Intrinsically Stretchable Nanostructured Silver Electrodes for Realizing

Subscriber access provided by Georgetown University | Lauinger and Blommer Libraries

Article

Intrinsically Stretchable Nanostructured Silver Electrodes for Realizing Efficient Strain Sensors and Stretchable Organic Photovoltaics Yang-Yen Yu, Chien-Hsun Chen, Chu-Chen Chueh, Chun-Ying Chiang, Jang-Hsing Hsieh, ChihPing Chen, and Wen-Chang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06963 • Publication Date (Web): 01 Aug 2017 Downloaded from http://pubs.acs.org on August 1, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Intrinsically Stretchable Nanostructured Silver Electrodes for Realizing Efficient Strain Sensors and Stretchable Organic Photovoltaics

Yang-Yen Yu, a,b* Chien-Hsun Chen,a Chu-Chen Chueh, c Chun-Ying Chiang, a Jang-Hsing Hsieh, a Chih-Ping Chen, a* and Wen-Chang Chen c* a

Department of Materials Engineering, Ming Chi University of Technology, New Taipei City 243, Taiwan. b Department of Chemical and Materials Engineering, Chang Gung University, Guishan Dist., Taoyuan City 33302, Taiwan c Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan

*Corresponding authors. [email protected]

E-mail:

[email protected];

[email protected];

KEYWORDS: organic photovoltaic, stretchable, power conversion efficiency, strain sensor, hybrid electrode

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 28

ABSTRACT

In this study, a new hybrid electrode featuring a high gauge factor of >30, decent stretchability (100% of the original conductivity can be retained after 50-cycle stretching under a 20% strain without pre-strain treatment), high transmittance (>70%) across 400 to 900 nm, and a good sheet resistance (dozens of nanometers) owing to their large diameters (being greater than 50 nm and up to several hundreds of nanometers)20 and inferior deformability in response to large tensile strains as a result of their poor adhesion on the substrates.15 To power these deformable electronic devices, developing compatible energy source is compulsory. Regarding this demand, organic photovoltaics (OPVs) has appealed to the research community due to their conformal coating capability, decent flexibility, and high power-perweight capability, which serves as powerful candidates for the deformable energy applications.2122

Developing compatible electrodes for stretchable OPVs has been regarded as one of the most

challenging issues. At present, high power conversion efficiencies (PCEs) of 12% has been accomplished in OPVs with proven stability for emerging applications.23-28 To fulfill the


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

development of deformable OPVs, the whole device stack including the constituent chargetransporting interlayers and photoactive layer, substrate, and electrodes need to be engineered to possess robust stretchability (the ability to resist pulling). In this regard, it is still challenging to develop high-performance stretchable OPVs despite a few demonstration of flexible or compressible OPVs.29 As a point of note, several factors, such as defective microstructures, mismatched Young’s modulus (elastic properties) between constituent interlayers, and poor interfacial adhesion, have made the stretchable OPVs more susceptible to fail under tensile strain than the conditions subjected to compressive strain and bending tests.12 On the other hand, the electrodes based on conducting polymers can afford a better stretchability, for which poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS, PH1000) was the most commonly used material. For example, Bao and Lipomi et al. have used a modified PH1000 to simultaneously serve as anode and hole-transporting layer (HTL) to realize a stretchable OPV on pre-strained polydimethylsiloxane (PDMS) substrate. Despite showing high stretchability, the device only yielded a mediocre PCE of 1.2%.10, 30 Very recently, we utilized PH1000 electrode on a 3MTM tape as the transparent substrate to construct a stretchable OPV with a configuration of 3MTM tape/PH1000/PTB7:PC71BM/eutectic gallium-indium (EGaIn) and demonstrated a much improved PCE of 3.82%. More intriguingly, this device can retain 80% of its original PCE after 50 cycles of stretching under a 20% strain. In spite of the demonstrated decent conductivity (100~150 Ω square−1) and mechanical durability,31 the modified PH1000 still does not possess a satisfactory stretchability since it must rely on the structural buckling (constructed on a pre-strained substrate) to accommodate the excessive tensile strain.32


