- Email: [email protected]
Easily available, low-cost 9,9′-bianthracene derivatives as efficient blue hosts and deep-blue emitters in OLEDs
Organic Electronics 66 (2019) 24–31
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Easily available, low-cost 9,9′-bianthracene derivatives as efficient blue hosts and deep-blue emitters in OLEDs
T
Zhanfeng Lia,d,∗, Guoyue Gana, Zhitian Lingb, Kunping Guob, Changfeng Sib, Xiang Lva, Hua Wangc, Bin Weib,∗∗, Yuying Haoa,∗∗∗ a
Key Laboratory of Advanced Transducers and Intelligent Control System, Ministry of Education and Shanxi Province, College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan, 030024, PR China b Key Laboratory of Advanced Display and System Applications, Ministry of Education, Shanghai University, Shanghai, 200072, PR China c Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Taiyuan, 030024, PR China d Center of Super-Diamond and Advanced Films (COSDAF) and Department of Chemistry, City University of Hong Kong, 999077, Hong Kong, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Fluorescent OLED 9,9′-bianthracene Deep-blue emitters Blue host
A series of alkyl substituted 9,9′-bianthracene derivatives (BAs) with easy availability and low cost have been applied as deep-blue emitters and blue host materials in fluorescent organic light-emitting diodes (OLEDs). The nearly perpendicular geometry configuration of 9,9′-bianthracene and the additional alkyl units with large steric hindrance in methyl substituted MBA and tert-butyl substituted TBBA would reduce the aggregation formation emitting at longer wavelength by intramolecular and intermolecular interaction, which endows both of the compounds with good thermal properties, and film-forming abilities as well as excellent electroluminescence performance. Nondoped pure deep-blue OLED using TBBA as the emitter showed good performance with EQE of 3.20% and high color purity with CIE (0.15, 0.06), which matched the EBU standard blue very well. Furthermore, these BAs worked as excellent host materials for p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSAph) dopant to get high-performance OLEDs with low turn-on voltages and high efficiencies, especially for MBA bearing two methyl substituents at 3- and 3′-positions of the 9,9′-bianthracene as the fluorescent host with a low turn-on voltage of 3.0 V, a high current efficiency of 16.54 cd/A and a high EQE of 9.47% with unobservable efficiency roll-off. Particularly, EQE can be maintained over 9.20% with very little variation of 0.54% in the range of luminance value of 1.54–13800 cd/m2 for the MBA-based device. The improved EQEs are due to the enhanced occurrence of singlet excitons in BAs-based devices.
1. Introduction Owing to their special features such as high efficiency, low cost, easy processing, and flexibility, organic light-emitting diodes (OLEDs) have received increasing attention recently and have already been applied in solid-state lighting technologies and in full-color flat-panel display applications. Furthermore, OLEDs are regarded as the most viable competitor for next-generation flexible displays and transparent displays [1]. However, the performance of blue/deep-blue OLEDs is usually inferior to that of green/red OLEDs because of blue emitters' intrinsic wide energy gaps, which is still one of the bottlenecks for the further application of OLEDs [1–5]. Though phosphorescent materials and thermally activated delayed fluorescence (TADF) materials can
harvest 100% internal quantum efficiency by utilizing both singlet and triplet excitons, they hardly gain breakthrough in deep-blue ones with Commission International de L'Eclairage (CIE) coordinates of y < 0.08, which match the National Television System Committee (NTSC) standard blue CIE (x, y) coordinates of (0.14, 0.08) for display applications [1,6–9], let alone the stringent European Broadcasting Union (EBU) standard blue with CIE coordinates of (0.15, 0.06) [10–21]. Meanwhile, high-performance deep-blue emitters can not only serve as energy donors (hosts) to create other visible emissions and white lights by means of energy transfer processes, but also play a key role in reducing the power consumption and broadening wide-gamut RGB (red, green, and blue) color space for vivid full-color displays. In addition to the above issue, the electroluminescence (EL)
∗ Corresponding author. Key Laboratory of Advanced Transducers and Intelligent Control System, Ministry of Education and Shanxi Province, College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan, 030024, PR China. ∗∗ Corresponding author. ∗∗∗ Corresponding author. E-mail addresses: [email protected] (Z. Li), [email protected] (B. Wei), [email protected] (Y. Hao).
https://doi.org/10.1016/j.orgel.2018.12.010 Received 1 September 2018; Received in revised form 8 December 2018; Accepted 10 December 2018 Available online 11 December 2018 1566-1199/ © 2018 Elsevier B.V. All rights reserved.
Organic Electronics 66 (2019) 24–31
Z. Li et al.
2. Experimental
properties of blue-emitting materials remain challenging, particularly in terms of easy availability, low cost, and high color purity. However, phosphorescent and TADF blue OLEDs suffer from a certain degree of efficiency roll-off owing to the triplet–triplet annihilation (TTA) process, which still remains unsatisfactory in the practical application [1,6,8,9,13]. Thus, traditional efficient fluorescent materials that perform low efficiency roll-off at high brightness are still considered to be promising for the realization of efficient and stable deep-blue devices [10,12,14,17,18,21–25]. Actually, a variety of deep-blue OLED materials using judicious molecular designs and potential starting precursor building blocks, such as fluorene and spirobifluorenes, anthracene and phenanthrene, imidazole, triphenylamine and carbazole, and other Nheterocycle-based emitters, have been developed [3]. Among them, anthracene is still the highest efficient emitter due to its unusual photoluminescence (PL) and EL properties. Many existing studies on blue emitters based on anthracene core that modified with sterically hindered substituents to inhibit molecular packing and reduce π–π* stacking can maintain a deep-blue emission [26–30]. In our previous work, a series of fluorinated 9,9′-bianthracene (BA) derivatives with nearly perpendicular geometry configuration were first presented as both highly efficient pure blue emitters and blue host materials for OLEDs [31–37]. These BA cored fluorescence emitters with particular twisted intramolecular charge-transfer (TICT) characteristics could realize electron–hole recombination via intramolecular conversion from CT excitons (immediate precursor) to radiative singlet excitons (final state), thus achieving the highly efficient blue OLEDs [32,37]. Another important issue that severely hinders the commercialization of OLEDs is to minimize the manufacture cost of materials and simplify the fabrication process. Herein, we designed and synthesized a series of easily available and low-cost 9,9′-bianthracene derivatives (BAs), 9,9′-bianthracene (BA), 9,9′-bi(3,3′-bimethyl)anthracene (MBA) and 9,9′-bi(3,3′-tert-butyl)anthracene (TBBA), as shown in Fig. 1. The nearly perpendicular geometry configuration of BA might prevent intramolecular extending of πelectron and suppress molecular packing in the solid state effectively. Furthermore, the additional two alkyl groups with large steric hindrance would help to reduce the aggregation formation emitting at longer wavelength by intermolecular interaction. TBBA-based nondoped device exhibited the best EL performance with external quantum efficiency (EQE) of 3.20% and the CIE coordinates of (0.15, 0.06), matching the requirement of EBU standard of CIE (0.15, 0.06) very well. When doping BAs into the host 4,4′-bis(N-carbazolyl)biphenyl (CBP) as a light-emitting layer, the devices achieved much higher EQEs of 3.46–4.56% with a pure blue emission at CIE (0.15–0.16, 0.05–0.08). Moreover, the use of BAs as the hosts of the blue-emitting dopant p-bis (p-N,N-diphenyl-aminostyryl)benzene (DSAph) resulted in high-performance OLEDs with very high EQEs of 5.72–9.47% and excellent current efficiency of 11.33–16.54 cd/A, especially the MBA-based device, exhibiting unobservable efficiency roll-off and maintaining over 9.24% with very little variation of 0.54% in the range of luminance value of 1.54–13800 cd/m2.
2.1. General information Commercially available reagents were used without further purification. 1H NMR was recorded on a Bruker 400 MHz NMR spectrometer. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried on a STA 409 PC instrument at a heating rate of 10 °C/min under argon. Photoluminescence (PL) pectra and photoluminescence quantum efficiency (Φf) were measured using a FLSP920 fluorescence spectrophotometer, both in solution and in the solid state. Absorption spectra were recorded by a Unico UV-2600 PCS spectrophotometer. Cyclic voltammetry (CV) was performed using a CHI 660E electrochemical workstation at a scan rate of 100 mV/s. All experiments were carried out in a three-electrode compartment cell with a Pt-sheet counter electrode, a glassy carbon working electrode, and a Pt-wire reference electrode. The supporting electrolyte used was 0.1 M tetrabutylammonium hexafluorophosphate ([Bu4N]ClO4) solution in dry acetonitrile. The cell containing the solution of the sample (1 mM) and the supporting electrolyte was purged with a nitrogen gas thoroughly before scanning for its oxidation and reduction properties. Ferrocene was used for potential calibration in each measurement. Density functional theory (DFT) calculations (B3LYP) were performed for the target compound with the 6-31G(d,p) basis set by using the Gaussian 09 program. Diffraction data were collected at 293(2) K using graphite monochromated Mo Ka radiation (λ = 0.71073 Å) on a BRUKER SMART APEX II CCD diffractometer. The collected frames were processed with the software SAINT+ and an absorption correction (SADABS) was applied to the collected reflections. The structure was solved by direct methods (SHELXTL-97) and refined by the fullmatrix-block least-squares method on F2. 2.2. Single-crystal X-ray analysis Crystallographic data for 9,9′-bi(3,3′-bimethyl)anthracene (MBA): C30H22, Mr = 382.48, crystal dimensions 12 × 15 × 23 mm3, monoclinic, space group P-1, Z = 4, a = 12.552 (9) Å, b = 13.432 (5), c = 15.079 (5), α = 115.095 (6)°, β = 101.146 (8)°, γ = 100.697 (9)°, U = 2153.9 (18) Å3, ρcalcd = 1.179 g/cm3, μ (Mo Kα) = 0.067 mm−1, F (000) = 808. A total of 7542 reflections were measured in the range 1.76 ≤ θ ≤ 25.05 (hkl indices: −14 ≤ h ≤ 7, −15 ≤ k ≤ 15, −17 ≤ l ≤ 17), 1646 unique reflections. The structure was refined on F2 to R1 = 0.1298, wR2 = 0.4781 (7542 reflections with I > 2σ (I), GOF = 1.001 on F2 for 450 refined parameters). CCDC number 1863484. 2.3. Synthesis The details of synthesis of 9,9′-bianthracene (BA) and 9,9′-bi(3,3′bimethyl)anthracene (MBA) are described in our previous work [31,37]. 9,9′-bi(3,3′-tert-butyl)anthracene (TBBA) were synthesized according to the procedure analogous to the preparation of compounds BA and MBA. 9,9′-bi(3,3′-tert-butyl)anthracene (TBBA). Yield: 82%. 1H NMR (CDCl3, 400 MHz): δ 1.35–1.60 (s, 18H), 7.00–7.18 (m, 7H), 7.39–7.61 (m, 3H), 8.00–8.19 (m, 4H), 8.56–8.70 (m, 2H). Anal. Calcd for C34H34: C, 92.31%; H, 7.69%. Found: C, 92.48%; H, 7.52%. 2.4. Device fabrication and testing The devices were fabricated using conventional vacuum deposition of the organic layer and cathode onto an indium tin oxide (ITO) coated glass substrate under a base pressure lower than 5.0 × 10−5 mbar. Prepared glass substrates were cleaned using detergent, de-ionized water, acetone, and isopropanol. Immediately prior to loading into a custom-made high vacuum thermal evaporation chamber, the substrates were treated to a UV-ozone environment for 15 min. Then,
