Stretchable Energy Storage and Conversion Devices

Transcription

1 Stretchable Electronics Stretchable Energy Storage and Conversion Devices Chaoyi Yan and Pooi See Lee * From the Contents 1. Introduction Stretchable Electrodes Stretchable Supercapacitors Stretchable Batteries Stretchable Solar Cells Summary and Outlook S tretchable electronics are a type of mechanically robust electronics which can be bended, folded, crumpled and stretched and represent the emerging direction towards next-generation wearable and implantable devices. Unlike existing electronics based on rigid Si technologies, stretchable devices can conform to the complex non-coplanar surfaces and provide unique functionalities which are unreachable with simple extension of conventional technologies. Stretchable energy storage and conversion devices are the key components for the fabrication of complete and independent stretchable systems. In this review, we present the recent progresses in the developments of stretchable power sources including supercapacitors, batteries and solar cells. Representative structural and material designs to impart stretchability to the originally rigid devices are discussed. Advantages and drawbacks associated with the fabrication methods are also analysed. Summaries of the research progresses along with future development directions for this exciting field are also presented Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 3443

2 reviews C. Yan and P. S. Lee 1. Introduction Stretchable electronics which can be stretched, twisted, bended and folded are a class of fundamentally different electronics from existing hard and rigid devices. [1,2 ] The remarkable features of stretchable electronics enable them to be compliant to complex non-coplanar surfaces and provide unique functionalities such as seamless integration with human organs and may have profound impacts to a wide range of practical applications including surgical and diagnostic implements, wearable consumer electronics, artificial sensory skin, etc. The research and developments of stretchable electronics started from the work on stretchable electrodes based on wrinkled metal films on elastomer substrates by Whitesides, [3 ] Wagner and their co-workers. [4 6 ] The integration of semiconductor components with metal electrodes by Rogers and co-workers resulted in stretchable circuits with practical functionalities [7 ] and ignited the rapid expansion of the field to more complex stretchable electronics. To date, a wide range of sophisticated electronics in stretchable from has been successfully demonstrated including but not limited to circuits, [7 11 ] light-emitting diodes (LEDs), [12 15 ] strain and pressure sensors, [16 18 ] epidermal electronics, [19 23 ] photodetectors [24 26 ] as well as power sources. The key focus of this Review will be on stretchable energy storage and conversion devices, which are of critical importance for the construction of complete, independent stretchable systems. The successful demonstration of stretchable power sources was only achieved since 2009 [27 ] and the field is still in its very primary stage with plenty of fascinating opportunities to be explored. In this Review, we focus on three major types of stretchable energy storage and conversion devices, including supercapacitors, batteries and solar cells. Supercapacitors store charges via electrical double layers or redox reactions. [28 ] Compared with batteries, they can deliver much higher power density but relative lower energy density. The first stretchable supercapacitor was reported by Yu et al. using buckled single walled carbon nanotube (SWCNT) macrofilms in [27 ] Consequent works on stretchable electrical double layer capacitors (EDLCs) and redox-type supercapacitors were demonstrated utilizing either elastomer or textile substrate. [29 32 ] Batteries are undoubtedly the dominant power sources especially for portable consumer electronics owing to their satisfactory energy density. The first battery in stretchable form was dry (zinccarbon) battery reported by Kaltenbrunner et al. in 2010, [33 ] followed by interests in other forms of batteries, such as Mg batteries, [34 ] alkaline batteries, [35,36 ] Li-ion batteries [37,38 ] and Ag-Zn batteries. [39 ] Solar cells are among the most promising candidates for clean energy applications, by converting solar energy into electricity. While the current market is dominated by rigid Si solar panels, it is desirable to fabricate stretchable solar cells to power future wearable electronic devices. The well-developed pre-strain strategy was naturally adopted for the fabrication of inorganic solar cells, where the rigid GaAs islands were connected with buckled, arc-shaped interconnections. [40 ] Attention was also drawn to organic solar cells due to the intrinsic stretchability of polymers. Following the first demonstration by Lipomi et al., [41 ] organic solar cells are continuously being studied and improved. [42 44 ] In this Review, we summarize the recent progresses in the field of stretchable supercapacitors, batteries and solar cells. We first present a summary of the strategies and materials used for stretchable electrode fabrication, which serve as the basis for the development of all power sources. Reports on respective devices are then reviewed and discussed, focusing on the innovative methods used to achieve stretchability. Finally, we present the discussions on the existing problems, challenges and possible directions for future research as well as the commercialization aspects of stretchable electronics. 2. Stretchable Electrodes 2.1. Strategies for Stretchable Electrodes In contrast to conventional rigid electronics on Si wafers, stretchable electronics are the soft form of electronics and can maintain their functionalities even after severe mechanical deformation. Several representative examples of stretchable electronics are shown in Figure 1. For example, the stretchable integrated silicon circuits can be punched with a glass tube but maintain their functionalities owing to the buckled Si nanostructures (Figure 1 a). [2,8 ] Figure 1 b shows a stretchable active matrix based on fluoride copolymer and SWCNT composite elastic conductors which can sustain high strains up to 134%. [45 ] The stretchable device in Figure 1 c is made of SWCNT strips sprayed onto polydimethylsiloxane (PDMS) substrate which acts like artificial skin to detect pressure and strains. [17 ] As one of the key components for stretchable devices, the electrodes have to be able to withstand such harsh mechanical requirements. Several strategies have been used to fabricate stretchable electrodes, and in general can be categorized into the following two groups: [2 ] (1) Structures that stretch; (2) Materials that stretch (see Table 1 ). Representative examples based on different strategies are shown in Figure 2. Creating structures that can be stretched is the most successful strategy for stretchable electronics, and the stretchable structures can be categorized into the following four types: (1.1) Non-coplanar buckled structures; (1.2) Coplanar serpentine structures; (1.3) Percolating nanostructured films; (1.4) Textiles. Creating buckled structures for originally rigid materials is an effective method to impart stretchability to a wide range of inorganic materials. [3 5,7,8,46 ] For example, functional components such as Au electrodes were deposited on pre-strained elastomer substrates, and buckled structures were created when the strain was released (Figure 2 a b). [3 5 ] The Au electrodes can accommodate large strains to maintain C. Y. Yan, Prof. P. S. Lee School of Materials Science and Engineering 50 Nanyang Avenue Nanyang Technological University Singapore, pslee@ntu.edu.sg DOI: /smll Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

