Two new medium-bandgap (MBG) donor-acceptor (D-A) conjugated polymers (PSTF and PDTS) with fluorobenzotriazole as an A unit and spiro[cyclopenta[1,2-b:5,4-b]dithiophene-4,9-fluorene] (STF) or dithienosilole (DTS) as the D unit are designed and synthesized as donor materials for polymer solar cell (PSC) applications. PSTF shows a broader absorption spectrum relative to PDTS reflecting an additional high-energy absorption band due to the conjugated thiophene side chains on STF moiety. Compared with PDTS, PSTF exhibits weaker - aggregation and lower lying HOMO level. Photovoltaic properties of the PSCs reveal that either PSTF or PDTS using PC61BM as acceptor exhibits better performances than that of ITIC as acceptor, which results from the simultaneously increased V-oc, J(sc), and FF of PC61BM-based PSCs. Moreover, when combined with PC61BM and ITIC, the PSTF-based PSCs exhibit an efficiency of 3.66% and 2.42%, respectively, which is 45% and almost 1.5 times higher than that of the PDTS-based PSCs, respectively. This can be ascribed to the obviously improved V-oc and FF of PSTF-based PSCs benefitted from the deeper HOMO level and better active layer morphology. Our work demonstrates that using spiro-annulated building block as donor unit to construct MBG D-A copolymers is an alternative and effective approach for achieving efficient donor materials in PSCs. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 2330-2343