Fig. 1. Molecular structures of BAs. 25
Organic Electronics 66 (2019) 24–31
Z. Li et al.
organic layers and a metal cathode layer were evaporated successively by using shadow masks. The entire organic layers and the Al cathode were deposited without exposure to the atmosphere. The deposition rates for the organic materials, and Al were typically 1.0 and 5.0 Å/s, respectively. The current density-voltage-luminescence characteristics were measured using a Keithley 2400 source meter and a PR-650 Spectra Colorimeter. The luminance and spectra of each device were measured in the direction perpendicular to the substrate.
solution. These phenomena imply that the emitters reported herein form a compact stacking structure in the thin film that benefits charge transport [38]. All of our compounds were highly emissive in the blue region with peaks in the range of 446–451 nm. According to the onset of the absorption in the thin film, the optical energy bandgaps (Egopt) of BA, MBA, and TBBA were determined to be 2.88, 2.99, and 2.90 eV, respectively. Their PL spectra were also studied in various solvents. As shown in Fig. S3, the PL spectra of the BAs exhibit red-shifts of 24–38 nm when the solvent polarity is raised from 2.4 (toluene) to 5.8 (acetonitrile). This phenomenon is consistent with a variety of the excited states from the locally excited state (LE) to an excited state with the strong CT character involving the TICT mechanism of the 9,9′bianthracene moieties [32,39]. The fluorescence quantum yields of these BA-based compounds in THF are determined by an integrating sphere and are listed in Table 1. The emission quantum yields are high in the region of 79–84%, indicating that these alkyl substituted BAbased compounds would have good efficient EL properties in OLED devices.
3. Results and discussion 3.1. Synthesis, structural characterization and theoretical computation The synthetic routes of BAs are shown in Fig. S1. This simple, onestep synthesis allows a large amount of BAs to be prepared on a multigram scale, which would help reduce the overall cost and increase the extent of devices' practical applications. Characterization was performed using 1H NMR and elemental analysis. The molecular structure of 3,3′-dimethyl-9,9′-bianthracene (MBA) was further confirmed by single crystal X-ray analysis (Fig. S2), and the crystallographic data and structure refinement parameters are summarized in the Experimental section. MBA crystallizes in the monoclinic space group P-1. The geometry and electronic structures of BAs were obtained using density functional theory (DFT) at the B3LYP/6-31 G (d) level in the Gaussian 03 program (Fig. 2). With a large dihedral angle (around 90°) between two anthracene moieties, the extremely twisted geometry configuration in BAs might prevent intramolecular extending of π-electron and suppress molecular packing in the solid state effectively. Furthermore, the additional two alkyl units with large steric hindrance would help to reduce the aggregation formation emitting at longer wavelength by intermolecular interaction. Apparently, the distributions of both the highest occupied molecule orbital (HOMO) and the lowest unoccupied molecule orbital (LUMO) of the alkyl substituted 9,9′-bianthracene derivatives (MBA and TBBA) were resided almost entirely on the bianthracene moieties while both HOMO and LUMO of BA without substitution delocalized only on one anthracene ring. This result indicates that the absorption and emission processes are mainly attributed to the π–π* transition of the two anthracene chromophores.
3.3. Electrochemical behavior The electrochemical properties and energy levels of the three compounds were investigated by cyclic voltammetry (CV). As shown in Fig. 4, the onset oxidation peaks (Eox) for BA, MBA, and TBBA are 0.58, 0.55, and 0.68 V against the Pt-wire reference electrode, respectively. The HOMO energy levels of BA, MBA, and TBBA are estimated to be −5.26 eV, −5.18 eV and −5.30 eV with regard to ferrocene. We failed to obtain the reduction potential of these compounds from the CV measurements, and thus their LUMOs were estimated from their HOMO and Egopt values to be −2.38, −2.19, and −2.40 eV, respectively. 3.4. Thermal properties The thermal properties of these emitters were determined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under nitrogen atomosphere and the results were shown in Fig. 5. The thermal stability or the glass-state durability of these organic compounds can be greatly enhanced by the introduction of alkyl substituents at 3- and 3′- position of the BA moieties. BA, MBA and TBBA showed high thermal stability with decomposition temperatures (Td) of 270, 308 and 324 °C and glass transition temperatures (Tg) of 98, 113 and 120 °C, in the order of increasing molecular weight. The high thermal values endow high morphologic stability of the amorphous phase in the film.
3.2. Optical and photoluminescent characteristics The photophysical data are collected in Table 1, and representative UV–vis absorption and photoluminescence (PL) spectra are shown in Fig. 3. The absorption spectra of all three BA molecules in the thin-film state show an obviously broader peak and slight red-shift of the peak compared to the absorption spectra of the BAs molecules in CH2Cl2
3.5. Carrier transporting characteristics Before characterizing the performance of these blue emitters in OLEDs, hole-only and electron-only devices were fabricated to investigate the charge transporting properties. The device configurations are shown as follows: ITO/MoO3 (8 nm)/BAs (80 nm)/MoO3 (8 nm)/Al (100 nm) and ITO/TPBi (10 nm)/BAs (80 nm)/TPBi (10 nm)/Al (100 nm). Herein, MoO3 and 1,3,5-tris(N-phenylbenzimidizol-2-yl) benzene (TPBi) function as hole- and electron-transporting materials, respectively. Fig. 6 shows the current density–voltage (J–V) curves of these single-carrier devices. The J–V characteristics of single-carrier devices demonstrated that all three molecules have bipolar transporting capacities with significant currents passing through the hole-only and electron-only devices. In addition, the three compounds show higher hole current density due to the good hole-transporting mobility of anthracene units in BAs. Notably, we find that MBA-based single-carrier devices are more sensitive to the voltage and show higher and faster currents for both holes and electrons than BA and TBBA counterparts, indicating that MBA bearing two relatively small methyl substituent at 3- and 3′-position of the 9,9′-bianthracene moiety is more likely to possess balanced charge-transporting ability, which is quite important
Fig. 2. The optimized geometries and the molecular orbital surfaces of the HOMOs and LUMOs for the BAs obtained at the B3LYP/6-31G level. 26
Organic Electronics 66 (2019) 24–31
Z. Li et al.
Table 1 Physical properties of BAs. Compound
λAbs max (nm) solna/filmb
λPL max(nm) solna/ filmb
Eoxc (V)
Φfd (%)
HOMO/LUMOexpe (Eg) (eV)
HOMO/LUMOcal (ΔEHOMO-LUMO) (eV)
Tg/Tm/Tdf (°C)
BA
334, 354, 372, 393 /333, 355, 378, 399 332, 352, 370, 390/338, 357, 376, 398 332, 351, 370, 391/338, 358, 378, 399
449/451
0.58
84
−5.26/–2.38 (2.88)
−5.21/–1.67 (3.54)
98/-/270
448/448
0.55
79
−5.18/–2.19 (2.99)
−5.13–1.57 (3.56)
113/236/308
451/446
0.68
83
−5.30/–2.40 (2.90)
−5.08/–1.62 (3.46)
120/190/324
MBA TBBA
a b c d e f
Measured in CH2Cl2. Measured in solid thin film on quartz plates. Measured in CH3CN. Absolute photoluminescence quantum yield determined in THF. Values from DFT calculation. Tg: glass-transition temperature; Tm: melting point; Td: decomposition temperature.
these BAs devices. The key device performance parameters and EL emission characteristics are summarized in Table 2. As can be seen in Fig. S4, the turn-on voltages of three devices were about 3.2 V. These lower turn-on voltages of BAs-based devices should result from the bipolar transporting properties of BAs as well as the relatively smaller injection barrier from TAPC and TPBi. The MBA- and TBBA-based nondoped devices showed deep-blue emission with the maximum emission wavelengths of 456 nm and 440 nm and the corresponding color coordinates of (0.15, 0.10) and (0.15, 0.06), respectively, as a result of the rigid nature and nonplanarity of alkyl substituted BAs with the relatively bulky substituents (methyl and tert-butyl groups) in the molecule to enhance the molecular distortion degree and suppress the formation of aggregation or π-π stacking in the solid state. In comparison with the corresponding PL of solid states, the EL emission peaks of
for a high EL efficiency. 3.6. Electroluminescence properties of OLEDs To evaluate the EL performances of BAs as blue emitters, we initially fabricated nondoped devices with a multilayer structure: ITO/HAT-CN (5 nm)/TAPC (40 nm)/BAs (20 nm)/TPBi (40 nm)/Liq (1 nm)/Al (120 nm). In these devices, 4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN) was used as hole injection layer, (1,1-bis[4[N,N-di(ptolyl)amino]phenyl]cyclohexane) (TAPC) was used as hole transporting layer (HTL), TPBi was used as electron transporting layer (ETL), LiF was used as electron injecting layer, and Al was used as cathode. Fig. 7 and Fig. S4 exhibit the EL spectrum and the current density–voltage–luminance–efficiency (J–V–L–η) characteristics of
Fig. 3. Absorption spectra of BAs in CH2Cl2 (a) and films (b), PL spectra of BAs in CH2Cl2 (c) and films (d). 27
