Organic photovoltaics (OPVs) is a promising third-generation PV technology due to its unique advantages including tuneable absorption spectrum, mechanical robustness, and low cost upon upscaling. A typical OPV active layer comprises a nanoscale blend of two different materials with an energy offset, referred to as the donor (D) and acceptor (A). As a result, those semi-crystalline blend films contain crystalline donor and acceptor domains, as well as amorphous D:A intermixed domains. A comprehensive understanding of active layer morphology is crucial to establishing the processing-structure-performance relationship. Grazing-incidence small angle X-ray scattering (GISAXS) measurement is effective in studying the morphology of crystalline domains as the high local electron density provides a strong contrast for X-ray scattering. On the other hand, the morphology within the amorphous D:A intermixed domains also plays an important role during photo charge generation, yet it cannot be probed by conventional X-ray scattering techniques due to the similar chemical structures of donor and acceptor materials.

In this work, we attempted to tackle this problem by combining grazing-incidence small-angle neutron scattering (GISANS) with targeted deuteration of a prototypical acceptor molecule, Y6. By replacing hydrogen atoms on side chains of Y6 molecules with deuterium atoms (Figure. 1a), the scattering power of d-Y6 is significantly enhanced (Figure. 1b) yet its morphological and electronic properties in thin films remain unchanged. Encouragingly, a new scattering feature was observed in the GISANS spectra of PM6:Y6 blend film at the large-q region (0.05-0.1 Å^-1, see Figure 3d). Such a feature was not observed in the GISAXS spectra of the same film (Figure 3c), suggesting that it does not originate from crystalline Y6 domains but rather from amorphous Y6 aggregates embedded within the D:A intermixed domains, as shown in Figure 3 e and f. Additionally, we found the average size of  Y6 aggregates is quite sensitive to processing conditions and larger aggregates are generally good for assisting charge extraction and suppressing charge recombination as determined via capacitance spectroscopy. We then extended our results by deuterating other high-performance acceptors. Encouragingly, we found that the ability to form short-range aggregates within the intermixed domains is exclusive for the state-of-art Y-series molecules as further confirmed by large-scale molecular dynamic simulations.

Overall, we successfully probed the nanomorphology within the amorphous intermixed domains of the OPV active layer for the first time by combining GISANS and targeted deuteration. The unique ability of Y-series molecules to aggregate within the intermixed domains underpins their morphology advantages. At last, our ability to probe and control amorphous phase nanomorphology opens up a new avenue for performance-driven morphology optimization of OPV active layers.

For more information, please refer to our recent publication in Nature Communications,  Nat. Commun., 2024, 15, 2784. https://www.nature.com/articles/s41467-024-47052-7.