A novel electron-accepting bis-lactam building block, 3,7-dithiophen-2-yl-1,5-dialkyl-1,5-naphthyridine-2,6-dione (NTDT), and a conjugated polymer P(NTDT-BDT) comprising NTDT as an electron acceptor and benzo[1,2-b:4,5-b']dithiophene (BDT) as an electron donor are designed and synthesized for producing efficient organic solar cells. The thermal, electronic, photophysical, electrochemical, and structural characteristics of NTDT and P(NTDT-BDT) are studied in detail and compared with those of the widely used bis-lactam acceptor 3,6-dithiophen-2-yl-2,5-dialkylpyrrolo [3,4-c]pyrrole-1,4-dione (DPPT) and its polymer P(DPPT-BDT). Compared to DPPT derivatives, NTDT and P(NTDT-BDT) exhibit remarkably higher absorption coefficients, deeper highest occupied molecular orbital energy levels, and more planar conformations. A bulk heterojunction solar cells based on P(NTDT-BDT) exhibit power conversion efficiency of up to 8.16% with high short circuit current (J(sc)) of 18.51 mA cm(-2), one of the highest J(sc) values yet obtained for BDT-based polymer. Thus, it is successfully demonstrated that the novel bis-lactam unit NTDT is a promising building block for use in organic photovoltaic devices.