Nanoparticle agglomeration in polymer-based solar cells.

We show that the morphology of polymer-based solar cells substantially changes after annealing using small angle neutron scattering. Phenyl-C61-butyric acid methyl ester (PCBM) is found reasonably well dispersed within the poly(3-hexylthiophene) (P3HT) rich phase after initial processing (spin coating). However, the PCBM structure coarsens after annealing, clearly evidenced by the increase in scattering intensity at a small wave vector. The change in morphology at the nanoscale is related to improved device performance and the simultaneous, contradictory, increase in photoluminescence.