Characterization of white electroluminescent devices fabricated using conjugated polymer blends

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Nam-Heon Lee Department of Physics, Inha University, Incheon 402-751, Korea

Changhee Lee School of Electrical Engineering and Computer Science, Seoul National University, Seoul 151-744, Korea

Yong-Bae Kim Liquid Crystal Research Center, Department of Chemistry, Kon-Kuk University, Seoul 143-701, Korea

Hong-Ku Shim Center for Advanced Functional Polymers, Department of Chemistry and School of Molecular Science (BK21), Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea (Received 4 November 2003; accepted 2 April 2004)

We report the characterization of white light emitting devices fabricated using conjugated polymer blends. Blue emissive poly[9,9-bis(4⬘-n-octyloxyphenyl)fluorene-2, 7-diyl-co-10-(2⬘-ethylhexyl)phenothiazine-3,7-diyl] [poly(BOPF-co-PTZ)] and red emissive poly(2-(2⬘-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene) (MEH-PPV) were used in the blends. The inefficient energy transfer between these blue and red light emitting polymers (previously deduced from the photoluminscence (PL) spectra of the blend films) enables the production of white light emission through control of the blend ratio. The PL and electroluminescence (EL) emission spectra of the blend systems were found to vary with the blend ratio. The EL devices were fabricated in the indium tin oxide [poly(3,4-ethylenedioxy-thiophene)-poly(styrenesulfonate)] (ITO/PEDOT-PSS)blend/LiF/Al configuration, and white light emission was obtained for one of the tested blend ratios.

I. INTRODUCTION

Conjugated polymers have attracted much research interest in science and technology in the past few decades as electro-active materials for diverse applications that include transistors,1–3 photovoltaic devices,4 nonlinear optical devices,5 and polymer light-emitting diodes (PLEDs).6–10 In particular, their use in electroluminescence applications has been extensively studied to realize thin, efficient, and stable displays with a wide viewing angle and fast response. Organic electroluminescence (EL) applications such as automotive, mobile, and television displays continue to make progress toward commercial production. Poly(fluorene)s (PFs) 11–15 and poly(1,4-phenylenevinylene)s (PPVs)16–20 are promising candidates for PLEDs. PPV derivatives are normally green and red emitting materials; the introduction of electron-donating or electron-withdrawing groups into these derivatives can be used to vary their emitting color. a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0261 J. Mater. Res., Vol. 19, No. 7, Jul 2004

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Although PFs are blue emitting materials, color tuning of these materials is easily obtained by copolymerization with low band gap comonomers.15 White light emitting diodes have attracted significant research interest due to their applications in full color displays combined with a color filter, in backlights for liquid crystal displays, and in meeting various other lighting requirements. White light emission requires the mixing

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