Novel heterofluorene-based hosts for highly efficient blue electrophosphorescence at low operating voltages

Organic Electronics 12 (2011) 1619–1624

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Letter

Novel heterofluorene-based hosts for highly efficient blue electrophosphorescence at low operating voltages Run-Feng Chen a, Guo-Hua Xie b, Yi Zhao b,⇑, Sheng-Lan Zhang a, Jun Yin a, Shi-Yong Liu b, Wei Huang a,⇑ a Key Laboratory for Organic Electronics and Information Displays (KOLED), Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications (NUPT), Nanjing 210046, China b State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, China

a r t i c l e

i n f o

Article history: Received 10 March 2011 Received in revised form 23 May 2011 Accepted 26 May 2011 Available online 23 June 2011 Keywords: Organic light-emitting diodes Host material Heterofluorene

a b s t r a c t A novel concept for molecular design of blue host materials for phosphorescent OLEDs (PHOLEDs) by combining carbazole and heterofluorene via structurally mimicking 4,40 N,N0 -dicarbazole-biphenyl (CBP) was presented. The carbazole end-capped heterofluorenes (CzHFs) prepared accordingly in high yields, were found to be ideal hosts for blue PHOLEDs. At the brightness level of 1000 and 5000 cd m2, the driving voltages are still lower than 4.5 V with the external quantum efficiencies retain as high as 18.1% and 16.8% respectively. Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.

1. Introduction Organic light-emitting diodes (OLEDs) have attracted considerable attention because of their great potential in high-quality next-generation flat-panel full-color displays and lighting technologies [1–3]. After two decades of intensive research since 1987 [4], OLEDs are now focused on the improvement of device stability and power efficiency [5]. Efficient and stable red and green OLEDs, which are two key components of white light, have met industry standards for operational lifetime and efficiency. However, the blue component, indispensable component in white emission as well as RGB emission for full-color display, is lagged far behind. With the introduction of triplet emitters in OLEDs to harness both singlet and triplet excited states, an internal quantum efficiency (IQE) of 100% can be achieved in phosphorescent OLEDs (PHOLEDs) [6,7], overcoming the limitation of 25% IQE in conventional ⇑ Corresponding authors. Tel.: +86 25 8586 6008; fax: +86 25 8586 6999. E-mail addresses: [email protected] (Y. Zhao), wei-huang@njupt. edu.cn (W. Huang).

fluorescent OLEDs. Hence, tremendous efforts have been made in the development of highly efficient blue PHOLEDs [8]. One of the most efficient blue phosphorescent dye has proven to be the commercially available iridium (III) 0 [bis(4,6-difluorophenyl)-pyridinato-N,C2 ] picolinate (FIrpic), which has a typical emission peak around 472 nm and a shoulder at 500 nm, and the corresponding Commission International de L’Eclairage (CIE) coordinates (0.17, 0.34) [9]. Recently, Kido et al. [10,11] reported a very efficient blue PHOLED based on FIrpic with recorded efficiencies of 55 and 46 lm W1 corresponding to external quantum efficiencies (EQE) of 26% and 25% at practical luminance of 100 and 1000 cd m2, respectively. However, in PHOLEDs, a host matrix is essential for phosphorescent emitters in order to reduce aggregation quenching and triplet–triplet annihilation [12]. To achieve efficient PHOLEDs with high current efficiency and low driving voltage, the choice of host material is of extreme importance. Generally, the energy bandgap (Eg) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and the triplet energy level (ET) of the host material should be higher than those of the triplet dopants to ensure exothermic

