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[Display omitted] • Excellent activity and selectivity of CO 2 -to-CH 4 conversion is realized by MIS. • The ultrathin insulator layer significantly expedites the interfacial electrons tunneling. • The MIS promotes rate-determining step from *COOH to *CO to produce CH 4. 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 CH 4 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 τ 2 determined via femtosecond transient absorption spectroscopy. The intermediates of paramount importance in CO 2 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 CH 4 than Cu-CTF. This work tamps the groundwork for conceptional roadmap in designing novel MIS photo-system for CO 2 conversion. [ABSTRACT FROM AUTHOR]