Nanoparticle concentration profile in polymer-based solar cells

Polymer-based solar cell performance is dictated by nanoscale structures since the active layer suffers from extremely short exciton diffusion lengths. To understand the structures' effect on cell performance we use neutron reflectivity to investigate the thin film morphology containing a 1 : 1 by weight blend of [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) nanoparticles and poly(3-hexylthiophene) (P3HT). Neutron reflectivity is uniquely suited to this system due to the large scattering length density contrast between the two components, other techniques do not have this contrast. Here we find a higher PCBM concentration at the substrate and near, but not at, the air interface. Annealing the active layer shows a qualitatively similar profile with slightly different interfacial concentrations. Regardless, it is clear the resulting morphology is not optimal for device performance as we show for working solar cells.