4

Page 5 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In this study, we describe a durable and deformable transparent electrode fabricated without the pre-strain treatment. We first adopted polyethyleneimine (PEI) as the metal-chelating layer and then fabricated the hybrid electrodes on a 3MTM tape with a multilayer structure of SiO2/PEI/Ag(12 nm)/modified PEDOT:PSS (Scheme 1). The 3MTM tape herein was used as an efficient stretchable and adhesive substrate and the thin solution-processed SiO2 layer was added to improve the light incoupling into the derived device. As for the inserted PEI layer, it can simultaneously improve the optoelectronic properties and stretchability of atop thin Ag film. Similarly, the Zonyl surfactant was added into PEDOT:PSS to enhance its mechanical stability.33 Consequently, such hybrid electrode was revealed to have high electrical conductivity (>105 S cm–1), high elastic moduli (ca. 102 GPa), high transmittance (>70%) across 400 to 900 nm, and decent stretchability (100% of the original conductivity can be retained after 50-cycle stretching under a 20% strain without pre-strain treatment), which is very suitable for serving as an efficient stretchable electrode.3 We manifested the good stretchability of this hybrid electrode resulted from the reversible phase separation endowed by the nano-cerebral morphology formed in Ag film (Scheme 1 a). When fabricating a resistive-type strain sensor using this hybrid electrode, a large gauge factor (GF ≥ 30) and stretchability (≥20%) can be obtained. This performance is rivaling with the results reported in the literatures.20 Since the hybrid electrode is constructed on a 3MTM tape, the whole device stack could be readily attached onto a human body to receive such information as the respiratory rate and the motion of physiological behavior, revealing its great merit in personalized health-care systems. Moreover, representative fullerene-based and all-polymer based OPVs using this electrode were fabricated and demonstrated superior stretchability. For the PTB7-th:PC71BM blend, the derived OPV can yield a decent PCEs of 6.0% along with high


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

deformability (surface buckling treatment as shown in Scheme 1 b was employed to reinforce the mechanical properties of the PTB7-th:PC71BM active layer) under a 20% tensile strain, for which 75% of its initial PCE can be retained after 50 deformation cycles. Meanwhile, the allpolymer OPV derived from the PTB7-th:N2200 blend can maintains over 96% of its original PCE after 50-cycles stretching (under a 20% strain) without using the wrinkled structure, benefitting from the better mechanical stretchability of N2200 over PC71BM. a)

b)

Scheme 1. Schematic representation of a) an intrinsically stretchable electrode and b) a derived stretchable OPV using surface buckling technology. Results and Discussion Optoelectronic properties of hybrid electrodes As discussed earlier, simultaneously high conductivity, mechanical durability, and high optical transmittance are the prerequisite properties for the electrodes used in stretchable OPVs.


6

Page 7 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Regarding the superior conductivity and ductility of Ag, we herein aim to develop a hybrid electrode based on ultra-thin metal film (UTMF). It has been documented that the conductivity and transmittance of an Ag UTMF will be largely declined when its thickness is lower than its corresponding percolation threshold (~10 nm) owing to the inhomogeneous film formation associated with the Volmer-Weber growth mode. 34-35 Hence, the optimization of the quality and film morphology of UTMF is the most critical part when developing a quailed UTMF-based electrode. Several methods have been reported to improve the optoelectronic properties of UTMFs to date.35-36 Among which, constructing a dielectric/UTMF/dielectric structure is the most widely adopted approach, for which solution-processed polymeric dielectric layers particularly attract our research interests. For instance, Lee et al. have employed a seeding layer based on the PEI polymer whose amino-functional groups can improve the compatibility with Ag and promotes its homogenous UTMF growth. Consequently, their fabricated hybrid electrode consisting of PEI (5 nm)/Ag UTMF (9 nm)/PEDOT:PSS (50 nm) can show a high visible transparency of > 90% and a Rs lower than 10 Ω square–1, facilitating the fabrication of efficient and flexible light-emitting diodes and OPVs.34 Considering the influence of the constituent interlayers’ thicknesses in relation to the resulting opto-electronic properties of the derived hybrid electrode, we herein first optimize the thicknesses of PEI and Ag layer on a 3MTM tape while fixing the thickness of modified PEDOT:PSS at 40 nm, for which 10% Zonyl surfactant was added to secure its mechanical properties,33 in order to guarantee the sufficient hole-transporting property and to avoid device shorting in the sequentially derived OPVs. Besides, this layer can also promote the light incoupling into the derived device.34


