The OLED inkjet printing uses solvents to melt OLED organic materials and subsequently prints materials on the surface of substrates to form R (red), G (green), and B (blue) organic light-emitting layers.
Inkjet Printing Polymer Materials
Due to the greater molecular weight of polymers, solution processing was mainly used to form films, such as spin coating or printing, while inkjet printing technology has proven to be the most efficient method for preparing
light-emitting polymer solutions. In 1990, Richard Friend et al. discovered the electroluminescence properties of polymers in Cavendish Laboratory at the University of Cambridge and invented the polymer light-emitting diode (PLED). Since then, PLED display has attracted great attention and demonstrated considerable potential for manufacturing next-generation flat-panel displays.
Inkjet Printing of Small Molecule Materials
At present, the efficiency (6–8cd/A) and the lifetime of polymer light-emitting devices (PLEDs) are generally low. Small-molecule light-emitting devices (SM-OLEDs) possess obvious performance advantages, such as higher efficiency (84cd/A) and a longer lifespan. Although PLEDs possess limitations in their application use, multilayer phosphorescent SM-OLEDs fabricated through the thermal evaporation process can achieve higher efficiency to overcome certain limitations.
In one study, Xia et al. created thin films of these traditional thermally evaporated small-molecule materials by inkjet printing and prepared phosphorescent small-molecule light-emitting devices with better performance. As a result, obtaining high-quality functional thin films remains necessary for fabricating efficient and long-life devices, while available small molecule materials have poor film-forming properties. During the drying process of the liquid film on the substrate, it is easy to form a discontinuous film. In this regard, there are two primary methods to improve the film-forming properties of small molecules. First, the molecular volume can be upregulated by modifying the alkyl chain length to design and synthesize molecules of good solubility and film-forming properties; Second, polymer materials can be added to small molecular materials to enhance film-forming properties.
In addition, modifying the physical and chemical properties of the substrate surface can improve the film-forming properties of materials. Sirringhaus et al. inkjet printed water-soluble materials on the surface of hydrophilic substrates patterned with hydrophobic materials and obtained high-resolution polymer electrodes. Hendriks et al. printed nano-silver ink wires on a hot-pressed engraved substrate surface. Since the contact angle was small, the ink was drawn into the channel by capillary action, especially the fluid properties of small molecule solutions mainly depend on the properties of solvents. Although the effect of solvents on the film-forming properties of small molecules has been extensively studied, they still remain complicated.
Inkjet Printed Cathodes
Similar to the principle of evaporating small molecules, the cathodes of OLED devices are generally produced through the vacuum evaporation process, but the evaporation equipment and masks required are expensive. Using inkjet printing technology to prepare cathodes can greatly reduce cost, but alternative problems such as uniform film formation within a large area remain a major challenge. Based on the preliminary research about the OLED display through the full printing process, the major factors toward development are printable cathode ink and large-area film formation technology. The main difficulty lies in the affinity between the cathode material and the organic functional layer, which is required to form a stable film for the printed cathode. Additionally, the fineness of the printed pattern must be guaranteed to provide a high-resolution image. As a result, the destruction of the bottom layer by the cathode paste must be avoided. The effective injection of carriers must be ensured to guarantee top-grade brightness and high display performance. These requirements should be met to ensure high brightness and high-efficiency display performance.
The OLED functional layer requires a uniform standard for film thickness and should retain optoelectronic properties for easy solvent drying and removal during the film formation process. Similarly, other additives in ink need also to be removed with minimum interference on the performance of the organic semiconductor film. This development of OLED display technology through inkjet printing not only furthers the development of inkjet printers and print heads but also attracts growing attention and research on related topics such as ink formulation, ink/substrate interface contact characteristics, and the drying process.
The solution for the inkjet printing OLED display is mainly composed of optoelectronic materials and solvents. It is necessary to consider the preparation of ink through the various processes related to fluid characteristics, degrees of spreading, and the drying process in film formation.
The printability of ink is primarily determined by the amount of viscosity, surface tension, and shear rate variation, while the molecular structure and molecular weight, solids content, and choice of solvent are the main factors that produce these physical parameters.