Fluorenone-based thermally activated delayed fluorescence materials for orange-red emission

Fluorenone-based thermally activated delayed fluorescence materials for orange-red emission

Organic Electronics 73 (2019) 240–246 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel ...

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Organic Electronics 73 (2019) 240–246

Contents lists available at ScienceDirect

Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Fluorenone-based thermally activated delayed fluorescence materials for orange-red emission

T

You-Jun Yu, Xun Tang, Hui-Ting Ge, Yi Yuan, Zuo-Quan Jiang*, Liang-Sheng Liao** Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Joint International Research Laboratory of Carbon-Based Functional Materials and Devices, Soochow University, Suzhou, Jiangsu, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Organic light-emitting diodes Thermally activated delayed fluorescence D-A structure Fluorenone Orange-red emission

Herein, two orange-red emitters, 3,6-bis(9,9-dimethylacridin-10(9H)-yl)-9H-fluoren-9-one (DMAC-FO) and 3,6di(10H-SP[acridine-9,9′-fluoren]-10-yl)-9H-fluoren-9-one (SPAC-FO), based on fluorenone have been designed and synthesized. As compared to the widely reported benzophenone acceptor, fluorenone has deeper lowest unoccupied molecular orbital (LUMO) because of its more conjugated skeleton and thus it can red-shift the emission maximum. The molecular simulation exhibits both emitters has separated highest occupied molecular orbitals (HOMOs) (@donors) and LUMOs (@acceptor), indicating they could act as thermally activated delayed fluorescence (TADF) materials which are further confirmed by the transient spectra. Consequently, external quantum efficiencies (EQEs) of 10.0% for DMAC-FO and 14.2% for SPAC-FO are achieved in the organic lightemitting diodes (OLEDs).

1. Introduction Recently, organic light-emitting diodes (OLEDs) have received extensive attention because of the potential in flat-panel display and nextgeneration solid lighting [1–4]. To design emitters which can utilize both singlet and triplet excitons has been one of the major subjects for improving the efficiency of OLEDs. The phosphorescent emitters achieved 100% internal quantum efficiency because of strong electron spin–orbit coupling in heavy metal complexes [5–9]. However, phosphorescence materials employing noble metals will increase the cost of manufacture [4,10,11]. Thus, it is necessary to develop new generation metal-free emitters with high internal quantum efficiency. In this regards, thermally activated delayed fluorescence (TADF) materials applied in OLEDs are one of the most promising candidates to resolve this issue [12–15]. As for TADF emitters, triplet excitons can be converted into singlet excitons via reverse intersystem crossing (RISC) process from T1 state to S1 state result from small energy gap between singlet and triplet excited states (ΔEST), which can realize 100% internal quantum efficiency in theory [16–19]. Furthermore, TADF emitters are cost-effective as it can be noble-metal-free molecules [20–22]. Mostly, the important factor, ΔEST, can be efficiently reduced by twisted conformational structure, which results in small overlap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) [23,24]. *

Therefore, most TADF emitters are constructed via donor-acceptor (D-A) architecture and the emission color can be finely tuned by varying the intensities of donor [25,26], acceptor [27–29] and their linking ways [30–33]. In the selection of donors and acceptors, organic chemists have offered various options, but it should be noted that the donors in TADF materials are only limited in arylamine species while the most reports concentrated on the development of various acceptors. Among them, reported ketone-based acceptors are benzophenone [34], 1,3-phenylenebis(phenylmethanone) [35], 1,4-phenylenebis(phenylmethanone) [36], anthracene-9,10-dione [37], benzene-1,3,5-triyltris(phenylmethanone) [38] and so on [4,39]. From these previous work, it is found that the mono-ketone derivatives covered emission range from blue to green. For instance, Adachi et al. reported a green TADF emitter, DMAC-BP, showing a maximum external quantum efficiency (EQE) of 18.9% [40]. Cheng et al. synthesized two benzoylpyridine-carbazole based emitters, DCBPy and DTCBPy, and the OLED devices based on it exhibited blue and green emission with EQEs of 24.0% and 27.2%, respectively [41]. With di(pyridinyl)methanone (DPyM) as acceptor, Cheng et al. reported two isometric TADF emitters, 2DPyM-mDTC and 3DPyM-pDTC. And the OLED device based on 3DPyM-pDTC showed EQE up to 31.9% with emission peak at 464 nm [42]. Furthermore, the diketone derivatives extended the emission to orange-red range. For example, Adachi et al. reported two diketone based derivatives named m-Px2BBP and p-Px2BBP and the devices

Corresponding author. Corresponding author. E-mail addresses: [email protected] (Z.-Q. Jiang), [email protected] (L.-S. Liao).

