AFM-IR Spectromicroscopy Unveils Hidden Phase Separation in Polymer-Polymer Blend Films for Photovoltaic Applications

Conjugated polymers belong to a class of organic semiconductors that are used in a broad range of optoelectronic applications such as organic solar cells and organic light-emitting diodes. Thin films of two or more conjugated polymers or small molecules are coated from a solution that undergoes phase separation during drying and forms multiscale structures. In state-of-the-art bulk heterojunction organic solar cells, electron-donating and electron-accepting molecules form a network of donor-rich and acceptor-rich phases, whose domain sizes, composition, and interconnectivity play an important role in their power conversion efficiency. While these mesoscale structures formed in bulk heterojunction blend films under some circumstances can be observed by conventional scanning probe microscopy techniques, the task of mapping the film morphology becomes increasingly difficult when the donor and acceptor molecules are more chemically similar. Here we use AFM-IR, a combination of AFM (atomic force microscopy) and IR (infrared) spectroscopy, to image, with nanometer resolution, the morphology of a blend film of a donor polymer, TQ1, and an acceptor polymer, N2200, by using their distinct chemical composition contrast. These composition maps expose an interpenetrating network of the polymers that could not be distinguished by topography or phase imaging. Moreover, the dependence of the film structures, visualized by AFM-IR, on the molecular weight of the N2200 acceptor and the donor:acceptor blend ratio could be rationalized using Hansen solubility parameters (HSP).