A series of novel pi-conjugated copolymers based on 2,2'-bis(1,3,4-thiadiazole) (BTDz) have been developed. Among them, the BTDz-based donor-acceptor alternating copolymer with the (E)-1,2-di(3-(2-ethylhexyl)thiophene)vinylene donor unit (PBTDzTV) exhibited a high solubility and high crystallinity. PBTDzTVs favorably self assembled, forming face-on and edge-on multibilayer structures in thin nanoscale films. The relative volume fractions of these structures varied depending on the polymer's molecular weight. The higher molecular weight polymer formed a higher volume fraction of the face-on structure; in particular, the polymer with a 26.6 kDa of number-average molecular weight made only the face-on structure. The device performance was improved as the polymer molecular weight and the volume fraction of the face-on structure increased. The bulk-heterojunction photovoltaic device based on PBTDzTV:PC71BM demonstrated the high power conversion efficiency (PCE) of 8.04% when the device was fabricated with the highest molecular weight polymer having the face-on structure.