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We report a newly synthesized blue fluorescent fluorinated 9,9'-spirobifluorene based host material Spiro-(3,5)-F and its application in organic light-emitting device. Spiro-(3,5)-F has a high thermal stability with the decomposition temperature of 395 °C and the glass transition temperature of 145 °C. An organic light-emitting device using Spiro-(3,5)-F as host and 4,4'-bis(9-ethyl-3-carbazovinylene)-1,1'-biphenyl as dopant material exhibits a low turn-on voltage of 3.5 V, a maximum current efficiency of 6.51 cd A−1, and a maximum external quantum efficiency of 3.85%, implying good energy transfer and device performances. Moreover, this device exhibited low efficiency roll-off at high luminance and stable blue emission.
© 2016 Optical Society of America
1. Introduction
Organic light-emitting device (OLED) has attracted much scientific and industrial interest owing to its promising applications in high quality flat panel displays and solid-state lighting [1–4]. Efficiency is crucial for OLED to be energy-saving and to have a long lifetime for display and solid state lighting applications. Comparing with red and green emitters in full-color displays and high quality white light emission, it is much more difficult to develop blue emitters with matching performance due to opposing requirements for optical and electrical properties. Saturated blue emission with wide energy gaps often implies low carrier mobility and higher carrier-injection barrier and thus inferior electrical performance [5,6]. Doping a blue emitter into a host material can significantly improve the device performance in terms of electroluminescence (EL) efficiency and emissive color, as well as the device operational stability [7–12]. The emission color of the doped emissive layer is determined by the dopant radiative decay if efficient transfer of energy and charge occurs from host to guest [10]. For example, many host materials for blue emitting OLEDs based on anthracene, di(styryl)arylene, dipyrenylbenzenes, oligofluorenes, tetra(phenyl)silyl, and oligoquinoline derivatives have been reported to date [13–18]. Among these, fluorene and its derivatives have been widely explored in the fabrication of highly efficient and stable OLEDs, due to their excellent photoluminescence, electroluminescence, electro-chemical properties, and good thermal stability [19–22]. Particularly, spirobifluorene compounds have excellent thermal and chemical stabilities and high quantum efficiencies as well as nondispersive ambipolar carrier transporting properties [8, 23–29]. In addition, incorporation of the strong electron-withdrawing fluorine atom is beneficial for adjusting the frontier molecular orbital energy levels, emission spectrum and fluorescence quantum yield, and promoting molecular organization and crystallization due to the interaction between C−F and H−C in fluorinated organic compounds, which may contribute to obtaining high efficiency EL [8,9, 14–16]. Li et al. reported pure blue OLEDs made from 9,9'-spirobifluorene end-capped varied fluorinated phenyl rings as host materials with maximum efficiencies up to 6.66 cd A−1 (external quantum efficiency of 4.92%) [8].
In this paper, we designed a new host material (Spiro-(3,5)-F) with a 9,9'-spirofluorene core and di-F substituent at the meta-position on the phenyl ring, compared to the mono-F substituent in Spiro-(3)-F, and used to construct highly efficient blue OLED. By fabricating an OLED with Spiro-(3,5)-F as a host material doped with the blue fluorescent dopant 4,4'-bis(9-ethyl-3-carbazovinylene)-1,1'-biphenyl (BCzVBi), we got an excellent blue EL performance with a maximum current efficiency of 6.51 cd A−1 and power efficiency of 3.97 lm W−1, and a maximum EQE of 3.85%. Moreover, the device presented small value of efficiency roll-off at high brightness up to 10000 cd m−2.
2. Experimental
Materials and instruments: The manipulation involving air-sensitive reagents was performed under an inert atmosphere of dry nitrogen. The 2,2′,7,7′-tetrabromo-9,9′-spirobifluorene (Spiro-4Br) was prepared according to the literature procedures [30], and other reagents in the scheme were used as received from commercial sources. 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−1 under argon. Photoluminescence (PL) and absorption spectra were recorded by a Horiba Jobin Yvon Fluoromax-4 spectrophotometer and a Unico UV-2600 PCS spectrophotometer, respectively. Cyclic voltammetry was performed using a CHI 660E electrochemical workstation at a scan rate of 100 mV s−1. 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 (Bu4NPF6) 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. All the potentials were reported relative to ferrocene-ferrocenium (Fc/Fc+) couple, whose oxidation potential was + 0.13 V relative to the reference electrode. The oxidation potentials were determined by taking the onset of the anodic potentials. The HOMO and LUMO values were estimated by using the following general equation: EHOMO = − (Eoxon − Eox, ferroceneon + 4.8) eV; ELUMO = EHOMO + Egopt, which were calculated using the internal standard ferrocene value of –4.8 eV with respect to the vacuum level.
