Pulsed-current versus constant-voltage light-emitting electrochemical cells with trifluoromethyl-substituted cationic iridium(III) complexes
We report on five cationic iridium(III) complexes with cyclometalating 2-(3'-trifluoromethylphenyl)pyridine and a diimine, [(C boolean AND N)(2)Ir(N boolean AND N)](PF6), N boolean AND N = 4,4'-R-2-2,2'-dipyridyl or 4,7-R-2-1,10-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 boolean AND 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 mu s. 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.