Enhanced triplet exciton generation in polyfluorene blends

Photoinduced absorption spectroscopy is used to study the intersystem crossing in a blend consisting of the fluorene-based conjugated polymers poly($9,9^{\prime }$-dioctylfluorene-$\mathit{co}$-benzothiadiazole) (F8BT) and poly( $9,9^{\prime }$-dioctylfluorene-$\mathit{co}$-bis-$N,N^{\prime }$-(4-butylphenyl)-bis-$N,N^{\prime }$-phenyl-1,4-phenylene-diamine) (PFB). The intersystem crossing efficiency is found to be $\sim 10$ times higher in F8BT:PFB than in F8BT alone. We attribute this effect to enhanced increased intersystem crossing in the charge-separated states formed at the polymer/polymer heterojunctions in the blend. Applying an electric field dissociates these states and thus reduces the rate of triplet state formation. We also perform time-dependent density functional theory quantum chemical calculations to determine the $T_1\to T_n$ absorption cross section and to estimate the triplet yield.