First, indium tin oxide (ITO) substrate pretreatment
(1) ITO surface flatness: ITO has been widely used in the manufacture of commercial display panels, which have the advantages of high transmittance, low resistivity and high work function. In general, ITO fabricated by RF sputtering is susceptible to poor process control factors resulting in surface irregularities, which in turn produce tipped materials or protrusions on the surface. In addition, the high temperature calcination and recrystallization process also produces a raised layer of about 10 to 30 nm on the surface. The path formed between the fine particles of the uneven layer provides the opportunity for holes to directly hit the cathode, and these intricate paths increase the leakage current. Generally, there are three methods to solve the influence of the surface layer. U is to increase the thickness of the hole injection layer and the hole transport layer to reduce leakage current. This method is mostly used for OLEDs (~200 nm) with thick PLEDs and hole layers. The second is to reprocess the ITO glass to make the surface smooth. The third is to use other coating methods to make the surface flatter.
(2) Increase of ITO work function: When holes are injected into HIL by ITO, excessive potential difference will cause Xiaoji energy barrier, making holes difficult to inject, so how to reduce the potential difference of ITO / HIL interface becomes ITO The focus of pre-processing. Generally, we use the O2-Plasma method to increase the saturation of oxygen atoms in ITO to increase the work function. The work function of ITO after O2-Plasma treatment can be increased from 4.8eV to 5.2eV, which is very close to the work function of HIL.
When the auxiliary electrode is added, since the OLED is a current driving component, when the external circuit is too long or too thin, a serious voltage gradient will be caused in the external circuit, so that the voltage actually falling on the OLED component is lowered, resulting in a decrease in panel luminous intensity. Since the ITO resistance is too large (10 ohm / square), it is easy to cause unnecessary external power consumption. Adding an auxiliary electrode to lower the voltage gradient becomes a shortcut to increase the luminous efficiency and reduce the driving voltage. Chromium (Cr: Chromium) metal is the material most commonly used as an auxiliary electrode, which has the advantages of good environmental factor stability and greater selectivity to the etching solution. However, its resistance value is 2 ohm / square at 100 nm of the film layer, which is still too large in some applications, so aluminum (Al: Aluminum) metal with a lower resistance value at the same thickness (0.2 ohm / square ) is another preferred option for the auxiliary electrode. However, the high activity of aluminum metal also causes problems in reliability. Therefore, multi-layered auxiliary metals have been proposed, such as Cr / Al / Cr or Mo / Al / Mo, but such processes add complexity and Cost, so the choice of auxiliary electrode materials has become one of the key points in the OLED process.