Poly{[2,7-(9,9-bis-(1-(2-(2-methoxyethoxy)ethoxy)ethyl)-fluorene)]-alt-[5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)]} (EO-PF-DTBT) was synthesized by Suzuki coupling reaction. The polymer is soluble in common organic solvent, such as toluene, THF, and chloroform, and it also shows solubility in polar solvent, such as cyclopentanone. Solar cells based on EO-PF-DTBT and PC61BM show maximum power conversion efficiency of 2.65% with an open circuit voltage (V-OC) of 0.86 V, a short circuit current density (J(SC)) of 6.10 mA/cm(2), and a fill factor of 51% under AM 1.5G illumination at 100 mW/cm(2), which is the best results for fluorene and 4,7-di-2-thienyl-2,1,3-benzothiadiazole copolymers and PC61BM blend. The 1,8-diiodooctane can work well to reduce the over-aggregated phase structure in polymer solar cells. Our results suggest that the introducing high hydrophilic side chain into conjugated polymer donor materials can tune the aggregation structure and improve the solar cells performances. (c) 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40478.