<p>The fabrication of heterostructures from two-dimensional materials is a key strategy for developing novel optoelectronic devices, yet performance is often limited by defective interfaces and inefficient charge collection at the contacts. While in-situ phase transformation of a parent crystal like SnS₂ can produce monolithic p-n junctions, previous work resulted in devices that did not function as true photodiodes. Here we show that by engineering the device architecture with a Platinum top contact, selected for its effective hole collection from p-type SnS, a high-performance vertical photodiode can be realized from a plasma-transformed SnS/SnS₂ heterojunction. The resulting Pt/SnS/SnS₂/Graphene vertical device exhibits excellent diode rectification with a significantly improved ideality factor of <i>n</i> = 1.51, indicating a high-quality interface. Critically, the heterojunction operates as an efficient photodiode under reverse bias and generating a large, stable photocurrent upon illumination. This enhanced performance is attributed to the effective separation of photogenerated carriers at the Type-II junction, followed by efficient collection of holes by the Pt contact and electrons by the graphene contact. This work demonstrates that strategic contact engineering is essential for unlocking the potential of phase-transformed 2D materials, providing a clear pathway from a material-level proof-of-concept to a functional, high-performance vertical photodiode.</p>