Efficient fluorescent white organic light-emitting diodes with blue ...

Organic Electronics 8 (2007) 29–36 www.elsevier.com/locate/orgel

Efficient fluorescent white organic light-emitting diodes with blue-green host of di(4-fluorophenyl)amino-di(styryl)biphenyl Jwo-Huei Jou a

a,*

, Chung-Pei Wang a, Ming-Hsuan Wu a, Po-Hsuan Chiang a, Hung-Wei Lin a, Hao Chun Li b, Rai-Shung Liu b

Department of Materials Science and Engineering, National Tsing Hua University, Hsin-Chu 30013, Taiwan, ROC b Department of Chemistry, National Tsing Hua University, Hsin-Chu 30013, Taiwan, ROC Received 15 June 2006; received in revised form 4 October 2006; accepted 15 October 2006 Available online 9 November 2006

Abstract Efficient fluorescent white organic light-emitting diodes are fabricated with the use of an efficient electro-fluorescence blue-green host material di(4-fluorophenyl)amino-di(styryl)biphenyl, doped with red dye 4-(dicyano-methylene)-2methyl-6-(julolidin-4-yl-vinyl)-4H-pyran. One resulting two-wavelength white emission device shows a maximum external quantum efficiency of 4.8% and a high power efficiency of 14.8 lm/W with 100 cd/m2 at 3.8 V. The high efficiency may be attributed to the high electroluminescence character of the host, relatively high host-to-guest energy transfer efficiency, and effective device architecture. 2006 Elsevier B.V. All rights reserved. Keywords: Organic light-emitting diodes; High-efficiency; Fluorescent; White

1. Introduction White organic light-emitting diodes (OLEDs) have attracted considerable attention due to their great potential for general purpose illumination [1,2] and flat-panel displays [3,4]. The emitting layer can be made of phosphorescent and/or fluorescent materials [3,5–12]. As reported, the power efficiency of phosphorescent white OLED can reach 36 lm/W, or 57 lm/W with an antireflective coating, while that of fluorescent white polymeric light-emitting diodes is 14–16 lm/W [9]. In comparison, the reported *

Corresponding author. E-mail address: [email protected] (J.-H. Jou).

power efficiency of fluorescent white OLED at 100 cd/m2 is 5.0 lm/W of two-wavelength or 7.2 lm/W of three-wavelength [10,11]. Fluorescent white OLEDs with improved power efficiency are thus being pursued. To effectively raise power efficiency, the device must be thin, have low carriers injection barriers, and possess effective carriers and excitons confining function [12,13]. More importantly, in the incomplete energy transfer guest-host system [14], the employed light-emitting host must have high electroluminescent (EL) efficiency and exhibit good energy transfer efficiency to the guest. Accordingly, this study fabricated a high efficiency fluorescent white OLED of two-wavelength

1566-1199/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2006.10.007

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J.-H. Jou et al. / Organic Electronics 8 (2007) 29–36

Scheme 1. Synthetic protocol of di(4-fluorophenyl)amino-di(styryl)biphenyl (DSB).

using a newly synthesized high electroluminescent blue-green host material of di(4-fluorophenyl)aminodi(styryl)biphenyl (DSB) doped with a red dye. The resulting white OLED has a maximum external quantum efficiency of 4.8% and a high power efficiency of 14.8 lm/W at 100 cd/m2. This high electroluminescent blue-green host DSB contains trans-4-diphenyl-amino-stilbene, a derivative of N-phenylstilbene. The N-phenylstilbene derivatives are generally highly fluorescent due to its effective reduction of cis-trans photoisomerization [15]. However, the introduction of N-phenyl substituent to the stilbene framework leads to more planar ground-state geometry, resulting in a less distorted structure with a larger chargetransfer character for the fluorescent excited states, but with an undesired red-shift in the absorption and fluorescent spectra [16]. To tune bluer and enhance fluorescent emission efficiency [17], inert

