Stretchable Displays: From Concept Toward Reality

Stretchable displays are now being developed using strategies involving intrinsically stretchable devices, wrinkled ultra-thin devices, and hybrid-type devices.

by Yongtaek Hong, Byeongmoon Lee, Eunho Oh, and Junghwan Byun

In the future, we may be able to remove a small display from our pocket and stretch it to create a larger screen, like something that would happen in a Hollywood movie. It seems exciting! If we want to do this with state-of-the-art, high-information (a few hundred ppi resolution) displays today and in the near future, we may find it very difficult. However, if we start with a low-resolution (maybe around 100 ppi) display and add a small percentage of stretching functionality, we might have more freedom in terms of form factor and various creative applications. Such a display could be attached conformably to any artistically designed, curved surfaces, even those with different curvatures in several directions, such as the inner or outer surfaces of vehicles, furniture, utensils, houses, and buildings.

In fact, these kinds of advanced-form-factor information displays have already been implemented through flexible display technology, going back more than 40 years. In 1974, Xerox Palo Alto Research Center (PARC) demonstrated the first flexible e-paper display, which could be flexed like paper while displaying digital images.¹ In 2005, nearly 30 years after that, Arizona State University opened the Flexible Display Center for development of flexible e-paper, and with support from HP, developed a prototype for an advanced flexible e-paper display in 2010. A few years later, this research then entered a new era with the development of flexible OLEDs. Nokia announced a concept mobile phone that used a flexible OLED display in 2008² and Sony demonstrated a 4.1-in. rollable OLED display in 2010.³ In recent years, many companies have demonstrated paper-thin, flexible OLED displays.^3,4 Notably, Samsung Display Company and LG Display introduced curved-edge smartphones and curved televisions to the market^5,6 respectively, both of which are based on flexible OLED display technology.

Deformable and stretchable displays are the next-generation technology step beyond the above-mentioned curved, bendable, and foldable displays. During Display Week 2017, Samsung demonstrated a 9.1-in. stretchable active-matrix OLED (AMOLED) display, which won a Best in Show award from the Society for Information Display.⁷ It is known that this display uses the so-called Kirigami-based stretchable technology,⁸ which involves cutting or making holes in a thin sheet in order to make it stretchable. The ensuing net or mesh-type structure enhances stretching functionality, as shown in Fig. 1. Figure 2 is an example of concave and convex displays based on this type of stretchable technology.⁷

In order to further improve stretching functionality and fabrication processes for higher-resolution displays, a few recently reported strategies for stretchable displays can potentially be adopted. These include making devices that are intrinsically stretchable, creating an entire system that is extremely thin and “wrinkled,” and engineering strain distribution for rigid-soft hybrid integration. Details of these strategies are described in the next section by focusing on stretchable light-emitting devices.

Fig. 1:  The stress distribution for Kirigami structures is determined here by finite element method (FEM) analysis.⁸

Fig. 2:  These photographs of a fabricated AMOLED prototype include (a) an un-deformed shape, and (b) and (c) convex and concave shapes produced through thermoforming processes.⁷

Technical Details

Intrinsically stretchable thin-film devices typically involve new conductor, insulator, and semiconductor materials or mechanically robust novel structures, which help provide stable operation for the fabricated devices, even under harsh tensile stress conditions. Stretchable electrodes have been implemented via wrinkled metal thin films,⁹ horseshoe-shaped silicon ribbons,¹⁰ and liquid-metal channels.¹¹ Facile fabrication of fine feature patterns and integration into high-resolution array formats remain among the technical challenges, while such electrodes show excellent conductive property and mechanical robustness when implemented in relatively large feature size. For stretchable thin-film transistors (TFTs), mechanically durable materials such as semiconductor single-walled carbon nanotubes (SWCNTs)¹² and organic semiconductors¹³ have been widely exploited for active layers with ion gel¹⁴ or elastomeric dielectric materials.¹⁵

Although whole devices based on the above technologies can be stretchable, their performance is typically lower than that of TFTs employed in current commercial products and their fabrication process is not yet compatible with typical display fabrication processes. However, there have been tremendous efforts toward overcoming such technical challenges, and fundamental frameworks to implement stretchable display systems have begun to be established. The bulk of these methodological efforts can be described in detail on the basis of three approaches: (1) intrinsically stretchable light-emitting devices, (2) wrinkling, ultra-thin, light-emitting devices, (3) hybrid stretchable devices integrated with stretchable electrodes.

