Fluorescent deep-blue and hybrid white emitting devices based on a naphthalene–benzofuran compound

Organic Electronics 14 (2013) 2023–2028

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Fluorescent deep-blue and hybrid white emitting devices based on a naphthalene–benzofuran compound X.H. Yang a,b, S.J. Zheng c, H.S. Chae c, S. Li c, A. Mochizuki c,⇑, G.E. Jabbour b,⇑ a

School of Physical Science & Technology, MOE Key Laboratory on Luminescence and Real-time Analysis, Southwest University, Chongqing 400715, PR China Solar and Alternative Energy Engineering Research Center, Physical Science and Engineering, KAUST, Thuwal, Saudi Arabia c Nitto Denko Technical Corporation, 501 Via Del Monte, Oceanside, CA 92058, USA b

a r t i c l e

i n f o

Article history: Received 18 November 2012 Received in revised form 22 January 2013 Accepted 3 March 2013 Available online 13 April 2013 Keywords: Organic light emitting devices Deep-blue emission Emitting dopant Hybrid white light emitting devices

a b s t r a c t We report the synthesis, photophysics and electrochemical properties of naphthalene– benzofuran compound 1 and its application in organic light emitting devices. Fluorescent deep-blue emitting devices employing 1 as the emitting dopant embedded in 4-40 -bis(9carbazolyl)-2,20 -biphenyl (CBP) host show the peak external quantum efficiency of 4.5% and Commission Internationale d’Énclairage (CIE) coordinates of (0.15, 0.07). Hybrid white devices using fluorescent blue emitting layer with 1 and a phosphorescent orange emitting layer based on an iridium-complex show the peak external quantum efficiency above 10% and CIE coordinates of (0.31, 0.37). Ó 2013 Published by Elsevier B.V.

1. Introduction Organic light emitting devices (OLEDs) have been intensively studied in recent years for applications in flat-panel displays and solid-state lighting [1]. Among three primary color devices, blue emitting devices warrant further improvement of the performance. Anthracene derivatives such as 2-methyl-9,10-di(2-naphthyl) anthracene have been widely used as the host materials, and recently as the dopant materials for blue emitting devices [2–10]. Other materials such as fluorene [11,12], triphenylene [13,14], quinoline [15,16], and pyrene [17–19] derivatives have also been studied. For example, blue emitting devices based on terfluorene showed the external quantum efficiency (EQE) of 5.3% [11], and incorporation of a non-planar spirobifluorene framework into the molecular structure of a commonly used blue emitting material 4,4-bis(2,2-diphenylvinyl)-1,1biphenyl significantly increased the glass transition temperature (Tg) and stabilized film morphology [12]; Jenekhe et al. [15] reported oligoquinoline derivatives based light ⇑ Corresponding authors. E-mail addresses: [email protected] (A. Mochizuki), [email protected] (G.E. Jabbour). 1566-1199/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.orgel.2013.03.012

emitting devices with the EQE of 6.6% and Commission Internationale d’Énclairage (CIE) coordinates of (0.15, 0.16); Properties of blue emitting devices employing pyrene derivatives were described by Suzuki et al. [17], Lee et al. [18], and Wu et al. [19], respectively. In the aspect of blue emitting dopant materials, distyrylarylenes have been commonly adopted [20]. The subsequent work described the use of unsymmetrical mono(styryl)amines with decreased conjugation compared to the parent distyrylamines as the dopant materials and the resultant light emitting devices showed the current efficiency of 5.4 cd/A and CIE coordinates of (0.14, 0.13) [21]. Despite significant advances in the synthesis of blue emitting materials, reports on efficient deep-blue emitters with y value of CIE coordinates lower than 0.1 have been rather scarce. Phosphorescent OLEDs possess higher efficiency than the fluorescent counterparts as both singlet and triplet excited states can contribute to light emission due to effective mixing of singlet and triplet excited states induced by strong spin–orbital coupling. Therefore, phosphorescent materials are favored especially for white OLEDs to boost the device efficiency. However, deep-blue phosphorescent material with long operational stability is still under construction. To circumvent this problem, hybrid white

