Intrinsically Elastomeric Polymer Light-Emitting Devices
Imagine an electronic display nearly as transparent as a window, or a curtain that illuminates a room, or a smartphone screen that doubles in size stretching like rubber. A recent demonstration of intrinsically elastomeric light-emitting devices suggests that such examples may soon become viable.
by Jiajie Liang and Qibing Pei
STRETCHABLE or ultra-flexible electronics and optoelectronics have emerged as alternative technologies for the next generation of electronic applications. These elasto-meric light-emitting display and solid-state lighting systems are fabricated on substrates that can be stretched, twisted, and folded – they are often referred to as skin-like displays.1 Companies such as Samsung, LG, and Sony have recently demonstrated prototypes of flexible displays and TV sets based on organic light-emitting diodes (OLEDs). However, true stretchability is much more demanding than flexibility.
While flexible displays normally need to withstand strains of no more than 1%, skin-like displays must be able to endure strains or shape changes of over 10%.2 The realization of stretchable displays would not only permit significantly more durable and even unbreakable devices to be created, it would also enable more exotic applications such as expandable and foldable screens for smartphones, electronic clothing, rollable or collapsible wallpaper-like lamps and curtains, bio-compatible light sources for in vivo or epidermal medical devices, and electronic skin in robotics.3–5
To fabricate stretchable or skin-like electronics, it is important to make the devices mechanically compliant and capable of stretching without undergoing physical damage.2 Strategies employing elastic interconnects and buckled or wavy materials that are otherwise unstretchable have been reported to fabricate stretchable semiconductor devices and displays.6–8
An alternative approach to realizing stretchable displays has been developed by a research group with the Department of Materials Science and Engineering at UCLA. This approach is based on a radically different strategy, whereby only elastomeric materials are employed to fabricate skin-like polymer OLEDs (PLEDs)1 An external deformation of the PLEDs causes more or less the same amount of strain in all constituent materials, including the electrodes, light-emitting semiconductor, and substrate. One of the major obstacles to fabricating skin-like PLEDs and displays had been the lack of an elastic transparent electrode that combines high visual transparency, good surface electrical conductivity, high stretchability, and high surface smoothness. These are all features essential to the fabrication of stretchable OLEDs.
Indium tin oxide (ITO) has been the ubiquitous transparent conductive electrode (TCE) material for practically all forms of displays. However, ITO routinely cracks under an applied strain of ~1%, limiting its application in flexible and stretchable optoelectronics.9 Several alternative materials, including carbon nanotubes (CNTs), graphene, and conducting polymers, have been investigated to replace ITO to make a stretchable TCE with varied success.10–12 These carbon- or polymer-based materials have relatively low electrical conductivity, and the sheet resistances of corresponding TCE are 1–2 orders of magnitude higher than that of ITO, making them unsuitable for OLEDs and organic solar cells. These materials also incur relatively high production costs.
Recently, percolation networks of metallic nanowires, such as silver nanowires, have shown promise in rivaling ITO in sheet resistance and visual transparency13,14 In addition, the mechanical compliance of silver-nanowire percolation networks could potentially be exploited as flexible and stretchable electrodes.15 However, silver-nanowire films coated directly on substrates have two critical drawbacks: surface height variations observed to be greater than 100 nm and weak bonding between the silver-nanowire networks and substrate such that mechanical scratches or repeated deformation could cause detachment of the silver nanowires and device failure. Cumbersome techniques, such as introducing polymer overcoating or hot-pressing, could alleviate these issues, but, in turn, introduce new issues such as reduced uniformity due to inadvertent removal of coating materials in some areas.
The research team at UCLA has developed a solution in the form of a transparent composite electrode comprising a thin percolation network of silver nanowires laid in the surface layer of rubber.1,16–18 This composite electrode meets all the requirements for the fabrication of high-performance OLEDs.19 The elastomeric OLEDs employ a pair of transparent composite electrodes sandwiching an electroluminescent polymer layer that is also elastomeric.1 The OLEDs have a maximum brightness of 2200 cd/m2 and a luminous efficacy of 11 cd/A (total emission from both sides) that are on par with OLEDs fabricated on ITO/glass using the same emissive polymer. Moreover, the elastomeric OLEDs exhibit rubbery elasticity at room temperature, are collapsible and twistable and can still function while stretching to more than twice the original size (strains as large as 120%). This material can also survive more than 1000 repeated continuous stretching cycles at 30% strain. Small stretching actually enhances its light-emitting efficacy as a result of enhanced electron injection and thus increased balance of electron and hole injections at small strains. The fabrication process is scalable and was readily adapted for the demonstration of a simple passive-matrix monochrome display containing multiple pixels.
