Optimizing non-equilibrium carrier concentration via thickness engineering in Py-COF/CdS S-scheme heterojunctions for enhanced photocatalytic hydrogen evolution

The rational design of S-scheme heterojunctions aims to leverage the built-in electric field (BEF) for efficient charge separation, yet the optimize the non-equilibrium carrier concentration in the heterojunction remains underexplored. Herein, we construct S-Scheme heterojunctions Py-COF/CdS and systematically investigate the correlation between CdS layer thickness and carrier dynamics, emphasizing carrier drift-diffusion balance under BEF. Theoretical and experimental analyses reveal that optimizing CdS thickness in S-Scheme heterojunction to match the depletion-layer width (for solid-solid/solution interfaces) can minimize bulk diffusion length, thereby maximizing non-equilibrium carrier concentration. When the bulk diffusion length for charges is optimized to approach zero, carrier extraction efficiency peaks due to BEF-dominated drift transport, yielding the highest charge carrier concentration for SF5, which was around 1.2-fold increase than that of SF1 and SF8. The enhanced hydrogen evolution rate was also achieved to 10.50 mmol h−1 g−1 with a 19.2 % apparent quantum efficiency. This work establishes, for the first time, a thickness-dependent carrier regulation framework, bridging the gap between delicate material design and non-equilibrium kinetics for high-performance photocatalytic systems.