands at around 348 nm were assigned to phenanthroimidazole unit [30]. MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA all showed blue emission in acetonitrile with maximum peaks at 425 nm, 428 nm and 423 nm, respectively. Obvi ously, ClPPIM-TPA show the longer emission wavelength because the π-conjugation along the phenanthroimidazole unit and the Cl-substitution benzene ring is enhanced due to the less steric resistance of chlorine atom. In addition, their emission spectra showed bath ochromic shift with increase of solvent polarity (Fig. S1), suggesting that the emissions are derived from the ICT transition. We also investigated the photophysical properties of three com pounds in thin films. The UV–Vis absorption and fluorescence emission spectra are shown in Fig. 1b. Their thin-film absorption spectra of these compounds all displayed slight red shift compared to their spectra in acetonitrile, indicating the existence of the enhanced intermolecular interactions [9]. In addition, the fluorescence spectra of MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA exhibited 11–27 nm red shifts in thin films, which further proved that the enhanced intermolecular in teractions occurred in aggregated state.
3.4. Theoretical calculations In order to understand the electronic structures of the three com pounds, Density Functional Theory (DFT) calculations were performed at B3LYP/6-31G (d) level in the Gaussian 09 program. Fig. 4 showed the electron density distributions of the frontier molecular orbitals. For three compounds, the electron densities of HOMO orbitals are mainly located on the phenanthroimidazole unit and triphenylamine unit, suggestting that the substituted group at the 1-position imidazole does not has significant effect on the their HOMO levels. The LUMOs for MePPIM-TPA and BuPPIM-TPA are mostly distributed on the phenan throimidazole moiety and the benzene moiety of the 2-position imid azole. However, the LUMO of ClPPIM-TPA is predominantly located on phenanthroimidazole moiety and the benzene moiety of the 1,2-position imidazole. 3.5. Carrier transporting properties To evaluate the carrier transporting properties of the three com pounds, we have fabricated hole-only and electron-only devices with a configuration of ITO/4,40 -Bis[N-(1-naphthyl)-N-phenyl amino]biphenyl (NPB) (20 nm)/MePPIM- TPA, ClPPIM-TPA or BuPPIM-TPA (40 nm)/ NPB (20 nm)/Al (100 nm) and ITO/TPBi (20 nm)/MePPIM-TPA, ClPPIM-TPA or BuPPIM-TPA (40 nm)/1,3,5-tris[N-(phenyl)benzimid azole]-benzene (TPBi) (20 nm)/LiF (0.5 nm)/Al (100 nm), respectively. Herein, NPB was employed to block electron injecting from cathode, whereas TPBi was used to block hole injection from the anode. Fig. 5 showed the current density-voltage curves of the hole-only and electrononly devices. It is obviously that three compounds all showed good bi polar transport property. In addition, they all showed much higher hole current density than electron current density due to the good holetransporting ability of TPA donor unit.
3.2. Thermal properties The thermal properties of three compounds were evaluated by TGA and DSC under nitrogen atmosphere. The results were presented in Fig. 2 and Table 1. The decomposition temperature (Td, 5% weight loss) were observed to exceed 356 � C, 406 � C and 433 � C for MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA, which indicate their good thermal stability and can prevent decomposition during the device fabrication. DSC analysis indicated that three compounds were amorphous solids with melting point (Tm) of 233 � C for MePPIM-TPA, 243 � C for ClPPIMTPA and 262 � C for BuPPIM-TPA, respectively. As depicted by the DSC curves in Fig. 2b, not only the glass transition temperatures (Tg) for MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA were observed at 101 � C, 118 � C and 122 � C, but also the crystallization temperatures (Tc) for MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA were determined at 188 � C, 205 � C and 197 � C. The high Tg and Tm value is beneficial for the stability of amorphous film during OLEDs fabrication and operation.
