Electroluminescent materials

Highly luminous, thermally stable and moisture-resistant light-emitting material derived from quadridentate ONNO-type ligands and a Group 10 metal were employed as emissive dopants in organic light-emitting devices. The dopants have molecular structures represented by the formula I and II: wherein M represents Group 10 metal (including platinum) and R1-R14 are each independantly selected from the group consisting of hydrogen; halogen; alkyl; substituted alkyl; aryl; substituted aryl, with substitutents selected from the group consisting of halogen, lower alkyl and recognized donor and acceptor groups.

FIELD OF THE INVENTION

The present invention relates to light-emitting materials, which can be deposited as a thin layer by vacuum deposition, and which can be used as effective dopants in organic light-emitting devices (OLEDs).

BACKGROUND OF THE INVENTION

The progress of light-emitting diode (LED) over the past two decades has primarily focused on inorganic types because early development in organic light-emitting devices (OLEDs) resulted in poor fabrication and packaging, and short lifetimes. Today, gallium arsenide-based LEDs in the market are commonly available with efficiencies in some spectral regions exceeding conventional filtered fluorescent lamps. However, in the development of light-emitting materials for display technology, inorganic semi-conductor materials are not compatible for large-area assembled displays.

Pope et al. at New York University demonstrated organic electroluminescence in the 1960s based on anthracene materials ( J. Chem. Phys . 38, 2042, (1963)). Much progress have been made since the discovery of the tris(8-hydroxyquinolato)aluminum (Alq 3 ) based thin film device by C. W. Tang et al. at Kodak ( Appl. Phys. Lett . 51, 913, (1987)). These contributed largely to the continuous discovery of new and improved electroluminescent materials. From small fluorescent molecules to conjugated polymers, many OLEDs have been shown to exhibit sufficient brightness, remarkable efficiencies, good operating lifetimes and desirable ranges of color emission.

Organic light-emitting devices containing metal complexes are of particular interest because of their unusual chemical and electronic properties. Some compounds bearing heavy metals exhibit potential advantages for OLEDs owing to their high internal quantum efficiencies. Conventionally, fluorescent materials are employed as dopants in emissive hosts. Singlet excitons (maximum theoretical internal quantum efficiency 25%) are formed after recombination of hole and electron to emit electroluminescence via dipole-dipole interaction through Forster mechanism (U.S. Pat. No. 6,310,360). Whereas, for heavy metal complexes, strong spin-orbit coupling can lead to singlet-triplet state mixing, which can result in high-efficiency electrophosphorescence in OLEDs (theoretical internal quantum efficiency up to 100%) ( Nature , 395, 151, (1998); Synthetic Metals , 93, 245, (1998); Appl. Phys. Lett . 77, 904, (2000)).

However, some phosphorescent materials have intrinsic disadvantages, such as saturation of emission sites due to excessively long lifetimes as well as triplet-triplet annihilation and concentration quenching arising from strong intermolecular interactions at high doping levels ( Phys. Rev. B . 60, 14422, (1999)).

Azomethine-aluminum/gallium complexes have been employed in OLEDs as emissive materials. The current density of the device containing azomethine-gallium complex is 1 mA/cm 2 at 10 V and the electroluminescence is greenish blue (U.S. Pat. No. 6,316,130).

It is therefore desirable to develop emissive dopant materials that can permit efficient energy transfer between the host and dopant in OLEDs, while causing little or no self-quenching even at sufficiently high doping concentrations.

SUMMARY OF THE INVENTION

Examples of objects of the present invention in embodiments thereof include:

The main objective of this invention is to prepare organic light-emitting devices (OLEDs) doped with new light-emitting materials. The devices exhibit low turn-on voltages and high luminance and efficiencies.

An object of the present invention is to provide thermally stable, moisture-resistant metal-chelated materials that can be deposited as a thin layer of known thickness by a vapor deposition process.

Further, the present invention concerns the design of high luminous dopants, which can be used at low concentration levels in light-emitting devices.

