Silicon (Si) and zinc oxide (ZnO)-based heterojunctions are optoelectronic devices that promise to provide good photonic performance. The n-ZnO  /  p-Si structure is investigated using two-dimensional numerical simulation by ATLAS Silvaco software. This study investigated the effects of p-Si substrate parameters, such as thickness, acceptor concentration, and minority carrier lifetime, on the photonic performances of n-ZnO  /  p-Si heterojunction, taking into account the interface states and defects in the ZnO emitter layer for a real simulated structure. The simulation results showed that the optimal parameters that allow for better photonic performance are a p-Si thickness of 250  μm, an acceptor concentration of 6  ×  10¹⁵  cm^−  3, and a minority carrier lifetime of 10^−  3  s. The obtained photovoltaic parameters are short circuit current density of J_SC  =  38.9  mA  /  cm², open circuit voltage of V_OC  =  0.54  V, fill factor (FF) of FF  =  59  %  , and conversion efficiency of η  =  12.36  %  .