**

https://doi.org/10.1016/j.orgel.2019.06.008 Received 19 March 2019; Received in revised form 16 May 2019; Accepted 4 June 2019 Available online 12 June 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.

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2.2. Synthesis

based on it achieved yellow and orange emission with peaks at 548 nm and 586 nm, respectively [35]. And a series of anthraquinone based TADF derivatives with orange to red emission were reported by same group [16]. Our group reported fused amine/carbonyl acceptor QAO and the device based on its derivative, QAO-Dad, achieved EQE of 23.9% with peak at 552 nm [43]. Although the diketone TADF derivatives can shift the emission to orange or red region, but they always need more complicated molecular structure. Rare mono-ketone based moieties have been exploited as TADF acceptors in constructing orangered emitters [39]. Herein, we designed and synthesized two orange-red TADF emitters, 3,6-di(10H-SP[acridine-9,9′-fluoren]-10-yl)-9H-fluoren-9-one (SPACFO) and 3,6-bis(9,9-dimethylacridin-10(9H)-yl)-9H-fluoren-9-one (DMAC-FO). In these two molecules, fluorenone was selected as acceptor unit because of its unique rigid structure and strong electron withdrawing ability [44,45]. Compared with benzophenone, the more rigid planar structure of fluorenone leads to extended electron delocalization and deeper LUMO level. As for the donor moieties, 9,9-dimethyl-9,10-hydroacridine (DMAC) and 10H-spiro[acridine-9,9′fluorene] (SPAC), were chosen due to their intrinsic strong electron donating abilities and large steric hindrance [46]. Therefore, fluorenone based emitters, DMAC-FO and SPAC-FO achieved orange-red emission. The highly twisted structure conformation leads to efficient separation of HOMO and LUMO which indicate TADF characteristics. The EQEs of DMAC-FO and SPAC-FO based devices surpassed the theoretical margin of conventional fluorescent device efficiency.

