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Transparent white organic light-emitting devices with a LiF / Yb: Ag cathode Tianyu Zhang, Letian Zhang, Wenyu Ji, and Wenfa Xie* State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun, 130012, China *Corresponding author:
[email protected] Received October 30, 2008; revised February 27, 2009; accepted March 4, 2009; posted March 9, 2009 (Doc. ID 103473); published April 7, 2009 Transparent white organic light-emitting devices with a LiF / Yb: Ag cathode are reported. The device with a 4 , 4⬘ , 4⬙ -tris(3-methylphenyl-phenylamino)-tripheny-lamine structure of indium-tin oxide/ 共40 nm兲 / N , N⬘-bis-(1-naphthyl)- N , N⬘ -diphenyl- 1 , 1⬘-biph-enyl-4 , 4⬘-diamine (NPB: 7 nm)/rubrene 共0.1 nm兲 / NPB共3 nm兲 / 2 , 2⬘ , 2⬙-(1,3,5-phenylene) tris(1-phenyl- 1H- benzimidazole 共30 nm兲/tris (8hydroxyquinoline) aluminum 共20 nm兲 / LiF 共0.5 nm兲 / Yb: Ag (15 nm, volume ratio 1:1) shows few differences in luminance and color purity from both sides. For instant, at a driving voltage of 10 V, the luminance and CIE coordinates of 1173/ 1488 cd/ m2 and 共0.3482, 0.3596兲 / 共0.3508, 0.3352兲 are obtained from top/bottom sides. CIE coordinates of the device from both sides are voltage independent, and the maximum total electroluminescence efficiencies of the device are 4.6 cd/ A. © 2009 Optical Society of America OCIS codes: 230.3670, 050.2230, 240.0310.
White organic light-emitting devices (WOLEDs) are considered as low-cost alternatives for backlights in liquid-crystal displays and illumination purposes. High-efficiency and high-luminance WOLEDs with different structure or materials have been reported [1–6]. Transparent organic light-emitting devices (TOLEDs) or top-emitting organic light-emitting devices (TEOLEDs) have become interesting because of their potential applications in active-matrix organic light-emitting devices and organic light-emittingdevice microdisplays. Besides, white TOLEDs may be integrated into automobile windshields, architectural windows, and eyewear. TOLEDs or TEOLEDs based on tris(8-hydroxyquinoline) aluminum 共Alq3兲 have also been reported [7–11]. However, there are a few reports on the white TEOLED or TOLED [12–15], and the differences in luminance and color purity for the emissions from both sides are bound to present in TOLED owing to an asymmetric structure of light propagation. Especially, the white TOLED with almost the same luminance and color purity from both sides is very rare owing to the low and uneven transmission of the cathode. Xie et al. reported a highly transparent green TOLED with LiF / Yb: Ag cathode [16]. The device shows almost the same luminance and color purity from both sides. In this Letter, a white TOLED (TWOLED) with LiF / Yb: Ag cathode is demonstrated. The TWOLED shows a little differences in luminance and color purity from its both sides. In addition, the CIE coordinates are voltage independent. We attribute it to the flat and high transmission of the cathode in the visible region. Device with a structure of ITO/4 , 4⬘ , 4⬙ -tris(3methylphenyl-phenylamino)-tripheny-lamine (mMTDATA: 40 nm兲 / N , N⬘ -bis-(1-naphthyl)- N , N⬘ -diphenyl- 1 , 1⬘-biph-enyl-4 , 4⬘-diamine (NPB: 7 nm)/ -(1,3,5rubrene 共0.1 nm兲 / NPB共3 nm兲 / 2 , 2⬘ , 2⬙ phenylene) tris(1-phenyl-1H- benzimidazole (TPBi: 30 nm兲 / Alq3 共20 nm兲 / LiF 共0.5 nm兲 / Yb: Ag (15 nm, 0146-9592/09/081174-3/$15.00
volume ratio 1:1) was fabricated by high-vacuum 共5 ⫻ 10−4 Pa兲 thermal evaporation without breaking the vacuum. m-MTDATA, 7 nm NPB, rubrene, 3 nm NPB, TPBI, and Alq3 were used as hole-injection layer, hole-transport layer, yellow-emitting layer, blue-emitting layer, hole-blocking layer, and electroninjection layer, respectively. Layer thickness of the deposited material was monitored using an oscillating quartz thickness monitor. Deposition rates of both the organic materials (except rubrene layer) and the metal films were controlled to be 1 – 2 Å / s, and the Yb:Ag cathode was coevaporated with a 1 to 1 volume ratio of Yb and Ag. Rubrene molecule was deposited on the NPB layer randomly and has an average thickness of 1 Å. The deposition rate of rubrene layer is 0.1 Å / s. Luminance–current–voltage characteristics and CIE coordinates of the devices were measured simultaneously with a programmable Keithley model 2400 power source and a Photoresearch PR650 spectrometer in air at room temperature. Figure 1 shows the current density–luminance– voltage characteristics of the TWOLED. The inset shows the photographs of emissions from both sides
