Three novel 1,8-naphthalimide-based small molecular acceptors (NI-A-C4, NI-A-C6 and NI-A-C8) were designed and synthesized. The LUMO levels of these three small molecules were high-lying, which significantly reduce the energy loss between the wide optical band gap polymer (WBGP) PBDTBT-C12 and the acceptors and result in a high open circuit voltage (V_oc) in solar cells. In addition, these three acceptors are planar, crystalline and H-aggregated in the solid state, which can facilitate the electron transport in blend films and lead to superior electron mobility and short circuit current (J_sc). A PCE of 4.05% with a V_oc of 1.08 V was obtained for PBDTBT-C12:NI-A-C6-based polymer solar cells (PSCs) in an active layer thickness of 35 nm. Such a PCE is comparable to that of PBDTBT-C12:PC₇₁BM-based optimized devices (4.07%) and better than devices with an active layer thickness of approximately 30 nm (2.72%). Besides, 35 nm is the thinnest active layer thickness for PSCs with a PCE above 4% and the absorption onset of PBDTBT-C12:NI-A-C6-blend films was as low as 630 nm, leading to a significantly high average visible transmittance up to 76.1%. Ultimately, a relatively high PCE and ultrahigh transmittance were achieved simultaneously, demonstrating that 1,8-naphthalimide-based small molecules are promising acceptors for tandem or semi-transparent PSCs.