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Efficient fluorescent red, green, and blue organic light-emitting devices with a blue host of spirobifluorene derivative
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Thin Solid Films 516 (2008) 5062 – 5068 www.elsevier.com/locate/tsf
Efficient fluorescent red, green, and blue organic light-emitting devices with a blue host of spirobifluorene derivative Rong-Ho Lee a,⁎, Yu-Wei Huang a , Ying-Yu Wang a , Hsien-Yu Chang b a
Department of Chemical and Material Engineering, National Yunlin University of Science & Technology, Yunlin 640, Taiwan, ROC b EChem Hightech CO., LTD, Hsin-Chu Industrial Park, Hu-Kou, Hsin-Chu, Taiwan, ROC Received 19 May 2007; received in revised form 20 December 2007; accepted 12 February 2008 Available online 19 February 2008
Abstract Efficient fluorescent blue, green, and red (RGB) organic light-emitting devices (OLEDs) were fabricated using a blue host material of pyrimidine-containing spirobifluorene derivative 2,7-bis[2-(4-tert-butylphenyl)pyrimidine-5-yl]-9,9′-spirobifluorene (TBPSF) doped with blue dye perylene, green dye 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l] pyrano[6,7,8-ij] quinolizin-11-one (C545T), and red dye 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), respectively. The brightness and current efficiency of the perylene doped blue device reached 10117 cd/m2 and 2.97 cd/A. Green emission of the C545T doped device reached 8500 cd/m2 and 13.0 cd/A. Red emission of the DCJTB doped device can be as high as 9000 cd/m2 and 2.0 cd/A, respectively. High color purity of the blue (Commission Internationale de L′Eclairage (CIEx,y) coordinates (CIE, x = 0.27, y = 0.24)), green (CIE, x = 0.19, y = 0.63) and red (CIE, x = 0.62, y = 0.37) emissions were achieved for RGB dyes doped TBPSF OLEDs. High brightness, large current efficiency, and good color purity of TBPSF-based RGB OLEDs were obtained by the configuration optimization device, such as inserting the hole and electron-injection materials, and suitable dopant content and light emitting layer thickness. © 2008 Elsevier B.V. All rights reserved. Keywords: Organic light emitting device; Fluorescence dye; Spirobifluorene derivative; Electroluminescence
1. Introduction Since Tang et al. first applied a light emitting layer doped with guest fluorescent dye to organic light-emitting devices (OLEDs), OLEDs have attracted growing interest for full color flat panel display applications [1]. The emitting color is primarily determined by the guest fluorescent dye molecule in the emitting layer. Researchers have made much progress in color emissions for fluorescent dye doped OLEDs [2–40]. Nevertheless, simultaneous high quality red, green, and blue (RGB) emissions are not easily achieved respectively from the RGB dopants using single host material via Forster energy transfer processes. Numerous red fluorescence dyes have been synthesized and studied for fabricating an excellent red OLED, including pyran, ⁎ Corresponding author. Tel.: +886 5 5342601x4625; fax: +886 5 5312071. E-mail address: [email protected] (R.-H. Lee). 0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.02.010
chromene, and isophorene containing 4-(dicyanomethylene)-2methyl-6-[4-(dimethylaminostyryl)-4H-pyran] (DCM) type red dyes [2–12], porphyrin-type macrocyclic red dyes [13–15], and squarylium dyes [16–18]. Among these red fluorescence dyes, DCM type dyes remain the most efficient material when applied for the red OLEDs, particularly for the dye namely (4(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9enyl)-4H-pyran) (DCJTB) [4,5]. However, the DCM type dye doped device with high doping concentration usually shows low brightness and efficiency due to concentration quenching [2]. This quenching effect is solved by the so-called “assist dopant or co-host” method [3,19,20]. 5,6,11,12-tetraphenylnaphthacene (Rubrene) was added as the assist dopant with DCM-type red emitter or co-host with the host material for red OLEDs fabrication [3,19,20]. Thus, DCJTB doped red device with good color purity, high brightness, and large current efficiency have been reported by several research groups and suitable for practical use [19,20]. In addition, highly efficiency DCJTB-based
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pure red electroluminescence (EL) was recently realized by doping the red dopant in green host Tri-(8-hydroxyquinoline) aluminum (Alq3) in the presence of an electron transporting/hole blocking layer (ETL/HBL) of 2,9-dimethyl-4,7-diphenyl-9,10phenanthroline [21]. On the other hand, the fluorescence dyes doped green OLEDs with excellent EL emission have also been studied extensively, including coumarin derivatives [1,22–24], quinacridone derivatives [25], quinoline derivatives [26], and carbazole derivatives [27]. The most frequently used green dopant among these green fluorescence dyes in the commercial OLED panel is the coumarin derivative dye 10-(2-benzothiazolyl)1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l] pyrano[6,7,8-ij] quinolizin-11-one (C545T) [23,24]. Although several green host-emitting materials have been developed by researchers [28], the green emitting material Alq3 is usually used as the host material for green fluorescence dyes doped OLEDs [1,23,24]. In addition to red and green fluorescence dyes, the blue emitter with excellent EL performance is also important for color OLEDs fabrication, with numerous blue fluorescence dyes having been synthesized and studied [29–40]. Hosokawal et al. report that bright blue OLEDs can be produced using a host emitting of 1,4-bis(2,2-diphenylvinyl)biphenyl with blue dopants [29]. A bright blue OLED based on the dopant of silylsubstituted ter-(phenylene-vinylene) derivative has been reported by Gao et al. [30]. A series of blue dopants of dipyrazolopyridine derivative based devices with high brightness, has been fabricated by Tao et al. [31]. Shi et al. show that a stable OLED can be produced using the guest-host of perylene derivative and 9,10-bis(2-naphthyl)anthracene (ADN) [32]. Danel et al. demonstrate that a series of blue emitting devices with high brightness can be fabricated using anthracene derivatives [33]. A highly efficient blue OLED based on a new anthracene derivative has been reported