Red organic electroluminescent materials

#This invention relates to red fluorescent materials that can be used as dopants in organic electroluminescent devices.

#One of the applications for using red fluorescent materials is in organic EL (electroluminescent) devices. Organic EL devices are known to be highly efficient and are capable of producing a wide range of colors. Useful applications such as flat-panel displays have been contemplated and commercialization is well underway. Representative of earlier organic EL devices are Gurnee et al., U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee, U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene,” RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner, U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. Typical organic emitting materials were formed of a conjugated organic host material and a conjugated organic activating agent having condensed benzene rings. The organic emitting material was present as a single layer medium having a thickness much above 1 micrometer. Thus, this organic EL medium was highly resistive and the EL device required a relatively high voltage (>100 volts) to operate.

#The most recent discoveries in the art of organic EL device construction have resulted in devices having the organic EL medium consisting of extremely thin layers (<1.0 micrometer in combined thickness) separating the anode and cathode. Herein, the organic EL medium is defined as the organic composition between the anode and cathode electrodes. Typically, organic EL devices are composed of three layers of organic molecules that are interposed between a transparent electrode and a metallic electrode. The three layers include an electron-transporting layer, a luminescent layer and a hole-transporting layer. The luminescent layer between the electron-transporting and hole-transporting layers provides an efficient site for the recombination of the injected hole-electron pair and resultant electroluminescence. A hole-injecting layer may be added to reduce the driving voltage. Optionally a hole- or electron-blocking layer may be added to improve the luminance efficiency. An organic EL device comprising four to six layers of organic molecules is thus obtained.

#These and other advantages of this invention can be better appreciated by reference to the following detailed description considered in conjunction with the drawings in which:

#FIG. 1 is a schematic diagram of the multilayer structures of the organic EL device according to the invention.

#FIG. 2 is a scheme for the synthesis of the red fluorescent dopants, DCJPR, of the invention.

#One particularly useful application of the compound is in EL devices. An EL device 10, which uses a compound according to the invention, is schematically illustrated in FIG. 1. The organic electroluminescent device (OLED) 10 comprises a transparent substrate 11, an anode layer 12, a hole-injecting layer 13, a hole-transporting layer 14, a luminescent layer 15, an electron-transporting layer 16, an electron-injecting layer 17 and a cathode layer 18. The transparent substrate 11 is glass or plastic. The anode layer 12 is transparent and electroconductive and is deposited on the surface of the substrate 11. A hole-injecting material is deposited on the surface of the anode layer 12 to form the hole-injecting layer 13. Subsequently, a hole-transporting material is deposited on the surface of the hole-injecting layer 13 to form a hole-transporting layer 14. The organic luminescent layer 15 made of luminescent material containing fluorescent dopants (a guest) is deposited on the surface of the layer 14. The electron-transporting layer 16 made of electron-transporting materials is deposited on the surface of the organic luminescent layer 15. Next, the electron-injecting layer 17 made of electron-injecting materials is deposited on the surface of the electron-transporting layer 16, and the cathode layer 18 made of metal is deposited on the surface of the electron-injecting layer 17 to form a cathode. The anode layer 12 is a p-type contact whereas the cathode layer 18 is an n-type contact.

#A power supply (19) with a negative end (−) and a positive end (+) provides an electric potential to the OLED device (10). The cathode layer 18 of the OLED device (10) is connected to the negative end (−) of power supply 19, and the anode layer 12 is connected to the positive end (+). When a potential is applied between the anode layer 12 and the cathode layer 18 by the power supply 19, the electron ejected from the n-type contact (cathode layer 18) will pass into the organic luminescent layer 15 through the electron-injecting layer 17 and organic electron-transporting layer 16. The hole ejected from the p-type contact (anode layer 12) will pass into the organic luminescent layer 15 through the organic hole-injecting layer 13 and organic hole-transporting layer 14. When the electron and hole recombined in the organic luminescent layer 15, energy is released in part as light, thus producing electroluminescence.

#The region where the hole and electron recombine is known as the recombination zone. The five-layer device structure is designed specifically to confine the recombination at the vicinity near the interface between the hole-transport and the electron-transport layer where the probability for producing electroluminescence is the highest. The recombination confinement scheme for two-layer device structure has been disclosed by Tang and Van Slyke (Applied Physics Letters, Vol.51, 913, 1987) and is done by choosing carrier injecting electrodes of suitable work-functions and transport materials of a proper carrier mobility. Away from this interface between the organic layers and in particular at or near the injecting electrodes, the recombination of hole and electron would generally be much less radiate due to the effect of radiate quenching by a conducting surface.