Three-Dimensional Nanoscale Imaging of Polymer Bulk-Heterojunction by Scanning Electrical Potential Microscopy and C-60(+) Cluster Ion Slicing

Abstract

Solution-processable fullerene and copolymer bulk-heterojunctions are widely used as the active layer of solar cells. It is known that the controlled phase-separation in the film provides a pathway for carrier transportation and is crucial to efficiency. In this work, scanning electrical potential microscopy (SEPM) is used to examine the surface distribution of [6,6]phenyl-C61-butyric acid methyl ester and poly(3-hexylthiophene), which form the bulk-heterojunction. Because the two components have different energies in the highest occupied molecular orbital (HOMO), the differences in contact potential yield strong contrast in SEPM. A cluster ion beam (C-60(+)) is used to remove the surface in order to determine the structure below, and SEPM is used to analyze the newly exposed surface. With the SEPM images acquired from different depth through the material stacked, a 3D volume image is obtained. It is demonstrated that using SEPM with cluster ion slicing is an effective tool for studying the 3D nanostructures of soft materials.