A new alternating copolymer (PSFDTBT) based on spirobifluorene, thiophene, and benzothiadiazole units has been synthesized. PSFDTBT has an optical band gap of 1.97 eV with the low-lying HOMO energy level at -5.4 eV. The hole mobility of the pristine PSFDTBT film spin-cast from o-dichlorobenzene (DCB) solution is 7.26 X 10(-3) cm(2) V-1 s(-1) with on/off ratios in the order of 10(5). Polymer solar cell devices based on the blend films of PSFDTBT and PC71BM show a high open-circuit voltage of 0.94 V and a power conversion efficiency of 4.6% without any post-treatment. All the device measurements were performed in air without encapsulation. This is the first report on spirobifluorene-based conjugated polymers used for polymer solar cells, demonstrating the great potential of spirobifluorene moiety as an electron-donating unit for the construction of main chain donor-acceptor alternating conjugated polymers for high performance polymer solar cells.