Influence of Molecular Excluded Volume and Connectivity on the Nanoscale Morphology of Conjugated Polymer/Small Molecule Blends

Over the past couple of decades, organic photovoltaics (OPV) based on conjugated polymer/fullerene derivative bulk heterojunctions have been extensively studied, resulting in single-junction efficiencies of order 10%. The need to push the efficiency toward 15% has resulted in the synthesis of a large number of non-fullerene electron acceptors with ever-increasing absorption coefficients in the red. Though some new acceptors have recently begun to be competitive with the fullerene, there is very little systematic understanding of which molecular geometries and spatial frontier orbital extent correlate with improved performance. One of the most important factors that determines the OPV efficiency is the nanoscale, phase-separated morphology of the blend. In this article, using a combination of resonant elastic X-ray scattering and elemental mapping, we investigate the influence of relatively small chemical changes to a nonplanar conjugated small molecule on the nanoscale morphology of the resulting polymer/molecule blend. We find that subtle modifications of the number and placement of peripheral functional groups can have an enormous influence on the length scale of phase separation. We then quantify the extent of phase separation by using the generalized indirect Fourier transform to convert resonant scattering intensities to pair-distance distribution functions. Our results point toward the large influence of the molecular excluded volume as a major morphology determinant. This work has implications for synthetic efforts to create non-fullerene electron acceptors that can substantially outcompete fullerenes in OPV devices