Intrinsically Stretchable Light-Emitting Devices

One of the key technologies of stretchable, light-emitting devices is the stretchable transparent electrode, which has been typically implemented by composite-based or organic transparent conductors.^16,17 The lower conductivity of those conductors has made the performance of light-emitting devices poor in comparison to those with conventional indium tin oxide (ITO) electrodes. Their operational stability is inferior, although they show excellent mechanical robustness. Furthermore, technical challenges in the fine-feature patterning of such transparent electrodes has limited their application to unit devices with polymer^16,18 light-emitting diodes (PLEDs). In addition, multi layer based light-emitting devices may not be a good candidate due to their mechanically fragile or vulnerable interfaces with the stretchable elastomer substrate. Therefore, most recently, easily deformable gel-based electrodes and an elastomer-compatible phosphor – especially zinc sulfide (ZnS) – have been widely used to fabricate highly stretchable electroluminescent (EL) devices.

In 2016, Professor Zhigang Suo and co-workers at Harvard University demonstrated extremely stretchable light-emitting devices composed of transparent ionic conductors and ZnS phosphor particles.¹⁹ They sandwiched phosphor particles between two layers of dielectric and ionic conductors, both of which were transparent and stretchable. The inserted phosphor particles did not compromise the stretching function of either layer. These intrinsically stretchable light-emitting devices were able to withstand a tensile strain of 1,500 percent without losing their light-emitting ability [Fig. 3(a)]. In addition, Professor Robert Shepherd and co-workers at Cornell University reported highly stretchable electroluminescent devices made with elastomer and ZnS composites.²⁰ These can be stretched up to nearly 400 percent. The Cornell team demonstrated multi-pixel electroluminescent displays fabricated via replica molding [Fig. 3(b)].

Despite these excellent achievements, there are still considerable technical issues to be addressed in terms of the practical implementation of intrinsically stretchable light-emitting devices. Because most of the intrinsically stretchable electronic components are based on the composite materials of functional filler and elastomeric matrices, they suffer from low performance and high-bias voltage. In addition, this approach has difficulty in patterning the light-emitting devices into a small pixel and integrating with the backplane for stretchable high-resolution display applications.

Fig. 3:  Electroluminescent devices with ionic gel conductors and ZnS phosphors depicted here include (a) extremely stretchable light-emitting devices composed of transparent ionic conductors and ZnS phosphor particles.¹⁹ and (b) highly stretchable electroluminescent devices made with elastomer and ZnS composites.²⁰

Ultra-Thin Light-Emitting Devices

As mentioned in the previous section, it might be difficult to fabricate OLEDs directly onto the surface of elastomeric materials, which typically have low surface energy and thus, poor adhesion property. However, if a device or system with multi layer structures is fabricated on a more favorable surface of significantly reduced thickness, it can then be transferred onto an elastomeric substrate. Buckling features can then be added to the whole system. Ultra-thin devices typically show mechanically peculiar characteristics compared to bulky ones. In general, inorganic layers in multi layer devices are vulnerable to bending in a small curvature due to brittleness or large Young’s modulus. On the other hand, if the whole device can be made into an ultra-thin foil with a few micrometers, the applicable bending radius can be reduced to even 10 um, allowing it to conformably contact any arbitrary-shaped surfaces.

In 2016, Professor Someya’s group at the University of Tokyo demonstrated ultra-thin and highly efficient PLEDs on a 1-um-thick parylene substrate (Fig. 4).²¹ Owing to a reduction in the total thickness to 3 um and positioning active layers on the neutral strain plane, multi layered PLED and even brittle ITO transparent conductors can withstand harsh bending conditions (bending radius ~100 um). Also, the existence of a flexible passivation layer with a multi layer structure of silicon oxynitride (SiON) and Parylene (a chemical vapor deposited poly(p-xylylene) polymer), greatly increased the half lifetime of the device from 2 to 29 hours. Interestingly, such ultra-thin foil-based devices can easily become stretchable when laminated onto a pre-stretched elastomeric substrate, such as an acrylic tape-silicone rubber sheet.

After release, wrinkled structures were formed on the laminated ultra-thin devices due to the modulus difference of each layer. These wrinkled structures act as a mechanical buffer, allowing the laminated device to be stretched with minimal change in performance when a large strain is applied to the substrate. The fabricated stretchable lighting device showed no degradation under repetitive 60% stretching (1,000 cycles). Although this ultra-thin foil strategy also shows excellent performance, there are still technical challenges for high-resolution, stretchable active-matrix display applications. This is due to the thickness of the entire display layer – including light-emitting device, TFT backplane, and passivation/encapsulation layer – which must be reduced to a few micrometers, while the same operation property and lifetime must be guaranteed even under stretching stress conditions.

