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ABSTRACT The selection of sensitizer and its existence in the blend films are important to the performance of all-polymer ternary solar cells. Herein, all-polymer ternary solar cell devices, which used poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′] dithiophene-alt-3-fluorothieno[3,4-b]thiophene-2-carboxy-late] (PTB7-Th) as donor, poly[[ N, N-bis(2-octyldodecyl)-napthalene-1,4,5,8-bis(dicarboximide)−2,6-diyl]-alt-5, 5′-(2,2′-bithiophene)] (N2200) as acceptor and poly[ N−900-hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole) (PCDTBT) as sensitizer, are successfully demonstrated. The intermolecular interaction between donor PTB7-Th and sensitizer PCDTBT may lead to aggregation of PTB7-Th which decreases domain sizes and enlarges D/A effective interface area. In addition, the PCDTBT molecules also extend light absorption and cascaded energy levels of the ternary blend system. As a result, with 15% PCDTBT we get a power conversion efficiency of 5.11%, almost 20% higher than control device due to more favored exciton dissociation and higher charge transport efficiency. This study reveals a promising way to achieve high efficiency all-polymer solar cells using a low-band gap polymer PCDTBT. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1811-1819 [ABSTRACT FROM AUTHOR]