Bright and efficient red emitting electroluminescent devices fabricated from ternary europium complexes

Abstract

The photophysical properties of two previously synthesized, highly efficient, bright red emitting complexes [Eu(btfa)₃DPEPO] (CIE, 0.672; 0.326) (Eu-1) and [Eu(nta)₃DPEPO] (CIE, 0.673; 0.326) (Eu-2) (btfa = 4,4,4-trifluoro-1-phenyl-1,3-butanedione, nta = 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butanedione and DPEPO = bis(2-(diphenylphosphino)phenyl)ether oxide) are reported. The complexes have been used as dopants to fabricate several single and double emitting layer (EML) electroluminescent (EL) devices. At the optimum doping concentration of 4 wt%, the single EML device of Eu-1 exhibited a pure red color with an external quantum efficiency (EQE) of 2.9% and a maximum brightness (B) = 1320 cd m⁻², maximum current efficiency (η_c) = 4.08 cd A⁻¹, maximum power efficiency (η_p) = 3.37 lm W⁻¹ with a very low turn-on voltage (V_turn-on) = 3.8 V. Importantly, the single EML device of Eu-2 at the optimum doping concentration exhibited pure red EL with EQE of 6.0% with the best EL performance: B = 2108 cd m⁻², η_c = 8.45 cd A⁻¹, and η_p = 6.98 lm W⁻¹ at a very low V_turn-on = 3.5 V. The performances of this device are among the best reported for devices incorporating a europium complex as a red emitter. Furthermore, the EL performances were achieved at the current density of 10 mA cm⁻², thereby alleviating a serious efficiency roll-off as a major obstacle to the development of Eu-complex based EL devices.