by Kan et al. [34,35]. Gebeyehu et al. report that a highly efficient deep-blue OLED can be fabricated using a spiroanthracene compound [36]. A bright blue emission OLED based on the phenanthroline derivative compound has been reported by Wang et al. [37]. Tao et al. report that highly efficient non-doped blue OLEDs can be produced based on fluorene derivatives [38]. A series of deep blue dopants based on mono(styryl)amine derivatives has been synthesized and characterized by Lee et al. [39]. New benzo[b] furans based blue OLEDs with high brightness have been studied by Hwu et al. [40]. Several blue emitters have been reported by researchers based on the above mention. Despite extensive RGB fluorescence dye development, considering EL performances and mass production cost of OLEDs, simultaneous high quality RGB light emitting respective from RGB dopants by a single host material is considerably more valuable for full-color OLED fabrication. However, only a few single host materials doped RGB emitters for obtaining efficient RGB OLEDs have been reported. Liu et al. fabricated the efficient RGB OLEDs using ADN as the host emitting material [41]. Wu et al. recently reported a blue host material of pyrimidine-containing spirobifluorene derivative 2,7-bis[2-(4tert-butylphenyl)pyrimidine-5-yl]-9,9′-spirobifluorene (TBPSF) doped with blue dye perylene [42]. High brightness and current
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efficiency were observed for the perylene doped TBPSF based blue device. Moreover, the TBPSF material shows a high Tg at temperature around 195 °C [42]. Nevertheless, high brightness of blue emission was only observed under high applied voltage. A high turn-on voltage of 10 V was observed for the TBPSF based devices. In addition to fluorescence dye intrinsic properties, OLED device configuration and fabrication process optimization are critical for obtaining excellent EL performances. Moreover, a well aligned energy level between the host material and RGB dopants facilitates the Forster resonance energy transfer, particularly important for the single host doped RGB emitter in OLEDs. This work reports efficient fluorescent RGB OLEDs fabrication using a blue host material of TBPSF respective doped with blue dye perylene, green dye C545T, and red dye DCJTB, C545T, and perylene are chosen as the RGB dopants of the single host doped devices due to the excellent aligned energy level between dopants and TBPSF and good vacuum deposition control of dopants. High brightness, large current efficiency, and good color purity of RGB doped TBPSF-based OLEDs, were expected by the configuration optimization device. 2. Experimental details 2.1. OLED materials and device configurations Fig. 1 shows the TBPSF chemical structure, synthesized according to the literature [42]. Configurations of OLED were ITO glass/CuPc/NPB/TBPSF/TPBI or Alq3/LiF/Al and ITO glass/CuPc or LiF/NPB/TBPSF:dopant/TPBI/LiF/Al. The energy diagram of multilayer OLED device is also shown in Fig. 1. Other organic materials were purchased from Aldrich
Fig. 1. Chemical structure of TBPSF and energy diagram of multilayer EL devices, respectively.
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Co. and used as received. Copper phthalocyanine (CuPc) and lithium fluoride (LiF) were used as the hole-injection layers (HIL). NPB and TPBI, which are abbreviations of N,N′-bis-(1naphthl)-diphenyl-1,1′-biphenyl-4,4′-diamine and 2,2′,2”(1,3,5-benzenetriyl)tris[1-phenyl-1H-benimidazole] were used as the hole-transporting (HTL) and HBL, respectively. Alq3 was used as the ETL. TBPSF was used as the light emitting layer (LEL) of host-emitting non-doped blue device or the host material of RGB dopants based devices. DCJTB, C545T, and perylene were used as the dopants of RGB devices, respectively. Rubrene was used as the co-host material of DCJTB doped red device. 2.2. OLEDs fabrication and EL property measurements Fabrication of OLEDs was conducted via high-vacuum thermal evaporation of the organic material onto pre-cleaned indium tin oxide (ITO)-coated glass. ITO-coated glass with a sheet resistance of 15 Ω/sq was purchased from Applied Film Corp. The ITO pretreatment includes a routine chemical cleaning using detergent and alcohol in sequence, followed by oxygen-plasma cleaning. Furthermore, a LiF layer is thermally deposited onto the ETL (TPBI or Alq3), followed by Al metal deposition as the top layer in a high vacuum chamber. After electrode deposition, the OLED was transferred from the evaporation chamber to a glove box purged by high purity nitrogen gas to keep oxygen and moisture levels below 1 ppm. The device was encapsulated by glass covers and sealed with UV-cured epoxy glue in the glove box. The cathode deposition rate was determined with a quartz thickness monitor (STM-100/ MF, Sycon). Thin film thickness was determined with a surface texture analysis system (3030ST, Dektak). The UV–vis spectra of organic thin films were measured using a Hewlett-Packard 8453 with a photodiode array detector. Photoluminescence (PL) and EL spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer. Current–voltage characteristics were measured on a programmable electrometer with current and voltage sources (Keithley 2400). Luminance was measured with a BM9 luminance meter (Topcon). 3. Results and discussion PL spectrum of the TBPSF in thin film state covers the wavelength range of 375 to 550 nm and exhibits two main peaks located at 425 and 455 nm [42]. The presence of PL emission peak covers the wavelength range of 375 to 550 nm suggestive of good host emitting material of RGB dopants, such as, perylene, C545T, and DCJTB materials. 3.1. EL properties of TBPSF-based non-doped blue OLEDs The EL properties of the TBPSF-based non-doped blue OLEDs A1 and A2 are shown in Fig. 2. AlQ3 and TPBI were used as the ETL for devices A1 and A2, respectively. CuPc and NPB were used as the HIL and HTL for both devices, respectively. Results indicate that the brightness and current efficiency of device A2 are much higher than device A1, even
Fig. 2. Electroluminescence properties of TBPSF based non-doped blue OLEDs (device configuration: ITO/CuPc(15 nm)/NPB(40 nm)/TBPSF(35 nm)/ETL (10 nm)/LiF(0.5 nm)/Al(100 nm); device A1: ETL =Alq3; device A2: ETL =TPBI).
