Visualizing Complex Multi-Length-Scale Morphology in Non-Fullerene Photovoltaics with Nitrogen K-Edge Resonant Soft X-Ray Scattering*

Multi-length-scale morphology in organic photovoltaics (OPVs) and other functional soft materials leads to high complexity, but often dictates functionality. Such mesoscale complexity in OPVs originates from the kinetically trapped nonequilibrium morphology, which determines the device charge generation and transport. Resonant soft x-ray scattering (RSoXS) has been revolutionary on the exploration of OPV morphology in the past decade. However, for non-fullerene OPVs, RSoXS analysis near the carbon K-edge (CK-RSoXS) has been difficult, due to the chemical similarity of materials used in active layers. An innovative approach is provided by nitrogen K-edge RSoXS (NK-RSoXS), utilizing the spatial and orientational contrasts from the cyano groups in the acceptor material, which allows for determination of the phase separation. Of particular importance is that NK-RSoXS, for the first time, clearly visualizes the combined feature sizes in PM6:Y6 blends from crystallization and liquid-liquid demixing, determining optoelectronic properties. NK-RSoXS also reveals that PM6:Y6:Y6-BO ternary blends with reduced phase separation size and enhanced material crystallization can lead to current amplification in devices.