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Solid‐state triplet–triplet annihilation upconversion (TTAUC) blue emission in an electroluminescence device (i.e., an organic light‐emitting diode (OLED)) is demonstrated. A conventional green fluorophore, tris‐(8‐hydroxyquinoline)aluminum (Alq₃), is employed as the sensitizer that generates 75% triplet under electrical pumping for the blue triplet–triplet annihilation emitter, 9,10‐bis(2′‐naphthyl) anthracene (ADN), with the heterojunction bilayer structure. The operation lifetime is elongated both for ADN blue (4.1x) and Alq₃green (34.8%) emission due to efficient use of excitons and separation of recombination and emission zone. To reduce the singlet quenching (SQ) of blue TTAUC signal by the Alq₃sensitizer with lower bandgap, 1‐(2,5‐dimethyl‐4‐(1‐pyrenyl)phenyl)pyrene (DMPPP) is inserted between the Alq₃and ADN as a triplet‐diffusion‐and‐singlet‐blocking layer. DMPPP exhibits triplet energy close to Alq₃and higher than ADN, as well as higher singlet energy than both Alq₃and ADN. It allows triplet diffusion from Alq₃to ADN, but blocks the SQ of the blue TTAUC signal by Alq₃. 86.1% intrinsic efficiency of TTAUC is demonstrated in this trilayer (Alq₃/DMPPP/ADN) OLED.