NON-POLYMERIC FLEXIBLE ORGANIC LIGHT EMITTING DEVICE

Abstract: An organic light emitting device (OLED) is disclosed for which the hole transporting layer, the electron transporting layer and/or the emissive layer, if separately present, is comprised of a non-polymeric material. A method for preparing such OLED's using vacuum deposition techniques is further disclosed.

Description: NON-POLYMERIC FLEXIBLE ORGANIC LIGHT EMITTING DEVICE RELATED APPLICATION This invention is related to provisional Application 60/024,001 dated August 12, 1996. FIELD OF THE INVENTION The subject invention is directed to flexible organic light emitting devices (OLED's) including a hole transporting layer and/or an electron transporting layer comprised of a vacuum- deposited, non-polymeric material. BACKGROUND OF THE INVENTION In one type of electrically controlled light emitting device, organic material is placed between a layer of conductive material that can inject electrons and a layer of conductive material that can inject holes. When a voltage of proper polarity is applied between the outer layers of conductive material, electrons from one layer combine with holes from the other so as to release energy as light that is, to produce electroluminescence (EL) . These devices are referred to as organic light emitting devices, OLED's. OLED's have been constructed from polymers so as to have a highly advantageous flexibility that enables them to be used for light weight, portable, roll-up displays or to be used for conformable displays wnich can be readily attached to windows, windshields or instrument panels that may have curved surfaces; "The Plastic LED: A Flexible Ligh -Emitting Device Using a Polyanilme Transparent Electrode" oy G. Gustafsson et al in "Synthetic Metals", 55-57 4123-4227 (1993). Even though there is a widespread application of vacuum-deposited, small- molecule-based heterostructural OLED's, which have Deen constructed on inflexible glass substrates and use ITO as the hole emitting layer, the devices of Gustafsson were fabricated using a polymer, that is, soluble semiconducting polymer poly ( 2 -methoxy , 5- (2 ' -ethyl -hexoxy) - 1 , 4 -phenylene-vmylene) (MEK-PPV) as the emissive layer, since the mecnanical oroperties of polymers were deemed to be unique with respect to making such devices It would be desirable if flexible OLED's could be fabricated having improved electroluminescent properties as well as the advantage of being readily fabricated using tne vacuum deoosition techniques typically used tor preparing OLED's SUMMARY OF THE INVENTION In accordance with this invention, a flexible OLED using small molecule based heterostructure of organic material is provided in which the hole transporting layer, the electron transporting layer, and/or the emissive layer, f separately present, includes a non-polymeric material, that is, a layer comprised of small molecules. The term "small molecules" is used herein to refer to molecules which are small in the sense that such molecules are not made up of a plurality of repeating molecular nits such as are present in a polymeric material Thus, for purposes of this invention, the term "small molecule" is intended to oe used intercnangeably with the term "non-polymeric ' In ract, the term "small molecules" may embrace relatively large molecules such as are typically used m the hole transporting layer, the electron transporting layer and/or the emissive layer that is present in an OLED. The subject invention is furtner directed to a method of fabricating flexible OLED' s wherein the nole transporting layer, the electron transporting layer and/or the emissive layer, .tf present, may oe prepared using vacuum deposition techniques, rather than using the less convenient fabrication technique such as employed by Gustafsson et al, tnat is, rather than spin coating a layer of polymer, such as polyamlme onto the flexible substrate Sucn vacuum deposition methods are particularly suitaole for use m fabricating the OLED's of tne subject invention since tr.e other layers of the OLED are also typically prepared using vacuum deposition tecnniques Integration of all the vacuum deposition steps into a single overall sequence of steps for fabricating the OLED, without requiring the use of solvents or removing the air sensitive layers from a vacuum chamber and exposing them to ambient conditions provides an additional especially beneficial advantage. Thus, the subject invention is directed to a method, wnerein the nole transporting, electron transporting, and/or separate emissive layer, if present, may be prepared using vacuum deposition steps BRIEF DESCRIPTION OF THE DRAWINGS Fig 1 is a cross section of one emjoodiment of a flexible OLED constructed in accorαance with this invention, Fig 2 contains graphs illustrating the current vs voltage characteristic of an OLED sucn as shown in Fig. 1 before and after repeated bending and the current vs voltage characteristic of a prior art OLED having a glass