The structure of the bulk heterojunction (BHJ) interface is surely critical to the performance of organic BHJ solar cells as this is the interface where charge separation and recombination occurs. Yet little is known about such interfaces: How sharp, diffuse, or rough are they and how does that depend on the materials used? What role does molecular orientation play? The latter, due to the intrinsic asymmetry of the electronic properties of organic molecules and their aggregates has been linked to solar cell performance with preferentially aligned edge-on aggregates relative to the electrodes typically improving performance. However, what is the role of such preferential molecular order relative to the BHJ interface? Does it even occur, how can it be measured and possibly controlled?

The recent development of a specialized tool, polarization dependent resonant soft x-ray scattering (P-SoXS) [1], has provided an opportunity to address these critical questions. It has been used to demonstrate control of molecular orientation at donor/acceptor interfaces in BHJ solar cells through choice of molecular chemistry (fluorination of backbone) and processing solvent. The device performance in several systems investigated depends almost exclusively on the relative orientation of the donor species at the BHJ interfaces and is largely uncorrelated to traditionally considered morphological traits, such as domain size and purity and material crystallinity. In previous measurements [2], correlations in performance based on the relative sharpness of the interface have been observed. Overall, the canonical paradigm that describes the complex device operation of organic solar cells will have to take the structure of the BHJ interface into consideration.