Blue Upconversion Organic Light‐Emitting Diodes Achieving 1000 cd m −2 With an Operating Voltage of 2.7 V by Suppressing Interfacial Exciton Quenching

ABSTRACT Triplet–triplet annihilation upconversion (TTA‐UC) provides an effective route for generating high‐energy excitons from low‐energy states, but its efficiency is intrinsically limited at low triplet density because first‐order decay pathways dominate over the bimolecular TTA process. Applying this concept to electroluminescence, TTA‐UC OLEDs suffer from high threshold current density and low efficiency under low current density for the same reason. Here, a triplet‐blocking strategy is introduced to confine charge‐transfer‐generated triplets within a thin TTA host layer, thereby suppressing MoO 3 ‐interfacial exciton quenching, increasing the steady‐state triplet population, and enabling efficient upconversion under low electrical injection. Incorporating a high‐T 1 exciton‐block layer between MoO 3 and 1,2‐ADN reduces the threshold current density from 20 to 1 mA cm −2 , while allowing thin‐film UC‐OLEDs to retain higher efficiency. As a result, the optimized device attains a luminance of 100 cd m −2 at 1.8 V and 1000 cd m −2 at 2.7 V, representing the lowest operating voltages reported for blue OLEDs.