Mark J. Childs

Passive and active matrix addressed polymer light-emitting diode displays

Polymer LEDs provide a new alternative to LCDs for many display applications, and are particularly attractive because of their high brightness, near-perfect viewing angle, and very fast response time. In this paper, the basic technology used to form the LED structures, and the performance of these devices is presented. Then, the fabrication and driving of passive addressed matrix displays formed using this technology is discussed. Finally, the necessity for active matrix addressing for larger size and higher resolution displays is demonstrated, and it is shown that this is best achieved using low temperature poly-Si technology. The state-of-the-art poly-Si technology used for active matrix addressed LED displays is described, with particular reference to transistor variation, and the resulting non-uniformities in images on displays. A variety of different addressing techniques, and pixel circuits can be used to drive the LEDs in the active matrix, and the performances of these schemes are compared. These include the basic current source circuit; the modified current source circuit; transistor current mirror circuits; and circuits with optical feedback and correction for uniformity variation. Consideration is given both to analogue and to digital drive methods.

32.1: Invited Paper: A Comparison of Pixel Circuits for Active Matrix Polymer/Organic LED Displays

In this paper measurements on several types of active matrix polymer LED (AMPLED) displays will be presented. The issues ofimage uniformity and polymer aging will be addressed by pixel circuit designs.

Towards large‐area full‐color active‐matrix printed polymer OLED television

Abstract— We have developed a new multi-head polymer OLED ink-jet-printing technology to make large-screen OLED television displays. This printer is used to make a 13-in.-diagonal 16:9-format polymer-OLED prototype driven by an LTPS active matrix with a pixel circuit which compensates for TFT threshold-voltage variations. A novel scrolling-bar addressing scheme is used to reduce motion artifacts and to make sparkling images with a high local peak brightness. The scalability of the polymer-OLED technology to larger sizes for television applications is discussed.

44.4: Distinguished Paper: Towards Large‐Area Full‐Color Active‐Matrix Printed Polymer OLED Television

We have developed a new multi-head Polymer OLED inkjet print technology to make large screen OLED television. This printer is used to make a 13″ diagonal 16:9 format Polymer OLED prototype driven by an LTPS active matrix with a pixel circuit which compensates for TFT threshold voltage variations. A novel scrolling-bar addressing scheme is used to make sparkling images with a high local peak brightness. Color processing is used to improve the overall perception of the image.

Improved optical feedback for OLED differential ageing correction

— To compete with LCDs and to meet standard display product specifications, OLED displays must have a high degree of tolerance to differential ageing or “burn-in.” A new optical feedback pixel circuit is presented that enables accurate differential ageing correction, can have low power consumption, and enables a high degree of non-uniformity correction. The circuit can be implemented in LTPS, and a-Si:H TFT technologies and circuits for both cases are shown. The a-Si:H approach is low cost and enables correction of both TFT threshold voltage drift and OLED degradation at the same time. The circuit analysis, operation, and technology will be described and results presented.

Advanced poly-LED displays

Philips have been actively developing polymer OLED (poly-LED) displays as a future display technology. Their emissive nature leads to a very attractive visual appearance, with wide viewing angle, high brightness and fast response speed. Whilst the first generation of poly-LED displays are likely to be passive-matrix driven, power reduction and resolution increase will lead to the use of active-matrix poly-LED displays. Philips Research have designed, fabricated and characterized five different designs of active-matrix polymer-LED display. Each of the five displays makes use of a distinct pixel programming- or pixel drive-technique, including current programming, threshold voltage measurement and photodiode feedback. It will be shown that hte simplest voltage-programmed current-source pixel suffers from potentially unacceptable brightness non-uniformity, and that advanced pixel circuits can provide a solution to this. Optical-feedback pixel circuits will be discussed, showing that they can be used to improve uniformity and compensate for image burn-in due to polymer-LED material degradation, improving display lifetime. Philips research has also been active in developing technologies required to implement poly-LED displays on flexible substrates, including materials, processing and testing methods. The fabrication of flexible passive-matrix poly-LED displays will be presented, as well as the ongoing work to assess the suitability of processing flexible next-generation poly-LED displays.