A carbazole-based dendritic host material for efficient solution-processed blue phosphorescent OLEDs

A carbazole-based dendritic host material for efficient solution-processed blue phosphorescent OLEDs

Dyes and Pigments 97 (2013) 286e290 Contents lists available at SciVerse ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/d...

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Dyes and Pigments 97 (2013) 286e290

Contents lists available at SciVerse ScienceDirect

Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig

A carbazole-based dendritic host material for efficient solutionprocessed blue phosphorescent OLEDs Wen Yang, Yuansheng Chen, Wei Jiang*, Xinxin Ban, Bin Huang, Yunqian Dai, Yueming Sun* School of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu 211189, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 October 2012 Received in revised form 14 December 2012 Accepted 17 December 2012 Available online 23 January 2013

A dendritic host material 4,4-bis[3,6-bis(3,6-di-tert-butylcarbazol-9-yl)-carbazol-9-yl]-biphenyl for solution-processed blue phosphorescent organic light-emitting devices was designed and synthesized. Owing to the decrease of the p conjugation length of biphenyl moiety in molecule structure, this carbazole derived derivative shows high triplet energy. Furthermore, the thermal, photophysical and electrochemical properties of 4,4-bis[3,6-bis(3,6-di-tert-butylcarbazol-9-yl)-carbazol-9-yl]-biphenyl were investigated. The high triplet energy of 4,4-bis[3,6-bis(3,6-di-tert-butylcarbazol-9-yl)-carbazol-9yl]-biphenyl ensures efficient energy transfer from the host to the triplet emitter iridium(III) bis(4,6difluorophenylpyridinato)picolinate. The single layer device using the carbazole derivatives as the host for iridium(III) bis(4,6-difluorophenylpyridinato)picolinate showed the maximum luminance efficiencies of 5.8 cd A1, and a maximum external quantum efficiency 2.8%. The efficiency of the carbazole derivative based device was almost 3 times higher than the corresponding 1,3-bis(9-carbazolyl)benzene based device. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: OLEDs Blue phosphorescence Dendrimer Host materials Carbazole CBP

1. Introduction In contrast to conventional fluorescent organic light-emitting diodes (OLEDs), the phosphorescent OLEDs (PhOLEDs) can reach to 100% of theoretical internal quantum efficiency by harvesting both singlet and triplet excitons, PhOLEDs are considered as promising candidates for the next generation large-size displays and solid-state lighting panels [1e5]. Usually, the phosphorescent emitters are doped into a suitable host to reduce self-aggregation quenching and tripletetriplet annihilation. It is essential that the triplet energy of the host material should higher than that of the phosphorescent emitter to prevent energy back transfer and to confine the electro-generated triplet excitons on the dopant molecules. Therefore, to achieve the high performance PhOLEDs, the development of efficient host materials is of great importance. For example, 4,40 -bis(9-carbazolyl)- 2,20 -biphenyl (CBP) is commonly used as a host material, but not for blue triplet emitters because of its lower triplet energy gap of 2.56 eV relative to those of blue triplet emitters such as iridium(III) bis(4,6-difluorophenylpyridinato)picolinate (FIrpic) (ET ¼ 2.62 eV) [6]. Recently, a series of structurally

* Corresponding authors. Tel./fax: þ86 25 52090621. E-mail addresses: [email protected], [email protected] (W. Jiang), [email protected] (Y. Sun). 0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2012.12.030

modified CBP-derivatives with high triplet energies for highly efficient blue phosphorescent OLEDs have been developed [7e13]. For example, 1,3-bis(9-carbazolyl)benzene (mCP) with a high triplet energy of 2.90 eV has been employed as a host material for blue phosphorescent OLEDs through replacing the biphenyl unit in CBP by a single benzene group in combination with a meta-relationship instead of a para-relationship between the N-position of carbazole and phenyl [8]. Gong et al. has recently reported two CBP isomers with high triplet energies (3.00 and 2.84 eV) by finely tuning the linking topology between the two carbazole units and biphenyl group from di-para-position to di-meta- or di-orhto-positions [7]. However, those CBP-derivatives also suffer from poor thermal and morphological stability due to the low molecular weight and may exhibit phase segregation and aggregation in the devices. During the search for host materials for blue PhOLEDs, a great deal of attention has been focused on carbazole-based molecules, owing to their high triplet energy and excellent hole transporting properties [14e32]. In our recent work, we proposed to modify CBP by the linking of two 3,6-di-tert-butylcarbazole moieties into the 3, 6 positions of the carbazole units to build a dendritic host material CzCBP. As a result, the new host exhibits excellent thermal and morphological stability. The HOMO energy level of Cz-CBP is raised and approaches the work function of poly(3,4-ethylenedioxythiophene) (PEDOT, 5.2 eV), which is believed to facilitate the hole injecting. Furthermore, the triplet energy of Cz-CBP is increased to 2.68 eV,

