A high molecular weight alternating naphthalene diimide–bithiophene copolymer (poly­{N,N′-bis­(2-octyldodecyl)-naphthalene-1,4,5,8-bis­(dicarboximide)-2,6-diyl-alt-5,5′-(2,2′-bithiophene)} (PNDIOD-T2)) has been prepared by direct (hetero)­arylation polymerization (DHAP). Its structure and properties were investigated in comparison with the Stille-prepared commercial analog, N2200. It was found that the new DHAP-derived PNDIOD-T2 and known N2200 have similar optical absorption spectra and HOMO/LUMO energy levels but slightly different 1H NMR spectra. Inverted all-polymer solar cells using PNDIOD-T2 as the acceptor and PCE12 or PCE10 as the donor polymer showed slightly superior performances compared to similar N2200 devices. All-polymer solar cells with 7.3% efficiency could be achieved with PNDIOD-T2 blends without the use of a processing additive. These results demonstrate that low-cost and eco-friendly DHAP-based n-type semiconducting polymers are promising for developing high performance all-polymer solar cells.