Synthesis and optical characterization of stable and highly luminescent poly-(9-vinylcarbazole)-aluminum-tris-(8-hydroxy
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MATERIALS RESEARCH
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Synthesis and optical characterization of stable and highly luminescent poly-(9-vinylcarbazole)-aluminum-tris(8-hydroxyquinoline) blends M. Allegrini, A. Arena, R. Girlanda, C. Pace, S. Patan`e, and G. Saittaa) Dipartimento di Fisica della Materia e Tecnologie Fisiche Avanzate, Istituto Nazionale per la Fisica della Materia (INFM) and Universit`a di Messina, Salita Sperone 31, I-98166, Messina, Italy (Received 15 May 1998; accepted 28 November 1998)
An electrochemical method used to obtain aluminum(III)-8 hydroxyquinoline (Alq3 ) blended into poly(N-vinylcarbazole) (PVK) matrix is described. The PVK-Alq3 blends show an intense green photoluminescence whose intensity increases by adding a small percentage of a substituted coumarin to the PVK-Alq3 solution. The material can easily be spin-coated to obtain homogeneous luminescent thin films suitable for optoelectronic applications. Spectrophotometry in the uv-vis-ir range, photoluminescence, and lifetime measurements are used to characterize the PVK-Alq3 films.
I. INTRODUCTION
Organic dyes and conjugated polymers have stimulated much research attention since the demonstration of their possible use as a low cost and highly processable active layer to obtain optoelectronic devices, such as polychromatic light emitting diodes1–5 and microcavities based on organic thin films sandwiched between highly reflective layers.6–9 Microcavity configuration has been used to obtain optically pumped solid-state lasers, and the achievement of electrically stimulated laser emission by solid-state organic compounds is actually under study.10–13 Most of the organic electro-optical devices use aluminum(III)-8 hydroxyquinoline (Alq3 ) as a highly luminescent electron-transporting medium.14 Because of the low solubility of the Alq3 powder, Alq3 films can be obtained only by vacuum deposition techniques. Here, we report on an approach that can be used to obtain homogeneous thin films of Alq3 dispersion in poly(Nvinylcarbazole) (PVK), with guest-to-host weight ratio within 4 : 1. The advantages of using soluble PVK-Alq3 blends instead of pure Alq3 , besides the increased processability, are the possibility to change the ground state absorption length and to minimize self-absorption processes by acting on the chromophore concentration. Further, the hole-transporting feature of PVK and the good electron mobility of Alq3 can be used to obtain bipolar single layer light emitting diodes. We observe that the luminescence intensity of PVK-Alq3 can be strongly increased by adding to the blends a small percentage of a soluble substituted coumarin, whose luminescence band is positioned below the PVK-Alq3 absorption threshold. The optical properties of PVKa)
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http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 6, Jun 1999
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Alq3 blends and coumarin-doped blends are investigated by means of spectrophotometric measurements in the uv-vis range. Infrared transmittance measurements are used t
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