Pulsed-current versus constant-voltage light-emitting electrochemical cells with trifluoromethyl-substituted cationic iridium(iii) complexes

Abstract

We report on five cationic iridium(III) complexes with cyclometalating 2-(3′-trifluoromethylphenyl)pyridine and a diimine, [(C⁁N)₂Ir(N⁁N)](PF₆), N⁁N = 4,4′-R₂-2,2′-dipyridyl or 4,7-R₂-1,10-phenanthroline (R = H, Me, tert-Bu, Ph), and characterize three of them by crystal structure analysis. The complexes undergo oxidation of the Ir–aryl fragment at 1.13–1.16 V (against ferrocene couple) and reduction of the N⁁N ligand at −1.66 V to −1.86 V, and have a redox gap of 2.84–2.99 V. The complexes exhibit bluish-green to green-yellow phosphorescence in an argon-saturated dichloromethane solution at room temperature with a maximum at 486–520 nm, quantum yield of 61–67%, and an excited-state lifetime of 1.2–4.3 μs. In two-layer spin-coated light-emitting electrochemical cells (LEC) operated at a constant-voltage (4 V) or a pulsed-current (100 A m⁻² per pulse; block wave, 1000 Hz; 50% duty), the complexes exhibit green-yellow electroluminescence with a maximum at 547–556 nm. The luminance and efficiency of LEC do not level off after peaking but decay; for example, the luminance of the devices after reaching the peak of 195–1094 cd m⁻² halves in 9–580 min. The best of the new LEC runs under pulsed-current driving and exhibits peak efficiencies of 16.8 cd A⁻¹ and 7.9 lm W⁻¹ and an EQE of 5.4% at a luminance of ≥834 cd m⁻². We find that the pulsed-current LEC offer the following advantages over the constant-voltage LEC: lower current, higher stability, faster turn-on, and higher efficiency at higher luminance.