Highly efficient bluish green phosphorescent organic light-emitting diodes based on heteroleptic iridium(III) complexes with phenylpyridine main skeleton

A new series of heteroleptic iridium(III) complexes, bis(2-phenylpyridinato-N,C2′)iridium (2-(2′,4′-difluorophenyl)-4-methylpyridine), (ppy)₂Ir(dfpmpy) and bis(2-(2′,4′-difluorophenyl)-4-methylpyridinato-N,C2′)iridium (2-phenylpyridine) (dfpmpy)₂Ir(ppy), have been synthesized by using phenylpyridine as a main skeleton for bluish green phosphorescent organic light-emitting diodes (PhOLEDs). The Ir(III) complexes showed high thermal stability and high photoluminescent (PL) quantum yields of 95% ± 4% simultaneously. As a result, the PhOLEDs with the heteroleptic Ir(III) complexes showed excellent performances approaching 100% internal quantum efficiency with a very high external quantum efficiency (EQE) of ∼27%, a low turn-on voltage of 2.4 V, high power efficiency of ∼85 lm/W, and very low efficiency roll-off up to 20,000 cd/m².     

Highly efficient solid-state dye-sensitized TiO2 solar cells via control of retardation of recombination using novel donor-antenna dyes

Two series of heteroleptic tris(bipyridyl)Ru(II) and bis(bipyridyl)(NCS)₂Ru(II) complexes have been synthesized and characterized. This is a part of a new concept of covalent linkage of donor-antenna groups, e.g., triphenylamine or N,N′-bis(phenyl)-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) to Ru(II) dye center. For the covalent attachment of donor units, a multi-step synthesis was carried out starting from 4,4′-dimethyl-2,2′-bipyridine followed by chlorination and Wittig reaction with donor aldehydes. This was followed either by a metallation reaction using bis(4,4′-dicarboxy-2,2′-bipyridyl)Ru(II)dichloride ((bpy(COOH)₂Ru(II)2Cl₂ 2H₂O) as precursor to get tris(bipyridyl) dyes or by a one pot synthesis starting from dichloro(p-cymene)Ru(II) dimer resulting in bis(bipyridyl)(NCS)₂ dyes. The complexes (bpy(COOH)₂)₂(bpyMe₂)Ru(II) 2PF₆ and (bpy(COOH)₂)(bpyMe₂)(NCS)₂Ru(II) without donor-antenna groups were also prepared to study and compare the properties. The influence of donor-antenna groups in these complexes was studied using UV–Vis spectroscopy and cyclic voltammetry. The heteroleptic complexes carrying donor groups show appreciably broad absorption ranges and extraordinarily high extinction coefficients. These high extinction coefficients are explained as due to the extended delocalization of π-electrons in the donor-antenna ligands. The HOMO/LUMO energy values obtained from cyclic voltammetry support the multi-step charge transfer cascade possible in these donor-antenna dyes. Examples of solid-state dye-sensitized solar cell utilizing these novel donor-antenna dyes revealed spectacular performances of power conversion efficiencies of up to 3.4%, for the dye carrying a TPD donor group as measured under AM 1.5 spectral conditions. This is attributed to highly efficient light harvesting of these novel dyes and the improved charge transfer dynamics at TiO₂–dye and dye–hole conductor interfaces.     

Effect of dendron generation on properties of self-host heteroleptic green light-emitting iridium dendrimers

A series of self-host heteroleptic green light-emitting iridium (Ir) dendrimers G1 and G2 have been synthesized under mild conditions with high yields, and their photophysical, electrochemical and electroluminescent properties are investigated in detail. Compared with the model compound G0, both G1 and G2 exhibit similar photophysical and electrochemical properties, indicating that the incorporation of carbazole dendrons via a flexible non-conjugated spacer can retain the independence of the emissive Ir core. However, the device performance gradually increases with the increasing dendron generation due to the reduced intermolecular interactions. As a result, a peak luminous efficiency of 17.2 cd/A has been obtained for the G2-based non-doped device, which is about 6 times that of G0. Further dispersing the dendrimer G2 into a host matrix, the efficiency can be improved to 29.2 cd/A.     

Deep blue phosphorescent organic light-emitting diodes with excellent external quantum efficiency

Highly efficient deep blue phosphorescent organic light-emitting diodes (PHOLEDs) using two heteroleptic iridium compounds, (dfpypy)₂Ir(acac) and (dfpypy)₂Ir(dpm), as a dopant and 9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazol-3-yl)diphenylphosphine oxide as a host material have been developed. The electroluminescent device of (dfpypy)₂Ir(dpm) at the doping level of 3 wt% shows the best performance with external quantum efficiency of 18.5–20.4% at the brightness of 100–1000 cd/m² and the color coordinate of (0.14, 0.18) at 1000 cd/m².     