Organic Electronics 66 (2019) 24–31
Z. Li et al.
(0.15, 0.06). The MBA- and TBBA-based devices also exhibited good performance with CIE coordinates of (0.16, 0.08) and (0.15, 0.07), respectively. All these values were in good agreement with the requirements of the NTSC standard blue CIE coordinates of (0.14, 0.08). Notably, the device with TBBA as the dopant, achieved the best EL performance, with a EQE as high as 4.56%, a CEmax of 4.16 cd/A, a PEmax of 3.84 lm/W and a pure blue emission at CIE (0.15, 0.07). The excellent EL efficiency of the TBBA-doped device can be attributed mainly to its high fluorescence quantum yield and the efficient energy transfer from CBP host to the TBBA dopant. The operation lifetime of TBBA-doped device was measured to study the stability of the TBBA. As observed in Fig. S7(a), the operation lifetime of approximately 42 min declined to 50%. After examination of the microscopic pictures of the device, the dark stains occurred at light-emitting areas. We note that such a rather short operational stability is not uncommon in blue devices with CIEy < 0.08, which has been convincingly attributed to annihilation between excited states (that is, exciton-exciton or excitonpolaron) in the EML that result in a hot (that is, multiply excited) exciton or polaron while the remaining state nonradiatively transitions to the ground state. The excellent deep-blue OLEDs based on BAs inspired us to further investigate the performance of sky blue OLEDs using BAs as host material and 3 wt% DSA-ph as a dopant. The OLEDs with the structure of ITO/HAT-CN (5 nm)/TAPC (40 nm)/BAFs: 3 wt% DSA-ph (20 nm)/ TPBi (40 nm)/Liq (1 nm)/Al (100 nm) were fabricated. Figs. 8 and 9 show the J–V–L–η characteristics of the DSAph-doped devices. The EL spectra of devices based on the compounds are shown in Fig. S8. The key device performance parameters and EL emission characteristics are summarized in Table 2. For the device with BAs as hosts, the EQE and current efficiency vs. current density were nearly flat. For instance, in the range of current density of 0.01–83.75 mA/cm2 (or luminance value of 1.54–13800 cd/m2), EQE (or CE) of MBA-based device can be maintained over 9.24% (or 15.84 cd/A) with very little variation of 0.54% (or 0.36%), exhibiting unobservable efficiency roll-off, as shown in Figs. 8 and 9. These results are rather intriguing, as most OLED devices show obvious efficiency roll-off. There are two different excitation mechanisms operating at the low and high current density, respectively. At low current density, the dominant excitation mechanism is in selfrecombination in the dopant as carrier trap (trap mechanism), while in high current density, the excitation mechanism is included not only in self-recombination in the dopant, but also the Förster energy transfer from host to dopant (energy transfer mechanism) [28,42–45]. Therefore, the MBA-based device can continue to maintain a relatively good efficiency at high brightness. Furthermore, the BA-, MBA- and TBBAbased devices exhibited very high EQEs of 5.72, 9.47 and 7.16% with CIE coordinates of (0.15, 0.33), (0.15, 0.26) and (0.15, 0.20),
Fig. 4. CV curves of the BAs in CH3CN.
BA without substituent showed a larger redshift (∼21 nm), which may be caused by the intermolecular interactions and the electrical field polarization in the excited states. TBBA-based nondoped device exhibited the best EL performance with a maximum current efficiency (CEmax) of 2.52 cd/A, a maximum power efficiency (PEmax) of 2.73 lm/ W and a EQEmax value of 3.20% with the CIE coordinates of (0.15, 0.06), which can be equivalent to that of reported high efficiency OLEDs in similar color gamut [4,10–12,15,18–21,24,36,37,40,41]. Key device performance parameters of the new compound TBBA and recently reported high performance nondoped deep-blue OLEDs are listed in Table S1. The doped OLED devices were constructed with the same device structures to further improve the device efficiency and color purity. Here, BA, MBA and TBBA were used as the guest (5 wt%) in 4,4′-bis(Ncarbazolyl)biphenyl (CBP) host, which were served as EML in doped devices. The PL of CBP exhibits large overlap with the absorption of BAs, demonstrating more efficient Förster energy transfer from CBP to BAs which can also lead to the high efficiency. The device characteristics are shown in Table 2, Figs. S5 and S6. The BAs-doped devices have low turn-on voltages no greater than 3.9 V. The CEmax and PEmax of these devices are in the range of 1.36–4.16 cd/A and 0.85–3.84 lm/ W, respectively. All the resulting EL spectra show an emission peak around 432–436 nm with narrow FWHM (full-width at half-maximum) of 56–68 nm, which are consistent with their PL emission in thin film. Compared with the nondoped device of BA, the doped device showed higher EQE of 3.46% and deeper blue emission with CIE coordinates of
Fig. 5. TGA (a) and DSC (b) curves of BAs. 28
Organic Electronics 66 (2019) 24–31
Z. Li et al.
Fig. 6. (a) Current density-voltage characteristics for the electron-only devices, (b) Current density-voltage characteristics for the hole-only devices.
Fig. 7. (a) Normalized EL spectra, (b) EQE-current density curves for BAs-based nondoped devices.
Table 2 EL performance of blue OLEDs. EML
Vona
λELb (nm)
Lmaxc (cd m−2)
CEd (cd A−1)
PEd (lm W−1)
EQEd (%)
FWHWb (nm)
CIE (x, y)b
BA MBA TBBA CBP:5%BA CBP:5%MBA CBP:5%TBBA BA:DSAph MBA:DSAph TBBA:DSAph MADN:DSAph
3.3 3.2 3.2 3.5 3.6 3.9 3.2 3.0 2.9 4.0
472 456 440 432 436 436 472 468 460 464
6470 3546 2013 1860 3742 2310 19200 13800 10900 –
3.95 2.26 2.52 1.36 2.76 4.16 12.06 16.54 11.33 9.7
3.87 1.92 2.73 0.85 2.40 3.84 5.59 16.57 10.45 5.5
2.47 2.51 3.20 3.46 3.60 4.56 5.72 9.47 7.16 –
56 66 75 56 56 68 53 55 60 –
(0.17, (0.15, (0.15, (0.16, (0.16, (0.15, (0.15, (0.15, (0.15, (0.16,
a b c d
0.22) 0.10) 0.06) 0.05) 0.08) 0.07) 0.33) 0.26) 0.20) 0.32) [42]
Turn-on voltage at 1 cd/m2. Values collected at 8 V. Maximum luminance. Values collected at a peak efficiency.
EQE of the OLEDs using MBA as the host for the DSAph dopant was 9.47% with excellent CIE coordinates of (0.15, 0.26) and low turn-on voltage of 3.0 V, as shown in Figs. 8 and 9 and Table 2. Our preliminary data reflect the prospects of BAs as a blue host fluorescent material. Due to the high photoluminescence quantum efficiencies of BAs which lead to the inefficiency of intersystem crossing (ISC) of S1→T1, we are not successful in obtaining their phosphorescent PL spectra at 77 K (Fig. S9) [27,46]. Similarly, we measured the transient PL in degassed THF and no delayed fluorescence was observed for these compounds (Fig. S10). These results exclude the possibility that BAs are TADF materials, in view of the fact that TADF materials in general show delayed fluorescence in the transient PL because the small singlet-triplet energy gap (ΔEST) leads to the possibility of T1→S1 through thermally assisted reverse intersystem crossing (RISC) [6,8]. The calculated HOMO and LUMO density maps of the BAs also do not support that they
respectively, and excellent CEmax of 12.06, 16.54 and 11.33 cd/A, respectively. It should be noted that the EQEs of all three OLEDs fabricated in this study are much higher than the conventional upper limit of 5%, indicating the upconversion ability of some triplet excitons to singlet states to generate light in these devices. Among blue fluorescent OLEDs, diphenylanthracene (ADN) derivatives are the most commonly referred hosts. Lee et al. reported that 2-methyl-9,10-di(2-napthyl)anthracene (MADN) as host material for DSAPh blue emitter to get the CEmax of 9.7 cd/A and the PEmax of 5.5 lm/W with CIE (x, y) = (0.16, 0.32) [42]. By using MBA as the fluorescent host, the PEmax was successfully enhanced to 16.57 lm/W, which is three times higher than the MADN:DSAph-based device. Moreover, all three OLEDs fabricated in this study showed higher color purity with CIE (0.15, 0.20–0.33) than our previous devices based on the fluorinated 3,3′-substituted 9,9′bianthracene derivatives (CIE (0.163–0.171, 0.384–0.408) [37]. The 29
Organic Electronics 66 (2019) 24–31
Z. Li et al.
Fig. 8. (a) Current density–voltage curves, (b) Brightness–voltage curves, (c) Current efficiency–current density curves, (d) Power efficiency–current density curves for BAs: 3% DSAph devices.
the dopant site, the available singlet excitons formed via Förster energy transfer from hosts BAs to dopant are greatly increased. Therefore, the BAs hosts can significantly enhance the singlet generation fraction in EL devices, as demonstrated in our previous work [32,37]. 4. Conclusions In conclusion, a series of easily available, low-cost 9,9′-bianthracene derivatives (BAs) have been applied as deep-blue emitters and blue host materials in fluorescent OLEDs. The incorporation of two alkyl into the 3- and 3′-positions of the twisted 9,9′-bianthracene structures would help to reduce molecular aggregation, which endows both MBA and TBBA with good thermal properties, and film-forming abilities as well as EL performance. Nondoped pure deep-blue OLED using TBBA as the emitter showed a high EQE of 3.20% with CIE coordinates (0.15, 0.06), which matched well with the requirement of EBU standard blue CIE coordinate of (0.15, 0.06). Moreover, the use of BAs as the hosts of the blue-emitting dopant DSAph resulted in high-performance OLEDs with very high EQEs of 5.72–9.47% and excellent CEs of 11.33–16.54 cd/A, especially the MBA-based device, exhibiting unobservable efficiency roll-off and high EQE over 9.24% at a high brightness of 13800 cd/m2. The outstanding features presented here reflect the prospects of BAs as a blue host fluorescent material and a deep-blue emitter.