3 Stretchable Energy Storage and Conversion Devices Figure 1. Representative examples of stretchable electronics. (a) Integrated silicon circuits with wavy structures deformed by a glass tube. Inset shows the wavy structures of two transistors. Reproduced with permission. [2 ] Copyright 2010, American Association for the Advancement of Science. (b) Rubber-like active matrix from fluoride copolymer and SWCNT composite elastic conductors. Reproduced with permission. [45 ] Copyright 2008, American Association for the Advancement of Science. (c) Strain and pressure sensors based on spray-coated SWCNT strips. Reproduced with permission. [17 ] Copyright 2011, Nature Publishing Group. the integrity of functional devices. [4 ] Analogous buckled arcshaped electrodes were used to impart stretchability to rigid Si devices. [2,25 ] Coplanar serpentine structures which can be Table 1. Strategies used to fabricate stretchable electrodes for energy storage and conversion devices. Strategy Examples Ref Structures that stretch Non-coplanar buckled structures 24,25 Coplanar serpentine structures 14,38 Percolating nanostructured films 27,48 Textiles29,32 Materials that stretch Intrinsically stretchable polymers 15,57 elongated to as high as 300% without mechanical failure were used to construct highly stretchable electronics. Stretchable Li-ion batteries [38 ] and LEDs [14 ] have been demonstrated using serpentine metal and graphene interconnects. There are also stretchable substrates based on percolating nanostructures, such as nanoparticles, nanowires, nanotubes or nanoplates. [15,33,47 52 ] For example, Yu et al. reported the fabrication of stretchable LEDs using flat CNT-embedded poly(tert-butylacrylate) (PtBA) elastomer electrodes. [53 ] The electrodes were flat without intentionally created buckles but they can sustain a high degree of strain due to its intrinsic stretchability of the percolating CNT networks. Analogously, percolating nanowire networks were demonstrated to possess excellent stretchability. [48,49,51 ] Note that although the nanostructured films are stretchable due to the inter-sliding, they are often made into buckled structures to improve the stretchability. [27 ] Apart from those nanostructures deposited on elastomer surface, they can also be embedded into the elastomer matrix to obtain stretchable composite electrodes. Typical examples include the printable elastic conductors based on SWCNT and fluorinated copolymer composite [12,45 ] as well as Ag nanowires [49,54 ] and Ag nanoparticles [55,56 ] dispersed in different polymer matrices. Textiles composed of 3D woven fibers are natural candidates for stretchable substrates. Depending on the textile/fabric types, they can typically be stretched to % without mechanical fracture. [29 ] The insulating textile substrates have to be coated with conducting elements such as SWCNTs, Ag film and polypyrrole (PPy) to serve as stretchable electrodes. For materials that stretched, we are mainly referring to polymers with flexible and stretchable molecular chains. Composites based on organic polymers and inorganic structures such as nanowires and nanoparticles are categorized as stretchable structures (as discussed above) since functional inorganic components are rigid and the stretchability originates from the percolating structure. Flat substrates composed of elastomer matrix and polymer functional components were demonstrated for stretchable LED applications. [15 ] The emitting layer is a flat film of polymer mixture with excellent intrinsic stretchability. PEDOT:PSS films directly deposited on PDMS substrates were also shown to have excellent stretchability up to 200%. [57 ] Although polymers have good stretchability, their properties such as electrical conductivity will degrade upon stretching. Thus, it is usually desirable to incorporate structural designs such as creating buckled polymer films to improve their stretchability. [41 ] 2.2. Materials for Stretchable Electrodes Conducting materials that have been used to fabricate stretchable electrodes for energy storage and conversion devices mainly include: (1). Carbon-based materials, such as CNT, carbon black; (2). Metal thin films, such as Au, Ag, Cu, Al, etc; (3). Conducting polymers, such as PEDOT:PSS, PPy, etc. The electrode categories based on materials are summarized in Table 2. Carbon-based materials with excellent chemical and thermal stabilities are ideal candidates for electrochemical 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

4 reviews C. Yan and P. S. Lee Figure 2. Strategies used to fabricate stretchable devices. Main strategy is to use structures that stretch, including (a b) non-coplanar buckled metal electrodes; Reproduced with permission. [3,4 ] Copyright 1998, Nature Publishing Group and Copyright 2003, American Institute of Physics; (c d) coplanar serpentine structures; Reproduced with permission. [38 ] Copyright 2013, Nature Publishing Group; (e f) percolating nanostructured films; Reproduced with permission; [26,27 ] (g h) textiles. Reproduced with permission. [35 ] Another strategy is based on the intrinsic stretchability of polymer materials (i j). Reproduced with permission. [15 ] energy storage applications where the electrodes have to withstand a variety of harsh environments such as strong acid, alkaline or corrosive solutions. Kaltenbrunner et al. used carbon black paste (mixed with silicone oil) as electrodes for dry batteries. [33 ] The mixture was casted into thin film as current collectors, which can sustain strains up to 100%. However, obvious cracks in the electrodes were observed at high Table 2. Stretchable electrode materials for energy storage and conversion devices. Materials Descriptions Ref Carbon CNT Percolating film for double-layer 27,31 supercapacitors Carbon black Casted thin film for alkaline battery 33 Metal film Cu/Al Electrodes for Li-ion battery 38 Ca/Ag Sputtered thin film for organic solar cells 42 Ag Au Coated on Nylon textile for alkaline battery Buckled Au structures for solar cells and battery 35 34,40 Polymer PEDOT:PSS Electrodes for organic solar cell 41,44 PPy Electrodeposited for redox 32 supercapacitors strain and result in significant performance degradations. Unlike 1D nanostructures such as CNTs and nanowires, carbon blacks made of microparticles have relatively poor interconnections and cannot well maintain their integrity and hence conductivity upon stretching. CNTs, on the other hand, form much better interconnections due to their onedimensional (1D) morphology and are considered as better candidates for stretchable electrodes. Yu et al. reported the fabrication of stretchable SWCNT electrode by laminating SWCNT macrofilm onto pre-stretched PDMS substrates. [27 ] Buckled SWCNT electrodes were produced when the prestrain was released, and the electrodes can be stretched to 70% with only moderate resistance increase (<4 times). Similar method was also employed by Niu et al. to fabricate buckled SWCNT electrode with an improved stretchability to 140%. [31 ] Lee et al. reported the fabrication of stretchable CNT-based electrode based on another strategy by dispersing the CNT fillers uniformly within the insulating PDMS matrix. [37 ] The PDMS host matrix was made porous to facilitate their applications in Li-ion batteries. An electrical conductivity of S cm 1 was achieved. The electrodes were flat but intrinsically stretchable. Textiles with woven structures are excellent candidates as stretchable substrates. However, since most natural (such as cotton) and synthetic textiles (such as Nylon) are insulating, they have to be made conductive for electrode applications. Hu et al. reported a facile and effective dyeing method to coat the textiles with Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

