Light emission from organic transistors holds strong potential for user-interactive functionality across applications ranging from wearable and biointegrated systems to neuromorphic electronics. However, conventional single-active-layer organic transistors suffer from inefficient charge carrier injection, resulting in high drain voltages (>80 V in field-effect devices and >3.5 V in electrochemical devices with p–i–n junctions) and narrow, spatially dynamic recombination zones (<75 μm). Here we report a single-active-layer electrochemical organic light-emitting transistor with drain-side electric double layer formation, achieving ultralow-voltage operation (<3.5 V) together with a wide, spatially pinned recombination zone. An ion transport enhancer in the light-emitting polymer channel induces an electric double layer at the drain electrode, overcoming limited electron injection without n-type doping. This mechanism enables flexible, large-area devices with a recombination zone width of 267 μm and a maximum luminance of 826 cd m−2 at 3.5 V, and operation with two 1.5-V batteries. This work advances single-active-layer OLETs and paves the way for organic electronic systems with intuitive, user-friendly visualization functions.