though current densities of both devices are nearly the same, suggesting that the electron-hole pair is more easily confined in the TBPSF layer as the ETL Alq3 was instead of TPBI. Although the difference of thin film ionization potential (Ip) of TBPSF (~ 6.1 eV) and Alq3 (Ip ~ 5.7 eV) is larger than the TBPSF and TPBI (6.2 eV), TPBI shows a better hole blocking (HB) characteristic than Alq3 [42]. The HB characteristic is presumably due to the better hole-transporting capacity of Alq3 than TPBI [42]. In addition, current efficiency starts to decay at elevated current densities or driving voltages. This phenomenon is usually observed in small molecule based OLED devices or light emitting polymer based devices [43–45]. Current efficiency decay is presumably due to unbalanced charges of electron and hole in the device at elevated driving voltages. Moreover, we supposed that partial device degradation due to high joule heat or defect formation in the device contributed to this phenomenon [46–48]. Brightness and current efficiency of the TPBSF based non-doped device A2 reached 7394 cd/m2 and 2.5 cd/A after device configuration optimization. Brightness turn-on voltage can turn down to 3.5 V. The EL spectra of A1 and A2 devices also varied with increasing applied voltage as shown in Fig. 3. The EL spectra for the A1 device cover the wavelength range of 400 to 600 nm and exhibits main and shoulder peaks located at 460 and 490 nm, respectively. The main peak is attributed to TBPSF emission. A shoulder peak at around 500 nm is partially contributed from Alq3 emission. Moreover, the EL spectra of device A1 did not change with increasing applied voltage. The electroluminescence CIE coordinates (x = 0.15, y = 0.20) of
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Fig. 3. Electroluminescence spectra of TBPSF based non-doped blue OLEDs under different applied voltages (devices A1 and A2).
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Fig. 4. Electroluminescence properties of perylene doped blue OLEDs using a blue host material of TBPSF (device configuration: ITO/CuPc(15 nm)/NPB (40 nm)/TBPSF: perylene (35 nm, perylene X %)/TPBI (10 nm)/LiF(0.5 nm)/Al (100 nm); device B1: perylene X = 3%; device B2: perylene X = 5%).
device A1 did not change as driving voltage increased from 8 V to 12 V. On the other hand, the EL spectra of device A2 slightly blue shifted with increasing applied voltage. A shoulder peak at around 500 nm was not observed for device A2. The electroluminescence CIE coordinates for device A2 changed only slightly (x = 0.15, y = 0.13) to (x = 0.15, y = 0.16). The TBPSF-based non-doped blue device A2 shows excellent color stability under changing applied voltages.
(x = 0.272, y = 0.242) to (x = 0.279, y = 0.240) for the perylene doped blue device as the driving voltage varies from 6 to 10 V. Perylene aggregation and concentration effects result in lower blue color purity compared to the non-doped TBPSF based device.
3.2. EL properties of perylene doped blue OLED
3.3. EL properties of C545T doped green OLED
The EL properties of perylene doped TBPSF blue devices B1 and B2 are shown in Fig. 4. Devices B1 and B2 were respectively doped with 3 and 5 wt.% of perylene in the LEL of TBPSF. CuPc and NPB were used as the HIL and HTL for both devices, respectively. Brightness turn-on voltage was observed at voltage nearly 2.8 V. Brightness and current efficiency varied significantly with increasing dopant contents. Higher brightness and greater current efficiency were obtained for device B2 as compared to devices B1. Brightness and current efficiency for device B2 can be as high as 10117 cd/m2 and 2.97 cd/A, respectively. The EL spectra vary with increasing applied voltage for device B2 as shown in Fig. 5. The EL spectra covering the wavelength range of 475 to 600 nm exhibit two main peaks located at 455 and 485 nm. This exhibition reveals that the emission of host material TBPSF completely converts into blue dopant perylene via the Forster energy transfer process. The electroluminescence CIE coordinates change only slightly from
The EL properties as a function of the applied voltage for the C545T-doped TBPSF green devices C1 and C2 are shown in
Fig. 5. Electroluminescence spectra of perylene doped blue OLED device B2 using a blue host material of TBPSF under different applied voltages.
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Fig. 6. Electroluminescence properties of C545T doped green OLEDs using a blue host material of TBPSF (device configuration: ITO/HIL/NPB(40 nm)/ TBPSF: C545T (35 nm; C545T: 1.5%)/TPBI (10 nm)/LiF(0.5 nm)/Al(100 nm); device C1: HIL = CuPc (15 nm); device C2: HIL = LiF (0.5 nm)).
Fig. 6. TPBI and NPB were used as the ETL and HTL for both devices, respectively. Device C2 with HIL LiF shows a higher current density than device C1 with HIL CuPc at a given voltage. Brightness turn-on voltage is observed nearly at 2.5 V for both devices C1 and C2. Maximal brightness and current efficiency of 28000 cd/m2 and 6.8 cd/A are observed for the device C1, respectively. Brightness for the device C2, reached 7000 cd/m2 at a driving voltage of 10 V. The maximal current efficiency (13.0 cd/A) occurred at a driving voltage of 2.7 V. Since introducing the electron accepting pyrimidine moiety, a
Fig. 7. EL spectra of C545T doped green OLED device C2 using a blue host material of TBPSF under different applied voltages.
Fig. 8. Electroluminescence properties of DCJTB doped red OLEDs using a blue host material of TBPSF (device configuration: ITO/LiF (0.5 nm)/NPB (40 nm)/TBPSF: rubrene: DCJTB (49:49:2; X nm)/TPBI (10 nm)/LiF(0.5 nm)/ Al(100 nm); device D1: X = 35; device D2: X = 25).
relatively large electron affinity (EA) of 3.0 eV is observed for the TBPSF. Such an EA value is comparable to that of the typical electron-transporting material Alq3 [42]. This suggests that TBPSF is a poor hole-transporting material. Moreover, inserting HIL LiF between ITO electrode and HTL NPB is more favorable for the hole-injection into light emitting layer as compared to that of the HIL CuPc [49]. As a result, device C2 has a better electron-hole balance than device C1. Higher brightness and larger current efficiency were observed for the
Fig. 9. EL spectra of DCJTB doped red OLEDs using a blue host material of TBPSF (device configuration: ITO/LiF (0.5 nm)/NPB(40 nm)/TBPSF: rubrene: DCJTB (50–0.5X:50–0.5X:X; 35 nm)/TPBI (10 nm)/LiF(0.5 nm)/Al(100 nm); device D2: X = 2%; device D3: X = 8%).
