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For solution-based small-molecule organic photovoltaic cells (SM BHJ), thermal annealing is often regarded as a very vital processing to yield high-efficiency devices. Here, the device with the photoactive p-DTS(FBTTh2)2:PC60BM blends has been studied by varying the annealing temperature. The highest power conversion efficiency (PCE), fill factor (FF), and short-circuit current ( Jsc) has been achieved after the device was annealed at the temperature of 80 °C. X-ray diffraction (XRD) displayed that the p-DTS(FBTTh2)2 tends to form big islands with well-defined crystalline structure (long coherence length) surrounded by small islands of the PC60BM after annealing, and reached the largest grain size of 27 nm after annealing at 80 °C. AFM images demonstrated the most sufficient phase separation within the photoactive blend layer after annealing at 80 °C, which provides good photogenerated excitons dissociation. Such improvements have proved beneficial to decrease the ohmic loss and recombination in carrier-transport process via measuring series resistance and shunt resistance as well as hole mobility. This work opens up a new way to understand the relationship between thermal annealing and molecular interaction from the viewpoint of lattice relaxation. [ABSTRACT FROM AUTHOR]