Fig. 9. EQE-current density curves for BAs: 3% DSAph devices.
are TADF materials, because a molecular design approach with donor–acceptor (D–A) type to decrease the ΔEST of emitters by separating the HOMO and LUMO is used to develop TADF emitters while electron density distribution of HOMO and LUMO in these non-D–A BAs are almost completely overlapped. Nevertheless, the TTA does not play a role in achieving high EQE, because the luminance increases linearly with increasing current density at low current injection (from 0.1 to 3 mA/cm2) and less than linearly at higher current injection (Fig. S11). The improved EQEs are due to the enhanced occurrence of singlet excitons in BAs-based EL devices. The BAs host molecule with a particular TICT excited state, which participates in a CT intersystem crossing mechanism, realizes transitions from the triplet to singlet CT-states. At
Acknowledgements We are grateful for support from the National Natural Scientific Foundation of China (Grant no. 61571317, 61775156, 61475109 and 61308093), Key Research and Development (International Cooperation) Program of Shanxi (201603D421042), and Platform and Base Special Project of Shanxi (201605D131038), and give our thanks to Prof. Ganglin Xue and Prof. Huaiming Hu (Northwest University, Xi'an, PR China) for their help in the single-crystal X-ray analysis and 30
Organic Electronics 66 (2019) 24–31
Z. Li et al.
Dr. Wencheng Chen (COSDAF) for the fruitful discussions.
2003–2010. [22] W. Sun, N. Zhou, Y. Xiao, S. Wang, X. Li, Chem. Asian J. 12 (2017) 3069–3076. [23] X.-L. Li, M. Liu, Y. Li, X. Cai, D. Chen, K. Liu, Y. Cao, S.-J. Su, Chem. Commun. 52 (2016) 14454–14457. [24] W.-C. Chen, G.-F. Wu, Y. Yuan, H.-X. Wei, F.-L. Wong, Q.-X. Tong, C.-S. Lee, RSC Adv. 5 (2015) 18067–18074. [25] H. Xu, P. Sun, K. Wang, J. Li, F. Wang, Y. Miao, H. Wang, B. Xu, W.-Y. Wong, J. Mater. Chem. C 5 (2017) 4455–4462. [26] M. Aydemir, G. Haykır, A. Battal, V. Jankus, S.K. Sugunan, F.B. Dias, H. Al-Attar, F. Türksoy, M. Tavaslı, A.P. Monkman, Org. Electron. 30 (2016) 149–157. [27] X. Tang, Q. Bai, T. Shan, J. Li, Y. Gao, F. Liu, H. Liu, Q. Peng, B. Yang, F. Li, P. Lu, Adv. Funct. Mater. (2018) 1705813. [28] C. He, H. Guo, Q. Peng, S. Dong, F. Li, J. Mater. Chem. C 3 (2015) 9942–9947. [29] M.J. Sung, H. Chubachi, R. Sato, M.-K. Shin, S.-K. Kwon, Y.-J. Pu, Y.-H. Kim, J. Mater. Chem. C 5 (2017) 1090–1094. [30] S. Sohn, M.-J. Kim, S. Jung, T.J. Shin, H.-K. Lee, Y.-H. Kim, Org. Electron. 24 (2015) 234–240. [31] Y. Yu, Z. Wu, Z. Li, B. Jiao, L. Li, L. Ma, D. Wang, G. Zhou, X. Hou, J. Mater. Chem. C 1 (2013) 8117–8127. [32] Y. Yu, B. Jiao, Z. Wu, Z. Li, L. Ma, G. Zhou, W. Yu, S.K. So, X. Hou, J. Mater. Chem. C 2 (2014) 9375–9384. [33] F. Gao, J. Ren, Z. Li, S. Yuan, Z. Wu, Y. Cui, H. Jia, H. Wang, F. Shi, Y. Hao, Opt. Mater. Express 5 (2015) 2468–2477. [34] Z. Li, Y. Sun, H. Li, C. Si, X. Lv, B. Jao, Z. Wu, Y. Cui, J. Yu, H. Wang, F. Shi, Y. Hao, Synthetic Met 217 (2016) 102–108. [35] Z. Li, W. Liu, Y. Yu, X. Lv, C. Si, Z. Wu, Y. Cui, H. Jia, J. Yu, H. Wang, F. Shi, Y. Hao, Dyes Pigments 122 (2015) 238–245. [36] Z. Li, X. Wang, X. Lv, C. Si, B. Wei, H. Wang, Y. Hao, RSC Adv. 7 (2017) 49125–49132. [37] C. Si, Z. Li, K. Guo, X. Lv, S. Pan, G. Chen, Y. Hao, B. Wei, Dyes Pigments 148 (2018) 329–340. [38] M. Liu, X.-L. Li, D.C. Chen, Z. Xie, X. Cai, G. Xie, K. Liu, J. Tang, S.J. Su, Y. Cao, Adv. Funct. Mater. 25 (2015) 5190–5198. [39] P. Zhang, W. Dou, Z. Ju, L. Yang, X. Tang, W. Liu, Y. Wu, Org. Electron. 14 (2013) 915–925. [40] D. He, Y. Yuan, B. Liu, D.-Y. Huang, C.-Y. Luo, F. Lu, Q.-X. Tong, C.-S. Lee, Dyes Pigments 136 (2017) 347–353. [41] Z. Huang, B. Wang, Q. Zhang, S. Xiang, X. Lv, L. Ma, B. Yang, Y. Gao, L. Wang, Dyes Pigments 140 (2017) 328–336. [42] M.-T. Lee, H.-H. Chen, C.-H. Liao, C.-H. Tsai, Appl. Phys. Lett. 85 (2004) 3301–3303. [43] S.J. Lee, J.S. Park, K.-J. Yoon, Y.-I. Kim, S.-H. Jin, S.K. Kang, Y.-S. Gal, S. Kang, J.Y. Lee, J.-W. Kang, S.-H. Lee, H.-D. Park, J.-J. Kim, Adv. Funct. Mater. 18 (2008) 3922–3930. [44] Y.H. Kim, D.C. Shin, S.H. Kim, C.H. Ko, H.S. Yu, Y.D. Chae, S.K. Kwon, Adv. Mater. 13 (2001) 1690–1693. [45] J. Littman, P. Martic, J. Appl. Phys. 72 (1992) 1957–1960. [46] P.-Y. Chou, H.-H. Chou, Y.-H. Chen, T.-H. Su, C.-Y. Liao, H.-W. Lin, W.-C. Lin, H.Y. Yen, I.-C. Chen, C.-H. Cheng, Chem. Commun. 50 (2014) 6869–6871.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2018.12.010. References [1] Y. Im, S.Y. Byun, J.H. Kim, D.R. Lee, C.S. Oh, K.S. Yook, J.Y. Lee, Adv. Funct. Mater. 27 (2017) 1603007. [2] W.-C. Chen, C.-S. Lee, Q.-X. Tong, J. Mater. Chem. C 3 (2015) 10957–10963. [3] X. Yang, X. Xu, G. Zhou, J. Mater. Chem. C 3 (2015) 913–944. [4] W.-C. Chen, Y. Yuan, Z.-L. Zhu, Z.-Q. Jiang, L.-S. Liao, C.-S. Lee, Adv. Optical Mater. 6 (2018) 1700855. [5] Z.-L. Zhu, M. Chen, W.-C. Chen, S.-F. Ni, Y.-Y. Peng, C. Zhang, Q.-X. Tong, F. Lu, C.S. Lee, Org. Electron. 38 (2016) 323–329. [6] M.Y. Wong, E. Zysman-Colman, Adv. Mater. 29 (2017) 1605444. [7] M. Sun, L. Zhu, W. Kan, Y. Wei, D. Ma, X. Fan, W. Huang, H. Xu, J. Mater. Chem. C 3 (2015) 9469–9478. [8] Q. Zhang, D. Tsang, H. Kuwabara, Y. Hatae, B. Li, T. Takahashi, S.Y. Lee, T. Yasuda, C. Adachi, Adv. Mater. 27 (2015) 2096–2100. [9] L.-S. Cui, Y.-M. Xie, Y.-K. Wang, C. Zhong, Y.-L. Deng, X.-Y. Liu, Z.-Q. Jiang, L.S. Liao, Adv. Mater. 27 (2015) 4213–4217. [10] Y. Yuan, J.-X. Chen, F. Lu, Q.-X. Tong, Q.-D. Yang, H.-W. Mo, T.-W. Ng, F.-L. Wong, Z.-Q. Guo, J. Ye, Z. Chen, X.-H. Zhang, C.-S. Lee, Chem. Mater. 25 (2013) 4957–4965. [11] W.-C. Chen, Y. Yuan, S.-F. Ni, Q.-X. Tong, F.-L. Wong, C.-S. Lee, Chem. Sci. 8 (2017) 3599–3608. [12] T. Shan, Y. Liu, X. Tang, Q. Bai, Y. Gao, Z. Gao, J. Li, J. Deng, B. Yang, P. Lu, Y. Ma, ACS Appl. Mater. Interfaces 8 (2016) 28771–28779. [13] C. Xiang, X. Fu, W. Wei, R. Liu, Y. Zhang, V. Balema, B. Nelson, F. So, Adv. Funct. Mater. 26 (2016) 1463–1469. [14] I. Kondrasenko, Z.-H. Tsai, K.-Y. Chung, Y.-T. Chen, Y.-Y. Ershova, A. DoménechCarbó, W.-Y. Hung, P.-T. Chou, ACS Appl. Mater. Interfaces 8 (2016) 10968–10976. [15] H. Shin, H. Jung, B. Kim, J. Lee, J. Moon, J. Kim, J. Park, J. Mater. Chem. C 4 (2016) 3833–3842. [16] J.-Y. Hu, Y.-J. Pu, F. Satoh, S. Kawata, H. Katagiri, H. Sasabe, J. Kido, Adv. Funct. Mater. 24 (2014) 2064–2071. [17] B. Liu, Z.-W. Yu, D. He, Z.-L. Zhu, J. Zheng, Y.-D. Yu, W.-F. Xie, Q.-X. Tong, C.S. Lee, J. Mater. Chem. C 5 (2017) 5402–5410. [18] J. Zhao, B. Liu, Z. Wang, Q. Tong, X. Du, C. Zheng, H. Lin, S. Tao, X. Zhang, ACS Appl. Mater. Interfaces 10 (2018) 9629–9637. [19] S.S. Reddy, V.G. Sree, H.-Y. Park, A. Maheshwaran, M. Song, S.-H. Jin, Dyes Pigments 145 (2017) 63–71. [20] A. Abdurahman, A. Obolda, Q. Peng, F. Li, Dyes Pigments 153 (2018) 10–17. [21] B. Liu, J. Zhao, C. Luo, F. Lu, S. Tao, Q. Tong A, J. Mater. Chem. C 4 (2016)
31
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Easily available, low-cost 9,9′-bianthracene derivatives as efficient blue hosts and deep-blue emitters in OLEDs
T
Zhanfeng Lia,d,∗, Guoyue Gana, Zhitian Lingb, Kunping Guob, Changfeng Sib, Xiang Lva, Hua Wangc, Bin Weib,∗∗, Yuying Haoa,∗∗∗ a
Key Laboratory of Advanced Transducers and Intelligent Control System, Ministry of Education and Shanxi Province, College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan, 030024, PR China b Key Laboratory of Advanced Display and System Applications, Ministry of Education, Shanghai University, Shanghai, 200072, PR China c Key Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Taiyuan, 030024, PR China d Center of Super-Diamond and Advanced Films (COSDAF) and Department of Chemistry, City University of Hong Kong, 999077, Hong Kong, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Fluorescent OLED 9,9′-bianthracene Deep-blue emitters Blue host
A series of alkyl substituted 9,9′-bianthracene derivatives (BAs) with easy availability and low cost have been applied as deep-blue emitters and blue host materials in fluorescent organic light-emitting diodes (OLEDs). The nearly perpendicular geometry configuration of 9,9′-bianthracene and the additional alkyl units with large steric hindrance in methyl substituted MBA and tert-butyl substituted TBBA would reduce the aggregation formation emitting at longer wavelength by intramolecular and intermolecular interaction, which endows both of the compounds with good thermal properties, and film-forming abilities as well as excellent electroluminescence performance. Nondoped pure deep-blue OLED using TBBA as the emitter showed good performance with EQE of 3.20% and high color purity with CIE (0.15, 0.06), which matched the EBU standard blue very well. Furthermore, these BAs worked as excellent host materials for p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSAph) dopant to get high-performance OLEDs with low turn-on voltages and high efficiencies, especially for MBA bearing two methyl substituents at 3- and 3′-positions of the 9,9′-bianthracene as the fluorescent host with a low turn-on voltage of 3.0 V, a high current efficiency of 16.54 cd/A and a high EQE of 9.47% with unobservable efficiency roll-off. Particularly, EQE can be maintained over 9.20% with very little variation of 0.54% in the range of luminance value of 1.54–13800 cd/m2 for the MBA-based device. The improved EQEs are due to the enhanced occurrence of singlet excitons in BAs-based devices.