3.6. Electroluminescence (EL) performance On the basis of above properties investigation of the three com pounds, the non-doped fluorescent OLEDs using them as emitters were fabricated with a device architecture of ITO/NPB (30 nm)/MePPIMTPA, ClPPIM-TPA or BuPPIM-TPA (40 nm)/TPBi (35 nm)/LiF (0.5 nm)/Al (100 nm). NPB and TPBi were used as hole transporting layer and electron transporting layer, respectively. As it is shown in Fig. 6a,
3.3. Electrochemical properties The cyclic voltammetry measurements were carried out in CH2Cl2 to investigate the electrochemical property of three compounds. As shown
Fig. 1. Normalized UV–Vis absorption spectra and PL spectra in acetonitrile solution (a) and in thin films (b) for MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA. 3
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Table 1 Physical properties of TPA-PIM. Ega (eV)
Comp
Td, Tg, Tc, Tm (� C)
λabs max(nm) CH3CN
film
CH3CN
film
MePPIM-TPA ClPPIM-TPA BuPPIM-TPA
356, 101, 188, 233 406, 118, 205, 243 433, 122, 197, 262
256, 348 257, 347 255, 354
369 362 365
425 428 423
452 439 442
a b c
λem max(nm)
3.03 3.07 3.09
HOMOb (eV) 4.96 5.03 5.01
LUMOc (eV) 1.93 1.96 1.92
The optical band gap (Eg) determined from the onset of the absorption, Eg ¼ 1240/λonset [31]. EHOMO ¼ -(Eonset þ 4.4eV). ELUMO ¼ EHOMO þ Eg.
Fig. 2. TGA (a) and DSC (b) curves of MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA.
the EL peaks are at 492 nm, 471 nm and 452 nm, respectively, which is redshifted comparing to the PL spectrum (Fig. 1b), mainly due the ag gregation states excited by electrical pumping [32]. The luminance-current density-voltage curve is illustrated in Fig. 6b. At the same operation voltage, the luminance of MePPIM-TPA is higher than the other two blue emitters, which is mainly owing to the excellent charge transport capability (Fig. 5) of MePPIM-TPA minimizing the energy barrier for formation of excitons. Although the current density of BuPPIM-TPA is highest among these three emitters, the luminance is less than other two emissive materials with the operation voltage increasing. The poor electron transport ability function as the main reason herein and the high current may be caused by the leakage cur rent. The maximum luminance reaches 1743 cd/m2 for MePPIM-TPA and 1013 cd/m2 for ClPPIM-TPA. The efficiency of these OLED devices is presented in Fig. 6 c. At low luminance, the external quantum effi ciency (EQE) and power efficiency of MePPIM-TPA is obviously higher than other two materials with the maximum value of 2.99% and 1.67 lm/w, which is equivalent to commonly used blue organic materials [16, 33]. In addition, the EQE of these three emitters decreases gradually with the optical bandgap increasing because of higher energy excitons generating inside organic materials which may cause more defects or optically induced degeneration. With increasing luminance over 100 cd/m2, the efficiency of ClPPIM-TPA based OLED devices is higher than MePPIM-TPA because of more balanced hole and electron injection as it is shown in Fig. 5 of CIPPIM-TPA. Similar to Fig. 6 b, the efficiency of BuPPIM-TPA became lower than other two materials at higher lumi nance where non-radiative recombination starts to dominate.
Fig. 3. Cyclic voltammetry curves of MePPIM-TPA, ClPPIM-TPA and BuP PIM-TPA.
4. Conclusions In this work, we successfully synthesized and characterized three bipolar blue light-emitting materials (MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA) containing phenanthroimidazole unit and triphenyl amine moiety. Three materials all show good thermal properties. The non-doped OLEDs based on MePPIM-TPA, ClPPIM-TPA and BuPPIMTPA all exhibit the blue electroluminescent peaking at 492 nm, 471 nm and 452 nm, respectively. The devices based on MePPIM-TPA and ClPPIM-TPA achieved the maximum luminance of 1743 cd/m2 and
Fig. 4. Calculated electron density distributions of the HOMO and LUMO orbits for MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA. 4
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Optical Materials 101 (2020) 109726
Fig. 5. Current density-voltage curves of the hole-only (a) and electron-only (b) devices for MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA.
