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

Published

September 22, 2011

Author(s)

Matthew R. Hammond, Regis J. Kline, Andrew A. Herzing, Lee J. Richter, David Germack, Hyun W. Ro, Christopher L. Soles, Daniel A. Fischer, Tao Xu, Luping Yu, Michael F. Toney, Dean M. DeLongchamp

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-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.

Citation

ACS Nano

Volume

5

Issue

10

Pub Type

Journals

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DOI Link

Citation

Hammond, M. , Kline, R. , Herzing, A. , Richter, L. , Germack, D. , Ro, H. , Soles, C. , Fischer, D. , Xu, T. , Yu, L. , Toney, M. and DeLongchamp, D. (2011), Molecular Order in High-Efficiency Polymer/Fullerene Bulk Heterojunction Solar Cells, ACS Nano, [online], https://doi.org/10.1021/nn202951e (Accessed October 13, 2025)