Narrow-bandgap n-type polymers are essential for advancing the development of all-polymer solar cells (all-PSCs). Herein, we developed a novel polymer acceptor PNT with π-extended 2-(3-oxo-2,3-dihydro-1H-cyclopenta[b]naphthalen-1-ylidene) malononitrile (CPNM) end groups. Compared to commonly used 2-(3-oxo-2,3-dihydro-1H-cyclopenta[b]naphthalen-1ylidene) malononitrile (IC) units, CPNM units have a further extended fused ring, providing the PNT polymer with extended absorption into the near-IR region (903 nm) and exhibiting a narrow optical bandgap (1.37 eV). Furthermore, PNT exhibits high electron mobility (6.79 × 10⁻⁴ cm² V⁻¹ S⁻¹) and a relatively high-lying lowest unoccupied molecular orbital (LUMO) energy level of −3.80 eV. When blended with PBDB-T, all-PSC achieves a power conversion efficiency (PCE) of 13.7% and a high short-circuit current density (J_SC) of 24.4 mA cm⁻², mainly attributed to broad absorption (600-900 nm) and efficient charge separation and collection. Our study provides a promising polymer acceptor for all-PSCs and demonstrates that π-extended CPNM units are important to achieve high-performance for all-PSCs.