Fig. 6. (a) EL spectra at a current density of 5 mA/cm2, (b) Current density-Voltage-luminance curves and (c) Power efficiency-current density-external quantum efficiency curves.
1013 cd/m2, maximum EQE of 2.99% and 2.7%, respectively. The present results demonstrate a promising combination of phenan throimidazole moiety and triphenylamine moiety to develop bipolar blue emitters.
Zhehan Wei: Investigation, Data curation. Lidan Wu: Software. Xiao meng Li: Data curation, Formal analysis, Investigation, Writing - orig inal draft, Writing - review & editing. Acknowledgements
Author agreement
We are grateful to the financial support from the Science and Tech nology Research Program of Education department of Jiangxi Province, China (No.: GJJ170928) and the National Natural Science Foundation of China (No.:61741510).
All authors have reviewed the manuscript and approved to submit to optical materials. Declaration of competing interest
Appendix A. Supplementary data
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Supplementary data to this article can be found online at https://doi. org/10.1016/j.optmat.2020.109726.
CRediT authorship contribution statement
References
Huanan Peng: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Software, Writing - original draft, Writing review & editing, Resources, Supervision, Project administration.
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6
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Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Efficient non-doped blue fluorescent OLEDs based on bipolar phenanthroimidazole-triphenylamine derivatives Huanan Peng a, *, 1, Zhehan Wei b, 1, Lidan Wu a, Xiaomeng Li b, ** a b
College of Chemistry and Environmental Sciences, Shangrao Normal University, Shangrao, 334001, PR China School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen, Shenzhen, Guangdong, 518172, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Fluorescent materials Phenanthroimidazole Triphenylamine Non-doped OLEDs Bipolar characteristic
Blue organic emissive materials are still the most important bottlenecks for the development of organic lightemitting diodes (OLEDs). To enrich the material library, herin, three bipolar phenanthroimidazole derivatives, namely MePPIM-TPA, ClPPIM- TPA and BuPPIM-TPA, are synthesized using triphenylamine as electron-donor and phenanthroimidazole as the electron-acceptor. The photophysical, thermal and electrochemical properties of three compounds are investigated with high decomposition temperature up to 350 � C, and strong blue emission. Single-carrier devices are fabricated to show that three compounds have good bipolar carrier transport characteristic. The non-doped fluorescent OLEDs devices using three compounds as emitting layers are fabricated among which the devices based on MePPIM-TPA achieved the maximum luminance of 1743 cd/m2, the maximum external quantum efficiency (EQE) of 2.99% which is relatively comparable to commonly used blue emitters.
