TECHNICAL FIELD

The present invention relates to a desiccant for an organic electroluminescent (EL) device, and a method of using the desiccant.

BACKGROUND ART

In organic electroluminescent (abbreviated below as “organic EL”) panels, because the panel elements are “self-emissive,” i.e., they emit light themselves, the panel does not require a separate light source such as the backlight in a liquid-crystal panel and thus has a low power consumption. Other characteristics of such panels include the ease of achieving a lower profile and a lighter weight. In addition, they reportedly have a higher picture quality than liquid-crystal panels. However, a drawback is that the elements are damaged by the ingress of even a trace amount of moisture into the panel (generating dark spots).

Because organic EL panels must be constructed so as to prevent moisture ingress to a high degree, a structure that hermetically seals the organic EL elements with, for example, a light-transmissive substrate made of glass or the like is generally used. This is a hollow, sealed structure in which a substrate of glass or the like has formed thereon an organic EL element constructed of an anode, an organic layer and a cathode arranged as successive layers and has joined thereto an opposing recessed glass member or the like, and is obtained by filling the interior with dried nitrogen gas or the like and bonding the end with a sealant. Various research aimed at carrying out the removal of moisture that has infiltrated into the organic EL panel during production has been reported, including methods which place a desiccant at the interior and methods which cover the organic EL element with a filler. Regardless of the method, cases in which the desiccant or filler has transparency are more effective because so-called top emission, in which light emitted by the organic EL device is extracted to the exterior from the top face, is possible.

Organic EL elements are formed for the most part by processes such as vapor deposition. However, an electrical short-circuit (shorting) between the cathode and the anode sometimes arises due to the presence of a small particle. When shorting arises, the elements around the particle causing the shorting must be separated with a laser or the like, which gives rise to problems such as partial loss of the emitted light and an increased number of fabrication steps.

Moreover, there is a pursuit of thinness in organic EL panels and, in addition to flat panels, other low-profile panels with a curved shape have attracted attention. Such low-profile panels require as well a tolerance to bending. Hence, not only thinness, but also flexibility is required.

Organic EL panels having a flexible structure that is foldable have also received a lot of attention. Thinness and flexibility are similarly required of the various constituent members, such as the light-transmitting substrate and also the desiccant, filler and the like.

Patent Document 1 discloses art which mixes a metal hydride as a desiccant into silicone rubber and places the mixture in an organic EL structure. The steps in this method are simple because filling the interior of the organic EL panel is not required. However, with a desiccant-containing silicone rubber, not only is it difficult to obtain light transmittance, when the silicone rubber is thinly formed, additional challenges include the presence of bubbles therein.

Patent Document discloses the use, as a transparent desiccant, of a mixture of a Group 2, Group 3 or Group 4 metal alkoxide with a transparent polymer that is inert to the metal alkoxide. In this method, there is a gradual loss of light transmittance as moisture absorption proceeds due to reaction of the metal alkoxide with moisture, and so maintaining the initial light transmittance is difficult. Also, in this method, a short circuit-suppressing effect is difficult to obtain.

PRIOR ART DOCUMENTS

Patent Documents

Patent Document 1: JP-A 2001-057287

Patent Document 2: JP-A 2009-095824

SUMMARY OF INVENTION

Technical Problem

The present invention was arrived at in light of the above circumstances. An object of the invention is to provide a transparent desiccant for an organic EL device which is highly transparent, making top emission possible, has a small growth in pixel shrinkage even at high temperature and high humidity, has the effects of suppressing the loss of element light-emitting regions (dark spot formation) and of suppressing electrical shorting, and has an excellent degassing ability in the material curing process. Another object is to provide a method of using such a desiccant.

Solution to Problem

The inventors have conducted extensive investigations aimed at achieving the above objects. As a result, they have found that by using the cured form of an organopolysiloxane composition having a specific make-up as a transparent desiccant for an organic EL device, the composition has an excellent degassing ability during curing, has a high transparency that makes top emission possible, suppresses the growth in pixel shrinkage under high temperature and high moisture, and is able to suppress dark spot formation and electrical shorting. This discovery ultimately led to the present invention.

Accordingly, the present invention provides the following transparent desiccant for an organic EL device and method for the use thereof.

1. A transparent desiccant for an organic EL device, the desiccant being a cured organopolysiloxane composition that contains:

(A) an alkenyl group-containing organopolysiloxane which includes a linear organopolysiloxane having at least two alkenyl groups per molecule,

(B) an organohydrogenpolysiloxane of formula (I) below
HaRbSiO(4-a-b)/2(I)
(wherein R is a monovalent hydrocarbon group of 1 to 10 carbon atoms, the subscript “a” is from 0.001 to 1.0 and the subscript “b” is from 0.7 to 2.1), and

(C) a hydrosilylation catalyst,

wherein the number of moles of silicon-bonded hydrogen atoms Y in component (B) and the number of moles of silicon-bonded alkenyl groups X in component (A) satisfy formula (2) below:
0.002 mole≤(Y−X)≤0.8 mole  (2).
2. A method of using a transparent desiccant for an organic EL device which includes the step of disposing and using the transparent desiccant for an organic EL device of 1 above in an organic EL panel.
3. The method of using a transparent desiccant for an organic EL device of 2 above, wherein the transparent desiccant is in the form of a sheet having a thickness of from 1.0 μm to 2 mm.
4. The method of using a transparent desiccant for an organic EL device of 2 or 3 above, wherein the desiccant is placed solely above an organic EL element.

Advantageous Effects of Invention

The transparent desiccant for an organic EL device of the invention is highly transparent, has a small growth in pixel shrinkage under high temperature and high humidity, and can suppress organic EL element degradation (dark spot formation). Moreover, because the transparent desiccant of the invention is highly transparent, it is suitable for top-emitting EL devices which transmit the light emitted by an organic EL element through the top side of the device.

Also, the inventive method of using the transparent desiccant for an organic EL device disposes the desiccant within an organic EL panel. In cases where it is necessary to thinly form the desiccant, it is possible to form the organopolysiloxane composition into a thin film having an excellent degassing ability in a curing step following application of the composition.

In addition, with regard to the transparent desiccant and method of use of the invention, the transparent desiccant need not be in contact with the organic EL element. The low-molecular-weight siloxane within the desiccant diffuses and accumulates within the organic EL panel, insulating the vicinity of small particles that have become present during formation of the organic EL element and making it possible to achieve the effect of suppressing electrical shorting between the cathode and the anode by the particles.

