Three D-pi-A copolymers containing thieno[3,2-b]thiophene (TT) bridge and BDT, carbazole, fluorene as D units and benzothiadiazole as A unit were synthesized and characterized. These copolymers of PBDT-tt-BT, PC-tt-BT and PF-tt-BT exhibited enough high thermal stabilites and good solubilites in chloroform and dichlorobenzene. Among the copolymers, with the increase of the electron-donating abilities of the D units from fluorene to carbazole further to BDT, the absorption spectra of PF-tt-BT shows blue shift and that of PBDT-tt-BT shows red shift comparing to that of PC-tt-BT in their solutions and films. Meanwhile, by electrochemical cyclic voltammetry measurements we found the HOMO levels vary in the same trench according to their electron-donating abilities. Under the illumination of AM 1.5G, 100 mW/cm(2), power conversion efficiency (PCE) of the PSCs based on these copolymers as donors and PC70M as acceptor were measured and PBDT-tt-BT shows a higher efficiency of 4.91% than PC-tt-ST and PF-tt-BT based devices mostly due to its higher hole mobility and broader absorption range. These results indicate that PBDT-tt-BT is a promising photovoltaic polymer donor material for efficient PSCs. (C) 2013 Elsevier Ltd. All rights reserved.