The microstructure of the donor and acceptor materials within a bulk heterojunction is critical to the performance of organic solar cells, significantly influencing charge generation, transport, and overall efficiency and stability. Here, we studied the role of the donor : acceptor ratio and the use of additives on the resulting microstructure in films and devices based on PM6 : Y6. A detailed study of the resulting structures, mainly by X-ray scattering, indicates that the molecular arrangement leading to higher efficiencies is correlated with the presence of a single, well-mixed phase. In this phase, the diffusion of the Y6 molecules is constrained within the PM6 matrix. Interestingly, this microstructure can be achieved either by tuning the composition or by incorporating a solvent additive. Consequently, either approach can be employed to enhance photovoltaic performance.