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

Abstract

Complex morphology in organic photovoltaics (OPVs) and other functional soft materials commonly dictates performance. Such complexity in OPVs originates from the mesoscale kinetically trapped non‐equilibrium state, which governs device charge generation and transport. Resonant soft X‐ray scattering (RSoXS) has been revolutionary in the exploration of OPV morphology in the past decade due to its chemical and orientation sensitivity. However, for non‐fullerene OPVs, RSoXS analysis near the carbon K‐edge is challenging, due to the chemical similarity of the 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 phase separation. NK‐RSoXS clearly visualizes the combined feature sizes in PM6:Y6 blends from crystallization and liquid–liquid demixing, while 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 the mesoscale complexity and open opportunities to understand heterogeneous systems.

Multi-Edge Resonant Tender X-ray Diffraction for Probing the Crystalline Packing of Conjugated Polymers

“…For polymers that already contain sulfur, chlorination offers the potential for resonant diffraction to be performed at multiple absorption edges, potentially providing more information, since the resonant diffraction effects at different edges will be sensitive to different local bonding environments and bonding geometries. Recent work in the soft X-ray regime has shown the efficacy of performing resonant scattering experiments at multiple edges . In this recent work, resonant soft X-ray scattering (R-SoXS) measurements were performed at both carbon (∼285 eV) and nitrogen (∼400 eV) K-edges, with the nitrogen K-edge measurements helping to unravel the complex nanoscale morphology of polymer:nonfullerene acceptor blends used in organic solar cells.…”

“…Recent work in the soft X-ray regime has shown the efficacy of performing resonant scattering experiments at multiple edges. 7 In this recent work, resonant soft X-ray scattering (R-SoXS) measurements were performed at both carbon (∼285 eV) and nitrogen (∼400 eV) K-edges, with the nitrogen K-edge measurements helping to unravel the complex nanoscale morphology of polymer:nonfullerene acceptor blends used in organic solar cells.…”

Spectroelectrochemically determined energy levels of PM6:Y6 blends and their relevance to solar cell performance

“…55 Regarding Y6 , it has a high tendency to pack with a face-on orientation when processed from different processing solvents. 14,24,56–59 Here, the literature also gives examples of how small changes on the side groups of Y6 can impact the morphology although solution and processing parameters are kept constant. 60 These results underline the importance of the chosen solvent for film preparation since the influence on the morphology and orientation in the film is strong and decides whether there is face-on, edge-on or mixed orientations.…”

“…60 These results underline the importance of the chosen solvent for film preparation since the influence on the morphology and orientation in the film is strong and decides whether there is face-on, edge-on or mixed orientations. 25–27,59,61 This difference in morphological orientation of PM6 in PM6 : Y6 films fabricated with CF:CN and o -xylene is reflected in the device performances (see Fig. 1(d) for the corresponding current density–voltage ( J – V ) characteristics and the Experimental section for the fabrication conditions).…”

Design Rules of the Mixing Phase and Impacts on Device Performance in High-Efficiency Organic Photovoltaics

“…Recently, the emerging of nonfullerene acceptors (NFAs) has greatly improved the power conversion efficiency (PCE) of OPV. Unlike fullerene derivatives, NFA usually mixes well with donor molecules to induce a large content of amorphous mixing domain in morphology, away from spinodal decomposition created nanostructure [18][19][20]. As a result, the mixing zone of D/A becomes the key morphological issue that needs to be further optimized to obtain high PCE in NFA-based OPVs.…”