1. Introduction
system to improve electroluminescence efficiency, and device fabrica tion turn out to be complicated and expensive [14,15]. Moreover, phase separation and energy transfer cannot be ignored in host-dopant systems [16,17]. In view of these issues, the development of blue fluorescent emitters for non-doped OLEDs still remains a subject of much current interest. Up to now, various organic molecules based on anthracene [18], fluorene [19], pyrene [20], fluoranthene [21], oxadiazoles [22], phos phineoxide [21], carbazole [23], triphenylamine [24] and phenan throimidazole [25,26] have been synthesized and employed as deep-blue emitters for efficient non-doped OLEDs. However, phenan throimidazole have been widely used an excellent building block for blue emitters due to its advantage, such as satisfactory colour purity, high carrier mobilities, excellent thermal stability, high fluorescence quantum, good film forming ability and simple synthesis method, as well as the ease of modification at the N1 and C2 positions [25]. Further more, phenanthroimidazole-based fluorophores usually have high triplet energy, and thus can be used as the host materials to structure blue, green and red PhOLEDs [25,27,28]. In this work, three bipolar phenanthroimidazole derivatives, namely
Since pioneering work carried out by Tang et al., in 1987, organic light-emitting diodes (OLEDs) have attracted much scientific and com mercial attention due to their potential application in the new genera tion display and lighting technologies [1–3]. As one of three typical emitting materials (red, green, and blue), blue emitting materials play a key role in full-color display and white lighting. They can not only cut down power consumptions and increase the color gamut, but also can serve as the host materials to produce other color light through energy transfer to low energy dopants [4–7]. However, the performance of blue emitting materials still inferior to red and green emitting materials owing to the intrinsic wide band-gap with relatively lower highest occupied molecular orbital (HOMO) energy levels and higher lowest unoccupied molecular orbital (LUMO) energy levels [8,9]. In recent years, more effort has been devoted to develop the blue phosphorescent materials and blue thermally activated delayed fluorescence (TADF) materials, which can reach 100% internal quantum efficiency in theory by utilizing 25% singlet and 75% triplet excitons [10–13]. Nevertheless, blue phosphorescent and TADF devices usually adopt a host-dopant
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Peng), [email protected] (X. Li). 1 These authors contributed equally to this paper. https://doi.org/10.1016/j.optmat.2020.109726 Received 8 November 2019; Received in revised form 21 January 2020; Accepted 27 January 2020 0925-3467/© 2020 Elsevier B.V. All rights reserved.
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MePPIM-TPA: Light yellow solid, yield 63%, m.p. 243–245 � C; 1H NMR(400 MHz, CDCl3): 8.85 (d, J ¼ 8.0 Hz, 1H), 8.75 (d, J ¼ 8.4 Hz, 1H), 8.69 (d, J ¼ 8.4 Hz, 1H), 7.72 (t, J ¼ 7.6 Hz, 1H), 7.62 (t, J ¼ 8.0 Hz, 1H), 7.50–7.27 (m, 9H), 7.24–7.02 (m, 10H), 6.94 (d, J ¼ 8.8 Hz, 2H), 2.53 (s, 3H, CH3); 13C NMR(100 MHz, CDCl3): 150.8, 148.3, 147.2, 139.9, 136.3, 130.8, 130.0, 129.3, 129.1, 128.8, 128.2, 128.1, 127.2, 126.2, 125.5, 125.0, 124.6, 124.1, 123.9, 123.4, 123.2, 123.1, 122.7, 122.1, 120.8, 21.5. FT-IR(KBr), ν(cm 1): 3058, 3030, 2900, 1590, 1515, 1465, 1379. HRMS: calcd for C40H29N3 [MþH]þ: 552.2440, found 552.2458. ClPPIM-TPA: Yellow solid, yield 51%, m.p. 240–242 � C; 1H NMR (400 MHz, CDCl3): 8.84 (d, J ¼ 8.0 Hz, 1H), 8.76 (d, J ¼ 8.4 Hz, 1H), 8.69 (d, J ¼ 8.4 Hz, 1H), 7.72 (t, J ¼ 8.0 Hz, 1H), 7.64 (t, J ¼ 8.4 Hz, 1H), 7.59–7.39 (m, 7H), 7.30–7.27 (m, 4H), 7.25–7.04 (m, 8H), 6.96 (d, J ¼ 8.8 Hz, 2H). 