White light from polymer light-emitting diodes ... - Yang Yang Lab

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A white light polymer light-emitting diode was demonstrated with a double layer configuration: poly[N,N -bis(4-butylphenyl)-N,N -bis(phenyl)benzidine] ...
APPLIED PHYSICS LETTERS 88, 163510 共2006兲

White light from polymer light-emitting diodes: Utilization of fluorenone defects and exciplex Q. J. Sun, B. H. Fan, Z. A. Tan, C. H. Yang, and Y. F. Lia兲 Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China

Y. Yang Department of Materials Science and Engineering, University of California, Los Angeles, California 90095-1595

共Received 1 December 2005; accepted 7 March 2006; published online 19 April 2006兲 A white light polymer light-emitting diode was demonstrated with a double layer configuration: poly关N , N⬘-bis共4-butylphenyl兲-N , N⬘-bis共phenyl兲benzidine兴 共poly-TPD兲 blended with poly 共N-vinylcarbazole兲 as both hole-transporting layer and electron-blocking layer, blue-emissive poly共9,9-dihexylfluorene-alt-co-2,5-dioctyloxy-para-phenylene兲 共PDHFDOOP兲 blended with green-emissive poly关6,6⬘-bi-共9,9⬘-dihexylfluorene兲-co-共9,9⬘-dihexylfluorene-3-thiophene-5⬘-yl兲兴 as an emissive layer. By annealing the emissive layer at a relatively high temperature, fluorenone defects were generated into PDHFDOOP, which formed an exciplex with poly-TPD, as a red emitter. The devices exhibit a maximum brightness of ⬃4800 cd/ m2 and a maximum luminous efficiency of ⬃3 cd/ A. Moreover, the Commission Internationale de L’Eclairage coordinates of the emitted light is close to that of pure white light and is insensitive to the applied voltages. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2197318兴 White light organic and polymer light-emitting diodes1–11 共WOLEDs and WPLEDs兲 are under active development for their potential applications as backlights for liquid crystal displays, full color displays, and solid-state lighting. WPLEDs fabricated by solution processing have the advantage of low-cost manufacturing and homogeneity in a large area. Several approaches including by using polymer blends,2–4 polymer doped with dyes,5–10 bilayer exciplex,11,12 and single polymer with different functional groups13 have been reported for achieving efficient WPLEDs. Polyfluorenes 共PFs兲 have been demonstrated to be good candidates for realizing WPLEDs.7–10 In such WPLEDs, PFs are a blue emitter and also function as the host due to its large band gap. Red light and green light are introduced through energy transfer process and/or charge trapping process from host PFs to guest molecules. However, the problem PFs have is its poor color stability. Its emission color changed from desired blue to bluish green, even to yellow.14–16 Recently, Gong et al.15 reported that the poor color stability in PFs was originated from the fluorenone defects which was generated by photo-oxidation, thermal oxidation, and metal catalysts during the device operation. Fluorenone defects are undesired component for pure blue light from PFs, however, it is a useful component for full color from PFs by properly utilizing it. In this letter, a WPLED was demonstrated by utilization of blue and green emitters from different PFs and a red emitter from an exciplex formed between hole-transporting layer and the fluorenone defects generated in PFs. The WPLEDs have a maximum brightness of ⬃4800 cd/ m2 at 20 V and a maximum luminous efficiency of ⬃3 cd/ A at 10 mA/ cm2. Moreover, the Commission Internationale de L’Eclairage 共CIE兲 coordinates of the white light are insensitive to bias voltages. a兲

Electronic mail: [email protected]