Synthesis of material: All reagents and solvents were used as purchased from commercial sources without further purification. Most of experiment methods were according to the published results [30]. As shown in Fig. 1, the fluorinated 9,9'-spirofluorene Spiro-(3,5)-F was synthesized by the Suzuki coupling reaction between brominated 9,9'-spirobifluorene (Spiro-4Br) and 3,5-difluorophenylboronic acid in the presence of a palladium catalyst. 2,2′,7,7′-Tetrakis(3,5-difluorophenyl)-spiro-9,9′-bifluorene (Spiro-(3,5)-F). 1.05 g, pale yellow solid, yield: 88%. Tm1 = 238 °C, Tm2 = 320 °C. 1H NMR (CDCl3, 400 MHz): δ 6.67-6.71 (m, 4H), 6.94-6.97 (m, 12H), 7.64-7.67 (m, 4H), 7.99-8.02 (d, J = 8.4 Hz, 4H). Anal. Calcd for C49H24F8: C, 76.96%; H, 3.14%. Found: C, 76.91%; H, 3.17%.
Devices fabrication and characterizations: OLEDs based on a multilayer structure have been fabricated onto patterned ITO coated glass substrates from XinYan Tech (thickness: 100 nm and sheet resistance: less than 20 Ω per square). The organic materials (from Aldrich and Lumtec) are deposited onto the ITO anode by sublimation under high vacuum (<1 × 10−3 Pa) at a rate of 0.2–0.3 nm s−1. Prior to organic layer deposition, ITO substrates were exposed to a UV-ozone flux for 10 min, following degreasing in acetone and isopropyl alcohol (IPA). The host material doped with BCzVBi in EML was used in co-evaporation technique. The thickness of each layer was determined by a quartz thickness monitor. The effective size of the OLED was 14 mm2. The voltage–current density (V–J) and voltage–brightness (V–L) as well as the current density–current efficiency (J–ƞc) and current density–power efficiency (J–ƞp) curve characteristics of devices were measured with a Keithley 2602 Source-Meter under ambient condition. The spectral emission was recorded with a SpectraScan PR650 spectrophotometer.
3. Results and discussions
Synthesis and structures: According to our previous report, the fluorescent molecule Spiro-(3,5)-F was synthesized using the Suzuki coupling reaction between brominated 9,9'-spirobifluorene (Spiro-4Br) and fluorinated phenylboronic acid in the presence of a palladium catalyst with high yield (Fig. 1). Beforehand, the key intermediate Spiro-4Br needed to be prepared from bromination of 9,9'-spirobifluorene in the presence of bromine [30]. Chemical structures of the intermediate and the product were confirmed from the corresponding 1H NMR and elemental analyses.
The DFT-optimized ground-state geometry (B3LYP/6-31G(d,p) method) reveals that the 9,9'-spirobifluorene moiety in Spiro-(3,5)-F possesses a cross-shaped molecular structure (Fig. 2). Furthermore, the peripheral fluorinated phenyl groups and the adjacent phenyl groups of the 9,9'-spirobifluorene core are twist-linked with a distorted angle of 68° because of steric repulsion between the substituted phenyl peri-H atoms and the hydrogen atoms of the phenyl ring attached to the 9,9'-spirobifluorene unit. The highly non-coplanar structure of Spiro-(3,5)-F can limit the intermolecular interactions and facilitate the formation of stable amorphous thin films. The electron density in the highest occupied molecular orbital (HOMO) of Spiro-(3,5)-F is almost concentrated on the two perpendicular spiro-9,9'-bifluorene moieties while that in the lowest unoccupied molecular orbital (LUMO) delocalize over the molecule. The calculated HOMO and LUMO of Spiro-(3,5)-F are −5.9 and −1.8 eV, respectively.
Fig. 2 The optimized geometries and the molecular orbital surfaces of the HOMO and LUMO for Spiro-(3,5)-F obtained at the B3LYP/6-31G(d,p) level.
Thermal properties: The good thermal stability of the compound is indicated by its high decomposition temperature (Td, corresponding to 5% weight loss) of 395 °C in the thermogravimetric analysis (TGA) (Fig. 3). And the glass transition temperature (Tg) determined through differential scanning calorimetry (DSC) is 145 °C for Spiro-(3,5)-F (inset of Fig. 3), further demonstrating its better thermal stability. The high Tg value can attribute to the rigid 9,9'-spirobifluorene core and the extending molecular system (see Fig. 2). Thus, the good thermal properties of the emitter would benefit to the process of vacuum deposition and operating stability of OLED devices.
