Localized surface plasmon (LSP) photophysical phenomena occurring in metal nanostructures are often presented as a method to effectively couple light into photovoltaic devices of sub-wavelength-scale thickness. However, the excitation of LSP is also associated with rapid energy dissipation leading to local heating, which affects the excitation energy pathway. We studied a system consisting of a planar gold nanoparticle (AuNP) array deposited at the surface of a semiconducting polymer thin film (P3HT). We observed heat transfer from laser pulse excited AuNPs into the P3HT, which was evidenced as a long-living thermochromic effect on transient optical absorption. By modeling of the ultrafast kinetics of exciton population evolution, we determined that their decay was caused by their mutual annihilation. The decay rate was controlled by a phonon-assisted one-dimensional diffusion mechanism with a diffusion constant of 2.2 nm ps. The transferred heat resulted in an increase of the diffusion constant by a factor of almost 2, compared to the control system of P3HT without AuNPs. These results are of practical use for the design of plasmon-enhanced optoelectronic devices.

The population of photogenerated species in organic semiconductors may decay due to their mutual annihilation upon collisions during their diffusive motion. The standard kinetic models for the population decay, n(t), assume a time-invariant diffusion coefficient, i.e., D(t) ≡ constant. This leads to a failure in predicting the experimentally observed temporal evolution of photogenerated species if it asymptotically approaches a power-law decay n(t) ∼ t –x , with x < 0.5. We have used a concept of the time-dependent diffusion coefficient and developed a novel mathematical method of its determination from decay collision rates obtained by transient optical absorption spectroscopy. We tested the applicability of this method on the interpretation of data of the decay of polaron population obtained experimentally by time-resolved transient absorption measurements on thin films of regioregular poly­(3-hexylthiophene), where we recently reported a power-law asymptote with x = 0.24. While we do not assume any microscopic origin of the time variance of D(t), we argue that, as the charge-carrier trapping states occupancy drops with decaying charge concentration, the carrier motion slows down. This argument is supported by a recent report on a molecular-scale model taking into account significant local anisotropy present in linear conjugated polymers. Our concept can be applied for the description of the evolution of species, like polarons or excitons, in various organic materials, provided their decay kinetics is controlled by a mutual annihilation during one- or three-dimensional diffusion.

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