Flexible Displays Require Flexible Electronics

Flexible Displays Require Flexible Electronics

Display Week 2016 provided numerous examples of advancements in flexible-display technologies.  But even though flexible displays are now in production, they are used in fixed formats encased in rigid packaging, so users have not experienced the actual physical flexibility.  In order for truly flexible displays to emerge, flexibility of the electronics is required, beyond the backplane and display driver electronics.  Clues to such developments could be found in many presentations at the annual event.

by Paul Semenza

WITH continued progress in the production of active-matrix backplanes on flexible substrates and the packaging (in particular, thin-film encapsulation) of flexible displays, the era of volume manufacturing of large full-color flexible flat-panel displays has finally arrived.  OLED displays are in the lead, and there is continued development of LCDs and EPDs, among others.  However, for the most part, such “flexible” displays have been used in rigid formats, such as in smart watches (which take advantage of the fact that flexible displays can be thin and light) and smartphones (which take advantage of curved or angled displays).  These form factors do not allow the display to be bent, folded, or stretched in any way.

In part, such rigidity reflects the need to utilize plastic and metal casings, as well as strengthened cover glass, to protect the flexible displays from damage.  In addition, a freely flexible display can in theory be bent, rolled, or folded an unknown number of times and in unpredictable ways, making it difficult to characterize failure mechanisms or predict mean time to failure.  However, with volume production, it is likely that the ruggedness and lifetime of flexible displays will improve, reducing the need for such protection.

Another reason that products with “flexible” displays are not able to take full advantage of their flexibility is that such displays are packaged with electronic systems that use metals, hard plastics, and rigid ceramics in component packaging, circuit boards, and connectors.  Thus, even though it may be possible to produce fully flexible display modules, these modules would need to be physically and electrically coupled to a rigid electronics package.  Early concept demonstrations of flexible displays in the mid-2000s, including Universal Display’s “roll-out” OLED display, in which the display rolled out of a tube that housed at least some of the electronics, and Polymer Vision’s fold-up reading device, in which a flexible EPD unfolded from a rigid housing, reflected this reality.  While neither of these concepts made it into production, similar configurations would be needed to house the circuitry were a product with a fully flexible display to come to market today.

Breaking Out of the Rigid Box

In order to unleash the full potential of flexible-display technology, electronics will also need to become flexible.  This could potentially include logic, memory, power, sensing, and communications functions, as well as circuit boards and batteries.  Broadly speaking, there are two ways to accomplish this – either replace devices that are currently made in bulk form (semiconductors and passive components) with devices manufactured through printing or other additive manufacturing processes onto flexible substrates or integrate thinned chips or other bulk devices with printed devices on flexible substrates.

Printing or other solution-processing deposition techniques are critical for flexible electronics, due to their ability to deposit electronic materials at low temperatures over large areas of flexible substrates such as plastic and polymer.  In the near term, replacing large-scale logic, memory, and processor semiconductors with printed versions is not feasible because printing techniques are not capable of depositing millions of transistors in an integrated device.  Thus, a combination of printed and semiconductor devices – referred to as flexible hybrid electronics – is the most likely path.  While such systems are not yet in production, presentations at Display Week 2016 indicated a breadth of research around creating flexible electronic devices using a variety of deposition techniques and materials.

State of the Art

Much of the effort on flexible electronics has been motivated by the desire to create active-matrix backplanes for flexible displays, particularly OLED displays and EPDs, as well as for thin-film PV and sensor arrays, using printing, coating, or other deposition methods for solution-based semiconductor materials.  Historically, the focus has been on organic semiconductor materials because they are suitable for low-temperature deposition and perform well under bending conditions.

As demonstrated by papers presented at Display Week, materials suppliers continue to develop organic semiconductors suitable for TFT-backplane fabrication through spin coating.  Merck reported on materials suitable for integration of spin-coating and photo-lithography in a paper titled “Photolithographic Integration of High-Performance Polymer TFTs” and has been working with flexible-electronics company FlexEnable to demonstrate full-color flexible LCDs, while BASF presented on the direct patterning of organic transistors through spin coating, but without the use of photoresist.  NHK described the fabrication of organic TFTs using coating and self-assembly, including the use of TFTs on thin paper substrates.  As an indication of additional applications for organic TFTs, researchers at the University of Tokyo and JST/ERATO reported on their use as flexible sensors in wearable and human-monitoring applications, in a variety of digital and analog circuit designs.

The application of alternative deposition techniques in conjunction with developments in soluble organic and inorganic semiconductor materials has expanded the potential production space for flexible electronics.  At Display Week, UC Berkeley presented results of work using gravure printing to create organic and transparent oxide-semiconductor TFTs as well as MEMS devices (Fig. 1), and NHK discussed using sputtering to deposit oxide transistors using tungsten and tin instead of gallium on a cured polyimide substrate.

Fig. 1:  In this conceptual view of gravure printing, ink is dispensed to fill wells in the gravure cylinder.  Excess ink is wiped using a doctor blade.  Ink is transferred to a passing substrate; the ink subsequently spreads and dries.  Source: UC Berkeley; 2016 SID Digest of Technical Papers.

In addition to organic and oxide semiconductors, single-walled carbon nanotubes (SWCNTs) have also shown promise for flexible-display applications.  Researchers from Seoul National University presented progress in ink-jet printing of SWCNT TFTs on polyester substrates with good performance under conditions of bending and illumination.

Flexibility Beyond the Display

Presentations at Display Week also included developments in flexible and stretchable electrodes, as well as integration of printed and bulk devices.  Work done at PARC with UC San Diego demonstrates the capability to combine silicon ICs (microcontroller and NFC) with printed components (photo and temperature sensors) with extrusion-printed interconnects.  Meanwhile, advances in the development of stretchable electrodes, which can connect to rigid devices, continues.  IMEC, Ghent University, Holst/TNO, and Panasonic reported in a joint paper on combining stretchable metallic electrodes with rigid LEDs (Fig. 2), and X-Celeprint, spun off from work done at the University of Illinois, presented a seminar on transferring micro-LEDs to flexible substrates using an elastometric stamp.

Fig. 2:  This conformable 64 × 45 RGB LED matrix is integrated into the sleeve of a t-shirt that is mounted on a mannequin.  Source: IMEC, Ghent University, Holst/TNO, and Panasonic; 2016 SID Digest of Technical Papers.

Other research points to using some of these techniques to create new electronic systems.  Corning and ITRI reported using gravure printing to fabricate metal-mesh grids on thin glass for antennas.  Finally, researchers at the Swiss research lab EPFL showed the use of stretchable metallization to create electronic skin.

Getting to Flexible

The momentum in flexible-display technology suggests that there will be growing production of such displays, if for no other reason than that they offer thin, light, and rugged form factors.  However, in order to realize the full potential of the flexible display – the ability to bend, roll, stretch, and, in general, conform to a wide variety of use cases, there is a need for flexibility in the associated electronics, whether it be in a wearable device, a smartphone, or a variety of devices envisaged as part of the Internet of Things.  It is likely that we will see increasing adoption of flexible electronics close to the display – driver electronics, communications, and graphics – and also in sensors designed for human or industrial monitoring.  Fully flexible electronics systems allowing the flexible display to escape the rigid packaging of the present will likely require hybrid approaches combining high-performance silicon devices with flexible displays and other components.  •

Paul Semenza (psemenza@gmail.com) is Director of Commercialization at the NextFlex Flexible Hybrid Electronics Manufacturing Institute.  From 1997 to 2014, he managed market research and consulting for DisplaySearch, Solarbuzz, iSuppli, and Stanford Resources.