Recombination kinetics of photoexcitations in films of MEH-PPV and MEH-PPV/C60 mixtures

We report on the recombination kinetics of photoexcited polarons and triplet excitons in films of poly(2-methoxy-5-(2′-ethyl-hexyloxy)- 1,4-phenylene vinylene) (MEH-PPV) and MEH-PPV/C60 mixtures. Experimentally, the dependence of the polaron and exciton photoin- duced absorption bands on the excitation intensity and modulation frequency was measured, covering a dynamic range of 4-5 orders of magnitude. We have found that in all cases the recombination kinetics can be successfully described as a “bimolecular dispersive” recom- bination, with the following characteristics of the photoinduced absorption, (a) Sublinear modulation frequency dependence (ω−α, α < l) at high frequencies of both in-phase and quadrature components, (b) Sublinear excitation intensity dependence (Iγ,γ < l), with decreasing ex- ponent as the modulation frequency decreases. The exponent α and the average recombination time vary with the polymer molecular weight.