In Situ GIWAXS Analysis of Solvent and Additive Effects on PTB7 Thin Film Microstructure Evolution during Spin Coating

• Published in: Advanced materials (Weinheim) Vol. 2017
• Main Authors: Manley, Eric F., Strzalka, Joseph, Fauvell, Thomas J., Jackson, Nicholas E., Leonardi, Matthew J., Eastham, Nicholas D., Marks, Tobin J., Chen, Lin X.
• Format: Journal Article
• Language: English
• Published: United States Wiley 09.10.2017
• Subjects: additive; in situ GIWAXS; MATERIALS SCIENCE; organic photovoltaic; PTB7; spin coating
• ISSN: 0935-9648; 1521-4095

The influence of solvent and processing additives on the pathways and rates of crystalline morphology formation for spin-coated semiconducting PTB7 (poly[[4,8-bis[(2- ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)- carbonyl]-thieno[3,4-b]thiophenediyl]]) thin films is investigated by in situ grazing incident wide angle x-ray scattering (GIWAXS) and optical reflectance, to better understand polymer solar cell (PSC) optimization approaches. In situ characterization of PTB7 film formation from chloroform (CF), chlorobenzene (CB), and 1,2-dichlorobenzene (DCB) solutions, as well as CB solutions with 1% and 3% v/v of the processing additives 1-chloronapthalene (CN), diphenyl ether (DPE), and 1,8-diiodooctane (DIO), reveals multiple crystallization pathways with: i) single-solvent systems exhibiting rapid (<3s) crystallization after a solvent boiling point-dependent film thinning transition, ii) solvent + additive systems exhibiting different crystallization pathways and crystallite formation times from minutes (CN, DPE) to 2 1.5 hours (DIO). Here, identifying crystalline intermediates has implications for bulk-heterojunction PSC morphology optimization via optimized spin-casting processes.