Extending the measurement of domain purity in organic photovoltaic blends

Resonant Soft X-ray Scattering (RSoXS) is an advanced structural characterization technique that combines small-angle x-ray scattering with soft x-ray spectroscopy to provide enhanced contrast and sensitivity to molecular orientation in soft matter thin films. Within the organic photovoltaic (OPV) community, the RSoXS technique is frequently employed to determine domain size and domain purity in blend films, which are often found to correlate with OPV power conversion efficiencies (PCE), with the common finding that increased purity leads to decreased electron-hole recombination and higher PCE. We will show that RSoXS contrast due to molecular orientation cannot be excluded from the measurement by energy selection alone, leading us to conclude that the conventional RSoXS purity measurement delivers a combination of compositional heterogeneity and orientational heterogeneity information.



In this presentation we will describe our efforts to deconvolute contributions from compositional heterogeneity and orientational heterogeneity in the RSoXS phase purity measurment. Using CyRSoXS, a GPU-accelerated voxel-based RSoXS simulator, we demonstrate the impact of molecular orientation and film surface roughness on the results of the conventional RSoXS domain size / phase purity assessment. This framework is validated using a synthetic model system, and then applied to an all-polymer OPV blend system to quantify the relative contributions of compositional, orientational, and vacuum scattering. We discuss these results in the context of developing more robust structure-property relationships: by isolating compositional and orientational aspects of morphology, both of which affect charge transport and recombination, we can accelerate the optimization of OPV blends for next generation organic electronics.