Wearable Power Source: A Newfangled Feasibility for Perovskite

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Wearable Power Source: A Newfangled Feasibility for Perovskite Photovoltaics Xiaotian Hu, Fengyu Li, and Yanlin Song ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00503 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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ACS Energy Letters

Wearable Power Source: A Newfangled Feasibility for Perovskite Photovoltaics Xiaotian Hu1,2, Fengyu Li1, Yanlin Song1,2*

1Key

Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of

Sciences (ICCAS), Beijing Engineering Research Center of Nanomaterials for Green Printing Technology, Beijing National Laboratory for Molecular Sciences (BNLMS), Beijing, 100190, P. R. China.

2University

*E-mail:

of Chinese Academy of Sciences, Beijing, 100049, P. R. China.

[email protected]

ACS Paragon Plus Environment

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ABSTRACT

With the booming advances in wearable electronics and perovskite photovoltaics, flexible perovskite solar cells (PSCs) have emerged as promising candidates for selfpower sources to sustainably drive the next-generation electronics. The wearable PSCs must simultaneously satisfy high power efficiency, light weight, environmental stability flexibility, stretchability and twistability, which are essential for practical applications. In this perspective, we lay out the key points for flourishing the wearable perovskite power source. We sequentially analyze the design, quality, printing methods and standardized tests of wearable PSCs. Then, representative progresses are summarized. Finally, we propose a comprehensive insight from both perspectives of optoelectronics and mechanics for future research in wearable PSCs, and outlook the possible integrated devices.

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As a fashionable artificial intelligence equipment, wearable electronic has attracted increasing attentions. Recently, promising applications, such as health monitoring, medical care, motion tracking and message communication, will profoundly improve the quality of human's daily life.1 The power source is the core component for harvesting and storing energy, which primarily influences the development from the perspectives of practicability and aesthetics. The current wearable electronics are usually equipped with a rigid battery. However, the rigid power source accounts for a large part of the total weight and affects the human conformability. Hence, it is significant to exploit the future wearable power source, which simultaneously provides light weight, flexibility, stretchability, twistability and sustainable output.

Recently, a self-power concept is developed to replacing the traditional batteries.2 Solar irradiance can be integrated perfectly with wearable electronics, because it is a clean, inexhaustible, and renewable energy source, especially plentiful out of doors.3 Among various wearable photovoltaics, perovskite solar cells are promising candidates with the rapid advances in photoelectric conversion efficiency (over 23%), long-term stability and


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module manufacture. Perovskites are materials with an ABX3 crystal structure, where ‘‘A’’ is a cation with 12-fold ‘‘X’’ coordination and ‘‘B’’ is 6-fold anion ‘‘X’’ coordinated, thereby making

BX6

octahedra.4

The

typical

device

is

a

sandwich

architecture:

Anode/pervskite/Cathode (Figure 1a). Specially, perovskite solar cells are low-cost, lightweight, suitable for roll-to-roll production. Compare with other organic photovoltaic absorbers, perovskite can release more strain energy with mechanical flexibility and durability. The polycrystalline perovskite exhibits much better nanoductility and fracture toughness than singlecrystalline one.5 Therefore, PSCs are more suitable for wearable power sources. In view of this promising feasibility, how to ultimately realize a high performance perovskite wearable power source is critical and significant for both fundamental research and commercial applications?

In this perspective, we summarize the key points for achieving a high performance wearable perovskite power source, as shown in Figure 1b. Then we highlight some important achievements on the design, quality, printing methods and standardized tests of wearable PSCs. Finally, future developments and integrated applications are prospected.


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Design of wearable PSCs. The device configuration is the core of wearable electronics. Due to the typical sandwich architecture and the outstanding performance of PSCs on rigid glass substrate, the planar configuration is first transplanted to the wearable PSCs. Several plastic substrates, commonly poly (ethylene terephthalate) (PET) and poly (ethylene naphthalate) (PEN), have been developed for the flexible planar device. Kaltenbrunner et al. demonstrate an ultra-lightweight flexible PSCs on the 1.4-µm-thick PET foils (Figure 2a).6 The planar PSCs show a power conversion efficiency (PCE) of 12% with a record power-per-weight of 23 W g−1 based on the whole device. These PSCs arrays can operate the aviation models for days under normal environmental condition. The light weight and desirable efficiency of planar flexible PSCs are feasible for wearable power sources. However, the flat substrate cannot meet the requirements of complicated deformation and conformability to skin or clothes.7 In order to fabricate more portable, foldable and wearable PSCs, textile structure is applied in PSCs. The double-twisted PSCs are prepared based on carbon nanotube fibers (Figure 2b).8 The perovskite and silver nanowires layers can easily deposit on the fibers with a good interfacial adhesion under stressing. These PSCs with a maximum PCE of 3.03% can maintain more than


