Plasma cleaning before ITO glass coating for OLED devices

OLED has undergone updates and iterations, and currently the multi-layer OLED devices mainly used in production are composed of five parts as shown in Figure 1, namely: cathode, anode, electron transport layer (ETL), hole transport layer (HTL), and organic light-emitting layer (EL). This multi-layer device structure has significantly better flexibility than single-layer and double-layer devices.

Figure 1 Schematic diagram of OLED device structure

The anode material of OLED devices is usually selected from materials with higher work functions, such as Indium Tin Oxide (ITO). The main function of ITO is to transmit current to excite organic luminescent materials, thereby generating brightness. The unique properties of this material allow OLED displays to be thinner and lighter than traditional liquid crystal displays (LCDs), providing higher flexibility, which is particularly important for bendable or foldable display devices.

Generally speaking, the OLED preparation process mainly includes substrate cleaning, plasma treatment, film formation, and packaging processes, as shown in Figure 2.

Figure 2 OLED Preparation Process

A flat and uniform thin film is a prerequisite for achieving high luminous efficiency and long service life of OLED devices, but small molecule materials usually have poor film-forming properties and are prone to forming discontinuous thin films during the solvent removal process after film formation. In order to improve the film-forming properties of small molecule materials and alter the surface properties of the coating substrate, it is one of the better film-forming methods.

Plasma cleaning of ITO coated glass substrates

Before coating the glass substrate, its surface needs to be cleaned. If the glass surface is contaminated, its surface free energy will decrease, which will cause the hole transport layer deposited on it to aggregate and result in uneven film formation.

Plasma cleaning is the process of using a power source to excite plasma to bombard the surface of a substrate. During the bombardment process, physical impact can be used to remove micro particles adsorbed on the substrate surface, which may also cause micro etching on the substrate surface; Therefore, it can be used for ultra precision cleaning of substrates. When plasma bombards pollutants on the surface of the substrate, it can also react with the pollutants to form volatile substances, which can be effectively removed by vacuum pumping to remove residual organic matter on the substrate surface. The biggest feature of plasma cleaning is that most types of substrates can be processed.

Plasma cleaning of ITO coated glass substrates can introduce polar hydroxyl (- OH) groups on the surface to improve wettability, making the ITO surface more compatible with organic layers and significantly enhancing the performance and lifespan of OLEDs.

After oxygen plasma cleaning, ITO will accumulate a layer of negatively charged oxygen on the surface, forming an interface dipole layer. With the increase of oxygen, the work function of ITO will also be improved. This greatly improves hole injection, endowing OLED devices with lower turn-on voltage and higher brightness and luminous efficiency.

As an inherent advantage, plasma cleaning can effectively remove nanoscale pollutants on the surface of ITO, ensuring a clean interface between the ITO electrode and the organic layer, thereby reducing device performance degradation and extending device lifespan.

Comparison of ITO glass before and after plasma cleaning

Expose the ITO glass substrate to plasma and clean for 1 minute. The decrease in the angle between water droplets and the surface indicates that plasma treatment significantly improves the wettability of ITO samples.

In summary, plasma cleaning of ITO glass surface can not only improve the work function of ITO, but also enhance the cleanliness and wettability of ITO surface. The increase in hydrophilicity is more conducive to the formation of hydrophilic organic active layer on its surface