Inkjet printing in display applications
For some time, flat panel manufacturers have been introducing new technologies. From the advent of the plasma screen technology to the latest state-of-the-art AM-OLED (Active Matrix Organic Light-Emitting Diode) displays, inkjet technology has contributed to their manufacture.
Liquid crystal displays (LCD)
Standard LCD (liquid crystal display) panels use colour filter technology. A colour filter is printed on the front plane of the flat panel in order to colour the light which is emitted when the TFTs (thin film transistors) allow each pixel to switch from the polarising to non-polarising state. Flat panel manufacturers need to place very small amounts of colour filter material into discrete areas of the front plane (known as pixels), defined by wells or troughs. The ink itself is a coloured polymer material, typically dissolved in an organic solvent. The front plane panels are often printed as large sheets and then cut to size afterwards. Displays are now printed at sizes up to 2.5 m x 2.5 m.
Typical printers for this application use a rotating printhead or bank of printheads such that the nozzle pitch of the print head can be aligned with the pitch of the pixels in the panel. The ink is typically delivered using a positive pressure of nitrogen, using a bag in a rigid bottle which connects the ink to the printer. A needle valve pierces the cap and inserts into the inner bag. A positive pressure of nitrogen is applied to the bottle forcing the contents of the inner ink bag to be driven upwards through the needle into the printer. The largest of the current printers can print at 1 metre per second.
In the last 7 years, several key flat panel manufacturers have developed AM-OLED emissive technology, which are now in production for small displays in hand held devices. This technology uses an active matrix TFT backplane and OLED emissive pixels. Some companies have demonstrated a fully inkjet printed AM-OLED display, where both the TFT layers and the OLED layers have been printed using inkjet technology. Printing the OLED display is a complex engineering challenge. A number of layers of material need to be deposited in pixels which have a typical size of 40 x 180 microns. The size of the pixels themselves is defined by the overall resolution the display will have (how many pixels per unit area of display). Typically, PC monitors have somewhere between 50 and 100 pixels per inch. For example, a 15-inch VGA monitor has a resolution of 640 pixels along a 12-inch horizontal line or about 53 pixels per inch.
Being able to place the right number of drops of the active materials into the pixels is a big enough challenge, let alone developing a process whereby the ink dries to deliver flat films of materials in the pixel. It is essential flat films are formed such that there are no electrical shorts and the panel operates with electrical efficiency. The display panels are made up of red, green and blue emissive materials, which are polymers that are soluble in organic solvents and can be deposited by inkjet printing. The polymers themselves are specifically designed semiconductor materials, which emit light when an electrical current is applied across them. There are two types of AM-OLED emissive devices, which are loosely termed as top emission (where a transparent cathode material is deposited) and bottom emission (where the emitted light is reflected from a non-transparent cathode out of the device). Both device types can have the functional layers applied by inkjet printing with the feature sizes being identical for each.
Some companies have incorporated inkjet printers into a standalone automated line for OLED manufacture, which are available for sale. Recent developments have been the application of OLED technology to lighting applications. Large areas of white emissive polymer materials can be applied by inkjet printing to form uniform lighting panels which can replace the fluorescent tubes in offices and stores.
Inkjet printing technology is already being employed in production of liquid crystal displays, and shows great promise for the product of OLED displays once these are on the market.