1566-1199/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.05.025

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energy transfer and confinement of triplet excitons on the triplet emitters. But, if the Eg of the host is too large, deep and strong charge traps will be induced on the dopant emitters, resulting in poor conductivity of the host and a high driving voltage of the device [13]. There exists also an inherent tradeoff between ET, Eg, and the p-conjugation length, since longer p-conjugation will lead to lower ET and Eg but higher charge mobility for lower driving voltage and higher device efficiency [14]. The large difference in HOMO and/or LUMO level between the charge-transporting and the emitting layers will also cause energetically unfavorable charge injection and a high driving voltage. Due to the large Eg and ET of the blue phosphorescent emitters, the search for a suitable host material is urgent and challenging. The host material used for blue PHOLEDs must have a large ET (>2.7 eV) which results in an even wider Eg (>3.0 eV) that may act as energy barriers for the carriers transport from nearby hole or electron-transporting layer to the emitting layer, leading to high driving voltage of the FIrpic-based devices [15]. One approach to solve the problematic high driving voltage of blue PHOLEDs is to design and synthesis suitable host materials that possess both the high triplet excited state, the superior carriertransporting property, and the appropriate energy levels (HOMO and LUMO). Up to now, researches directed toward the molecular design of host materials with high ET, relatively narrow Eg, and good carrier-transporting properties were rather limited [8,16–17]. Heterofluorenes (HF) [18] such as nitrogafluorene (carbazole, NF) [19], sulfurafluorene (SF) [20], silafluorene (SiF) [21–23], germafluorene (GeF) [24], and phosphafluorene (PF) [25] have received a number of interests in many fields due to their particular physical and optoelectronic properties, since the breakthrough of their synthetic methods [26]. Recently, we designed and successfully prepared a serial of carbazole end-capped heterofluorenes (CzHFs) via Ullmann CAN coupling reaction in high yields [27] by mimicking the molecular structure of 4,40 -N,N0 -dicarbazole-biphenyl (CBP). CBP is the most popular and famous carbazole-based host material for green and red phosphorescent emitters, but its ET (2.56 eV) [13] is too low for blue emitters even the sky-blue phosphor, FIrpic. Our innovation is to connect the biphenyl of CBP with the bridging heteroatoms, which results in a planar heterofluorene in the centre of CBP [28]. And, this marriage of carbazole and heterofluorene in a manner as that in CBP is expected to produce interesting host materials for blue PHOLEDs with the more desirable localization and energy level of HOMOs and LUMOs and also higher ET (3.0 eV) compared with those of CBP, with the aid of the great impact of the heteroatoms on the optoelectronic properties [18]. This heterofluorene-based host material benefits both from the heterofluorene which has very particular optoelectronic properties and from the carbazole which is very popular building block for host materials. In this paper, FIrpic-based PHOLEDs using meta-CzHFs (m-CzHFs) as the host material were fabricated and the devices exhibit driving voltages lower than 3.9 V at the luminance of 1000 cd m2. With such low driving voltages, the power efficiency of the CzHF-based devices are dramatically enhanced up to 30.9 and 30.2 lm W1 (with very little

variation) at the practical operation brightness from 100 to 1000 cd m2. The high efficiency with the EQE of 18.4%, and the highly stable CIE (0.16, 0.31) of the m-CzHFs-based PHOLEDs at a wide range of brightness were also observed. This novel concept to combine carbazole and heterofluorene by structurally mimicking CBP turns out to be very effective in the molecular design of the high-performance host materials. 2. Results and discussion 2.1. Synthesis and photophysical properties of m-CzHFs CzHFs were synthesized as reported in our previous publication [27]. An optimized Ullmann CAN crosscoupling reaction between the carbazole and dibromoheterofluorene was adopted to attach the 9-position of the carbazole moiety to the terminal ends of the heterofluorene cores in high yields (70%) [29]. CzHFs have good solubility in common solvents and excellent thermal properties (see Table 1) with the glass-transition temperature (Tg) above 97 °C and that of m-CzSF and m-CzOF are even higher than 110 °C, which are significantly improved in comparison with that of CBP (Tg = 62 °C) [30]. The melting point (Tm) of these materials are also high, which are all above 200 °C, and m-CzSF and m-CzOF are even nearly 300 °C. High Tg is highly desirable for host materials in OLEDs to prevent morphological changes and to suppress the formation of aggregation upon exposure to the heat produced during the device operation. The m-CzHFs show two main absorption bands both in THF solution and in solid film (see Fig. S7a in S.I.), the first absorption band around 290 nm can be assigned to the carbazole-centered n-p⁄ transition, while the other longer absorption around 320–340 nm is attributed to the p–p⁄ transition of the entire conjugated backbone. The optical energy band gaps of the m-CzHFs determined from the onset of the UV–vis spectra in solid film are very similar (3.34–3.41 eV), which is very close to that of CBP (3.4 eV). The PL spectra of the host materials are between 350 and 500 nm, which are well overlapped with the metal-to-ligand charge-transfer absorption peaks of the FIrPic around 380 and 420 nm [15], indicating that efficient and rapid Förster energy transfer from the host materials to the blue phosphorescent emitter. 2.2. Triplet energy and molecular simulations From the highest-energy 0–0 phosphorescence emissions of m-CzHFs (see Fig. S7b in S.I.), we estimated that their values of ET are 2.84, 2.97, and 2.87 eV respectively, as listed in Table 1. Because m-CzHFs possesses significantly higher ET than CBP resulted from the bridging of the heteroatoms, we would expect that (1) m-CzHFs can be good host for FIrpic (2.62 eV), (2) the energy of the triplet excitons would be transferred effectively from the host to the guest emitter, and (3) the reversing pathway (backward energy transfer) was prohibited. The HOMO and LUMO levels of m-CzHFs were measured using cyclic voltammetry (CV) (see Table 1 and Fig. S8 in S.I.). m-CzHFs

