We have designed and synthesized three nona-coordinated organoeuropium complexes (OEuCs) with the general formula [Eu(btfa) 3(FurTerPy)] (Eu1), [Eu(btfa) 3(ThioTerPy)] (Eu2), and [Eu(btfa) 3(NapTerPy)] (Eu3) by employing a primary 4,4,4-trifluoro-1-phenyl-1,3-butanedione (btfa) antenna ligand and three functionalized 2,2′:6′,2″-terpyridine (TerPy) ligands bearing different electron-donating groups at the 4′ position, namely, FurTerPy = 4′-(furan-2-yl)-2,2′:6′,2″-terpyridine; ThioTerPy = 4′-(thiophen-2-yl)-2,2′:6′,2″-terpyridine; and NapTerPy = 4′-(naphthalen-1-yl)-2,2′:6′,2″-terpyridine. The detailed photophysical properties of Eu1, Eu2, and Eu3 were analyzed using both experimental and computational methods. By analyzing the experimental and time-dependent density functional theory (TD-DFT) data in conjunction with the Lanthanide Luminescence Software Package (LUMPAC), we further elucidated the energy transfer processes in the OEuCs. Finally, the complexes were tested as the emitting layer (EML) in a multilayered device to fabricate red organic light-emitting diodes (R-OLED). Through optimization and device engineering, we have achieved a remarkable electroluminescence (EL) performance of maximum current efficiency (η c) = 12.32 cd/A, maximum power efficiency (η p) = 11.73 lm/W, and maximum external quantum efficiency (EQE max) = 10.20%, respectively, for the Eu3-based double-EML OLED. To the best of our knowledge, this is the highest reported overall EL performance among the OEuCs until now.