Photo and Electroluminescent Properties of a Porphyrin-Poly( p -Phenylene Vinylene) Derivative
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Lisboa, Portugal 3Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of
Cambridge, Pembroke Street, Cambridge, CB2 3RA, UK 4Department of Chemistry, Imperial College, London SW7 2AZ, UK ABSTRACT The solid state luminescent properties of new statistical copolymers of 2methoxy-5-(2´-ethylhexyloxy)-1,4-phenylene vinylene and a porphyrin-substituted phenylene vinylene are reported. The photoluminescence, PL, emission shows an efficient energy transfer to the porphyrin, in spite of the small absorption coefficient of the porphyrin in the wavelength range of the poly [2-methoxy-5-(2´ethylhexyloxy)-1,4-phenylene vinylene], MEH-PPV, emission. At a porphyrin content of 0.25%, by weight, the emission is already dominated by the porphyrin. The PL efficiency of these copolymers is significantly reduced from the value of 13% for MEH-PPV down to 3% for the copolymer with 10.84% of porphyrin. Efficient energy transfer is also observed in the emission spectra of light-emitting diodes. 1. INTRODUCTION The polymer based light-emitting diodes (LEDs) have now achieved perfomance levels (efficiency and lifetime) good enough to be used in “real” devices (see Refs 1 and 2, as recent reviews). In order to prepare full colour displays, active research on polymers with efficient emission in the red, green and blue is still being carried out. We describe here the work we have carried out in the search for a red emitting polymer. This particular approach is based on the use of an efficient electroluminescent polymer, poly[2-methoxy-5-(2´-ethylhexyloxy)-1,4-phenylene vinylene] or MEH-PPV, which emits in the orange [3], in conjunction with a tetraphenylporphyrin derivative with saturated red emission. We have explored both the blending of the porphyrin chromophore with the neat MEH-PPV [4] and the statistical copolymers poly[2-methoxy-5-(2´-ethylhexyloxy)-1,4-phenylene vinylene-co-prophyrinsubstituted diallkoxyphenylene vinylene], CPn (see Fig. 1). 2. EXPERIMENTAL The synthesis of the porphyrin moiety and of the copolymers will be reported elsewhere[5]. The photoluminescence, PL, efficiency was determined in films deposited on fused quartz (Spectrosil B), according to a reported procedure [6]. Single-layer LEDs were prepared by spin coating of ITO-coated glass with the polymer solutions in chloroform, followed by thermal deposition of aluminium or
calcium cathodes at a base pressure of ≈ 8x10-6 mbar, defining pixel areas of about 4 mm 2. Double-layer devices with an additional hole-injection/transport layer (HTL), inserted between the ITO and the emissive polymer layer, were prepared similarly. The additional HTL layer consisted either of PEDOT:PSS (Bayer AG) (≈40-45 nm thick) or poly(p-phenylene vinylene), PPV (thickness ≈ 30-35 nm). The PPV layer was prepared via precursor route [7]. Electroluminescence, EL, and PL spectra were obtained with an UV-enhanced spectrograph (Oriel Instaspec IV) with a single monochromator. 3. RESULTS 3.1. CHARACTERISATION OF THE COPOLYMERS The molecular weight (determined by GPC with r
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