5 Stretchable Energy Storage and Conversion Devices SWCNT inks. [29 ] Considering the 3D porous structures of the textiles and strong interactions between cellulose fiber surface and SWCNTs, the textiles became conductive immediately after a simple dipping into the SWCNT ink. Highly conductive textiles with sheet resistance below 1 Ω sq 1 were obtained. Significantly, the textiles have outstanding mechanical and chemical stabilities required for device applications. No obvious resistance variations were observed after water washing, heat treatment (200 C for 6 h), acid treatment (4 M HNO 3 for 30 min) and alkaline treatment (2 M KOH for 30 min). The textiles can be stretched to 140% even with improved conductivity upon stretching due to the enhanced contacts between conductive fibers. Metals are definitely the materials of choice for the majority existing nanoelectronic devices. However, in the domain of stretchable devices, it is important to make new designs to impart stretchability to the rigid metal films which in general can only sustain strain <1%. [38 ] One successful strategy is to create buckled metal structures using pre-strain method. For example, Wang et al. reported the fabrication of buckled Au films on PDMS substrates for Mg battery applications. [34 ] The PDMS substrates was pre-stretched before Au film deposition and then released after deposition. The substrate induced strain will create buckles in the Au films and impart stretchability to the originally rigid metal films. The electrode is stable when re-stretched to its pre-strain, but cracks may appear upon further stretching. The authors showed that the Au film could maintain its integrity up to 70% strain, and there were very limited degradations upon repeated stretching until 30% strain. Non-coplanar arcshaped Au interconnects were also used by Lee et al. to connect the rigid, non-stretchable functional units. [40 ] The rigid GaAs microscale solar cells were connected with wavy Au interconnects. Up stretching, the arc-shaped interconnects can deform to accommodate the strain. This is an efficient method despite that it is difficult to achieve stretchability >30% without sacrificing other performances such as areal coverage. [40 ] Serpentine metal interconnects were shown to be excellent stretchable electrodes. Xu et al. reported the design of serpentine Cu and Al electrodes for Li-ion batteries. [38 ] A mechanical design of self-similar serpentine interconnects with two levels of stretchable geometries imparted the devices with excellent stretchability up to 300%. There are almost no resistance changes upon stretching since all the strains were effectively accommodated by the structural deformation instead of stretching the metal layer itself. Analogous to CNT coated textiles, metal layer such as Ag was coated on textiles to make stretchable conductive electrodes. Ag coated Nylon fabrics (commercially available) were used as the substrates for alkaline batteries. [35 ] The conductive fabric showed excellent stretchability up to 150%. The resistance of a fresh Ag fabric was 0.59 Ω cm 1, and was reduced to Ω cm 1 after 1000 cycles to 100% strain due to the enhanced contact between fibers as have also been observed in SWCNT coated textiles, but further stretching to 150% will increase the resistance to 1.5 Ω cm 1, which is still quite acceptable for practical applications. Both the carbon-based and metal-based electrodes are fabricated by imparting stretchability to rigid materials through structural designs, conducting polymers can be used as stretchable electrodes with intrinsic stretchability. PEDOT:PSS is the most commonly used conducting polymers for stretchable electrodes especially in solar cells. The polymer layer itself is stretchable up to 200% [57 ] and the stretchability can be further enhanced by creating buckled structures on pre-stretched substrates. Lipomi et al. reported the fabrication of buckled PEDOT:PSS films on pre-stretched (20%) PDMS substrates. [41 ] The electrode was stable upon continuous stretching to 20% and there were negligible device degradations, although re-stretching beyond the prestrain would lead to cracks in the films and hence performance degradations, as have also been observed in metal films. Yue et al. used another type of conducting polymer PPy for stretchable electrodes and the stretchability was achieved by coating PPy on stretchable fabric substrates. [32 ] The PPy layers were coated by polymerization onto the Nylon fabrics. The electrodes were used for supercapacitor applications and they can sustain at strains up to 60%. Discussions above are limited to electrodes used for energy storage and conversion devices. There are more materials available for the fabrication of stretchable electrodes, such as graphene, Ag nanowires, Au nanoplates, etc. Stretchable graphene electrodes were first reported by Kim et al. using high-quality graphene transferred from Cu foil to PDMS substrate. [58 ] The stretchable graphene electrodes can work efficiently within uniaxial strain up to 30%, with stable resistance within 10%. Ag nanowires are considered as excellent candidates for stretchable and potentially transparent electrodes. For example, percolating Ag nanowire films on stretchable Ecoflex substrates have been reported to have excellent stretchability and resistance retention upon stretching. [48 ] Ag nanowires were also embedded in elastomers such as PDMS matrix to achieve excellent stretchability for sensor [49 ] and actuator [54 ] applications. Au microplates have been synthesized and employed for stretchable electrode applications. [59 ] The interconnected Au microplates can maintain conducting paths even when stretched up to 200%. 3. Stretchable Supercapacitors 3.1. Double-Layer Supercapacitors EDLC is a type of supercapacitors that store energy using electrical double layers, that is, adsorption and desorption of charged ions on the surfaces. Most commonly used active materials for double-layer supercapacitors are carbon-based materials such as SWCNTs, graphene and mesoporous carbon, due to their excellent electrical conductivity and high surface areas. [60 ] The carbon-based materials are also natural choices for stretchable supercapacitors, with proper structural engineering to impart stretchability to the devices. Yu et al. reported the work on stretchable supercapacitors using buckled SWCNT films on stretchable PDMS substrates where the SWCNT film served as both electroactive materials and current collectors. [27 ] The buckled macrofilms was created using a pre-strain strategy. Schematics of the fabrication processes are shown in Figure 3a. The elastomeric 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

