An organic thin-film el device

Abstract: An organic thin-film electroluminescence (=EL) device is provided. This device includes as a main constituent of an organic luminescent thin-film layer sandwiched between a couple of electrodes at least one of which is transparent, a naphthalimide derivative represented by the formula (I): wherein R₁, R₂ and R₃ are identical or different from each other and are selected from the group consisting of hydrogen, an alkyl group, an aryl group, a substituted alkyl group, a substituted aryl group, an alkoxyl group, an amino group and an acylamino group.

Description: BACKGROUND OF THE INVENTION Field of the invention The present invention relates to an organic thin-film electroluminescence (=EL) device for use particularly in a planar light source, a display or the like. Description of the prior art An EL device fabricated of organic substance has been expected to realize a low-cost large-surface multicolored display device. For example, a condensed multi-ring aromatic compound such as anthracene, perylene or the like is made into a thin film by a Langmuir-Blodgett (LB) method, a vacuum evaporation plating method or the like to produce a DC driving organic thin-film EL device and study its luminescent properties. Such prior art organic thin-film EL device, however, requires high driving voltage, whereas its luminous brightness and luminous efficiency are inferior to those of inorganic thin-film EL devices. Further, it shows considerable degradation of luminescent properties and thus is far from practical. Recently, however, a new type of an organic thin-film EL device having a double-layer structure of organic thin films was reported to attract keen attention [Applied Physics Letters, Vol. 51, No. 12, pages 913 - 915 (1987)]. According to the report, this organic thin-film EL device has a double-layer structure as shown in the attached Fig. 1 comprising an organic luminescent thin-film layer 24 fabricated of strongly fluorescent metal chelate complex and an organic hole injection layer 23 fabricated of amine series compound, which layers are sandwiched between an indium-tin-oxide (ITO) transparent electrode 22 and a backplate 25, and owing to such double-layer structure can exhibit bright green luminescence. In effect, brightness of some hundreds cd/m² reportedly was obtained at DC driving voltage of 6 - 7 V, and maximum brightness was 1,000 cd/m² at 10 V. Further, it shows maximum luminous efficiency of 1.5 lm/W, which is near to practical levels. This device however is of a type driven by a DC current and so, in order to gain such high brightness, requires stable flow of DC current larger than 100 mA/cm². The organic luminescent layer of the material used heretofore is not satisfactory to realize such stable flow of a large current, owing to degradation of the luminescent layer by electric conduction and morphological change in the luminescent thin-film which cause reduction of electron and hole injection into the organic luminescent layer. SUMMARY OF THE INVENTION An object of the present invention therefore is to present an improved organic thin-film EL device having an increased luminous efficiency and a reduced degradation rate of luminous brightness. The present invention provides an organic thin-film EL device comprising at least one organic luminescent thin-film layer and at least one organic electric-charge transporting thin-film layer sandwiched between a couple of electrodes at least one of which electrodes is transparent, said organic luminescent thin-film layer containing as a main constituent naphthalimide derivative (I) wherein R₁ is hydrogen or a substituted or unsubstituted alkyl or aryl group and R₂ and R₃ are selected independently from hydrogen and alkyl, aryl, substituted alkyl, substituted aryl, alkoxy, amino and acylamino groups. US-A-4,672,265 discloses an electroluminescent device formed by lamination of at least two electroluminescent layers, but does not describe or teach the presence of any charge transporting layer or any use of naphthalimide compounds for the electroluminescent materials. Naphthalimide compounds used in the present invention are known as luminophores from GB-A-2,183,667 and SU-A-371,422, but the latter do not describe or teach their use for the material of an organic luminescent thin layer which together with an organic charge transporting thin-film layer forms an organic thin-film EL device. The main feature of the present invention resides in the use of naphthalimide derivative (I) as for the luminescent layer and so there are no substantial limitations to materials which constitute elements other than the luminescent layer. Some examples of fabrication of the organic thin-film EL device of the present invention are illustrated in the attached Figs. 1 and 2. In the example of Fig. 1, an organic luminescent thin-film layer 14 is sandwiched between an ITO transparent electrode 12 on a glass substrate 11 and a backplate 15, with an organic hole injection layer 13 additionally placed between the ITO transparent electrode 12 and the organic luminescent thin-film layer 14 to make up a double-layer structure. In the example of Fig. 2, an organic electron injection layer 16 is placed between the organic luminescent thin-film layer 14 and the backplate 15, in addition to the organic hole injection layer 13 shown in Fig. 1. In any of