Photophysical properties: The UV and photoluminescence (PL) spectra of Spiro-(3,5)-F in CH2Cl2 solution are shown in Fig. 4. The compound absorbs in the UV spectral region below 360 nm with maximum 340 nm, which can be attributed to π−π* transitions of the conjugated aromatic rings. The similarities between the absorption spectra of dilute solution and thin film suggests that the conformation of the solid state of Spiro-(3,5)-F is similar to that of the solution state Spiro-(3,5)-F, which is attributed to its non-coplanar structure to block aggregation effectively. From the onset of the film absorption spectrum, the energy band gap (Eg) of Spiro-(3,5)-F in thin film was calculated to be 3.5 eV. In the case of Spiro-(3,5)-F dilute solution, a maximum PL at wavelength λmax = 380 nm and a vibronic PL shoulder at wavelength λs = 364 nm were observed, while the PL peaks in spin-coated thin films were significantly red-shifted to 410 and 395 nm, respectively. And the full width at half maximum (FWHM) in the film state (ca. 65 nm) is apparently larger than that in solution (ca. 44 nm), which could be due to the presence of weak interactions between the molecules (packing effect) in the solid state. We measured the fluorescence quantum yields (Φf) of Spiro-(3,5)-F in a dilute CH2Cl2 solution using quinine sulfate as a standard (Φf = 0.56 in 1.0 M H2SO4 solution) at room temperature. Spiro-(3,5)-F exhibited high Φf of 0.91, which is comparable to the F-substituted SFs (i.e. Spiro-(2)-F, Spiro-(3)-F, Spiro-(4)-F, Spiro-(2,4)-F and Spiro-(3,4,5)-F) [8].
Fig. 4 (a) Absorption and PL spectra of Spiro-(3,5)-F in solution and solid state, (b) Absorption spectra of BCzVBi and PL spectra of Spiro-(3,5)-F.
Electrochemical properties: Cyclic voltammetry (CV) was used to investigate the electrochemical properties of Spiro-(3,5)-F. From the onset of its oxidation potential with respect to that of ferrocene according to the equation: HOMO = −(4.8 + Eoxon) eV (Fig. 5), the HOMO energy level was determined to be –6.1 eV. As no clear reduction curve was observed, the LUMO energy level was calculated to be –2.6 eV from the HOMO energy level and Eg. As expected, the fluorinated compound Spiro-(3,5)-F had lower HOMO and LUMO values than the pure hydrocarbon material 4-phenyl-9,9'-spirobifluorene (4-Ph-SBF) and its constituting building block 9,9'-spirobifluorene (SBF) for the highly electronegative fluorine substituents in Spiro-(3,5)-F [29]. The resultant LUMO energy level can facilitate an effective injection of minor carriers, electron, into the emissive layer due to low lying LUMO energy level of host that exhibit a 0.1 eV barrier from electron transporting material, 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi). The small energy gap between the emissive and elctron-transporting layer suggests the efficient charge transfer in the OLED; thus, low turn-on voltage and high luminescence of the device could be expected.
Electroluminescence properties: We first characterized the EL performance of Spiro-(3,5)-F as a blue emitter by fabricating a non-doped OLED with a configuration of ITO/HAT-CN (5 nm)/TAPC (40 nm)/emitting layer (EML) (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (150 nm). In this device, HAT-CN (dipyrazino[2,3-f:2',3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile) was used as a carrier generation layer (CGL); TAPC (1,1-bis[4-[N,N′-di(p-tolyl)amino]phenyl]cyclohexane) was used as a hole transporting layer (HTL); TPBi was used as an electron transporting layer (ETL) and exciton blocking layer; and LiF was used as an electron injection layer (EIL). The device configurations and the energy diagrams were shown in Fig. 6. The current density–voltage–luminance–efficiency (J–V–L–ƞ) characteristics were shown in Fig. 7 and Fig. 8(b). The non-doped blue OLED showed pure blue emission characteristics of CIE (0.20, 0.19) with the emission peaking around 468 nm at 7 V, which was nearly identical to the corresponding PL spectra obtained in the thin-film state (Fig. 4), indicating that the EL emission was mainly contributed from the fluorescence of the blue emitter and the formation of excimer or exciplex was effectively suppressed during the EL process. The device has a turn-on voltage (at a brightness of 1 cd m−2) of 4.5 V, a maximum current efficiency of 0.62 cd A−1, which were comparable to those previously reported for non-doped fluorescence blue-light-emitting OLEDs based on other non bipolar spiro configured compounds [31–33].