6 reviews C. Yan and P. S. Lee Figure 3. Buckled SWCNT macrofilms for stretchable supercapacitors. (a) Schematic processes for the fabrication of buckled SWCNT films on PDMS substrates. (b) Cross-sectional SEM image showing the buckled structures. Reproduced with permission. [27 ] (c) Resistance variations with strain for buckled SWCNT films and flat SWCNT film. (d) Charge-discharge curves before and after 10 times stretching. (e) Specific capacitances variations with strain up to 120%. Reproduced with permission. [31 ] PDMS substrate was stretched and SWCNT macrofilm was laminated onto the substrate surface after UV treatments. The substrate was released to its original length and buckled structures were created due to the compression strain. A typical cross-sectional view of the buckled SWCNT film on substrate is shown in Figure 3 b. Note that the SWCNT film attached well to the underlying elastomeric substrate and no obvious delamination was observed, due to the strong interactions between functional groups on SWCNT surface and substrate surface after UV treatment. Two recent reports also use similar strategies to create buckled SWCNT macrofilms for stretchable supercapacitors. [30,31 ] The buckled structures allow excellent stretchability with little degradation of electrode resistance until the prestrain. Figure 3 c shows the resistance change of buckled and flat SWCNT film versus tensile strain. [31 ] While the resistance of flat SWCNT increased >3 times at a strain of 90%, the resistance for buckled SWCNT film remained almost constant at strain up to 140%. The buckled macrofilm exhibited stable electrochemical performances against repeated stretching, with no obvious resistance changes after 10 stretching cycles (Figure 3d). Specific capacitances in the range of F g 1 were reported based on pure SWCNT stretchable electrodes. [27,31 ] For double layer supercapacitors, the resistance largely determines the performances. Hence, the performances of buckled SWCNT films remained stable upon stretching (Figure 3 e) due to the excellent stretchability. The performances even increased slightly upon stretching to 120%, probably due to the electrochemical activation of the SWCNT films. Another strategy to fabricate stretchable supercapacitors is to use the stretchable characteristics of textiles. Textile fibers have hierarchical structures with interwoven 3D arrangements, functional groups and high porosity, all of which make them ideal substrates for stretchable supercapacitors. Hu et al. reported a facile and effective method to fabricate stretchable supercapacitors by dyeing the textiles. [29 ] The cotton sheet was dipped into SWCNT ink (10 mg ml 1 in aqueous solution) and was quickly coated with dense SWCNTs due to the strong adhesion forces between fibers and SWCNTs ( Figure 4 a). The coating method is readily scalable for future reel-to-reel productions. The textiles are uniformly coated with SWCNT films and became highly conductive after the coating process. The textiles retain their 3D porous structures as can be seen in Figure 4 c. Figure 4 d is a magnified view of the fibril surfaces showing the entangled SWCNTs on the surface. Sheet resistance of the conductive fabric can be controlled by chemical treatments of the substrates and dipping cycles. The sheet resistance for SWCNT on nitric acid treated cotton textile can reach <1 Ω sq 1. The resistance change upon stretching for the conductive textiles is shown in Figure 4 e, which showed unusual characteristics compared with those SWCNTs on buckled elastic conductors. While the SWCNT macrofilms on top of elastomeric substrates became more resistive upon stretching, resistance of the textiles first decreases upon stretching until a high strain of 140% and then increase upon further stretching. Upon initial stretching, the resistance keeps decreasing due to the improvement of the mechanical and hence electrical contacts between fabric fibers. Upon further stretching beyond 140%, the resistance starts to increase probably due to the severe inhomogeneous deformation at large strains, as well as the reduced cross section areas. Note that textiles worked well at a strain as high as 230% (Figure 4 e). Two symmetric conductive textiles were used as active electrodes and one blank textile as separator in the two-electrode testing system. Specific capacitances Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

7 Stretchable Energy Storage and Conversion Devices Figure 4. Stretchable conductive textiles coated with SWCNTs. (a) Dyeing process by dipping cotton fabric into SWCNT ink. (b) A uniformly coated fabric showing the scalability of the method. (c) SEM images of the 3D woven structures. (d) Enlarged view of the SWCNT coated fiber surface. (e) Resistance change of the conductive textile upon stretching to 230%. (f) Specific capacitance of the supercapacitors before and after stretching. Reproduced with permission. [29 ] Copyright 2010, American Chemical Society. of 140 F g 1 (at 20 µa cm 2 ) and 80 F g 1 (at 20 ma cm 2 ) were obtained, which were 2 3 time higher than comparative samples on polyethylene terephthalate (PET) substrates. The performances are comparable to the highest specific capacitances achieved with SWCNTs on rigid substrates. [61 ] No obvious changes were observed after repeated stretching to 120% strain for 100 cycles (Figure 4 f). Stretchable double-layer supercapacitors based on SWCNTs films on both elastic PDMS substrates and textile substrates have excellent stretchability, electrochemical and mechanical stability. The specific performances especially for SWCNT on textile substrates are superior compared with previous reports of pure SWCNT devices due to the 3D porous fabric structures which are beneficial for electrolyte penetration and utilization of electroactive materials. However, the key drawback of those double-layer devices is the low specific capacitance and energy density and this can potentially be improved by the incorporation of redox materials Redox Supercapacitors Double-layer devices have excellent stability since there are no ion insertion and extraction but suffer from low energy density. The stretchable double layer supercapacitors typically have specific capacitances in the range of F g 1. [29,31,62 ] However, the specific capacitances (C sp ) for redox type supercapacitors (also called pseudo-capacitors) were shown to be much higher due to the redox reactions. For example, Co(OH) 2 /zeolite composites exhibited specific capacitances exceeding 3000 F g 1. [63 ] Note that redox reactions are associated with both redox supercapacitors and batteries. The difference is that there is phase change in battery reactions, that is, the electrode materials react with ions from electrolytes and are converted into new phases. However, in redox supercapacitors, the ions/atoms simply cling to the atomic structure of electrode materials without making or breaking of chemical bonds. [64 ] That is, the ions/atoms are chemically intercalated in batteries but physically intercalated in redox supercapacitors. Redox supercapacitors usually exhibit much higher reaction rates and thus power densities than batteries. It is desirable to explore stretchable redox supercapacitor with higher energy density than EDLCs and higher power density than batteries. One straightforward method is to incorporate redox materials into stretchable CNT films discussed above. Hu et al. continued their work by depositing MnO 2 nanoparticles on the SWCNTs films for stretchable redox supercapacitors with enhanced performances. [29 ] The MnO 2 nanoparticles were electrodeposited onto the SWCNT coated textiles. SWCNTs served as electron paths to improve the overall conductivity of the film ( Figure 5a) and MnO 2 nanoparticles served as redox materials to improve the specific capacitances. The textiles kept their porous structures after MnO 2 electrodeposition and the as-deposited MnO 2 nanoparticles had good electrical contacts with SWCNTs (Figure 5 a b). The areal capacitance was significantly enhanced by 24 times reaching 0.41 F cm 2 after MnO 2 deposition, which is much higher than those reports based on planar SWCNT films. [65 67 ] The redox supercapacitors also showed excellent electrochemical stabilities with little degradation after cycles (Figure 5 d). But no testing under stretched states was performed in the reference. One of the advantages of redox supercapacitors, as demonstrated, is that the areal and specific capacitances can be significantly improved. Possible issues may arise from the electrochemical and mechanical stabilities. It is possible that the SWCNT/ MnO 2 composite electrodes are mechanically less stable than pure SWCNT electrodes when subjected to repeated sliding and grinding between the fibers. The loss of active materials will lead to performance degradation after long-term stretching and cycling tests. Another work on stretchable redox supercapacitor was recently reported based on conductive polymer coated textiles. [32 ] PPy, a commonly used conductive polymer with good capacitive performances, was deposited via chemical polymerization onto stretchable Nylon fabrics. A sheet resistance of 149 Ω sq 1 was achieved with optimized deposition processes. Device performances of the redox polymer supercapacitors were also tested. In a two-electrode system at a current density of 1.0 A g 1, the C sp increased from F g 1 (0% strain) to 117.6, 119.6, F g 1 at 20%, 40% and 60% strain, respectively. The C sp decreased less than 10% after 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