The molecular structures of the polymers in our study were shown in Fig. 1. Poly共9,9-dihexylfluorene-altco-2,5-dioctyloxy-para-phenylene兲 共PDHFDOOP兲 and poly 关6,6⬘-bi-共9,9⬘-dihexylfluorene兲-co-共9⬘-dihexylfluorene-3thiophene-5⬘-yl兲兴 共PFT兲 were used as emissive layer 共EML兲. They were synthesized via a Suzuki coupling reaction17 and used with further purification. Cyclic voltammetry was carried out to estimate the highest occupied molecular orbital 共HOMO兲 and the lowest unoccupied molecular orbital 共LUMO兲 energy levels18 of the polymers. The HOMO and LUMO of PDHFDOOP were −5.7 and −2.1 eV, respectively; the HOMO and LUMO of PFT were −5.4 and −2.7 eV, respectively. Poly关N , N⬘-bis共4-butylphenyl兲共poly-TPD兲 and poly N , N⬘-bis共phenyl兲benzidine兴 共N-vinylcarbazole兲 共PVK兲 were used as hole-transporting layer 共HTL兲. Poly-TPD was purchased from American Dye Source, Inc., and PVK was purchased from Aldrich Chem. Co.

FIG. 1. Chemical structures of the polymers used in this study.

0003-6951/2006/88共16兲/163510/3/$23.00 88, 163510-1 © 2006 American Institute of Physics Downloaded 04 May 2006 to 164.67.192.183. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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FIG. 2. Current density-voltage and brightness-voltage characteristics of the double-layer EL devices. The inset is the luminous efficiency-current density characteristics.

WPLEDs were fabricated with a double-layer configuration of indium tin oxide 共ITO兲/poly共ethylenedioxythiophene PEDOT兲 共25 nm兲 / HTL 共70 nm兲 / EML 共60 nm兲 / Ca 共10 nm兲 / Al 共150 nm兲. The HTL, poly-TPD with PVK 共weight ratio= 1 : 1兲, was prepared by spin-coating from chlorobenzene solution on top of PEDOT polystyrene sulphonate, followed with an annealing treatment at 110 ° C for ca. 30 min. The EML, PDHFDOOP with PFT 共weight ratio = 1 : 1%兲, was prepared by spin coating from toluene solution on top of HTL, followed with an annealing treatment at 120 ° C for ca. 1 h. The cathode Ca overcoated with Al was thermal vacuum deposited through a shade mask at a pressure of ⬍1 ⫻ 10−6 Torr. The current density-voltage and brightness-voltage characteristics of the double-layer devices are shown in Fig. 2. The inset of Fig. 2 shows a characteristic of the luminous efficiency versus current density. The devices were turned on at ⬃5 V. The devices exhibited a maximum of brightness of 4800 cd/ m2 at 20 V, with a maximum of luminous efficiency of ⬃3 cd/ A at 10 mA/ cm2. The electroluminescence 共EL兲 spectrum observed from the double-layer devices is shown in Fig. 3. The inset of Fig. 3 is the CIE coordinates of white light from the devices biased at different voltages. The CIE coordinates of white light are 共0.31, 0.32兲, 共0.29, 0.31兲, and 共0.27, 0.30兲 at 9.5, 14, and 17 V, respectively. Biased at these voltages, the brightness are 100, 1000, and

Appl. Phys. Lett. 88, 163510 共2006兲

FIG. 4. 共a兲 EL spectra of the double-layer devices with different HTLs or EMLs: device I: ITO/ PEDOT/ PVK/ PDHFDOOP/ Ca/ Al, device II: ITO/ PEDOT/poly-TPD/ PDHFDOOP/ Ca/ Al, device III: ITO/PEDOT/polyTPD+ PVK共1 : 1兲 / PDHFDOOP/ Ca/ Al, and device IV: ITO/PEDOT/polyTPD/ fluorenone+ PMMA/ Ca/ Al. The EMLs in all devices were subject to annealing at 120 ° C for 1 h. 共b兲 EL spectra of device II made with the EMLs annealed at different temperatures for 1 h.