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higher than the value of the commonly used triplet blue-emitter FIrpic, which enables its application as a host for blue phosphorescent dopants. The influence of the carbazole moieties on the thermal stability, electrochemical properties, photophysical behavior, and EL performance of Cz-CBP host material is investigated.

collected with a Photo Research PR705 Spectrophotometer. All measurements of the devices were carried out in ambient atmosphere without further encapsulations.

2. Experimental

The geometrical and electronic properties of CBP and Cz-CBP were performed with the Gaussian 03 program package. The calculation was optimized at the B3LYP/6-31G(d) level of theory. The molecular orbitals were visualized using Gaussview.

2.1. General information All reactants and solvents, unless otherwise stated, were purchased from commercial sources and used as received. 1H NMR and 13 C HMR spectra were measured on a Bruker ARX300 NMR spectrometer with tetramethylsilane as the internal standard. Elemental analysis was performed on an Elementar Vario EL CHN elemental analyzer. Mass spectrometry was performed with a Thermo Electron Corporation Finnigan LTQ mass spectrometer. Absorption spectra were recorded with a UVevis spectrophotometer (Agilent 8453) and PL spectra were recorded with a fluorospectrophotometer (Jobin Yvon, FluoroMax-3). TGA was recorded with a Netzsch simultaneous thermal analyzer (STA) system (STA 409PC) under a dry nitrogen gas flow at a heating rate of 10  C min1. Glass-transition temperature was recorded by DSC at a heating rate of 10  C min1 with a thermal analysis instrument (DSC 2910 modulated calorimeter). Cyclic voltammetry was performed on a Princeton Applied Research potentiostat/galvanostat model 283 voltammetric analyzer in CH2Cl2 solutions (103 M) at a scan rate of 100 mV s1 with a platinum plate as the working electrode, a silver wire as the pseudo-reference electrode, and a platinum wire as the counter electrode. The supporting electrolyte was tetrabutylammonium hexafluorophosphate (0.1 M) and ferrocene was selected as the internal standard. The solutions were bubbled with a constant argon flow for 10 min before measurements.

2.3. Quantum chemical calculations

2.4. Materials Compound 3,6-Bis(3,6-di-tert-butylcarbazol-9-yl)-carbazole was prepared according to published procedures [33]. Synthesis of 4,4-bis[3,6-bis(3,6-di-tert-butylcarbazol-9-yl)carbazol-9-yl]-biphenyl (Cz-CBP). 4,4 - Dibromobiphenyl (1.0 g, 3.2 mmol), 3,6-bis(3,6-di-tert-butylcarbazol-9-yl)-carbazole (5.4 g, 7 mmol), copper(I) iodide (0.06 g, 0.3 mol), 18-crown-6 (0.08 g, 0.3 mmol), K2CO3 (3.18 g, 30 mmol), and DMPU (10 mL) were added sequentially to a sealed tube under nitrogen and heated at 180  C in oil bath for 48 h. Then the mixture was cooled to room temperature and filtered. The filtrate was extracted with dichloromethane and water. After the organic layer was dried by anhydrous MgSO4 and filtered, the product was purified by chromatography (silica gel, nhexane/CH2Cl2, 10:1) to give 1.79 g (35.2%) of a white solid which was further recrystallized three times from n-hexane/CH2Cl2 to give 0.51 g (10.6%) of Cz-CBP. 1H NMR (500 MHz, CDCl3, d): 8.28 (s, 4H), 8.18 (s, 8H), 8.11-7.96 (m, 4H), 7.92 (d, J ¼ 8.0 Hz, 4H), 7.81-7.47 (m, 8H), 7.49 (d, J ¼ 7.2 Hz, 8H), 7.37 (d, J ¼ 7.5 Hz, 8H), 1.48 (s, 72H). 13 C NMR (500 MHz, CDCl3, d): 142.7, 140.2, 131.2, 129.0, 128.9, 127.7, 127.6, 126.1, 126.0, 124.2, 123.6, 123.2, 119.4, 116.3, 111.2, 111.1, 109.1, 34.8, 32.1. MS (MALDI-TOF) [m/z]: calcd for C116H1116N6, 1594.2; found, 1593.9. Anal. Calcd. for C116H1116N6 (%): C, 87.39; H, 7.33; N 5.27. Found: C, 87.50; H, 7.26; N 5.20.