8 reviews C. Yan and P. S. Lee Figure 5. (a) Schematic diagrams of the MnO 2 nanoparticle coated conductive fibers. (b) SEM image of the MnO 2 nanoparticles wrapped by SWCNT conducting paths. (c) Charge-discharge curves for bare SWCNT double layer devices and SWCNT-MnO 2 redox devices. (d) Cyclic stability of the stretchable redox supercapacitors. Reproduced with permission. [29 ] Copyright 2010, American Chemical Society. being stretched to 100% for 1000 times. The devices have excellent mechanical properties with good adhesion of PPy to the Nylon fabric, but there is still much room to improve the specific capacitances which was relatively low even compared with pure SWCNT supercapacitors (140 F g 1 on textiles). [29 ] As have also been widely observed for redox polymer supercapacitors, the electrochemical cycling stability needs to be improved in future studies. 4. Stretchable Batteries 4.1. Dry (Zinc-Carbon) Batteries Zn, carbon black and xanthan gel) served as anode and MnO 2 paste (made of MnO 2, carbon black and electrolyte paste) served as cathode. The anode and cathode materials were immersed in electrolyte (NH 4 Cl, ZnCl 2 and DI water) to assemble the complete battery. Figure 6 a is a schematic diagram of the alkaline battery. Sample images at both relaxed and stretched states are shown in Figure 6 b c, respectively. The circuit is made of two batteries connected in series to drive a LED device. One problem with the reported carbon black electrode is that they would crack at high strain (Figure 6 c) and result in inferior performances. The load curves of the circuits are shown in Figure 6 e, with short circuit currents in the range of ma. It is also obvious that the short circuit current decreased significantly at 50% strain, but the open circuit voltage remained almost the same. More detailed characterizations of the relationship between short circuit current, open circuit voltage and strain are shown in Figure 6 f. Within the strain ranges tested (0 100%), the open circuit voltage is less sensitive to strain, but the short circuit current depends heavily on strain. In the first stretching cycle, the current levels decreased severely upon stretching, probably due to the cracking of the electrodes, which results in dispatching of the active cathode/anode and consequently less efficient utilization of the active materials. The device structures especially current collects have to be improved to get better performances. While the electrode based on carbon black pastes cracked into discontinuous pieces upon stretching (Figure 6 c), electrodes based on 1D SWCNTs have excellent stretchability with little conductivity degradation upon stretching. Other electrodes such as buckled metal structures may also serve as promising alternatives to the carbon black current collectors. Dry batteries, also called zinc-carbon batteries although carbon is not the electrochemically active component, are commonly found in most household, low-duty electronics. Dry batteries typically have nominal voltages of 1.5 V with Zn and MnO 2 as active components, analogous to alkaline batteries which will be discussed later. However, dry batteries use mixtures of ZnCl 2 and NH 4 Cl as electrolytes (pure ZnCl 2 for heavy duty purpose) instead of alkaline solutions as in alkaline batteries. Dry batteries are considered as cheap, safe, non-toxic and environmental friendly choices of power sources. Kaltenbrunner et al. reported the first fabrication of dry batteries in stretchable form, [33 ] as shown in Figure 6. Carbon pastes (100 µm thick, made of carbon black and silicon oil) were used as the stretchable current collectors for both cathode and anode of the dry batteries. Zn paste (made of 4.2. Mg Batteries Analogous to the well-known Li-ion batteries, Mg batteries store charges via the insertion and extraction of Mg ions. Since Mg ions carry two-electron charge, they are believed to have improved volumetric energy density than Li-ion batteries. However, much less attention was drawn to Mg batteries primarily due to the fact that no promising electrode materials and electrolytes were identified for Mg batteries and the demonstrated performances are yet satisfactory. Wang et al. reported the fabrication of stretchable Mg batteries based on structured conducting polymers. [34 ] The electrodes were made of electrodeposited PPy on buckled Au electrodes produced from the pre-stretching strategy. The elastomeric substrate was first stretched and then Au film was deposited after plasma surface treatment. The substrate Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

9 Stretchable Energy Storage and Conversion Devices cracks (Figure 7 e). Battery was assembled using stretchable electrode PPy (Viii) as cathode, Mg plate as anode and phosphate buffered saline (PBS) as electrolyte. Due to the excellent stretchability of the functional cathode, the battery performed well in both relaxed and stretched states. A cell voltage of 1.12 V was obtained after 2320 in situ stretching cycles to 30% strain (Figure 7f). Figure 6. (a) Schematic of the stretchable alkaline battery structures. Images of the alkaline battery in relaxed (b) and stretched (100% strain) state (c). (d) Output curves of the battery at 0% and 50% strain. (e) Short circuit current and open circuit voltage as a function of applied strain. Reproduced with permission. [33 ] was released and buckled Au film with good stretchability was obtained. Functional PPy film was electrodeposited onto the buckled Au electrodes as cathodes in Mg batteries. The buckled morphologies of the Au electrode before and after PPy electrodeposition are shown in Figure 7 a and b, respectively. Figure 7 c is the resistance change of three types of electrodes upon stretching. Sample PPy (V) was prepared by direct PPy deposition on buckled Au film, and sample PPy (Viii) was prepared by first stretching the buckled Au electrode and then electrodeposit PPy on the flattened substrate. The stretchability after PPy deposition became worse for both samples, but sample PPy (Viii) exhibited relatively better stretchability than sample PPy (V) and was used for subsequent tests. Figure 7 d is the cyclic resistance change of PPy (Viii) subjected to 2000 cycles of stretching to 30% strain. The electrodes were quite stable with less than 1 time resistance increase after cycling and the film itself also maintained its morphology without obvious defects such as 4.3. Li-Ion Batteries The battery market is now dominated by Li-ion batteries mainly due to their high energy density and light weight. The energy density of Li-ion battery is 3 4 time as high as Ni-Cd batteries and 2 3 time as high as Ni-metal hydride batteries. [68,69 ] Moreover, they are also advantageous in terms of high open circuit voltage, no memory effect, low self-discharge rate and environmental benignity, all of which make them the most popular batteries for a wide range of applications especially for portable consumer electronics. [68,69 ] Thus, it is highly desirable to fabricate stretchable Li-ion batteries to power future stretchable electronics. Lee et al. reported the fabrication of porous and stretchable polymer-cnt composite electrode for Li-ion battery applications. [37 ] The porous stretchable electrodes were fabricated by a controllable phase separation method. PDMS, poly(methyl methacrylate) (PMMA), CNT and block copolymer sources were mixed and cured, after which PMMA was removed leaving micropores in the PDMS matrix. Those CNTs exposed on the surface of the pores are active for energy storage, but those fully embedded in the PDMS matrix was inactive for electrochemical reactions and only contributed to electron conductions. Schematic diagram of porous substrates is shown in Figure 8a and images showing the porous structures of the substrates are shown in Figure 8 b d. Properties of the porous substrates, such as pore size, pore distributions, electrical conductivity, facture strength and Young s modulus can be tuned by controlling mass ratio of PDMS/PMMA/block copolymer and annealing temperature. Those parameters were carefully optimized to obtain the best performances. For example, with all the other fabrication processes remain the same (CNT content 5%, PMMA content 20%, annealing temperature 75 C), the pore diameters depended on the amount of PDMS-b-PMMA block copolymer (Figure 8 c: 0.1%; Figure 8 d: 1%). The Li-ion battery was assembled using this stretchable porous electrode as anode, Li metal as cathode and conventional LiPF 6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

