Increasing social demands for clean and cheap renewable energy sources have been promoting the researches on polymer photocells based on bulk heterojunction composites consisting of conjugated polymers and fullerenes. The replacement of traditional chloroaromatic solvents such as chlorobenzene with a halogen-free solvent 1,2,4-trimethylbenzene (TMB), which enables the preparation of bulk heterojunction composites with unmodified fullerenes, potentially reduces the economic and environmental costs of organic bulk heterojunction photocells. In this study, photovoltaic characteristics of PTB7:C-70 bulk heterojunction photocells prepared with TMB at low light intensity are studied. It has been found that the power conversion efficiency (PCE) of a photocell with ITO/PEDOT:PSS/PTB7:C-70/Al structure whose PCE is approximately 3 % under AM1.5G 1 sun (100 mW/cm(-2)) illumination increases with decreased illumination light intensity, being 5.9 % at 10(-3) sun illumination. An analysis of current-voltage characteristics suggests that the increased PCE is mainly due to increased shunt resistance and decreased saturation current of the diode in the model. The results indicate that the solution-processed organic photocell based on unmodified fullerene prepared with halogen-free solvent is a promising candidate for environmental-friendly device for indoor light harvesting.