2.2.1. Synthesis of 3,6-bis(9,9-dimethylacridin-10(9H)-yl)-9H-fluoren-9one (DMAC-FO) Place a mixture of 3,6-dibromo-9H-fluoren-9-one (1.02 g, 3 mmol), 9,9-dimethyl-9,10-dihydroacridine (1.47 g, 7 mmol), Pd2(dba)3 (0.110 g, 0.12 mmol), t-Bu3PHBF4 (0.035 g, 0.12 mmol), t-BuONa (1.681 g, 15 mmol) in a 100-ml two necked flask under an argon atmosphere. Then 50 mL dry toluene was added. After stirring for 18 h at 110 °C, the reaction mixture was cooled to room temperature, then filtered using dichloromethane (DCM) as wash solvent. The filtrate was reduced by evaporation. The crude product was purified by column chromatography on silica gel (eluent: PE/DCM = 1:1, v/v), 1.27 g orange-red solid powder was obtained. Yield 71%. 1 H NMR (600 MHz, CDCl3) δ = 7.95 (d, J = 7.8 Hz, 2H), 7.46 (d, J = 4.0, 4H), 7.45 (d, J = 1.5 Hz, 2H), 7.34 (d, J = 7.8, 1.7 Hz, 2H), 7.00 (t, J = 7.7, 1.6 Hz, 4H), 6.95 (t, J = 7.5, 1.2 Hz, 4H), 6.47 (d, J = 8.1, 1.2 Hz, 4H), 1.66 (s, 12H). 13C NMR (150 MHz, CDCl3) δ 191.47, 148.04, 146.50, 145.83, 140.16, 133.30, 131.36, 126.71, 126.47, 125.39, 122.55, 121.48, 114.83, 36.15, 30.93. MS (MALDITOF) m/z = 595.500 [M+]. Elem. Anal. Calcd for C43H34N2O: C, 86.84; H, 5.76; N, 4.71; O, 2.69; found: C, 86.25; H, 5.96; N, 4.69. 2.2.2. Synthesis of 3,6-di(10H-spiro[acridine-9,9′-fluoren]-10-yl)-9Hfluoren-9-one (SPAC-FO) Place a mixture of 3,6-dibromo-9H-fluoren-9-one (0.41 g, 1.2 mmol), 10H-spiro[acridine-9,9′-fluorene] (0.83 g, 2.5 mmol), Pd2(dba)3 (0.044 g, 0.048 mmol), t-Bu3PHBF4 (0.014 g, 0.048 mmol), tBuONa (0.673 g, 6 mmol) in a 100-ml two necked flask under an argon atmosphere. Then 50 mL dry toluene was added. After stirring for 18 h at 110 °C, the reaction mixture was cooled to room temperature, then filtered using DCM as wash solvent. The filtrate was reduced by evaporation. The crude product was purified by column chromatography on silica gel (eluent: PE/DCM = 1:1, v/v), 0.71 g orange solid powder was obtained. Yield 85%. 1H NMR (600 MHz, CD2Cl2) δ 8.10 (d, J = 7.0 Hz, 2H), 7.84 (d, J = 7.2 Hz, 4H), 7.78 (s, 2H), 7.57 (d, J = 7.2 Hz, 2H), 7.39 (m, J = 12.2, 6.8 Hz, 8H), 7.27 (t, J = 7.3 Hz, 4H), 6.96 (t, J = 7.8 Hz, 4H), 6.59 (t, J = 7.6 Hz, 4H), 6.54 (d, J = 8.8 Hz, 4H), 6.38 (d, J = 8.4 Hz, 4H). 13C NMR (150 MHz, CDCl3) δ 191.51, 156.48, 147.84, 146.72, 140.52, 139.24, 134.03, 133.02, 128.41, 128.17, 127.67, 127.37, 127.17, 125.73, 124.99, 124.19, 121.19, 119.94, 114.44, 77.22, 77.01, 76.80, 56.67. MS (MALDI-TOF) m/z = 838.603[M+]. Elem. Anal. Calcd. for C63H38N2O: C, 90.19; H, 4.57; N, 3.34; O, 1.91; found: C, 89.74; H, 4.84; N, 3.30.

2. Experimental section 2.1. Chemicals and instruments All chemicals and reagents were used as received from commercial resources without further purification. Toluene used in synthetic routes were purified by PURE SOLV (Innovative Technology) purification system. 1H NMR and 13C NMR spectra were measured on a Bruker 400 spectrometer at room temperature. Mass spectra and time of Flight MSMALDI (MALDI-TOF) were performed on a Thermo ISQ mass spectrometer using a direct exposure probe and Bruker Autoflex II/Compass 1.0, respectively. Elemental analyses (C, H and N) were measured using VARIO EL III elemental analyzer. UV–vis absorption spectra were recorded on a PerkinElmer Lambda 750 spectrophotometer. Photoluminescence (PL) spectra and phosphorescent spectra were performed on Hitachi F-4600 fluorescence spectrophotometer. Differential scanning calorimetry (DSC) was performed on a TA DSC 2010 unit at a heating rate of 10 °C/min under nitrogen. The glass transition temperature (Tg) was determined from the second heating scan. Thermogravimetric analysis (TGA) was performed on TA SDT 2960 instrument at a heating rate of 10 °C/min under nitrogen, the temperature at 5% weight loss was used as the decomposition temperature (Td). The PLQY was measured using Hamamatsu C9920-02G in nitrogen atmosphere. Transient spectra and time-resolved spectra were obtained by using Quantaurus-Tau fluorescence lifetime measurement system (C11367-03, Hamamatsu Photonics Co.) in vacuum. The electrochemical measurement was made using a CHI600 voltammetric analyzer. A conventional three-electrode configuration consisting of a platinum working electrode, a Pt-wire counter electrode, and an Ag/ AgCl reference electrode were used. The solvent in the measurement was CH2Cl2, and the supporting electrolyte was 0.1 M tetrabutylammonium hexafluorophosphate. Ferrocene was added as a calibrant after each set of measurements, and all potentials reported were quoted with reference to the ferrocene-ferrocenium (Fc/Fc+) couple at a scan rate of 100 mV/s. The geometrical and electronic properties were calculated based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT) approaches at the B3LYP/631G (d) level with the use of the Gaussian 09 program [47,48].