3000 cd/ m2, respectively. The small change of CIE coordinates at the brightness from 100 to 3000 cd/ m2 demonstrated that the white light is less sensitive to the applied voltages. There are three different features in the white light obtained from the double-layer devices. Blue emission was from PDHFDOOP. Green emission was from PFT which was due to the intramolecular energy transfer from difluorene units to fluorene-thiophene units. Both blue and green emissions were clearly interpreted from the EL spectra of the PLEDs made with pure PDHFDOOP and pure PFT 共shown in Fig. 3兲. In order to investigate the low-energy 共1.85– 2.15 eV兲 red emission from the double-layer devices, PLEDs fabricated with the same configuration and different materials were characterized. No red emission was observed from the PLEDs made with poly-TPD, PVK, poly-TPD blended with PVK, and PDHFDOOP blended with PFT. These indicate that red emission from the double-layer PLEDs was not from either pure HTL or pure EML, and imply that it was probably from the interface between HTL and EML. Figure 4共a兲 shows the EL spectra of the doublelayer PLEDs with different HTLs or EMLs. Only blue emission was observed from the device 共device I兲 with PDHFDOOP as EML and PVK as HTL. This illustrates that the red emission was not related to PVK. Replacing PVK by polyTPD or a blend of poly-TPD with PVK 共Devices II and III兲, the red emission then appeared in the EL spectra besides the blue emission from PDHFDOOP. These results indicate that the red emission is probably from an exciplex associating with poly-TPD and PDHFDOOP. From energy levels of poly-TPD and PDHFDOOP shown in Fig. 5, it is hard to believe that the red emission is from the interface between poly-TPD and PDHFDOOP because there is large energy gap between LUMO of poly-TPD and HOMO of PDHFDOOP. To further investigate what kind of exciplex is formed, the device was fabricated by using poly-TPD as HTL, fluorenone molecule blended with poly共methyl methacrylate兲 共PMMA兲 as EML 共device IV兲. In device IV, blue emission is from poly-TPD. Green emission peaked at ⬃510 nm from fluorenone molecule itself was not observed. A red emission, which is in agreement very well

FIG. 3. EL spectrum of the double-layer EL device. For a good comparison, EL spectra of single-layer EL devices made separately from PDHFDOOP and PFT are also shown in it. The inset shows the CIE coordinates of the double-layer EL devices at different bias voltages. Downloaded 04 May 2006 to 164.67.192.183. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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FIG. 5. Energy levels of the materials involved in the experiments.

with those observed from devices II and III, was observed. The energy gap between the HOMO of poly-TPD 共−5.1 eV兲 and the LUMO of fluorenone 共−3.1 eV兲 is ⬃2.0 eV, which is perfectly matched the red emission band. Therefore, the red emission from device IV is attributed to an exciplex formed between poly-TPD and fluorenone molecules. Analogically, we concluded that the red emission feature observed from the double-layer WPLEDs was from the exciplex formed between poly-TPD and fluorenone defects generated in PDHFDOOP. Fluorenone defects in PFs can be generated by different ways.15 In our devices, the fluorenone defects were induced by thermal annealing of EMLs during the device preparation. Figure 4共b兲 shows the EL spectra from device II with the EMLs annealed at different temperatures. At an annealing temperature less than 70 ° C, there is no exciplex emission appeared in the EL spectra. When annealing temperature was 100 ° C, an exciplex emission appeared. The intensity of the exciplex emission was increased with increasing annealing temperature. These results suggest that a relatively high annealing temperature is essential to the formation of the fluorenone defects, and thus, is an important factor for the demonstration of white light from the double-layer devices. The mechanism for generation of white light from the double-layer device structure in our study is proposed as follows. By controlling the annealing temperature of the EML, fluorenone defects are generated in PDHFDOOP. The injected/transported holes 共from poly-TPD兲 and injected electrons 共from Ca/ Al兲 can recombine on the main chain of PDHFDOOP to produce blue light and/or on the main chain of PFT to produce green light. Or they can partially be trapped by PFT with a green emission. In parallel, the injected/transported holes were in the HOMO of poly-TPD and injected electrons were trapped in the LUMO of fluorenone defects, followed by radiative recombination to produce a red light. In the double-layer WPLEDs, PVK is functionalized as electron-blocking layer 共EBL兲共Ref. 19兲 rather than HTL because its LUMO 共−1.9 eV兲 is 0.2 eV higher than that of PDHFDOOP and its HOMO 共−5.4 eV兲 is 0.3 eV

