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

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• 作者：Matthew R. Hammond ; R. Joseph Kline ; Andrew A. Herzing ; Lee J. Richter ; David S. Germack ; Hyun-Wook Ro ; Christopher L. Soles ; Daniel A. Fischer ; Tao Xu ; Luping Yu ; Michael F. Toney ; Dean M. DeLongchamp
• 刊名：ACS Nano
• 出版年：2011
• 出版时间：October 25, 2011
• 年：2011
• 卷：5
• 期：10
• 页码：8248-8257
• 全文大小：1004K
• 年卷期：0
• ISSN：1936-086X

文摘

We report quantitative measurements of ordering, molecular orientation, and nanoscale morphology in the active layer of bulk heterojunction (BHJ) 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 π-stacking direction within well-ordered regions, revealing a moderate preference for π-stacking in the vertical direction (“face-on”). 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.