A　metal–insulator-semiconductor　(MIS)　ternary　photo-system　was　intricately　crafted　through　precise　amalgamation　polyvinylpyrrolidone　(PVP)-capped　metal　Cu　with　typical　covalent　triazine　framework　CTF-1　via　electrostatic　self-assembly.　The　2　%　Cu-PVP-CTF　exhibited　an　impressive　CH4　yield　of　80.7　μmol·g−1·h−1　with　selectivity　of　96.8　%　under　visible　light,　representing　a　2.3-fold　and　112-fold　improvement　over　Schottky-type　Cu-CTF　and　pristine　CTF-1,　respectively.　In-situ　XPS　and　VASP-diff　calculations　unfolded　that　the　ultrathin　PVP　insulating　layer　significantly　expedited　interfacial　charges　tunneling,　corroborated　by　smaller　lifetime　τ2　determined　via　femtosecond　transient　absorption　spectroscopy.　The　intermediates　of　paramount　importance　in　CO2　reduction　like　*COOH　and　*HCHO　were　meticulously　monitored　by　in-situ　Fourier　infrared　spectroscopy.　DFT　calculations　elucidated　that　Cu-PVP-CTF　was　notably　more　adept　at　facillitating　the　rate-determining　step　(*COOH　→　*CO)　to　produce　CH4　than　Cu-CTF.　This　work　tamps　the　groundwork　for　conceptional　roadmap　in　designing　novel　MIS　photo-system　for　CO2　conversion.　©　2024　Elsevier　B.V.