PIERS ONLINE, VOL. 4, NO. 3, 2008 351 Small Nano-dot Incorporated High-efficiency Phosphorescent Blue Organic Light-emitting Diode Jwo-Huei Jou 1, Wei-Ben Wang 1, Mao-Feng Hsu 1, Chi-Ping Liu 1, Cheng-Chung Chen 1, Chun-Jan Wang 1, Yung-Cheng Tsai 1, Jing-Jong Shyue 2, Sung-Cheng Hu 3, Chung-Che Chiang 4, and He Wang 5 1 Department of Materials Science and Engineering, National Tsing Hua University Hsin-Chu, Taiwan 30013, China 2 Research Center for Applied Sciences Academia Sinica 128 Academia Rd., Sec. 2 Nankang, Taipei 115, Taiwan, China 3 Chung-shan Institute of Scince and Technology, Armament Bureau, M. N. D. No. 481, Sec. Chia An, Zhongzheng Rd., Longtan Shiang, Taoyuan County 325, China 4 Department of Applied Chemistry, National Chi Nan University Nantou Hsien, Taiwan 545, China 5 Department of Materials Science and Engineering, Tsinghua University Beijing 100084, China Abstract High efficiency phosphorescent blue organic light-emitting diode (OLED) was obtained by incorporating small amino or hydroxyl functional group-modified polymeric nano-dot ( or ) in the hole transporting layer (HTL), poly (ethylenedioxythiophene): poly (styrene sulfonic acid) (PEDOT:PSS). The device comprised a 1250 Å anode layer of indium tin oxide, a 350 Å HTL of PEDOT:PSS doped with or, a 400 Å blue emissive layer composed of a molecular host of 4,4 -bis (carbazol-9-yl) biphenyl doped with 14 wt% blue dye of bis (3,5-difluoro-2-(2-pyridyl)-phenyl-(2-carboxypyridyl) iridium (III), a 320 Å electron-transporting layer of 2, 2, 2 -(1, 3, 5-benzenetriyl)-tris (1-phenyl-1-H-benzimidazole), a 7 Å electron-injection layer of lithium fluoride and a 1500 Å cathode layer of aluminum. The resultant power efficiency at 100 cd/m 2, for example, was increased from 12.0 to 25.9 lm/w, an increase of 116%, as 7 wt% of 8 nm in size was added. By employing 7 wt%, the power-efficiency was 21.7 lm/w. The resultant luminance markedly increased with the incorporation of the PND. Whilst,the corresponding current density continuously decreased. These results indicate that the marked efficiency improvement may be attributed to a better balance of carrier-injection resulted from the hole-blocking-function possessed and the hole-trapping-function possessed, which respectively exhibited positive and negative charge on the surface. Moreover, the chromaticity coordinate at 100 cd/m 2, for example, was (0.19, 0.34), barely changed in the presence of the nano-dots. Importantly, since the nano-dot was not employed in the emissive layer, the same concept may be applied to fluorescent blue or other OLEDs. 1. INTRODUCTION Organic light-emitting diodes (OLEDs) are increasingly attracting interest because of their high potential as flat-panel displays and for liquid-crystal-display backlighting and area illumination. [1 4] These applications require highly efficient OLEDs. Numerous approaches have been reported to improve the efficiency, such as the use of electroluminescence (EL) efficient phosphorescent and/or fluorescent materials [4], coupled with appropriate device architectures. Efficient devices typically possess optimized device-thickness, low carrier-injection-barrier, effective carrier/excitonconfinement, highly efficient host-to-guest energy-transfer and balanced carrier-injection. [4 14] Recently, the incorporation of quantum- or nano-dot in the emissive or another layer has been found to be effective for some OLED devices. [13 18] However, the mechanism of this improvement is not yet clear. A homogeneous distribution of the embedded nano-dots may also be crucial, which restrains the use of a dry-process for their incorporation. In order to obtain high efficiency, OLED devices must frequently be kept relatively thin, which would consequently limit the use of large nano-dots. In this letter, we present phosphorescent blue OLEDs with marked efficiency-improvement obtained by incorporating small amino or hydroxyl functional group-modified polymeric nano-dot ( or ) in the hole transporting layer (HTL), poly(ethylenedioxythiophene): poly(styrene
