Synthesis and Optoelectronic Properties of Novel Fluorene-based Copolymer for Red Light-emitting Diodes

Transcription

1 Synthesis and Optoelectronic Properties of Novel Fluorene-based Copolymer for Red Light-emitting Diodes Synthesis and Optoelectronic Properties of Novel Fluorene-based Copolymer for Red Light-emitting Diodes Zhitian Liu 1, a, Shuangqiang Hu 1, Baolong Yang 1, Shuangquan Wu 1, Zixing Wang 2, Chi Zhang 1, and Zhao Hu 1 1 School of Material Science and Engineering, Wuhan Institute of Technology, Wuhan , China 2 Shanghai University, Shanghai, , China Summary Novel copolymer derived from ethoxylated fluorene and 4,7-dithienyl-2,1,3-benzothiadiazole was successfully synthesized by palladium(0)-catalyzed Suziki coupling reactions. Optoelectronic properties of the copolymer were characterized by elemental analysis, NMR, UV absorption, photoluminescence (PL), and electroluminescence (EL). The films of the copolymer showed DBT-dominant red emissions. EL devices of the copolymer with a sandwich configuration of ITO/Polymer/Al displayed exclusive DBT emissions peaked at around 655 nm with the CIE coordinate of (0.67, 0.32). The polymer layer contained a conjugated copolymer that emits red light, trimethylolpropane trimethacrylate (TMPTMA), a low vapor pressure highly crosslinkable liquid, and lithium trifluoromethane sulfonate (LiTf) at a weight ratio of 20:40:1. A dynamic p-i-n junction was formed in thin films of light emitting polymers admixed with an ionically conductive medium. The PLEC device was turned on at relative low voltage 3.2 V and the brightness reached up to 1200 cd/m 2 at 8 V. Keywords: Conjugated polymer; polymer light-emitting diodes; optical properties 1. Introduction During the past 30 years, conjugated polymers with a variety of structures have drawn broad attention due to their important applications in polymeric light-emitting diodes (PLEDs), photovoltaic cells (PVCs), and field effect transistors (FETs) 1-5. Polymer light-emitting diodes (PLEDs) have attracted considerable interest because of their potential application in flat panel displays 3,6 using a low-cost process such as ink-jet printing, screen printing, or roll lamination onto flexible substrates 7,8. Some important factors, such as higher photoluminescence (PL) efficiency of the polymer, balanced injections of holes and electrons into the emissive layer, and balanced transports and re-combinations of holes and electrons in the emissive layer, play the major roles in the improvement of the PLED efficiency a able.ztliu@gmail.com Smithers Rapra Technology, 2012 Due to the difficulty in fabricating ultrathin, high-quality polymer films and the requirement for hermetic encapsulation, which limit commercialization of PLEDs. These limitations are caused by relatively high charge-injection barriers at the electrode/polymer interfaces and the low carrier mobility in the luminescent polymers, so we require multi-layered device architectures to balance the injection of electrons and holes. Reactive metals or metal salts are commonly used as the electron injection materials in PLEDs. These materials are sensitive to oxygen or moisture, moreover they are not very compatible with a solution process. Polymer light-emitting electrochemical cells (PLECs) were thus introduced to overcome these problems. PLECs have a simple device structure, using high workfunction cathodes which remain stable in air, and active polymer film consists of dissolved salts and ionic conducting components in addition to the electroluminescent conjugated polymers, In an idealized device, a dynamic p-i-n junction is formed when a bias voltage equal to or greater than the band gap of the conjugated polymer is applied. This junction formation effectively reduces the charge injection barriers and enhances the balance of electron and hole injections During the recent years, polyfluorene (PF) and its copolymers have become the promising emitters to be utilized in the fabrications of commercial PLEDs because of their highly efficient photoluminescence (PL) and electroluminescence (EL), excellent thermal and oxidative stability, and good solubility in common organic solvents Emission color of polyfluorenes can be changed over an entire visible region by introducing narrow-band-gap comonomer into the polyfluorene backbone. Most widely Polymers & Polymer Composites, Vol. 20, Nos. 1 & 2,