13C NMR (100 MHz, CDCl3): 150.9, 148.6, 147.1, 137.5, 135.8, 130.5, 130.4, 130.1, 129.4, 129.2, 128.2, 127.8, 127.3, 127.1, 126.4, 125.7, 125.2, 124.9, 124.2, 123.6, 123.2, 123.1, 122.9, 122.8, 121.8, 120.6. FT-IR(KBr), ν(cm 1): 3059, 3034, 1590, 1518. HRMS: calcd for C39H26ClN3 [MþH]þ: 572.1894, found 572.1915. BuPPIM-TPA: Light yellow solid, yield 60%, m.p. 258–261 � C; 1H NMR (400 MHz, CDCl3): 8.79 (d, J ¼ 8.0 Hz, 1H), 8.67 (d, J ¼ 8.4 Hz, 1H), 8.61 (d, J ¼ 8.4 Hz, 1H), 7.64 (t, J ¼ 7.2 Hz, 1H), 7.55 (t, J ¼ 8.0 Hz, 1H), 7.51 (d, J ¼ 8.4 Hz, 2H), 7.42–7.34 (m, 5H), 7.18–7.08 (m, 7H), 7.00–6.93 (m, 5H), 6.85 (d, J ¼ 8.8 Hz, 2H), 1.36 (s, 9H). 13C NMR (100 MHz, CDCl3): 153.2, 150.9, 148.3, 147.3, 136.1, 130.1, 129.6, 129.3, 129.1, 128.6, 128.3, 128.2, 128.1, 127.2, 126.9, 126.2, 125.5, 124.9, 124.6, 124.0, 123.4, 123.1, 123.0, 122.7, 121.6, 120.8, 35.0, 31.4. FT-IR (KBr), ν(cm 1): 3056, 3036, 2900, 2867, 1590, 1507, 1471, 1385. HRMS: calcd for C43H35N3 [MþH]þ: 594.2909, found 594.2932.
MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA in Scheme 1, were designed and synthesized employing triphenylamine as electron-donor and phenanthroimidazole as the electron-acceptor. Three compounds all exhibited good thermal stability with glass transition temperature (Tg) of 101 � C, 118 � C and 122 � C, decomposition temperature (Td) of 356 � C, 406 � C and 433 � C, respectively. They also showed blue emission in solution and thin films. The single-carrier devices based these com pounds presented bipolar properties. Moreover, three compounds also were investigated for potential application as non-doped emitting ma terials in blue OLEDs. The present results should provide valuable in formation for designing highly efficient blue fluorescent emitters. 2. Experimental 2.1. Materials and instruments All reagents and solvents were purchased from commercial sources and used without further purification. 4-(Diphenylamino)benzaldehyde was synthesized according to previous reports [29]. Melting points were obtained with a Yanaco micro melting point apparatus and are uncorrected. Infrared (IR) spectra were recorded on a Nicolet 6700 infrared spectrometer. 1H and 13C NMR spectra were determined on a Bruker 400 MHz spectrometer utilizing CDCl3 as sol vents and tetramethylsilane (TMS) as internal standard. Mass spectra determinations were performed on an Agilent Technologies 6545 Q-TOF LCMS spectrometer. UV–Vis absorption spectra and fluorescence emis sion spectra in solution were recorded on a Varian Cary 60 spectro photometer and a Hitachi F-7000 fluorescence spectrophotometer, respectively. Thermogravimetric analysis (TGA) were performed on a Mettler Toledo TGA/DSC1 thermal analysis system under a nitrogen atmosphere at a heating rate of 10 � C/min. Differential scanning calo rimetery (DSC) measurement were carried out on a Mettler Toledo DSC3 instrument with a heating rate of 20 � C/min. Cyclic voltammetry were recorded on a CHI660C Electrochemical Workstation in CH2Cl2 con taining 0.1 M tetrabutylammounium hexafluorophosphate (Bu4NPF6) as the supporting electrolyte. The electrodes consist of a glassy carbon working electrode, a Pt wise auxiliary electrode, and a non-aqueous Ag/ Agþ reference electrode.