PIERS ONLINE, VOL. 4, NO. 3, 2008 352 sulfonic acid) (PEDOT:PSS). The effect of the concentration of these two polymeric nano-dot (PND) on the electroluminescent (EL) characteristics of the resultant devices was examined. The resultant power efficiency at 100 cd/m 2, for example, was increased from 12.0 to 25.9 lm/w, an increase of 116%, as 7 wt% of 8 nm in size was added. By employing 7 wt%, the power-efficiency was 21.7 lm/w. Al TPBI Blue emissive layer PEDOT:PSS with/ Binding Energy (ev) LUMO 5.2 ITO 2.3 5.1 2.9 2.7 4.3 LiF/Al ITO Blue emission HOMO PEDOT:PSS with/without PND 6.0 CBP 6.2 TPBi FIrpic (3.2, 5.8) Figure 1: The schematic device structure and energy-level diagram of the phosphorescent blue OLEDs. 2. EXPERIMENTAL Figure 1 shows the schematic device structure and energy-level diagram of the phosphorescent blue OLEDs studied. The device comprises a 1250 Å anode layer of indium tin oxide (ITO), a 350 Å hole-injection layer of PEDOT:PSS doped with PNDs, a 400 Å blue emissive layer, a 320 Å electrontransporting layer of 2, 2, 2 -(1, 3, 5-benzenetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), a 7 Å electron-injection layer of lithium fluoride (LiF) and a 1500 Å cathode layer of aluminum (Al). The blue emissive layer was composed of a molecular host of 4, 4 -bis(carbazol-9-yl) biphenyl (CBP) doped with 14 wt% blue dye of bis(3, 5-difluoro-2-(2-pyridyl)-phenyl-(2-carboxypyridyl) iridium (III) (FIrpic). The emission area of all the resultant devices was 25 mm 2 and only the luminance in the forward direction was measured. The resultant electro-luminescent characteristics were determined by using Minolta CS100A luminance meter and KEITHLEY 2400 source meter. All the measurements were carried out at the ambient condition. SiO 2 H2 N H2 N SiO 2 2 nm Figure 2: The schematic molecular structures of the studied and also shown is the TEM image of the. The PNDs were prepared by hydrolysis and condensation of sodium metasilicate. [19] To examine the doping effect, and with size of 8 nm were synthesized. Figure 2 shows a schematic illustration of the molecular structure and transmission electron microscopic (TEM) image of the synthesized PND. The resultant or also exhibited positive or negative charge as determined by the value of their zeta potential measured with a Nano ZS ZEN-3600.
PIERS ONLINE, VOL. 4, NO. 3, 2008 353 3. RESULT AND DISCUSSION Figure 3 shows the power efficiency of the blue OLEDs with and without the incorporation of the 8 nm or. The power efficiency increased as the PND was employed. Without the incorporation of PND, the power efficiency at 100 cd/m 2, for example, was 12.0 lm/w. The power efficiency became 25.9 lm/w, an increase of 116%, as was added. By employing, the power efficiency was 21.7 lm/w. Power Efficiency (lm/w) 30 20 10 0 4 5 6 7 Current Density (ma/cm 2 ) 10 1 10 0 10-1 10-2 10-3 4 5 6 7 Luminance (cd/m 2 ) 10 3 10 2 10 1 10 0 4 5 6 7 Figure 3: Doping effects of the 8 nm PND on the power efficiency, current density and luminance of the blue OLEDs. Figure 3 also shows the effects of the employed PND on the current density and luminance of the blue OLEDs. The current density decreased as or was added, indicating that the PND had effectively reduced the injection of hole-carrier. The incorporation of titanium oxide nano-dot in a separated layer of a green OLED was found to enhance the injection of hole caused by tunneling effect as revealed by the marked increase of current density and decrease of turnon voltage. [11 13] However, in the present work the turn-on voltage did not change much with the incorporation of PND with various different concentrations, revealing the absence of tunneling effect. The size of the PND, 8 nm, was much smaller than the 35 nm thickness of the PEDOT:PSS HTL, so that the PND was presumably well embedded within the HTL. The turn-on voltage described herein was defined as the voltage at which the luminance is equal to or greater than 10 cd/m 2. The resultant luminance, especially at voltage between 4.5 to 6 V, did not decrease, but increased obviously with the incorporation of or. This indicates that higher carrierrecombination efficiency was resulted from the addition of the PND, since its corresponding current density was comparatively lower than that of its counterpart incorporation. The effect of concentration of PND on the EL characteristics of the blue OLEDs was shown in Table 1. The power efficiency at 100 cd/m 2, for example, increased from 12.0 to 20.3 lm/w as 0.7 wt% was added. It was further increased to 25.9 lm/w as 7.0 wt% was incorporated. By increasing the PND concentration to 70 wt%, the power efficiency dropped to 15.4 lm/w. Similarly, the power efficiency was strongly depended on the concentration of the incorporated. Moreover, the chromaticity coordinate at 100 cd/m 2, for example, was (0.19, 0.34), barely changed in the presence of the PND, as also shown in Table 1. These results indicate that the marked efficiency improvement may be attributed to a better balance of carrier-injection resulted from the hole-blocking-function possessed and the hole-
