Superbly Efficient and Stable Ultrapure Blue Phosphorescent Organic Light‐Emitting Diodes with Tetradentate Pt(II) Complex with Vibration Suppression Effect

Blue phosphorescent organic light‐emitting diodes (PHOLEDs) are on the brink of commercialization for decades. However, the external quantum efficiency (EQE) and operational lifetime of PHOLEDs are not yet reached industrial standards. Here, a novel tetradentate Pt(II) emitter with a spirofluorene onto the carbazole unit that minimizes the vibration modes, corresponding to the structural relaxation during the de‐excitation, called the vibration suppression effect is reported. This modification reduces the intensity of the second peak in the spectrum and Shockley–Read–Hall recombination by blocking direct hole injection into the emitter while enhancing Förster resonance energy transfer, resulting in 451 h of LT50 (the time until a 50% decrease in initial luminance at 1000 cd m−2) and 25.1% of the maximum EQE (EQEmax). Thanks to the vibration suppression effect, an extremely narrow full width at half a maximum of 22 nm is obtained. In phosphor‐sensitized thermally activated delayed fluorescent OLED, ultra‐pure blue emission with Commission internationale de l′Eclairage (CIE) coordinates of (0.136, 0.096) is obtained with 28.1% of EQEmax. Furthermore, 50.3% of the EQEmax and 589 h of LT70 are simultaneously recorded with the two‐stack tandem PHOLED, which is the highest EQEmax among 2‐tandem and bottom‐emission PHOLEDs with CIEy