Fig. 4:  An ultrathin PLED device is outlined at top, with its stretchable application shown at lower left and results at right.²¹

Stretchable Hybrid Display Based on Inorganic LEDs

As an alternative strategy, many research groups have demonstrated stretchable hybrid displays, in which the conventional inorganic LEDs (iLEDs) are interconnected by stretchable electrodes.²² Since iLEDs in die or packaged form are mass-produced by conventional semiconductor processes, if an appropriate integration method is developed for making high-resolution displays, this approach can be one of the key technologies for stretchable display implementation in a facile manner. The intrinsic rigidity of iLEDs, however, often causes severe failure – mainly at the interface between rigid chips and soft materials – due to modulus mismatch under tensile strain conditions. Therefore, in order to utilize this hybrid strategy, it is necessary to selectively manage stress distribution over the whole system.

In 2010, Professor John A. Rogers and co-workers demonstrated a micro-inorganic light-emitting diode (μ-iLED) and photodetector array with mechanically designed layouts [Fig. 5(a)].²² μ-iLEDs and interconnects were fabricated on the polyimide (PI) ribbons by using conventional patterning methods such as etching and mask-assisted deposition processes. The μ-iLEDs with the interconnects were then transfer-printed onto a pre-stretched elastomer substrate. Since the iLEDs were firmly adhered to the substrate, they acted as well-defined islands, which were connected by the serpentine interconnects that were slightly floated in an out-of-plane direction. Such a structure allowed the researchers to demonstrate an μ-iLED array composed of 6 × 6 iLEDs with a stretchable function of nearly 50%.

Due to facile and scalable properties, the integration of iLEDs into a stretchable array format has attracted commercial interest, mainly in low-resolution dot-matrix displays. In 2015, researchers at Holst Centre and CMST, associated with IMEC, developed a stretchable 45 × 80 RGB iLED display with meander interconnects and rigid island strategy [Fig. 5(b)].²³ Thirty-six-hundred RGB iLEDs were placed at strain-relief rigid islands, which provided mechanically stable protection for the contacts between chips and interconnects. In addition, having the meander interconnects embedded in a polyurethane film allows this small pixel-pitch (3 mm) display to show excellent performance, with a stretchable function of up to 10%.

Fig. 5:  These examples of stretchable hybrid display technology using iLEDs include (a) a micro-inorganic light-emitting diode (μ-iLED) and photodetector array with mechanically optimized layout.²² and (b) a stretchable 45 × 80 RGB iLED display with meander interconnects.²³

Since the number of meandering or serpentine structures is directly related to the stretchability of the interconnects, such lateral stretchable structures can limit potential increase in display resolution. If buckling electrodes can be combined with the island and interconnect strategy in a facile manner, display resolution can be further improved. In addition, it is important to develop a fabrication process to obtain the patterned buckling electrodes directly on the stretchable substrate. In 2017, Professor Hong’s group reported a fully printable, strain-engineered soft platform for customizable wearable systems including passive-matrix iLED displays.²⁴

Figure 6 shows the key concept involving the strain-engineered stretchable platform implemented by embedding inkjet-printed rigid islands (PRIs) into the soft matrix and inkjet-printing stretchable interconnects between the islands. Since the printing processes can be used for PRI formation, universal or customized PRI arrays can be easily implemented. On a rigid glass substrate, a sacrificial layer and a thin PDMS layer are spin-coated. After curing both layers, PRIs are inkjet-printed in various forms. After relatively thick PDMS layers are coated and cured, the PRI-embedded PDMS layer is detached from the glass substrate.

Figure 6(a) shows a comparison between typical and stretchable universal PCBs. They are conceptually similar, in that, for a given universal PRI array, we can freely choose the target PRIs for chip placement according to to-be-implemented electronic systems. In addition, we can pre-design the PRI array for chip placement instead of using the universal format, as shown in Fig. 6(b). In order to pro-vide more freedom on routing topology, we also developed a program that can produce image files for various interconnects according to the user’s intention. The image files are uploaded onto the printer, so that the inkjet-printer can draw the interconnects between the selected PRIs.

