Customizing Phase Separation in Non-Fullerene-Based Organic Photovoltaics: The Impact of Interfacial Morphology

Abstract

Organic photovoltaics (OPVs) show continuous fast improving power conversion efficiency (PCE) to date, owing to the newly developed materials and their functionalization. Nevertheless, precise morphological control in the active layer remains challenging. Particularly, manipulating the degree of phase separation appears as a key avenue to reduce geminate charge recombination. Herein, we report a strategy to tailor the mixing/pure phase in OPVs via terminal engineering of the non-fullerene acceptor (NFA) materials. We selectively design an asymmetric (1OM) and a symmetric (2OM) derivative of Y6-BO. Our further analysis shows a stepwise tailored degree of phase separation in the active layer when paring the above three acceptors with D18 as donor. Interestingly, 1OM not only suppressed the excessive crystallization and domain growth of D18, but also facilitated the formation of a well-balanced phase-separated morphology (Y6-BO>1OM>2OM) with continuous donor and acceptor domains. Such results render 1OM-based OPV with PCE of 19.1%, higher than that of Y6-BO (17.5%) and 2OM (18.2%), with additionally reduced geminate charge recombination revealed by ultra-fast transient absorption spectroscopy. We believe our findings highlight the importance of interfacial compatibility and morphology control in achieving efficient OPV cells.