Solvent-morphology-property relationship of PTB7: PC₇₁BM polymer solar cells

The influence of three different solvents and a solvent additive on the morphology and photovoltaic performance of bulk heterojunction films made of the copolymer based on thieno[3,4-b]thiophene-alt-benzodithiophene unit PTB7-F40 blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM) is investigated. Optical microscopy and atomic force microscopy are combined with X-ray reflectivity and grazing incidence small and wide-angle X-ray scattering (GISAXS and GIWAXS, respectively), enabling the characterization of the morphology of the whole photoactive film. The detailed study reveals that different length scales of PCBM clusters are observed using different solvents, while adding a solvent additive results in the PCBM clusters being selectively dissolved. Vertical and lateral phase separation occurs during spin coating and depends on the solvent used. A hierarchical morphology is detected within the bulk film through GISAXS measurements. Furthermore, GIWAXS shows that a rather amorphous film with low crystallinity was probed, which substantiates that high crystallinity is not necessarily required for high performance organic solar cells. Different models for the morphology are proposed through the combination of all the findings and correlated with the corresponding device properties. Consequently, the solvent-induced different device performance is mainly ascribed to the varied lateral structure sizes, whereas the highest device performance is a result of the smallest average multilength scale lateral structure sizes with the smallest length scale matching the exciton diffusion length.