2.2. Device fabrication and performance measurements 3. Results and discussion In a general procedure, indium-tin oxide (ITO)-coated glass substrates were pre-cleaned carefully and treated by UV ozone for 4 min. A 40 nm poly(3,4-ethylenedioxythiophene) doped with poly(styrene-4-sulfonate)(PEDOT:PSS) aqueous solution was spin coated onto the ITO substrate and baked at 210  C for 10 min. The substrates were then taken into a nitrogen glove box, where FIrpicdoped host:1,3-bis[4-tert-butylphenyl0-1,3,4-oxidiazolyl]phenylene (OXD-7) layer was spin coated onto the PEDOT:PSS layer from 1,2-dichloroethane solution and annealed at 120  C for 30 min. The substrate was then transferred into an evaporation chamber, where the Cs2CO3/Al bilayer cathode was evaporated at evaporation rates of 0.2 and 10  A/s for Cs2CO3 and Al, respectively, under a pressure of 1  103 Pa. The current-voltage-brightness characteristics of the devices were characterized with Keithley 4200 semiconductor characterization system. The electroluminescent spectra were

3.1. Synthesis and characterization Scheme 1 shows the synthetic routes and structure of the newly synthesized dendrimer Cz-CBP. The intermediate 3,6-bis(3,6-ditert-butylcarbazol-9-yl)-carbazole was obtained according to previously methods [33]. Subsequently, the aromatic CeN coupling reactions of 4,4-dibromobiphenyl with 3,6-Bis(3,6-di-tert-butylcarbazol-9-yl)-carbazole led to Cz-CBP with a yields of 35.2%. Finally, the dendrimer was purified by the silica column method and recrystallization three times from n-hexane/CH2Cl2, yielding a very pure white powder. 1H NMR, 13C NMR, matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, and elemental analysis were employed to confirm the chemical structures of Cz-CBP.

Scheme 1. Synthesis and molecular structure of Cz-CBP.

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3.2. Thermal analysis The thermal stabilities of Cz-CBP was investigated by thermal gravimetric analyses (TGA) and differential scanning calorimetry (DSC) under nitrogen atmosphere at a heating rate of 10  C min1. As shown in Fig. 1, TGA measurement reveals its high thermal decomposition temperature (Td), corresponding to 5% weight-loss) of 475  C. This value is about 100  C above the CBP [24] which is attributed to the high molecular weight of Cz-CBP. The DSC trace exhibits a clear glass transition temperature (Tg) of 287  C during the second heating scans, which is much higher than the value of CBP (62  C) [30]. The thermal analysis results clearly demonstrate the additional 3,6-di-tert-butylcarbazole moieties greatly enhances the thermal stability of Cz-CBP, which facilitates the forming of amorphous films through solution processing. 3.3. Photophysical properties The photophysical properties of Cz-CBP in CH2Cl2 were analyzed using UVevis absorption and photoluminescence (PL) spectra. As shown in Fig. 2, the absorption spectra of Cz-CBP exhibits three absorption bands centered at 245e350 nm, which are nearly identical to those of the outer-layer carbazole monomer and transitions between the inner-layer carbazole and biphenyl units, thus can be attributed to the pep* transitions. Upon UV excitation, the PL spectrum of Cz-CBP has lost vibronic structure and the emission peaks red-shifted to 402 nm. The band gap was calculated from the edge of the Uvevis absorption peak, giving a value of 3.42 eV for CzCBP. The phosphorescence spectra measured from a frozen 2methyltetrahydrofuran matrix at 77 K are also shown in Fig. 3. The triplet energy of Cz-CBP was determined to be 2.68 eV by the highest energy 0e0 phosphorescent emission in 2methyltetrahydrofuran at 77 K, which is sufficiency high enough to serve as the appropriate hosts for FIrpic (2.62 eV). 3.4. Electrochemical analysis and theoretical calculations Cyclic voltammetry (CV) was performed to investigate the electrochemical properties of the compounds. As shown in Fig. 3, during the anodic scan in dichloromethane, Cz-CBP showed reversible oxidation behavior, which can be attributed to the introduction of two tert-butyl groups at the 3,6 positions of outlayer carbazole units [34]. In contrast, when the electrochemically

Fig. 1. TGA and DSC traces of Cz-CBP.