fluoro-substituent is introduced to the Ph2N-containing oligo(arylenevinylene). Scheme 1 shows the structure and synthesis of the new blue-green host DSB. 2. Experimental 2.1. Synthesis of di(4-fluorophenyl)amino-di(styryl)biphenyl 2.1.1. Synthesis of 4-[bis(4-fluorophenyl)amino]benzaldehyde To a toluene solution of bis(4-fluorophenyl)amine (2.00 g, 9.75 mmol) was added 2-(4-bromophenyl)-1,3-dioxolane (2.23 g, 9.75 mmol), Pd(OAc)2 (43.8 mg, 0.19 mmol), P(t-Bu)3 (78.9 mg, 0.39 mmol), sodium tert-butoxide (1.03 g, 10.72 mmol). The reaction mixture was stirred under reflux condition at 110 C for 12 h; after cooling to room tem-


perature, 1 N HCl (20 mL) and acetone (30 mL) were added, stirred for another 1 h; the mixture was extracted with ethyl acetate, washed with aqueous NaCl solution, dried with MgSO4, and concentrated to yield a dark brownish solid. Chromatography (hexanes : EtOAc = 8:1, Rf = 0.30) afforded 4-[bis(4-fluorophenyl)amino]benzaldehyde (1) as a yellowish solid (2.65 g, 88%). 1H NMR (CDCl3, 600 MHz): d 9.78 (s, 1H), 7.65 (d, J = 8.2 Hz, 2H), 7.13–7.11 (m, 4H), 7.03 (t, J = 8.0 Hz, 4H), 6.90 (d, J = 8.2 Hz, 2H). 13C NMR (CDCl3, 150 MHz): d 190.3, 160.1 (d, JCF = 244.8 Hz), 153.3, 141.9, 131.4, 129.0, 128.0 (d, JCF = 8.0 Hz), 118.3, 116.7 (d, JCF = 22.7 Hz). HRMS (70 eV): calcd for C19H13F2NO: 309.0965, found: 309.0965. Anal. calcd for C19H13F2NO: C, 73.78; H, 4.24; N, 4.53. found: C, 73.76; H, 4.24; N, 4.54. 2.1.2. Synthesis of tetraethyl biphenyl-4,4 0 diylbis(methylene)diphosphonate To a neat triethylphosphite liquid (3.98 g, 23.89 mmol) was added 4,4 0 -bis (chloromethyl)biphenyl (1.50 g, 5.97 mmol). The reaction mixture was stirred at 150 C for 12 h; after cooling to room temperature, the reaction mixture was purified by bulb to bulb distillation to afford tetraethyl biphenyl-4,4 0 -diylbis(methylene)diphosphonate (2) as a white solid (2.63 g, 97%). 1H NMR (CDCl3,

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400 MHz): d 7.53 (d, J = 8.0 Hz, 4H), 7.36 (d, J = 8.0 Hz, 4H), 4.06–4.02 (m, 8H), 3.18 (d, J = 22.0 Hz, 4H), 1.26 (t, J = 6.8 Hz, 12H). 13C NMR (CDCl3, 100 MHz): d 139.0, 130.5, 130.0, 126.8, 61.8, 33.1 (d, JCP = 135.8 Hz), 16.1. HRMS (70 eV): calcd for C20H28O6P2: 426.1361, found: 426.1364. Anal. calcd for C20H28O6P2: C, 58.15; H, 7.10. found: C, 58.13; H, 7.11. 2.1.3. Synthesis of di(4-fluorophenyl)aminodi(styryl)biphenyl (DSB) To a THF solution of compound 2 (1.63 g, 3.58 mmol) was added sodium tert-butoxide (0.76 g, 7.91 mmol) at 0 C. The reaction mixture was stirred under ice bath for 1 h; a THF solution (5 mL) of compound 1 was added. After 20 h, 5 mL of NH4Cl solution was added. The reaction mixture was extracted with CH2Cl2, dried with MgSO4 and evaporated in vacuum. The residue was purified by recrystallization in mixed solution of CH2Cl2 and hexane affording DSB (2.14 g, 82%) as a light-green solid. 1H NMR (CDCl3, 600 MHz): d 7.59 (d, J = 8.4 Hz, 4H), 7.54 (d, J = 8.4 Hz, 4H), 7.37 (dd, J = 8.8, 2.4 Hz, 4H), 7.09 (s, 2H), 7.06–7.03 (m, 12H), 7.02 (s, 2H), 6.99–6.94 (m, 8H). 13C NMR (CDCl3, 150 MHz): d 158.9 (d, JCF = 241.9 Hz), 147.5, 139.4, 136.6, 131.2, 128.1, 127.4, 127.0, 126.7, 126.5, 126.1 (d, JCF = 7.7 Hz), 122.3, 116.2 (d, JCF = 22.5 Hz).

