Two acceptor-donor1-donor2-donor1-acceptor (A-D-1-D-2-D-1-A)-type narrow bandgap small molecules BDT(IDT-IC)(2) and BDT(IDT-IC-2F)(2) with indacenodithiophene-benzodithiophene-indacenodithiophene (BDT-(IDT)(2)) as donating cores and 2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (IC) or 2-(5,6-difluoro-3-oxo-2,3-di-hydro-1H-inden-1-ylidene) malononitrile (2FIC) as end groups, have been synthesized and investigated as non-fullerene acceptors in solution-processed polymer solar cells (PSCs). The optical bandgaps of BDT(IDT-IC)(2) and BDT(IDT-IC-2F)(2) are 1.59 eV and 1.54 eV, respectively. Using PBDB-T as donor, the power conversion efficiencies (PCEs) of 4.98% and 6.21% are achieved by PSCs based on BDT(IDT-IC)(2) and BDT(IDT-IC-2F)(2), respectively.