The detailed molecular structure of a polymer:fullerene blend used in bulk heterojunction solar cells has been established and correlated with macroscopic photovoltaic device properties.  Using a combination of x-ray diffraction (XRD) techniques, molecular mechanics (MM) simulations, two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) spectroscopy, and infrared absorption spectroscopy, we determine the molecular packing within the unit cell of the self-assembled poly-(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT): phenyl-C-butyric acid methyl ester (PCBM) bimolecular crystal.  The molecular packing reveals that the conformation of the electron-donating PBTTT polymer is significantly disrupted by incorporation of the electron-accepting PCBM molecules, which introduce twists and bends along the polymer backbone, curved polymer side chains, and one-dimensional (1D) fullerene channels for electron transport.  These results reveal that strong polymer-fullerene interactions dictate the local structure in polymer:fullerene blends, thus affecting important processes in organic solar cells such as exciton disassociation, recombination, and charge transport.

Integration of molecular mechanics simululations with X-ray diffraction to determine molecular packing in bulk heterojunction solar cells

243rd National Meeting (2012)

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