Abstract: It is a critical proposition to efficiently convert CO₂ into hydrocarbon fuel utilizing photocatalytic technology. However, the insufficient thermodynamic potential of photogenerated carriers and the sluggish multi-proton coupled electron transfer (PCET) process severely hinder the formation of CH₄ and other hydrocarbons. Hence, we constructed an all-organic S-scheme heterojunction photocatalyst (CN/UPDI-x) with large π-delocalization via π-π interactions, with CO and CH₄ yields of 34.10 and 4.55 µmol g^-1 h^-1, where the CH₄ yields were 12.3 and 11.7 times higher than those of pristine CN and UPDI, respectively. The S-scheme heterojunction improves the separation efficiency of photogenerated electron-hole pairs, preserves the highly oxidizing holes required for accelerating water oxidation for H* production, and enables a substantial accumulation of high-energy electrons that drive the conversion of reaction intermediates. Moreover, the extensive π-electron delocalization system formed by CN and UPDI offers an efficient pathway for the rapid transport of photogenerated electrons. The high-efficiency supply of H* coupled with CO₂ adsorbed on the CN at the heterojunction interface to form intermediate *CHO. This intermediate is further transformed into CH₄ through a multi-step hydrogenation process. This work provides novel perspectives for the design and development of organic polymer semiconductor photocatalysts applicable to environmental protection and clean energy production.

Key words: All-organic S-scheme heterojunction, PCET, π-π, interactions, Photocatalytic CO₂ reduction