OLED technology refers to the phenomenon that organic semiconductor materials and luminescent materials emit light through carrier injection and recombination under the drive of the electric field. As a solid-state self-luminous technology, OLED does not need the combination of LCD and LED backlight source, so the thickness of the TV can be made very thin, the viewing angle is wider, the power consumption is lower, the color is richer, and it can be produced on different flexible substrate materials such as plastic and resin to achieve soft screen, so it is more and more popular with consumers.
In the manufacturing process of OLED, “evaporation” is the essence and difficulty. It refers to a coating technology that uses a resistor or electron beam to heat the material to be evaporated in a high vacuum state and uses a crucible in a vacuum cavity to heat the OLED material to evaporate the atoms and reach and adhere to the surface of the substrate.
In OLED, in addition to the luminescent material, even the metal electrode is evaporated in this way, so the quality of the crucible directly affects the yield of the OLED process. If an inappropriate crucible is selected, the material in the crucible will not be completely evaporated, the boiling point required by the equipment will not be reached, and it will not be able to adhere to the surface of the substrate. It will not only waste time and manpower, but also cause the evaporation material to be unable to be effectively applied, the process yield will deteriorate, and the cost burden of raw materials will be increased. Therefore, choosing a good crucible is a good start.
Material selection of crucibles for OLED evaporation
Commonly used supports are: W, Mo, Ta, high-temperature resistant metal oxides, ceramics or graphite crucibles, etc., but it is also necessary to pay attention to the possible reaction between the support material and the evaporated material. At present, OLED evaporation can use three types of crucibles: tantalum, graphite, and PBN. The following is a description of the PBN crucible:
Pyrolytic boron nitride (PBN)
Pyrolytic boron nitride (PBN) is an advanced ceramic material of the hexagonal system, ivory white, non-toxic, with a purity of up to 99.999%, acid and alkali resistance, oxidation resistance, good thermal conductivity, good surface density, high-temperature resistance, no pores, and easy processing. It is made by chemical vapor deposition under high temperature and high vacuum conditions, using boron-containing gas (BCl3 or B2H6) as raw material. However, because B2H6 is highly toxic, BCl3 is currently used as raw material. The boron-containing gas undergoes pyrolysis (1500~1800℃) and reacts with NH3 in a high-temperature reaction chamber to form solid boron nitride.
The growth process of PBN material is similar to “falling snow”, that is, the hexagonal BN snowflakes grown in the reaction are continuously piled on the heated graphite matrix (core mold). As time goes by, the accumulation layer thickens, forming a PBN shell. The demolding and removal is an independent, pure PBN component, and the remaining part is the PBN coating. The same method can also be used to prepare PBN sheets.
The high purity of the PBN crucible is because no sintering agent is added during its preparation process. Therefore, the operating temperature under vacuum is as high as 1800 degrees, and the operating temperature under atmosphere protection can reach up to 2100℃ (usually nitrogen or argon). It is currently used mostly in evaporation/molecular beam epitaxy (MBE)/GaAs crystal growth and other applications. However, due to the slow deposition speed, the price of PBN crucibles is quite expensive, and most of them are small-sized crucibles.
How to choose?
Of course, it is necessary to start from the actual situation and combine the production process requirements to make the selection. For example, different evaporation materials have different crucibles that can be matched, because not every crucible can be matched to the boiling point required by the material, so it must be tailored according to the material characteristics to achieve the purpose of fully evaporating the material. In addition, the density and thermal conductivity of the crucible material are also determined according to the actual production situation. Therefore, the crucible for OLED evaporation generally needs to be customized to achieve good results.