Splitting salt water via sunlight into molecular oxygen and hydrogen for use as fuel or as an energy carrier is a clear pathway toward renewable energy. Monolayer MoS2 and WS2 are promising materials for the energetically demanding water oxidation reaction, absorbing ∼10% of incident light in the visible spectrum and possessing chemical stability and band edges more positive than the oxidation potential of water. A heterostructure of MoS2/WS2 forms a type-II heterojunction, supporting fast separation of the photogenerated charge carriers across the junction. Here, we show the role played by defects in determining the efficiency of the photon-driven oxidation process. By reducing the defects in this material system, it is possible to obtain an incident photon-to-current conversion efficiency (IPCE) of ∼1.6% and a visible-light-driven photocurrent density of 1.7 mA/cm2 for water oxidation. The efficiency is one order of magnitude higher than that of photoelectrocatalytic hydrogen reduction and water oxidation supported by liquid-phase exfoliated transition-metal dichalcogenides (TMDs). This result has been achieved with chemically vapor deposited (CVD) MoS2/WS2 heterojunctions, in the form of 100 μm large flakes assembled to form thin films. The large flakes sizes, compared to liquid-phase exfoliated materials (normally <5 μm), and thus the low edge flake density, and the flakes’ atomically sharp and clean interfaces between the flakes are responsible for reducing charge carrier recombination. These results show a general approach to the scalable synthesis of high-crystal-quality low-dimensional semiconductor photoelectrodes for solar energy conversion systems. It also shows the uniqueness of the CVD synthesis process of these materials, which can lead to high quality materials without the need of any postsynthesis treatments.