Highly Efficient Organic Light‐Emitting Diode Based on a Hidden Thermally Activated Delayed Fluorescence Channel in a Heptazine Derivative

: An orange-red organic light-emitting diode containing a heptazine derivative exhibits high performance with a maximum external quantum efficiency of 17.5 ± 1.3% and a peak luminance of 17000 ± 1600 cd m⁻² without any light out-coupling enhancement. The high electroluminescence performance can be ascribed to the presence of an efficient up-conversion channel from the lowest triplet state to the lowest singlet state.