The practice of combining electron rich (donor, D) and electron deficient (acceptor, A) moieties to make low-band gap co-polymers has led to dramatic efficiency gains in organic photovoltaics (OPV). The flexibility afforded by combining different D and A building blocks to make distinct co-polymers allows for the tuning of the optoelectronic properties of the active layer to improve device performance. This tuning is currently predicted to a reasonable extent using electronic structure methods based on vacuum phase calculations. However, device properties such as spectral broadening and hole mobility depend critically on morphology. Such considerations need to be addressed by methods beyond calculations on isolated molecules. One can predict the morphology of a polymer film using classical, atomistic molecular dynamics (MD) simulations and appropriate force fields. Here, we report results from large-scale (volumes greater than 1000 nm<sup>3</sup>) MD simulations for a variety of high power conversion efficiency DA co-polymers. The internal packing and relative positions of D and A moieties were established via the analysis of radial distribution functions, inter-ring dihedral distributions and computed GIWAXS spectra. The effect of polymer packing on electronic structure was also analyzed using quantum chemical methods such as time dependent density function theory in order to determine the spectral broadening that occurs once a polymer is placed within a film. In addition, generalized inter-site electronic coupling distribution functions were computed using Corresponding Orbital Transformation for each pair of sites within the film. The relationship of these calculations to measured hole mobilities in neat polymers is described. Finally, we discuss the implications of these results for understanding structure-property relations in OPV.

Atomistic simulations of donor-acceptor polymer morphologies for high-efficiency organic photovoltaic

249th National Meeting (2015)

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