Organic solar cells (OSCs) based on donor-acceptor blends have shown a rapid improvement in power conversion efficiency (PCE) now approaching, for small cells, those of the state-of-the art commercial solar modules. However, performance degradation remains one of the most critical impediments for OSC technology commercialization. Ternary solar cells where a third component, for instance an acceptor, is added to a non-fullerene acceptor-polymer donor blend are an effective approach for improving both OSC efficiency and long-term stability. Here, we study the role of two fullerene acceptors, ET18 and PCBM, as the third component in P-D:Y6 blends. These fullerene derivatives significantly enhance the cell stability, which retained > 90% of their initial PCEs (13-14%) even after storage in air for 6 months, compared to only similar to 20% retention for the binary devices. GIWAXS, AFM, in situ impedance spectroscopy and femtosecond transient absorption spectroscopy measurements reveal that the enhanced stability of the ternary devices results from a more robust blend morphology reducing charge recombination in the ternary devices during aging.