Fig. 1. Current density–luminance–voltage characteristics of the TWOLED. The inset shows the photographs of the emissions from both sides at a voltage of 10 V. © 2009 Optical Society of America
April 15, 2009 / Vol. 34, No. 8 / OPTICS LETTERS
at 10 V. Pure white emissions are observed from both sides of the device, and there is a little difference in luminance from its both sides. For example, at a driving voltage of 10 V, the luminance from the top and the bottom side are 1173 and 1488 cd/ m2, respectively. Figure 2 shows the electroluminescence (EL) spectra of the device from both sides at 10 V. There are two primary peaks at 448 and 560 nm in the EL spectrum from the top side and two at 440 and 560 nm from the bottom side. We attribute this to the filtering effect by the Yb:Ag film. The emission peaks at 440 (or 448) and 560 nm are from NPB and rubrene. The ultrathin rubrene layer inserted between 7-nm-thick NPB and 3-nm-thick NPB can trap the hole injection from the ITO anode and the electron injection from the cathode; thus, electron–hole recombination in rubrene leads to yellow emission from rubrene. Furthermore, TPBI can block holes efficiently owing to the 0.8 eV energy difference in the highest occupied molecular orbital between NPB and TPBI, thus, electron–hole recombination in NPB leads to blue emission from NPB. Consequently, white emission is obtained by combining blue with yellow emission. It also can be seen that the EL intensity from the top side is less than that from the bottom side, and the intensity decrease of NPB emission is more than that of rubrene emission. To study the reason for the difference in EL spectra from both sides of the TWOLED, the transmission of the ITO–glass substrate and the TWOLED are compared in Fig. 3. The transmission of the substrate is ⬎80% at the visible region, and the TWOLED with Yb:Ag cathode is ⬎60% at the wavelength above 450 nm. We also can see that the transmission spectrum 共 ⬎ 450 nm兲 of the TWOLED with Yb:Ag cathode is flat. The transmission of the device is 58% at 440 nm and 63% at 560 nm. This indicated that more light emitted from rubrene can pass thought the Yb:Ag cathode than from NPB. Furthermore, the transmission of the device decreases rapidly at short wavelengths 共 ⬍ 440 nm兲 with the decrease of the wavelength. This will filter the NPB emission. As a result, the EL peak of NPB from the bottom side is at 440 nm, while that from the top side is at 448 nm,
Fig. 2. EL spectra of the device from both sides of the TWOLED at 10 V.
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Fig. 3. Transmission spectra of ITO glass substrate, TWOLED with Yb:Ag cathode and Mg:Ag cathode.
and the EL intensity of the NPB emission from the top side is less than that from the bottom side. Figure 4 shows the CIE coordinates from both sides of the TWOLED. It can be seen that the CIE coordinates from both sides are almost the same. The CIE coordinates of the top side are (0.3482, 0.3596) at 10 V, the color temperature (CT) is 4900 K, and the color rendering index (CRI) is 60. Also, the CIE coordinates of the bottom side are (0.3508, 0.3352) at 10 V, CT= 4700 K, and CRI= 65. The CRI of the WOLED is low because two complementary colors are utilized to obtain white emission. If using a red– green–blue stacked structure to obtain white emission, the CRI of the WOLED can be improved. Another excellent performance of the device is the voltage-independent characteristics of the CIE coordinates from both sides. With a change of voltage from 4 to 20 V, the CIE coordinates change from (0.3587, 0.3722) to (0.3657, 0.3811) for the top side and from (0.3628, 0.3482) to (0.3657, 0.3524) for the bottom side. Figure 5 shows the EL efficiency of the TWOLED. The maximum EL efficiency from top and bottom sides of the TWOLED are 1.96 and 2.64 cd/ A, respectively. The maximum total EL efficiency of the device is 4.6 cd/ A.
