title: Solvent-Morphology-Property Relationship of PTB7:PC71BM Polymer Solar Cells.
abstract: 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.
authors.affiliation: Physik-Department, Lehrstuhl für Funktionelle Materialien, Technische Universität München , James-Franck-Str. 1, 85748 Garching, Germany.
authors.forename: Shuai
authors.identifier: http://orcid.org/0000-0003-0759-7172
authors.initials: S
authors.lastname: Guo
authors.affiliation: Physik-Department, Lehrstuhl für Funktionelle Materialien, Technische Universität München , James-Franck-Str. 1, 85748 Garching, Germany.
authors.forename: Weijia
authors.identifier: http://orcid.org/0000-0003-0164-9322
authors.initials: W
authors.lastname: Wang
authors.affiliation: Munich School of Engineering, Technische Universität München , Lichtenberg Str. 4, 85748 Garching, Germany.
authors.forename: Eva M
authors.identifier: http://orcid.org/0000-0002-0151-5562
authors.initials: EM
authors.lastname: Herzig
authors.affiliation: Physik-Department, Lehrstuhl für Funktionelle Materialien, Technische Universität München , James-Franck-Str. 1, 85748 Garching, Germany.
authors.forename: Anna
authors.initials: A
authors.lastname: Naumann
authors.affiliation: Department of Engineering, University of Cambridge , 9 JJ Thomas Avenue, Cambridge CB3 0FA, United Kingdom.
authors.forename: Gregory
authors.initials: G
authors.lastname: Tainter
authors.affiliation: Deutsches Elektronen-Synchrotron at DESY , Notkestr. 85, 22603 Hamburg, Germany.
authors.forename: Jan
authors.initials: J
authors.lastname: Perlich
authors.affiliation: Physik-Department, Lehrstuhl für Funktionelle Materialien, Technische Universität München , James-Franck-Str. 1, 85748 Garching, Germany.
authors.forename: Peter
authors.identifier: http://orcid.org/0000-0002-9566-6088
authors.initials: P
authors.lastname: Müller-Buschbaum
keywords: PTB7; crystallinity; morphology; organic solar cells; solvent