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R.-F. Chen et al. / Organic Electronics 12 (2011) 1619–1624 Table 1 Thermal and Electrochemical properties, triplet energy (ET), and dipole moment of m-CzHFs. Compound

CBP [30] m-CzNF m-CzOF m-CzSF

Thermal analysis [°C]

Electrochemical analysis [eV]

Tg

Tm

HOMO

LUMO

Eg

ET [eV]

62 97 114 121

– 213 285 293

5.5 5.34 5.40 5.42

2.0 2.22 2.26 2.26

3.7 3.12 3.14 3.16

2.56 2.84 2.97 2.87

Dipole moment

0.0003 4.80 1.62 1.65

Fig. 2. (a) External quantum efficiency–Luminance curves. Inset: the corresponding normalized EL spectra; (b) Luminance-voltage-current density (L-V-J) curves; (c) and (d) Current efficiency (c) and Power efficiency (d) versus current density curves; e) CIE (x, y) versus luminance curves of m-CzHFs-based PHOLEDs.

have very close HOMOs (5.34 – 5.42 eV) and LUMOs (2.22  2.26 eV), and their band gaps are also quite similar (3.12–3.22 eV). Their higher band gaps than that of FIrpic (3.1 eV) ensure the exothermic energy transfer from the host to the guest. It is interesting that m-CzHFs have ETs higher than that of CBP although their bandgaps are lower, showing the great modification abilities of the heteroatom linkage. The higher HOMOs, lower LUMOs, but still higher ETs, compared with those of CBP, make CzHFs attractive host materials for blue triplet emitters. To gain insight into the electronic states of m-CzHFs, molecular simulations via ab initio density-functional theory (DFT) calculation were performed. The results show that their HOMOs are mainly determined by carbazole, whereas LUMOs are predominately localized on and are primarily determined by the central heterofluorenes (see S.I. Table S2). The separated electron density distribution between the HOMO and LUMO of CzHFs is preferable for

efficient hole- and electron-transporting properties and the prevention of reverse energy transfer, enabling CzHFs the excellent host materials for PHOLEDs [31]. 2.3. Device performance of the m-CzHFs as hosts for FIrpic To evaluate the performance of the m-CzHFs as host materials for blue phosphorescent emitters, we fabricated the devices with the following configurations (see Fig. 1): ITO/MoOx(2 nm)/m-MTDATA:MoOx(15 wt.%, 30 nm)/m-MTDATA(10 nm)/Ir(ppz)3(10 nm)/m-CzHFs:FIrpic (10 wt.%, 10 nm)/BPhen (40 nm)/LiF(1 nm)/Al (100 nm). In these devices, MoOx acts as hole-injecting layers, 4,40 ,400 -tris(3-methylphenylphenylamino)-triphenylamine (m-MTDATA) as hole-transporting layer (HTL), tris(phenylpyrazole) Iridium (Ir(ppyz)3) as electron-blocking layer (EBL), and 4,7-dipheyl-1,10-phenanthroline (BPhen) as hole-blocking (HBL) and electron-transporting layer

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Table 2 Performance of m-CzHFs-based PHOLEDsa.

a b c d

Host compoud

EL [nm]

gext [%]

Vonb [V]

gc [cd/A]

gp [lm/W]

Lc (cd/m2)

CIE, (x,y)d

m-CzNF m-CzOF m-CzSF

468 472 468

9.0 17.8 18.4

3.0 3.0 3.2

17.3 36.4 34.8

17.0 31.2 29.6

4186 9504 9884

(0.157,0.301) (0.160,0.316) (0.159,0.306)

Maximum external quantum efficiency (gext), current efficiency (gc), and power efficiency (gp). The applied voltage (Von) required for brightness of 10 cd m2. The luminescence (L) at 5 V. CIE (x,y) at the brightness of 1000 cd m2.

