By the advent of information age, plenty of information is generated every minute. To deal with this whenever and wherever you want, electronic products are getting smaller and more portable. That means that the display with portability and high resolution for vivid movie is expected appositely. As a result, the Cathode Ray Tube(CRT) display which is bulky and heavy is being replaced with Flat Panel Display(FPD). The Liquid Crystal Display (LCD) is currently the dominant technology due to its light weight, low power consumption and easy operation. But LCD still has drawbacks in view angle, response time and brightness. As a result, the Organic Light-Emitting Diode(OLED) which has solutions for the drawbacks of LCD has come into the spotlight as an alternative for LCD. Now PMOLED already has been commercialized but, for the OLED to replace the LCD, AMOLED should be realized. AMOLED needs more TFTs compared to LCD due to its driving method and this reduces aperture ratio. As a result, to enhance an aperture ratio, top emission type that has nothing to do with the number of TFTs needs to be applied to AMOLED. For that, the technology on stable reflective anode, transparent cathode and transparent encapsulation should be guaranteed.
Basically, for cathode, it is desirable to use metals having a low-work function for the easy electron-injection. These include Al, Mg, Ca, Ag, Mg:Ag, and LiF:Al. However, because metals with a low-work function are always susceptible to atmospheric oxidation, alloys with stable metals having relatively high-work function or low-work function metals with a protective layer are used for cathode. And to be transparent in visible light, to a thickness of less than optical skin depth and not to damage the underlying organic layer, by thermal evaporation method, cathode is fabricated.
In this study, the Ca/Ag double layer has been fabricated for the transparent cathode in Top Emission OLED by thermal evaporation to a thickness of less than optical skin depth. Ca is suitable for easy electron-injection due to its low work function (2.9~3.0 eV) and Ag is suitable for the protective layer of reactive Ca due to its high stability and easy deposition. Besides its relatively low resistivity(1.6 μΩ㎝) is a help to high electrical conductivity for an electrode. Ca and Ag are deposited one after another without vacuum breaking. The structure and optical properties of the Ca/Ag double layer are studied.
The structure of the Ca/Ag double layer is studied. The existence of Ag and $Ca(OH)_2$ and the fact that the Ca/Ag double layer is actually the (Ca+O)/(Ag+Ca) double layer are confirmed. And in Ca layer, there are a large portion of $Ca(OH)_2$ and a small portion of CaO. From the facts that the large portion of extra Ca is in the whole layer and there is no Ca peak in the result of XRD, these are concluded that in Ag layer, Ca is intermixed with Ag in amorphous state and in Ca layer, Ca exists in amorphous state. That is to say, in upper layer, there are Ag in crystalline state and the intermixing of Ag and Ca in amorphous state. And in lower layer, there are $Ca(OH)_2$ in crystalline state and Ca and CaO in amorphous state. This structure is formed as follows : in vacuum, the Ca/Ag double layer is formed and after the sample is exposed to air, $H_2O$ in air is reacted with Ca through Ag layer. And then Ca is changed to $Ca(OH)_2$ accompanying the volume expansion. At this time, a small amount of Ca is changed to CaO and large amount of Ca is remained untouched. By the volume expansion in this change, Ca which has been remained untouched is pushed to the Ag layer. For the reasons that the ratio of Ag and Ca is constant and no oxygen is detected in the upper layer, the solubility of Ca in Ag is about 35 % and Ca is intermixed with Ag in amorphous state.
Above a certain thickness of Ag layer, transmittance of the Ca/Ag double layer is reduced as the thickness of Ca layer increases. On the other hand, below a certain thickness of Ag layer, transmittance of the Ca/Ag double layer is enhanced as the thickness of Ca layer increases. Among the latter cases, especially when Ag film does not fully cover the Ca layer, high transmittance(average over the 70 % in visible light) and reasonably low sheet resistance(8.5 Ω/□) is acquired. In this case, in the Ag layer, O is detected and Ca is much more than Ag compared to other cases in which Ag film fully covers the Ca layer. And according to XPS, there are $Ca(OH)_2$ and CaO in the Ag layer. Thanks to $Ca(OH)_2$, CaO and the holes in the Ag layer, large aperture is acquired and this leads high transparency. Also not fully covered but continuous Ag film leads good electrical conductance.
The Ca/Ag double layer has been fabricated for the transparent cathode in TEOLED. The structure of this layer is as follows : (1) in upper layer, there are crystalline Ag and the mixture of Ag and Ca and (2) in lower layer, there are crystalline $Ca(OH)_2$ and amorphous Ca and CaO. In case that Ag film does not fully cover the Ca layer, $Ca(OH)_2$ and CaO are also in upper layer. This leads better transmittance same as holes in upper layer and at the same time, reduces sheet resistance. But sheet resistance is low enough for this layer to be applied as the transparent cathode for TEOLED.