Organic light emitting devices (OLEDs) are comprised of several organic layers in which one of the layers is comprised of an organic material that can be made to electroluminesce by applying a voltage across the device. C. W. Tang et al., Appl. Phys. Lett. 51, 913 (1987). Certain OLEDs have been shown to have sufficient brightness, range of color and operating lifetimes for use as a practical alternative technology to LCD-based full color flat-panel displays. S. R. Forrest, P. E. Burrows and M. E. Thompson, Laser Focus World, February 1995. Since many of the thin organic films used in such devices are transparent in the visible spectral region, they allow for the realization of a completely new type of display pixel in which red (R), green (G), and blue (B) emitting OLEDs are placed in a vertically stacked geometry to provide a simple fabrication process, a small R-G-B pixel size, and a large fill factor.
A transparent OLED (TOLED), which represents a significant step toward realizing high resolution, independently addressable stacked R-G-B pixels, was reported in U.S. Pat. No. 5,703,436, Forrest et al. This TOLED had greater than 71% transparency when turned off and emitted light from both top and bottom device surfaces with high efficiency (approaching 1% quantum efficiency) when the device was turned on. The TOLED used transparent indium tin oxide (ITO) as the hole-injecting electrode and a Mg--Ag--ITO electrode layer for electron-injection. A device was disclosed in which the ITO side of the Mg--Ag--ITO electrode layer was used as a hole-injecting contact for a second, different color-emitting OLED stacked on top of the TOLED. Each layer in the stacked OLED (SOLED) was independently addressable and emitted its own characteristic color, red or blue. This colored emission could be transmitted through the adjacently stacked transparent, independently addressable, organic layer, the transparent contacts and the glass substrate, thus allowing the device to emit any color that could be produced by varying the relative output of the red and blue color-emitting layers.
U.S. Pat. No. 5,703,745, Forrest et al, disclosed an integrated SOLED for which both intensity and color could be independently varied and controlled with external power supplies in a color tunable display device. U.S. Pat. No. 5,703,745, thus, illustrates a principle for achieving integrated, full color pixels that provide high image resolution, which is made possible by the compact pixel size. Furthermore, relatively low cost fabrication techniques, as compared with prior art methods, may be utilized for making such devices.
Such devices whose structure is based upon the use of layers of organic optoelectronic materials generally rely on a common mechanism leading to optical emission. Typically, this mechanism is based upon the radiative recombination of a trapped charge. Specifically, OLEDs are comprised of at least two thin organic layers between an anode and a cathode. The material of one of these layers is specifically chosen based on the material's ability to transport holes, a "hole transporting layer" (HTL), and the material of the other layer is specifically selected according to its ability to transport electrons, an "electron transporting layer" (ETL). With such a construction, the device can be viewed as a diode with a forward bias when the potential applied to the anode is higher than the potential applied to the cathode. Under these bias conditions, the anode injects holes (positive charge carriers) into the HTL, while the cathode injects electrons into the ETL. The portion of the luminescent medium adjacent to the anode thus forms a hole injecting and transporting zone while the portion of the luminescent medium adjacent to the cathode forms an electron injecting and transporting zone. The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, a Frenkel exciton is formed. These excitons are trapped in the material which has the lowest energy. Recombination of the short-lived excitons may be visualized as an electron dropping from its conduction potential to a valence band, with relaxation occurring, under certain conditions, preferentially via a photoemissive mechanism.
The materials that function as the ETL or HTL of an OLED may also serve as the medium in which exciton formation and electroluminescent emission occur. Such OLEDs are referred to as having a "single heterostructure" (SH). Alternatively, the electroluminescent material may be present in a separate emissive layer between the HTL and the ETL in what is referred to as a "double heterostructure" (DH).
In a single heterostructure OLED, either holes are injected from the HTL into the ETL where they combine with electrons to form excitons, or electrons are injected from the ETL into the HTL where they combine with holes to form excitons. Because excitons are trapped in the material having the lowest energy gap, and commonly used ETL materials generally have smaller energy gaps than commonly used HTL materials, the emissive layer of a single heterostructure device is typically the ETL. In such an OLED, the materials used for the ETL and HTL should be chosen such that holes can be injected efficiently from the HTL into the ETL. Also, the best OLEDs are believed to have good energy level alignment between the highest occupied molecular orbital (HOMO) levels of the HTL and ETL materials.
In a double heterostructure OLED, holes are injected from the HTL and electrons are injected from the ETL into the separate emissive layer, where the holes and electrons combine to form excitons.
Various compounds have been used as HTL materials or ETL materials. HTL materials mostly consist of triaryl amines in various forms which show high hole mobilities (.about.10.sup.-3 cm.sup.2 /Vs). There is somewhat more variety in the ETLs used in OLEDs. Aluminum tris(8-hydroxyquinolate) (Alq.sub.3) is the most common ETL material, and others include oxidiazol, triazol, and triazine.
