Blue Light-Emitting Electrochemical Cells with Ir(Ⅲ) Complex
Authors: Iwai Kazuma , Aoki Atsushi 
Published: December 2020

Light-emitting electrochemical cells (LECs) are one of promising technology for light emitting devices. LECs have some advantages which are single layer structure with air-stable metal electrodes and to use solution process for the device fabrication. Full color emission which means red, green and blue lights is needed for the light application. Phosphorescent Ir(Ⅲ) complex have potential to emit full color because iridium complex has a wide energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In particular, archetypal cationic Ir(Ⅲ) complex [Ir(ppy)2(bpy)]+, where ppy and bpy stands for 2-phenylpyridine (Hppy) as the cyclometalating ligand and 2,2’-bipyridine (bpy) as the ancillary ligand, respectively. Green, yellow, orange and red emitting cationic Ir(Ⅲ) complexes have been synthesized so far. In contrast, the number of blue emitting cationic Ir(Ⅲ) complexes are limited. Blue light emission is essential for full color emission. Widening energy gap of Ir(Ⅲ) complexes causes blue shift. For example, Fluorine element which is electron-deficient substituents have been attached cyclometalating ligands to stabilize HOMO. However, fluorine substitution degrades the electrochemical and thermal stability of the complex. It is necessary to find a cyclometalating ligand without fluorine substitution for stable blue light emission.

1-(2,6-dimethylphenyl)-3-methyl-5-phenyl-1H-1,2,4-triazole (dphtz) was synthesized as a cyclometalating ligand for blue light emitting cationic Ir(Ⅲ) complex. 2-(1Hpyraol-1- yl)pyridine (pzpy) was used as an ancillary ligand. [Ir(dphtz)2(pzpy)](TFSI) was synthesized in a three-step reaction. First, the chloro-bridged iridium dimers, [Ir(dphtz)2Cl]2 was synthesized from IrCl3・nH2O and dphtz. Second, [Ir(dphtz)2(pzpy)]Cl was synthesized from [Ir(dphtz)2Cl]2 and pzpy. Third, LiTFSI was added to [Ir(dphtz)2(pzpy)]Cl. Cl- was replaced by TFSI-, and [Ir(dphtz)2(pzpy)](TFSI) was synthesized[1].

Absorption and photoluminescence (PL) spectrum of [Ir(dphtz)2(pzpy)](TFSI) were measured in CH3CN solution to characterize photophysical property. Absorption spectrum indicates that the complex has absorption under 399 nm, resulting that the bandgap is calculated to be 3.11 eV. PL spectrum indicates blue emission with peak wavelength of 466 nm. The peak wavelength is blue-shifted by nearly 98 nm compared with archetypal cationic Ir(Ⅲ) [Ir(ppy)2(bpy)](TFSI) (λ= 564 nm) of our previous work. Cyclic voltammetry in solution was performed to investigate the electrochemical property of the complex. The voltammogram shows that the oxidation potential is 1.40 V and reduction potential is -1.77 V vs. Ag/Ag+. From these two values, the theoretical drive voltage of LECs using the complex can be calculated to be 3.17 V. In addition, the complex exhibits reversible oxidation process and irreversible reduction process in CH3CN. These properties would cause unbalanced electron and hole injection of LECs device.

LECs using the complex was fabricated. The device structure consists of ITO/complex:20 wt％ EMIMTFSI:6.25 wt％ PMMA(polymethylmethacrylate) /LiF (0.5 nm)/Ag (75 nm). EL spectrum shows blue light emission with centered 440 nm. However, the blue emission did not keep for a long time. After a minute, the emission color changed gray EL with centered 463 nm. Under a constant driving current of 2.0 mA, the LECs provide a peak brightness of 9.9 cd/m2. This low luminance may be due to unbalanced electron and hole injection. Under a constant driving current of 2.0 mA, the driving voltage shows 3.95 V, whereas after a minute, it exceeds 4.5 V, indicating that [Ir(dphtz)2(pzpy)](TFSI) was degraded, and LECs does not function optimally.

In conclusion, the blue light emitting complex in this study shows poor luminance and short lifetime for LECs. In order to get higher luminance and longer lifetime, we will improve the chemical stability of the Ir(Ⅲ) complex and the balance of the electron and hole injection into the emitting layer of LECs.

Reference

[1] X.Wang et al., Inorg.Chem., 58, 12132 (2019).