Polarized Electroluminescence in Double Layer LEDs with Perpendicularly Oriented Polymers
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Polarized Electroluminescence in Double Layer LEDs with Perpendicularly Oriented Polymers A. Bolognesi, C. Botta, D Facchinetti, M. Jandke 1, P. Strohriegl 1, K. Kreger 1, S. Blumstengel 2 Istituto di Chimica delle Macromolecole, CNR, Via E. Bassini 15, 20133 Milano, Italy 1 Makromolekulare Chemie I, Univerität Bayreuth D-95440 Bayreuth, Germany 2 Dip. Scienza dei Materiali, Università Bicocca Via Cozzi 53, 20125 Milano, Italy
ABSTRACT We have prepared and characterized a light emitting diode (LED) with the active film formed by two polymers emitting in two different regions: in the yellow-green and in the red range of the visible spectrum. The two polymers are polyparaphenylenevinylene (PPV) and poly[3-(6-methoxyhexyl)thiophene] (P6OMe) respectively. P6OMe was spin coated on a PPV layer previously oriented and was also oriented by means of the rubbing technique in the direction perpendicular to the PPV orientation. The polarized electroluminescence obtained from this device shows yellow-green emission from PPV, or red from P6OMe depending on the orientation of the analyzer. INTRODUCTION The interest in light emitting diodes (LED) able to provide polarized light is increasing in these last years, thanks to the availability of low cost techniques suitable to align the chains of conjugated polymers [1] generally used as active materials in organic LEDs. The strong anisotropy of the conjugated chains, and their easy processability, allows preparation of well oriented thin films able to provide polarized electroluminescence (EL) with low cost technology. Polarized EL is useful for many applications, e.g., as backlights in liquid crystal displays [2], as recently reported in the literature [3]. The most used methods for aligning polyconjugated systems have been recently described and summarised in a review article by Grell and Bradley [4]. Depending on the optical properties of the polymers and on the orienting technique, different emission colours and EL anisotropies (REL) were obtained. For red emission, poly(3-alkylthiophenes) (PAT) oriented through a combination of rubbing and thermal annealing have shown anisotropies REL ≈ 8 [5]. The aim of this work is to study the feasibility of a double layer device, where the two active materials are formed, respectively, by two oriented polymers, whose orientation direction is orthogonal, and which can emit simultaneously in different regions of the visible spectrum. We demonstrate that the anisotropy of the polymers is not lost when they are oriented perpendicularly, obtaining polarized light in a large spectral region, extending from the green to the red. The emitted light, observed through a polarizer, can be varied from green to red by simply rotating the polarization axis, obtaining polarized light of variable colour. This peculiar device design is particularly appealing as it can increase the versatility of organic LEDs providing polarized light with easily variable colour emission.
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EXPERIMENTAL DETAILS PPV was prepared according to reference 6, while details o
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