[en] Tunable photo-response is highly desirable by photodiodes for future optoelectronic applications. As compared to bulk semiconducting materials, the atomically thin two-dimensional (2D) materials may be one of the potential candidates to fabricate such adaptive photodiodes, since they possess not only excellent but also widely tunable optoelectronic properties. The most extensively applied device structure for the 2D materials based photodiodes is the vertically aligned van der Waals heterostructure. However, fabricating the vertical 2D material heterostructures is usually complicated, involving manually stacking multiple 2D material flakes together, which is undesirable for industry applications. In this work, we developed a vertical MoO₃/MoS₂ heterojunction for photodetection and photovoltaic applications. The device used MoS₂ and its oxidation layer of MoO₃ as the n- and p-type regions, respectively, which can greatly simplify the fabrication process of 2D vertical heterojunctions. Moreover, the device exhibited prominent photo-response with photo-responsivity of 670 mA W⁻¹, detectivity of 4.77  ×  10¹⁰ Jones and power conversion efficiency (PCE) of 3.5% under 0 V bias. The device also presented efficient gate tunability on photocurrent with on/off ratio of 10³. This research provides an alternative way to fabricate 2D materials based vertical heterojunctions for optoelectronic applications with tunable photo-responses. (paper)