Two electron-donating sides chain (octylthio and methoxy) were strategically attached on BDTT unit to develop two new polymers (PBDTT-OS-TT-EF and PBDTT-OS-TT-CF) as donor materials for polymer solar cells. PBDTT-OS-TT-EF and PBDTT-OS-TT-CF show broad absorption spectra with narrow optical band gap of 1.52 and 1.56 eV, and low-lying HOMO energy level of -5.34 and -5.46 eV, respectively. A high V-oc of 0.92 V was obtained from the PSC devices based on PBDTT-OS-TT-EF. Particularly, the V-oc of PSC device based on PBDTT-OS-TT-CF is as high as 1.00 V, which is the highest value for the devices based on TT-base polymers. The results indicate that the engineering two electron-donating side chains in BDTT unit is an effective strategy to narrow the band-gap and manipulate the energy levels of polymers, which can be potentially applied to design polymer donor materials with high V-oc values. (C) 2016 Elsevier B.V. All rights reserved.