Phase Separation and Molecular Intermixing in Polymer-Fullerene Bulk Heterojunction Thin Films

The phase separation and molecular intermixing in poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) bulk heterojunction thin films are investigated as a function of the overall PCBM content. The structural length scales, phase sizes, and molecular miscibility ratio in bulk heterojunction films are probed with grazing incidence small-angle neutron scattering (GISANS). The PCBM content is varied between 9 and 67 wt %. For the symmetric P3HT/PCBM ratio, which is typically highly efficient in photovoltaic devices, a structure size of 20 nm, the largest PCBM phases, and 18 vol % dispersed PCBM in the amorphous P3HT phase are found. The molecularly dispersed PCBM content is found to increase with the overall PCBM content. Absorption measurements complement the GISANS investigation.