10 reviews C. Yan and P. S. Lee Figure 7. Mg batteries based on buckled Au-PPy electrodes. SEM images of buckled Au (a) and Au-PPy (b) electrodes. (c) Resistance versus applied strain for pure Au, PPy (V) and PPy (Viii) electrodes. (d) Resistance change upon repeated stretching to 2000 cycles. (e) Morphology of the battery electrodes after repeated stretching cycles. (f) Discharge curves of the Mg battery after 2320 in situ stretching cycles to 30% strain. Reproduced with permission. [34 ] as electrolyte. Relationship between electrical conductivity, pore diameter and battery specific capacity is shown in Figure 8 e. Optimum performance of 190 mah g 1 was achieved (based on weight of CNT) and the capacity based on the complete weight of PDMS-CNT is 7 mah g 1. Due to the fact that only part of the CNTs contribute to the charge storage, the obtained capacity of 190 mah g 1 is less than that for CNT powders ( 250 mah g 1 ). Although the porous PDMS/ CNT composite electrode is stretchable, the performances in stretched states were not reported. The performances may degrade slightly upon stretching due to the increased resistance. The stretchability might be inferior due to the existence of micropores and make the electrodes easy to rupture upon mechanical stretching. Xu et al. reported another strategy using stretchable serpentine interconnects to fabricate Li-ion batteries which can be stretched up to 300% strain. [38 ] Structural schematics of the Li-ion batteries are shown in Figure 9a. The stretchability was achieved by the design of self-similar serpentine interconnects between battery pads. The serpentine interconnects are composed of three columns of wires connected by two horizontal straight lines (Figure 9 b). The short wavelength serpentine within each column is referred as the first level and the second level corresponds to the large-scale serpentine shape with long wavelength. Upon stretching, the second level unravelled first until a strain of 150% for the specific layout with little deformation for the first level. Upon further stretching beyond 150% strain, the first level began to unravel until the point of 300% where they were fully stretched. Finite element analysis (FEA) results and experimental observations of the symmetric serpentine contacts are shown in Figure 9 c. The interconnects were stretched but maintained their integrity until the strain of 300%, which is an excellent example of structural design enabled stretchability. The batteries were assembled using LiCoO 2 cathode, Li 4 Ti 5 O 12 anode and LiPF 6 /EC/DMC/polyethylene glycol (PEG) gel electrolyte. Charge-discharge curves showed no obvious performance changes in relaxed and stretched states. The battery performances such as output power remained almost constant within the strain range of 0% 300%. Digital images of the battery powering a red LED light is shown in Figure 9 f (relaxed state and biaxially stretched to 300%), verifying their proper functioning upon stretching. The nearly constant battery performances up to very high strain verified the feasibility of using serpentine interconnects to accommodate strain. The unique interconnect structures with spring within a spring design enable a level of stretchability that is >4 times larger than previous reports but meanwhile maintain an areal coverage as high as 50%. Further development and improvements of the batteries will depend on the research of active cathode and anode materials. As for the structural design of the stretchable devices, balances between stretchability and areal coverage have to be taken into consideration especially when areal coverage is important (for such as solar cells), since it is obvious that majority of the areas were not utilized when the devices was stretched (Figure 9f) Alkaline Batteries Alkaline battery is a similar battery family to dry batteries which have >100 years of history but still being heavily used nowadays. Although gradually replaced by Li-ion batteries, alkaline batteries still find their presences in many consumer electronics, with an annual production over 10 billion units worldwide. Alkaline batteries in stretchable form was first Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

11 Stretchable Energy Storage and Conversion Devices of 7.75 mah (3.875 mah cm 2, electrode area 2 cm 2 ) was obtained. The discharge capacity was stable within 0 100% strain, indicating that good contacts between the particles and the fabric were maintained in stretched states. The slight fluctuations in discharge are within experimental errors. Images of the devices tested are shown in Figure 10 h i. The devices can be stretched up to 150% and twisted to 90 and maintain their functionality. Figure 8. Intrinsically stretchable porous electrodes for Li-ion batteries. (a) Schematic of the active and inactive CNTs within the porous electrodes. (b) Example of a porous CNT-PDMS electrode. (c d) Controlling the pore diameter within the PDMS matrix. The PDMS-b-PMMA block copolymer amount is 0.1% for (c) and 1% for (d), with all the other fabrication methods unchanged. (e) Optimizing the battery performance through pore diameter and electrical conductivity control. (f) Battery example lightening a red LED. Reproduced with permission. [37 ] demonstrated by Gaikwad et al. based on conductive textile substrates [35 ] and a further development was demonstrated by Kettlgruber et al. very recently. [36 ] The commercially available conductive and stretchable textile made of Ag coated Nylon threads served as the substrate. The woven structures of textiles naturally impart excellent stretchability to the fabric. A sample of the commercial conductive fabric is shown in Figure 10 a. Top view and cross-sectional view of the stretchable battery structure is shown in Figure 10 b and c, respectively. An elastomeric casing was used to contain the stretchable anode, cathode and polymer gel electrolyte. Figure 10 d f show the microscale structural changes upon stretching. The woven structures can be stretched biaxially until 100% strain without any mechanical failure or damage. Device performances of the alkaline battery at 0%, 50% and 100% strains are shown in Figure 10 g. A discharge capacity 4.5. Silver-Zinc Batteries We recently demonstrated the successful fabrication of fully stretchable silver-zinc (Ag-Zn) batteries based on embedded nanowire elastic conductors. [39 ] Ag-Zn batteries are now being used in both small scale such as coin cells and large scale for military and aerospace applications. Compared with alkaline batteries and dry batteries, Ag-Zn system can deliver much higher energy and power densities. The performances of Ag-Zn batteries are comparable or even better (especially in terms of power density) than the current market leader Li-ion batteries. The wide applications of Ag-Zn battery are primarily hindered by the high cost of Ag. However, Ag-Zn batteries are intrinsically safe due to the usage of aqueous electrolyte and are free from flammability problems that have plagued the Li-ion batteries. Safety issue would become an overweighing consideration especially when the batteries are used for future wearable and implantable electronics. Ag nanowire (AgNW) electrodes with embedded structures in PDMS substrates were used as the Ag cathode. [ 39 ] Zn anode was fabricated by Zn electroplating on stretchable AgNW electrodes ( Figure 11 a). Fully stretchable Ag-Zn batteries were assembled using an in-plane structure with 10 M KOH as electrolyte (Figure 11 b). The full battery was shown to be highly stretchable (up to 80%) and could maintain their functionalities in both relaxed and stretched states. First the performances of AgNW cathodes were characterized using a two-electrode system with Zn plate as counter and reference electrode. CV curves of the AgNW electrodes at 0% and 80% strains (scan rate 5 mv s 1 ) are shown in Figure 11 d. Obvious redox peaks were observed and can be indexed to the reactions between Ag, Ag + and Ag 2+ states. [ 39 ] It is also evident that the current increased when the AgNW electrodes were stretched to 80%, and the effect of strain on the electrode capacity is shown in Figure 11 e. The capacity is relatively stable within 0% and 40% strains (at both current densities of 1 ma cm Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

