Improved efficiency by doping blue iridium (III) complex in europium complex-based light-emitting diodes

We demonstrated high-efficiency red organic light-emitting diodes (OLEDs) employing a europium complex, Eu (III) tris(thenoyltrifluoroacetone) 3,4,7,8-tetramethyl- 1,10-phenanthroline (Eu(TTA)₃(Tmphen)), as an emitter and a blue electrophosphorescent complex, Iridium (III) bis[4,6-di-fluorophenyl-pyridinato-N, C²] picolinate (FIrpic), as an assistant dopant codoped into 4,4-N,N-dicarbazole-biphenyl (CBP) host as an emissive layer. A pure red electroluminescence (EL) only from Eu³⁺ ions at 612 nm with a full width at half maximum of 3 nm was observed and the EL efficiency was significantly enhanced. The maximum EL efficiency reached 7.9 cd A^-1 at 0.01 mA cm^-2 current density, which is enhanced by 2.8 times compared with electrophosphorescence-undoped devices. The large improvements are attributed to energy transfer assistance effects of FIrpic, indicating a promising method for obtaining efficient red OLEDs based on rare-earth complexes.