Optical characterization using ms transient photoinduced absorption in poly(9,9-dihexylfluorene-co-benzothiadiazole)

Abstract

We have used transient photoinduced absorption (PA) in the ms time regime to study the recombination kinetics of long lived photoexcitations in poly(9,9-dihexylfluorene-co-benzothiadiazole) (F6BT). The PA spectrum shows a high energy band (HE) peaked at 1.44 eV associated with triplet excitons, and accompanied by shoulders on both sides. We show how to obtain the temperature dependence of the recombination process by fitting the ms transient PA decay of the HE band to the solution of the full bimolecular rate equation. From the fits we extract the temperature dependence of the amplitude, the monomolecular lifetime, and the bimolecular recombination coefficient. Both the amplitude and the monomolecular lifetime decrease almost linearly with increasing temperature, whereas the bimolecular recombination coefficient has a very strong non-Arrhenius temperature dependence, i.e. β=β 0 exp[(T/T 0) 2] , with β 0=1.0×10 −16 cm 3/s and a characteristic temperature T 0=82 K. A model for this unusual temperature dependence is proposed.