Figure 6(b) shows examples of various topologies for the interconnects of a given PRI array. Since the printing process is performed on the pre-strained PRI-embedded PDMS substrate, two-dimensional wrinkles are formed after curing the printed Ag electrode, followed by releasing the applied pre-strain, as shown in Fig. 6(b). A magnified image of the Ag electrode with two-dimensional wrinkles is shown in Fig. 7(a). Figure 7 shows a stretchable hybrid 5 × 7 iLED array display implemented on a PRI-embedded platform. Since iLEDs and their bonding areas are located on each PRI, any applied external strain is effectively absorbed by the wrinkled electrodes. The tensile strain on the rigid island areas is kept low enough to protect the mounted iLEDs, even under a large biaxial tensile strain. Moreover, stable bonding between the iLEDs and the inkjet-printed stretchable electrodes is achieved by using pure and conductive epoxy materials. Due to optimized rigid island configuration and robust chip bonding, this stretchable passive matrix display showed great reliability under a biaxial tensile strain of 30% [Fig. 7(b)] and a great working stability in harsh environments [Fig. 7(c). All the processes from the inkjet printing of rigid islands to the bonding of iLEDs are sequentially performed in situ. It is also noted that in such an array form, it is necessary to fabricate crossovers at the cross-section area of vertical and horizontal interconnects. As shown in Fig. 7(a), the printed PDMS crossovers performed well and showed stable operations under various tensile stress conditions. Finally, the programmed microcontroller units that are directly mounted on the elastomeric substrate enabled the display to operate in a stand-alone mode without any extra wires for the data communications.

Although stretchable hybrid displays have several advantages, such as relatively high in-air stability, potentially low-cost process, and facile scalable integration, they have their limits in terms of realizing high-resolution information displays at this point. However, if the micro iLED transfer process is combined with the substrate engineering and correspondingly processed TFT backplane, stretchable hybrid displays should have a promising future for various high-resolution applications.

Fig. 6:  The concept of stretchable universal/customized PCB platforms and various inkjet-printed routing topologies is represented in (a), by a comparison between typical and stretchable universal PCB concepts, and in (b), by two examples of inkjet-printed routing for a given customized PRI array. Custom-developed automatic routing programs enable change and optimization of the interconnect patterns.

Fig. 7:  Stretchable hybrid iLED displays implemented on PRI-embedded skin-like elastomeric platforms are depicted and described as follows: (a) optical images of a stretchable passive-matrix display and its enabling technologies; (b) operating input signals of an iLED before and after deformation under 30% biaxial strain; and (c) photographs of the stretchable passive-matrix display operated under various conditions.²⁴

Challenges and Future Directions

Beginning with flexible e-paper displays, deformable displays have undergone numerous innovations and evolved into various forms, from curved to stretchable. Stretchable display prototypes have been developed using strategies of intrinsically stretchable devices, wrinkled ultra-thin devices, and hybrid-type devices. Although only low-resolution displays have been demonstrated up to now, there is potential in implementing high-resolution ones when the key enabling technologies developed for the stretchable displays are appropriately combined.

For example, in order to obtain operational stability and image quality with stretchable functionality as well, the active pixel area needs to be free from deformation under the external tensile stress. In fact, the proper strain engineering strategy can be adopted by either thinning and wrinkling or controlling stress distribution. However, based on our previous research,²⁴ there is still some amount of strain (< 2%) in the pixel area even though the optimized strain engineering is used. Therefore, somewhat intrinsically stretchable devices need to be adopted for stretchable high-resolution displays. In addition, stretchable passivation and encapsulation layers are among the biggest challenges for stretchable OLED displays. If other light-emitting devices, such as iLEDs, which are stable in air, can be used, the requirement of gas permeation of the layers is less demanding, and stretchable passivation and encapsulation layers can thus be implemented more easily.

There is no doubt that we will see early commercialization of stretchable low-resolution displays or stretchable wearable patch devices based on a combination of the aforementioned three strategies. Hollywood makes people dream, and the Society for Information Display makes those dreams come true!

References

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²⁴J. Byun, B. Lee, E. Oh, H. Kim, S. Kim, et al., “Fully printable, strain-engineered electronic wrap for customizable soft electronics,” Scientific Reports 7, 45328, 2017.  •

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Yongtaek Hong, Byeongmoon Lee, Eunho Oh, and Junghwan Byun are with Seoul National University. Hong can be reached at yongtaek@snu.ac.kr.