In this study, we introduce a design methodology aimed at enhancing the ideality factor of SnO_x-based p-n diodes, fabricated fully at room temperature using ion-beam-assisted deposition. We demonstrate that this method can fabricate both p- and n-type SnO_x semiconductor films with mobility exceeding 1 cm²V^-1s^-1, which is beneficial for constructing high-performance p-n junction devices. Despite exhibiting satisfactory rectification properties, the diode ideality factor of 21.1 substantially deviates from the ideal value. By correlating the electrical and optical properties of the films, obtained via Hall and ab-sorption measurements, with numerical drift-diffusion simulations, we identify that the suboptimal ideality factor is due to an intermediate-phase layer between the p-type and n-type layers. Our findings suggest that reducing the thickness of the interfacial layer effectively lowers the ideality factor to 5.1, providing a promising strategy for enhancing the performance of SnO_x-based p-n diodes.