Stretchable Transparent Composite Electrodes
Silver nanowires with a length-to-diameter aspect ratio of approximately 500 were used to form a conductive silver-nanowire network with high electrical conductivity and mechanical compliancy. A schematic representation of the manufacturing process for stretchable silver-nanowire-based composite electrodes appears in Fig. 1(a). Basically, a dispersion of silver nanowires in isopropanol was coated on glass substrates utilizing Meyer rod or airbrush spraying. The resulting transparent conductive coating on release substrates (glass or PET) was then overcoated with a precursor solution comprising a siliconized urethane acrylate oligomer, an ethoxylated bisphenol A dimethacrylate, and a photo-initiator. The coatings were cured under UV and then peeled off as free-standing silver-nanowire polyurethane acrylate (PUA) composite electrodes. The resulting composite electrodes fabricated by thisin situ substrate formation and transfer method generally had a smooth surface with roughness lower than 5 nm [Fig. 1(b)]. Due to the large aspect ratio of the silver nanowires used, with silver being the most conductive metal, a conductive percolation network was formed using an extremely low coating density of nanowires. The inter-nanowire contact resistance was reduced by thermal treatment that forged nanowire–nanowire fused joints. A low sheet resistance of 15 Ω/sq. could be obtained with a coating density of only 130-mg silver nanowires per square meter area. This all-solution-based fabrication process is scalable to produce large-area sheets [Fig. 1(c)].
Fig. 1: At top (a) is a schematic illustration of the manufacturing process of an elastic transparent silver-nanowire-PUA composite electrode. An atomic force microscopy (AFM) topographic image of the silver-nanowire–PUA composite electrode appears in (b). And (c) is an optical image of a silver-nanowire–PUA composite electrode (25 cm × 15 cm, within the red dashed rectangle).
The transmittances for the neat PUA matrix and silver-nanowire–PUA composite electrodes with various silver-nanowire coating densities, and thus sheet resistances are shown in Fig. 2(a). The silver-nanowire–PUA composite electrode with a resistance of 15 Ω/sq. exhibits a transmittance higher than 81% in the range of 500–1000 nm, which is comparable to those of ITO/glass and better than commercial ITO/PET electrodes.
To test the bonding force between the silver nanowires and PUA matrix, Scotch adhesive tape was applied to the conductive surface of the silver-nanowire–PUA composite electrode and peeled off. After 100 such tests, the sheet resistance of the silver-nanowire–PUA composite electrode remained unchanged, indicating good bonding force between silver nanowires and PUA. The strong bonding between the PUA elastomer matrix and silver nanowires is also beneficial in preventing long-range motion or sliding of the silver nanowires and in preserving the nanowire– nanowire junction during large-strain deformation of the composite electrodes.
The resistance evolution of silver-nanowire–PUA composite samples during continuous stretching-relaxing cycles between 0 and 30% linear strain is shown in Fig. 2(b). The baseline sheet resistance for the silver-nanowire–PUA sample only increased from 15 to 45 Ω/sq. after 1500 stretching–relaxing cycles. The sheet resistance at 30% strain also showed a gradual increase with stretching cycles and reached 85 Ω/sq. only after 1500 cycles, which is still lower than most freshly prepared transparent electrodes based on carbon nanotubes, graphene, or conducting polymers without stretching. The improvement in stretchability is possible due to the unique microstructure of the composite electrode in which the silver-nanowire percolation network is embedded within an elastomeric matrix. The latter prevents sliding or long-range drift of the nanowires.
Fig. 2: At left (a) are the transmittance spectra of a neat PUA film and silver-nanowire–PUA composite films with a specified sheet resistance (thickness ~150 µm). At right (b) is a graph representing transient resistance measured during 1500 cycles of stretching-relaxing between 0% and 30% strains for a 15 Ω/sq. silver-nanowire–PUA composite electrode.
Moreover, the composite electrodes can be stretched to as much as 100% strain while sheet resistance remains below 1 kΩ/sq. Having a smooth conductive surface is critically important for the fabrication of thin-film electronic devices such as OLEDs or polymer solar cells. This requirement has been particularly challenging for new electrode materials replacing ITO/glass. The elastomeric composite electrodes have a conductive surface that replicates the surface of the release substrates. Using glass as the release substrates, the surface roughness of the composite electrodes was found to be less than 5 nm. No cracks, voids, or buckling patterns were observable on the surface. Stretching–releasing cycles to 30% strain did not significantly increase the roughness.