12 reviews C. Yan and P. S. Lee Figure 9. Stretchable Li-ion batteries using serpentine interconnects. (a) Exploded view of the battery layers. (b) Illustrations of the serpentine interconnects used for interconnects. (c) FEA and experiment results of the deformation of the interconnects at 50% and 300% strain. (d) Charge-discharge curves of the batteries at relaxed (black) and 300% strain (red). (e) Output power as a function of applied biaxial strain. (f) Operation of the battery at relaxed state and biaxially stretched to 300% strain. Reproduced with permission. [38 ] Copyright 2013, Nature Publishing Group.F and 10 ma cm 2 ) but increase slightly at higher strains (60% and 80%) probably due to the freshly exposed AgNW surfaces when the electrodes were heavily stretched. The AgNW electrodes were also found to be highly stable without obvious performance degradations after 1000 cycles owing to the unique embedded structure where the AgNWs underneath the surficial layer were protected by the PDMS matrix from the corrosion of electrolyte. Figure 11 f is the charge/discharge curves of the full battery showing the proper function of fully battery in relaxed and stretched states. One issue with the Ag-Zn battery is that Zn electrode is quite reactive in alkaline electrolytes leading to hydrogen evolution. Possible solutions to overcome this problem are chemical modifications of Zn electrodes or alkaline electrolyte to minimize the undesired side reactions. [ 70,71 ] Figure 10. Alkaline batteries based on stretchable conductive textiles. (a) Example of the commercially available Ag-coated Nylon fabric. (b) Top view and (c) cross-sectional view of the device structures. (d f) Microstructures of the textiles in relaxed state (d), X-axial 100% strain (e) and Y-axial 100% strain (f). (g) Discharge curves of the device at 0%, 50% and 100% strain. (h i) Images of the device in relaxed state and 150% strain and 90 twisted state. Reproduced with permission. [35 ] Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

13 Stretchable Energy Storage and Conversion Devices Figure 11. (a b) Cross-sectional schematic diagrams of Zn electrodes and full batteries. The Zn electrodes were fabricated by Zn electroplating on AgNW electrodes. (c) Optical images of the fully stretchable Ag-Zn battery at 0% and 80% strain. (d) CV curves for pure AgNW electrode at 0% and 80% strain (scan rate 5 mv s 1 ). (e) The effect of strain on AgNW electrode capacity. (f) Charge/discharge curves of the full battery at 0% and 80% strain (current density 1 ma cm 2 ). Reproduced with permission. [39 ] 5. Stretchable Solar Cells 5.1. Inorganic Solar Cells Solar cell is a type of important energy conversion devices converting the almost infinite solar energy into electricity. Inorganic solar cells are more efficient but also more expensive than organic solar cells. While crystalline Si is by far the most prevalent materials for solar cells, other common inorganic material options include cadmium telluride (CdTe), copper indium gallium selenide (CIGS), gallium arsenide (GaAs), indium gallium nitride (InGaN) and the emerging DSSCs. For example, although both the material and production costs (via metalorganic vapour phase epitaxy) of GaAs cells are much higher than Si cells, the highly efficient GaAs multijunction cells found important applications in satellites and space explorations. They are also emerging as low cost alternatives in terms of cost per kwh when used in concentrators. [72 ] Lee et al. reported the fabrication of stretchable inorganic solar cells based on GaAs microcells with buckled Au interconnects. [40 ] Rigid GaAs microcells were fabricated using the conventional thin film technology. The microcell arrays fabricated on Si substrate were transferred to and bonded with PDMS substrate with the help of SiO 2. Consequently, Au interconnects were transferred to the microcell arrays to connect the individual cells. After transferring and release of the prestrain, the Au interconnects buckled into non-coplanar arc-shaped bridges. They can effectively accommodate the strain during later stretching and maintain the functionality of the devices. Optical and SEM images providing direct observations of interconnects movements are shown in Figure 12 a d. But the devices cannot be further stretched when the interconnects are flattened, which imposes stretching limits (usually <30%) to devices based on this technology. It is possible to revise the device layouts (such as longer Au interconnects) to improve the stretchability, but they will adversely lead to lower areal coverage which is undesirable for solar cell applications. The open circuit voltage, short circuit current, fill factor and energy convention efficiency of each microcell are 0.91 V, 88 µa, 0.79 and 13%, respectively. As expected, the open circuit voltage increases about 7 times ( 6.4 V) when seven microcells are connected in series in the relaxed state. Cycling tests involving repeated biaxial stretching to 20% followed by complete relaxation leave the performance unchanged. The efficiency ( 12.5%) and fill factor (FF, 0.78) of the interconnected microcells are constant over more than 500 cycles. Related devices based on flat substrates offer similar attractive fatigue response, but they do not allow, Figure 12. (a-b) Optical images and (c-d) SEM images showing the stretching of the buckled Au interconnects. (e) I V curves for 1 cell, 7 cells connected in series in relaxed and stretched states. (f) Efficiency and filling factors as a function of stretching cycles. Reproduced with permission. [40 ] 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

