WHITE LIGHT EMITTING ZINC BASED OLED Field of the invention
The present invention relates to a white light emitting zmc based Organic Light Emitting Diode The present invention also relates to a method for the manufacture of a white light emitting diode Background of the invention:
An OLED device mcludes a substrate, an anode, a hole-transporting layer made of an organic compound, an organic luminescent layer with suitable dopants, an organic electron transport layer, and a cathode OLEd devices are attractive because of their low driving voltage, high luminance, wide-angle viewing and capabihty for full-color flat emission displays Tang et al descnbed this multilayer OLED device m their U S Pat Nos 4,769,292 and 4,885,211
Efficient white light producmg OLED devices are considered as low cost alternative for several applications such as paper-thin light sources back hghts in LCD displays, automotive dome, etc Ideally white light LED's should have Commission International d'Eclairage (CIE) chromaticity coordinates of about (033, 0 33) In any event, m accordance with this disclosure, white light is that light which is perceived by a user as having a white color
In the fall of 1996, Nichia of Japan introduced a new white LED, Product No
NSPW310AS This product uses broadband yellow emitting (~580nm) Yttrium
Alumimum garnet doped with Cerium to convert a blue LED mto a white LED with a
full width a half maximum of 160 nm The Aluminium Garnet phosphor is coated
directly on the LED The entire device is encapsulated m a clear plastic lens The
emission of the Garnet contains enough orange emission to produce a hybrid white
light at a color temperature of about 8000 K, a color rendermg index (CRI) of about
77, and a device luminous efficacy of approximately 5 lumens per watt (Im/w) of
mput electric power The color temperature of the white LED produced by Nichia can be lowered using more phosphor However, the system efficiency gets reduced There is a need for a phosphor coated LED having a higher device luminous efficacy, there is also a need for organic device to give white hght It is another object of this invention to provide an efficient and stable white light producing OLED device with simple structure and which can be reproduced in manufacturing environment It has been found quite unexpectedly that white hght producmg OLED devices with high luminance efficiency and operational stability can be obtained by employing TPD as a hole transport layer, and Zn (BTZ) as the electron transport as well as the emissive layer
The following patents and publications disclose the preparation of organic OLED devices capable of emitting white hght, comprising a hole transport layer and an organic luminescent layer, and interposed between a pair of electrodes
Kido et al., in Science, Vol 267, p 1332 (1995) and m Applied Physics Letters Vol 64, p 815 (1994) report a white hght producing OLED device In this device three emissive layers with different charge carrier transport properties, each emitting blue, green or red hght, are used to generate blue, green or red hght, are used to generate white hght This device has good electroluminescent characteristic, but device with multi layer is difficult to control during large-scale manufacturing
White hght producing OLED devices have been report before by J Shi (U S Pat No 5,683,823) wherein, the luminescent layer mcludes a red and blue light-emitting materials uniformly dispersed m a host emitting material
Sato et al. m JP07,142,169 discloses an OLED device, capable of emitting
white hght, is made by stickmg a blue light-emitting layer next to the hole transport
layer and followed by a green light-emitting layer havmg a region containing a red
fluorescent layer
Liftman et. al. mUS Pat No 5,405,709 discloses another white light emitting device, which is capable of emitting white hght m response to hole-electron recombmation, and compnses of a fluorescent material m visible range from bluish green to red
Recently, Deshpande et al., m Applied Physics Letters, Vol, 75, p 888 (1999) published white OLED device usmg red, blue and green luminescent layers separated by a hole-blocking layer However, these OLED devices required precised control of electronics due to large number of layer structure, making the process difficult to control for large-scale manufacturing Also emission color vanes due to differences m the lifetimes of the three different colors and also the concentrations of the dopant dye Objects of the invention
The mam object of the mvention is therefore to provide a white hght emitting device compnsmg of a hole transport layer and a single emissive/electron layer encapped between two electrodes, which overcomes the disadvantages of the prior art given above Summary of the Invention:
The present mvention, therefore, provides a white light emitting device, compnsmg hole transport layer and a smgle emissive/electron layer encapped between two electrodes
In one embodiment of the mvention, the white hght emitting device of the mvention comprises
a) a substrate,
b) an anode deposited over the substrate,
c) a hole transport layer deposited over the hole injecting layer,
d) a light -emitting layer/electron transport layer, deposited directly on the hole transport layer,
e) a cathode deposited over the electron transport layer,
The object of the invention is achieved by an organic light-emitting diode (OLED) device which produces substantially white light, comprising
a) A substrate
b) An anode deposited over the substrate
c) A hole transport layer deposited over the anode
d) A light -emitting layer deposited directly on the hole transport layer,
e) A cathode deposited over the electron transport layer Brief description of the drawings:
FIG 1 depicts a prior art organic hght-emittmg device,
FIG 2 depicts another prior art multi-layer organic light-emitting device,
FIG 3 depicts a white hght producmg OLED device accordmg to the present
invention, FIG 4 shows PL spectra of the electro transport/emissive material FIG 5 shows the electro lummescence spectra of the OLED device FIG 6 shows the current density versus voltage curve for the OLED device FIG 7 shows a graph of electroluminescence at various current densities Detailed description of the invention
The emission layer of an OLED comprises of an organic -metallic material that produces hght, known as electroluminescence, as a result of electro-hole recombination in the layer Hereinafter, the term organic will be taken to mclude both purely organic and organometallic matenals In the simplest construction of the prior art, shown in FIG 1, an emission layer A is sandwiched between an anode B and a
