Varying the donor-acceptor ratio is a common technique in optimising organic photovoltaic (OPV) device performance. Here we fabricate poly(3-hexylthiophene) (P3HT): phenyl C-61 butyric acid methyl ester (PCBM) nanoparticle OPVs with varied donor-acceptor ratios from 1:0.5 to 1:2. Device performance increases with PCBM loading from 1:0.5 to 1:1, then surprisingly from 1:1 to 1:2 the performance plateaus, unlike reported trends in bulk heterojunction (BHJ) OPVs where device performance drops significantly as the donor:acceptor ratio increases beyond 1:1. Scanning transmission X-ray microscopy (STXM) measurements reveal core-shell nanoparticles for all donor:acceptor ratios with a systematic increase in the PCBM nanoparticle core volume observed as the PCBM loading is increased. This increases the functional PCBM domain size available for exciton harvesting, contrary to the result observed in BHJ OPV devices where increasing the PCBM loading does not lead to an increase in functional PCBM domains. In addition, STXM measurements reveal that the core-shell nanoparticles have core and shell compositions that change with PCBM loading. In particular, we observe that the PCBM component in the nanoparticle shell phase increases from a concentration that is below the percolation limit to one that is close to the optimal weight fraction for charge transport. This increase in the functional PCBM volume is reflected in an increase in PCBM photocurrent calculated from external quantum efficiency (EQE) measurements. (C) 2015 Elsevier B.V. 411 rights reserved.