Device performance enhancement of polymer solar cells by nanoparticle self-assembly

We show that it is possible to assemble a sparse mono-layer of Fe3O4 nanoparticles (NPs) at cathode interface of a polymer solar cell based on poly(3-hexylthiophene): [6,6]-phenyl-C-60-butyric acid methyl ester (P3HT: PCBM) through the synergic effect of strong convective outflow, surface energy, Fe3O4 NPs concentration and active layer thickness. When the distance between those Fe3O4 NPs is smaller than the size of P3HT, the P3HT is excluded from the inter-particle space, and fullerene molecules fill in the space to build electron transport pathways improving charge transport and collection near cathode interface proved by transmission electron microscopy and X-ray photoelectron spectroscopy. The power conversion efficiency of the devices is improved up to 20%. The bulk morphology of light absorbing layers is not affected by the addition of Fe3O4 NPs as demonstrated by neutron and X-ray scattering results.