The OLED layer of an OLED panel is sandwiched between an anode and a cathode, which emits light by passing current through the OLED layer. In general, the anode uses a transparent electrode, the cathode uses a metal electrode, and light is emitted from the anode surface. Although there have been precedents for trial production of transparent organic EL panels, most of the cathodes use transparent electrodes.
There are two problems with past practice. One is that it is difficult to form transparent electrodes such as oxides on the OLED layer, and mass production technology cannot be established. The other is that the light emitted from the OLED layer is emitted not only from the anode side but also from the cathode side, so the display content can be seen not only from the front side of the panel but also from the back side. Taking the use of the home screen of a mobile phone as an example, the user "does not want to see the screen display content from the back".
Therefore, TDK has developed new construction panels that avoid the above problems. First of all, in order to use the existing production technology to achieve mass production as soon as possible, the cathode directly uses a metal electrode. On this basis, in order to be able to see the back, instead of using the cathode to cover all the pixels, the line width of the cathode is thinned and only a part of the pixel is covered. With the part not covered by the cathode, the transparent function that can see the rear is realized. TDK named this panel structure "fine-wire electrode structure". Of the pixels of the organic EL layer, only the portion covered by the cathode emits light. However, because the cathode is an opaque metal electrode, the light emitted by the OLED layer exits only from the anode side. Therefore, it is difficult to see the display content from the back of the display.
In order to put it into practical use as soon as possible, TDK has adopted a color display method that combines white OLED and color filters instead of blue materials with a short service life. In general, the use of color filters reduces the transmittance of the panel. However, since the color filter also adopts a structure in which the line width is narrowed and covers only part of the pixels like the cathode, the influence of the decrease in the transmittance of the panel is small. The brightness of this transparent OLED panel is 150cd/m2 and the transmittance is 40%.
Currently, TDK is producing such transparent OLED panels at "TDK Micro Device" in Kita Ibaraki, Ibaraki Prefecture. There are two production lines. Although it was forced to suspend production after the Great East Japan Earthquake on March 11, the two production lines have resumed operations on April 8 and May 9, respectively.