substrate in place of the flexible polyester substrate and Fig 3 contains grapns illustrating the optical power vs current characteristics of an OLED such as shown in Fig 1 before and after repeated bending and the current vs voltage characteristic of a prior art OLED used m a glass substrate in place of the flexiole polyester substrate Fig 4 shows atomic force microscope (AFM) images of a typical ITO-coated polyester substrate film wherein (a) snows t.ne ITO (top) substrate and (b) snows the polyester (bottom) suostrate surface Tne height range of the images was -50 nm (nanometers) Fig 5 shows a photograph of an array of nine unpackaged 1 cm 2 vacuum-deposited, non-polymeric flexible OLED's One device in contact with the probe arm is shown operating in air in a well-iilummated room at normal video display brightness (-100 cd/m 2 ) DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The suDject invention will now be described m detail for specific preferred embodiments of the invention, it being understood that these embodiments are intended only as illus t rative examples and the invention is not to be limited thereto. As illustrative errtDoαiments of the subject invention, the sub j ect OLED's may be incorporated into a single heterostructure or in a double heterostructure . The materials, methods and apparatus for preparing the single and double heterostructures are disclosed, for example, in U.S. Patent No. 5,554,220, which is herein incorporated m its entirety by reference The subject invention as disclosed herein may be used in conjunction with co-pending applications, "Hign Reliability, High Efficiency, Integratable Organic Light Emitting Devices and Methods of Producing Same,-" "Novel Materials for Multicolor LED's,-" "Electron Transporting and Light Emitting Layers Based on Organic Free Radicals," "Multicolor Display Devices,-" "Red-Emitting Organic Light Emitting Devices (LED's) , " and "High Efficiency Organic Light Emitting Device Structures," each of said co-pendmg application being filed on December 23, 1996, and being herein incorporated m their entirety by reference The suo ect invention may also be used m conjunction with co-pending U.S. Serial Nos. 08/354,674; 08/613,207; 08/632,316; 08/632,322; and 08/693,359; which are also herein incorporated in their entirety by reference. The single or double heterostructures, as referred to herein, are intended solely as examples showing how- an OLED embodying the subject invention may be fabricated without m any way intending the invention to be limited to the particular sequence or order of making the layers shown. For example, a single heterostructural OLED of the subject invention includes a flexible substrate, which is preferably transparent, a first electrode, which may typically be an indium tin oxide (ITO) anode layer, a hole transporting layer, an electron transporting layer, a second electrode layer, for example, a metal cathode layer of Mg.Ag, and a metal protective layer, for example, made of a layer of Ag, for protecting tne Mg Ag cathode layer from atmospheric oxidation A double heterostructure would also include an additional layer containing an emissive material This additional layer is herein referred to as a "separate emissive layer" so as to distinguish it from tne otner layers, since the hole transporting layer and the electron transporting layer can be made to produce electroluminescent emission without the need for this separate emissive layer Althougn not limited to the thickness ranges recited herein, the ITO anode layer may be about 1000 A (1 A = 10 3 cm) to greater than about 4000 A thιcκ, the nole transporting layer about 50 A to greater than aoout 1000 A thick, the layer containing emissive material aoout 50 A to aoout 200 A thick, tne electron transporting layer about 50 A to about 1000 A thick; and each metal layer, about 50 A to greater than about 100 A thick, or substantially thicker if the cathode layer is not intended to be transparent The ability to achieve highly flexible displays vacuum- deposited molecular organic materials, whic-h have stable electroluminescent properties, αepenαs on, inter alia , tne following two factors. First, the molecular bonds responsible for the mechanical properties of the thin films comprising the OLED needs to be reasonably tolerant of the stress applied to the structure on oending, and, second, the substrates needs to be sufficiently flat and uniform such that mechanical defects are not formed during growth or flexing Concerning the first factor, virtually all organic materials used in vacuum-deposited OLED's are held together oy highly flexible van der Waals bonds. Previously, it has been shown, Y. Zhang and S. R Forrest, Phys . Rev Lett 71, 2765 (1993), that the bonding of aromatic molecules similar to those used in OLED's is highly compressible. For example, it was snown that the compressibility of the van der Waals -bonded naphthalene-based molecular crystal NTCDA nas a roughly 20- times-higher