6

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1000 bending cycles without a distinct degradation. Meanwhile, Li et al. develop a layerby-layer coaxial textile PSCs on a stainless-steel fiber via the dip-coating method (Figure 2c).9 These fiber-shaped PSCs exhibit a PCE of 3.3% with high flexibility. To further improve the PCE of devices, a flat interface on ribbon-like shape PEN/ Indium tin oxide (ITO) electrode is delivered.10 The coaxial PSCs achieve a high PCE of 9.49% with longtime stability. Hence, although the PSCs based on textile structure show excellent flexibility and conformability, the performances of those are far away from the planar PSCs. The coverage and crystal size of perovskite films are key points for further enhancing the PCE.

Recently, the planar structure has become the mainstream of wearable PSCs due to the rapid advances in PCE and stability of rigid devices. However, the common electrodes, such as PET/ITO and PEN/ITO, negatively affect the mechanical stability and flexibility of the wearable PSCs due to the limited ductility of polymer substrates and intrinsic brittleness of ITO. In order to improve the ductility, the thickness of PET substrates is designed to decrease under 100 μm (Figure 3a).11 At the same time, other substrates are


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incorporated into wearable PSCs. Yu et al. report the flexible PSCs fabricated on abundant, low-cost, and biocompatible cellulose papers (Figure 3b).12 The paper based PSCs exhibit the highest PCE of 9.05% with superior durability. These ultrathin substrates can substantially reduce device weight, providing a good sense of wearing. In addition, inspired from the metal foil based DSSCs, the titanium and copper foils suffer a favorable ductility for wearable PSCs. Watson et al. firstly use the commercial titanium foil into flexible PSCs, yielding a PCE of 10.3% (Figure 3c).13 Up to now, the best performance of wearable PSCs based on titanium and copper substrate has increased to 13.07%14 and 12.80%,15 respectively. Interestingly, these metal foils also provide an opportunity to the self-grown transport layer for simplifying the preparation process.

From the perspective of mechanical endurance, the brittle ITO is the Achilles' heel for the wearable PSCs. Great efforts for alternatives have been performed in recent years. Silver nano-grids and meshes are firstly exploited,16 but the node-resistance and roughness of the electrode limit the photoelectric performance. Yang et al. demonstrate an embedded Ag-mesh/conductive polymer composite film on PET substrate to resolve


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these issues (Figure 3d).11 This hybrid electrode delivers a low sheet resistance of 3 Ω/□ with a 82–86% transmission in visible region. More importantly, the semi-embedded structure shows excellent flexibility even under a narrow bending. Furthermore, Song et al.

and

Zheng

et.

al.

successfully

use

the

pure

poly(3,4-

ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) for highly conductive films (Figure 3e).17,18 The PEDOT:PSS ink can be easily applied various printing methods for roll-to-roll production. Benefiting from the film-transfer lamination technique, these PEDOT:PSS films are also suitable for the top electrodes. It is noteworthy that the same material is used for both bottom as well as top electrodes is in favor of the mechanical stability. Besides, carbon nanotubes and graphene are incorporated in wearable PSCs.19,20 The flexible devices based on carbon nanotubes and graphene electrodes show a PCE of 5.38% and 16.8%, respectively. Due to the favorable mechanical endurance, especially the two dimensional graphene (Figure 3f), the wearable PSCs maintain overwhelming bending stability. There is 85% original performance under a bending radius of 2 mm after 5000 bending cycles, which verify the possibility for wearable and foldable photovoltaic applications.


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Quality of perovskite films. There is a huge difference in crystallinity of perovskite on different substrates. Thus achieving high quality perovskite films on wearable substrates is a crucial factor for the device performance. As for printing process, tailoring the composition21 and incorporating the additives into perovskite are efficient approaches to improve the quality of perovskite films. Compared with the former, doping method is more simple and repeatable. For example, Yang et al. develop an additive assistant strategy, the dimethyl sulfide evidently enhances the grain size and crystallinity of perovskite films because of the declining crystallization rate (Figure 4a).22 With this method, the PCE of flexible PSCs increases to 18.4%, which is the highest reported value for small-area devices. Moreover, inspired by the “brick-and-mortar” microstructure and crystallization mechanism from the nacre, the antithetic soluble composite additives are reported recently(Figure 4b).23 Among the additives, the insoluble component can promote heterogeneous nucleation, and the soluble component suppresses the crystallization rate due to an interaction with lead halide. The perovskite films with the additives grow a perpendicular micro-parallel structure, which eliminate the defects of carrier transfer from horizontal grain boundaries. More importantly, this approach shows great universality on