2 Zhitian Liu, Shuangqiang Hu, Baolong Yang, Shuangquan Wu, Zixing Wang, Chi Zhang, and Zhao Hu used narrow-band-gap comonomers are a variety of aromatic heterocycles containing N or S atoms, such as thiophene 19 and benzothiadiazole 20. Compared with a high efficiency green polyfluorene emitter based on fluorene and benzothiadiazole, the high-efficiency polymer emitter with a saturated red emission remains a great challenge. In this paper, a novel copolymer (EOPF-DBT, show in Figure 1) derived from ethoxylated fluorene and 4,7-dithienyl- 2,1,3-benzothiadiazole(DBT) was successfully synthesized by palladium(0)-catalyzed Suziki coupling reactions. Ethoxylate attached on the fluorene side group was introduced to promote ionic conductivity necessary for the operation of LECs 21. Narrowband-gap DBT unit was introduced as an efficient exciton trap for PLEC. Optoelectronic properties including UV absorption, PL and EL of the polymer were examined. EL devices of the copolymer with a sandwich configuration of ITO/Polymer/Al displayed exclusive DBT emissions peaked at around 655 nm with the CIE coordinate of (0.67, 0.32). The PLEC device was turned on at relative low voltage 3.2 V and the brightness reached up to 1200 cd/m 2 at 8 V. spectra were recorded on an HP 8453 spectrophotometer. The PL spectra of the copolymer were obtained on a Jobin Yvon Fluorolog-3 spectrofluorometer. 2.3 Synthesis of monomers 2,7-bis(4,4,5,5-tetramethyl- 1,3,2-dioxaborolan-2-yl)-9,9- dioctylfluorene, 2,7-dibromo-9-(4- (2-(2-(2-methoxyethoxy)ethoxy) phenyl)-9-phenyl-fluorene and 4,7-bis(5-bromo-2-thienyl)-2,1,3- benzothiadiazole were synthesized according to literature Polymerization The polymerizations were carried out by palladium(0)-catalyzed Suzuki coupling reactions with equivalently molar ratio of the diboronic ester monomer to the dibromo monomers under dry argon protection. A typical procedure for the polymerization of the random copolymer EOPF-DBT is shown in Figure 2. Figure 1. Chemical structure of the copolymer EOPF-DBT EOPF-DBT was synthesized by the conventional palladium(0)-catalyzed Suzuki coupling reactions with equivalently molar ratio of the diboronic ester monomer to the dibromo monomers under dry argon protection. Carefully purified 2,7-bis(4,4,5,5- tetramethyl-1,3,2-dioxaborolan-2- yl)-9,9-dioctylfluorene (321 mg, 0.5 mmol), 2,7-dibromo-9-(4-(2-(2-(2- methoxyethoxy)ethoxy)phenyl)-9- phenyl-fluorene (275 g, 0.43 mmol), 4,7-bis(5-bromo-2-thienyl)-2,1,3- benzothiadiazole (32 mg, 0.07 mmol), (PPh 3 ) 4 Pd(0) (3.5 mg), and several drops of Aliquat 336 (~150 mg) were dissolved in a mixture of toluene (10 ml) and aqueous 2 M Na 2 CO 3 (2 ml). The solution was refluxed with vigorous stirring for 24 h. At the end of polymerization, small amount of phenylboronic acid (20 mg) was added, followed by bromobenzene (200 mg). The mixture was then poured into vigorously stirred methanol. The precipitated solid was filtered 2. Experiment 2.1 Materials All manipulations involving airsensitive reagents were performed under an atmosphere of dry argon. All reagents, unless otherwise specified were obtained from Aldrich or TCI America Co. Trimethylolpropane trimethacrylate (SR9035) was purchased from Sigma- Aldrich and purified by silica column chromatography before use. Figure 2. Typical procedure for the polymerization of EOPF-DBT 2.2 Instrumentation 1H and 13C NMR spectra were recorded on a Bruker AV 300 spectrometer with tetramethylsilane (TMS) as the internal reference. UV-vis absorption 28 Polymers & Polymer Composites, Vol. 20, Nos. 1 & 2, 2012

3 Synthesis and Optoelectronic Properties of Novel Fluorene-based Copolymer for Red Light-emitting Diodes and washed with acetone to remove oligomers and catalyst residues. Red powder of EOPF-DBT was obtained. 2.5 PLEC Fabrication and Characterization The PLEC was fabricated with a configuration of ITO/Polymer/Al. ITO glass substrates were ultrasonically cleaned for 30 mins each with acetone, detergent water and isopropanol. The cleaned ITO glass substrates were dried at 80 C. The polymer was dissolved at room temperature in anhydrous THF with the polymer concentration fixed at 2.5 mg/ml. The polymer electrolyte consists of SR9035 and lithium trifluoromethanesulfonate (LiTf). The luminescent material and the polymer electrolyte were dissolved in their common solvent, anhydrous THF. Suitable amounts of these master solutions were then mixed together to form an LEC solution with different ratio of polymer. The solution was spincoated directed onto ITO substrates at 2000 rpm. The samples were heated at 70 C to remove the residual solvent. Polymer film thickness was measured by a dektak profilometer. An aluminum top electrode with 100 nm thickness was evaporated by an thermal metal evaporation system at mbar and 4 Å/s deposition rate. A shadow mask was used to define each device area as 12 mm 2. A Keithley 2400 sourcemeter with Keithley 2000 multimeter and calibrated Si photodiode was used to turn on the LECs while simultaneously recording the device electroluminescent (EL) intensity. The EL spectra were obtained by a Photoresearch PR655 spectrometer. All devices processing procedures and testing were performed inside a MBraun double glove-box/evaporator system filled with dry nitrogen. films of EOPF-DBT. The absorption spectra of the copolymer measured in solution display two absorption peaks. The absorption peak appeared at ca. 389 nm is consistent with those reported for poly(9,9-dihexylfluorene) homopolymer (388 nm). This peak can be attributed to the fluorene segment in the copolymer, and the additional long wavelength absorption bands at ca. 522 nm, can be attributed to the DBT unit incorporated into the polyfluorene main chain, which has narrower band gap than the fluorene units. Compared to the solution, there is almost similar absorption spectrum for EOPF-DBT in thin films. Both absorption spectra are dominated by the fluorene units. 3.2 Photoluminescence Properties Photoluminescence spectra of the copolymer in a dilute THF solution at a concentration of M and in thin films are shown in Figure 4. The PL spectrum of EOPF-DBT exhibits two dominant well-resolved peaks of PFO emission at 416 and 440 nm, and also there is longer weak signature of Figure 3. UV absorption spectra of THF solutions and thin film Figure 4. Photoluminescence spectra of THF solutions and thin film 3. Result and discussion 3.1 UV Absorption Figure 3 shows the UV-vis absorption spectra of the THF solutions and thin Polymers & Polymer Composites, Vol. 20, Nos. 1 & 2,

4 Zhitian Liu, Shuangqiang Hu, Baolong Yang, Shuangquan Wu, Zixing Wang, Chi Zhang, and Zhao Hu DBT emission at 633 nm. Since DBT compounds show a larger Stokes shift relative to the PFO and normally emit red lights. Compared with the PL emission in solution, the intensity of the blue emission from PFO segments is largely quenched and the red emission from the DBT units is dominant, due to the contributions of intra- and intermolecular energy transfers in the solid state 24. The PL spectra has some red shift to 651 nm in thin film. 3.3 Electroluminescence Properties PLECs with EOPF-DBT as the emissive layer were fabricated with Figure 5.The EL spectrum of copolymer a configuration of ITO/ EOPF- DBT:SR9035:LiTf (20:40:1)/Al. The EL spectra are shown in Figure 5. The copolymer show the exclusive narrow band gap DBT unit emissions, demonstrating that the DBT units are efficient exciton traps. The PLEC emits red light peaked at 655 nm with the CIE coordinate of (0.67, 0.32). The EL performance of the I-V-B curve is shown in Figure 6. The LEC was turned on at relative low voltage 3.2 V and the brightness reached up to 1200 cd/m 2 at 8V. We also use this polymer as emissive layer for fully bendable and transparent polymer light emitting devices, the device performance is in progress. 4. Conclusions We have successfully synthesized a light emitting conjugated polymer (EOPF-DBT) derived from ethoxylated fluorene and 4,7-dithienyl-2,1,3- benzothiadiazole, which is soluble in common organic solvents and can be spin cast into uniform thin films. The thin films exhibit intense red fluorescence. PLECs with EOPF- DBT as the emissive layer were fabricated with a configuration of ITO/ EOPF-DBT:SR9035:LiTf (20:40:1)/ Al. The LEC device was turned on at 3.2 V and brightness reached up to 1200 cd/m 2 at 8 V. PLEC emits red light peaked at 655 nm with the CIE coordinate of (0.67, 0.32). The optimized performance is in progress and will be reported at the conference. The device performance of fully bendable and transparent polymer light emitting devices with this polymer as emissive layer is in progress. Acknowledgements This work was partially supported by National Natural Science Foundation of China (Grant No ) and the Key Laboratory for Advanced Displays and System Application, Ministry of Education (Shanghai University) (Project No ). Figure 6. I-V-B curve of red PLEC References 1. Burroughes J.H., Bradley D.D.C., Brown, A.R., Marks, R.N., Mackay, K., Friend, R.H., Burns, P.L., and Holmes, A.B., Nature, 347 (1990) Braun D. and Heeger A.J., Appl. Phy. Lett., 58 (1991) Kraft A., Grimsdale A.C., and Holmes A.B., Angew. Chem. Int. Ed., 37 (1998) Yu G., Gao J., Hummelen J.C., Wudl F., and Heeger A.J., Science, 270 (1995) Bao Z., Dodabalapur A., and Lovinger A.J., Appl. Phys. Lett., 69 (1996) Akcelrud L., Prog. Polym. Sci., 28 (2003) Polymers & Polymer Composites, Vol. 20, Nos. 1 & 2, 2012

