<p>This study employs mesoscopic modeling to investigate the mesostructure of block copolymers as active layers in bulk heterojunction polymer solar cells (BHJ-PSCs). We systematically analyze the morphology of various donor–acceptor (D-A) systems as a function of key parameters: the volumetric ratio between donor and acceptor blocks (<i>φ</i>_D/<i>φ</i>_A), processing temperature (<i>T</i>), and the Flory–Huggins parameter (<i>χ</i>_DA), which quantifies the compatibility between components. Using P3HT as a common donor block, we evaluate two distinct acceptor polymers: fullerene derivative PC₆₁BM and non-fullerene derivative PFTBT. The mesoscopic interaction parameters (<i>a</i>_DA) were derived from the temperature-dependent <i>χ</i>_DA&#xa0;values. Afterward, the self-assembly of the corresponding linear block copolymer families was simulated using Dissipative Particle Dynamics. Our results demonstrate that the resulting nanoscale morphology is predominantly governed by the thermodynamic compatibility between the donor and acceptor blocks, providing key insights for the rational design of high-performance BHJ-PSCs.</p> Graphical abstract <p></p>