14 reviews C. Yan and P. S. Lee Figure 13. (a) Schematic of the stretchable solar cell fabrication process using the pre-strain strategy. SEM images of the functional films when at 20% pre-strain before release (b), released to 0% strain (c) and over-stretched beyond 20% (d). (e) I V curves for devices on PDMS, glass and ITO substrates. (f) I V curve variations when subjected to 0% 22.2% strain during the 1st stretching cycle. (g) I V curves in the 2nd and 11th cycles. Reproduced with permission. [41 ] simultaneously, similar levels of coverage and stretchability without leading to high interfacial stress that can cause adverse effects on the module such as the bowing effect Organic Solar Cells Compared with inorganic solar cells, the soft organic solar cells usually have inferior performances and poor environmental stability. But they are advantageous in terms of low costs, structural flexibility and ease of production from solution processes. They are considered as good candidates for future solar energy conversions if the efficiency and device stability can be improved, which are also the prime research focuses for organic solar cells. Notably, the soft, flexible and stretchable characteristics of organic polymers allow ready construction of stretchable solar cells. Lipomi et al. reported the fabrication of stretchable organic solar cells using mechanical buckling strategy. [41 ] The fabrication method is shown in Figure 13 a, a typical method based on pre-strain induced buckling technology. A PDMS substrate is stretched to a desired extend (such as 20%), and then functional layers (PEDOT:PSS and P3HT:PCBM) were spun coated consequently onto the substrate. Compliant top electrode (liquid metal EGaIn) was deposited before releasing the prestrain. Buckled structures were obtained after releasing the prestrain. SEM images of the films at pre-strain of 20%, relaxed to 0% strain and overstretched beyond 20% are shown in Figure 13 b d, respectively. As can be clearly viewed the functional films became buckled after releasing the prestrain. Further stretching beyond the prestrain led to cracks in the film. Photovoltaic properties of the devices are shown in Figure 13 e g. Figure 13 e plots the comparison of three typical devices on ITO, bare glass and PDMS substrates. The performance is better on ITO glass due to the improved electron conduction assisted by ITO layer and performances on rigid glass and soft PDMS are similar. Figure 13 f shows the performance at different strains during the first stretching cycle (0% to 22.2% strain). Note that both the short circuit current and open circuit voltage increased upon stretching. Since the active area of the device would increase upon stretching, this can in part explain the increase of short circuit current. But the continuous increases of open circuit voltage are not fully understood at the moment. [41 ] The device performances were stabilized after the first stretching cycle (Figure 13 g). No obvious changes were observed during the consequent stretches. The following figures of merit under a flux of 95 mw cm 2 were obtained: J sc = 7.4 ma cm 2, V oc = 415 mw, fill factor (FF) = 0.38 and power conversion efficiency (PCE) = 1.2%. Liquid metal EGaIn was used in several reports as compliant stretchable electrodes for ease of demonstration. [15,17 ] But they need to be replaced with corresponding metal film electrodes for practical applications, such as Ca/Ag electrodes [42 ] and probably other more stable electrodes with suitable work functions. The conversion efficiency can be improved by the employment of better top electrodes, more efficient conjugated polymers, antireflective coatings, etc. A higher stretchability is also desirable. The maximum achievable stretchability using this method was 20% 30%, due to the usage of simple clipping method and permanent stretching of PDMS substrates when heated. The stretchability can be improved by using method such as spray coating for the functional layer deposition instead of spin coating. The adhesion of active layers to the substrates has to be considered at higher pre-strain to prevent the delamination problem. Martin et al. reported the fabrication of ultrathin, lightweight and stretchable organic solar cells with overall device thickness less than 2 µm. [42 ] Schematic of the ultrathin solar cells is shown in Figure 14 a. Starting from an ultrathin PET substrate of 1.4 µm thick, PEDOT:PSS (150 nm), P3HT:PCBM (200 nm) and top metal electrode Ca/Ag (115 nm) were deposited consequently. The total device thickness is 1.87 um. The device was so thin that it could be wrapped conformally around a human hair (Figure 14 b). While the device after layer depositions was flat on the supporting glass substrate, they became wrinkled after compressing the released ultrathin free-standing device. The ultrathin solar cell could be stretched from the wrinkled states to flat states with proper maintenance of their functionalities. The extreme conformability of the ultrathin organic solar cells is demonstrated by deforming the device with a plastic tube of 1.5 mm tip diameter (Figure 14 d). Performances of the organic solar Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

15 Stretchable Energy Storage and Conversion Devices Figure 14. (a) Device structure of the ultrathin organic solar cells. (b) Wrapping the ultrathin solar cells around a human hair. (c) Compress the originally flat device by 50%. (d) Ultrathin solar cell on a plastic tip with 1.5 mm diameter. (e) I V curves of the solar cells subjected to 0% 80% compression. (f) Typical figures of merits of the solar cells after 22 compression cycles to 50%. Reproduced with permission. [42 ] Copyright 2012, Nature Publishing Group. cells upon compression are shown in Figure 14 e. The short circuit current depends heavily on the compression but open circuit voltage is relative constant. Short circuit current can be affected by the integrity of the functional films and electrodes. For example, if cracks appear during the stretching process, the films ruptured into patches and only part of the films can be utilized and this will lead to short circuit current decrease. The open circuit voltage also depends slightly on the stretching process, as have also been observed in previous report. [41 ] The performance change of the ultrathin solar cells against repeated stretching is shown in Figure 14 f. The devices are relatively stable but still exhibited 27% degradation after 22 stretching cycles. 6. Summary and Outlook Stretchable electronics is an emerging field for the next-generation wearable technologies, and stretchable power sources hold the key to scalable applications of portable electronic devices. To date, only very limited works have been demonstrated in the area of stretchable energy storage and conversion devices, much more efforts are needed to explore high performance devices and integrate them into complete and hybrid functional systems. One of the key challenges is to fabricate reliable electrodes for the stretchable devices. Electrodes for supercapacitors and batteries are required to be highly conductive with excellent electrochemical stability. The demonstrated examples such as SWCNT coated on textiles have good conductivity and electrochemical stability. However, the purified SWCNTs with good conductivity are very expensive; those multi-wall carbon nanotubes (MWCNTs) are cheap but have non-ideal conductivities. Stretchable electrodes based on Au also suffer from high material and production costs. It is desirable to search for alternatives that have good conductivity and electrochemical stability. Nanostructured film based on cheap metal nanowires is one of the potential options. Recently, oxidation-resistant Cu-Ni alloyed nanowires were synthesized and used for transparent electrodes, [73 ] which serve as promising alternatives due to their low costs and excellent stabilities. Multilayered electrodes with combined advantages such as good electrical conductivity of metal nanowires (such as Cu nanowires) and electrochemical stability of carbon nanostructures (such as graphene, MWCNTs) are also potential candidates for stretchable 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim