In summary, this study reported the synthesis of two naphthalene imide small molecules, namely NDT-1 and NDT-2, which were successfully used as ETMs in OSCs. The introduction of ester functional groups at the π-bridge units in NDT-2 resulted in (1) an enlarged molecular dihedral angle, improved alcohol solubility, enhanced film conductivity and film-forming quality; (2) suppression of charge recombination, enhancement of electron mobility and reduction of series resistance, leading to a significant increase in J_SC; (3) almost no effect on V_OC and FF. Ultimately, OSC devices based on PM6:BTP-eC9 with NDT-2 as the ETM achieved a remarkable PCE of 17.1% (a V_OC of 0.839 V, a J_SC of 28.2 mA cm^-2 and FF of 72.1%), demonstrating superior photovoltaic performance compared to NDT-1 (PCE = 13.2%). This work discovered that the ester-modified NI molecules exhibit excellent alcohol solubility and film-forming ability, providing valuable insights for the development of NI molecules as efficient ETMs used in high-efficient OSCs