A series of eight new donor-acceptor conjugated copolymers based on thiazolo[5,4-d]thiazole (TT) were synthesized by Suzuki and Stile polycondensations, and their optical, electronic structure, field-effect charge transport, and photovoltaic properties were investigated. The number-average molecular weight was moderate to high (20-103 kDa) for four of the new copolymers and low (<10 kDa) for the rest. The optical band gap (1.7-2.2 eV) and the ionization potential (4.9-5.7 eV) of the copolymers could be tuned by varying the strength of the electron-donating comonomers. X-ray diffraction analysis of the neat TT polymer thin films showed that they exhibit a lamellar crystalline morphology with a (100) d-spacing of 1.449-1.881 nm. The polymers had field-effect hole mobilities of 2 X 10(-3)-3 X 10(-2) cm(2)/(V s). Although fullerene (PC71BM)-based bulk heterojunction solar cells showed that most of the new polymers had low photovoltaic power conversion efficiency (<2.4%), the standout polymer PDEHTT had a high efficiency of 5.7%. X-ray diffraction analysis and TEM imaging of PDEHTT/PC71BM blend films showed a bicontinuous nanoscale morphology with a crystalline polymer nanofiber (15-20 nm width) network.