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wearable devices, a high quality perovskite film is achieved on polydimethylsiloxane (PDMS) substrate. Meanwhile, because both of the additives filled into the boundaries are elastomer, the “brick-and-mortar” microstructure exhibits excellent mechanical flexibility. As a result, a 56.02 cm2 area wearable solar power source with a 7.91% certified certified can easily power commercial wearable devices in a variety of body movements. Moreover, it is important to balance the photovoltaic performance with flexibility during the growth of perovskite crystals. Despite the quality of perovskite absorber, the interfacial engineering, particularly the bottom interface, is significant for wearable PSCs. A series of metal oxide nanoparticles have been applied in flexible PSCs as the interfacial layers. The metal oxide nanoparticles suffer low-temperature processability for the flexible substrates, and the mobility of these materials is higher than organic interfacial layers. Furthermore, all the metal oxide nanoparticles can serve as a mechanical buffer layer between transparent electrode and perovskite layer. For the p-i-n PSCs, tungsten oxide and nickel oxide nanoparticles are prepared as the anode interfacial layer (AIL).24,25 Due to their high workfunction, the PSCs based on the nanoparticles exhibit a higher open-circuit voltage than traditional


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PEDOT:PSS AIL. Additionally, SnO2 and TiOx nanoparticles act as cathode interfacial layer (CIL) in n-i-p wearable PSCs.26,27 Recently, Yan et al. develop a SnO2 layer with a water vapor post-treatment, leading an improved conductivity (Figure 4c). With this SnO2 CIL, the flexible PSCs show the best PCE of 18.36% without an evident hysteresis. In spite of the material selection of interfacial layers, Song et. al. demonstrates a nanocellular PEDOT:PSS scaffold to architect optical resonant cavity and mechanical buffer layer in flexible PSCs (Figure 4d).18 We successfully fabricated hysteresis-free, flexible and large-scale (1 cm2) with a recorded PCE of 12.3 %. It is believed that this printable nano-scaffold provides a mechanical design for new laminated devices. Thus, the bulk crystallinity of perovskite as well as interfacial engineering is significant for achieving highperformance wearable PSCs.

Printing methods of wearable PSCs. The reproducibility from the lab-scale chip to largescale module is an inexorable trend for commercialization. Spin-coating is the most commonly used method, but it easily causes a waste of materials and is difficult to achieve large-scale high quality perovskite crystals. Blade coating the perovskite films, which


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regard as a prototype of further slot-die printing, is investigated (Figure 5a).28 Hest et al. use the methylamine/acetonitrile mixed solvent to fabricate the perovskite films via blade coating and slot-die printing. The crystal growth of perovskite is well controlled without the intermediate state during the printing and thermal annealing processes. As a result, the all slot-die coated flexible PSCs demonstrate a PCE of 14.1%. Moreover, the slot-die printing technique in roll-to-roll process is investigated. The key point for roll-to-roll printing is how to rapidly removal the precursor solvent without the anti-solvent treatment. Vak et al. develop a fully slot-die printing PSCs via the two-step method (Figure 5b).29 The pinhole-free PbI2 films are printed by gas-quenching treatment. And the hole and electron transporting layers are also fabricated via the slot-die coating. Thus, the best PCE of 11.96% is obtained under ambient conditions, indicating the possibility of mass production of flexible PSCs. Moreover, the one-step method for perovskite deposition is demonstrated. Galagan et al. introduce an additive of 2-butoxyethanol in perovskite precursor (Figure 5c).30 With this strategy, a uniform perovskite layer is printed with a suitable wettability. At the same time, the additive can accelerate the crystallization process, which leads high quality crystals. As a result, a manufacture of printable flexible


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PSCs with the stabilized PCE of 13.5% is realized. In addition, high-precision and patterned preparation of perovskite film is another important issue for wearable devices. Song et al. have reported that inkjet printing is promising for fabricating homogeneous perovskite film and devices.31

Encapsulation is usually the final step of printing process for wearable PSCs. From long-time stability, the sealing layer should prevent the moisture and oxygen from accessing into perovskite layer. Moreover, the whole device structure should be adapted to complex body movements. Recently, Song et. al. develop a “sandwish” structure for planar PSCs (Figure 5d).23 The PEDOT:PSS/PDMS top electrode is incorporated to replace the conventional metal electrode. Due to both of the top and bottom electrodes are soft polymer, the mechanical neutral surface of the device shifts from top electrode to the perovskite layer, which can release much more stress. Meanwhile, the hydrophobic PDMS exhibits good water and oxygen resistance to protect perovskite layer. Besides, the compactness of encapsulation is investigated by Weerasinghe et al (Figure 5e).32 The partially and completely encapsulated devices are tested under ambient conditions. It is


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found that the degradation of device performance is firstly caused at the interfaces. An adhesive contact between each layer is significant, especially for flexible devices. Moreover, due to the toxicity of Pb-based perovskite materials, the encapsulation layer must prevent the leakage of Pb element.