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OLEDs, which combine blue fluorophore with green/red phosphors, have been proposed [22–27]. In such devices, fluorescent blue emitter is located within device carrier recombination zone, where radiative decay of singlet excitons leads to fluorescent blue emission. Triplet excitons can be harvested by peripheral phosphorescent emitters located within a triplet exciton diffusion length from carrier recombination zone. Typically, a thin interfacial layer is incorporated in between the fluorescent and phosphorescent regions to regulate the exciton distribution [22]. In this paper, we report the synthesis and characterization of a naphthalene–benzofuran compound 1. Light emitting devices employing CBP: 1 as the emissive layer show the peak EQE of 4.5% and CIE coordinates of (0.15, 0.07). Furthermore, hybrid white OLEDs combining CBP: 1 fluorescent blue emitting layer with CBP: iridium complex phosphorescent orange emitting layer show the peak EQE above 10% and CIE coordinates of (0.31, 0.37). 2. Experimental section All non-aqueous reactions were carried out under dry argon atmosphere unless stated otherwise. All reagents and solvents were received from Aldrich or Gelest and were used as received unless indicated otherwise. The NMR spectra data were collected using a JEOL ECL-400. Fluorescence measurements were performed on a Jobin Yvon Fluromax 3 luminescence spectrometer. Phosphorescence spectrum was measured in 2-MeTHF at 77 K. UV–vis spectra were recorded on a Varian Cary 50 Scan spectrophotometer. DSC measurements were carried out on a Seiko Exstar 6000. Heating and cooling rates were 10 °C/min. Cyclic voltammetry (CV) was performed using an Autolab type-II potentiostat/galvanostat model. Anhydrous DMF degassed was used as the solvent under a nitrogen atmosphere, and 0.1 M tertra(n-butyl)-ammonium hexafluorophosphate (NBu4PF6) was used as the supporting electrolyte. An Ag wire, a Pt wire and glassy carbon electrode were used as the reference electrode, the counter electrode, and the working electrode, respectively. The redox potentials were referred to a ferrocene/ferrocenium (Cp2Fe/Cp2Fe+) redox couple as an internal standard for calibration. CV scan was recorded with a scan rate of 100 mV/s.

2.1. OLED fabrication Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) filtered through a 0.2 lm filter was spin-coated onto the precleaned substrates, giving a 55 nm thick film. All other layers were deposited in a glove-box-hosted vacuum deposition system at a pressure of 105 Pa. A 30 nm 4,40 ,400 -tris(N-carbazolyl) triphenylamine (TCTA) layer and a 40 nm 1,3,5-tris(phenyl-2-benzimidazolyl)benzene (TPBI) layer were employed as holetransporting layer and electron-transporting layer. A cesium fluoride layer with a nominal thickness of 1 nm and a 100 nm aluminum layer served as electron injection layer and cathode. The emissive layers were made by co-evaporation of the host and dopant materials. The deposition rates for organic materials, CsF and Al were 0.06, 0.0015 and 0.1 nm/s, respectively. Individual devices had areas of 0.14 cm2. Electroluminescence spectra were measured with an Ocean Optics HR4000 spectrometer, and current density–voltage–luminance measurements were performed using a Keithley 2400 SourceMeter, a Newport 2832-C power meter, and 818 UV detector. 3. Results and discussion 3.1. Synthesis and characterization of 1 A mixture of 1,4-dibromonaphthalene (1.0 g, 3.5 mmol), benzofuran-2-ylboronic acid (1.19 g, 7.3 mmol), Pd(dppf)Cl2 (0.267 g, 0.36 mmol), and KF (1.27 g, 22 mmol) in toluene/water (20 mL/10 mL) was degassed and heated at 130 °C overnight. The resulting mixture was worked up with water/dichloromethane. The organic phase was collected and dried over Na2SO4, then loaded on silica gel and purified by flash column (hexanes/ethyl acetate 8:1). The blue fluorescent fraction was collected and a white solid was obtained (0.50 g, in 44% yield) after removal of solvents. LCMS (APCI positive): Calcd for C26H17O2 (M + H): 361.1; Found: 361. 1H NMR (400 MHz, CDCl3): 8.58 (m, 2H), 7.98 (s, 2H), 7.68 (d, 2H), 7.62 (m, 4H), 7.32 (m, 4H), 7.15 (s, 2H). As 1 was synthesized in one step Suzuki coupling reaction, the cost is low and the toxicity is estimated to be low compared to materials containing heavy metal atoms or halogens (see Scheme 1).

Fig. 1. Optical properties of 1.

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Fig. 1 shows the optical properties of 1. A noticeable absorption band in the wavelength range between 250 and 400 nm is observed, while the emission spectrum of 1 chloroform solution shows a peak at ca. 438 nm. The emission spectrum of 1 solid film exhibits a ca. 20 nm red-shift compared to that in solution, which may be associated with enhanced intermolecular interactions in the solid state. Triplet energy of 1 can be estimated from the first peak of phosphorescence spectrum measured at 77 K

to be 2.43 eV. (Signals from fluorescence (the shoulder at ca. 440 nm) and excitation light (sharp peak at ca. 400 nm) are included in the spectrum.) Triplet energy of 1 is lower than that of CBP (2.56 eV) [28], indicating that triplet excited states are confined on 1 in the CBP: 1 film. There is no pronounced glass transition discerned in both heating and cooling regimes in DSC measurements. Cyclic Voltammogram of 1 exhibits a reversible reduction and irreversible oxidation with the onset of the

Table 1 Summary of properties of 1.