compressibility than most ductile metals such as In or Al . C. Kittei, Solid State Physics, 4th ed . (Wiley, New York, 1971) p. 143 While not intending to be limited to the theory of wny the suoject invention is capaole of producing stable electroluminescence, such considera ions help to explain why the molecular materials disclosed herein are sufficiently ductile to undergo significant stress without cracking . The second factor, that the substrates used be sufficiently flat, was established through the use of images produced by an atomic force microscope. These images, sucn as shown in Fig. 4, shows that the ITO surface had a rms roughness of only 1.8 n , whereas the polyester surface of the flexible was somewhat rougher, with a rms value of 2.8 nm. Although there was some variation from substrate to substrate, ITO surface roughness did not exceed 3.6 nm. In either case, the substrates were sufficiently smooth ( .e., the height of the surface features was a small fraction of the total device thickness) such that no significant damage was observed for the subject OLED heterostructure on growth or bending. Based on sucn considerations, the subject devices grown on flexible substrates were found to have efficiencies comparable with conventional vacuum-deposited OLED's grown on glass and, furthermore, such devices were found to be mechanically robust . This invention will now be described in detail with respect to showing how certain specific representative embodiments thereof will be made, the materials, apparatus and process steps being understood as examples that are intended to be illustrative only. In particular, the invention is not intended to be limited to the methods, materials, conditions, process parameters, apparatus and the like specifically recited herein. EXAMPLES In the cross section of the particular embodiment of the invention illustrated in Fig. 1, a flexible substrate 2 for the device is comprised of any suitable flexible polymer sheet such as a polyester. Preferably, the flexible substrate is capable of being bent to a radius of curvature down to 0.5 cm or less. The flexible substrate 2 is precoated with a thin film 4 of indium tin oxide, ITO, such as is available from Southwall Technologies, Inc., 1029 Corporation Way, Palo Alto, Calif., 943C3, Part No. 903-6011. In this particular embodiment, the hole transport layer 6 was comprised of N,N'- diphenyl-N,N " ' -bis (3 - ethylpheny) l-l ' bιphenyl-4 , 4 'diamine (TPD) , and the electron transporting layer 8 was comprised of tris- (8 -hydroxyqumoline) aluminum (Alq 3 ) Other non-polymeric materials, such as known m the art for preparing hole transporting layers, electron transporting layers and emissive layers may also be used The hole conducting layer 6 of TPD is formed on top of tne hole injecting ITO layer 4, and a light emitting layer 8 of Alq, is formed on the layer 6. Alternatively, a single layer can be used in place of the layers 6 and 8 in which TPD and Alq 3 are combined A layer 10 of Mg-Ag is formed on the Alq 3 layer 8, and a layer 12 of Ag s formed on the Mg-Ag layer 10 A power supply 14 is connected between the Ag layer 12 and the precoated ITO layer 4 Although the layers could have different thicknesses than those shown in the table below, the thicknesses shown produced the characteristics illustrated by the graphs of F gs 2 and 3. Layer No Laver Thickness layer 2 175 μM layer 6 800 A layer 8 800 A layer 10 1500 A layer 12 500 A In accordance with another aspect of the invention, the light emitting device of Fig. 1 may be fabricated- as follows. In tms example, the substrate 2 is a 175 μM thick (1 μM = 10" 6 meters) transparent polyester sheet precoated with a transparent, conducting ITO thin film 4 The thickness of the flexible substrate may be either substantially tnicker or thinner depending on the needs of the particular application which the OLED is used The sheet resistance of the ITO thin film 4 was 60 Ω/D, and the transparency of the coated substrate was -80% throughout the visible spectrum Prior to the deposition of the organic film, the substrate 2 was ultrasonically cleaned in detergent for two minutes, then rinsed with deiomzed water Next, it was rinsed m 2- propanol , nεld at room temperature for two to three minutes, and then ooiled in 2-propanol again for two to three minutes, followed by drying with a blow gun using a stream of filtered dry nitrogen An 800 A thick layer 6 of tne nole conducting material, TPD, was deposited by thermal evaporation in a vacuum of <4 x 10" 7 Torr, followed DV the deposition of a 800 A thick Alq 3 layer The top electrode consisted of a 1500 A thick layer of Mg-Ag and a 500 A thick Ag cap deposited through a shadow mask A conventional device on an ITO- precoated glass substrate was simultaneously fabricated for comparison using identical cleaning ana deposition procedures The sneet resistance and transparency of tne ITO-precoated glass substrate was 20 Ω/D and -90%, respectively Fig. 2 shows the