New light-emitting materials derived from quadridentate ONNO-type ligands, and a Group 10 metal (including platinum) were prepared as illustrated by formula I and II:

wherein M represents Group 10 metal (including platinum) and R 1 -R 14 are each independently selected from the group consisting of hydrogen; halogen; alkyl; substituted alkyl; aryl; substituted aryl, with substitutents selected from the group consisting of halogen, lower alkyl and recognized donor and acceptor groups.

Embodiments of the present invention includes, but is not limited to, OLEDs comprising heterostructures for producing electroluminescence which contain anode (ITO glass substance), hole transport layer (NPB( -naphthylphenylbiphenyl amine)), matrix emissive layer host material (beryllium bis(2-(2 -hydroxyphenyl)pyridine) (Bepp 2 )) with different concentration of dopants as illustrated by formula I and II herein , charge transport layer (lithium fluoride) and cathode (aluminum metal).

The preferred embodiment as an effective dopant in the OLEDs herein is:

The present invention provides new materials for applications as emissive dopants in electroluminescent devices. The invention includes the synthetic methods for these novel complexes plus their use as light-emitting materials. The devices of the present invention can be applied to field of display, light-emitter, display board for sign lamp, or light source for liquid crystal display.

DETAILED DESCRIPTION OF THE INVENTION

The inventions are generally related to syntheses, spectral characterization, phosphorescence, light-emitting properties of the new light-emitting materials, and their applications in OLEDs. The examples are set forth to aid in an understanding of the inventions but are not intended to, and should not be interpreted to, limit in any way the invention as set forth in the claims which follow thereafter.

The examples given illustrate the synthetic methods of ligands 1 a and 2 a , and the platinum complexes 1 b and 2 b . The quadridentate ONNO-type ligands 1 a and 2 a were prepared by modification of a literature procedure ( J. Chem. Soc., Perkin Trans . 2, 863, (1998)). Another example of the preparation of ONNO-type ligands has been reported (U.S. Pat. No. 6,177,419).

The spectral characteristics of the platinum complexes 1 b and 2 b according to this invention are shown in Table 1. For complex 1 b , strong absorption bands at 250-350 nm ( 38400-17500 dm 3 mol 1 cm 1 ) and a moderately intense absorption band at max 398 nm ( 10800 dm 3 mol 1 cm 1 ) are observed (FIG. 1 ). In addition, a broad absorption band can be found at ca. 480 nm (2800 dm 3 mol 1 cm 1 ). For complex 2 b (FIG. 1 ), several vibronic transitions at 291-375 nm ( 39200 to 24700 dm 3 mol 1 cm 1 ) and a broad band at 504 nm ( 7200 dm 3 mol 1 cm 1 ) are observed in CH 2 Cl 2 .

TABLE 1 UV/vis absorption data of 1b and 2b in CH 2 Cl 2 Complexes max /nm ( /10 4 dm 3 mol 1 cm 1 ) 1b 255 (3.80), 315 (1.75), 400 (0.82), 480 (0.25), 505 (0.22) 2b 291 (3.92), 315 (3.40), 325 (3.23), 352 (2.58), 375 (2.47), 420 (0.52), 488 (0.67), 504 (0.72) The photoluminescence (PL) of the platinum complexes 1 b and 2 b in solution and as thin film are summarized in Table 2. In FIG. 2 , the 298 K structureless emission of complex 1 b are observed at 595 and 599 nm in CH 2 Cl 2 and as thin film respectively. The PL properties of complex 2 b in solution and as thin film are shown in FIG. 3 . The emission maximum of 2 b in thin film is shifted by 1704 cm 1 compared to that in solution. Meanwhile, complexes 1 b and 2 b exhibit lifetimes of 1.9 and 5.3 s in CH 2 Cl 2 and luminescent quantum yields of 0.1 and 0.6 (with Ru(bpy) 3 Cl 2 as reference standard) respectively.