Fig. 2. Normalized absorption, emission and phosphorescence (77 K) spectra of CzCBP.

active sites (3,6 positions of carbazole) are left unblocked, the oxidation processes of CBP are not reversible, with the oxidation potential gradually shifting to lower potentials and the current increasing during repeated CV scans (inset of Fig. 3) [24]. No reduction waves were detected. On the basis of the onset potentials for oxidation, the HOMO energy of Cz-CBP was estimated to be 5.30. Compared with CBP (HOMO ¼ 5.51 eV), it is believed that the holes are easily injected from the PEDOT:PSS to the emitting layer. The LUMO level was calculated from the HOMO and bandgap from UVevis, giving a value 1.98 eV for Cz-CBP. The effect of the substitution on CBP of the carbazole units was also investigated with the density functional theory (DFT) calculations. The calculated geometries of Cz-CBP and CBP in Fig. 4 show that the outer-layer carbazole units are significantly twisted against the biphenyl groups. The geometrical characteristic of Cz-CBP can effectively prevent intermolecular interactions between pe systems and thus suppress molecular recrystallization, which improves the morphological stability of the thin film. In the case of CBP, the dihedral angle of carbazole and biphenyl unit was 37.7, while the dihedral angle of Cz-CBP increased to a value of 41.0 . The

Fig. 3. Oxidation part of the CV curves of CBP and Cz-CBP in CH2Cl2 solutions.

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Fig. 4. Optimized geometries and calculated HOMO and LUMO density maps for CBP and Cz-CBP.

extent of conjugation between the central biphenyl units was shortened by the increase dihedral angles of Cz-CBP, which leads to improvement of the triplet energy gap. This calculated result is in agreement with the values measured by the low temperature phosphorescence spectra. In addition, the HOMO and LUMO level of CBP is localized on the carbazole units and the biphenyl core, respectively. However, for Cz-CBP, its LUMO level is similarly localized on the biphenyl core, while the HOMO is mostly distributed over the three outer-layer carbazole units. It was confirmed by CV results that the introduction of carbazole units linked

through the 3, 6 positions of the carbazole unit can lead to the increasing of the HOMO levels. This design strategy should be extended to other will-designed host materials of low-lying HOMO levels [27]. 3.5. Electroluminescent properties The single-layer blue electrophosphorescent device with the configuration of ITO/PEDOT:PSS/Host:OXD-7(30 wt%):FIrpic(10 wt %)/Cs2CO3/Al was fabricated by solution-processing methods. In this device, PEDOT:PSS was used as the hole-injection layer; the electron-transporting material OXD-7 was mixed into the host materials to facilitate the electron transport in the emitting layer [35,36]; FIrpic with an optimized concentration of 10 wt% was used as a dopant emitter; and Cs2CO3 was used as the electron-injection layer. Current density-voltage-luminance and luminous efficiencycurrent density characteristics are shown in Fig. 5. The device with Cz-CBP as host displays a turn-on voltages of 5.6 V and a maximum luminance of 8600 cd m2 (13.5 V), achieves a maximum current efficiency of 5.8 cd A1, and the corresponding maximum external quantum efficiency of 2.8%. The efficiencies of Cz-CBP showed nearly 3 times higher than the reported values of mCP with the maximum current efficiency of 1.8 cd A1 and maximum external quantum efficiency of 1.0% with the same device configuration [18]. This performance is probably due to the excellent thermal stability of the dendritic host Cz-CBP, which significantly enhances the capability of forming stable amorphous thin films. The normalized electroluminescence (EL) spectrum of device was shown in Fig. 5(b). As clearly seen, the spectra peak at 474 nm, and the EL bands are nearly identical with the Commission Internationale de L’Eclairage (CIE) coordinates of (0.16, 0.36) for a pure blue shade. Hence, the EL originates only from the FIrpic, indicating a complete energy transfer from the blend hosts to the FIrpic. 4. Conclusions

Fig. 5. (a) Current density-voltage-luminance characteristics, (b) luminance efficiency versus current density plots and EL spectra for the Cz-CBP devices.

In conclusion, we have designed and synthesized a solutionprocessable dendritic host material based on the CBP molecule. The molecular structure of Cz-CBP shows a more twist configuration and decreases the p conjugation of biphenyl moiety. As a result, Cz-CBP possesses high triplet energy of 2.68 eV and excellent thermal and morphological stability with high glass transition temperature of 287  C. Utilizing the dendrimer as host material, high-performance single-layer blue PhOLEDs has been successfully fabricated by spin coating, which is almost 3 times higher than the corresponding mCP-based devices.

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Acknowledgments The authors thank Dr. Zhao Yan for OLEDs fabrication and measurement. This work was supported by the National Natural Science Foundation of China (Grant No.51103023, 21173042 and 21201034) and the National Basic Research Program of China (Grant No.2013CB932900).

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