Fig. 1. Schematic illustration of the structure of the two-wavelength fluorescent WOLEDs using the novel blue host DSB, also shown molecular structure of the three blue host. The inset shows solution photoluminescence images of DSB, BANE and ADN in tetrahydrofuran.

0.382) 0.403) 0.420) 0.448) – (0.313, (0.341, (0.382, (0.425, 0.302) 0.421) 0.429) 0.441) 0.459) (0.188, (0.373, (0.308, (0.424, (0.461, 4.2 3.7 3.7 3.8 3.9 a

The maximum power/current efficiency is defined as the resultant value obtained at luminance P10 cd/m2.

8.0 15.0 14.3 15.6 15.2 6.3 13.7 13.2 14.8 13.9 10.7 15.8 16.3 16.9 16.8 7.5 15.4 15.7 16.5 16.3 8,550 20,300 22,000 24,600 20,000 DSB III

0 0.10 0.12 0.15 0.20

(0.155, 0.102) – (0.432, 0.430) (0.401, 0.402) (0.457,0.447) (0.412, 0.405) (0.487,0.475) (0.440, 0.424) 8.0 5.5 5.6 5.4 0.6 10.2 12.1 13.8 0.3 7.3 8.1 9.1 1.2 13.9 14.8 15.3 0.8 12.8 13.2 14.0 1,900 12,000 15,300 16,500 ADN II

0 0.10 0.15 0.20

– (0.419, 0.429) (0.428, 0.432) (0.455, 0.452) 0.122) 0.433) 0.440) 0.457) (0.155, (0.427, (0.436, (0.464, V

9.5 5.0 5.1 5.5 0.8 7.9 9.6 10.0

cd/A lm/W

0.6 4.8 6.0 7.1 3.6 9.9 11.7 13.4 1.2 6.6 7.8 8.3 6,500 35,000 40,000 41,500 BANE

0 0.10 0.15 0.20

Power efficiency/current efficiency/driving voltage at 100 cd/m2 Maximum power/current efficiency (lm/W)/(cd/A)a

I

The luminance and CIE chromatic coordinates of the resulted OLEDs were measured by using Minolta CS-100 luminance-meter. The electro-luminescence and photo-luminescence spectra were measured using a Hitachi F-4500 fluorescence spectrophotometer. The ultraviolet visible (UV–visible) absorption spectra were measured using a Hitachi U-3010 UV–visible spectrophotometer. The highest occupied molecular orbital (HOMO) energy levels of the organic materials studied were calculated from their oxidation potentials measured by a cyclic voltammetry [23], while the corresponding lowest unoccupied molecular orbital energy levels were estimated based on their HOMO energy levels and the lowest-energy absorption edge of the UV–visible

Maximum luminance (cd/m2)

2.3. Measurement

DCM2(%)

The deposition source of the white emission layer was prepared via solution-mixing [11] as followed. The composing dye and host were first separatively dissolved in tetrahydrofuran. After complete dissolution, the resultant solutions were mixed to form a host solution uniformly dispersed with the desired doping dye. The resulted dye-dispersed host solution was then vacuum-dried at 80 C for 60 min prior to vapor-deposition. The device was fabricated by vapor-deposition using an indium tin oxide coated glass substrate (Merck Display Technologies, Ltd.) with a sheet resistance of 13 X/square and a thickness of ˚ . The substrate was cleaned in ultrasonic 1250 A baths of detergent, de-ionized water, acetone and isopropyl alcohol in turn, and then treated with the boiling hydrogen peroxide. The resulted substrate was then purged with nitrogen. The respective organic layers and the cathode layer were deposited at 2 · 105 Torr using resistively heated tantalum and tungsten boats. All the organic layers were ˚ /s. The deposited at rates ranging from 1 to 3 A ˚ ˚ 5 A lithium fluoride and 1500 A aluminum were ˚ /s, subsequently deposited at rates of 0.1 and 10 A respectively. The emission area of the device was 8 mm2, and only the luminance in the forward direction was measured.

