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

Complex morphology in organic photovoltaics (OPVs) and other functional soft materials often dictates performance output. Such complexity in OPVs originates from the kinetically trapped nonequilibrium state on mesoscale, 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 due to its enhanced chemical and orientation sensitivity. However, for non-fullerene OPVs, RSoXS analysis near the carbon K-edge (CK-RSoXS) has been challenging, 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 materials, 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. Nitrogen is common in organic semiconductors and other soft materials, and the strong and directional N 1s-->π* resonances make NK-RSoXS a powerful tool to uncover mesoscale complexity and open opportunities to understand heterogeneous systems.