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OLED (Organic Light Emitting Diodes) are light emitting systems, made of an organic thin film between two conductors - and the organic compound emits light when electrical current is applied. OLEDs are used to make display and lighting panels. The main components of an OLED display are the OLED emitter sandwiched between the electrodes, the backplane, and the encapsulation layer. The use of an encapsulation layer is fundamental for the device protection and to ensure the required lifetime, since OLED materials show sensitivity to oxygen and moisture.
In the presence of water, the device degradation occurs via formation and evolution of black spots on the light emitting surface and progressive pixel shrinkage. This is due to chemical interactions with the oxidizing agent, which progressively limit the emission area and, hence, the light output of the OLED. These degradation phenomena can also lead to delamination of the electrode film with generation of further defects, i.e., more entry ports for moisture and mechanical failure.
The presence of oxygen also gives rise to the formation of black spots, even though the degradation kinetics is slower than with water by more than three orders of magnitude. Oxygen can be responsible of the thermal oxidation of the metallic cathode and of the oxidation of the organic material when a bias is applied.
The encapsulation layer in OLEDs has the role to reduce the ingress of water and oxygen from environmental atmosphere. An effective protection of the OLED device requires upper limits for Water Vapour Transmission Rate (WVTR) in the order of 10-6 g/m2/day. Similarly, to reach a minimum lifetime of 10,000 hours, the oxygen transmission rate (OTR) should be in the range of 10-5 to 10-3 cm3/m2/d. These requirements are about two orders in magnitude more stringent than the capabilities of conventional barrier films.
In its standard configuration, the encapsulation for glass-based OLED displays is obtained by sealing the display in inert atmosphere by means of a sealant (usually an epoxy-based adhesive, with good barrier properties) in combination with a getter for water (that can capture the gas molecules crossing the sealant layer in the long run and penetrating from the leaks inherent to the device construction). The getter function can also be performed by specific active fillers and/or active sealants and serves the purpose of enhancing the barrier properties.
From saes, an advanced functional materials group
September 23, 2022
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