Table 1 Electroluminescent characteristics of the two-wavelength fluorescent WOLEDs with the different blue hosts doped with trace amounts of the red dye

2.2. Device fabrication

Host

HRMS (70 eV): calcd for C52H36F4N2: 764.2815, found: 764.2811. Anal. calcd for C52H36F4N2: C, 81.66; H, 4.74; N, 3.66. found: C, 81.63; H, 4.74; N, 3.69.

CIE 1931 (x, y) chromatic coordinates at 100 cd/m2 and 10,000 cd/m2


No.

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absorption spectra. A Keithley 2400 electrometer was used to measure the current–voltage (I–V) characteristics. All the devices were characterized without encapsulation and all the measurements were carried out in the ambient condition. The resultant blue-green DSB is doped with a red dye of 4-(dicyano-methylene)-2-methyl-6-(julolidin4-yl-vinyl)-4H-pyran (DCM2) to generate a twospectrum white emission. To clarify the effects of host materials with different EL efficiency and energy transfer efficiency on device performance, a comparison is made with two other blue light-emitting hosts, namely 1-butyl-9,10-naphthalene-anthracene (BANE) and 9,10-di(2-naphthyl)-anthracene (ADN). Fig. 1 illustrates the device architecture and molecular structures of the three hosts, and the inset shows their solution photoluminescence (PL) images in tetrahydrofuran. 3. Result and discussion Table 1 lists the effects of different hosts on device EL characteristics. The DSB host composing device, that doped with 0.15% of DCM2, exhibits maximum external quantum efficiency of 4.8% and power efficiency of 16.5 lm/W with 22 cd/m2 at 2.7 V or 14.8 lm/W with 100 cd/m2 at 3.8 V, whose CIEx, y coordinates are (0.424, 0.441) at 100 cd/m2 and (0.382, 0.420) at 10,000 cd/m2. Meanwhile, the ADN counterpart exhibits maximum external quan-

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tum efficiency of 3.7% and power efficiency of 13.2 lm/W with 11 cd/m2 at 4.5 V or 8.1 lm/W with 100 cd/m2 at 5.6 V, whose CIEx, y coordinates are (0.457, 0.447) at 100 cd/m2 and (0.412, 0.405) at 10,000 cd/m2, and, the BANE counterpart exhibits maximum external quantum efficiency of 3.4% and power efficiency of 7.8 lm/W with 15 cd/m2 at 3.8 V or 6.0 lm/W with 100 cd/m2 at 5.1 V, whose CIEx, y coordinates are (0.436, 0.440) at 100 cd/m2 and (0.428, 0.432) at 10,000 cd/m2. All three devices with the different hosts show relatively good efficiency performance owing to having thin device layers, low carriers injection barriers and good carriers and excitons confining function. For example, the energy barriers for injection of holes and electrons to inject the DSB host are 0.16 eV and 0.03 eV, respectively, as shown in Fig. 2 Moreover, the energy barriers for injection of holes and electrons into the ADN host are 0.1 eV and 0.2 eV, respectively, while those for the BANE host are 0.2 eV and 0.1 eV, respectively. From the perspective of hole injection barrier, the best host is ADN (0.1 eV), followed by DSB (0.16 eV), and the least favorable host is BANE (0.2 eV). Whilst, from the viewpoint of electron injection barrier, the best host is DSB (0.03 eV), followed by BANE (0.1 eV), and finally ADN (0.2 eV). Though the ADN composing device has the lowest hole injection barrier, it has the highest electron injection barrier. On the other hand, the DSB composing device

Fig. 2. HOMO/LUMO energy-level diagram of the hole-transporting/emission/electron-transporting trilayered structure of the WOLEDs using the three different hosts.

