Small-molecule-based non-fullerene acceptors (NFAs) are emerging as a new field in organic photovoltaics, due to their structural versatility, the tunability of their energy levels, and their ease of synthesis. High-efficiency polymer donors have been tested with these non-fullerene acceptors in order to further boost the efficiency of organic solar cells. Most of the polymer:fullerene systems are optimized with solvent additives for high efficiency, while little attention has been paid to NFA-based solar cells so far. In this report, the effect of the most common additive, 1,8-diiodooctane (DIO), on PTB7-Th:PC71BM solar cells is investigated and it is compared with non-fullerene acceptor 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno-[1,2-b:5,6b′]di-thiophene (ITIC) devices. It is interesting that the high boiling solvent additive does have a negative impact on the power conversion efficiency when PTB7-Th is blended with ITIC acceptor. The solar cell devices are studied in terms of their optical, photophysical, and morphological properties and find out that PTB7-Th:ITIC devices with DIO results in coarser domains, reduced absorption strength, and slightly lower mobility, while DIO improves the absorption strength of the PTB7-Th:PC71BM blend film and increase the aggregation of PC71BM in the blend, resulting in higher fill factor and Jsc.