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

Recent interest in π-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′-ethylhexyl)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.