One of society's challenges is to develop a clean, renewable, and sustainable energy source. Organic photovoltaics (OPVs) offer great potential as a low-cost solution to this need because of their ability to be solution processed over large areas. Currently, OPVs has surpassed the estimated 10% power conversion efficiency needed to achieve economic viability, but further improvement and large-area utilization is predicated on a deeper understanding of their complex microstructure and how this affects the device performance.

This aim of this dissertation is to develop a deeper understanding of the temperature dependent miscibility and interdiffusion within polymer-fullerene based organic solar cells. This initial understanding was extended to include strategies to control the polymer-fullerene miscibility within OPVs and the effect on the OPV performance. Ultimately, the polymer-fullerene miscibility can be used to control the microstructure and power conversion efficiency of the polymer-fullerene BHJ OPVs.