An efficient light-harvesting ruthenium dye for solar cell application

A novel, heteroleptic ruthenium dye comprising a vinyl group between the carboxylate and bipyridine segments as well as extended π-conjugation of the ancillary ligand, employing alkyl-bithiophene, was synthesized. The dye displayed a remarkably high absorption coefficient of 2.51 × 10⁴ M⁻¹ cm⁻¹ (at 562 nm) for its metal-to-ligand charge transfer band. The photo-to-current conversion efficiency of the corresponding dye-sensitized solar cell was 9.12% under AM 1.5 (100 mW/cm²) irradiation. Furthermore, owing to both the very strong metal-to-ligand charge transfer band and the large number of dye molecules adsorbed on the TiO₂ electrode, the conversion efficiency of the dye-sensitized cell was >7.5% at a light intensity ≤198 mW cm⁻².     

Efficient blue-green organic light-emitting diodes based on heteroleptic tris-cyclometalated iridium(III) complexes

The blue-green organic light-emitting diodes based on heteroleptic tris-cyclometalated iridium(III) complexes containing the F₂-ppy (2,4-difluorophenylpyridine) and ppy (2-phenylpyridine) ligands were fabricated. Ir(ppy)₃ has been known to have a high phosphorescence efficiency in electroluminescence owing to its strong metal-to-ligand-charge transfer (MLCT) excited state, whereas the luminous efficiency of Ir(F₂-ppy)₃ was found to be low due to weak MLCT. Herein, we report two heteroleptic phosphorescent blue-green emitters, Ir(ppy)₂(F₂-ppy) and Ir(ppy)(F₂-ppy)₂, that exhibit emission peaks at 502 nm and 495 nm, respectively. The maximum luminous efficiencies of the devices with Ir(ppy)₂(F₂-ppy) and Ir(ppy)(F₂-ppy)₂ were 8.93 cd/A and 13.80 cd/A, respectively. The quantum efficiency of the device containing Ir(ppy)(F₂-ppy)₂ was 3.63% at J = 10 mA/cm².     

Syntheses, crystal structures and properties of sterically congested heteroleptic complexes of group 10 metal ions with p-tolylsulfonyl dithiocarbimate and 1,2-bis (diphenylphosphino) ethane

Three novel heteroleptic complexes of the type cis- [ML(dppe)] [M = Ni(II), Pd(II), Pt(II); L = p-tolylsulfonyl dithiocarbimate; dppe = 1,2-bis(diphenylphosphino)ethane] have been prepared and characterized. X-ray crystallography revealed the close proximity of one of the ortho phenyl protons of the dppe ligand to the metal in the Ni(II) complex showing existence of the less common C–H?Ni anagostic interactions observed for the first time in the dithio-phosphine mixed-ligand systems. The platinum complex showed a strong photoluminescence emission near visible region in CH₂Cl₂ solution.     

Atomic layer deposition of pure In2O3 films for a temperature range of 200–300 °C using heteroleptic liquid In(DMAMP)2(OiPr) precursor

In₂O₃ films were deposited by atomic layer deposition (ALD) using a newly synthesized heteroleptic In precursor, In(DMAMP)₂(OⁱPr), and O₃ at 150–300 °C. Self-limiting growth characteristics were exhibited for a wide ALD temperature range of 200–300 °C and growth rate of 0.029–0.033 nm/cycle. At a low temperature of 150 °C, the amorphous In₂O₃ film was deposited, while polycrystalline In₂O₃ films were achieved at 200–300 °C. The In₂O₃ films grown in this ALD temperature range had high densities of 7.0–7.2 g/cm³, which are comparable to those of bulk In₂O₃. At all growth temperatures (150–300 °C), no carbon or nitrogen impurities were detected, suggesting high reactivity of the In(DMAMP)₂(OⁱPr) precursor. The ALD In₂O₃ films showed n-type electronic property with high electron concentrations of 1.6 × 10²⁰–3.6 × 10²⁰/cm³ and a Hall mobility of 31–39 cm²/V·s.     

Single dopant white electrophosphorescent light emitting diodes using heteroleptic tris-cyclometalated Iridium(III) complexes

We report single dopant single emissive layer white organic electroluminescent (EL) device based on the heteroleptic tris-cyclometalated iridium(III) complex, Ir(dfppy)₂(pq), as the guest, where dfppy and pq are 2-(2,4-difluorophenyl) pyridine and 2-phenylquinoline, respectively, and 1,4-phenylenesis(triphenylsilane) (UGH2) as the host. The maximum luminous and power efficiencies of the device were 11.00 cd/A (J = 0.05 mA/cm²) and 5.60 lm/W (J = 0.001 mA/cm²), respectively. The CIE coordinates of the device with Ir(dfppy)₂(pq) are (0.443, 0.473) and the EL spectrum of device shows emission band at 473 and 544 nm, at the applied voltage of 12 V. The similar phosphorescent decay rate of two ligands can lead to emit luminescence in two ligands at the same time.