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device C2 in comparison with device C1 as the driving voltage below 5 V. However, excess amount of hole-injection is not helpful for the charge balance of device C2. Therefore, the current efficiency of device C2 decreases significantly as driving voltage increased above 5 V. The EL spectra vary with increasing applied voltage for device C2 are shown in Fig. 7. The absorption band of C545T nearly overlaps with the EL band of TBPSF based device A2 [23,24,42]. This overlap reveals a well aligned energy level between the green dopant C545T and host material TBPSF, favorable for the Forster resonance energy transfer between the dopant and host materials. The maximal emission peak of the C545T-doped TBPSF device C2 occurs at a wavelength of 510 nm. No emission peak of TBPSF is observed for the device C2, indicating that TBPSF emission is completely absorbed by the dopant C545T via the Forster energy transfer processes. The electroluminescence CIE coordinates change only slightly from (x = 0.19, y = 0.62) to (x = 0.20, y = 0.63) as driving voltage varies from 8 to 10 V. A similar EL spectrum is observed for the device C1. 3.4. EL properties of DCJTB-doped red OLED DCJTB doped red devices were fabricated using TBPSF and rubrene with equal deposition ratio as co-host materials. Two percent of DCJTB was doped into a light-emitting layer as the red emitter. EL properties of the 2% DCJTB-doped TBPSF red devices D1 and D2 with different LEL thickness (35 and 25 nm) are shown in Fig. 8. TPBI and NPB were used as the ETL and HTL for both devices, respectively. A higher current density and larger brightness were obtained for the device D2 compared with device D1 due to the reduction of light emitting layer thickness. However, thinner light emitting layer thickness results in lower current efficiency for device D2 compared with device D1. This is attributed to excess carriers, and less carrier recombination sites in the light-emitting layer. Reduced light emitting layer thickness is also favorable for reduced brightness turn-on voltage from 3 V to 2.4 V. Maximal brightness of 9000 cd/m2 was obtained for the device D2. The DCJTB doped TBPSF device exhibited excellent red emission performance due to good TBPSF to DCJTB Forster energy transfer using the rubrene co-host. Two and eight percents of DCJTB were doped into a lightemitting layer containing TBPSF and rubrene with equal deposition ratio as the red emitter for the devices D2 and D3, respectively. The EL spectra of devices D2 and D3 are shown in Fig. 9. The absorption band of DCJTB partially overlaps with the EL band of TBPSF based device A2 [19,20,42]. Therefore, using rubrene as the co-host material facilitates Foster energy transfer from host material TBPSF to the DCJTB dopant. Maximal EL emission peak of 2% DCJTB-doped TBPSF based device D2 occurred at a wavelength of 590 nm as shown in Fig. 9. The electroluminescence CIE coordinates (x = 0.53–54, y = 0.45–46) were obtained as a driving voltage range from 8 V to 12 V. Yellow–orange emission was observed for the 2% DCJTB-doped TBPSF device D2. Device D2 shows lower color purity than the one based on the host material of Alq3 [19,20]. However, the EL emission of host material TBPSF at
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wavelength of 450 nm almost disappears due to the completely absorbed of DCJTB via Foster energy transfer process. In fact, the color purity of DCJTB doped TBPSF device was dopant concentration dependent. Fig. 9 shows the maximal emission peak red shift from 590 nm to 620 nm with increasing DCJTB concentration from 2% to 8%. The electroluminescence CIE coordinates (x = 0.62, y = 0.37) of deep red emission were observed for the 8% DCJTB doped TBPSF device D3. As in most cases, brightness and current efficiency decreased significantly with increasing red dopant concentration due to the concentration quenching effect. 4. Conclusion Efficient fluorescent RGB OLEDs have been produced using a blue host material of TBPSF doped with respective RGB dyes. High brightness, large current efficiency, and good color purity of TBPSF-based RGB OLEDs are due to excellent energy transfer from the TBPSF host material to guest dopants. EL performances of OLED emissions are determined by respective RGB dopants. TBPSF is an excellent blue host material for RGB doped OLEDs. RGB doped TBPSF-based devices have potential for commercial application of flat panel displays. Acknowledgment The authors thank the Ministry of Economic Affairs of Taiwan, ROC, for financial support. References [1] C.W. Tang, S.A. VanSlyke, C.H. Chen, J. Appl. Phys. 65 (1989) 3610. [2] V. Bulovic, A. Shoustikov, M.A. Baldo, E. Bose, V.G. Kozlov, M.E. Thompson, S.R. Forrest, Chem. Phys Lett. 287 (1998) 455. [3] Y. Hamada, H. Kanno, T. Tsujioka, H. Takahashi, T. Usuki, Appl. Phys. Lett. 75 (1999) 1682. [4] C.H. Chen, C.W. Tang, J. Shi, K.P. Klubek, Thin Solid Films 363 (2000) 327. [5] J. Feng, F. Li, W. Gao, G. Cheng, W. Xie, S. Liu, Appl. Phys. Lett. 81 (2002) 2935. [6] B. Chen, X. Lin, L. Cheng, C.S. Lee, W.A. Gambling, S.T. Lee, J. Phys., D, Appl. Phys. 23 (2001) 30. [7] M. Mitsuya, T. Suzuki, T. Koyama, H. Shirai, Y. Taniguchi, M. Satsuki, S. Suga, Appl. Phys. Lett. 77 (2000) 3272. [8] X.H. Zhang, B.J. Chen, X.Q. Lin, O.Y. Wong, C.S. Lee, H.L. Kwong, S.T. Lee, S.K. Wu, Chem. Mater. 13 (2001) 1565. [9] B.J. Jung, C.B. Yoon, H.K. Shim, L.M. Do, T. Zyung, Adv. Fun. Mater. 11 (2001) 430. [10] C.Q. Ma, Z. Liang, X.S. Wang, B.W. Zhang, Y. Cao, L.D. Wang, Y. Qiu, Synth. Met. 138 (2003) 537. [11] X.T. Tao, S. Miyata, H. Sasabe, G.J. Zhang, T. Wada, M.H. Jiang, Appl. Phys. Lett. 78 (2001) 279. [12] J. Li, D. Liu, Z. Hong, S. Tong, P. Wang, C. Ma, O. Lengyel, C.S. Lee, H.L. Kwong, S. Lee, Chem. Mater. 15 (2003) 486. [13] P.E. Burrows, S.R. Forrest, S.P. Silbey, M.E. Thompson, Appl. Phys. Lett. 69 (1996) 2959. [14] Y. Sakakibara, S. Okutsu, T. Enokida, T. Tani, Thin Solid Films 363 (2000) 29. [15] X.H. Zhang, Z.Y. Xie, F.P. Wu, L.L. Zhou, O.Y. Wong, C.S. Lee, H.L. Kwong, S.T. Lee, S.K. Wu, Chem. Phys. Lett. 382 (2003) 561. [16] T. Mori, K. Miyachi, T. Kichimi, T. Mizutani, Jpn. J. Appl. Phys. 33 (1994) 6594.