2.3. OLEDs fabrication and measurement The OLED devices were fabricated on the pre-patterned ITO glass substrate with sheet resistance of 15 square 1. Before device fabrication, the glass substrates were cleaned with acetone, isopropyl alcohol and deionized water, and then treated with O2 plasma for 5 min. Then, the organic layers were deposited in a thermal evaporation under a precalibrated rate of 0.1 nm/s. Finally, a cathode composed of 0.5 nm LiF and 100 nm Al were deposited through a shadow mask. The emission area of the OLEDs was 9 mm2, which defined the overlapping area be tween ITO and Al electrodes. The electroluminescence (EL) spectra and current density–voltage (J–V) characteristics of the OLEDs were measured by using an Ocean Optics fiber optic spectrometer and an Agilent 4156C semiconductor parameter analyzer, respectively. The luminance data were obtained with an SRI-RL-5000 spectral luminance meter. To calculate the external quantum efficiency (EQE), the OLEDs were placed directly onto the surface of a large integrating sphere and all emitted photons from the glass side were captured.
2.2. Synthesis and characterization The synthetic routes of MePPIM-TPA, ClPPIM-TPA and BuPPIMTPA were shown in Scheme 1. A mixture solution of 9, 10-phenanthre nequinone (1 mmol), 4-(diphenylamino)benzaldehyde (1 mmol), substituted benzenamine (1 mmol) and ammonium acetate (1 mmol) in ethanol (10 mL) was stirred and heated at 100 � C for 8 h. After completion of the reaction, the reaction mixture was cooled to room temperature. The precipitated solid was filtered, washed with ethanol, and dried by vacuum to afford the products MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA.
Scheme 1. Synthetic routes of MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA. 2
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3. Results and discussion
in Fig. 3, they all exhibited reversible oxidation process. The oxidation onset potentials were observed at 0.56eV, 0.63eV and 0.61eV for MePPIM-TPA, for ClPPIM-TPA and for BuPPIM-TPA, respectively. According to the oxidation onset potentials, the HOMO energy levels of three compounds were calculated to be 4.96eV, 5.03eV and 5.01eV, and then the LUMO energy levels were determined to be 1.93eV, 1.96eV and 1.92eV from the calculated HOMOs and the energy gaps estimated from the onset of UV–vis absorption spectra. The HOMO and LUMO values of three compounds were summarized in Table 1.
3.1. Optical properties The photophysical properties of three compounds were studied in CH3CN by the UV–vis absorption and fluorescence emission spectra, as shown in Fig. 1a. The key optical parameters of them are summarized in Table 1. It was clearly observed that three compounds exhibit two similar absorption bands. The strong absorption peaks at around 255 nm were attributed to π-π* electronic transition of the benzene ring, whereas the longer wavelength absorption bands at around 348 nm were assigned to phenanthroimidazole unit [30]. MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA all showed blue emission in acetonitrile with maximum peaks at 425 nm, 428 nm and 423 nm, respectively. Obvi ously, ClPPIM-TPA show the longer emission wavelength because the π-conjugation along the phenanthroimidazole unit and the Cl-substitution benzene ring is enhanced due to the less steric resistance of chlorine atom. In addition, their emission spectra showed bath ochromic shift with increase of solvent polarity (Fig. S1), suggesting that the emissions are derived from the ICT transition. We also investigated the photophysical properties of three com pounds in thin films. The UV–Vis absorption and fluorescence emission spectra are shown in Fig. 1b. Their thin-film absorption spectra of these compounds all displayed slight red shift compared to their spectra in acetonitrile, indicating the existence of the enhanced intermolecular interactions [9]. In addition, the fluorescence spectra of MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA exhibited 11–27 nm red shifts in thin films, which further proved that the enhanced intermolecular in teractions occurred in aggregated state.