Fig. 4. CIE coordinates from both sides of the TWOLED.
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from both sides, and the CIE coordinates of the device are voltage independent. We attribute this to the flat and high transmission of the Yb:Ag cathode in the visible region. This result provides a guidance in the fabrication of TWOLEDs with optimum performance, such as luminance and color purity from both sides. This work was supported by the National Natural Science Foundation of China (NSFC) (grants 60606017, 60707016, 60723002) and National High Technology Research and Development Program of China (grant 2006AA03A162). References Fig. 5. EL efficiency of the TWOLED.
For comparison, a TWOLED having the same organic layers with Mg:Ag (15 nm, volume ratio 10:1) cathode was fabricated. We found the TWOLED with Mg:Ag cathode gives considerably different luminance from its both sides. At a driving voltage of 10 V, the luminance from the top and bottom sides of the TWOLED with Mg:Ag cathodes are 375 and 2714 cd/ m2, respectively. In other words, only a 27% luminance difference exists between the top and the bottom sides of the TWOLED with Yb:Ag cathode, while over 600% luminance difference exists between the two sides of the TWOLED with Mg:Ag cathode. We also found the TWOLED with Yb:Ag cathode delivers about the same CIE coordinates from both sides, whereas the TWOLED with Mg:Ag cathode shows considerably different CIE coordinates from its both sides. For instance, at a driving voltage of 10 V, the CIE coordinates from the top/bottom side of the TWOLEDs with Yb:Ag and Mg:Ag cathode are 共0.3482, 0.3596兲 / 共0.3508, 0.3352兲 and 共0.3177, 0.3187兲 / 共0.3713, 0.3683兲, respectively. The considerable differences in luminance and color purity from both sides of the TWOLED with Mg:Ag cathode are due to the low and uneven transmission of Mg:Ag cathode (Fig. 3). In summary, we demonstrated a transparent WOLED with a LiF / Yb: Ag cathode. The device shows little difference in luminance and color purity
1. B. W. D’Andrade, M. E. Thompson, and S. R. Forrest, Adv. Mater. (Weinheim, Ger.) 14, 147 (2002). 2. B. W. D’Andrade, J. Brooks, V. Adamovich, M. E. Thompson, and S. R. Forrest, Adv. Mater. (Weinheim, Ger.) 14, 1032 (2002). 3. Y. R. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson, and S. R. Forrest, Nature 440, 908 (2006). 4. W. F. Xie, Y. Zhao, C. N. Li, and S. Y. Liu, Semicond. Sci. Technol. 20, 326 (2005). 5. F. I. Wu, P. I. Shih, Y. H. Tseng, C. F. Shu, Y. L. Tung, and Y. Chi, J. Mater. Chem. 17, 167 (2007). 6. B. W. D’Andrade, R. J. Holmes, and S. R. Forrest, Adv. Mater. (Weinheim, Ger.) 16, 624 (2004). 7. B. J. Chen, X. W. Sun, and S. C. Tan, Opt. Express 13, 937 (2005). 8. G. Gu, V. Bulovic´, P. E. Burrows, S. R. Forrest, and M. E. Thompson, Appl. Phys. Lett. 68, 2606 (1996). 9. A. Yamamori, S. Hayashi, T. Koyama, and Y. Taniguchi, Appl. Phys. Lett. 78, 3343 (2001). 10. G. Parthasarathy, C. Adachi, P. E. Burrows, and S. R. Forrest, Appl. Phys. Lett. 76, 2128 (2000). 11. G. Parthasarathy, P. E. Burrows, V. Khalfin, V. G. Kozlov, and S. R. Forrest, Appl. Phys. Lett. 72, 2138 (1998). 12. S. F. Hsu, C. C. Lee, S. W. Hwang, and C. H. Chen, Appl. Phys. Lett. 86, 253508 (2005). 13. H. Kanno, Y. Sun, and S. R. Forrest, Appl. Phys. Lett. 86, 263502 (2005). 14. W. Y. Ji, L. T. Zhang, R. X. Gao, L. M. Zhang, W. F. Xie, H. Z. Zhang, and B. Li, Opt. Express 16, 15489 (2008). 15. C. J. Lee, R. B. Pode, J. I. Han, and D. G. Moon, Appl. Surf. Sci. 253, 4249 (2007). 16. W. F. Xie, K. C. Lau, C. S. Lee, and S. T. Lee, Thin Solid Films 515, 6975 (2007).