2.3. Fabrication and characterization of the OLED devices The OLED devices were fabricated on the indium-tin oxide (ITO) coated transparent glass substrates, the ITO conductive layer has a thickness of ca. 100 nm and a sheet resistance of ca. 30 Ω per square. The substrate was cleaned with ethanol, acetone and deionized water several times, and then dried in an oven, finally exposed to UV ozone for 15 min. All of the organic materials and metal layers were deposited under a vacuum of ca. 10−6 Torr. Identical OLED devices were formed on each of the substrate and the emission area of 0.09 cm2 for each device. The electroluminescence (EL) performances of the devices were measured with a PHOTO RESEARCH SpectraScan PR 655 PHOTOMETER and a KEITHLEY 2400 SourceMeter constant current source at room temperature. 3. Results and discussion 3.1. Molecular synthesis and simulation The synthetic routes for DMAC-FO and SPAC-FO were illustrated in 241

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Scheme 1. Synthetic routes of DMAC-FO and SPAC-FO.

and 467 °C respectively. Meanwhile, glass transition temperature (Tg) is 134 °C for DMAC-FO. The Tg of SPAC-FO were not found, which may due to the more rigid structure of SPAC than DMAC [49,50]. The replacement of DMAC by SPAC greatly improves the thermal stability. The high Td and Tg values benefit from the rigid molecular structure, which are highly desired in electroluminescence device [51]. The electrochemical properties were measured by cyclic voltammetry (CV) curves (shown in Fig. 2c). The HOMO/LUMO levels of DMAC-FO and SPAC-FO were −5.29/-2.98 and −5.39/-3.05 eV, respectively. Which were calculated based on the onset of the oxidation curves and the onset of the absorption bands.

Scheme 1. DMAC-FO and SPAC-FO were obtained in good yields by one-step Buchwald–Hartwig amination reaction between 3,6-dibromo9H-fluoren-9-one and 9,10-dihydroacridine derivatives. Moreover, two compounds were confirmed by 1H NMR, 13C NMR, mass spectrometry and elemental analysis. To have a better understanding the relationship between molecular configuration and optical properties, the configurations and energy levels of DMAC-FO and SPAC-FO were evaluated by DFT and TD-DFT at the B3LYP/6-31G(d) level. The calculation results are shown in Fig. 1. Both DMAC-FO and SPAC-FO possess similar twisted structure with nearly vertical dihedral angles between 9,10-dihydroacridine based donor and fluorenone acceptor moieties. And HOMO and LUMO are well separated and mainly located at donor and fluorenone, respectively. Moreover, these calculation results indicated that DMAC-FO and SPAC-FO may have TADF nature.

3.3. Photophysical properties The photophysical properties of DMAC-FO and SPAC-FO were studied and summarized in Table 1. The absorption and fluorescence spectra in solution of two emitters are shown in Fig. 3. Both DMAC-FO and SPAC-FO exhibited similar absorption bands. The weak broad absorption bands of them between 340 nm and 530 nm are caused by intramolecular charge transfer (ICT) from 9,10-dihydroacridine based donors to fluorenone. The strong absorption bands below 340 nm are attributed to localized absorption of fluorenone and corresponding

3.2. Thermal and electrochemical properties Thermal properties were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under nitrogen atmosphere (shown in Fig. 2a and b). The decomposition temperature (Td, corresponding to 5 wt% loss) of DMAC-FO and SPAC-FO are 377

Fig. 1. Chemical structures and calculation results of DMAC-FO and SPAC-FO. 242

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Fig. 2. (a) TGA and (b) DSC curves of DMAC-FO and SPAC-FO and (c) cyclic voltammogram curves in CH2Cl2 of DMAC-FO, SPAC-FO and Ferrocene for oxidation.