Standardized tests of wearable PSCs. Although great achievements about flexible and wearable PSCs have been reported, it still lacks a series standards of efficiency, environmental stability and mechanical stability for practical solar power sources. The efficiency of PSCs has a close relationship with the effective area, the area should be defined by a mask or an aperture. In order to sufficient power for a supply, the effective area should be over 1 cm2 for future researches on flexible PSCs chips. An international standard about efficiency testing conditions, just like the crystalline silicon photovoltaics, should be established.33 Furthermore, the reliability of wearable solar modules is also important. The continue efficiency of wearable solar modules under outdoor lighting is critical as well as under AM 1.5 G sunlight. The wearable PSCs should output enough power under cloudy or low-light conditions. The angular dependence of the PCE should


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also be considered. Flexible PSCs are more sensitive to temperature, light soaking, UVlight and moisture, etc. Even the encapsulated devices, the moisture and oxygen are more permeable to ingress into the flexible PSCs. In addition, a normative protocol for stability tests is needed, several attentions, for example encapsulation, ambient environment, light sources, cycling and transparency, should be considered. The authentic and dependable stability data is the basis for practical wearable power sources. Then, the mechanical stability tests also need to be standardized. Usually, the actual employment of wearable PSCs is usually not at a plane. So the efficiency under bending, stretching and twisting tests should be considered. Appropriate mechanical parameters should be developed to guide the design of devices, such as end-to-end distance, bending angle, bending radius and bending cycles for bending tests.34 As for the stretchable tests, the stretched length and proportion should be definite. More importantly, the mechanical state (under stressing or restored) and the testing position (center or edge) of the samples should be provided.


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Conclusion and outlook. In recent years, the wearable PSCs have been continuously blossomed, and provided remarkable opportunities for new application areas. The excellent efficiencies of flexible solar cells and modules have been over 18% and 10%, respectively. Some promissing applications for wearable PSCs can be expected, for example, attachments on bags, clothes, caps as well as the integration into watch straps and bangles. An assumptive 100 mW power output of the wearable PSCs can act as a supplementary source for daily electronics, such as smartwatch, mobile phone and fitbit etc. Apart from the above achievements in wearable device structure, flexible electrode, optimization of perovskite crystals and interfaces, several challenges are still remained.

(i) It is imperative to achieve a light weight for per watt, which indicates that both efficiency and stability require improvement compared with rigid devices. Currently, although textile PSCs provide the feasibility to integrate with cloth and skin, the planar device is the optimal choice due to its high efficiency and simple roll-to-roll processability. So the weight of substrate and sealing layer needs a reduction as much as possible. The ultra-thin transparent rubber and cellulose paper are promising candidates. Attaching


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patterned PSCs on the other parts of the device is also a good choice. Meanwhile, the crystal growth of perovskite on different substrates needs to accurately control. Due to the toxicity of Pb-based perovskite materials, it is important to fabricate high quality leadfree perovskite, such as FASnI3, crystals on wearable substrates.

(ii) Regarding the wearable conformability of devices, the flexibility of electrode and perovskite layer should be improved. The substitutes should synchronously satisfy high conductivity and transmittance with excellent flexibility. In addition, the stretchable and twistable capabilities of perovskite films are vital and fascinating concerns. Inspired from other wearable electronics, incorporation of the physical patterned structures, such as wire/cable patterns, origami structure, and bridge island design is a solution. On the other hand, reducing the Young modulus of perovskite crystals via new materials design and molecular synthesis could be another approach.

(iii) The large-area reproducibility via roll-to-roll printing is also significant. Anti-solvent treatment and vacuum-assisted method will meet challenges for the roll-to-roll printing process. A scalable printing method to achieve large-area high quality perovskite layer is


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highly desired. Air-assisted process, multicomponent solvent and infrared post-treatment incorporated in roll-to-roll printing might be meaningful researches in the future. The design of wearable module is also important. Software and hardware integration from other wearable devices should be considered.

