Comparing electroluminescence efficiency and photoluminescence quantum yield of fluorene-based pi-conjugated copolymers with narrow band-gap comonomers

Abstract

Recent interest in pi-conjugated statistical copolymers, as well as their analog alternating copolymer, is due to their relatively high efficiencies as red light-emitting diodes and photovoltaic cells when used as active semiconductors. An example of this type of copolymer is poly {9,9-bis(2'-ethylhexyl)fluorene-2,7-diyl-co-2,5-bis(2-thienyl-1-cyanovinyl)-1-(2'-ethylhexyloxy)-4-methoxybenzene-5 '',5 '''-diyl}, which is synthesized from monomers 2,7-dibromo-9,9-bis(2'-ethy[hexyl)fluorene and 2,5-bis(2-(5'-bromothienyl)-1 -cyanovinyl)-1-(2 ''-ethylhexyloxy)-4-methoxybenzene. Accordingly, this study investigates the exciton dynamics of these materials as model copolymeric systems at systematically varied comonomer ratios. For a more quantitative exploration of the copolymer systems more quantitatively, ultraviolet-visible absorption and photoluminescence spectroscopy were utilized for diluted solutions and spin-cast thin films. The photoluminescence quantum yield was calculated from the time-integrated spectroscopic data. The yield values are discussed in terms of a primary kinetics model and used to clarify the different exciton dissipation behaviors of solutions and films. Moreover, the fundamental data are compared with previously reported electroluminescence efficiencies. (C) 2009 Elsevier B.V. All rights reserved.