Recently, all-polymer solar cells (all-PSCs) have become an important organic photovoltaic technology, ascribing to their unique characteristics of high stability and mechanical endurance. However, the morphology control between polymer donor and polymer acceptor suffers from tough difficulties, resulting from the nature of rigid planarity and chain entanglement in the conjugated polymer backbones. In this work, we utilize an additive, 1-chloro-naphthalene (CN), to regulate polymer chain stacking and orientation in D18:PY-IT system, resulting in the formation of versatile nano-scale polymer fibrillization between donor and acceptor phases. Consequently, the CN-modified D18:PY-IT blend film shows improved molecular stacking characteristics and distinct nano-scale bi-continuous phase separation. Attributing to the incorporation of CN additive in a bulk-heterojunction (BHJ) D18:PY-IT system, it exhibits higher photovoltaic performance than the as-cast and only thermal annealing (TA) treated devices, where the CN-based device provides a power conversion efficiency (PCE) of 17.31%, an open-circuit voltage (V_OC) of 0.955 V, a short-circuit current density (J_SC) of 24.16 mA·cm⁻², and a fill factor (FF) of 74.99%, respectively. This is one of the highest photovoltaic performances reported in the D18:PY-IT based binary BHJ all-PSCs. Hence, it is evident that the morphology in all-PSCs can be feasibly modulated via incorporating appropriate additive into active layer for achieving excellent photovoltaic performance.