William A. Barrow

The impact of the transient response of organic light emitting diodes on the design of active matrix OLED displays

Much of the organic light emitting diode (OLED) characterization published to date addresses the high current regime encountered in the operation of passively addressed displays. Higher efficiency and brightness can be obtained by driving with an active matrix, but the lower instantaneous pixel currents place the OLEDs in a completely different operating mode. Results at these low current levels are presented and their impact on active matrix display design is discussed.

High rate roll to roll atomic layer deposition, and its application to moisture barriers on polymer films

Atomic layer deposition has been shown to provide high quality single layer moisture barrier films on polymer substrates, but conventional pulse-based processes are too slow to be commercially feasible. One way to overcome this speed limitation is to avoid the need to pulse and purge precursors by moving the substrate between zones containing the precursors, passing through intermediate purge zones. Recently, several groups have reported various approaches to accomplishing this, including the approach discussed here in which the flexible web is passed between precursor zones in a serpentine pattern. Al2O3 and TiO2 barrier films 12 to 20 nm thick with water vapor transmission rates in the range of 10−4 g/m2/day have been demonstrated for web speeds in excess of 1 m/s on 100 mm wide polyethylene terephthalate web. Scale-up of this process to 300 mm wide web in a system capable of depositing 10–20 nm of film in a single pass is currently under way. This scale-up effort and the potential for very high volume,...

A Polysilicon Active Matrix Organic Light Emitting Diode Display with Integrated Drivers

The design of an active matrix organic light emitting diode (AMOLED) display using a polysilicon thin film transistor pixel is described. Characteristics of the OLED response in the low current regime are described and their impact on the design of integrated driver circuitry is discussed. Integrated data and select scanners which generate the signals necessary for data capture and pixel calibration are presented.

Active matrix organic light emitting diode pixel design using polysilicon thin film transistors

Organic light emitting diodes (OLEDs) are presently of great interest due to their potential application in high efficiency displays. However, as the number of pixels in a passively addressed display increases, the time available to drive each pixel decreases, the peak pixel brightness increases and the OLED drive current increases. Thin film transistors made with polysilicon can be used to generate a constant current source at each pixel. Each pixel is programmed to provide a constant current throughout the entire frame time, eliminating the high currents encountered in the passive matrix approach. However, polysilicon thin film transistors vary widely in their initial output characteristics due to the nature of the polysilicon crystal growth. A description of some of the important considerations in the design of such an AMOLED pixel and results on improved pixel designs are presented.

Degradation mechanisms in organic light emitting diodes

Experimental and theoretical results are presented on the lifetime of organic light emitting diodes (OLEDs) for active matrix display applications. DC aging tests on the OLEDs show that the driving voltage increases under forward bias and then reverse its trend when the bias polarity is reversed, which reproduce our previous test under AC conditions. Furthermore, the voltage seems to be able to relax slowly toward its initial value when the device bias is reset to zero after a long forward bias stress. The mobile ions are proposed to be the origin of the observed voltage shifts. By solving a system of transient equations governing the mobile ion motion under an external field, we obtained the transient mobile ion distributions and their contribution to the driving voltage. Several cases were studied. We found that the mobile ion model with reasonable assumptions could very well explain the experimental results. Furthermore, by comparison between the data and simulation, the possibility of the initial mobile ion sources can be narrowed.