MEH-PPV의 합성과 전기-광학적 특성에 관한 연구

권승호, 백상윤, 박지호, 유재수

중앙대학교 화학공학과

Synthesis of MEH-PPV and Electrical and Optical Properties of its Single Layer Device

Seung Ho Kwon, Sang Yoon Paik, Ji Ho Park, Jae Soo Yoo

Dept. of Chemical Engeering, Chung-Ang University

Introduction

Conducting polymers suggested the possibility of combining the electronic and optical properties of semiconductors and metals with the alternative mechanical properties and processing advantages of polymers[1]. Recently, poly(2,5-dialkoxy-1,4-phenylenevinylene)s have been of interest as conducting polymers, since their synthesis is facile and different length side-groups can be incorporated into the polymer to give varying physical properties. Fully-conjugated polymers, which are soluble in organic solvents, can be prepared by using suitably long side chains[2]. Soluble MEH-PPV can work as a highly efficient light-emitting material in single layer electroluminescent device. A number of synthetic approaches for soluble dialkoxy PPVs has been investigated, including dehydrohalogenation[3], elimination from sulfonium salts[3], and aryl/ethylene coupling via Heck- or Suzuki-reactions[4]. Polymers synthesized by these methods contain small amounts of structural defects as a incomplete elimination, cross linking or other side reactions during polymerization[5].

Electroluminescent device by polymer promises a new and exciting technology of emissive type. The advantage of organic emitting materials over classical semiconductors is in the possibility of designing new materials with different band gaps and electron affinities. The color range of emission from organic LEDs can be varied over the range of visible spectrum, which offers the opportunity for multi-colored emission from a single layer[6]. Polymer LEDs are low voltage devices with turn-on voltages as low as 3V and operating voltages below 10V[7].

In this study, we synthesized poly[2-methoxy-5- (2-ethyl hexyloxy)-1,4 -phenylene vinylene], MEH-PPV, as conjugated polymer and characterized its electrical and optical properties through fabricating a single layer device.

Experimental

A. Synthesis

Monomer, 2,5-bis(chloromethyl)-1-methoxy-4-(2-ethylhexyloxy)benzene was prepared in a two step procedure from 4-methoxy phenol and sodium methoxide with 2-ethylhexyl bromide and subsequent vacuum distillation. The resulting 1-methoxy-4-(2-ethylhexyloxy)benzene was bischloromethylated by the reaction of formalin with hydrogen chloride to obtain the desired monomer.

The monomer was polymerized to MEH-PPV in anhydrous tetrahydrofuran by the addition of a solution of freshly sublimed potassium tert-butoxide under nitrogen with vigorous stirring at room temperature. The mixture was stirred at room temperature for 2 days before precipitation into methanol. The polymer was isolated by filtration through glass filter with a silica gel bed, and then dried at 45℃ in vacuum oven.

B. Preparation of polymer EL device

Diode device were fabricated on ITO-coated glass. ITO-coated glass substrates with a surface resistance of 16□/sq were cleaned in ultra-sonic bath of trichloroethylene(TCE), acetone, and methanol, and then dried in a stream of nitrogen. In fabricating the device having a single layer, the PEDOT/PSS(Bayer) as conducting polymer was spin-coated on the patterned substrates by photolithography. After baking, Polymer film was also formed by spin-coating using chloroform solvent containing MEH-PPV. After drying under Ar gas over 12 hours, Ca as cathode was deposited by thermal deposition method. The thickness of film was measured by Tancor Alpha Step 2000. The electroluminescent properties of fabricated devices were investigated by Keithley 236 as source measure unit and CS-100(Minolta) photometer.

Results and discussion

Synthetic route of MEH-PPV was shown in Fig. 1. Number average molecular weight(Mn) and polydispersity of synthesized MEH-PPV were 306,749g/g-mol and 2.43, respectively.

Fig. 1 Synthetic route of MEH-PPV

Its glass transition temperature was 105.53℃, as shown in Fig. 1. Figure 2 shows IR spectrum of synthesized MEH-PPV.

Fig. 2 Analysis of glass transition temperature(Tg) by DSC

Fig. 3 FT-IR spectrum of synthesized MEH-PPV

The device structure was a single layer sandwiched between electrodes. Figure 2 shows the molecular structure of synthesized MEH-PPV and the cell configuration.

Fig. 4 Configuration of MEH-PPV light-emitting diodes.

The PEDOT/PSS was introduced on ITO for improving the luminance and power efficiency. Polymer film was 900Å thick and Calcium as a cathode 1000Å thick.  In investigating the fabricated device, the turn-on voltages was 1.7V, and the power efficiency was 2.79 lm/W at the current density of 0.58mA/mm2 at DC 3.5V. The luminance of about 3000 cd/m2 was achieved at DC 5.5V, as shown in Fig. 5(a). Figure 5(b) shows the character imaging operated at DC 5V.

(a)                                 (b)

Fig. 5 (a)Electro-optical properties of fabricated device and

(b)character imaging operated at DC 5V

In this study, MEH-PPV as conjugated polymer was synthesized and a single layer device using it was successfully fabricated in order to investigate the electrical and optical properties of polymer EL device. Synthesis of MEH-PPV and the key factors determining the electroluminescent properties of device such as luminance, power efficiency, and lifetime will be discussed in detail.

Reference

[1] US patent, 5,189,136, Feb. 23, 1993.

[2] G. J. Sarnecki, P. L. Burn, R. H. Friend, and A. B. Holms, Synthetic          Metals, 55-57, 1993.

[3] F. Wudl, P. M. Allemand, G, Srdanov, Z. Ni, D. Mcbranch, ACS Symp.        Ser, 445, 1991.

[4] F. Koch, W. Heitz, Macromol. Chem. Phys., 198(1531), 1997.

[5] S. Pfeiffer, H. H. Horhold, Synthetic Metals, 101, 109-110, 1999.

[6] Karg. et. al., in 38th Electronic Materials Conference, Santa Barbara CA,      26-28, June, 1996.

[7] J. C. Scott, J. H. Kaufman, P. J. Brock, R. Dipietro, J. Salem, and J. A.      Goitia, J. Appl. Phys., 79(5), 1 March, 1996.[7]

Acknowledgement

This work was supported by Intelligent Microsystem Center(IMC) in 21C

Frontier R&D Program of Ministry of Science and Technology.