1. 2. 3. 4.

kab (nm)

kem (nm)

kfilm (nm)

ET1 (eV)

HOMO (eV)

LUMO (eV)

Eg(Opt) (eV)

359

438

463

2.43

5.57

2.6

3.0

Absorption and emission spectra of 1 chloroform solution Emission spectrum of drop-casting neat film Triplet energy (ET1) measured at 77 K in 2-MeTHF LUMO level measured by cyclic voltammetry, HOMO level estimated from band gap energy, Eg from UV–vis spectroscopy and LUMO energy

Table 2 Summary of device characteristics. Each device is listed by number and structure. Data is measured at which a forward light output of approximately 500 cd m2 is obtained. Parameters listed are drive voltage (Bias), current density (J), external quantum efficiency (EQE), power efficiency (PE), luminous efficiency (LE), and CIE coordinates. @ 500 cd m2

Device

(1) TCTA/CBP: 3% 1/TPBI/CsF/Al (2) TCTA/CBP: 5% 2/CBP/CBP: 3% 1/TPBI/CsF/Ala (3) TCTA/CBP: 5% 2/CBP/CBP: 3% 1/TPBI/CsF/Al

J (mA/cm2)

EQE (%)

PE (lm/W)

LE (cd/A)

CIE

7.5 5.8 5.9

20.9 5.2 4.2

3.4 4.8 5.8

1.0 5.1 7.0

2.3 9.5 13.1

(0.15, 0.07) (0.25, 0.25) (0.28, 0.30)

The thickness of 2 doped CBP layer for devices 2 and 3 is 10 nm and 15 nm, respectively.

Intensity (a.u) 300

400

500

600

3000 150

-2

1

Luminance (cd m )

-2

Current density (mA cm )

a

Bias (V)

100 1500 50

0 0

5

Wavelength (nm)

0 10

Voltage (V) 5

EQE (%)

4 3 2 1 0 0

50

100

150

Current density (mA cm-2) Fig. 2. Properties of devices with the structure of PEDOT:PSS/TCTA (30 nm)/CBP: 3% 1 (30 nm)/TPBI (40 nm)/CsF/Al, (a) EL spectra; (b) current density (solid symbols)–voltage–luminance (open symbols) characteristics; and (c) EQE-current density characteristics.

X.H. Yang et al. / Organic Electronics 14 (2013) 2023–2028

2.0

10 nm orange layer @ 6 V (0.25, 0.25) 15 nm orange layer @ 6 V (0.28, 0.30) 15 nm orange layer @ 8 V (0.31, 0.37)

1.5 1.0 0.5 0.0 400

500

600

700

800

Wavelength (nm) 150

8000 6000

100 4000 50 2000

Luminance (cd/m2)

To evaluate the electroluminescence (EL) properties of 1, we fabricate light emitting devices with the following configuration: PEDOT:PSS/TCTA (30 nm)/CBP:3% 1 (30 nm)/ TPBI (40 nm)/CsF/Al. Fig. 2a shows the EL spectrum of devices. The EL spectrum shows an emission peak at ca. 440 nm, similar to the emission spectrum of 1 in solution. The CIE coordinates of devices are (0.15, 0.07), very close to the National Television System Committee (NTSC) blue color standard (0.14, 0.08). It is worth noting that the EL spectrum of devices is independent of drive voltage. Fig. 2b shows the current density–voltage–luminance properties of devices. Turn-on voltage of devices (0.1 cd/m2) is 3.3 V and the drive voltage for 500 cd/m2 is 7.5 V. As shown in Fig. 2c, the peak EQE of devices is 4.5%. At 500 cd/m2, EQE/ power efficiency (PE) values of 3.4%/0.97 lm/W are measured. Measurements on device stability are planed to understand the structure–property relationship. Hybrid WOLEDs using 1 as fluorescent blue emitter and iridium(III) complex [Ir(L(1))2(acac)] (HL(1) = 5-trifluoromethyl-2-[3-(N-phenylcarbazolyl)]pyridine, Hacac = acetylacetone) 2 [31,32] as phosphorescent orange emitter have been made. Fig. 3a presents the EL spectra of devices with the structure of PEDOT:PSS/TCTA (30 nm)/CBP:5% 2 (10 nm)/CBP (2 nm)/CBP:3% 1 (10 nm)/TPBI (40 nm)/CsF/ Al and PEDOT:PSS/TCTA (30 nm)/CBP:5% 2 (15 nm)/CBP (2 nm)/CBP:3% 1 (10 nm)/TPBI (40 nm)/CsF/Al. The EL spectra of devices consist of blue emission from 1 and orange emission from 2. Relative intensity of blue emission with respect to that of orange emission decreases with an increase in the thickness of CBP: 2 layer, which may be related to enhanced capture of triplet excitons by 2. In addition, the intensity of orange emission grows more rapidly than that of blue emission with increasing drive voltage, presumably attributed to the shift of device carrier recombination zone toward the TCTA/CBP: 2 interface at high drive voltages. Devices with a 15 nm 2 doped CBP layer show the CIE coordinates of (0.31, 0.37) and Color Rendering Index (CRI) of 52 at 8 V. The reason for the low CRI value is that EL spectra fail to completely cover the whole visible light region, as typically found in two-color WOLEDs. The operational voltage of hybrid WOLEDs is similar to that of fluorescent blue devices, as shown in Fig. 3b. Fig. 3c displays the EQE-current density characteristics. For devices with a 15 nm CBP: 2 layer, the peak EQE is 10.7%, which drops to ca. 6.0% at a luminance of 500 cd/m2. The peak EQE of devices with a 10 nm orange emitting layer is ca. 8%. Diminished triplet energy transfer to 2 may account for the reduced EQE of devices with a 10 nm orange emitting layer, which is consistent with the

