Length Scale Coordination Between Effective Electron Diffusion and Light‐Emitting Boosts Electrochemiluminescence Efficiency

Electrochemiluminescence (ECL), the emission of light excited by electrochemical reactions, has drawn attention as a powerful tool for bioassays and environmental monitoring. Nonetheless, a limited understanding of the bulk electron transport (ET) process in solid‐state ECL emitters, particularly the mismatch between the electron diffusion length ( L n ) at the photoelectrode and the light‐emitting layer thickness ( d ), significantly undermines the ECL efficiency ( Φ ECL ) and impedes practical applications. Herein, we report L n /d as a quantitative impedance‐derived kinetic‐matching descriptor of bulk electron transport in solid‐state ECL systems, and an empirical L n / d range (0.9–1.2) for achieving high ECL efficiency is identified from the slope behavior of temporal ECL evolution curves. This strategy was further confirmed experimentally across different ECL systems, including metal‐organic frameworks (MOF), poly(heptazine imide) (PHI), and carbon nitride with tunable band structures. The optimal L n / d ratio range enabled superior electron transport and mass transfer, yielding a record‐high cathodic ECL efficiency for carbon nitride (CN) and enabling a visual tetracycline (TC) sensor with an excellent detection range and limit.