A cyclometalated platinum (II) complex, Pt04, was designed and synthesized as a blue dopant for solution-processed phosphorescent (Ph) and phosphor-sensitized fluorescence (PSF) OLEDs. A multidentate N-heterocyclic ligand framework coordinates the Pt04 core. The molecular structure also includes bulky tert-butyl-substituted aryl groups, which effectively reduce intermolecular aggregation and prevent excimer formation. Solution-processed Ph-OLEDs using Pt04 as the emitter reached a maximum external quantum efficiency (EQE) of 18.3%. They achieved a current efficiency (CE) of 24.7 cd A⁻¹ and a power efficiency (PE) of 15.5 lm W⁻¹. When Pt04 served as a triplet sensitizer in solution-processed PSF-OLEDs, the devices showed an improved EQE of 28.7%, along with stable blue emission at CIE (0.134, 0.147). These results show how effective molecularly engineered Pt sensitizers can be for high-efficiency blue OLEDs through solution processing.