Intrinsically Elastomeric PLEDs
To fabricate elastomeric PLEDs, a polymer light-emitting electrochemical cell (PLEC) architecture was employed. The PLEC was selected, instead of the conventional OLED architectures, because of the simplicity of the PLEC device structure, which does not require specific electrode work functions for charge injections20,21 The research group’s recent work on the fabrication of high-performance and fully solution-processed PLECs by spin-coating, rod-coating, and/or blade-coating at ambient conditions advanced the fabrication of the PLECs to the point where it is compatible to a low-cost roll-to-roll process.22 The process of fabricating elastomeric PLEC, as illustrated in Fig. 3, started with spin-coating a thin layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) on a silver-nanowire–PUA composite electrode as an anode. The thin PEDOT layer helped protect the PUA matrix from solvent attack in the subsequent coating of the electroluminescent polymer layer. The electroluminescent polymer layer consisted of a blend of a yellow light-emitting polymer (SuperYellow), ethoxylated trimethylolpropanetriacrylate (ETPTA), polyethylene oxide (PEO), and lithium trifluoromethane sulfonate (LiTf).
Fig. 3: This simplified rendering shows the fabricating process for a stretchable PLED device based on a pair of silver-nanowire–PUA composite electrodes (not to scale). The processes are all-solution based (AgNW = silver nanowire).
SuperYellow was selected for its very high molecular weight, which is suitable for large-strain stretchability. ETPTA was chosen for its capability to conduct ions and to polymerize to form a highly cross-linked polymer network that ceases to conduct ions. This property is important for the formation of a stable PIN junction.23,24 PEO, an ionic conductor widely used for solid electrolytes, was added to enhance the stretchability of the cross-linked ETPTA network. LiTf is a widely used salt in solid electrolytes. In the PLEC, LiTf provides ionic dopants to dope SuperYellow in the formation of a PIN junction. A second silver-nanowire–PUA composite electrode (as cathode) was stacked onto the emissive polymer layer, face down and laminated to complete the device fabrication.
The PLEC was initially driven at a constant current to establish a PIN junction in the emissive polymer layer. The pre-charged PLEC can subsequently be operated like a conventional OLED with rapid turn-on. Characteristic performance for the PLEC is presented in Fig. 4. Light emission in this device turns on at 6.8 V and reaches a peak brightness of 2200 cd/m2 at 21V. The luminous efficacy reaches 5.7 cd/A at the maximum brightness. The driving voltage at 10, 120, and 320 cd/m2 brightness is 9, 14, and 16 V, respectively. These voltages are in the same range as typical polymer-based OLEDs. The PLEC light-emission efficiency continuously increases with the drive voltage and brightness, which indicates unbalanced injections of electrons and holes. The injection balance is enhanced at higher current density.
Both charge-injection electrodes of the PLEC are transparent, and the emissive layer is semitransparent. The PLEC is thus semitransparent, as can be seen in Fig. 4(c). Light produced in the electroluminescent polymer layer escapes from both surfaces of the device with nearly identical luminance and efficacy. The actual maximum external current efficacy of the stretchable PLEC at the maximum brightness thus should account for emissions from both surfaces and adds up to 11.4 cd/A. The calculated external quantum efficiency is 4.0%. This performance is comparable to state-of-the-art PLEC based on SuperYellow and fabricated on ITO/glass substrate as anode and evaporated aluminum as cathode.23,24 The device is bendable and can be folded around a 400-µm-thick piece of cardboard without causing any damage to its mechanical integrity or electrical properties [Fig. 4(c)].
Fig. 4: At upper left (a) are shown the current-density–luminance–driving-voltage characteristics of an elastomeric PLEC device. (b) depicts the luminous-efficacy characteristics of the device. At lower left (c) are photographs of the PLEC (original emission area: 3.0 mm × 7.0 mm) unbiased, biased at 12 V, deformed to show light emission from both surfaces and folded around a piece of cardboard 400 µm thick.
