Enhancement of Electroluminescence Intensity from Dye-Doped Poly(3-Alkylthiophene) Light Emitting Diode with Different Alkyl-Side-Chain Length

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Abstract
Electroluminescent (EL) diodes utilizing poly(3-alkylthiophene) (P3AT) containing fluorescent dyes have been fabricated and their marked enhancement of emission characteristics has been discussed. Enhancement of EL efficiency has been observed for diodes with P3AT doped with PBD (2-(4-biphenylyl)-5-(4- tert-butylphenyl)-1,3,4-oxadiazole), PPCP (1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene) or BPPC (N,N'-bis(2,5-di- tert-butylphenyl)-3,4,9,10-perylenedicarboximide), and more than 500 times higher emission efficiency is obtained in PPCP-doped P3AT. Based on the result of thickness dependence of EL efficiency in PBD-doped P3AT, the layer thickness at which maximum efficiency is obtained is found to be smaller than that in undoped P3AT. The mechanism of emission enhancement by doping of dye molecules into P3AT with different alkyl-side-chain lengths has been discussed in terms of carrier trapping and the energy band diagram.