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Thin Solid Films 516 (2008) 5062 – 5068 www.elsevier.com/locate/tsf
Efficient fluorescent red, green, and blue organic light-emitting devices with a blue host of spirobifluorene derivative Rong-Ho Lee a,⁎, Yu-Wei Huang a , Ying-Yu Wang a , Hsien-Yu Chang b a
Department of Chemical and Material Engineering, National Yunlin University of Science & Technology, Yunlin 640, Taiwan, ROC b EChem Hightech CO., LTD, Hsin-Chu Industrial Park, Hu-Kou, Hsin-Chu, Taiwan, ROC Received 19 May 2007; received in revised form 20 December 2007; accepted 12 February 2008 Available online 19 February 2008
Abstract Efficient fluorescent blue, green, and red (RGB) organic light-emitting devices (OLEDs) were fabricated using a blue host material of pyrimidine-containing spirobifluorene derivative 2,7-bis[2-(4-tert-butylphenyl)pyrimidine-5-yl]-9,9′-spirobifluorene (TBPSF) doped with blue dye perylene, green dye 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l] pyrano[6,7,8-ij] quinolizin-11-one (C545T), and red dye 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), respectively. The brightness and current efficiency of the perylene doped blue device reached 10117 cd/m2 and 2.97 cd/A. Green emission of the C545T doped device reached 8500 cd/m2 and 13.0 cd/A. Red emission of the DCJTB doped device can be as high as 9000 cd/m2 and 2.0 cd/A, respectively. High color purity of the blue (Commission Internationale de L′Eclairage (CIEx,y) coordinates (CIE, x = 0.27, y = 0.24)), green (CIE, x = 0.19, y = 0.63) and red (CIE, x = 0.62, y = 0.37) emissions were achieved for RGB dyes doped TBPSF OLEDs. High brightness, large current efficiency, and good color purity of TBPSF-based RGB OLEDs were obtained by the configuration optimization device, such as inserting the hole and electron-injection materials, and suitable dopant content and light emitting layer thickness. © 2008 Elsevier B.V. All rights reserved. Keywords: Organic light emitting device; Fluorescence dye; Spirobifluorene derivative; Electroluminescence
1. Introduction Since Tang et al. first applied a light emitting layer doped with guest fluorescent dye to organic light-emitting devices (OLEDs), OLEDs have attracted growing interest for full color flat panel display applications [1]. The emitting color is primarily determined by the guest fluorescent dye molecule in the emitting layer. Researchers have made much progress in color emissions for fluorescent dye doped OLEDs [2–40]. Nevertheless, simultaneous high quality red, green, and blue (RGB) emissions are not easily achieved respectively from the RGB dopants using single host material via Forster energy transfer processes. Numerous red fluorescence dyes have been synthesized and studied for fabricating an excellent red OLED, including pyran, ⁎ Corresponding author. Tel.: +886 5 5342601x4625; fax: +886 5 5312071. E-mail address: [email protected] (R.-H. Lee). 0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.02.010
chromene, and isophorene containing 4-(dicyanomethylene)-2methyl-6-[4-(dimethylaminostyryl)-4H-pyran] (DCM) type red dyes [2–12], porphyrin-type macrocyclic red dyes [13–15], and squarylium dyes [16–18]. Among these red fluorescence dyes, DCM type dyes remain the most efficient material when applied for the red OLEDs, particularly for the dye namely (4(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9enyl)-4H-pyran) (DCJTB) [4,5]. However, the DCM type dye doped device with high doping concentration usually shows low brightness and efficiency due to concentration quenching [2]. This quenching effect is solved by the so-called “assist dopant or co-host” method [3,19,20]. 5,6,11,12-tetraphenylnaphthacene (Rubrene) was added as the assist dopant with DCM-type red emitter or co-host with the host material for red OLEDs fabrication [3,19,20]. Thus, DCJTB doped red device with good color purity, high brightness, and large current efficiency have been reported by several research groups and suitable for practical use [19,20]. In addition, highly efficiency DCJTB-based
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pure red electroluminescence (EL) was recently realized by doping the red dopant in green host Tri-(8-hydroxyquinoline) aluminum (Alq3) in the presence of an electron transporting/hole blocking layer (ETL/HBL) of 2,9-dimethyl-4,7-diphenyl-9,10phenanthroline [21]. On the other hand, the fluorescence dyes doped green OLEDs with excellent EL emission have also been studied extensively, including coumarin derivatives [1,22–24], quinacridone derivatives [25], quinoline derivatives [26], and carbazole derivatives [27]. The most frequently used green dopant among these green fluorescence dyes in the commercial OLED panel is the coumarin derivative dye 10-(2-benzothiazolyl)1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l] pyrano[6,7,8-ij] quinolizin-11-one (C545T) [23,24]. Although several green host-emitting materials have been developed by researchers [28], the green emitting material Alq3 is usually used as the host material for green fluorescence dyes doped OLEDs [1,23,24]. In addition to red and green fluorescence dyes, the blue emitter with excellent EL performance is also important for color OLEDs fabrication, with numerous blue fluorescence dyes having been synthesized and studied [29–40]. Hosokawal et al. report that bright blue OLEDs can be produced using a host emitting of 1,4-bis(2,2-diphenylvinyl)biphenyl with blue dopants [29]. A bright blue OLED based on the dopant of silylsubstituted ter-(phenylene-vinylene) derivative has been reported by Gao et al. [30]. A series of blue dopants of dipyrazolopyridine derivative based devices with high brightness, has been fabricated by Tao et al. [31]. Shi et al. show that a stable OLED can be produced using the guest-host of perylene derivative and 9,10-bis(2-naphthyl)anthracene (ADN) [32]. Danel et al. demonstrate that a series of blue emitting devices with high brightness can be fabricated using anthracene derivatives [33]. A highly efficient blue OLED based on a new anthracene derivative has been reported by Kan et al. [34,35]. Gebeyehu et al. report that a highly efficient deep-blue OLED can be fabricated using a spiroanthracene compound [36]. A bright blue emission OLED based on the phenanthroline derivative compound has been reported by Wang et al. [37]. Tao et al. report that highly efficient non-doped blue OLEDs can be produced based on fluorene derivatives [38]. A series of deep blue dopants based on mono(styryl)amine derivatives has been synthesized and characterized by Lee et al. [39]. New benzo[b] furans based blue OLEDs with high brightness have been studied by Hwu et al. [40]. Several blue emitters have been reported by researchers based on the above mention. Despite extensive RGB fluorescence dye development, considering EL performances and mass production cost of OLEDs, simultaneous high quality RGB light emitting respective from RGB dopants by a single host material is considerably more valuable for full-color OLED fabrication. However, only a few single host materials doped RGB emitters for obtaining efficient RGB OLEDs have been reported. Liu et al. fabricated the efficient RGB OLEDs using ADN as the host emitting material [41]. Wu et al. recently reported a blue host material of pyrimidine-containing spirobifluorene derivative 2,7-bis[2-(4tert-butylphenyl)pyrimidine-5-yl]-9,9′-spirobifluorene (TBPSF) doped with blue dye perylene [42]. High brightness and current
