Importance of domain purity and molecular packing in effi cient solution-processed small-molecule solar cells

Organic solar cells made from a solution-processed blend of electron-donating and electron-accepting small molecules have been demonstrated to be viable alternatives to their conjugated polymer-based or evaporated small molecule counterparts. As in polymer-based devices, controlling and understanding the surprisingly complex nanoscale morphology of the active layer in molecular bulk heterojunction (BHJ) devices remains a principal challenge. For most BHJ systems, improved solar cell performance has been achieved by varying the film-processing conditions often leading to a distinctly different morphology from those seen in lower performing devices. In addition to size and composition variation (purity), molecular ordering relative to the dominant, discrete donor-acceptor interface can also be a critical structure parameter that impacts performance