Optical studies of excited-state relaxation in poly(9,9-dihexylfluorene-co-benzothiadiazole)

Abstract

We have studied the optical properties of poly(9,9-dihexylfluorene-co-benzothiadiazole) using absorption, photoluminescence as well, as cw and transient photoinduced absorption (PA) techniques. The PA spectrum shows a high-energy (HE) band peaked at 1.44 eV accompanied by shoulders on both sides. To study the recombination kinetics of the photoexcitations associated with the HE band we measured the intensity and frequency dependence of the PA signal, and modeled the results by analytically solving the full bimolecular rate equation using harmonic analysis. As an important result of these calculations we found that the experimentally observed linear intensity dependence at low pump intensities can be explained by including a monomolecular term into the rate equation, in contrast to the superlinear intensity dependence predicted for pure bimolecular recombination. Furthermore, the value of the monomolecular lifetime $\ensuremath{\tau}$ can directly be estimated from the ratio of the in-phase and out-of-phase signals in the linear regime, and we also show how to estimate the bimolecular recombination coefficient from the saturation value of the out-of-phase signal. To study the temperature dependence of the recombination process we measured the transient decay of the HE band, and by fitting the results to the full bimolecular rate equation we extracted the temperature dependence of the amplitude, the monomolecular lifetime, and the bimolecular recombination coefficient. The monomolecular lifetime decreases almost linearly with increasing temperature, and the bimolecular recombination coefficient has a very strong non-Arrhenius temperature dependence i.e., $\ensuremath{\beta}={\ensuremath{\beta}}_{0}\mathrm{exp}[{(T/T}_{0}{)}^{2}],$ with ${\ensuremath{\beta}}_{0}=1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}{\mathrm{cm}}^{3}/\mathrm{s}$ and a characteristic temperature ${T}_{0}=82\mathrm{K}.$ A model for this unusual temperature dependence is proposed.