Green indium phosphide (InP)-based quantum dot light-emitting diodes (QD-LEDs) still suffer from low efficiency and short operational lifetime, posing a critical challenge to fully cadmium-free QD-LED displays and lighting1–3. Unfortunately, the factors that underlie these limitations remain unclear and, therefore, no clear device-engineering guidelines are available. Here, by using electrically excited transient absorption spectroscopy, we find that the low efficiency of state-of-the-art green cadmium-free QD-LEDs (which ubiquitously adopt the InP–ZnSeS–ZnS core–shell–shell structure) originates from the ZnSeS interlayer because it imposes a high injection barrier that limits the electron concentration and trap saturation. We demonstrate, both experimentally and theoretically, that replacing the currently widely used ZnSeS interlayer with a thickened ZnSe interlayer enables improved electron injection and depressed leakage simultaneously, allowing to achieve a peak external quantum efficiency of 26.68% and T95 lifetime (time for the luminance to drop to 95% of the initial value) of 1,241 h at an initial brightness of 1,000 cd m–2 in green InP-based QD-LEDs emitting at 543 nm—exceeding the previous best values by a factor of 1.6 and 165, respectively3,4. Replacing ZnSeS with a thickened ZnSe layer in green InP-based QD-LEDs improves the efficiency and lifetime, boosting electron injection and reducing leakage, enabling 26.68% external quantum efficiency and 1,241 h T95 lifetime.