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 28

Hybrid electrodes with varied Ag thicknesses (9, 12, and 15 nm) on the PEI-coated substrates from different concentrations (0.3, 0.5, 1, 3, and 5 wt%) were accordingly studied. Table 1 and Fig. S1 summarizes their corresponding Rs and visible transmittance [T (%)] at a wavelength of 500 nm. As shown, Rs of the hybrid electrodes is clearly decreased as increasing the Ag thickness for all kind of PEI-coated substrates. Whereas, increasing the thickness of PEI seeding layer lowers the resulting Rs of the derived electrodes. Note that no electrical properties can be detected for some of the hybrid electrodes with a 9-nm-thick Ag film, which can be attributed to the aforementioned issue of inhomogeneous film formation especially on an elastomeric substrate whose dimensions might change under vacuum. The T(%) of the hybrid electrodes is clearly increased when using a thinner Ag film for all kind of PEI-coated substrates, wherein the T(%) of the 9-nm-thick Ag electrodes can be improved to 76%. However, it obviously showed an optimized thickness for the PEI layer in the studied hybrid electrode. These results reflect the improved compatibility at the substrate/Ag interface, which can be interpreted as the consequence of increased the chelating ability or nucleation inducing ability between Ag and PEI34, thereby improving the optoelectronic properties of the electrodes as discussed earlier. Table 1. Optoelectronic properties (Rs (Ω square–1) and T (%)) of the studied hybrid electrodes.

PEI (wt%)

0.3

0.5 N.A. (53%)

1

3

5

9nm

N.A. (53%)

1083±35 (67%) 526±8 (76%) 504±8 (66%)

12nm

87±13 (44%) 74±10 (48%) 53±8 (53%)

47±4 (61%)

42±4 (51%)

15nm

75±14 (38%) 63±9 (43%)

37±3 (50%)

33±2 (47%)

46±7 (47%)


8

Page 9 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


As mentioned, there is a trade-off between conductivity and transmittance of the UTMF. Besides, the reflectance of incoming light also adversely affects the overall transmittance of UTMF-based electrode. We therefore measured the reflectance of the fabricated 3 wt% PEI/12nm Ag hybrid electrode from the tape side. As shown in Figurer S2a, an average of 16% across 400 to 850 nm was observed. To mitigate this optical loss, we inserted an additional SiO2 layer at the PEI/substrate interface, for which it was similarly deposited through facile solution process and can serve as an effective antireflection layer as demonstrated in our previous work.37-38 The detailed process for this layer was described in the Supporting Information. As can been seen in Figure S2a, the reflectance of the hybrid electrode can be largely reduced to 6% after using this additional layer, which accordingly increase its overall transmittance from 60 to 70% as portrayed in Figure S2b. This result affirms the enhanced light incoupling effect introduced by the antireflective SiO2 layer. Presented in Figure S3 are the AFM images of the PEI-coated 3MTM tape structures, in which the used precursor concentration of PEI is varied. As shown, the surface roughness (Ra) for the 0.3, 0.5, 1, 3, and 5 wt% samples was 1.1, 1.1, 1.8, 3.0, and 3.6 nm, respectively. The corresponding thicknesses of the PEI layers (measured by AFM) were 14, 21, 37, 45, and 90 nm for the 0.3, 0.5, 1, 3, and 5 wt%, respectively. The corresponding AFM images of the sequentially evaporated Ag with a thickness of 12 nm (tape/SiO2/PEI/Ag) are presented in Figure 1. As shown, the Ra for the Ag UTMF varies from 10 nm to 34 nm when deposited on different PEI-coated tape substrates. We notice that the deformation of underlying elastomer during vacuum evaporation might contribute to the large Ra as observed herein. However, there is a clear trend that a thicker PEI seeding layer can afford the formation of a smoother Ag UTMF.


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a)

b)

c)

Ra=34nm

Ra =26nm

Ra =21nm

d)

e)

Ra =12nm

Ra =10nm

Page 10 of 28

Figure 1. Topographical AFM images (10 µm × 10 µm) of a 12-nm-thick Ag UTMF deposited on the PEI-coated tope substrates with varied PEI thickness of concentrations of a) 14 (0.3 wt%), b) 21 (0.5 wt%), c) 37 (1wt%), d) 45 (3 wt%), and e) 90 nm (5 wt%), respectively. Besides the changes in Ra, the textures of these evaporated Ag layers are also different. It suggests the changed nucleation behavior of Ag on these substrates, which can be traced to the PEI layers. As shown, the thinner PEI layer seems to afford morphology consisting of rough and uneven distribution of the large Ag strip, which might result from the large discrepancy in elastic moduli between the elastomer and Ag. As mentioned, the dimensions of the elastomer changed under high vacuum. When the elastomer recovered its originally structural dimensions back at atmospheric pressure, it could cause the morphological change of the Ag layer. Fortunately, this negative effect can be mitigated by increasing the thickness of PEI layer. As stated, the Ra of the deposited Ag layer is gradually decreased as the thickness of PEI layer increases. Moreover, it shows a more uniform morphology consisting of smaller cerebral patterns or granular-like