Fig. 7 (a) Current density-voltage curves, (b) Brightness-voltage curves, (c) Current efficiency-current density curves, and (d) Power efficiency-current density curves.
Fig. 8 (a) Normalized EL spectra of BCzVBi-doped device using Spiro-(3,5)-F recorded at various driving voltages, and (b) External quantum efficiency-current density curves for Spiro-(3,5)-F devices.
To further enhance the device performances, Spiro-(3,5)-F was then employed as the host material doped with BCzVBi in EML. The optimized doping concentration of BCzVBi is 10 wt%. Since the spectral overlap efficiency between the PL spectra of Spiro-(3,5)-F and absorption spectra of BCzVBi is very high (Fig. 4(b)), Förster energy transfer was efficient from the host Spiro-(3,5)-F to the dopant BCzVBi [34]. The CIE coordinates of BCzVBi-doped device were (0.15, 0.24) at 7 V, which showed pure blue emission characteristic with peak at 476 nm without emission of the host material, suggesting an effective energy transfer from Spiro-(3,5)-F to the BCzVBi dopant [8]. The EL spectra of the device also presented good color stability with nearly the same CIE coordinates over the whole voltage range (Fig. 9). From the current density-voltage-brightness (J–V–L) curves (Fig. 7), the maximum luminance value of 14362 cd m−2 is presented at 13.0 V, and a low turn-on voltage (at a brightness of 1 cd m−2) of 3.5 V is obtained, revealing small carrier injection barriers in the device. The device shows high maximum current, power, and external quantum efficiencies of 6.51 cd A−1, 3.97 lm W−1, and 3.85%, respectively. This observation with the high efficiencies reflects a complete energy transfer from the host to the dopant. Moreover, the current efficiency and EQE of the device is maintained at 6.46 cd A−1 and 3.83%, respectively, at a high brightness of 1000 cd m−2. It was worth noting that high efficiencies were achieved at practical luminance between 100 and 1000 cd m−2, which indicates that the device has great potential for practical applications [35,36]. Upon increasing to illumination-relevant luminance of 10000 cd m−2, the blue-emitting device still shows a high current efficiency of 5.29 cd A−1 and an EQE of 3.83% with a roll-off value of below 5%. The performance of the Spiro-(3,5)-F device was comparable with previously reported results for the SFs-based fluorescence blue light-emitting OLEDs [8]. The low roll-off of these efficiencies with an increasing brightness or current density can be attributed to two facts: (i) energy transfer from the Spiro-(3,5)-F host to the BCzVBi dopant was highly efficient because of the good energy level match; and (ii) the HOMO and LUMO energy levels of the Spiro-(3,5)-F host are well matched with the neighboring TAPC HTL and the TPBi ETL (Fig. 6). The charge trapping property of the emitting dopants should also be an advantage to control the charge balance and suppress the efficiency roll-off.
Fig. 9 CIE coordinates of EL spectra in response to the driving voltage of BCzVBi-doped device from 4 V to 9 V.
4. Conclusions
A fluorescent blue host material based on fluorinated 9,9'-spirobifluorene was successfully prepared and used to construct blue OLEDs. The blue host showed excellent thermal stabilities (Td = 395 °C and Tg = 145 °C) and pronounced PL efficiencies. The device made using the blue fluorescent dopant BCzVBi had a maximum luminance of 14362 cd m−2, and a maximum current efficiency of 6.51 cd A−1, a maximum EQE of 3.85% and CIE coordinates of (0.15, 0.24). Moreover, this device exhibited low efficiency roll-off at high luminance and stable blue emission. According to these characteristics, the blue emitting fluorinated 9,9'-spirobifluorene material has sufficient potential for fluorescence OLED applications.
Acknowledgment
We are grateful for support from the National Natural Scientific Foundation of China (Grant Nos. 61471052, 61308093, and 61571317), the National High Technology Research and Development Program (“863”Program) of China (Grant no. 2015AA016901), the New Teachers' Fund for Doctor Stations (Grant no. 20131402120020), the Science and Technology on Information Transmission and Dissemination in Communication Networks Laboratory (ITD-U14005/KX142600012), the Doctoral Scientific Fund of MOE of China (No. 20120005110010), and the Fund of State Key Laboratory of Information Photon. & Opt. Comm (BUPT).
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