Resonant Soft X-ray Scattering for Organic Photovoltaics.

Abstract

Resonant Soft X-ray Scattering (RSoXS) has emerged as a powerful technique for probing the morphology in organic bulk heterojunction (BHJ) solar cells, frequently employed as a measurement of phase purity and compositional length scales. Here we use the National Institute of Standards and Technology RSoXS Simulation Suite to systematically examine how structural features common to BHJs would contribute to RSoXS patterns in the PM6:Y6 BHJ system. Starting from experimentally determined anisotropic optical constants, we simulate scattering from controlled morphological variations including compositional heterogeneity, interfacial sharpness, surface roughness, and molecular orientation. Our results demonstrate that noncompositional features can cause increases in scattering intensity exceeding those from compositional phase separation. Surface roughness of just a few nanometers produces substantial scattering due to the high contrast between organic materials and vacuum, and molecular orientation effects─whether random, interface-aligned, or independently correlated─can dramatically influence pattern intensity and shape. However, each structural feature exhibits a distinct energy-dependent scattering signature across the carbon K-edge, suggesting that careful analysis of the complete spectral response could enable deconvolution of multiple contributions. These findings provide a broader interpretation of the excellent correlations between RSoXS measurements and BHJ solar cell device performance, while highlighting the potential of forward simulation approaches to leverage the full information content of energy-dependent RSoXS measurements.