Highly-efficient solution-processed organic light-emitting diodes using cationic iridium complexes with a pyridine-imidazole type ancillary ligand as the dopants

Abstract

Phosphorescent cationic iridium complexes have emerged as promising dopants for solution-processed OLEDs, but further enhancing their device performances to the level achieved by neutral iridium complexes has remained challenging. Herein, we report a series of cationic iridium complexes, namely [Ir(ppz)2(Mepyim)]PF6 (1), [Ir(Mephtz)2(Mepyim)]PF6 (2), [Ir(tPhTAZ)2(Mepyim)]PF6 (3) and [Ir(CF3-dPhTAZ)2(Mepyim)]PF6 (4) for highly-efficient solution-processed OLEDs, which use 1-phenyl-1H-pyrazole (ppz), 1,3-dimethyl-5-phenyl-1H-1,2,4-triazole (Mephtz), 3,4,5-triphenyl-4H-1,2,4-triazole (tPhTAZ), and 3,4-diphenyl-5-(trifluoromethyl)-4H-1,2,4-triazole (CF3-dPhTAZ) as the cyclometalating (C^N) ligands, respectively, and 2-(1-methyl-1H-imidazole-2-yl)pyridine (Mepyim) as the ancillary (N^N) ligand. Their chemical structures have been verified by X-ray crystallography and their photophysical and electrochemical properties have been comprehensively characterized. In lightly doped poly(methylmethacrylate) films, complexes 1–4 afford green-blue to sky-blue emission, with high phosphorescent efficiencies at 0.75–0.98; in particular, complex 2 exhibits not only a near-unity phosphorescent efficiency (0.98) but also a short triplet lifetime (1.19 μs), because of its rather high radiative decay rate (8.2 × 105 s−1). Theoretical calculations reveal that the emission originates from either charge-transfer states (Ir/C^N → N^N) or C^N/N^N-centered 3π−π* states. By using complexes 1–4 as the dopants, solution-processed OLEDs with a simple double-layer structure afford efficient green-blue to sky-blue electroluminescence, with external quantum efficiencies (EQEs) at 6.8%–18.9%. In particular, the green-blue OLED based on complex 2 shows not only a high peak EQE of 18.9% but also a small efficiency roll-off (EQE = 16.5% at 1000 cd m−2), which is the best-performing device based on cationic iridium complex reported so far.