A halogen-free solvent 1,2,4-trimethylbenzene dissolves both unmodified fullerenes and conjugated polymers such as PTB7-Th. This unique feature enables the study on the effect of the substituent of fullerene in polymer:fullerene bulk heterojunction solar cells under low-light illumination. Although the power conversion efficiency (PCE) of the PTB7-Th:C(70)solar cell was higher than that of the PTB7-Th:C-70-PCBM solar cell under one sun illumination, the latter showed higher PCE than the former under 10(-3)sun illumination. This result comes from the fact that C-70-PCBM yields higher open-circuit voltage (V-OC) at one sun and smaller dependence ofV(OC)on illumination light intensity than C-70.