Tuning the side chains of conjugated polymers is a simple, yet effective strategy for modulating their structural and electrical properties, but their impact on n-type conjugated polymers has not been studied extensively, particularly in the area of all-polymer solar cells (all-PSCs). Herein, the effects of side chain engineering of P(NDI2OD-T2) polymer (also known as Polyera Activink N2200) are investigated, which is the most widely used n-type polymer in all-PSCs and organic field-effect transistors (OFETs), on their structural and electronic properties. A series of naphthalenediimide-bithiophene-based copolymers (P(NDIR-T2)) is synthesized, with different side chains (R) of 2-hexyldecyl (2-HD), 2-octyldodecyl (2-OD), and 2-decyltetradecyl (2-DT). The P(NDI2HD-T2) exhibits more noticeable crystalline behaviors than P(NDI2OD-T2) and P(NDI2DT-T2), thereby facilitating superior 3D charge transport. For example, the P(NDI2HD-T2) shows the highest OFET electron mobility (1.90 cm(2) V-1 s(-1)). Also, a series of all-PSCs is produced using different electron donors of PTB7-Th, PTB7, and PPDT2FBT. The P(NDI2HD-T2) based all-PSCs produce much higher power conversion efficiency (PCE) irrespective of the electron donors. In particular, the PTB7-Th: P(NDI2HD-T2) forms highly ordered, strong face-on interchain stackings, and has better intermixed bulk-heterojunction morphology, producing the highest PCE of 6.11% that has been obtained by P(NDIR-T2) based all-PSCs to date.