Contact Angle Behavior for Fullerene/Porphyrin Mixtures on Si surfaces*

Fullerene/porphyrin mixtures are of great interest in bulk heterojunction organic solar cells. Here we study the morphology of the phase separation which occurs when [6,6]-phenyl-C$_{61}$-butyric acid methyl ester (PCBM) and tetranitro-zinc phthalocyanine (tn-ZnPc), are deposited onto silicon (111) substrates, including the individual domain length scales, shapes and wetting angles. tn-ZnPC forms small clusters on the Si surface with a contact angle of approximately 15\r{ }, while PCBM forms compact clusters on broad ($\sim $0.5 um diameter) ``wetting-layer'' disks, with the cluster contact angle of $\sim $19\r{ }. Interestingly, a 50{\%} mixture shows topography qualitatively similar to that for PCBM, but with a larger contact angle of 22\r{ }, indicating that the mixture wets the interface less than either pure component alone.