The PLEC device can be uniaxially stretched up to 120% strain with uniform bright emission across the entire luminous area at strains up to 120% (Fig. 5). When biased at 12 V, the PLEC shows an initial increase of brightness from 0 to 20% strain, then decreases as the strain is further increased. Interestingly, the luminous efficacy shows a 200% increase, from 1.0 cd/A before stretching to 3.0 cd/A at 40% strain. It levels off up to 80% strain and then begins to decrease, but still remains at a fairly high value of 2.1 cd/A at 120% strain, which is still 100% higher than its original value. An investigation of charge injections indicates that the increasing efficacy with strain probably results from a more balanced injection of electrons and holes when the device was under strain. The PLEC can be repeatedly stretched between 0 and 30% strain for 1000 continuous cycles. As shown in Fig. 5(c), the luminous efficacy of the PLEC drops rapidly in the first 100 cycles and then stabilize in subsequent cycles. Since 30% strain at room temperature is sufficient for most biomedical or bio-inspired applications, the elastomeric PLEC is a true skin-like light emitter.
Fig. 5: The chart at upper left (a) depicts current-density and luminance characteristics of a PLEC device at 12 V with increasing strains. Chart (b) shows current-efficacy characteristics of the device with strain and chart (c) plots the luminous efficacy at 0% strain during 1000 continuous cycles of stretching–relaxing between 0 and 30% strains. In (d) are photographs of a PLEC (original emission area: 5.0 mm × 4.5 mm) biased at 14 V at specified strains.
OLEDs generally require hermetic sealing. Packaging is thus another important issue to address for skin-like OLEDs. The fabrication and operation of the stretchable PLECs described above were conducted in a nitrogen-protected glove box with oxygen and moisture contents both kept below 0.5 ppm. To take the devices out of the glove box, a thermally cross-linked polyurethane (TCPU) was used to sandwich the device as shown in Fig. 6(a). This encapsulated device, with uniform lighting area, could still be stretched repeatedly, stretched, and wrapped around a person’s finger [Fig. 6(b)] and twisted [Fig. 6(c)]. The storage lifetime of this TCPU-sealed device in air was only about 1 week. There do not appear to be any elastomeric materials capable of hermetically blocking moisture and oxygen. However, several recent and encouraging developments that may lead to the eventual development of a stretchable barrier material include elastomers incorporated with graphene25 and layered silicate.26
Fig. 6: At top (a) is a depiction of the lamination of an elastomeric PLEC device between two layers of a polyurethane packaging material. Below are images of an encapsulated PLEC device that is (b) stretched, bent, and wrapped around a finger and (c) formed into a twisted shape.
The fabrication process of the stretchable PLEDs could be adapted to demonstrate pixel-ated displays. Fig. 7(a) illustrates the process needed to fabricate a simple pixelated display employing the same technique described above except that the silver-nanowire-based composite anode and cathode are patterned into rows and columns and the two patterned electrodes are aligned at a 90° angle. Figure 7(b) includes optical images of an encapsulated elastomeric display consisting of 5 × 5 pixels. The display is semitransparent – the background logo can be clearly seen through it. Stretching at a 10% strain does not affect the uniform light emission of each pixel, and the pixels can be selectively addressed [Fig. 7(b)]. It is possible to pattern the silver-nanowire traces to line widths under 100 µm, and researchers are working on higher-resolution skin-like displays.
Fig. 7: A schematic illustration (a) at top left and a top-view illustration at top right depict an encapsulated elastomeric PLEC display consisting of 5 × 5 pixels. Below (b, left to right) are photographs (with UCLA logos part of the background surface) of a stretchable display unstretched with 5 × 5 pixels all turned off, stretched with all pixels turned on, and stretched with selected pixels turned on (pixel size without stretching is 1 mm × 1 mm).
Prospects for Skin-Like OLED Displays
High-performance elastomeric OLEDs can be fabricated through a relatively simple, all-solution-based process. A key development is the elastomeric transparent composite electrodes that combine high optical transmittance, surface electrical conductivity, surface smoothness, and rubbery elasticity, all essential for the fabrication of organic thin-film electronic devices. The ability to form a light-emitting PIN junction in situ in the emissive polymer layer simplifies the OLED device architecture and thus allows the fabrication of the skin-like PLEDs and displays. There are plenty of opportunities to further increase the performance and stretchability.
There are still major technical challenges to be overcome before skin-like PLEDs and displays can be commercialized, such as synthesis of transparent sealing materials, synthesis of elastomeric electroluminescent polymers, an increased device lifetime to at least thousands of hours, and the development of stretchable TFTs. Once these are overcome, the skin-like PLEDs and displays will lead to a bright future where information and lighting are provided in various thin, stretchable, or conformable form factors, or are invisible when not needed. Such technologies will definitely enable a number of very exciting products along the way.
Acknowledgments
The work reported here was supported by the Air Force Office of Scientific Research (FA9550-12-1-0074) and National Science Foundation (ECCS-1028412). The authors thank Zhi Ren and Kwing Tong for their assistance.
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