current-voltage characteristics of a 1 mm diameter flexible device prior to bending, curve 16, after repeated bending (4 to 5 times) over a small radius of curvature (-0.5 cm), curve 18, and the conventional device on a glass substrate, curve 20 All the current/voltage curves are shown to follow the power law dependenc-e of current -on- voltage At lower voltages, the current/voltage curves indicate onmic behavior, while at nigner voltages, the curves follow I = V 1 * 1 with m=7, suggestive of trap-limited conduction typical of OLED's. The power law dependence was observed for at least four orders of magnitude change in current in the high current region. There was no obvious change m the current/voltage characteristics after the device was repeatedly flexed. The turn-on voltages (defined as the voltages at which the current due to ohmic and trap limited conduction are equal) of the three curves was almost identical (-6.5V), while the leakage current at low voltages of the flexible device was even less than that of the conventional device, and was not increased after bending. This indicated that the ITO film precoated on the flexible substrate is sufficiently uniform such that current shunt paths between the too and bottom contacts 12 and 4 are not induced after bending, even for very thin film (-1600 A molecular organic structures) . The light output power versus current (L-I) characteristics of the flexible device before and after Dending, and of the conventional device are shown m Fig. 3 by graph 22. Curve 24 illustrates the L-I output of a standard device having a glass substrate. The external quantum efficiency of the flexible device was 0.20%, and that of the conventional device was 0.14%. In both cases, the efficiency was calculated from light emitted only in the forward scattering direction. This considerably underestimates the true quantum efficiency but is useful for comparing between devices. Once again, the quantum efficiency of the flexible device was demonstrated not to be affected by repeated bending. The fact tha-t there was no appreciable change in either the I-V or L-I characteristics after the device was flexed indicated that the ITO contact, the organic layers, and the alloy top contact were not significantly affected by bending even over a small radius of curvature . Large-area ^ -1 cm 2 ) devices were also fabricated by similar methods. As in the case of the smaller devices, the large devices were also be bent over radii of -0.5 cm without apparent degradation. That these larger areas can be achieved indicates that flexible OLED's can be used m large, roll-up, or conformable flat panel displays. This, in conjunction with the fact that the ITO-precoated substrate is available m large spools, indicates that flexible, OLED-based displays can be mass manufactured on a roll-to-roll basis by use of suitable volume growtn technologies such as organic vapor phase deposition. Failure modes of the large-area device were also studied. If on the convex side of a curved substrate tne device can be bent without failure even after a permanent fold occurs m the Dolyestεr film. If on the concave side tne device remains operational when bent over a radius of curvature down to 0.5 cm. At smaller radii, cracks propagate through the device, and curren -shunt paths are created between ootto and top contacts after further bending. When ITO-precoated substrates are similarly bent, the same cracking phenomenon is observed, from which it can be inferred that the cracks occur in the ITO rather than in the OLED itself. In conclusion, vacuum-deposited, van der Waais-bonded, non- polymeric flexible OLED's, such as illustrated in Fig. 5, have been fabricated using an ITO-precoated transparent polyester film as the substrate. It has been shown that an ITO thin film, when precoated on a flexible suostrate, provides a flat, highly transparent, conductive, flexible contact suitable for OLED applications This hole- injecting ITO-coated substrate may also oe used with OLED's comprising polymeric nole transporting, electron transporting, and/or emissive layers comprised of polymers In addition, performance similar to that disclosed herein is expected if non-polymeric devices are vacuum deposited on polymeric, transparent hole- injecting contacts sucn as polvamline, which may be useful if even greater flexibility is required m certain applications Although a particular OLED structure of Fig 1 has oeen descrioed, it is to oe unαerstood that any OLED structure having layers that are vacuum formed could be formed on a flexible polymeric suostrate m accordance with tnis invention Those of skill in the art may recognize certain modifications to the various embodiments of the invention, which modifications are meant to oe covered oy tne spirit and scope of the appendeα claims