these examples, it is possible to obtain a device having very excellent luminous efficiency by using the above mentioned naphthalimide derivative for the organic luminescent thin-film layer in accordance with the present invention. It is particularly effective to employ as the material of the hole injection layer 13, as shown in Figs. 1 and 2, provided on the ITO electrode 12, which is transparent and is a hole injection electrode, an aromatic tertiary amine series compound or a phthalocyanine series compound, which shows the property of a p-type semiconductor, because efficiency of hole injection from the ITO electrode 12 and transport efficiency in the hole injection layer 13 are increased. Similarly, it is possible and effective to increase efficiency of electron injection from the backplate 15 and transport efficiency in the electron injection layer 16. As for the transparent electrode, it is possible to use any ordinarily used one, for example, ITO, SnO₂:Sb, ZnO:Al, Au, etc. may be mentioned. As for the backplate, In, Mg, Ag, Mg:Ag, etc. may be mentioned. As for the naphthalimide derivative to be used in accordance with the present invention, it is possible to use the following compounds, but these compounds are only examples and by no means limit the scope of the derivatives: N-(2',4'-xylyl)-4-aminonaphthalimide, 4-aminonaphthalimide, N-methyl-4-aminonaphthalimide, N-ethyl-4-aminonaphthalimide, N-propyl-4-aminonaphthalimide, N-n-butyl-4-aminonaphthalimide, 4-acetylaminonaphthalimide, N-methyl-4-acetylaminonaphthalimide, N-ethyl-4-acetylaminonaphthalimide, N-propyl-4-acetylaminonaphthalimide, N-n-butyl-4-acetylaminonaphthalimide, N-methyl-4-methoxynaphthalimide, N-ethyl-4-methoxynaphthalimide, N-propyl-4-methoxynaphthalimide, N-n-butyl-4-methoxynaphthalimide, N-methyl-4-ethoxynaphthalimide, N-ethyl-4-ethoxynaphthalimide, N-propyl-4-ethoxynaphthalimide, and N-n-butyl-4-ethoxynaphthalimide. In accordance with the present invention, it becomes possible to improve luminous efficiency 1.5 to 2 times that of the prior art organic thin-film EL device, as shown in the embodiments of the present invention shown hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 and 2 respectively show cross sectional views of some examples of organic thin-film EL devices having various structures, to which the present invention can be applied. DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention will be explained below further in detail with respect to its embodiment shown as a working example. Example 1 As for the organic luminescent material, a naphthalimide derivative UFG-1, a product of Nippon Kayaku Co., Ltd., Japan was used. As shown in Fig. 1, an ITO transparent electrode 12 was formed on a glass substrate 11, and then a thin film of 500 Å thickness of 1,1-bis(4-N,N-ditolylaminophenyl)cyclohexane and a thin film of 1,000 Å thickness of said naphthalimide were plated thereon by vacuum evaporation at 10⁻⁷ Torr. vacuum to form a hole injection layer 13 and an organic luminescent thin-film layer 14 respectively. Finally, a backplate 25 was formed thereon by depositing indium to a thickness of 1,500 Å by an electron beam source evaporation plating method to obtain an organic thin-film EL device. The obtained device was tested to determine its luminescent properties in dry nitrogen atmosphere. It was found that bright yellow luminescence of 200 cd/m² was obtained by applying DC voltage of about 7 V. This corresponds to improvement of luminous brightness and efficiency of 1.5 to 2 times those of a prior art device. According to aging tests performed under application of an electric current density of 0.5 mA/cm², the above device showed a brightness half-value period of not less than 500 hours, whereas the prior art device showed one of 100 to 300 hours. As explained above, the present invention presents an organic thin-film EL device having considerably improved luminescent properties and reliability, which is very effective for use as a thin-film luminescent device operative by a low driving voltage and thus is highly valuable in the industry.

Claims: An organic thin-film EL device comprising at least one organic luminescent thin-film layer and at least one organic electric-charge transporting thin-film layer sandwiched between a couple of electrodes at least one of which electrodes is transparent, said organic luminescent thin-film layer containing as a main constituent naphthalimide derivative (I) wherein R₁ is hydrogen or a substituted or unsubstituted alkyl or aryl group and R₂ and R₃ are selected independently from hydrogen and alkyl, aryl, substituted alkyl, substituted aryl, alkoxy, amino and acylamino groups. A device according to claim 1 wherein at least one organic electric-charge transporting thin-film layer consists of a hole injection layer made of an organic hole transporting material to give a double-layer structure. A device according to claim 1 wherein at least one organic electric-charge transporting thin-film layer consists of a hole injection layer made of an organic hole transporting material and an electron injection layer made of an organic electron transporting material to give a triple-layer structure.

Inventors: ISHIKO MASAYASU, UTSUGI KOJI, ISHIKO, MASAYASU, UTSUGI, KOJI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1990
Main classification: C09K11
IPC codes: H05B33, C09K11, H01L51