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efficiency were observed for the perylene doped TBPSF based blue device. Moreover, the TBPSF material shows a high Tg at temperature around 195 °C [42]. Nevertheless, high brightness of blue emission was only observed under high applied voltage. A high turn-on voltage of 10 V was observed for the TBPSF based devices. In addition to fluorescence dye intrinsic properties, OLED device configuration and fabrication process optimization are critical for obtaining excellent EL performances. Moreover, a well aligned energy level between the host material and RGB dopants facilitates the Forster resonance energy transfer, particularly important for the single host doped RGB emitter in OLEDs. This work reports efficient fluorescent RGB OLEDs fabrication using a blue host material of TBPSF respective doped with blue dye perylene, green dye C545T, and red dye DCJTB, C545T, and perylene are chosen as the RGB dopants of the single host doped devices due to the excellent aligned energy level between dopants and TBPSF and good vacuum deposition control of dopants. High brightness, large current efficiency, and good color purity of RGB doped TBPSF-based OLEDs, were expected by the configuration optimization device. 2. Experimental details 2.1. OLED materials and device configurations Fig. 1 shows the TBPSF chemical structure, synthesized according to the literature [42]. Configurations of OLED were ITO glass/CuPc/NPB/TBPSF/TPBI or Alq3/LiF/Al and ITO glass/CuPc or LiF/NPB/TBPSF:dopant/TPBI/LiF/Al. The energy diagram of multilayer OLED device is also shown in Fig. 1. Other organic materials were purchased from Aldrich
Fig. 1. Chemical structure of TBPSF and energy diagram of multilayer EL devices, respectively.
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Co. and used as received. Copper phthalocyanine (CuPc) and lithium fluoride (LiF) were used as the hole-injection layers (HIL). NPB and TPBI, which are abbreviations of N,N′-bis-(1naphthl)-diphenyl-1,1′-biphenyl-4,4′-diamine and 2,2′,2”(1,3,5-benzenetriyl)tris[1-phenyl-1H-benimidazole] were used as the hole-transporting (HTL) and HBL, respectively. Alq3 was used as the ETL. TBPSF was used as the light emitting layer (LEL) of host-emitting non-doped blue device or the host material of RGB dopants based devices. DCJTB, C545T, and perylene were used as the dopants of RGB devices, respectively. Rubrene was used as the co-host material of DCJTB doped red device. 2.2. OLEDs fabrication and EL property measurements Fabrication of OLEDs was conducted via high-vacuum thermal evaporation of the organic material onto pre-cleaned indium tin oxide (ITO)-coated glass. ITO-coated glass with a sheet resistance of 15 Ω/sq was purchased from Applied Film Corp. The ITO pretreatment includes a routine chemical cleaning using detergent and alcohol in sequence, followed by oxygen-plasma cleaning. Furthermore, a LiF layer is thermally deposited onto the ETL (TPBI or Alq3), followed by Al metal deposition as the top layer in a high vacuum chamber. After electrode deposition, the OLED was transferred from the evaporation chamber to a glove box purged by high purity nitrogen gas to keep oxygen and moisture levels below 1 ppm. The device was encapsulated by glass covers and sealed with UV-cured epoxy glue in the glove box. The cathode deposition rate was determined with a quartz thickness monitor (STM-100/ MF, Sycon). Thin film thickness was determined with a surface texture analysis system (3030ST, Dektak). The UV–vis spectra of organic thin films were measured using a Hewlett-Packard 8453 with a photodiode array detector. Photoluminescence (PL) and EL spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer. Current–voltage characteristics were measured on a programmable electrometer with current and voltage sources (Keithley 2400). Luminance was measured with a BM9 luminance meter (Topcon). 3. Results and discussion PL spectrum of the TBPSF in thin film state covers the wavelength range of 375 to 550 nm and exhibits two main peaks located at 425 and 455 nm [42]. The presence of PL emission peak covers the wavelength range of 375 to 550 nm suggestive of good host emitting material of RGB dopants, such as, perylene, C545T, and DCJTB materials. 3.1. EL properties of TBPSF-based non-doped blue OLEDs The EL properties of the TBPSF-based non-doped blue OLEDs A1 and A2 are shown in Fig. 2. AlQ3 and TPBI were used as the ETL for devices A1 and A2, respectively. CuPc and NPB were used as the HIL and HTL for both devices, respectively. Results indicate that the brightness and current efficiency of device A2 are much higher than device A1, even
Fig. 2. Electroluminescence properties of TBPSF based non-doped blue OLEDs (device configuration: ITO/CuPc(15 nm)/NPB(40 nm)/TBPSF(35 nm)/ETL (10 nm)/LiF(0.5 nm)/Al(100 nm); device A1: ETL =Alq3; device A2: ETL =TPBI).
though current densities of both devices are nearly the same, suggesting that the electron-hole pair is more easily confined in the TBPSF layer as the ETL Alq3 was instead of TPBI. Although the difference of thin film ionization potential (Ip) of TBPSF (~ 6.1 eV) and Alq3 (Ip ~ 5.7 eV) is larger than the TBPSF and TPBI (6.2 eV), TPBI shows a better hole blocking (HB) characteristic than Alq3 [42]. The HB characteristic is presumably due to the better hole-transporting capacity of Alq3 than TPBI [42]. In addition, current efficiency starts to decay at elevated current densities or driving voltages. This phenomenon is usually observed in small molecule based OLED devices or light emitting polymer based devices [43–45]. Current efficiency decay is presumably due to unbalanced charges of electron and hole in the device at elevated driving voltages. Moreover, we supposed that partial device degradation due to high joule heat or defect formation in the device contributed to this phenomenon [46–48]. Brightness and current efficiency of the TPBSF based non-doped device A2 reached 7394 cd/m2 and 2.5 cd/A after device configuration optimization. Brightness turn-on voltage can turn down to 3.5 V. The EL spectra of A1 and A2 devices also varied with increasing applied voltage as shown in Fig. 3. The EL spectra for the A1 device cover the wavelength range of 400 to 600 nm and exhibits main and shoulder peaks located at 460 and 490 nm, respectively. The main peak is attributed to TBPSF emission. A shoulder peak at around 500 nm is partially contributed from Alq3 emission. Moreover, the EL spectra of device A1 did not change with increasing applied voltage. The electroluminescence CIE coordinates (x = 0.15, y = 0.20) of
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Fig. 3. Electroluminescence spectra of TBPSF based non-doped blue OLEDs under different applied voltages (devices A1 and A2).