In a word, it is still a long way to achieve commercial wearable perovskite photovoltaics. Future research should pay more attention to a comprehensive design philosophy from both perspectives of optoelectronics and mechanics. Particularly, the insight gained regarding wearable device structure and flexible perovskite materials would enable future integrated applications with supercapacitors, recyclable batteries and artificial intelligence equipment.


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AUTHOR INFORMATION

*E-mail: [email protected]

ORCID Xiaotian Hu: 0000-0001-5483-8800 Fengyu Li: 0000-0003-2481-6111Yanlin Song: 0000-0002-0267-3917

Notes The authors declare no competing financial interests. Readers are welcome to comment on the online version of the paper.

Biographies

Xiaotian Hu received his B.S. and M.S. degree from School of Materials Science and Engineering, Nanchang University in 2016. Currently he is a Ph.D. candidate in Institute of Chemistry, Chinese Academy of Sciences. His research interests include flexible solar cells, printable and wearable electronics.


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Fengyu Li is a full professor of Jinan University. He got his Ph.D. degree from Institute of Chemistry, Chinese Academy of Sciences in 2008. His research interests include photonic crystal materials, multi-analyte sensing, flexible electronics, printed assembly, 3D printing manufacture.

Yanlin Song received his Ph.D. degree from the Department of Chemistry at Peking University in 1996. He is currently a professor and director of Key Laboratory of Green Printing, Chinese Academy of Sciences. His research interests include information function materials, photonic crystals, printed electronics, and green printing materials and technologies.

His

lab

web

page

can

be

found

at

the

following

website:

http://ylsong.iccas.ac.cn/

ACKNOWLEDGMENT

Y. L. Song thank the National Nature Science Foundation of China (Grant Nos. 51773206 and 51473173), the National Key R&D Program of China (Grant Nos. 2018YFA0208501, 2016YFC1100502, 2016YFB0401603, and 2016YFB0401100), the K.C.Wong Education


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Foundation, and the External Cooperation Program of BIC, Chinese Academy of Sciences, Grant No. GJHZ201948.

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Figure 1. (a) Schematic of the standard structure of wearable PSCs. (b) The key points for achieving high performance wearable perovskite power source.


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Figure 2. (a) A representative planar structure of flexible PSCs. The photographs show the corresponding device chips. Reprinted from ref 6. (b) Structure of each layer in the double-twisted fibrous perovskite solar cells. The corresponding SEM images of


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perovskite (left) and Ag (right) nanowire network. Reprinted from ref 8. (c) Cross-sectional SEM image of the fiber-shaped PSCs based on the stainless steel. Reprinted from ref 9.


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Figure 3. The choice of substrates for the planar flexible PSCs: (a) based on polymer, reprinted from ref 11. (b) based on paper, reprinted from ref 12. (c) based on metal, reprinted from ref 13. Indium-free alternatives for transparent electrodes: (d) Metal mesh composite, reprinted from ref 11. (e) Conductive polymer, reprinted from ref 17. (f) Graphene, reprinted from ref 20.


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Figure 4. Crystallization control of perovskite and interfacial engineering. (a) The top-view SEM images of PVK films with and W/O DS. Reprinted from ref 22. (b) The top-view and cross-section SEM images of PVK films with and W/O SBS–PU. Reprinted from ref 23. (c) The AFM image of SnO2 on ITO/PET substrate. Reprinted from ref 27. (d) The SEM image of the NC-PEDOT:PSS layer and the corresponding energy level diagram of PSCs. Reprinted from ref 18.


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Figure 5. Large-area printing methods for PSCs. (a) Schematic representation of blade coating. Reprinted from ref 28. (b) Schematic illustration of slot-die coating with a gasquenching process for the fabrication of PbI2 layer. Reprinted from ref 29. (c) Photograph images of the roll-to-roll coated perovskite layer and an example of the manufactured


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flexible devices. Reprinted from ref 30. Encapsulation for PSCs: (d) PDMS sealing layer, reprinted from ref 23. (e) Schematic representation of “partial” and “complete” encapsulation architectures. Reprinted from ref 32.

Quotes

It is important to balance the photovoltaic performance with flexibility during the growth of perovskite crystals. The wearable PSCs should output enough power under cloudy or low-light conditions. The angular dependence of the PCE should also be considered. An assumptive 100 mW power output of the wearable PSCs can act as a supplementary source for daily electronics, such as smartwatch, mobile phone and fitbit etc Inspired from other wearable electronics, incorporation of the physical patterned structures, such as wire/cable patterns, origami structure, and bridge island design is a solution.


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Wearable Power Source: A Newfangled Feasibility for Perovskite

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