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has no barrier for electron injection though having a slightly higher hole injection barrier than ADN. The better balance between hole and electron injections in the DSB composing device may, at least partly, explain its better device efficiency. Importantly, the electroluminescent (EL) efficiency of DSB is the highest among all hosts, which is 6.3 lm/W with 100 cd/m2 at 4.2 V, while those of ADN and BANE at 100 cd/m2 are 0.3 lm/W at 8.0 V and 0.6 lm/W at 9.5 V, respectively. The resultant better EL efficiency indicates DSB to be a higher EL-efficiency molecule with a sound carriermobility, as revealed by its higher current density

shown in Fig. 3. The comparatively high EL efficiency of the DSB host itself may also explain the resulting high device efficiency. The efficiency of energy transfer from host to guest can be revealed by the spectral overlap between the emission of the host and the absorption of the dopant [16]. Fig. 4 shows the solution photoluminescence (PL) spectra of DSB, ADN, and BANE in tetrahydrofuran and the UV–visible absorption spectra of the red dye DCM2. The spectral peaks of the DSB, BANE and ADN hosts are located at 470, 442 and 436 nm, respectively, while that of red dye DCM2 is located at 500 nm. A great-

Fig. 3. J–V characteristics of the resulted OLEDs, with different hosts with and without the red-dopant DCM2.

Fig. 4. Photoluminescence spectra of the host molecules of DSB, BANE and ADN with and without the dopant of DCM2, and ultraviolet–visible absorption spectra of the dopant molecule of DCM2.


est spectral overlap is observed between the PL spectrum of DSB and the UV–visible spectrum of DCM2, plausibly indicating a highest Forster energy-transfer efficiency, by realizing that the doping concentration is relatively trace in these devices [17]. Energy transfer efficiency can also be realized via the PL experiments [18–20]. Fig. 4 shows the PL spectra of the three hosts with and without the doping of DCM2. All the PL peaks of the three hosts drop and red-shift on the addition of DCM2, confirming the occurrence of incomplete energy trans-

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fer. The decrease in the peak intensity and the extent of red-shift are the most pronounced for the DSB host, indicating a comparatively best energy transfer efficiency. This, coupled with the high electroluminescence efficiency of pure DSB host, the most favorable electron-injection and -confining characteristics, as mentioned above, may explain why the DSB composing device exhibits the highest power efficiency as revealed. Fig. 5 shows the effect of DCM2 concentration on the power efficiency and luminance of the DSB employed white OLEDs. The maximum luminance

Fig. 5. The effects of the blue emitting host of DSB doped with the different concentration of the red dye of DCM2 on the resulting power efficiency and luminance of the two-wavelength WOLEDs.

Fig. 6. Doping-concentration effects of the red dye of DCM2 on the electroluminescence spectra of the two-wavelength WOLEDs.

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of the resultant device increases with the increase of dopant concentration. Fig. 6 shows the effect of DCM2 concentration on the electroluminescence spectrum. The spectra show two emission peaks at 470 nm from the blue host of DSB and at 560 nm from the red dopant of DCM2. The emission at 10,000 cd/m2, for example, changes from CIEx, y of (0.283, 0.365) to (0.425, 0.448) as the DCM2 concentration is increased from 0.10% to 0.20%. By doping 0.12% of DCM2, white emission is obtained with a maximum luminance of 22,000 cd/ m2 at 9.5 V and a maximum power efficiency of 15.7 lm/W with 19 cd/m2 at 2.8 V or 13.2 lm/W with 100 cd/m2 at 3.7 V. The CIE coordinates of the device blue-shift from (0.385, 0.418) to (0.332, 0.388) as brightness increases from 100 to 10,000 cd/m2. The power efficiency at 100 cd/m2, however, decreases from 14.8 to 13.9 lm/W as the DCM2 concentration increases from 0.15% to 0.20%. This decrease may be attributed to the concentration-quenching phenomenon [5], due to the increasing formation of guest molecule aggregates [21], carrier trapping [22] at higher dopingconcentrations. 4. Conclusion To summarize, we have fabricated an efficient fluorescent white organic light-emitting diode with the use of a blue-green host material of DSB, which has a relatively high EL efficiency and fairly good energy transfer efficiency to the red dye of DCM2. Additionally, the device is thin and has a hole-transporting/emission/electron-transporting tri-layered structure with relatively low carrier-injection barriers and efficient carriers and excitons confining function. The resulting two-wavelength white emission device shows a maximum external quantum efficiency of 4.8% and a high power efficiency of 14.8 lm/W with 100 cd/m2 at 3.8 V. Acknowledgements The authors thank Prof. C. H. Cheng of Department of Chemistry, National Tsing Hua University, for help with the EL spectra measurements. The blue hosts of BANE and ADN were provided by

