Abstract Nonmixed planar heterojunction (PHJ) small-molecule organic photovoltaics (OPVs) with 96% internal quantum efficiency (at 595 nm) and 4.77% power conversion efficiency (PCE) have been demonstrated. In addition to boron subphthalocyanine chloride (SubPc) and C60 as electron donor and acceptor materials, respectively, PHJ OPVs contain an ultrathin (2 nm) buffer layer of bis-(naphthylphenylaminophenyl)fumaronitrile (NPAFN) between the indium tin oxide (ITO) anode and the donor layer (SubPc). Compared with copper phthalocyanine (CuPc) or α-naphthylphenylbiphenyl diamine (NPB) buffer layers, the NPAFN buffer layer blocks the exciton diffusion from the SubPc electron donor layer to the ITO anode more effectively and considerably improves the short circuit current (JSC) from 5.96 (without an NPAFN layer) to 7.70 mA/cm2 (with a 4-mm-thick NPAFN layer ). In addition, experimental results indicated that the NPAFN buffer layer reduces the crystallization, or stacking, of the SubPc electron donor, thereby limiting the reverse saturation current and elevating the open circuit voltage (VOC) from 1.01 (without an NPAFN layer) to 1.08 V (with a-2-nm thick NPAFN layer). However, series resistance (RS) of the OPV monotonically increases with increasing NPAFN layer thickness. The performance of the OPV is optimized when the NPAFN buffer layer thickness is 2 nm. Compared with a SubPc–C60 PHJ OPV without an NPAFN buffer layer, the PCE of a OPV with a buffer layer increases by 22% from 3.96% to 4.77%, with a concurrent increase in JSC (from 5.96 to 7.02 mA/cm2) and VOC (from 1.01 to 1.08 V). However, a decrease in RS (from 10.21 to 14.95 Ω cm2) and in fill factor (from 65% to 63%) is also observed.