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Publication Citation: Molecular Order in High-Efficiency Polymer/Fullerene Bulk Heterojunction Solar Cells

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Author(s): Matthew R. Hammond; Regis J. Kline; Andrew A. Herzing; Lee J. Richter; David Germack; Hyun Wook Ro; Christopher L. Soles; Daniel A. Fischer; Tao Xu; Luping Yu; Michael F. Toney; Dean M. DeLongchamp;
Title: Molecular Order in High-Efficiency Polymer/Fullerene Bulk Heterojunction Solar Cells
Published: September 22, 2011
Abstract: We report quantitative measurements of ordering, molecular orientation, and nanoscale morphology in the active layer of bulk heterojunction organic photovoltaic cells based on a thieno[3,4-b]thiophene-alt-benzodithiophene copolymer (PTB7), which has been shown to yield very high power conversion efficiency when blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). A surprisingly low degree of order was found in the polymer - far lower in the bulk heterojunction than in pure PTB7. X-ray diffraction data yielded a nearly full orientation distribution for the polymer -stacking direction within well-ordered regions, revealing a moderate preference for -stacking in the vertical direction (,face-onŠ). By combining molecular orientation information from polarizing absorption spectroscopies with the orientation distribution of ordered material from diffraction, we propose a model describing the PTB7 molecular orientation distribution (ordered and disordered), with the fraction of ordered polymer as a model parameter. This model shows that only a small fraction (≈ 20%) of the polymer in the PTB7/PC71BM blend is ordered. Energy-filtered transmission electron microscopy shows that the morphology of PTB7/PC71BM is composed of nanoscale fullerene-rich aggregates separated by polymer-rich regions. The addition of diiodooctane (DIO) to the casting solvent, as a processing additive, results in smaller domains and a more finely interpenetrating BHJ morphology, relative to blend films cast without DIO.
Citation: ACS Nano
Volume: 5
Issue: 10
Pages: pp. 8248 - 8257
Research Areas: Polymers, Electronics, Materials Science, Solar
DOI: http://dx.doi.org/10.1021/nn202951e  (Note: May link to a non-U.S. Government webpage)