Quantifying the Degree of Phase Separation in the Photoactive Layer of Polymer Solar Cells

Through a combination of energy filtered transmission electron microscopy (EFTEM) and grazing-incidence small angle X-ray scattering (GISAXS), we have quantified the domain size and composition of polythiophene-fullerene solar cells. The differences in elemental composition between polythiophene and fullerene allows us to unambiguously image the mesoscale structure of the photoactive layer of high-performance polymer solar cells through EFTEM elemental mapping. Furthermore, the contrast in our images is quantitatively related to the composition in each of the phases. GISAXS provides a robust measure of the domain size, and is in good agreement with our EFTEM data. We find that both the domain size and composition are a strong function of processing temperature, and consequently suggests how the mesoscale structure of the photoactive layer might affect device performance.