  Materials' microstructure strongly influences its performance and is thus a
critical aspect in design of functional materials. Previous efforts on
microstructure mediated design mostly assume isotropy, which is not ideal when
material performance is dependent on an underlying transport phenomenon. In
this article, we propose an anisotropic microstructure design strategy that
leverages Spectral Density Function (SDF) for rapid reconstruction of high
resolution, two phase, isotropic or anisotropic microstructures in 2D and 3D.
We demonstrate that SDF microstructure representation provides an intuitive
method for quantifying anisotropy through a dimensionless scalar variable
termed anisotropy index. The computational efficiency and low dimensional
microstructure representation enabled by our method is demonstrated through an
active layer design case study for Bulk Heterojunction Organic Photovoltaic
Cells (OPVCs). Results indicate that optimized design, exhibiting strong
anisotropy, outperforms isotropic active layer designs. Further, we show that
Cross-sectional Scanning Tunneling Microscopy and Spectroscopy (XSTM/S) is as
an effective tool for characterization of anisotropic microstructures.
