Photophysics and photochemistry of conjugated polymer nanoparticles

Abstract

Nanoparticles prepared by the reprecipitation of conjugated polymers are an exciting development in the field of biological imaging and fluorescence sensing. The colloidal stability of these hydrophobic polymers in aqueous suspension was found to originate from the surface charge density of ~15 mC/m², which is in part attributed to negatively charged functional groups produced by polymer oxidation. The suitability of these nanoparticles in traditional conjugated polymer applications, such as polymer photovoltaics, was also investigated using femtosecond fluorescence upconversion and transient absorption spectroscopies. Polarisation resolved studies of energy transfer in highly compact nanoparticles and extended polymer conformations showed the nanoparticles exclusively undergo intermolecular energy transfer. These observations were supported by energy transfer simulations on polymer structures obtained from coarse-grained molecular dynamics simulations. Next, the recombination of polarons in composite rr-P3HT/PCBM nanoparticles, dominated by geminate recombination below 10 wt% PCBM, was demonstrated using a one dimensional diffusion model. This model also yielded the P3HT domain size of ~5 nm, which confirms these nanoparticles can serve as a model system for probing charge generation and recombination processes in device-like environments. Finally, the power dependent exciton decay in highly ordered nanowires yielded an exciton diffusion length of 11 ± 3 nm, which is toward the upper limit of diffusion lengths reported for annealed P3HT films. This data indicates the gentle solution based crystallisation of nanowires is a promising route for enhancing the performance of bulk heterojunction devices.