cathode C The emission layer can be a single pure material wit high lummescence
efficiency A well known material for this purpose is tns (8-quinohnolato-Nl, 08) aluminum (Alq), which produces excellent green electroluminescence A glass substrate coated with indium tin oxide (ITO) provides mechanical support for the OLED and for electrical leads connecting the OLED to a source of electrical current Layers labeled cathode, anode and emissive layer together comprise the OLED m Figure 1 The substrate also acts as anode and is transparent to the electroluminescent light, allowing the emitted hght to be viewed The term transparent refers to the ability to transmit no less than 80 percent of the electroluminescent hght When the cathode and anode are connected to a source of electrical current (not shown), holes are mjected from the anode and electrons are mjected from the cathode, and they recombine m the emission layer to produce electroluminescent hght
In a more elaborate of the prior art, shown m FIG 2, an emission layer 4 is situated between a hole- transport layer 5 and an electron transport layer 6 Each of these layers is composed predominantly of organic materials The emission/hole transport layer junction functions as the primary site for electron-hole recombmation and emission of the resulting electroluminescent hght In this respect, the functions of the individual organic layers are distinct and can therefore be optimized mdependently Thus, the emission layer can be optimized for a desirable EL color and high luminance efficiency
The OLED device can be viewed as a diode, which is forward biased when the
anode is at a higher potential than the cathode The anode and cathode of the organic
OLED device can each take any convement conventional form, such as any of the
vanous forms disclosed by Tang et al U S Pat No 4,885, 211 Operatmg voltage can
be substantially reduced when usmg a low work function cathode an a high-work
function anode The preferred cathodes are those constructed of a combination of a
metal having a work function less than 4 0 eV and one other metal, preferably a metal
having a work function greater than 4 OeV the Mg Ag of Tang et al U S Pat No 4,885,211 constitutes one preferred cathode construction The Al Mg cathodes of VanSlyke et al US Pat No 5,059,862 is another preferred cathode construction Hung et al in U S Pat No 5,776,622 has disclosed the use of a LiF/Al bilayer to enhance electron injection in organic OLED devices Cathodes made of eitherMg Ag, Al Li, AL Mg or LiF/Al are opaque and displays cannot be viewed through the cathode
The substrate in the pnor art shown in Figure 3 can be constructed of glass, quartz, or a plastic material and can, optionally, incorporate additional layers servicing additional functions such as color-filter layers to remove unwanted spectral components from the electroluminescent light
Matenal useful m the hole -transport layer of this invention mclude tertiary amines as taught m Van Slyke m U S Pat No 4,539,507 They can be used as the sole matenal of an undoped hole transport layer or an undoped sublayer of a hole-transport layer Matenals of this class are cunently prefened Most preferred ones are denvatives of tnphenyl diamine compound
The mvention and its advantages are further illustrated by the specific examples, which follows FIG 3 show the white light producmg OLED device structure prepared according to the present mvention and graphs of vanous parameters of then- operations The mvention and it advantages are further illustrated by the specific examples, which follow FIG 3 depicts an OLED of the present mvention This OLED is similar to the OLED of the pnor art show m FIG 2 The electron transport layer also serves the purpose of the emissive layer FIG 4 depicts the photo luminescence spectra of the emissive/electron transport matenal showing emission component when excited at its absorption maxima FIG 5 depicts the EL
spectra of the present mvention m accordance to present device shown m FIG 3
FIG 6 shows the CURRENT DENSITY - LUMINESCENCE - VOLTAGE charactenstics of the device when prepared as m FIG 3 The thickness of the hole-transport layer is between 50 nm and about 150 mm. The thickness of the emission layer is between 50 nm and about 200 nm. The thickness of the cathode layer is between 50 nm and 200 nm

WE CLAIM
1. An organic light-emitting diode (OLED) device which produces
electroluminescence with broad spectral width suitable for white light
applications, comprising:
a) a glass substrate
b) an anodic substance deposited over the substrate;
c) a hole transport cum injecting layer deposited over the anodic substance;
d) a light-emitting layer/electron transport layer, deposited directly on the hole transport layer;
e) a cathodic substance deposited over the election transport layer;

2. The OLED device of claim 1 wherein the hole transport layer comprises deposited Zinc complexes of benzthiazoles and derivatives thereof substituted with cyano, halo, alkoxy and other functional group with either electron withdrawing or electron donating groups on phenoxide as well as benothiazole group with selective materials such as TPD and material for hole transporting function.
3. The OLED device according to claim 1 wherein the cathodic substance is selected from the group consisting of high electropositive metal and combination of metals selected from the group consisting of Ca, Au, Mg, Li, Al, In, Mg: In, Lif/Al, Mg: Ag, Al: Mg, and AL: Li, and mixtures thereof
4. The OLED device according to claim 1 where in the anode is transparent.
5. The OLED device according to claim 2 wherein the electron transport layer is transparent.
6. The OLED device according to claim 2 wherein thickness of the electron transport layer/white light emissive layer is between 50mm -200nm.
7. The OLED device according to claim 1 wherein thickness of the hole transport
layer is between 20nm -150nm.
8. The OLED device according to claim 1 wherein the hole transport layer includes an aromatic tertiary amine.
9. An organic light- emitting diode device which produces substantially broad spectral characteristics for white light applications.
10. An organic Kght- emitting diode device substantially as described hereinbefore and with reference to the accompanying drawings.