Photoinduced absorption in an alternating polyfluorene copolymer for photovoltaic applications

We present a detailed study of a novel alternating polyfluorene copolymer, poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3-benzo-thiadiazole)], and its blends with the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester, using continuous wave photoinduced absorption (PA) techniques. We also present the use of phase information from the PA measurements for estimating the number of different photoexcitation types present in the PA spectra as well as their lifetimes. In all blends the PA spectra show a broad high-energy PA band ranging from ∼1 to 2 eV as well as a low-energy band at ∼0.35 eV. We find two kinds of excitations present at 80 K, polarons showing dispersive recombination with lifetimes of roughly a millisecond, and a second kind of photoexcitations tentatively assigned to coulombically bound intrachain polaron pairs showing practically non-dispersive recombination with a lifetime of around 20 μs.