lower than that of poly-TPD. High performance of the WPLEDs partially resulted from poly-TPD as HTL and PVK as EBL. In summary, a method was developed for achieving white light from PLEDs by using PFs as both blue and green emitters and the exciplex formed between fluorenone defectes generated in PFs and HTL as a red emitter. The generation of the fluorenone defects was induced by annealing EML at a relatively high temperature. The white light WPLEDs have a maximum brightness of ⬃4800 cd/ m2 at 20 V and a maximum luminous efficiency of 3 cd/ A at 10 mA/ cm2. Moreover, the CIE coordinates of the white light are insensitive to biased voltages. The authors thank Dr. W. L. Yu 共Dow Chemicals, Inc.兲 for valuable discussions. This work was supported by the Ministry of Science and Technology of China 共Grant No. 2002CB613404兲 and NSFC 共Grant Nos. 50373050, 20421101, and two bases project, Grant No. 60440420149兲. One of the authors Q.J.S. is grateful to the support of Postdoctor Science Foundation of China. B. W. D’Andrade and S. R. Forrest, Adv. Mater. 共Weinheim, Ger.兲 16, 1585 共2004兲. M. Granstrom and O. Inganas, Appl. Phys. Lett. 68, 147 共1996兲. 3 Y. H. Xu, J. B. Peng, Y. Q. Mo, Q. Hou, and Y. Cao, Appl. Phys. Lett. 86, 3502 共2005兲. 4 G. Ho, H. Meng, S. Lin, S. Horng, C. Hsu, L. Chen, and S. Chang, Appl. Phys. Lett. 85, 4576 共2004兲. 5 J. Kido, H. Shionoya, and K. Nagai, Appl. Phys. Lett. 67, 2281 共1995兲. 6 I. Tanaka, M. Suzuki, and S. Tokito, Jpn. J. Appl. Phys., Part 1 42, 2737 共2003兲. 7 Q. Xu, H. M. Duong, F. Wudl, and Y. Yang, Appl. Phys. Lett. 85, 3357 共2004兲. 8 J. H. Kim, P. Herguth, M. Kang, A. K.-Y. Jen, Y. Tseng, and C. Shu, Appl. Phys. Lett. 85, 1116 共2004兲. 9 X. Gong, W. Ma, J. C. Ostrowski, G. C. Bazan, D. Moses, and A. J. Heeger, Adv. Mater. 共Weinheim, Ger.兲 16, 615 共2004兲. 10 X. Gong, D. Moses, A. J. Heeger, and S. Xiao, J. Phys. Chem. B 108, 8601 共2004兲. 11 D. D. Gebler, Y. Z. Wang, J. W. Blatchford, S. W. Jessen, D. K. Fu, T. M. Swager, A. G. MacDiarmid, and A. J. Epstein, Appl. Phys. Lett. 70, 1644 共1997兲. 12 C. L. Chao and S. A. Chen, Appl. Phys. Lett. 73, 426 共1998兲. 13 G. Tu, Q. Zhou, Y. Cheng, L. X. Wang, D. G. Ma, X. B. Jing, and F. S. Wang, Appl. Phys. Lett. 85, 2172 共2004兲. 14 M. Strukelj, R. H. Jordan, and A. Dodabalapur, J. Am. Chem. Soc. 118, 1213 共1996兲. 15 X. Gong, P. K. Iyer, D. Moses, G. C. Bazan, and A. J. Heeger, Adv. Funct. Mater. 13, 325 共2003兲. 16 M. Gaal, E. J. W. List, and U. Sherf, Macromolecules 36, 4236 共2003兲. 17 W. L. Yu, J. Pei, Y. Cao, W. Huang, and A. J. Heeger, Chem. Commun. 共Cambridge兲 1999, 1837. 18 Q. J. Sun, H. Q. Wang, C. H. Yang, and Y. F. Li, J. Mater. Chem. 13, 800 共2003兲. 19 I. D. Parker, Q. Pei, and M. Marrocco, Appl. Phys. Lett. 65, 1272 共1994兲. 1

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