Two types of small molecule nonfullerene acceptors (IDICO1 and IDICO2) based on 2,2 '-((2Z,2 ' Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b ']dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IDIC) are synthesized by attaching octyl side-chains onto terminal end groups. The alkyl substitution increases the lowest unoccupied molecular orbitals (-3.81 to -3.86 eV) of the two acceptors, compared to that of IDIC (-3.94 eV). Interestingly, the IDICO1 and IDICO2 films have higher integrated absorption coefficients (1.49 x 10(7) cm(-1)) than the IDIC (1.29 x 10(7) cm(-1)) film. Also, the electron mobilities of IDICO1 and IDICO2 are approximately twice as high as that of IDIC. The terminal octyl substitution also improves the miscibility with a donor polymer (PBDB-T) to form well-intermixed blends with a decreased pi-pi stacking distance. As a result, their photovoltaic devices exhibit significant improvements in both the open-circuit voltage and short-circuit current density, compared to those of the reference PBDB-T:IDIC device, exhibiting maximum power conversion efficiencies of up to 9.64%, 20.4%, and 1.68% under 1-sun, 1000-lx LED, and halogen lamp illumination, respectively, which are significantly higher than those of PBDB-T:IDIC (7.2%, 11.7%, and 1.2%, respectively). It is worth noting that a maximum power density of 141.4 mu W cm(-2) is achieved for the PBDB-T:IDICO2-based device under a halogen lamp, which is the highest value reported to date among those achieved under indoor lighting conditions.