10

Page 11 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


domains on the thicker PEI layer. This result validates that the PEI as the seeding layer can effective reduce the percolation threshold of Ag UTMF and therefore improve its optoelectronic properties. Consequently, our fabricated hybrid electrode substrate derived form 3 wt% PEI/12nm Ag can show a high transmittance of > 70% in the visible range and a low sheet resistance of ~ 50 Ω square–1. Mechanical properties of hybrid electrodes To evaluate the potential of the fabricated hybrid electrode for the applications in deformable electronics, we first examined their mechanical properties by analyzing their Rss after 1, 5, 10, 20, and 50 deformation cycles of stretch/release from 0 to 20% tensile strain. Besides, optical microscopy (OM) was employed to trace the morphological changes after applying tensile strains. Note that we tested the mechanical properties without applying any pre-strain treatment. As summarized in Table S1, the hybrid electrodes prepared derived from thin PEI (1 wt%) layer displayed a poor stretchability regardless of the thickness of Ag layer. In contrast, the hybrid electrodes using thicker PEI layers (3 and 5 wt%) exhibited much better mechanical properties, wherein the sheet Rss almost remained intact after 50-cycle stretching tests. For example, the 5 wt% PEI/12-nm Ag electrode showed Rss of 42 ± 5, 45 ± 3, 45 ± 7, 45 ± 7, and 43 ± 6 Ω square–1 after 1, 5, 10, 20, and 50 cycles of deformation testing, respectively. The corresponding OM images of the fabricated electrodes after deformation test were presented in Figures S4-S6. As can be clearly seen, the 0.3 wt% PEI/12-nm Ag electrode showed severe cracks after just 1-cycle deformation (Figures S4a and b). Similar problem was encountered for the 0.3-1 wt% PEI/9-15 nm Ag electrodes (Figures S4-S6) although the degree


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 28

of cracks was reduced after increasing the thickness of PEI layer from 14 to 37 nm as shown in Figure S4a-g. Whereas, further increasing the thickness of PEI layer to over 45 nm could afford better stretchability. As shown in Figures S4h–o, there are no cracks observed in the films (12-nm Ag) even after 50 cycles of stretch/release under a 20% strain. Same behavior was evident for the hybrid electrodes using 9- and 15-nm Ag films (Figures S5 and S6). These observations are consistent with the changes in conductivity after stretching tests. That said, the failure in Rs is rooted in the appearance of cracks. To elucidate the mechanism for the decent stretchability of these hybrid electrodes, the surface AFM study subjected to various strain conditions of the electrodes derived from 45-nmthick PEI layer were investigated. As shown in Figure 2a–d, when applying the tensile strains of 5, 10, 15, and 20% to a 3 wt% PEI/12-nm Ag electrode, the porous size and density between the Ag cerebellar patterns is increased and is proportional to the magnitude of applied strains. Accordingly, the Rs of the electrode increased from 47 ± 4 Ω square–1 initially to 110 ± 26 and 432 ± 48 Ω square–1 under stretching at 10 and 20% elongation, respectively. However, these hybrid electrodes will retrieve their original morphology after the stretching tests as shown in Figures 2e and 2f, thereby recovering the originally low Rss. The cartoon images provided in Figure 2g illustrates the possible mechanism for the superior stretchability of our fabricated electrodes, wherein the reversible phase separation was endowed by the nano-cerebral morphology formed in Ag layer. This phenomenon is believed to correlate with the chelating interactions between the PEI and the Ag clusters. Nevertheless, different to the similar results reported in the literature,34 the employed PEI layer herein not only functioned as a


