The most effective and potential approach to improve the performance of heterojunction photodetectors is to obtain favorable interfacial passivation by adding an insertion layer. In this paper, MoOx/Al2O3/n-Si heterojunction photodetectors with excellent photocurrents, responsivity and detectivity were fabricated, in which alumina acts as a tunneling passivation layer. By optimizing the post-annealing treatment temperature of the MoOx and the thickness of the ultra-thin Al2O3, the photodetector achieved a ratio of photocurrent to dark current of 3.1 × 105, a photoresponsivity of 7.11 A W−1 (@980 nm) and a detective of 9.85 × 1012 Jones at −5 V bias. Besides, a self-driven response of 0.17 A W−1 and a high photocurrent/dark current ratio of 2.07 × 104 were obtained. The result demonstrated that optimizing the interface of heterojunctions is a promising way to obtain a heterojunction photodetector with high-performance. [ABSTRACT FROM AUTHOR]