Two donor-acceptor (D--A) type naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole (NT)-based conjugated copolymers (CPs), namely, PBDT-TT-DTNT-HD and PBDT-TT-DTNT-OD, containing different side chain length (2-hexyldecyl, HD and 2-octyldodecyl, OD) anchoring to thiophene pi-bridge between the two-dimensional (2D) 5-((2-butyloctyl)thieno[3,2-b]thiophen-2-yl)benzo[1,2-b:4,5-b]dithiophene (BDT-TT) unit and NT moiety are developed and fully characterized. The resultant two copolymers exhibited broader absorption in wide range of 300-820 nm and obviously deepened E-HOMO of approximately -5.50 eV. The effects of side chain length on film-forming ability, absorption, energy levels, aggregation, dielectric constant (epsilon(r)), mobility, morphology, and photovoltaic properties are further systematically investigated. It was found that the side chain length had little impact on solution-processability, absorption, energy levels, and aggregation in CB solution of resultant CPs. However, tinily increasing side chain length promoted to form the more ordered structure of neat polymer film even if the corresponding epsilon(r) decreased. As a result, the side-chain-extended PBDT-TT-DTNT-OD:PC71BM-based device achieved 32% increased FF than that of PBDT-TT-DTNT-HD:PC71BM and thus the PCE was significantly raised from 3.99% to 5.21%, which were benefited from 2 times higher SCLC hole mobility, more favorable phase separation, and improved exciton dissociation. These findings could provide an important and valuable insight by side chain modulation for achieving efficient PSCs. (C) 2018 Wiley Periodicals, Inc.