12

Page 13 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nucleation-induced layer but also as acted as a binding layer to improve the adhesion of Ag on the tape substrate, thereby simultaneously securing the electronic properties of the derived hybrid electrodes after the stretching tests and the overall stretchability. As shown in Scheme 1a, the functional amine group in PEI serving as a ligand could donate an electron lone pair to the Ag atom to form plausible coordination between them. It therefore can be envisioned that the first few layer of Ag atoms are covalently bonded onto the PEI surface, ensuring the sufficient adhesion between the sequentially deposited Ag and the PEI.39-40 In this study, we used an 3M tape as the efficient stretchable adhesive substrate, embedded a solution-processed SiO2 layer to increase the optical transparency, applied a thick PEI layer (45 nm) to improve the electrode’s stretchability and optoelectronic properties, and used the Zonyl surfactant as a plasticizer in the PEDOT:PSS to improve its mechanical stability. By combing all these functional layers, we eventually describe a promising candidate electrode for stretchable electronics.

a)

b)

c)

Ra =28nm

Ra=26nm

Ra=26nm

d)

e)

f)

Ra=27nm

Ra=21nm

Ra=23nm


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 28

g)

Figure 2. AFM topographical images (10 µm × 10 µm) of 3M/SiO2/3wt% PEI/12-nm Ag electrodes subjected to a) 5, b) 10, c) 15, and d) 20% tensile strain; after e) 20 and f) 50 strain/release (20%) cycles; g) cartoon representation of the stretchability mechanism of our electrodes. In a previous report, a hybrid electrode consisting of PEI (5 nm)/flat Ag (9 nm)/PEDOT:PSS (50 nm) has been used to fabricate a transparent flexible OPV device.34 However, to realize deformable OPVs, the whole structure—the transparent substrate, electrodes, active layers, and interfacial layers—must display robust stretchability (the ability to resist pulling). It apparently faces a greater manufacturing challenge to fabricate a stretchable OPV. To realize such hybrid electrode for stretchable electronics applications, we herein optimized the hybrid structure by tuning the thicknesses of both Ag layer (9, 12, and 15 nm) and PEI layer (14, 21, 37, 45, and 90 nm) (Table 1). Primarily, no electrical property was observed for the hybrid electrode using a 9nm-thick Ag film and a 14-nm-thick PEI layer. It is because the dimensions of the employed elastomeric substrate would change under vacuum, as evidenced by the distinct Ag deposition behavior prepared on the regular high-density substrates (glass or PEN). While the PEI thickness reached the threshold value of 45 nm, the conductivity of the hybrid electrode is increased along


14

Page 15 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with an enhanced mechanically stretchable property (Table S1) due to the nano-granular-like morphology formed in Ag. For the optimized electrode, ~100% of the original conductivity can be retained after 50-cycle stretching under a 20% stain without pre-strain treatment, highlighting its potential for realizing deformable optoelectronic devices. The stability of the fabricated electrode towards air and moisture is next examined since it is an important index for practical applications. The optimized electrode was stored in a box with a related humidity of 75% and we measured its conductivity over various periods of storage time in air. The resistance of electrode showed negligible change after 2-week storage, suggesting its respectable robustness against air and moisture. Performance of strain sensors using the fabricated hybrid electrodes To realize the efficacy of our fabricated hybrid electrodes in stretchable optoelectronics, we first fabricated a resistive-type strain sensor using a 3 wt% PEI/12 nm Ag electrode. The GF [(∆R/R0)/Ɛ, where Ɛ is the strain (∆L/L) and R is the resistance] was first recorded by measuring the resistance under variable strains.41 Figure 3a displays the relationship between the resistance and strain, in which high GFs (31 at 15% strain; 40 at 20% strain) can be clearly evaluated. In principle, a good strain sensor must simultaneously display high stretchability, large GFs for high sensitivity, good response and recovery times, and robustness.20, 42-44 Our preliminary results suggested a high GF and reasonable stretchability can be achieved using our fabricated hybrid electrode. We further examine its practical applications in wearable sensors to monitor the respiratory rate and the motion of physiological behavior. Figure 3b exhibited the results of resistance testing on an index finger, wherein an increase in resistance occurred when the finger was bent thanks to the morphological change in Ag