Labeltek Inc and Department of Chemistry, National Tsing Hua University, respectively. This work was supported by the University System of Taiwan and National Science Council, Taiwan, Republic of China for financially supporting in part this research under Grants No. 92J0162J4 and NSC92-2216-E-007-027, respectively. References [1] A.R. Duggal, J.J. Shiang, C.M. Heller, D.F. Foust, Appl. Phys. Lett. 80 (2002) 3470. [2] B.W. D’Andrade, S.R. Forrest, Adv. Mater. 16 (2004) 1585. [3] J. Kido, M. Kimura, K. Nagai, Science 267 (1995) 1332. [4] Z. Shen, P.E. Burrows, V. Bulovic´, S.R. Forrest, M.E. Thompson, Science 276 (1997) 2009. [5] C.W. Tang, S.A. Vanslyke, C.H. Chen, J. Appl. Phys. 85 (1989) 3610. [6] B.W. D’Andrade, R.J. Holmes, S.R. Forrest, Adv. Mater. 16 (2004) 624. [7] Y. Shao, Y. Yang, Appl. Phys. Lett. 86 (2005) 073510. [8] Y.S. Huang, J.H. Jou, W.K. Weng, J.M. Liu, Appl. Phys. Lett. 80 (2002) 2782. [9] R.F. Service, Science 310 (2005) 1762. [10] J.H. Jou, Y.S. Chiu, R.Y. Wang, H.C. Hu, C.P. Wang, H.W. Lin, Org. Electron. 7 (2006) 8. [11] J.H. Jou, Y.S. Chiu, C.P. Wang, R.Y. Wang, H.C. Hu, Appl. Phys. Lett. 88 (2006) 193501. [12] M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, Y. Taga, Appl. Phys. Lett. 79 (2001) 156. [13] Z.Y. Xie, L.S. Hung, S.T. Lee, Appl. Phys. Lett. 79 (2001) 1048. [14] R.S. Deshpande, V. Bulovic, S.R. Forrest, Appl. Phys. Lett. 75 (1999) 888. [15] D.H. Waldeck, Chem. Rev. 91 (1991) 415. [16] J.S. Yang, S.Y. Chiou, K.L. Liau, J. Am. Chem. Soc. 124 (2002) 2518. [17] R.M. Gurge, A.M. Sarker, P.M. Lahti, B. Hu, F.E. Karasz, Macromolecules 30 (1997) 8286. [18] M. Pope, C.E. Swenberg, Electronic Process in Organic Crystals and Polymers, Second ed., Oxford University Press, New York, 1999. [19] X. Gong, J.C. Ostrowski, D. Moses, G.C. Bazan, A.J. Heeger, Adv. Func. Mater. 13 (2003) 439. [20] L. Yan, N.J. Watkins, S. Zorba, Y. Gao, C.W. Tang, Appl. Phys. Lett. 81 (2002) 2752. [21] V. Bulovic, R.S. Deshpande, M.E. Thompson, S.R. Forrest, Chem. Phys. Lett. 308 (1999) 317. [22] Y.T. Tao, C.W. Ko, E. Balasubramaniam, Thin Solid Films 417 (2002) 61. [23] S. Janietz, D.D.C. Bradley, M. Grell, C. Giebeler, M. Inbasekaran, E.P. Woo, Appl. Phys. Lett. 73 (1998) 2453.