A novel naphtho[1,2-c:5,6-c']bis([1,2,5]thiadiazole)-based narrow-bandgap pi-conjugated polymer is designed for application in polymer solar cells. Remarkable power conversion efficiencies over 10% can be achieved based on both conventional and inverted device architectures with thick photoactive layers, which are processed by using chlorinated or nonhalogenated solvents, suggesting its great promise toward practical applications based on high-throughput roll-to-roll processing.