A Refined Bulk P–I–N Structure in All-Polymer Solar Cells To Achieve 20.1% Efficiency and Improved Stability

All-polymer solar cells (all-PSCs) have shown good potential for achieving balanced power conversion efficiency (PCE) and operational stability. However, precise control of the morphology remains challenging. Here, we constructed a bulk p-i-n structure with a regulated i-region by incorporating a shamrock-shaped nonfullerene acceptor, AQI4. This ternary formulation resulted in optimized energy-level stairing, enhanced exciton dissociation, and reduced energy loss. In blended thin film fabrication, a binary solvent of chlorobenzene (CB):o-xylene (OXY) was used, which enhanced the crystallization of the polymers and formed a high-density fibril network. The i-region width was reduced to less than 2 nm. This structural refinement enabled efficient charge transport through tie-chains between polymer crystallizations, thereby suppressing charge recombination. As a result, the organic solar cell (OSC) devices achieved a PCE of 20.1% (certified as 19.5%) and retained good stability. The <i>T</i>₈₀ lifetime of over 1800 h in the OSC structure was recorded. Notably, the binary solvent CB:OXY also contributed to a feasible printing fabrication; the 18.4 cm² OSC mini-modules achieved a PCE of 16.5%. Our results suggest that the shape of the guest molecule may be a key point to modulate the bulk p-i-n structure and improve the performance of all-PSCs.