An organic EL device is constituted by placing at least a thin layer of an organic light-emitting material (light-emitting layer) between a pair of electrodes facing to each other. In this organic EL device, electrons injected into a light-emitting layer from a cathode directly or through an electron-injecting layer and holes injected into the light-emitting layer from an anode directly or through a hole-injecting layer recombine in the light-emitting layer, whereby light is emitted.
As means for improving the light emission properties of the organic EL device based on the above light-emitting mechanism, it is known to select or improve the organic light-emitting material, to improve the film quality of the light-emitting layer or to select or improve a cathode material. Of these, the means of selecting or improving the cathode material is generally aimed at the improvement of the efficiency of injection of electrons into the light-emitting layer for improving the light emission properties. It is therefore being attempted to use a variety of electrically conductive metals, alloys or intermetallic compounds having small work functions as a material for the cathode.
For example, U.S. Pat. No. 3,173,050 and U.S. Pat. No. 3,382,394 disclose organic EL devices whose cathode is formed of alkali metal such as Na-K alloy. The organic EL devices disclosed in these U.S. Patents are desirable in that the quantum efficiency is high (see RCA Review Vol. 30, page 322), while they are not practical since alkali metals and alloy formed of alkali metals alone are highly active and chemically unstable.
It is therefore variously proposed to form cathodes from metals other than alkali metals. For example, U.S. Pat. No. 4,539,507 discloses an organic EL device whose cathode is formed of In. JP-A-3-231970 discloses an organic EL device whose cathode is formed of Mg-In alloy. Further, European Patent 0278757 discloses an organic EL device provided with a cathode formed of a layer containing a plurality of metals other than alkali metals, at least one of which metals other than alkali metals has a work function of 4 eV or less (Mg-based electrode formed of Mg and one of Ag, In, Sn, Sb, Te and Mn, such as Mg-Ag electrode).
Further, various organic EL devices whose cathodes are formed of mixtures of at least 11 at. % of metals having excellent electron-injecting properties such as Li, Na, Ca and Sr with relatively stable metals such as Mg, Al, In and Sn are disclosed in Autumn Meeting held in 1991 (Polymer Preprints, Japan, Vol. 40, No. 10, page 3,582). However, the organic EL devices disclosed in the Meeting are inferior to an organic EL device having an Mg-Ag electrode as a cathode in efficiency (power conversion efficiency).
As described above, there have been various proposals for forming cathodes from metals having electroninjecting properties, while these cathodes are poor in chemical stability with time, and organic EL devices having these cathodes have been unsatisfactory in power conversion efficiency.
It is an object of the present invention to provide a novel organic EL device having a cathode having high stability with time and improved power conversion efficiency.