Flexible and Curved Displays: Out of the Lab and into the Field
August 5, 2014, Emerging Display Technology Conference, San Jose, CA—Paul Semenza from NPD DisplaySearch, Bob Senior from Canatu, and Sri Srinivas from Raydiance discussed the emerging flexible and curved displays. The drivers for non-planar displays are greater flexibility in design and the conflicting demands for smaller and lighter versus larger screens.
Semenza opened with a summary of the basic possible configurations, and noted that all the apps for flexible displays need greatly improved ruggedness. Curved displays can be a mixture of flexible and fixed form factor, or fixed form factor with added stress to make it non-planar. LCD can be on a panes that is bent at the edges. All flat panel display technologies are sheared on the edges of the glass.
LG and Samsung are now producing curved smart phones at the lower range with OLED on flexible substrates. The curved shapes are fixed with a low radius of curvature. The new form factors extends the battle between LCD and LED for displays.
Some issues for curved displays include light leakage and black uniformity. A variable and repeated curve is a flexible display. The ultimate in flexible will be in foldable materials with a bend radius of 0.1 mm. The flexible displays will need to survive thousands of flexes, so robustness and reliability are key metrics for the technologies.
The technology for the manufacturing will need to find impermeable substrates to prevent oxygen and water incursions. The backplanes will need to include the TFTs, capacitors, and connectors on the flexible materials. The front plane materials will also have to handle the encapsulation and the touch interfaces. All of the materials for the substrate, backplane, frontplane, encapsulation, and the display materials will have to be integrated into a manufacturable integrated process. In addition, the need for a controlled gap in the LCD or OLED cells will continue.
The candidate substrate materials are glass, polymers, and metal foils. Glass and polymers are the most likely, since the foils can create short circuits. the physical stress on the devices will change the device performance. Polymers will need barrier coatings and might have some adhesion issues. The organic materials are better suited for flexible displays, but have mobility and uniformity issues. The need for encapsulation makes the metal foils better as barriers, but they are opaque.
The various processes and materials offer may potential alternatives. The thin film materials can be metal oxides, nitrides, carbides, oxy-nitrides, etc. The touch interface can use metal mesh, silver nano-wires, conductive polymers, carbon nano-tumes, or graphene. Any touch material has to meet the requirements for flexibility, transparency, durability, and be better than ITO.
There are already products with curved, bendable, rollable, and foldable displays in demonstration products and on the market. The high-resolution, high performance requirements with good interactivity will enable new form factors for mobile and fixed display devices.
Senior addressed the wearable markets. So far, there have been lots of devices and most made early transfers to a drawer. The existing devices are big and rigid, look like toys, and are definitely not fashionable. The look of being “not cool” and requiring a smart phone for functionality makes this generation of wearable devices candidates for early withdrawal.
Although there have been demonstrations and prototypes of OLED on flexible substrates, no products are on the market with these capabilities. The ITO for the touch interface is too brittle for a rollable display, and daylight readability is poor.
Despite all the shortcomings, the market is slated to grow soon. One key factor is to enable touch controls on 3-D shapes. Direct dry printing with carbon nano-bulbs bonded as a lamination film is a fusion of nanotubes and buckyballs, and provides a conductive material that is capable of hundreds of thousands of 180° flexes. This technology enables touch on any surface with very low haze and reflectance due to the black color of the carbon materials.
Srinivas proposed changes in tooling to address the cover glass manufacturing. Laser processing allows for non-contact cutting and shaping for materials like cover glass. The laser can produce 2.5-D chamfers and tight radius cutouts. The laser cutter enables the use of thinner materials, since the glass doesn’t have to support the pressure of contact cutting tools.
The laser machining is also capable of working on sapphire which is harder than glass, and still leaves clean edges. A laser tool can create holes, notches, grooves, and polished edges with no post processing except for a wash step. These combined capabilities can reduce manufacturing costs by reducing the number of steps and eliminating caustic chemicals.