TABLE 2 PL properties of complexes 1b and 2b in solution and as thin film Complexes (Measuring medium/ Emission Temperature) (Maximum/nm) Lifetime ( s) Quantum yield 1b (CH 2 Cl 2 /298K) 595 1.9 0.1 1b (Thin Film/298K) 599 / / 2b (CH 2 Cl 2 /298K) 586 5.3 0.6 2b (Thin Film/298K) 651 / / concentration 1 10 5 mol dm 3 The TGA thermograms of complexes 1 b and 2 b are shown in FIG. 4 . Both the complexes demonstrate high thermal stabilities in nitrogen and air at heating rate of 15 C./min. Complex 2 b is stable up to 536 C. in nitrogen and 379 C. in air. The on-set temperatures of 1 b are at 438 C. in nitrogen and 382 C. in air. These observations reveal that these light-emitting materials can be sublimed and stable at vacuum deposition conditions in preparation of OLEDs.

The device A was assembled as follows: indium tin oxide (ITO) electrode with sheet resistance of 20 /square on glass substrate, a hole transport material NPB ( -naphthylphenylbiphenyl amine) with thickness of 500 , an emitting layer made of mixture of 0.3 wt. % complex 1 b and blue luminescent material Bepp 2 (beryllium bis(2-(2 -hydroxyphenyl)pyridine) with 400 thickness, an enhanced charge transport layer LiF with thickness of 15 , and aluminum layer with 2000 thickness. The metal and organic layers were laminated in sequence under 5 10 6 mbar without breaking vacuum between different vacuum deposition processes. The layers were deposited at rates of 2 or 5 per second. The emissive area of the device as defined by overlapping area of cathode and anode was 3 3 mm 2 . The ITO coated glass slides were cleaned with organic solvents (acetone-isopropanol-methanol), deionized water, followed by ultra-violet-ozone cleaner. EL spectra and current density-voltage-luminance characteristics of the devices were measured with a spectrophotometer and a computer-controlled direct-current power supply respectively at room temperature.

For these examples, the device external efficiencies increase when the doping concentration levels of complex 1 b were adjusted from 2 to 0.3 wt %. The specific examples are further illustrated as follows:

The performances of device A with 0.3 wt % doping level of complex 1 b are shown in FIG. 6 . Two intense EL emissions at 453 and 540 nm are observed when the device was driven under forward bias. The current density-voltage-luminance characteristics curves of device A are also shown. The turn-on voltage is approximately 6-7 V. The maximum efficiency of the device was 4.1 cd/A at luminance of 2849 cd/m 2 . The maximum luminance of 9325 cd/m 2 was obtained at driving voltage of 10 V. The EL color of device A is yellow (CIE coordinates: x 0.33, y 0.47).

The performances of device B with 1.0 wt % doping level of complex 1 b are shown in FIG. 7 . The device exhibits an intense EL emission peak at 546 nm and a weak emission at 457 nm. The onset voltage of device B was approximately at 6-7 V. The efficiency and maximum luminance were 1.9 cd/A at luminance of 1927 cd/m 2 and 6563 cd/m 2 at driving voltage of 9.5 V respectively. The EL color of device B is yellow (CIE coordinates: x 0.39, y 0.54).

Device C with 2.0 wt % doping level of complex 1 b exhibits an intense EL emission peak at 548 nm with an extremely weak emission at around 450 nm (FIG. 8 ); EL efficiency of 1.5 cd/A was detected. Luminance of 6450 cd/m 2 was observed at driving voltage of 12 V. The EL color of device C is yellow (CIE coordinates: x 0.42, y 0.56).

Typically, doping level of greater than 5% is reported to achieve dopant emission in organic or polymeric light-emitting devices. In this invention, the OLEDs show virtually complete emission of complex 1 b when the doping level is around 2% and the efficiencies of devices increase from 1.5 to 4.1 cd/A when the doping levels of complex are decreased from 2.0 to 0.3 wt %.