Colloidal quantum dot (CQD) based photoelectrochemical cells (PECs) for solar-driven hydrogen (H2) generation offer an avenue to convert solar energy into chemical fuel. So far, recent advances in CQD-based PEC performance have relied on the use of defect passivation strategies. Specifically, shell-isolated defect passivation in ternary I–III–VI CQDs has been shown to contribute to lowered trap state densities, strengthened effective charge separation, and improved photovoltaic performance. Here we present a kind of heavy metal-free Cu-doped AgIn5S8/ZnS (CAIS/ZnS) core/shell CQD that shows a remarkable photocatalytic efficiency with an engineered shell thickness. In particular, we add a capping ZnS layer to protect the CAIS/ZnS CQD surface in photoanodes and thus enable a dual defect passivation strategy based on the homojunction combination. This strategy passivates surface defects and enhances photogenerated charge transportation in order to maintain high catalytic efficiency, and the additional ZnS layer suppresses energy transfer and nonradiative recombination for better charge injection and greater stability performance of PEC devices. As a result, we achieved a saturated photocurrent density of 23.40 mA cm−2 for a CAIS/ZnS CQD-based PEC measured at 0.75 V versus RHE under 3 suns (that is 3 times the standard solar irradiance of 100 mW cm−2), as compared to 10.60 mA cm−2 under 1 sun, which is a record for heavy metal-free CQD-based photoelectrodes for solar driven hydrogen generation. Importantly, our PEC devices maintained over 50% of their original photocurrent density after 6800 s of aging under 1 sun at 0.5 V versus RHE. Our work reveals a new platform for the development of high-efficiency photocatalysts based on I–III–VI CQDs.