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Printed electronics is a silent revolution within the electronics industry that is gaining momentum, particularly in electronic displays. These displays are not only seen as low-cost alternatives to existing display technology, but are also seen as enablers of entirely new display-equipped applications.
Two emerging imaging technologies poised to displace LCD displays are electrophoretic displays, also known as electronic paper displays (EPDs), and organic LEDs (OLEDs). EPD technologies are voltage driven and optically passive, as are LCDs, and OLEDs are current driven and optically emissive in the manner of conventional LEDs.
These new display technologies will be accompanied by changes in display driver-circuit architecture and technology. For example, E Ink's electronic ink technology (Fig. 1) is based on a surface containing fluid-filled microcapsules approximately 50 µm in diameter and situated between upper and lower electrodes. Each capsule contains positively charged white particles and negatively charged black particles that can be moved to opposite sides of the capsule using an electric field created by these electrodes.
This is quite different from the operation of an LCD, in which floating polarized crystals are merely twisted by the electric field. Instead, each microcapsule can be thought of as an optical integrator, allowing each pixel to pass through a continuous range of “grayscale” states while transitioning between fully white and fully black over a time interval of approximately 1 s, according to Holly Gates, senior hardware engineer at E Ink.
In a further departure from LCD technology, lateral electric fields can be applied to each microcapsule, leading to the formation of adjacent clusters of black and white particles on both the upper and lower surfaces within the capsule. This allows subpixel addressing for the fully white and black states.
To support these functions, a split electrode is positioned beneath each microcapsule in addition to a grounded transparent planar electrode situated above all the microcapsules, and the driver must be able to supply dual polarity voltages to each element of the split lower electrodes. The driving waveforms are also more complicated, because each pixel is usually “cleared” with a dc pulse and then driven with the opposite polarity for a specific time to achieve its unique grayscale state for the new image.
The 15 V applied between the electrodes of an EPD is often supplied with a regulated charge pump. In many cases, LCD driver designs can be adapted to support this electrical requirement, Gates stated. However, addressing and controlling each pixel is obviously quite different between the two technologies. Driver ICs for EPDs are available commercially, but usually begin as custom ASICs with the possibility of evolving into standardized commercial products. At present, E Ink and Dialog Semiconductor have partnered to develop commercial driver components for E Ink's electronic ink displays.
While E Ink's technology is well suited for electronic book applications, where low power and high readability are required, OLEDs (Fig. 2) have emerged in cell phones, digital cameras and PDAs where LCDs currently dominate. According to Julie Brown, CTO for Universal Display Corp. (UDC), OLEDs are a strong technology contender in such applications, which are demanding ever-increasing video functionality, lower cost and longer battery life. Because OLEDs are formed of individually addressable pixels that function much like conventional LEDs, have inherently fast switching speeds and excellent color saturation, the technology is well suited for information displays, and OLEDs are being developed by several technology developers such as UDC. UDC's OLED displays are manufactured in Asia.
OLED driver topologies are relatively straightforward. The two configurations for OLED displays are the passive matrix and the active matrix. While a passive matrix is simpler in construction, electrical parasitics within the display sometimes require the driver to supply a prepulse to increase pixel switching speed.
In an active matrix, each pixel is driven by a dedicated thin film transistor (TFT). According to Brown, organic TFTs (OTFTs) that are integrated within the OLED display are still under basic research. Therefore, a separate backplane containing conventional TFTs implemented in silicon is placed behind the active matrix OLED display. The TFT backplane also includes integrated resistors and capacitors implemented in low-temperature polysilicon or amorphous silicon.
Unlike E Ink's electronic ink technology, which is a reflective display technology, OLED displays are transducers, converting electricity to light with high efficiency. Like many output transducers, they are subject to the effects of aging. Brown stated that UDC has explored integrated photodetectors embedded within each pixel that can be used to compensate for aging by providing light-intensity feedback to the driver circuits. This same configuration might also enable a type of optical latch, keeping a pixel energized once it has been triggered. This could simplify the drive electronics for static display applications.
Clare Micronix, now a subsidiary of IXYS, has developed driver components for both electronic ink and OLED display technologies. The MXED302 is designed for 96 × 64 passive matrix RGB-color OLED display panels. This device features tightly matched RGB current drivers as well as grayscale capabilities. The MX834 is a display-driver IC specifically designed for EPDs using E Ink's electronic ink technology.
Many new disposable applications will be enabled with printable display technology, including advanced electronic greeting cards. Printable batteries are well suited for driving printed electronics displays in these applications. Len Allison, vice president of business development at Thin Battery Technologies (TBT), stated that many display applications have modest capacity requirements in the intermediate range of 20 mAh to 50 mAh, and TBT's technology has been used by at least one client in an electrophoretic display application.
Given the rapid progress of printed electronic technology, designers may read about future developments in printed display drivers from a flexible medium other than paper.