A well documented cause of OLED failure is thermally induced deformation of the organic layers (e.g. melting, crystal formation, thermal expansion, etc.). This failure mode can be seen in the studies that have been carried out with hole transporting materials, K. Naito and A. Miura, J. Phys. Chem. (1993), 97, 6240-6248; S. Tokito, H. Tanaka, A. Okada and Y. Taga. Appl. Phys. Lett. (1996), 69, (7), 878-880; Y. Shirota, T. Kobata and N. Noma, Chem. Lett. (1989), 1145-1148; T. Noda, I. Imae, N. Noma and Y. Shirota, Adv. Mater. (1997), 9, No. 3; E. Han, L. Do, M. Fujihira, H. Inada and Y. Shirota, J. Appl. Phys. (1996), 80, (6) 3297-701; T. Noda, H. Ogawa, N. Noma and Y. Shirota, Appl. Phys. Lett. (1997), 70, (6), 699-701; S. Van Slyke, C. Chen and C. Tang, Appl. Phys. Lett. (1996), 69, 15, 2160-2162; and U.S. Pat. No. 5,061,569.
Organic materials that are present as a glass, as opposed to a crystalline or polycrystalline form, are desirable for use in the organic layers of an OLED, since glasses are capable of providing higher transparency as well as producing superior overall charge carrier characteristics as compared with the polycrystalline materials that are typically produced when thin films of the crystalline form of the materials are prepared. However, thermally induced deformation of the organic layers may lead to catastrophic and irreversible failure of the OLED if a glassy organic layer is heated above its T.sub.g. In addition, thermally induced deformation of a glassy organic layer may occur at temperatures lower than T.sub.g, and the rate of such deformation may be dependent on the difference between the temperature at which the deformation occurs and T.sub.g. Consequently, the lifetime of an OLED may be dependent on the T.sub.g of the organic layers even if the device is not heated above T.sub.g. As a result, there is a need for organic materials having a high T.sub.g that can be used in the organic layers of an OLED.
The most common hole transporting material used in the HTL of OLEDs is a biphenyl bridged diamine, N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1-biphenyl-4,4'-diamine (TPD) having the chemical structure: ##STR1##
This material has a good hole mobility and efficiently transfers holes to aluminum tris (8-hydroxyquinoline) in a simple single heterostructure OLED. However, TPD has a melting point of 167.degree. C. and a glass transition temperature of 65.degree. C. If a device prepared with TPD is heated above 65.degree. C., the glass transition temperature, catastrophic and irreversible failure results. In order to increase the glass transition temperature of the HTL, several groups have explored different modifications to the basic structure of TPD, Naito et al.; Tokito et al.; Shirota et al.; Noda et al. (Adv. Mater.); Han et al.; Noda et al. (Appl. Phys. Lett.); Van Slyke et al.; and U.S. Pat. No. 5,061,569. While these studies have led to materials with T.sub.g values as high as 150.degree. C., they have not led to an understanding of why certain structural modifications increase T.sub.g, while other modifications may not affect T.sub.g at all or may even lower T.sub.g. Still other modifications may produce a material not having a glass transition temperature at all or a material not having the combination of properties that is suitable for use in an HTL. For example, replacing the amine groups of TPD with carbazole groups to produce 4,4'-di(N-carbazolo)diphenyl (CBP), having the chemical structure: ##STR2## increases the melting point to 285.degree. C. However, the material shows no glass transition. Further changes in the basic structure of TPD can increase the T.sub.g value even higher, but the materials often have poorer hole transporting properties than TPD, i.e. OLEDs made with these high temperature materials give poor device properties in OLEDs compared to TPD.
U.S. Pat. No. 5,061,569 discloses hole transporting materials comprised of at least two tertiary amine moieties and further including an aromatic moiety containing at least two fused aromatic rings attached to the tertiary amine nitrogen atoms. Out of the large number of compounds encompassed by the broadly disclosed class of compounds recited, U.S. Pat. No. 5,061,569 fails to disclose how to select those compounds which have a high glass transition temperature. For example, the naphthyl derivatives do make stable glasses. One such molecule is 4,4'-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (.alpha.-NPD), having the chemical structure: ##STR3##
The present inventors' measurements show that .alpha.-NPD has a T.sub.g of 100-105.degree. C., which is substantially higher than the T.sub.g of 65.degree. C. of TPD. This material has excellent hole conduction properties, and the T.sub.g of 100-105.degree. C. is higher than the T.sub.g of TPD of about 65.degree. C. OLEDs prepared with NPD have electrical properties very similar to those prepared with TPD. However, 4,4'-bis[N-(2-naphthyl)-N-phenyl-amino]biphenyl (.beta.-NPD), having the structure: ##STR4## has been generally understood to have a T.sub.g which is substantially lower than .alpha.-derivative. Apparently because of this purportedly low and anomalous difference between T.sub.g of the .alpha.- .beta.-derivatives, there had been no known reports of using the .beta.-derivative as the hole transporting material of an OLED.
It would be desirable if OLED's could be fabricated from glassy charge carrier materials having improved temperature stability, while still providing luminescent characteristics comparable to prior art compounds. As used herein, the term "charge carrier layer" may refer to the hole transporting layer, the electron transporting layer or the separate emissive layer of an OLED having a double heterostructure. In addition, it would be useful to have a method for selecting and preparing such glassy charge carrier materials having improved temperature stability, as characterized, in particular, by glassy charge carrier materials having a high glass transition temperature.
In addition, there is a general inverse correlation between the T.sub.g and the hole transporting properties of a material, i.e., materials having a high T.sub.g generally have poor hole transporting properties. Using an HTL with good hole transporting properties leads to an OLED having desirable properties such as higher quantum efficiency, lower resistance across the OLED, higher power quantum efficiency, and higher luminance. There is therefore a need for a HTL having a high hole mobility and a high glass transition temperature.