Control of the nanoscale morphology of the donor−acceptor material blends in organic solar cells is critical for optimizing the photovoltaic performances. The influence of intrinsic (acceptor materials) and extrinsic (donor:acceptor weight ratio, substrate, solvent) parameters was investigated, by atomic force microscopy (AFM) and electron tomography (ET), on the nanoscale phase separation of blends of a low-band-gap alternating polyfluorene copolymers (APFO-Green9) with [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM). The photovoltaic performances display an optimal efficiency for the device elaborated with a 1:3 APFO-Green polymer:[70]PCBM weight ratio and spin-coated from chloroform solution. The associated active layer morphology presents small phase-separated domains which is a good balance between a large interfacial donor−acceptor area and continuous paths of the donor and acceptor phases to the electrodes.