The energy difference between a singlet exciton and twice of a triplet exciton, Delta E-SF, provides the thermodynamic driving force for singlet exciton fission (SF). This work reports a systematic investigation on the effect of Delta E-SF on SF efficiency of five heteroacenes in their solutions. The low-temperature, near-infrared phosphorescence spectra gave the energy levels of the triplet excitons, allowing us to identify the values of Delta E-SF, which are -0.58, -0.34, -0.31, -0.32, and -0.34 eV for the thiophene, benzene, pyridine, and two tetrafluorobenzene terminated molecules, respectively. Corresponding SF efficiencies of the five heteroacenes in 0.02 M solutions were determined via femtosecond transient absorption spectroscopy to be 117%, 124%, 140%, 132%, and 135%, respectively. This result reveals that higher Delta E-SF is not, as commonly expected, always beneficial for higher SF efficiency in solution phase. On the contrary, excessive exoergicity results in reduction of SF efficiency in the heteroacenes due to the promotion of other competitive exciton relaxation pathways. Therefore, it is important to optimize thermodynamic driving force when organic materials for high SF efficiency.