(ETL). From Fig. 2, holes can be effectively injected into the host with small barrier (<0.3 eV) and be blocked by BPhen with the energy barrier about 0.9 eV; whilst the electron is injected into the host with the similar barrier as that of hole (0.3–0.4 eV) and is blocked by Ir(ppz)3 with the barrier over 0.5 eV. In such a device construction, the holes and electrons can be effectively confined in the host layer to form host excitons, which subsequently transfer exothermicly to FIrpic for the blue phosphorescence. The device performance, as summarized in Table 2, is extraordinary with low turn-on voltages (<3.0 V), and high maximum external quantum efficiency, current efficiency, and power efficiency, up to 18.4%, 36.4 cd A1, and 31.2 lm W1, respectively. At a brightness level of 1000 and 5000 cd m2, the driving voltages are still lower than 4.5 V with the external quantum efficiencies retain as high as 18.1% and 16.8%. The performance is comparable with the previously reported highest values for FIrpic-based devices [10,11]. From Fig. 2a, the electroluminescence (EL) spectra of the m-CzHFs-based devices show identical FIrpic emission without any residue emission from the host and/or adjacent layer, even at high current densities, indicating complete energy transfer from the host to the guest upon electrical excitation and excellent confinement of charge carriers and excitons in the emitting layer. FIrpic is a greenish-blue emitter with a typical emission peak around 472 nm and a shoulder at 500 nm with CIE (x, y) around (0.17, 0.34), depending on the emission band profile [15]. One advantage of the EL spectra of our devices is that they all displayed bluer emission with an emission peak at 468 (m-CzNF and m-CzSF) and 472 nm (m-CzOF) and a vibrational peak around 495–498 nm with a narrow full-width at half maximum (FWHM) of 55 nm and a CIE (x, y) around (0.16, 0.30). The bluer emission and improved color purity are probably attributed to the excellent miscibility and morphological stability of CzHFs, which effectively separated and clamped the doped FIrpic, depressing the ground-state association and leaving single molecule emission of the dopant in the solid solution (host) [15]. Fig. 2b displays the current density–voltage-luminance characteristics of these devices and obvious advantages of our devices lie in the very low turn-on and driving voltages. Remarkably, the driving voltages required for brightness of 10, 100, and 1000 cd m2 are 3.0–3.2, 3.4–3.5, and 3.8– 4.0 V with corresponding current density of 1.1–1.2, 1.3– 1.4, 1.9–3.3 mA cm2, respectively. And at 5 V, our devices show high luminance from 4186 to 9884 cd/m2. With such low driving voltages and current densities, the efficiency of the CzHFs-based PHOLEDs is dramatically enhanced. In

Fig. 2(c and d), the highest current efficiency of 36.4 cd A1 is reached at 4.0 V, with the corresponding brightness of 1613 cd m2 and current density of 2.24 mA cm2, while the highest power efficiency of 31.2 lm W1 is observed at 3.6 V, with brightness of 360 cd m2 and current density of 1.02 mA cm2. The efficiency roll-off of the devices is greatly reduced in the blue PHOLEDs with m-CzHFs as the host material (see Fig. 2a). For example, the external quantum efficiency of the m-CzSF-based device is 15.8%, 18.4% (maximum), 18.2%, and 15.3% at 100, 791, 1000, and 5000 cd m2, respectively, suggesting the balanced charge transporting in our devices. Color stability of PHOLEDs with different host materials were monitored according to luminance, as shown in Fig. 2e. The color coordinate of (0.16, 0.30) was little affected within the luminance range, indicating good color stability of the devices. And, their normalized EL spectra are identical over the range of the current density from 10 to 50 mA cm2, which further assures the stability of our devices. The low driving voltage, high efficiency, and high stability of the m-CzHFs-based devices at practical brightness are mainly resulted from the low charge injection barrier, the balanced charge transport, and the reduced triplet–triplet annihilation of the CzHFsbased devices due to the proper device structural design (see Fig. 1). The m-CzNF-based device shows lower brightness, efficiency, and color stability due to two main reasons. One is its high dipole moment (see Table 1) which results in low carrier mobilityand increased triplet–triplet annihilation [32]. The other is the alkyl linked to the N atom, which influences the molecular packing of the thin film, resulting in low charge mobility. The relatively lower Tg of m-CzNF (97 °C) is also an important reason for its lower device performance. m-CzOF and m-CzSF have comparable device performances due to their similar photophysical and electrochemical properties. Considering the higher triplet energy of m-CzOF, it may find further applications in deep blue PHOLEDs. In comparison with the device results of Seidler [33] and Wu [34] based on the host materials that are very similar to m-CzNF (with only different substituents on the carbazole core), which showed efficiencies up to 15%, 31 cd A1, and 28 lm W1 under a different device structure, the performance of our m-CzNF based device is a little lower, suggesting that our devices based on m-CzOF and m-CzSF can be further improved by optimizing device structures. The high performance of the FIrpic-based blue PHOLEDs using m-CzHFs as the host material in this research proves the validity of our approach to design host materials by connecting the biphenyl of CBP with heteroatoms.

