Controlled mutual diffusion between fullerene and conjugated polymer nanopillars in ordered heterojunction solar cells

Abstract

A new approach is presented to control the nanomorphology of organic solar cells in a predictable, controllable, and easily-scalable way. The nanoimprint lithography (NIL) is combined with a subsequent molecular diffusion step controlled by thermal annealing. The new approach is realized by using nanointerdigitated donor-acceptor structure, consisting of poly(3-hexylthiophene-2,5-diyl) nanopillar arrays surrounded by phenyl-C61-butyric acid methyl ester. Subsequent thermal annealing leads to vertically aligned ordered quasi-bulk heterojunctions with hierarchical nanostructure. The changes are studied in nanostructural and electrical properties of the pillar samples using scanning probe microscopy. In addition, grazing-incidence small and wide angle X-ray scattering yield detailed quantitative information on the molecular- to domain-scale nanostructures. The changes in crystal size, chain orientation, and domain composition as a function of thermal anneal temperature and time are obtained. In addition, the conductive scanning force microscopy in quantitative imaging mode, applied to the pillar-based samples for the first time, allows us to establish a clear relationship between nanomorphology, nanoelectrical property, and macroscale device performance. It is believed that the NIL combined with controlled molecular diffusion is a powerful method, which could be easily extended to other materials and processes to realize a whole variety of other hierarchical nanomorphologies.