3.4. Theoretical calculations In order to understand the electronic structures of the three com pounds, Density Functional Theory (DFT) calculations were performed at B3LYP/6-31G (d) level in the Gaussian 09 program. Fig. 4 showed the electron density distributions of the frontier molecular orbitals. For three compounds, the electron densities of HOMO orbitals are mainly located on the phenanthroimidazole unit and triphenylamine unit, suggestting that the substituted group at the 1-position imidazole does not has significant effect on the their HOMO levels. The LUMOs for MePPIM-TPA and BuPPIM-TPA are mostly distributed on the phenan throimidazole moiety and the benzene moiety of the 2-position imid azole. However, the LUMO of ClPPIM-TPA is predominantly located on phenanthroimidazole moiety and the benzene moiety of the 1,2-position imidazole. 3.5. Carrier transporting properties To evaluate the carrier transporting properties of the three com pounds, we have fabricated hole-only and electron-only devices with a configuration of ITO/4,40 -Bis[N-(1-naphthyl)-N-phenyl amino]biphenyl (NPB) (20 nm)/MePPIM- TPA, ClPPIM-TPA or BuPPIM-TPA (40 nm)/ NPB (20 nm)/Al (100 nm) and ITO/TPBi (20 nm)/MePPIM-TPA, ClPPIM-TPA or BuPPIM-TPA (40 nm)/1,3,5-tris[N-(phenyl)benzimid azole]-benzene (TPBi) (20 nm)/LiF (0.5 nm)/Al (100 nm), respectively. Herein, NPB was employed to block electron injecting from cathode, whereas TPBi was used to block hole injection from the anode. Fig. 5 showed the current density-voltage curves of the hole-only and electrononly devices. It is obviously that three compounds all showed good bi polar transport property. In addition, they all showed much higher hole current density than electron current density due to the good holetransporting ability of TPA donor unit.
3.2. Thermal properties The thermal properties of three compounds were evaluated by TGA and DSC under nitrogen atmosphere. The results were presented in Fig. 2 and Table 1. The decomposition temperature (Td, 5% weight loss) were observed to exceed 356 � C, 406 � C and 433 � C for MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA, which indicate their good thermal stability and can prevent decomposition during the device fabrication. DSC analysis indicated that three compounds were amorphous solids with melting point (Tm) of 233 � C for MePPIM-TPA, 243 � C for ClPPIMTPA and 262 � C for BuPPIM-TPA, respectively. As depicted by the DSC curves in Fig. 2b, not only the glass transition temperatures (Tg) for MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA were observed at 101 � C, 118 � C and 122 � C, but also the crystallization temperatures (Tc) for MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA were determined at 188 � C, 205 � C and 197 � C. The high Tg and Tm value is beneficial for the stability of amorphous film during OLEDs fabrication and operation.
3.6. Electroluminescence (EL) performance On the basis of above properties investigation of the three com pounds, the non-doped fluorescent OLEDs using them as emitters were fabricated with a device architecture of ITO/NPB (30 nm)/MePPIMTPA, ClPPIM-TPA or BuPPIM-TPA (40 nm)/TPBi (35 nm)/LiF (0.5 nm)/Al (100 nm). NPB and TPBi were used as hole transporting layer and electron transporting layer, respectively. As it is shown in Fig. 6a,
3.3. Electrochemical properties The cyclic voltammetry measurements were carried out in CH2Cl2 to investigate the electrochemical property of three compounds. As shown
Fig. 1. Normalized UV–Vis absorption spectra and PL spectra in acetonitrile solution (a) and in thin films (b) for MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA. 3
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Table 1 Physical properties of TPA-PIM. Ega (eV)
Comp
Td, Tg, Tc, Tm (� C)
λabs max(nm) CH3CN
film
CH3CN
film
MePPIM-TPA ClPPIM-TPA BuPPIM-TPA
356, 101, 188, 233 406, 118, 205, 243 433, 122, 197, 262
256, 348 257, 347 255, 354
369 362 365
425 428 423
452 439 442
a b c
λem max(nm)
3.03 3.07 3.09
HOMOb (eV) 4.96 5.03 5.01
LUMOc (eV) 1.93 1.96 1.92
The optical band gap (Eg) determined from the onset of the absorption, Eg ¼ 1240/λonset [31]. EHOMO ¼ -(Eonset þ 4.4eV). ELUMO ¼ EHOMO þ Eg.