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R

X

N

X = N X = O ,S

N

m -CzNF

R = octyl

m - C zOF

R = double lone pairs

, m -CzSF

1.7 1.9

2.0

5.8

5.34

5.40

2.26

5.42

BPhen

2.6

CBP (2.56eV)

m-CzSF (2.87eV)

5.1

2.26

m-CzOF (2.98eV)

FIrpic

2.9

m-CzNF (2.86eV)

5.1

Ir(ppz)3

M-MTDATA

2.22

5.5 6.5

Fig. 1. Device structure and energy level diagrams of the m-CzHFs-based PHOLEDs.SYNOPSIS TOC.

3. Conclusions In summary, m-CzHFs of m-CzNF, m-CzOF, and m-CzSF, which possess superior localization and energy level of HOMOs and LUMOs and also higher ET (3.0 eV) compared with CBP, were reported for use as blue phosphorescent hosts. Their superiorities in high-efficiency and low-voltage blue PHOLEDs have been demonstrated. The extraordinary device performance of m-CzOF was observed with low turn-on voltages (<3.0 V), and high maximum external quantum, current, and power efficiencies, up to 18.4%, 36.4 cd A1, and 31.2 lm W1, respectively. At the brightness level of 5000 cd m2, the driving voltages are still lower than 4.5 V with the external quantum efficiencies retain as high as 16.8%. The important concept of combining carbazole and heterofluorene via structurally mimicking CBP has proven to be successful in the rational molecular design of the high-performance host materials for PHOLEDs. m-CzHFs that can be prepared easily in high yields, are ideal host materials for blue PHOLEDs and their excellent device performance enables them to be good choice in the next-generation flat-panel displays and lighting technologies. 4. Experimental The OLEDs were fabricated using a vacuum thermal evaporation chamber with high-vacuum (104 Pa) at a rate of 0.1–0.2 nm/s. The devices without encapsulation

were measured in ambient atmosphere at room temperature. Electroluminescent (EL) spectra and CIE corrdinates of the devices were measured by a PR650 spectroscan spectrometer, as well as the luminance-voltage and current–voltage characteristics were measured simultaneously with a programmable Keithley 2400 voltagecurrent source. The cyclic voltammetry (CV) measurements were performed at room temperature on a CHI660E system in a typical three-electrode cell. The phosphorescence spectra of the compounds were measured using an Edinburgh LFS920 fluorescence spectrophotometer at 77 K, with a 5 ms delay time. All computations were done with Gaussian03 program package with B3LYP/6–31G(d) for structure optimizations and vibrational analysis. Acknowledgment This work was financially supported by the National Basic Research Program of China (973 Program, 2009CB930601 and 2010CB327701), National Natural Science Foundation of China (20804020, 60976019, 20974046, and 60977024), and Scientific Research Foundation of Nanjing University of Posts and Telecommunications (NY210017). References [1] M.C. Gather, A. Köhnen, K. Meerholz, Adv. Mater. 23 (2011) 233–248. [2] Q. Wang, D. Ma, Chem. Soc. Rev. 39 (2010) 2387. [3] U. Mitschke, P. Bauerle, J. Mater. Chem. 10 (2000) 1471.

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