Claims: What Is Claimed Is: 1. A flexible organic light emitting device comprising: an organic light emitting device comprising a flexible substrate layer and: a) a hole transporting layer comprised of a non-polymeric material or b) an electron transporting layer comprised of a non- polymeric material . 2. The flexible organic light emitting device of claim 1 wherein said organic light emitting device includes a hole transporting layer comprised of a non-polymeric material and an electron transporting layer comprised of a non-polymeric material . 3. The flexible organic lignt emitting device of claim 1 wherein sai orσanic light e ittir.α cevice furtner includes a separate emissive layer comprised of a on- poiymeric material . 4. The flexible organic light emitting device of claim 1 wherein said flexible substrate is transparent. 5. The flexible organic light emitting device of claim 1 wherein said flexible substrate is polymeric. 6. The flexible organic light emitting device of claim 1 wherein said flexible substrate comprises a polyester. - 1 ύ. - 7. The flexible organic iignt emitting device of claim 1 wherein said organic light emitting device further includes an anode layer comprised of indium tin oxide. 8. The flexible organic light emitting device of claim 1 wherein said hole transporting layer comprises N,N'- diphenyl -N, N ' -bis ( -me hylpheny) 1 - 1 ' bιphenyl-4 , ' diamine 9. The flexible organic light emitting device of claim 1 wherein said electron transporting layer comprises tris- ( 8 -hydroxyqumoime) aluminum. 10. A flexible organic light emitting device comprising: a flexible suostrate, an anode layer on said flexible suostrate, a hole transporting layer on said anode layer, an electron transporting layer on said hole transporting layer and a catnode aver on said electron transoortmc iaver, wherein said hole transporting layer cr said e e; transporting layer is compriseα or a r.o -polymeric material . A flexible organic light emitting device comprising: a flexible substrate, a cathode layer on said flexible suostrate, a hole transporting layer on said cathode layer, an electron transporting layer on said nole transporting layer and an anode layer on said electron transporting layer, wherein said hole transporting layer or said electron transporting layer is comprised of a non-polymeric material . 12. A method of preparing a flexible organic light emitting device comprising: fabricating an organic light emitting devxce on a flexible substrate wherein the fabrication of said organic light emitting device includes the step of vacuum depositing a nole transporting layer or the step of vacuum depositing an electron transporting layer. 13. The method according to claim 12 wherein said fabrication step includes the steps of vacuum depositing a hole transporting layer and of vacuum depositing an electron transporting layer. 14. The method according to claim 12 wherein said fabrication step further includes the step of vacuum depositing a separate emissive layer. 15. A method of preparing a display device comprising: incorporating a flexible organic light emitting device into a display device wherein said flexible organic light emitting device is fabricated on a flexible substrate using a method that includes the step of vacuum depositing a hole transporting layer or the step of vacuum depositing an electron transporting layer. 15. A display incorporating tne fiexiole organic light emitting device of any one of claims 1 through 11 17 A vehicle incorporating the flexible organic iignt emitting device electron transporting layer of any one of claims 1 througn 11. lb. A computer incorporating the flexible organic light emitting device electron transporting layer of any one of claims 1 through 11. 19. A television incorporating the flexible organic light emitting device electron transporting layer of any one of claims 1 througn 11. 20. A printer incorporating tne fiexiole organic light emitting device electron transporting layer of any one of claims 1 through 11 21. A wall, tneater or stadium screen incorporating tne flexible organic light emitting device electron transporting layer of any one of claims 1 through 11 22. A billboard or a sign incorporating tne flexible organic light emitting device of any one of claims 1 through 11. 23. A flexible organic light emitting device comprising a flexible, transparent polyester subs-trate, an anode layer on said fiexiole, transparent polyester substrate, said anode layer oemg comprised of indium tin oxide ; a hole transporting layer on said anode layer, said hole transporting layer being comprised of N, N' -dipheny -N, N' ■ bis (3-methylpheny) 1-1 ' biphenyi -4,4' diamme , an electron transporting layer on said nole transporting layer, said electron transporting layer being comprised of tris- (8-hydroxyqumolιne) aluminum; and a cathode layer on said electron transporting layer.

Inventors: BURROWS PAUL, FORREST STEPHEN R, GU GONG, BURROWS, PAUL, FORREST, STEPHEN, R., GU, GONG
Applicants: UNIV PRINCETON, THE TRUSTEES OF PRINCETON UNIVERSITY
Application year: 1997
Main classification: H01J1
IPC codes: C09K11, G09F9, G09F13, H01J1, H01L51, H05B33