1. Introduction Owing to their special features such as high efficiency, low cost, easy processing, and flexibility, organic light-emitting diodes (OLEDs) have received increasing attention recently and have already been applied in solid-state lighting technologies and in full-color flat-panel display applications. Furthermore, OLEDs are regarded as the most viable competitor for next-generation flexible displays and transparent displays [1]. However, the performance of blue/deep-blue OLEDs is usually inferior to that of green/red OLEDs because of blue emitters' intrinsic wide energy gaps, which is still one of the bottlenecks for the further application of OLEDs [1–5]. Though phosphorescent materials and thermally activated delayed fluorescence (TADF) materials can
harvest 100% internal quantum efficiency by utilizing both singlet and triplet excitons, they hardly gain breakthrough in deep-blue ones with Commission International de L'Eclairage (CIE) coordinates of y < 0.08, which match the National Television System Committee (NTSC) standard blue CIE (x, y) coordinates of (0.14, 0.08) for display applications [1,6–9], let alone the stringent European Broadcasting Union (EBU) standard blue with CIE coordinates of (0.15, 0.06) [10–21]. Meanwhile, high-performance deep-blue emitters can not only serve as energy donors (hosts) to create other visible emissions and white lights by means of energy transfer processes, but also play a key role in reducing the power consumption and broadening wide-gamut RGB (red, green, and blue) color space for vivid full-color displays. In addition to the above issue, the electroluminescence (EL)
∗ Corresponding author. Key Laboratory of Advanced Transducers and Intelligent Control System, Ministry of Education and Shanxi Province, College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan, 030024, PR China. ∗∗ Corresponding author. ∗∗∗ Corresponding author. E-mail addresses: [email protected] (Z. Li), [email protected] (B. Wei), [email protected] (Y. Hao).
https://doi.org/10.1016/j.orgel.2018.12.010 Received 1 September 2018; Received in revised form 8 December 2018; Accepted 10 December 2018 Available online 11 December 2018 1566-1199/ © 2018 Elsevier B.V. All rights reserved.
Organic Electronics 66 (2019) 24–31
Z. Li et al.
2. Experimental
properties of blue-emitting materials remain challenging, particularly in terms of easy availability, low cost, and high color purity. However, phosphorescent and TADF blue OLEDs suffer from a certain degree of efficiency roll-off owing to the triplet–triplet annihilation (TTA) process, which still remains unsatisfactory in the practical application [1,6,8,9,13]. Thus, traditional efficient fluorescent materials that perform low efficiency roll-off at high brightness are still considered to be promising for the realization of efficient and stable deep-blue devices [10,12,14,17,18,21–25]. Actually, a variety of deep-blue OLED materials using judicious molecular designs and potential starting precursor building blocks, such as fluorene and spirobifluorenes, anthracene and phenanthrene, imidazole, triphenylamine and carbazole, and other Nheterocycle-based emitters, have been developed [3]. Among them, anthracene is still the highest efficient emitter due to its unusual photoluminescence (PL) and EL properties. Many existing studies on blue emitters based on anthracene core that modified with sterically hindered substituents to inhibit molecular packing and reduce π–π* stacking can maintain a deep-blue emission [26–30]. In our previous work, a series of fluorinated 9,9′-bianthracene (BA) derivatives with nearly perpendicular geometry configuration were first presented as both highly efficient pure blue emitters and blue host materials for OLEDs [31–37]. These BA cored fluorescence emitters with particular twisted intramolecular charge-transfer (TICT) characteristics could realize electron–hole recombination via intramolecular conversion from CT excitons (immediate precursor) to radiative singlet excitons (final state), thus achieving the highly efficient blue OLEDs [32,37]. Another important issue that severely hinders the commercialization of OLEDs is to minimize the manufacture cost of materials and simplify the fabrication process. Herein, we designed and synthesized a series of easily available and low-cost 9,9′-bianthracene derivatives (BAs), 9,9′-bianthracene (BA), 9,9′-bi(3,3′-bimethyl)anthracene (MBA) and 9,9′-bi(3,3′-tert-butyl)anthracene (TBBA), as shown in Fig. 1. The nearly perpendicular geometry configuration of BA might prevent intramolecular extending of πelectron and suppress molecular packing in the solid state effectively. Furthermore, the additional two alkyl groups with large steric hindrance would help to reduce the aggregation formation emitting at longer wavelength by intermolecular interaction. TBBA-based nondoped device exhibited the best EL performance with external quantum efficiency (EQE) of 3.20% and the CIE coordinates of (0.15, 0.06), matching the requirement of EBU standard of CIE (0.15, 0.06) very well. When doping BAs into the host 4,4′-bis(N-carbazolyl)biphenyl (CBP) as a light-emitting layer, the devices achieved much higher EQEs of 3.46–4.56% with a pure blue emission at CIE (0.15–0.16, 0.05–0.08). Moreover, the use of BAs as the hosts of the blue-emitting dopant p-bis (p-N,N-diphenyl-aminostyryl)benzene (DSAph) resulted in high-performance OLEDs with very high EQEs of 5.72–9.47% and excellent current efficiency of 11.33–16.54 cd/A, especially the MBA-based device, exhibiting unobservable efficiency roll-off and maintaining over 9.24% with very little variation of 0.54% in the range of luminance value of 1.54–13800 cd/m2.
2.1. General information Commercially available reagents were used without further purification. 1H NMR was recorded on a Bruker 400 MHz NMR spectrometer. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried on a STA 409 PC instrument at a heating rate of 10 °C/min under argon. Photoluminescence (PL) pectra and photoluminescence quantum efficiency (Φf) were measured using a FLSP920 fluorescence spectrophotometer, both in solution and in the solid state. Absorption spectra were recorded by a Unico UV-2600 PCS spectrophotometer. Cyclic voltammetry (CV) was performed using a CHI 660E electrochemical workstation at a scan rate of 100 mV/s. All experiments were carried out in a three-electrode compartment cell with a Pt-sheet counter electrode, a glassy carbon working electrode, and a Pt-wire reference electrode. The supporting electrolyte used was 0.1 M tetrabutylammonium hexafluorophosphate ([Bu4N]ClO4) solution in dry acetonitrile. The cell containing the solution of the sample (1 mM) and the supporting electrolyte was purged with a nitrogen gas thoroughly before scanning for its oxidation and reduction properties. Ferrocene was used for potential calibration in each measurement. Density functional theory (DFT) calculations (B3LYP) were performed for the target compound with the 6-31G(d,p) basis set by using the Gaussian 09 program. Diffraction data were collected at 293(2) K using graphite monochromated Mo Ka radiation (λ = 0.71073 Å) on a BRUKER SMART APEX II CCD diffractometer. The collected frames were processed with the software SAINT+ and an absorption correction (SADABS) was applied to the collected reflections. The structure was solved by direct methods (SHELXTL-97) and refined by the fullmatrix-block least-squares method on F2. 2.2. Single-crystal X-ray analysis Crystallographic data for 9,9′-bi(3,3′-bimethyl)anthracene (MBA): C30H22, Mr = 382.48, crystal dimensions 12 × 15 × 23 mm3, monoclinic, space group P-1, Z = 4, a = 12.552 (9) Å, b = 13.432 (5), c = 15.079 (5), α = 115.095 (6)°, β = 101.146 (8)°, γ = 100.697 (9)°, U = 2153.9 (18) Å3, ρcalcd = 1.179 g/cm3, μ (Mo Kα) = 0.067 mm−1, F (000) = 808. A total of 7542 reflections were measured in the range 1.76 ≤ θ ≤ 25.05 (hkl indices: −14 ≤ h ≤ 7, −15 ≤ k ≤ 15, −17 ≤ l ≤ 17), 1646 unique reflections. The structure was refined on F2 to R1 = 0.1298, wR2 = 0.4781 (7542 reflections with I > 2σ (I), GOF = 1.001 on F2 for 450 refined parameters). CCDC number 1863484. 2.3. Synthesis The details of synthesis of 9,9′-bianthracene (BA) and 9,9′-bi(3,3′bimethyl)anthracene (MBA) are described in our previous work [31,37]. 9,9′-bi(3,3′-tert-butyl)anthracene (TBBA) were synthesized according to the procedure analogous to the preparation of compounds BA and MBA. 9,9′-bi(3,3′-tert-butyl)anthracene (TBBA). Yield: 82%. 1H NMR (CDCl3, 400 MHz): δ 1.35–1.60 (s, 18H), 7.00–7.18 (m, 7H), 7.39–7.61 (m, 3H), 8.00–8.19 (m, 4H), 8.56–8.70 (m, 2H). Anal. Calcd for C34H34: C, 92.31%; H, 7.69%. Found: C, 92.48%; H, 7.52%. 2.4. Device fabrication and testing The devices were fabricated using conventional vacuum deposition of the organic layer and cathode onto an indium tin oxide (ITO) coated glass substrate under a base pressure lower than 5.0 × 10−5 mbar. Prepared glass substrates were cleaned using detergent, de-ionized water, acetone, and isopropanol. Immediately prior to loading into a custom-made high vacuum thermal evaporation chamber, the substrates were treated to a UV-ozone environment for 15 min. Then,