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Fig. 4. Electroluminescence properties of perylene doped blue OLEDs using a blue host material of TBPSF (device configuration: ITO/CuPc(15 nm)/NPB (40 nm)/TBPSF: perylene (35 nm, perylene X %)/TPBI (10 nm)/LiF(0.5 nm)/Al (100 nm); device B1: perylene X = 3%; device B2: perylene X = 5%).
device A1 did not change as driving voltage increased from 8 V to 12 V. On the other hand, the EL spectra of device A2 slightly blue shifted with increasing applied voltage. A shoulder peak at around 500 nm was not observed for device A2. The electroluminescence CIE coordinates for device A2 changed only slightly (x = 0.15, y = 0.13) to (x = 0.15, y = 0.16). The TBPSF-based non-doped blue device A2 shows excellent color stability under changing applied voltages.
(x = 0.272, y = 0.242) to (x = 0.279, y = 0.240) for the perylene doped blue device as the driving voltage varies from 6 to 10 V. Perylene aggregation and concentration effects result in lower blue color purity compared to the non-doped TBPSF based device.
3.2. EL properties of perylene doped blue OLED
3.3. EL properties of C545T doped green OLED
The EL properties of perylene doped TBPSF blue devices B1 and B2 are shown in Fig. 4. Devices B1 and B2 were respectively doped with 3 and 5 wt.% of perylene in the LEL of TBPSF. CuPc and NPB were used as the HIL and HTL for both devices, respectively. Brightness turn-on voltage was observed at voltage nearly 2.8 V. Brightness and current efficiency varied significantly with increasing dopant contents. Higher brightness and greater current efficiency were obtained for device B2 as compared to devices B1. Brightness and current efficiency for device B2 can be as high as 10117 cd/m2 and 2.97 cd/A, respectively. The EL spectra vary with increasing applied voltage for device B2 as shown in Fig. 5. The EL spectra covering the wavelength range of 475 to 600 nm exhibit two main peaks located at 455 and 485 nm. This exhibition reveals that the emission of host material TBPSF completely converts into blue dopant perylene via the Forster energy transfer process. The electroluminescence CIE coordinates change only slightly from
The EL properties as a function of the applied voltage for the C545T-doped TBPSF green devices C1 and C2 are shown in
Fig. 5. Electroluminescence spectra of perylene doped blue OLED device B2 using a blue host material of TBPSF under different applied voltages.
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Fig. 6. Electroluminescence properties of C545T doped green OLEDs using a blue host material of TBPSF (device configuration: ITO/HIL/NPB(40 nm)/ TBPSF: C545T (35 nm; C545T: 1.5%)/TPBI (10 nm)/LiF(0.5 nm)/Al(100 nm); device C1: HIL = CuPc (15 nm); device C2: HIL = LiF (0.5 nm)).
Fig. 6. TPBI and NPB were used as the ETL and HTL for both devices, respectively. Device C2 with HIL LiF shows a higher current density than device C1 with HIL CuPc at a given voltage. Brightness turn-on voltage is observed nearly at 2.5 V for both devices C1 and C2. Maximal brightness and current efficiency of 28000 cd/m2 and 6.8 cd/A are observed for the device C1, respectively. Brightness for the device C2, reached 7000 cd/m2 at a driving voltage of 10 V. The maximal current efficiency (13.0 cd/A) occurred at a driving voltage of 2.7 V. Since introducing the electron accepting pyrimidine moiety, a
Fig. 7. EL spectra of C545T doped green OLED device C2 using a blue host material of TBPSF under different applied voltages.
Fig. 8. Electroluminescence properties of DCJTB doped red OLEDs using a blue host material of TBPSF (device configuration: ITO/LiF (0.5 nm)/NPB (40 nm)/TBPSF: rubrene: DCJTB (49:49:2; X nm)/TPBI (10 nm)/LiF(0.5 nm)/ Al(100 nm); device D1: X = 35; device D2: X = 25).
relatively large electron affinity (EA) of 3.0 eV is observed for the TBPSF. Such an EA value is comparable to that of the typical electron-transporting material Alq3 [42]. This suggests that TBPSF is a poor hole-transporting material. Moreover, inserting HIL LiF between ITO electrode and HTL NPB is more favorable for the hole-injection into light emitting layer as compared to that of the HIL CuPc [49]. As a result, device C2 has a better electron-hole balance than device C1. Higher brightness and larger current efficiency were observed for the
Fig. 9. EL spectra of DCJTB doped red OLEDs using a blue host material of TBPSF (device configuration: ITO/LiF (0.5 nm)/NPB(40 nm)/TBPSF: rubrene: DCJTB (50–0.5X:50–0.5X:X; 35 nm)/TPBI (10 nm)/LiF(0.5 nm)/Al(100 nm); device D2: X = 2%; device D3: X = 8%).