PIERS ONLINE, VOL. 4, NO. 3, 2008 354 Table 1: The effects of size and concentration of PND and thickness of ETL on the EL characteristics of the blue OLEDs. Type of PND (wt%) Concentration of PND (wt%) Driving voltage (V) Power efficiency (lm/w) CIE 1931 chromatic coordinates (x, y) at 100 cd/m 2 max. at 100 cd/m 2 at 1000 cd/m 2 0.0 4.6 12.0 12.5 (0.19, 0.34) (0.18, 0.34) 0.7 4.5 20.3 21.1 (0.19, 0.34) (0.18, 0.34) 7.0 4.5 25.9 28.4 (0.19, 0.34) (0.18, 0.34) 70.0 4.6 15.4 18.9 (0.19, 0.34) (0.18, 0.34) 0.7 4.6 18.2 18.4 (0.19, 0.34) (0.18, 0.34) 7.0 4.5 21.7 22.4 (0.19, 0.34) (0.18, 0.34) 70.0 4.6 13.6 15.3 (0.19, 0.34) (0.18, 0.34) trapping-function possessed, which respectively exhibited positive and negative charge on the surface. Importantly, since the nano-dot was not employed in the emissive layer, the same concept may be applied to fluorescent type OLEDs. 4. CONCLUSIONS In conclusion, a novel small PND was synthesized and added in the hole transporting layer, PE- DOT:PSS, to markedly improve the efficiency of phosphorescent blue OLEDs. The device efficiency was strongly dependent on the concentration of the PND. The resultant power efficiency at 100 cd/m 2, for example, was increased from 12.0 to 25.9 lm/w, an increase of 116%, as 7 wt% of 8 nm in size was added. By employing 7 wt%, the power-efficiency was 21.7 lm/w. These results indicate that the marked efficiency improvement may be attributed to a better balance of carrier-injection resulted from the hole-blocking-function possessed and the hole-trappingfunction possessed. Importantly, since the nano-dot was not employed in the emissive layer, the same concept may be applied to fluorescent type OLEDs. ACKNOWLEDGMENT This work was financially supported under NSC95-2221-E-007-128-MY3, AFOSR-AOARD-05-0488 and BD96013P. REFERENCES 1. Kido, J., M. Kimura, and K. Nagai, Science, Vol. 267, 1332, 1995. 2. Duggal, R., J. J. Shiang, C. M. Heller, and D. F. Foust, Appl. Phys. Lett., Vol. 80, 3470, 2002. 3. Forrest, S. R., Org. Electron., Vol. 4, 45, 2003. 4. D Andrade, W. and S. R. Forrest, Adv. Mater., Vol. 16, 1585, 2004. 5. Lee, M. T., C. H. Liao, C. H. Tsai, and C. H. Chen, it Adv. Mater., Vol. 17, 2493, 2005. 6. Hung, L. S. and C. H. Chen, Mat. Sci. Eng. R., Vol. 39, 143, 2002. 7. Adamovich, V. I., S. R. Cordero, P. I. Djurovich, A. Tamayo, M. E. Thompson, B. W. D Andrade, and S. R. Forrest, Org. Electron., Vol. 4, 77, 2003. 8. Xie, Z. Y., L. S. Hung, and S. T. Lee, Appl. Phys. Lett., Vol. 79, 1048, 2001. 9. Jou, J. H., Y. S. Chiu, C. P. Wang, R. Y. Wang, and H. C. Hu, Appl. Phys. Lett., Vol. 88, 193501, 2006. 10. Poon, C. O., F. L. Wong, S. W. Tong, R. Q. Zhang, C. S. Lee, and S. T. Lee, Appl. Phys. Lett., Vol. 83, 1038, 2003. 11. Zhu, F., B. Low, K. Zhang, and S. Chua, Appl. Phys. Lett., Vol. 79, 1205, 2001. 12. Deng, Z. B., X. M. Ding, S. T. Lee, and W. A.Gambling, Appl. Phys. Lett., Vol. 74, 2227, 1999. 13. Zhang, Z. F., Z. B. Deng, C. J. Liang, M. X. Zhang, and D. H. Xu, Displays, Vol. 24, 231, 2003.
PIERS ONLINE, VOL. 4, NO. 3, 2008 355 14. Caruge, J.-M., J. E. Halpert, V. Bulovic, and M. G. Bawendi, Nano Lett., Vol. 6, 2991, 2006. 15. Carter, S. A., J. C. Scott, and P. J. Brock, Appl. Phys. Lett., Vol. 71, 1145, 1997. 16. Bliznyuk, V., B. Ruhstaller, P. J. Brock, U. Scherf, and S. A. Carter, Adv. Mater., Vol. 11, 1257, 1999. 17. Kim, Y. K., K. Y. Lee, O. K. Kwon, D. M. Shin, B. C. Sohn, and J. H. Choi, Synth. Met., Vol. 207, 111 112, 2000. 18. Oey, C. C., A. B. Djurisic, C. Y. Kwong, C. H. Cheung, W. K. Chan, J. M. Nunzi, and P. C. Chui, Thin Solid Films, Vol. 492, 253, 2005. 19. Hsu, Y. G., K. H. Lin, and I. L. Chiang, Mater. Sci. Eng., B87, Vol. 31, 2001. 20. Tsai, Y. C., J. H. Jou, Appl. Phys. Lett., Vol. 89, 243521, 2006. 21. Aziz, H. and Z. D. Popvic, Chem. Mater., Vol. 16, 4522, 2004.