Fig. 1. Molecular structures of BAs. 25
Organic Electronics 66 (2019) 24–31
Z. Li et al.
organic layers and a metal cathode layer were evaporated successively by using shadow masks. The entire organic layers and the Al cathode were deposited without exposure to the atmosphere. The deposition rates for the organic materials, and Al were typically 1.0 and 5.0 Å/s, respectively. The current density-voltage-luminescence characteristics were measured using a Keithley 2400 source meter and a PR-650 Spectra Colorimeter. The luminance and spectra of each device were measured in the direction perpendicular to the substrate.
solution. These phenomena imply that the emitters reported herein form a compact stacking structure in the thin film that benefits charge transport [38]. All of our compounds were highly emissive in the blue region with peaks in the range of 446–451 nm. According to the onset of the absorption in the thin film, the optical energy bandgaps (Egopt) of BA, MBA, and TBBA were determined to be 2.88, 2.99, and 2.90 eV, respectively. Their PL spectra were also studied in various solvents. As shown in Fig. S3, the PL spectra of the BAs exhibit red-shifts of 24–38 nm when the solvent polarity is raised from 2.4 (toluene) to 5.8 (acetonitrile). This phenomenon is consistent with a variety of the excited states from the locally excited state (LE) to an excited state with the strong CT character involving the TICT mechanism of the 9,9′bianthracene moieties [32,39]. The fluorescence quantum yields of these BA-based compounds in THF are determined by an integrating sphere and are listed in Table 1. The emission quantum yields are high in the region of 79–84%, indicating that these alkyl substituted BAbased compounds would have good efficient EL properties in OLED devices.
3. Results and discussion 3.1. Synthesis, structural characterization and theoretical computation The synthetic routes of BAs are shown in Fig. S1. This simple, onestep synthesis allows a large amount of BAs to be prepared on a multigram scale, which would help reduce the overall cost and increase the extent of devices' practical applications. Characterization was performed using 1H NMR and elemental analysis. The molecular structure of 3,3′-dimethyl-9,9′-bianthracene (MBA) was further confirmed by single crystal X-ray analysis (Fig. S2), and the crystallographic data and structure refinement parameters are summarized in the Experimental section. MBA crystallizes in the monoclinic space group P-1. The geometry and electronic structures of BAs were obtained using density functional theory (DFT) at the B3LYP/6-31 G (d) level in the Gaussian 03 program (Fig. 2). With a large dihedral angle (around 90°) between two anthracene moieties, the extremely twisted geometry configuration in BAs might prevent intramolecular extending of π-electron and suppress molecular packing in the solid state effectively. Furthermore, the additional two alkyl units with large steric hindrance would help to reduce the aggregation formation emitting at longer wavelength by intermolecular interaction. Apparently, the distributions of both the highest occupied molecule orbital (HOMO) and the lowest unoccupied molecule orbital (LUMO) of the alkyl substituted 9,9′-bianthracene derivatives (MBA and TBBA) were resided almost entirely on the bianthracene moieties while both HOMO and LUMO of BA without substitution delocalized only on one anthracene ring. This result indicates that the absorption and emission processes are mainly attributed to the π–π* transition of the two anthracene chromophores.
3.3. Electrochemical behavior The electrochemical properties and energy levels of the three compounds were investigated by cyclic voltammetry (CV). As shown in Fig. 4, the onset oxidation peaks (Eox) for BA, MBA, and TBBA are 0.58, 0.55, and 0.68 V against the Pt-wire reference electrode, respectively. The HOMO energy levels of BA, MBA, and TBBA are estimated to be −5.26 eV, −5.18 eV and −5.30 eV with regard to ferrocene. We failed to obtain the reduction potential of these compounds from the CV measurements, and thus their LUMOs were estimated from their HOMO and Egopt values to be −2.38, −2.19, and −2.40 eV, respectively. 3.4. Thermal properties The thermal properties of these emitters were determined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under nitrogen atomosphere and the results were shown in Fig. 5. The thermal stability or the glass-state durability of these organic compounds can be greatly enhanced by the introduction of alkyl substituents at 3- and 3′- position of the BA moieties. BA, MBA and TBBA showed high thermal stability with decomposition temperatures (Td) of 270, 308 and 324 °C and glass transition temperatures (Tg) of 98, 113 and 120 °C, in the order of increasing molecular weight. The high thermal values endow high morphologic stability of the amorphous phase in the film.
3.2. Optical and photoluminescent characteristics The photophysical data are collected in Table 1, and representative UV–vis absorption and photoluminescence (PL) spectra are shown in Fig. 3. The absorption spectra of all three BA molecules in the thin-film state show an obviously broader peak and slight red-shift of the peak compared to the absorption spectra of the BAs molecules in CH2Cl2
3.5. Carrier transporting characteristics Before characterizing the performance of these blue emitters in OLEDs, hole-only and electron-only devices were fabricated to investigate the charge transporting properties. The device configurations are shown as follows: ITO/MoO3 (8 nm)/BAs (80 nm)/MoO3 (8 nm)/Al (100 nm) and ITO/TPBi (10 nm)/BAs (80 nm)/TPBi (10 nm)/Al (100 nm). Herein, MoO3 and 1,3,5-tris(N-phenylbenzimidizol-2-yl) benzene (TPBi) function as hole- and electron-transporting materials, respectively. Fig. 6 shows the current density–voltage (J–V) curves of these single-carrier devices. The J–V characteristics of single-carrier devices demonstrated that all three molecules have bipolar transporting capacities with significant currents passing through the hole-only and electron-only devices. In addition, the three compounds show higher hole current density due to the good hole-transporting mobility of anthracene units in BAs. Notably, we find that MBA-based single-carrier devices are more sensitive to the voltage and show higher and faster currents for both holes and electrons than BA and TBBA counterparts, indicating that MBA bearing two relatively small methyl substituent at 3- and 3′-position of the 9,9′-bianthracene moiety is more likely to possess balanced charge-transporting ability, which is quite important
Fig. 2. The optimized geometries and the molecular orbital surfaces of the HOMOs and LUMOs for the BAs obtained at the B3LYP/6-31G level. 26
Organic Electronics 66 (2019) 24–31
Z. Li et al.
Table 1 Physical properties of BAs. Compound
λAbs max (nm) solna/filmb
λPL max(nm) solna/ filmb
Eoxc (V)
Φfd (%)
HOMO/LUMOexpe (Eg) (eV)
HOMO/LUMOcal (ΔEHOMO-LUMO) (eV)
Tg/Tm/Tdf (°C)
BA
334, 354, 372, 393 /333, 355, 378, 399 332, 352, 370, 390/338, 357, 376, 398 332, 351, 370, 391/338, 358, 378, 399
449/451
0.58
84
−5.26/–2.38 (2.88)
−5.21/–1.67 (3.54)
98/-/270
448/448
0.55
79
−5.18/–2.19 (2.99)
−5.13–1.57 (3.56)
113/236/308
451/446
0.68
83
−5.30/–2.40 (2.90)
−5.08/–1.62 (3.46)
120/190/324
MBA TBBA
a b c d e f
Measured in CH2Cl2. Measured in solid thin film on quartz plates. Measured in CH3CN. Absolute photoluminescence quantum yield determined in THF. Values from DFT calculation. Tg: glass-transition temperature; Tm: melting point; Td: decomposition temperature.
these BAs devices. The key device performance parameters and EL emission characteristics are summarized in Table 2. As can be seen in Fig. S4, the turn-on voltages of three devices were about 3.2 V. These lower turn-on voltages of BAs-based devices should result from the bipolar transporting properties of BAs as well as the relatively smaller injection barrier from TAPC and TPBi. The MBA- and TBBA-based nondoped devices showed deep-blue emission with the maximum emission wavelengths of 456 nm and 440 nm and the corresponding color coordinates of (0.15, 0.10) and (0.15, 0.06), respectively, as a result of the rigid nature and nonplanarity of alkyl substituted BAs with the relatively bulky substituents (methyl and tert-butyl groups) in the molecule to enhance the molecular distortion degree and suppress the formation of aggregation or π-π stacking in the solid state. In comparison with the corresponding PL of solid states, the EL emission peaks of
for a high EL efficiency. 3.6. Electroluminescence properties of OLEDs To evaluate the EL performances of BAs as blue emitters, we initially fabricated nondoped devices with a multilayer structure: ITO/HAT-CN (5 nm)/TAPC (40 nm)/BAs (20 nm)/TPBi (40 nm)/Liq (1 nm)/Al (120 nm). In these devices, 4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN) was used as hole injection layer, (1,1-bis[4[N,N-di(ptolyl)amino]phenyl]cyclohexane) (TAPC) was used as hole transporting layer (HTL), TPBi was used as electron transporting layer (ETL), LiF was used as electron injecting layer, and Al was used as cathode. Fig. 7 and Fig. S4 exhibit the EL spectrum and the current density–voltage–luminance–efficiency (J–V–L–η) characteristics of
Fig. 3. Absorption spectra of BAs in CH2Cl2 (a) and films (b), PL spectra of BAs in CH2Cl2 (c) and films (d). 27
