Posted: January 13, 2014
In many flat panel displays, each pixel is addressed by a voltage applied between two parallel electrodes, and depending on the type of display, one or both of the electrodes must be transparent to visible light. A thin film of tin-doped indium oxide (ITO), a wide-bandgap semiconductor, is most often used to create transparent electrodes.
Liquid Crystal Display (LCD) Applications:
In conventional LCDs, the quality of the image degrades as the viewing angle increases. To correct for viewing angle problems, modern LCDs often incorporate birefringent organic thin films, also known as compensators, to improve contrast and gray-scale stability. While viewing angle is improved, the organic films are susceptible to UV degradation and heat distortion, and must be UV cured, cut, and then laminated onto polarizer sheets, which is an expensive and time consuming process.
University of Alberta researchers have employed a novel deposition technique to create ITO thin films that are comprised of microscopic columns exhibiting large form-birefringence. By using the new birefringent ITO, the function of the compensators can be integrated into the ITO layer, eliminating the need for the organic birefringent films currently used in LCDs. The technology will reduce cost and will offer considerably thinner displays, which is of interest for mobile applications.
When liquid crystals are in contact with the birefringent ITO, their alignment is related to the orientation and nature of the microstructural ITO columns. This may eliminate the need for a separate polymer alignment layer.
Organic Light Emitting Diode (OLED) Display Applications:
There may be an opportunity to combine a birefringent quarter-wave plate (used in circular polarization filters) and a transparent electrode into a single thin-film layer in an OLED to improve contrast, simplify the OLED display design and reduce manufacturing costs. Extremely thin OLED displays could potentially be manufactured.
Liquid Crystal on Silicon (LCoS) Display Applications:
When used in a projection system, LCoS displays are subjected to very high light intensities that have a tendency to degrade traditional organic films. For this reason, wire grid polarizers are used in place of drawn polymers. To improve the contrast ratio, films with a small retardance (typically ~ 20nm) must be added to the polarizers. Since organic retarders degrade, the University’s thin films could provide a stable replacement.
In addition, LCOS polyimide liquid crystal orientation layers are subjected to a photochemical degradation with high intensity light exposure. Since the University’s ITO films also orient LCs and are resistant to photochemical breakdown, degradation may be avoided.
- Combines the characteristics of a transparent electrode and a birefringent coating into a single thin-film.
- May eliminate the need for a separate polymer alignment layer.
- Could be used to generate thinner displays.
- Extends the life of LCDs and OLEDs.
The advantages of the technology could be realized by any company that applies ITO coatings to glass substrates for incorporation into flat panel displays.