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

UTMF in response to the tensile strain. As shown, the derived sensor can sensitively monitor finger motion under different bending degrees. Note that similar values of (R – R0)/R0 in the range from 1.50 to 1.77 can be obtained after 50 cycles of stretching testing. As a result, the relative resistance under finger motion was kept almost constant for the first 10 cycles (from 80 to 82 Ω at straight and from 200 to 210 Ω at bending, respectively) and was only increased slightly over the next 50 cycles (Figure 3c). Figure 3d showed the results of sensing the respiratory rate, wherein the resistance clearly changes with inspiratory breathing. The relative resistance under inspiratory breathing was almost constant for inhaling (from 56 to 60 Ω) and for exhaling (from 120 to 135 Ω). After 50 cycles of stretching tests, the value of (R – R0)/R0 only showed a small increase from 1.14 to 1.25. It is worthwhile to note that there is a typically trade-off between high GF (sensitivity) and stretchability for strain sensors.20 High sensitivity of a strain sensor generally requires considerable and reversible structural deformation, while high stretchability demands the morphology to be intact after large degrees of elongation. Therefore, it is challenging to simultaneously meet these two requirements.45-46 Unlike most stretchable strain sensors reported previously, our fabricated strain sensor possesses a high GF and show decent stretchability (>20%) simultaneously, suggesting its great merits in the stress-sensing applications.20 Further device optimization is currently under investigation in our group.


16

Page 17 of 28

a)

b)

25

2.0

20

1.6

(R-R0)/R0

(R-R0)/R0

15 10

1.2 0.8 0.4

5

0.0

0 0

5

10

15

20

25

0

5

Strain (%)

10

15

20

Time(s)

c)

d)

4

2.0 1.6

(R-R0)/R0

3 (R-R0)/R0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

1.2 0.8 0.4

1

0.0

0 0

30

60

90

120

150

0

50

100

150

200

250

Time(s)

Time(s)

Figure 3. a) Relationship between resistance and strain. b, c) Changes in resistance under b) different bending degrees and c) continuous finger motion. d) Relative resistance under inspiratory breathing. 2.4 OPV performance and mechanical properties With the successful demonstration of an efficient strain sensor using our fabricated hybrid electrode, we further investigate its efficacy in fabrication efficient stretchable OPVs. We herein used the representative PTB7-th:PC71BM blend as the studied bulk-heterojunction (BHJ) layer


17


and

the

studied

device

is

fabricated

in

tape/SiO2/PEI/Ag/PEDOT:PSS/PTB7-th:PC71BM/EGaIn.

a The

of

3MTM

regarding

device

configuration details

fabrication is described in the Experimental Section. Note that we used ImageJ software to calculate the actual active areas of the devices. Figure 4 a and S7 present the current density–voltage (J–V) characteristics of the optimized stretchable OPV devices and the relevant photovoltaic parameters, including open-circuit voltage (Voc), short-circuit (Jsc), and fill factor (FF), were summarized in Table 2. For a control device, a PCEAVG of 5.32% with a Voc of 0.76 V, a Jsc of 12.41 mA cm–2, and a FF of 0.56 can be obtained. Note that this lower performance than the results reported for PTB7-th:PC71BM BHJ in the literature is attributed to the lower transmittance and conductivity of our hybrid electrode than those of the conventional ITO–based electrodes.47 a)

b) Pristine

5

10

20

0

Current density(mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

pristine cycle 5 cycle 10 cycle 20 cycle 50

-2

-4

-6

-8

-10

-12 0.0

0.2

0.4

0.6

0.8

50

Voltage(V)


18

Page 19 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c)

Figure 4. a) J–V curves and b) OM images (40×) of stretchable PTB7-th:PCBM-OPV devices that had been subjected to various numbers of deformation cycles; and c) cartoon representation of PCBM and all-polymer based devices before (left) and after subjected to 50 cycles of stretching (right). Table 2. OPVs parameters of devices based on the various testing conditions.

Entry

Jsc (mA cm−2)

Voc (V)

FF (%)

PCE(%)

