Organic EL display assembly

Details Organic EL display assembly Organic EL display assembly

1999-06-11

2001-09-04

Thomas, Alexander S. (Department: 1772)

Stock material or miscellaneous articles

Sheet including cover or casing

Filled with gas other than air; or under vacuum

C428S076000, C428S690000, C428S917000, C313S512000

Reexamination Certificate

active

06284342

ABSTRACT:

This invention relates to an organic electroluminescent (EL) display assembly using an organic compound, and more particularly, to an organic EL display assembly wherein a sealing space for protecting an organic EL structure on a substrate contains a desiccant.
BACKGROUND OF THE INVENTION
Recently, active research works have been made on organic EL devices. As a basic configuration, the organic EL device includes a hole injecting electrode, a thin film formed thereon by evaporating a hole transporting material such as triphenyldiamine (TPD), a light emitting layer deposited thereon of a fluorescent material such as an aluminum quinolinol complex (Alq
3
), and a metal electrode or electron injecting electrode formed thereon from a metal having a low work function such as magnesium. Such organic EL devices are attractive in that they can achieve a very high luminance ranging from several 100 to several 10,000 cd/m
2
with a drive voltage of approximately 10 volts.
Organic EL devices are very sensitive to moisture. Contact with moisture can cause serious problems including separation between the light emitting layer and the electrode layer, deterioration of constituent materials, formation of non-light-emitting zones known as dark spots, and reduction of the luminous area. As a result, the EL devices fail to maintain light emission of the desired quality.
One known solution to this problem is to shield an organic EL multilayer structure on a substrate from the exterior by tightly securing a gas-tight casing or sealing layer to the substrate so as to enclose the EL structure, as disclosed in JP-A 5-36475, 5-89959, and 7-169567.
Despite the provision of such casings or sealing layers, moisture gradually penetrates in the interior with the lapse of drive time whereby the devices deteriorate in several aspects, such as a drop of light emission luminance, generation or enlargement of dark spots, and reduction of the luminous area. Eventually the devices become unusable because of failure of light emission.
It was also proposed to accommodate the organic EL structure in a gas-tight casing in which a desiccant is contained. For example, JP-A 3-261091 discloses diphosphorus pentoxide (P
2
O
5
) as the desiccant. However, P
2
O
5
tends to absorb moisture and becomes liquid (deliquescence) and thus forms phosphoric acid, which can be detrimental to the organic EL structure. The introduction of P
2
O
5
into the casing requires a careful limited procedure. This technique is impractical.
JP-A 6-176867 discloses fine powdery solid desiccants such as zeolite, active alumina, silica gel, and calcium oxide. The desiccants of the type that physically adsorbs water, typically zeolite, release the once adsorbed water by the heat associated with the light emission of the organic EL device. The lifetime of the device is not fully long.
JP-A 9-148066 discloses desiccants in the form of compounds capable of chemically adsorbing moisture and maintaining a solid state even after moisture absorption, for example, alkali metal oxides, alkaline earth metal oxides, sulfates, and metal halides. Because of the chemical adsorption, once water is adsorbed, these compounds no longer release the water. The lifetime of the device is increased, but not to a satisfactory extent.
SUMMARY OF THE INVENTION
Therefore, an object of the invention is to provide an organic EL display assembly which is easy to fabricate, is protected from moisture and other deleterious elements, deteriorates little with time, and maintains the initial performance over a long period of time and hence, has a long lifetime.
According to the invention, there is provided an organic electroluminescent (EL) display assembly comprising a gas-tight casing, an organic EL structure received in the casing, and a desiccant disposed in the casing out of contact with the organic EL structure. The organic EL structure includes a hole injecting electrode, an electron injecting electrode, and at least one organic layer disposed between the electrodes. The desiccant is comprised of calcium hydride, strontium hydride or a mixture of calcium hydride and strontium hydride, preferably calcium hydride. Preferably, the gas-tight casing includes a substrate on which the organic EL structure is laid up, a sealing member disposed above and spaced from the organic EL structure, and a sealing adhesive for bonding the sealing member to the substrate. The space between the sealing member and the organic EL structure is preferably filled with nitrogen.
The organic EL display assembly of the invention includes an organic EL device in the form of an organic EL structure having a hole injecting electrode, an electron injecting electrode, and at least one organic layer disposed between the electrodes. The organic EL structure is received in a gas-tight casing. Calcium hydride and/or strontium hydride is disposed in the casing as a desiccant out of contact with the organic EL structure. By containing calcium hydride and/or strontium hydride as a desiccant in the gas-tight casing, the EL device is significantly increased in lifetime. This is because the desiccant picks up the moisture (or atmospheric water vapor) that will penetrate within the casing after its sealing so that the concentration of moisture within the casing is minimized.
Calcium hydride and strontium hydride pick up moisture according to the following reaction scheme.
CaH
2
+2H
2
O→Ca(OH)
2
+2H
2
SrH
2
+2H
2
O→Sr(OH)
2
+2H
2
Since the compounds (or hydroxides) resulting from reaction with water are stable, the once picked up water is never released again. The use of calcium hydride and strontium hydride as the desiccant has the additional advantage that hydrogen generates along with water removal so that the interior of the gas-tight casing becomes a reducing atmosphere, which prevents the electrodes, especially the electron injecting electrode, from deterioration. Consequently, the lifetime of the EL device is further prolonged. The lifetime is long enough as compared with the use of the compounds described in JP-A 9-148066. Since an equimolar amount of hydrogen evolves upon adsorption of water vapor as understood from the reaction scheme, the change of the internal pressure within the gas-tight casing is little or nil, which is advantageous in maintaining the seal. The compounds described in JP-A 9-148066 are disadvantageous in this respect too.
Moreover, the desiccant used herein maintains a solid state even after moisture absorption. As long as the desiccant is disposed out of contact with the organic EL structure, the desiccant can exert no deleterious effect thereon. The desiccant can be easily contained in the casing.


REFERENCES:
patent: 5048301 (1991-09-01), Sabin et al.
patent: 5909081 (1999-06-01), Eida et al.
patent: 0 776 147 (1997-05-01), None
patent: 0 859 539 (1998-08-01), None
patent: 3-261091 (1991-11-01), None
patent: 5-36475 (1993-02-01), None
patent: 5-89959 (1993-04-01), None
patent: 6-176867 (1994-06-01), None
patent: 7-169567 (1995-07-01), None
patent: 9-148066 (1997-06-01), None
patent: 0914 8066 (1997-06-01), None
patent: 10-214683 (1998-08-01), None
patent: 10-275679 (1998-10-01), None

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