Fig. 2. TGA (a) and DSC (b) curves of MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA.
the EL peaks are at 492 nm, 471 nm and 452 nm, respectively, which is redshifted comparing to the PL spectrum (Fig. 1b), mainly due the ag gregation states excited by electrical pumping [32]. The luminance-current density-voltage curve is illustrated in Fig. 6b. At the same operation voltage, the luminance of MePPIM-TPA is higher than the other two blue emitters, which is mainly owing to the excellent charge transport capability (Fig. 5) of MePPIM-TPA minimizing the energy barrier for formation of excitons. Although the current density of BuPPIM-TPA is highest among these three emitters, the luminance is less than other two emissive materials with the operation voltage increasing. The poor electron transport ability function as the main reason herein and the high current may be caused by the leakage cur rent. The maximum luminance reaches 1743 cd/m2 for MePPIM-TPA and 1013 cd/m2 for ClPPIM-TPA. The efficiency of these OLED devices is presented in Fig. 6 c. At low luminance, the external quantum effi ciency (EQE) and power efficiency of MePPIM-TPA is obviously higher than other two materials with the maximum value of 2.99% and 1.67 lm/w, which is equivalent to commonly used blue organic materials [16, 33]. In addition, the EQE of these three emitters decreases gradually with the optical bandgap increasing because of higher energy excitons generating inside organic materials which may cause more defects or optically induced degeneration. With increasing luminance over 100 cd/m2, the efficiency of ClPPIM-TPA based OLED devices is higher than MePPIM-TPA because of more balanced hole and electron injection as it is shown in Fig. 5 of CIPPIM-TPA. Similar to Fig. 6 b, the efficiency of BuPPIM-TPA became lower than other two materials at higher lumi nance where non-radiative recombination starts to dominate.
Fig. 3. Cyclic voltammetry curves of MePPIM-TPA, ClPPIM-TPA and BuP PIM-TPA.
4. Conclusions In this work, we successfully synthesized and characterized three bipolar blue light-emitting materials (MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA) containing phenanthroimidazole unit and triphenyl amine moiety. Three materials all show good thermal properties. The non-doped OLEDs based on MePPIM-TPA, ClPPIM-TPA and BuPPIMTPA all exhibit the blue electroluminescent peaking at 492 nm, 471 nm and 452 nm, respectively. The devices based on MePPIM-TPA and ClPPIM-TPA achieved the maximum luminance of 1743 cd/m2 and
Fig. 4. Calculated electron density distributions of the HOMO and LUMO orbits for MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA. 4
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Optical Materials 101 (2020) 109726
Fig. 5. Current density-voltage curves of the hole-only (a) and electron-only (b) devices for MePPIM-TPA, ClPPIM-TPA and BuPPIM-TPA.
Fig. 6. (a) EL spectra at a current density of 5 mA/cm2, (b) Current density-Voltage-luminance curves and (c) Power efficiency-current density-external quantum efficiency curves.
1013 cd/m2, maximum EQE of 2.99% and 2.7%, respectively. The present results demonstrate a promising combination of phenan throimidazole moiety and triphenylamine moiety to develop bipolar blue emitters.
Zhehan Wei: Investigation, Data curation. Lidan Wu: Software. Xiao meng Li: Data curation, Formal analysis, Investigation, Writing - orig inal draft, Writing - review & editing. Acknowledgements
Author agreement
We are grateful to the financial support from the Science and Tech nology Research Program of Education department of Jiangxi Province, China (No.: GJJ170928) and the National Natural Science Foundation of China (No.:61741510).
All authors have reviewed the manuscript and approved to submit to optical materials. Declaration of competing interest
Appendix A. Supplementary data
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Supplementary data to this article can be found online at https://doi. org/10.1016/j.optmat.2020.109726.
CRediT authorship contribution statement
References
Huanan Peng: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Software, Writing - original draft, Writing review & editing, Resources, Supervision, Project administration.
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