Ultrafast dynamics of newly synthesized low band gap polymers polyfluorene-thiophene-benzothiadiazole are studied. Additional carbazole groups are added to the side chain to modify its morphological and electronic characters. In solution, ultrafast interband excitation energy transfer is found to be faster in polymer without carbazole side group. In film form, fast quenching happens for exciton-exciton annihilation and charge recombination. The existence of carbazole group dramatically affects the excitation behavior of the film. The polaron-pair recombination was resolved dominant with carbazole group present in side chain. (C) 2011 Elsevier B. V. All rights reserved.