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device C2 in comparison with device C1 as the driving voltage below 5 V. However, excess amount of hole-injection is not helpful for the charge balance of device C2. Therefore, the current efficiency of device C2 decreases significantly as driving voltage increased above 5 V. The EL spectra vary with increasing applied voltage for device C2 are shown in Fig. 7. The absorption band of C545T nearly overlaps with the EL band of TBPSF based device A2 [23,24,42]. This overlap reveals a well aligned energy level between the green dopant C545T and host material TBPSF, favorable for the Forster resonance energy transfer between the dopant and host materials. The maximal emission peak of the C545T-doped TBPSF device C2 occurs at a wavelength of 510 nm. No emission peak of TBPSF is observed for the device C2, indicating that TBPSF emission is completely absorbed by the dopant C545T via the Forster energy transfer processes. The electroluminescence CIE coordinates change only slightly from (x = 0.19, y = 0.62) to (x = 0.20, y = 0.63) as driving voltage varies from 8 to 10 V. A similar EL spectrum is observed for the device C1. 3.4. EL properties of DCJTB-doped red OLED DCJTB doped red devices were fabricated using TBPSF and rubrene with equal deposition ratio as co-host materials. Two percent of DCJTB was doped into a light-emitting layer as the red emitter. EL properties of the 2% DCJTB-doped TBPSF red devices D1 and D2 with different LEL thickness (35 and 25 nm) are shown in Fig. 8. TPBI and NPB were used as the ETL and HTL for both devices, respectively. A higher current density and larger brightness were obtained for the device D2 compared with device D1 due to the reduction of light emitting layer thickness. However, thinner light emitting layer thickness results in lower current efficiency for device D2 compared with device D1. This is attributed to excess carriers, and less carrier recombination sites in the light-emitting layer. Reduced light emitting layer thickness is also favorable for reduced brightness turn-on voltage from 3 V to 2.4 V. Maximal brightness of 9000 cd/m2 was obtained for the device D2. The DCJTB doped TBPSF device exhibited excellent red emission performance due to good TBPSF to DCJTB Forster energy transfer using the rubrene co-host. Two and eight percents of DCJTB were doped into a lightemitting layer containing TBPSF and rubrene with equal deposition ratio as the red emitter for the devices D2 and D3, respectively. The EL spectra of devices D2 and D3 are shown in Fig. 9. The absorption band of DCJTB partially overlaps with the EL band of TBPSF based device A2 [19,20,42]. Therefore, using rubrene as the co-host material facilitates Foster energy transfer from host material TBPSF to the DCJTB dopant. Maximal EL emission peak of 2% DCJTB-doped TBPSF based device D2 occurred at a wavelength of 590 nm as shown in Fig. 9. The electroluminescence CIE coordinates (x = 0.53–54, y = 0.45–46) were obtained as a driving voltage range from 8 V to 12 V. Yellow–orange emission was observed for the 2% DCJTB-doped TBPSF device D2. Device D2 shows lower color purity than the one based on the host material of Alq3 [19,20]. However, the EL emission of host material TBPSF at
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wavelength of 450 nm almost disappears due to the completely absorbed of DCJTB via Foster energy transfer process. In fact, the color purity of DCJTB doped TBPSF device was dopant concentration dependent. Fig. 9 shows the maximal emission peak red shift from 590 nm to 620 nm with increasing DCJTB concentration from 2% to 8%. The electroluminescence CIE coordinates (x = 0.62, y = 0.37) of deep red emission were observed for the 8% DCJTB doped TBPSF device D3. As in most cases, brightness and current efficiency decreased significantly with increasing red dopant concentration due to the concentration quenching effect. 4. Conclusion Efficient fluorescent RGB OLEDs have been produced using a blue host material of TBPSF doped with respective RGB dyes. High brightness, large current efficiency, and good color purity of TBPSF-based RGB OLEDs are due to excellent energy transfer from the TBPSF host material to guest dopants. EL performances of OLED emissions are determined by respective RGB dopants. TBPSF is an excellent blue host material for RGB doped OLEDs. RGB doped TBPSF-based devices have potential for commercial application of flat panel displays. Acknowledgment The authors thank the Ministry of Economic Affairs of Taiwan, ROC, for financial support. References [1] C.W. Tang, S.A. VanSlyke, C.H. Chen, J. Appl. Phys. 65 (1989) 3610. [2] V. Bulovic, A. Shoustikov, M.A. Baldo, E. Bose, V.G. Kozlov, M.E. Thompson, S.R. Forrest, Chem. Phys Lett. 287 (1998) 455. [3] Y. Hamada, H. Kanno, T. Tsujioka, H. Takahashi, T. Usuki, Appl. Phys. Lett. 75 (1999) 1682. [4] C.H. Chen, C.W. Tang, J. Shi, K.P. Klubek, Thin Solid Films 363 (2000) 327. [5] J. Feng, F. Li, W. Gao, G. Cheng, W. Xie, S. Liu, Appl. Phys. Lett. 81 (2002) 2935. [6] B. Chen, X. Lin, L. Cheng, C.S. Lee, W.A. Gambling, S.T. Lee, J. Phys., D, Appl. Phys. 23 (2001) 30. [7] M. Mitsuya, T. Suzuki, T. Koyama, H. Shirai, Y. Taniguchi, M. Satsuki, S. Suga, Appl. Phys. Lett. 77 (2000) 3272. [8] X.H. Zhang, B.J. Chen, X.Q. Lin, O.Y. Wong, C.S. Lee, H.L. Kwong, S.T. Lee, S.K. Wu, Chem. Mater. 13 (2001) 1565. [9] B.J. Jung, C.B. Yoon, H.K. Shim, L.M. Do, T. Zyung, Adv. Fun. Mater. 11 (2001) 430. [10] C.Q. Ma, Z. Liang, X.S. Wang, B.W. Zhang, Y. Cao, L.D. Wang, Y. Qiu, Synth. Met. 138 (2003) 537. [11] X.T. Tao, S. Miyata, H. Sasabe, G.J. Zhang, T. Wada, M.H. Jiang, Appl. Phys. Lett. 78 (2001) 279. [12] J. Li, D. Liu, Z. Hong, S. Tong, P. Wang, C. Ma, O. Lengyel, C.S. Lee, H.L. Kwong, S. Lee, Chem. Mater. 15 (2003) 486. [13] P.E. Burrows, S.R. Forrest, S.P. Silbey, M.E. Thompson, Appl. Phys. Lett. 69 (1996) 2959. [14] Y. Sakakibara, S. Okutsu, T. Enokida, T. Tani, Thin Solid Films 363 (2000) 29. [15] X.H. Zhang, Z.Y. Xie, F.P. Wu, L.L. Zhou, O.Y. Wong, C.S. Lee, H.L. Kwong, S.T. Lee, S.K. Wu, Chem. Phys. Lett. 382 (2003) 561. [16] T. Mori, K. Miyachi, T. Kichimi, T. Mizutani, Jpn. J. Appl. Phys. 33 (1994) 6594.
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