Facile Method for the Investigation of Temperature-Dependent C60 Diffusion in Conjugated Polymers

We developed a novel all-optical method for monitoring the diffusion of a small quencher molecule through a polymer layer in a bilayer architecture. Experimentally, we injected C-60 molecules from a C-60 layer into the adjacent donor layer by stepwise heating, and we measured how the photoluminescence (PL) of the donor layer becomes gradually quenched by the incoming C-60 molecules. By analyzing the temporal evolution of the PL, the diffusion coefficient of C-60 can be extracted, as well as its activation energy and an approximate concentration profile in the film. We applied this technique to three carbazole-based low-bandgap polymers with different glass temperatures with a view to study the impact of structural changes of the polymer matrix on the diffusion process. We find that C-60 diffusion is thermally activated and not driven by WFL-type collective motion above T-g but rather by local motions mediated by the sidechains. The results are useful as guidance for material design and device engineering, and the approach can be adapted to a wide range of donor and acceptor materials.