Organic Electronics 66 (2019) 24–31
Z. Li et al.
(0.15, 0.06). The MBA- and TBBA-based devices also exhibited good performance with CIE coordinates of (0.16, 0.08) and (0.15, 0.07), respectively. All these values were in good agreement with the requirements of the NTSC standard blue CIE coordinates of (0.14, 0.08). Notably, the device with TBBA as the dopant, achieved the best EL performance, with a EQE as high as 4.56%, a CEmax of 4.16 cd/A, a PEmax of 3.84 lm/W and a pure blue emission at CIE (0.15, 0.07). The excellent EL efficiency of the TBBA-doped device can be attributed mainly to its high fluorescence quantum yield and the efficient energy transfer from CBP host to the TBBA dopant. The operation lifetime of TBBA-doped device was measured to study the stability of the TBBA. As observed in Fig. S7(a), the operation lifetime of approximately 42 min declined to 50%. After examination of the microscopic pictures of the device, the dark stains occurred at light-emitting areas. We note that such a rather short operational stability is not uncommon in blue devices with CIEy < 0.08, which has been convincingly attributed to annihilation between excited states (that is, exciton-exciton or excitonpolaron) in the EML that result in a hot (that is, multiply excited) exciton or polaron while the remaining state nonradiatively transitions to the ground state. The excellent deep-blue OLEDs based on BAs inspired us to further investigate the performance of sky blue OLEDs using BAs as host material and 3 wt% DSA-ph as a dopant. The OLEDs with the structure of ITO/HAT-CN (5 nm)/TAPC (40 nm)/BAFs: 3 wt% DSA-ph (20 nm)/ TPBi (40 nm)/Liq (1 nm)/Al (100 nm) were fabricated. Figs. 8 and 9 show the J–V–L–η characteristics of the DSAph-doped devices. The EL spectra of devices based on the compounds are shown in Fig. S8. The key device performance parameters and EL emission characteristics are summarized in Table 2. For the device with BAs as hosts, the EQE and current efficiency vs. current density were nearly flat. For instance, in the range of current density of 0.01–83.75 mA/cm2 (or luminance value of 1.54–13800 cd/m2), EQE (or CE) of MBA-based device can be maintained over 9.24% (or 15.84 cd/A) with very little variation of 0.54% (or 0.36%), exhibiting unobservable efficiency roll-off, as shown in Figs. 8 and 9. These results are rather intriguing, as most OLED devices show obvious efficiency roll-off. There are two different excitation mechanisms operating at the low and high current density, respectively. At low current density, the dominant excitation mechanism is in selfrecombination in the dopant as carrier trap (trap mechanism), while in high current density, the excitation mechanism is included not only in self-recombination in the dopant, but also the Förster energy transfer from host to dopant (energy transfer mechanism) [28,42–45]. Therefore, the MBA-based device can continue to maintain a relatively good efficiency at high brightness. Furthermore, the BA-, MBA- and TBBAbased devices exhibited very high EQEs of 5.72, 9.47 and 7.16% with CIE coordinates of (0.15, 0.33), (0.15, 0.26) and (0.15, 0.20),
Fig. 4. CV curves of the BAs in CH3CN.
BA without substituent showed a larger redshift (∼21 nm), which may be caused by the intermolecular interactions and the electrical field polarization in the excited states. TBBA-based nondoped device exhibited the best EL performance with a maximum current efficiency (CEmax) of 2.52 cd/A, a maximum power efficiency (PEmax) of 2.73 lm/ W and a EQEmax value of 3.20% with the CIE coordinates of (0.15, 0.06), which can be equivalent to that of reported high efficiency OLEDs in similar color gamut [4,10–12,15,18–21,24,36,37,40,41]. Key device performance parameters of the new compound TBBA and recently reported high performance nondoped deep-blue OLEDs are listed in Table S1. The doped OLED devices were constructed with the same device structures to further improve the device efficiency and color purity. Here, BA, MBA and TBBA were used as the guest (5 wt%) in 4,4′-bis(Ncarbazolyl)biphenyl (CBP) host, which were served as EML in doped devices. The PL of CBP exhibits large overlap with the absorption of BAs, demonstrating more efficient Förster energy transfer from CBP to BAs which can also lead to the high efficiency. The device characteristics are shown in Table 2, Figs. S5 and S6. The BAs-doped devices have low turn-on voltages no greater than 3.9 V. The CEmax and PEmax of these devices are in the range of 1.36–4.16 cd/A and 0.85–3.84 lm/ W, respectively. All the resulting EL spectra show an emission peak around 432–436 nm with narrow FWHM (full-width at half-maximum) of 56–68 nm, which are consistent with their PL emission in thin film. Compared with the nondoped device of BA, the doped device showed higher EQE of 3.46% and deeper blue emission with CIE coordinates of
Fig. 5. TGA (a) and DSC (b) curves of BAs. 28
Organic Electronics 66 (2019) 24–31
Z. Li et al.
Fig. 6. (a) Current density-voltage characteristics for the electron-only devices, (b) Current density-voltage characteristics for the hole-only devices.
Fig. 7. (a) Normalized EL spectra, (b) EQE-current density curves for BAs-based nondoped devices.
Table 2 EL performance of blue OLEDs. EML
Vona
λELb (nm)
Lmaxc (cd m−2)
CEd (cd A−1)
PEd (lm W−1)
EQEd (%)
FWHWb (nm)
CIE (x, y)b
BA MBA TBBA CBP:5%BA CBP:5%MBA CBP:5%TBBA BA:DSAph MBA:DSAph TBBA:DSAph MADN:DSAph
3.3 3.2 3.2 3.5 3.6 3.9 3.2 3.0 2.9 4.0
472 456 440 432 436 436 472 468 460 464
6470 3546 2013 1860 3742 2310 19200 13800 10900 –
3.95 2.26 2.52 1.36 2.76 4.16 12.06 16.54 11.33 9.7
3.87 1.92 2.73 0.85 2.40 3.84 5.59 16.57 10.45 5.5
2.47 2.51 3.20 3.46 3.60 4.56 5.72 9.47 7.16 –
56 66 75 56 56 68 53 55 60 –
(0.17, (0.15, (0.15, (0.16, (0.16, (0.15, (0.15, (0.15, (0.15, (0.16,
a b c d
0.22) 0.10) 0.06) 0.05) 0.08) 0.07) 0.33) 0.26) 0.20) 0.32) [42]
Turn-on voltage at 1 cd/m2. Values collected at 8 V. Maximum luminance. Values collected at a peak efficiency.
EQE of the OLEDs using MBA as the host for the DSAph dopant was 9.47% with excellent CIE coordinates of (0.15, 0.26) and low turn-on voltage of 3.0 V, as shown in Figs. 8 and 9 and Table 2. Our preliminary data reflect the prospects of BAs as a blue host fluorescent material. Due to the high photoluminescence quantum efficiencies of BAs which lead to the inefficiency of intersystem crossing (ISC) of S1→T1, we are not successful in obtaining their phosphorescent PL spectra at 77 K (Fig. S9) [27,46]. Similarly, we measured the transient PL in degassed THF and no delayed fluorescence was observed for these compounds (Fig. S10). These results exclude the possibility that BAs are TADF materials, in view of the fact that TADF materials in general show delayed fluorescence in the transient PL because the small singlet-triplet energy gap (ΔEST) leads to the possibility of T1→S1 through thermally assisted reverse intersystem crossing (RISC) [6,8]. The calculated HOMO and LUMO density maps of the BAs also do not support that they
respectively, and excellent CEmax of 12.06, 16.54 and 11.33 cd/A, respectively. It should be noted that the EQEs of all three OLEDs fabricated in this study are much higher than the conventional upper limit of 5%, indicating the upconversion ability of some triplet excitons to singlet states to generate light in these devices. Among blue fluorescent OLEDs, diphenylanthracene (ADN) derivatives are the most commonly referred hosts. Lee et al. reported that 2-methyl-9,10-di(2-napthyl)anthracene (MADN) as host material for DSAPh blue emitter to get the CEmax of 9.7 cd/A and the PEmax of 5.5 lm/W with CIE (x, y) = (0.16, 0.32) [42]. By using MBA as the fluorescent host, the PEmax was successfully enhanced to 16.57 lm/W, which is three times higher than the MADN:DSAph-based device. Moreover, all three OLEDs fabricated in this study showed higher color purity with CIE (0.15, 0.20–0.33) than our previous devices based on the fluorinated 3,3′-substituted 9,9′bianthracene derivatives (CIE (0.163–0.171, 0.384–0.408) [37]. The 29
Organic Electronics 66 (2019) 24–31
Z. Li et al.
Fig. 8. (a) Current density–voltage curves, (b) Brightness–voltage curves, (c) Current efficiency–current density curves, (d) Power efficiency–current density curves for BAs: 3% DSAph devices.
the dopant site, the available singlet excitons formed via Förster energy transfer from hosts BAs to dopant are greatly increased. Therefore, the BAs hosts can significantly enhance the singlet generation fraction in EL devices, as demonstrated in our previous work [32,37]. 4. Conclusions In conclusion, a series of easily available, low-cost 9,9′-bianthracene derivatives (BAs) have been applied as deep-blue emitters and blue host materials in fluorescent OLEDs. The incorporation of two alkyl into the 3- and 3′-positions of the twisted 9,9′-bianthracene structures would help to reduce molecular aggregation, which endows both MBA and TBBA with good thermal properties, and film-forming abilities as well as EL performance. Nondoped pure deep-blue OLED using TBBA as the emitter showed a high EQE of 3.20% with CIE coordinates (0.15, 0.06), which matched well with the requirement of EBU standard blue CIE coordinate of (0.15, 0.06). Moreover, the use of BAs as the hosts of the blue-emitting dopant DSAph resulted in high-performance OLEDs with very high EQEs of 5.72–9.47% and excellent CEs of 11.33–16.54 cd/A, especially the MBA-based device, exhibiting unobservable efficiency roll-off and high EQE over 9.24% at a high brightness of 13800 cd/m2. The outstanding features presented here reflect the prospects of BAs as a blue host fluorescent material and a deep-blue emitter.