3.4. Electroluminescence properties

donor units, which were confirmed by absorption of fragments DMAC, SPAC and fluorenone (shown in Fig. S1). Upon the photoexcitation of the ICT absorption, DMAC-FO and SPAC-FO showed orange-red emission in dilute toluene solution at room temperature with peaks at 600 and 588 nm, respectively. The blue-shifted emission of SPAC-FO is caused by the different donating groups. The electron donating strengths of SPAC is weakened by the aromatic substitution, while the methyl substitution enhances the electron donating abilities of DMAC [52]. Photoluminescence spectra were also investigated under low temperature to evaluate the photophysical properties. ΔEST values were calculated based on the onset of the low temperature fluorescence and phosphorescence spectra. Fig. 3b and c shows that both lowest singlet and triplet states exhibit charge transfer dominant characteristic. The experimental values of ΔEST for DMAC-FO and SPAC-FO were 0.14 and 0.13 eV, respectively. Such small ΔEST lead to efficient RISC process for harvesting triplet excitons. To further confirm the TADF characteristic, the transient decay curves and PL quantum yields (PLQYs) of DMAC-FO and SPAC-FO in doped 4,4′-di(9H-carbazol-9-yl)-1,1′-biphenyl (CBP) films were measured and summarized in Table 1. The transient decay curves for 2 wt% DMAC-FO and 10 wt% SPAC-FO in CBP at room temperature are shown in Fig. 4. The lifetimes of prompt fluorescence are 16.8 ns for DMAC-FO and 16.9 ns for SPAC-FO, while the lifetimes of delayed fluorescence for DMAC-FO and SPAC-FO are 1.6 μs, respectively. Time resolved spectra were also measured and shown in Fig. S2. These phenomena are similar to the reported literature and demonstrate the TADF nature of DMACFO and SPAC-FO [20–22]. PLQY values of DMAC-FO and SPAC-FO in CBP at different doping ratio were measured in nitrogen atmosphere. The PLQY values of DMAC-FO in CBP with 2 wt%, 5 wt%, 10 wt% doping ratios are 0.43, 0.40 and 0.37, respectively. And for SPAC-FO in CBP with 5 wt%, 10 wt%, 15 wt% doping ratios are 0.48, 0.49 and 0.47. The higher PLQY values of SPAC-FO than DMAC-FO reflect that SPAC has more stable conformation than DMAC [53]. Furthermore, the PLQY values in different doping ratios suggest 2 wt% for DMAC-FO and 10 wt % for SPAC-FO are most suitable for device fabrication.

To investigate the electroluminescence properties, DMAC-FO and SPAC-FO were explored as TADF emitters doping in CBP in devices with the multilayer structure of ITO/HAT-CN (10 nm)/TAPC (40 nm)/TCTA (10 nm)/X wt% emitter: CBP (20 nm)/B4PyMPM (50 nm)/Liq (2 nm)/ Al (120 nm). Herein, ITO and Al were anode and cathode, respectively. 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (HAT-CN) and 8-Hydroxyquinolinolato-lithium (Liq) were used as hole and electron injection layer. 1,1-Bis[4-[N,N-di(p-toly)amino]pnenyl]cyclohexane (TAPC) and 4,6-Bis(3,5-di(pyridin-4-yl)phenyl)-2-methylpyrimidine (B4PyMBM) were employed as hole and electron transporting layer. 4,4′,4″-Tris(carbazol-9-yl)-triphenylamine (TCTA) was selected as excitons block layer and CBP was chosen as host materials, respectively. The device performance for DMAC-FO and SPAC-FO at different doping ratio are shown in Figs. S3 and S4 and summarized in Table S1. The optimized doping ratio for DMAC-FO and SPAC-FO were 2% and 10%, respectively [53]. The current density-voltage-luminescence characteristics, current density dependence of current/power/external quantum efficiency and electroluminescence spectra of optimized devices are shown in Fig. 5. And key parameters of device performance are summarized in Table 2. The devices based on two emitters realized orange-red emission. The DMAC-FO and SPAC-FO based devices achieved EL peak at 600 nm with Commission Internationale de L’ Eclairage (CIE) coordinates of (0.56, 0.44) and 588 nm with CIE coordinates of (0.53, 0.46), respectively. Moreover, the device based on SPAC-FO achieved better device performance with the EQEmax/CEmax/PEmax of 14.2%, 28.0 cd A−1 and 29.0 lm W−1, respectively. The device performances well consistent with the PLQY values, which indicates efficient RISC process and high exciton utilization in the device. 4. Conclusions In summary, two fluorenone based TADF molecules named DMACFO and SPAC-FO have been designed and synthesized. Two emitters