5 Synthesis and Optoelectronic Properties of Novel Fluorene-based Copolymer for Red Light-emitting Diodes 7. Gustafsson G., Cao Y., Treacy G.M., Klavetter F., Colaneri N., and Heeger A.J., Nature, 357 (1992) Friend R.H., Gymer R.W., Holmes A.B., Burroughes J.H., Marks R.N., Taliani C., Bradley, D.D.C., Dos Santos D.A., Bredas J.L., Logdlund M., and Salaneck W.R., Nature, 397 (1999) Forrest S.R., Bradley D.D.C., and Thompson M.E., Adv. Mater., 15 (2003) Cao Y., Parker I.D., Yu G., Zhang C., and Heeger A.J., Nature (London), 397 (1999) Yu G., Zhang C., Yang Y., and Heeger A.J., Science, 269 (1995) Pei Q.,Yang Y., Yu G., Zhang C., and Heeger A.J., J. Am. Chem. Soc., 118 (1996) Pei Q. and Heeger A.J., Nature Materials, 7 (2008) Yu Z., Sun M., and Pei Q., J. Phys. Chem. B, 113 (2009) Grice A.W., Bradley D.D.C., Bernius M.T., Inbasekaran M., Wu W.W., Woo E.P., Appl. Phys. Lett., 73 (1998) Ma W., Iyer P.K., Gong X., Liu B., Moses D., Bazan G.C., and Heeger A.J., Adv. Mater., 17 (2005) Li Y., Ding J., Day M., Tao Y., Lu J., and Diorio M., Chem. Mater., 16 (2004) Wu F.I., Shih P.I., Shu C.F., Tung Y.L., and Chi Y., Macromolecules, 38 (2005) Larmat F., Reynolds J.R., Reinhardt B.A., Brott L.L., and Clarson S.J., J. Polym. Sci., Part A, Polym. Chem., 35 (1997) Inbasekaran M., Woo E.P., Wu W.S., and Bernius M.T., PCT application W A1, Yang Y. and Pei Q.B., J. Appl. Phys., 81 (1997) Ranger M., Rondeau D., and Leclerc M., Macromolecules, 30 (1997) Tang C., Liu F., Xia Y.J., Xie L.H., Wei A., Li S.B., Fan Q.L., and Huang W., J. Mater. Chem., 16 (2006) Hou Q., Xu Y.S., Yang W., Yuan M., Peng J.B., and Cao Y., J. Mater. Chem., 12 (2002) Polymers & Polymer Composites, Vol. 20, Nos. 1 & 2,

6 Zhitian Liu, Shuangqiang Hu, Baolong Yang, Shuangquan Wu, Zixing Wang, Chi Zhang, and Zhao Hu 32 Polymers & Polymer Composites, Vol. 20, Nos. 1 & 2, 2012