Intensity (a.u)

3.2. OLED device studies

observation of the reduced orange emission intensity in the EL spectrum. Assuming a Lambertian emission pattern, the peak PE of devices with a 15 nm CBP: 2 layer is 25 lm/W. Optimization of the layer thickness and selection of interlayer material with balanced charge-transport property are currently underway to improve the device performance (see Table 2). CBP preferentially transports holes, therefore the carrier recombination zone of devices is expected to be close to the CBP: 1/TPBI interface [33]. Singlet excitons decay radiatively followed by Förster energy transfer from CBP to 1, generating fluorescent blue emission; CBP triplet excitons can diffuse and transfer energy to 2, yielding phosphorescent orange emission. Triplet energy transfer from 1 to 2 is probably restricted since preceded energy transfer from 1 to CBP is required, which is endothermic (DE = 0.13 eV). As a result, some of 1 triplet excited states are wasted. Selection of fluorescent blue dopant materials possessing

Current density (mA/cm2)

reduction potential at ca. 2.2 V referred to ferrocene/ferrocenium redox couple (4.8 eV below the vacuum level [29]). The Lowest Unoccupied Molecular Orbital (LUMO) and Highest Occupied Molecular Orbital (HOMO) levels of 1 are located at ca. 2.6 and 5.6 eV deduced from the cyclic voltammetry measurements and the band gap energy obtained from the absorption spectrum. Table 1 summarizes some properties of 1. Comparison of the energy levels of 1 with those of CBP [30] suggests that 1 serves as a hole trap when embedded in CBP host (see Table 1).

0

0 0

3

6

9

Voltage (V) 12

8 EQE (%)

2026

4

0 0.01

0.1 1 10 Current density (mA/cm2)

100

Fig. 3. Properties of devices with the structure of PEDOT:PSS/TCTA (30 nm)/CBP: 5% 2 (10 nm)/CBP (2 nm)/CBP: 3% 1 (10 nm)/TPBI (40 nm)/ CsF/Al (squares) and PEDOT:PSS/TCTA (30 nm)/CBP: 5% 2 (15 nm)/CBP (2 nm)/CBP: 3% 1 (10 nm)/TPBI (40 nm)/CsF/Al (circles), (a) EL spectra; (b) current density (solid symbols)–voltage–luminance (open symbols) characteristics; and (c) EQE-current density characteristics.

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B(OH) 2 Br

Br

O O Pd(dppf )Cl2, KF toluene/water 132 C

O

Exact Mass: 360.12

Scheme 1. Synthesis of 1.

larger triplet energy than the host material would facilitate triplet energy transfer to phosphorescent emitter and improve the device efficiency as reported in recent literature [26]. Intensified triplet–triplet and triplet–polaron interactions at high current densities on one hand reduce the efficiency of phosphorescent light emitting devices, and decrease the number of triplet excited states reaching the CBP: 2 layer on the other hand [24], which is postulated to be the main reason for a fast roll-off of the device EQE with increasing current density.

[8]

[9]

[10]

[11]

4. Conclusion In conclusion, we report the synthesis and characterization of a deep-blue emitting naphthalene–benzofuran compound. Fluorescent deep-blue emitting devices employing this compound as the emitting dopant show the peak EQE of 4.5% and CIE coordinates of (0.15, 0.07). Hybrid white organic light emitting devices with the peak EQE exceeding 10% are enabled by using this compound and an iridium-complex as fluorescent blue and phosphorescent orange emitters, respectively.

[12]

[13]

[14]

[15]

Acknowledgements Financial support by National Natural Science Foundation of China (Grant No. 61177030), the Chinese Ministry of Education under the Program for New Century Excellent Talents in Universities (Grant No. NCET-11-0705), the start-up Grant (SWU111057) from Southwest University, KAUST, and FiDiPro program, Academy of Finland, is acknowledged.

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[19]

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