Table 1 Thermal, photophysical properties of DMAC-FO and SPAC-FO. Compound

Tg[a]/Td[b][oC]

λabs[c][nm]

λPL[c][nm]

HOMO[d]/LUMO[e][eV]

Eg[f][eV]

ES/ET[g][eV]

ΔEST[h][eV]

τp[i][ns]

τd[i][μs]

PLQY[i][%]

DMAC-FO SPAC-FO

134/377 –/467

300/459 310/464

600 588

−5.29/-2.98 −5.39/-3.05

2.31 2.34

2.40/2.26 2.46/2.33

0.14 0.13

16.8 16.9

1.6 1.6

43 49

a b c d e f g h i

Measured by TGA at heating rate of 10 °C min−1 under a N2 atmosphere. Measured by DSC according to the heat-cool-heat-cool procedure. Measured in 1 × 10−5 toluene solution at room temperature. Calculation from CV curves. LUMO = HOMO + Eg. Calculation from the onset of UV–vis absorption spectra. Calculation from the onset of low temperature (77 K) fluorescence and phosphorescence spectra. ΔEST=ES - ET. Measured in doped CBP films in nitrogen atmosphere. 243

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Fig. 3. (a) Absorption spectra and emission spectra (in toluene) of DMAC-FO and SPAC-FO at room temperature and low temperature fluorescence and phosphorescence specture at 77 K of (b) DMAC-FO and (c) SPAC-FO.

exhibited orange-red emission with emission peaks at 600 nm for DMAC-FO and 588 nm for SPAC-FO due to the strong electron withdrawing abilities of fluorenone. TADF properties of DMAC-FO and SPAC-FO have been demonstrated by calculations and experiment measurements. The EQEmax values of devices based on DMAC-FO and SPAC-FO were 10.0% and 14.2%, respectively. These results indicate effective RISC process for the utilization of triplet excitons. This work demonstrates that fluorenone with rigid planar structure can be exploited as acceptor to construct TADF materials with long wavelength emission.

Acknowledgements The authors acknowledge financial support from the National Natural Science Foundation of China (Grant No. 61575136, 21572152 and 51873139) and the National Key R&D Program of China (Grant No. 2016YFB0400700). This project is also funded by the Collaborative Innovation Center of Suzhou Nano Science and Technology (Nano-CIC), by the Priority Academic Program Development of Jiangsu Higher

Fig. 4. (a) PL spctra and (b) transient decay curves of 2 wt% DMAC-FO and 10 wt% SPAC-FO in CBP.

Fig. 5. OLED device performances of DMAC-FO and SPAC-FO. 244

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Table 2 Key performance parameters of optimized OLED devices based on DMAC-FO and SPAC-FO. Emitter

λEL[a][nm]

CEmax.[b][cd A−1]

PEmax.[c][lm W−1]

EQEmax.[d][%]

CIE[e][x,y]

DMAC-FO SPAC-FO

600 588

19.5 28.0

15.2 29.0

10.0 14.2

(0.56, 0.44) (0.53, 0.46)

a b c d e

Obtained at 1 mA/cm2 Maximum current efficiency. Maximum power efficiency. Maximum external quantum efficiency. Obtained at 1000 cd m−2

Education Institutions, by the “111” Project of the State Administration of Foreign Experts Affairs of China, and by the Yunnan Provincial Research Funds on College-Enterprise Collaboration (NO. 2015IB016).

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Appendix A. Supplementary data

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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2019.06.008.

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