1-0

12.41±0.54

0.76±0.02

0.56±0.07

5.32±0.58

1-5

11.60

0.76

0.53

4.67

1-10

11.03

0.74

0.49

3.98

1-20

10.89

0.76

0.35

2.90

1-50

10.33

0.73

0.36

2.71

2-0

12.02±0.14

0.78±0.00

0.61±0.02

5.69±0.25

2-5

12.11

0.78

0.62

5.83

2-10

11.82

0.78

0.58

5.37


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 28

2-20

11.78

0.76

0.57

5.13

2-50

11.43

0.73

0.54

4.47

1− Device without the pre-strain treatment; 2− device with the pre-strain treatment To understand the stretchability of our fabricated OPVs, their mechanical properties were then investigated. First, the device was deformed at a 20% strain and then was released to the original state, followed by different numbers of cycles. Figure S7a plots the J–V characteristics of the OPVs after being subjected to 5, 10, 20, and 50 stretching cycles. The pristine cell had a PCE of 6.0%, with a value of Voc of 0.78 V, a value of Jsc of 12.0 mA cm–2, and an FF of 64%. After 5, 10, 20, and 50 stretching cycles, the corresponding PCE for the device is 4.67, 3.98, 2.90, and 2.71%, respectively. This result indicates an unsatisfactory stretchability, for which only 45% of its initial PCE was kept after 50 stretching cycles under a 20% strain. The loss in efficiency mainly stems from the quickly degraded FF. The main reason can be attributed to the poor stretchability of the BHJ layer as evidenced in Figure S7b, wherein severe cracks can be clearly observed after a few stretch/release cycles probably due to the brittleness of PC71BM.48 In this regard, we adopted the strategy of using a surface buckling structure to improve the mechanical properties of the fabricated OPVs provided this scaffold can relieve the stress to mitigate its impact on the BHJ (Scheme 1).30 Fig. 4a showed the J–V curves of stretchable PTB7-th:PCBM-OPV devices with pre-strain (20%) treatment. The PCEs of the pre-strain devices measured after stretching cycles under a 20% strain were greatly improved, in which this device delivered PCEs of 5.83, 5.37, 5.13, and 4.47 % after 5, 10, 20, and 50 tensile-release cycles as summarized in Table 2 (entry 2-0). Over 75% of its initial PCE can be retained after 50 stretching cycles from 0 to 20%. Figure 4b presents their corresponding OM images after different cycles of tensile-release tests under a 20% strain. We observed cracks on site after 20


20

Page 21 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


cycles of the tensile-release test, manifesting the efficiency of surface buckling scaffold to reinforce the mechanical properties of BHJ layers. Besides the device engineering, we, on the other hand, replaced PC71BM in the BHJ composition to intrinsically improve the mechanical property of BHJ. Recently, Kim et al. demonstrated the better ductility of all-polymer blends than the fullerene-based counterparts.49 This result inspires us to fabricate an all-polymer OPV using our fabricated electrode. We herein use the representative polymeric acceptor, N2200, to pair with PTB7-th to form the targeted BHJ. In order to realize the improvement of the intrinsically mechanical property of the allpolymer blend, we fabricate the OPV without using pre-strain scaffold. Intriguingly, as portrayed in Figure S7c, the fabricated PTB7-th/N2200 OPV showed a respectable stretchability. This device displayed PCEs of 2.02, 2.05, 2.00, and 1.96 % after 5, 10, 20, and 50 cycles of the tensile-release test. That said, the device can retain over 95% of its initial PCE after 50 stretching cycles from 0 to 20%. Figure S7d presents their corresponding OM images, wherein no clear cracks on site appear after 50 stretch/release cycles. Figure 4c showed the cartoon representation of PCBM and all-polymer based devices. Because of the brittleness of PC71BM, when the number of stretches and the strain exceeds the strength of the entanglement of the PTB7-th molecular chain, it will produce cracks, and crack propagation in the blend film will result in the failure of performance. In all polymer device, the entanglement of polymer chains between the PTB7-th and N2200 will reinforcement the mechanical property of BHJ layer thus the device retained its initial PCE after 50 cycles of strain from 0 to 20%. Nevertheless, there is still considerable room to improve in our device performance, which warrants further investigation and device optimization, such the substitution of EGaIn,


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 28

employment of proper interfacial layer, and better targeted non-fullerene BHJ systems, which is currently under investigation in our group. Conclusion We have developed a highly stretchable and transparent hybrid Ag electrode on an elastomeric tape via rationally designed nanostructures and applied to realize efficient strain sensors and stretchable OPVs. The hybrid electrode is consisted of a multilayer structure of tape/SiO2/PEI/thermally evaporated Ag (12 nm)/modified PEDOT:PSS, wherein the SiO2 layer serves as an anti-reflective layer to improve the light incoupling into the derived devices while the PEI layer serves as a chelating agent to improve the mechanical and electronic properties of the Ag UTMF and simultaneously acted as an adhesive layer to increase the adhesion between Ag and the tape. We demonstrated that this electrode can simultaneously possess a high GF (>30), a decent stretchability (100% of its original conductivity retained after 50 cycles of stretching at 20% strain), visible transparency (>70% at wavelengths from 400 to 900 nm), and low sheet resistance (

Intrinsically Stretchable Nanostructured Silver Electrodes for Realizing

Recommend Documents