Diffusion-controlled charge separation in conjugated polymer heterojunctions for visible light-driven hydrogen production

• Developed metal-free GCN-organic polymer heterojunctions for efficient H 2 evolution. • Identified a shift from type I to type II charge separation by modifying the D-A structure. • Superior hydrogen evolution rate, significantly outperforming conventional materials. • Dominance of diffusion-controlled charge separation, revealing critical electronic interactions within the heterojunctions. Internal electric fields are commonly considered the primary driving force behind charge separation in heterojunction structures for photocatalysis, while diffusion-controlled mechanisms have been less explored. Here, we develop two heterojunctions combining GCN with different D-A conjugated polymers, enabling type II and type I charge transfer mechanisms for photocatalytic H 2 evolution. A minor structural unit modification induces a transition from type I to type II charge separation. Theoretical calculations and experimental findings indicate that electron density variations between D-A conjugated polymers and GCN enable a diffusion-controlled charge separation, overwhelming the impact of internal electric fields. The BSO-T/GCN heterojunction, driven by effective diffusion-controlled charge separation, exhibits an enhanced photocatalytic HER (19.36 mmol/g/h), achieving a 13.6- and 7.0-fold enhancement over GCN (1.42 mmol/g/h) and BSO-T (2.78 mmol/g/h), respectively. The superior HER is significantly better than other D-A conjugated polymers/GCN heterojunction materials. These discoveries underscore the significance of diffusion-controlled charge separation and provide guidelines for developing high-performance heterojunction photocatalysts.