Dispersive nonlinear dynamics of photoexcitations in π-conjugated polymers

We examine the recombination kinetics for long-lived photoexcitations in a series of π-conjugated polymer films using modulation frequency (ω) and excitation intensity (I) dependencies of the photoinduced absorption (PIA). The PIA in these films shows a variety of dependencies on ω and I. We analyze the data using monomolecular, bimolecular and defect-limited recombination processes that lead to saturation. Using generalized kinetics variables we found characteristic plots for all recombination processes, and show that a complete identification is possible only by examining the PIA dependencies both near and away from the steady state. In particular, the bimolecular recombination process shows a superlinear intensity dependence away from the steady state; on the contrary, dispersive kinetics leads to sublinear dependence even for this case.