Fig. 9. EQE-current density curves for BAs: 3% DSAph devices.
are TADF materials, because a molecular design approach with donor–acceptor (D–A) type to decrease the ΔEST of emitters by separating the HOMO and LUMO is used to develop TADF emitters while electron density distribution of HOMO and LUMO in these non-D–A BAs are almost completely overlapped. Nevertheless, the TTA does not play a role in achieving high EQE, because the luminance increases linearly with increasing current density at low current injection (from 0.1 to 3 mA/cm2) and less than linearly at higher current injection (Fig. S11). The improved EQEs are due to the enhanced occurrence of singlet excitons in BAs-based EL devices. The BAs host molecule with a particular TICT excited state, which participates in a CT intersystem crossing mechanism, realizes transitions from the triplet to singlet CT-states. At
Acknowledgements We are grateful for support from the National Natural Scientific Foundation of China (Grant no. 61571317, 61775156, 61475109 and 61308093), Key Research and Development (International Cooperation) Program of Shanxi (201603D421042), and Platform and Base Special Project of Shanxi (201605D131038), and give our thanks to Prof. Ganglin Xue and Prof. Huaiming Hu (Northwest University, Xi'an, PR China) for their help in the single-crystal X-ray analysis and 30
Organic Electronics 66 (2019) 24–31
Z. Li et al.
Dr. Wencheng Chen (COSDAF) for the fruitful discussions.
2003–2010. [22] W. Sun, N. Zhou, Y. Xiao, S. Wang, X. Li, Chem. Asian J. 12 (2017) 3069–3076. [23] X.-L. Li, M. Liu, Y. Li, X. Cai, D. Chen, K. Liu, Y. Cao, S.-J. Su, Chem. Commun. 52 (2016) 14454–14457. [24] W.-C. Chen, G.-F. Wu, Y. Yuan, H.-X. Wei, F.-L. Wong, Q.-X. Tong, C.-S. Lee, RSC Adv. 5 (2015) 18067–18074. [25] H. Xu, P. Sun, K. Wang, J. Li, F. Wang, Y. Miao, H. Wang, B. Xu, W.-Y. Wong, J. Mater. Chem. C 5 (2017) 4455–4462. [26] M. Aydemir, G. Haykır, A. Battal, V. Jankus, S.K. Sugunan, F.B. Dias, H. Al-Attar, F. Türksoy, M. Tavaslı, A.P. Monkman, Org. Electron. 30 (2016) 149–157. [27] X. Tang, Q. Bai, T. Shan, J. Li, Y. Gao, F. Liu, H. Liu, Q. Peng, B. Yang, F. Li, P. Lu, Adv. Funct. Mater. (2018) 1705813. [28] C. He, H. Guo, Q. Peng, S. Dong, F. Li, J. Mater. Chem. C 3 (2015) 9942–9947. [29] M.J. Sung, H. Chubachi, R. Sato, M.-K. Shin, S.-K. Kwon, Y.-J. Pu, Y.-H. Kim, J. Mater. Chem. C 5 (2017) 1090–1094. [30] S. Sohn, M.-J. Kim, S. Jung, T.J. Shin, H.-K. Lee, Y.-H. Kim, Org. Electron. 24 (2015) 234–240. [31] Y. Yu, Z. Wu, Z. Li, B. Jiao, L. Li, L. Ma, D. Wang, G. Zhou, X. Hou, J. Mater. Chem. C 1 (2013) 8117–8127. [32] Y. Yu, B. Jiao, Z. Wu, Z. Li, L. Ma, G. Zhou, W. Yu, S.K. So, X. Hou, J. Mater. Chem. C 2 (2014) 9375–9384. [33] F. Gao, J. Ren, Z. Li, S. Yuan, Z. Wu, Y. Cui, H. Jia, H. Wang, F. Shi, Y. Hao, Opt. Mater. Express 5 (2015) 2468–2477. [34] Z. Li, Y. Sun, H. Li, C. Si, X. Lv, B. Jao, Z. Wu, Y. Cui, J. Yu, H. Wang, F. Shi, Y. Hao, Synthetic Met 217 (2016) 102–108. [35] Z. Li, W. Liu, Y. Yu, X. Lv, C. Si, Z. Wu, Y. Cui, H. Jia, J. Yu, H. Wang, F. Shi, Y. Hao, Dyes Pigments 122 (2015) 238–245. [36] Z. Li, X. Wang, X. Lv, C. Si, B. Wei, H. Wang, Y. Hao, RSC Adv. 7 (2017) 49125–49132. [37] C. Si, Z. Li, K. Guo, X. Lv, S. Pan, G. Chen, Y. Hao, B. Wei, Dyes Pigments 148 (2018) 329–340. [38] M. Liu, X.-L. Li, D.C. Chen, Z. Xie, X. Cai, G. Xie, K. Liu, J. Tang, S.J. Su, Y. Cao, Adv. Funct. Mater. 25 (2015) 5190–5198. [39] P. Zhang, W. Dou, Z. Ju, L. Yang, X. Tang, W. Liu, Y. Wu, Org. Electron. 14 (2013) 915–925. [40] D. He, Y. Yuan, B. Liu, D.-Y. Huang, C.-Y. Luo, F. Lu, Q.-X. Tong, C.-S. Lee, Dyes Pigments 136 (2017) 347–353. [41] Z. Huang, B. Wang, Q. Zhang, S. Xiang, X. Lv, L. Ma, B. Yang, Y. Gao, L. Wang, Dyes Pigments 140 (2017) 328–336. [42] M.-T. Lee, H.-H. Chen, C.-H. Liao, C.-H. Tsai, Appl. Phys. Lett. 85 (2004) 3301–3303. [43] S.J. Lee, J.S. Park, K.-J. Yoon, Y.-I. Kim, S.-H. Jin, S.K. Kang, Y.-S. Gal, S. Kang, J.Y. Lee, J.-W. Kang, S.-H. Lee, H.-D. Park, J.-J. Kim, Adv. Funct. Mater. 18 (2008) 3922–3930. [44] Y.H. Kim, D.C. Shin, S.H. Kim, C.H. Ko, H.S. Yu, Y.D. Chae, S.K. Kwon, Adv. Mater. 13 (2001) 1690–1693. [45] J. Littman, P. Martic, J. Appl. Phys. 72 (1992) 1957–1960. [46] P.-Y. Chou, H.-H. Chou, Y.-H. Chen, T.-H. Su, C.-Y. Liao, H.-W. Lin, W.-C. Lin, H.Y. Yen, I.-C. Chen, C.-H. Cheng, Chem. Commun. 50 (2014) 6869–6871.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2018.12.010. References [1] Y. Im, S.Y. Byun, J.H. Kim, D.R. Lee, C.S. Oh, K.S. Yook, J.Y. Lee, Adv. Funct. Mater. 27 (2017) 1603007. [2] W.-C. Chen, C.-S. Lee, Q.-X. Tong, J. Mater. Chem. C 3 (2015) 10957–10963. [3] X. Yang, X. Xu, G. Zhou, J. Mater. Chem. C 3 (2015) 913–944. [4] W.-C. Chen, Y. Yuan, Z.-L. Zhu, Z.-Q. Jiang, L.-S. Liao, C.-S. Lee, Adv. Optical Mater. 6 (2018) 1700855. [5] Z.-L. Zhu, M. Chen, W.-C. Chen, S.-F. Ni, Y.-Y. Peng, C. Zhang, Q.-X. Tong, F. Lu, C.S. Lee, Org. Electron. 38 (2016) 323–329. [6] M.Y. Wong, E. Zysman-Colman, Adv. Mater. 29 (2017) 1605444. [7] M. Sun, L. Zhu, W. Kan, Y. Wei, D. Ma, X. Fan, W. Huang, H. Xu, J. Mater. Chem. C 3 (2015) 9469–9478. [8] Q. Zhang, D. Tsang, H. Kuwabara, Y. Hatae, B. Li, T. Takahashi, S.Y. Lee, T. Yasuda, C. Adachi, Adv. Mater. 27 (2015) 2096–2100. [9] L.-S. Cui, Y.-M. Xie, Y.-K. Wang, C. Zhong, Y.-L. Deng, X.-Y. Liu, Z.-Q. Jiang, L.S. Liao, Adv. Mater. 27 (2015) 4213–4217. [10] Y. Yuan, J.-X. Chen, F. Lu, Q.-X. Tong, Q.-D. Yang, H.-W. Mo, T.-W. Ng, F.-L. Wong, Z.-Q. Guo, J. Ye, Z. Chen, X.-H. Zhang, C.-S. Lee, Chem. Mater. 25 (2013) 4957–4965. [11] W.-C. Chen, Y. Yuan, S.-F. Ni, Q.-X. Tong, F.-L. Wong, C.-S. Lee, Chem. Sci. 8 (2017) 3599–3608. [12] T. Shan, Y. Liu, X. Tang, Q. Bai, Y. Gao, Z. Gao, J. Li, J. Deng, B. Yang, P. Lu, Y. Ma, ACS Appl. Mater. Interfaces 8 (2016) 28771–28779. [13] C. Xiang, X. Fu, W. Wei, R. Liu, Y. Zhang, V. Balema, B. Nelson, F. So, Adv. Funct. Mater. 26 (2016) 1463–1469. [14] I. Kondrasenko, Z.-H. Tsai, K.-Y. Chung, Y.-T. Chen, Y.-Y. Ershova, A. DoménechCarbó, W.-Y. Hung, P.-T. Chou, ACS Appl. Mater. Interfaces 8 (2016) 10968–10976. [15] H. Shin, H. Jung, B. Kim, J. Lee, J. Moon, J. Kim, J. Park, J. Mater. Chem. C 4 (2016) 3833–3842. [16] J.-Y. Hu, Y.-J. Pu, F. Satoh, S. Kawata, H. Katagiri, H. Sasabe, J. Kido, Adv. Funct. Mater. 24 (2014) 2064–2071. [17] B. Liu, Z.-W. Yu, D. He, Z.-L. Zhu, J. Zheng, Y.-D. Yu, W.-F. Xie, Q.-X. Tong, C.S. Lee, J. Mater. Chem. C 5 (2017) 5402–5410. [18] J. Zhao, B. Liu, Z. Wang, Q. Tong, X. Du, C. Zheng, H. Lin, S. Tao, X. Zhang, ACS Appl. Mater. Interfaces 10 (2018) 9629–9637. [19] S.S. Reddy, V.G. Sree, H.-Y. Park, A. Maheshwaran, M. Song, S.-H. Jin, Dyes Pigments 145 (2017) 63–71. [20] A. Abdurahman, A. Obolda, Q. Peng, F. Li, Dyes Pigments 153 (2018) 10–17. [21] B. Liu, J. Zhao, C. Luo, F. Lu, S. Tao, Q. Tong A, J. Mater. Chem. C 4 (2016)
31