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Co-optimization of Adhesion and Power Conversion Efficiency of Organic Solar Cells by Controlling Surface Energy of Buffer Layers Inhwa Lee, Jonghyeon Noh, Jung-Yong Lee, and Taek-Soo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10398 • Publication Date (Web): 29 Sep 2017 Downloaded from http://pubs.acs.org on October 1, 2017

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ACS Applied Materials & Interfaces

Co-optimization of Adhesion and Power Conversion Efficiency of Organic Solar Cells by Controlling Surface Energy of Buffer Layers Inhwa Lee1, Jonghyeon Noh2, Jung-Yong Lee2*, and Taek-Soo Kim1* 1

Department of Mechanical Engineering, KAIST, Daejeon, 34141, South Korea

2

Graduate School of Energy, Environment, Water and Sustainability (EEWS), KAIST, Daejeon, 34141, South Korea

KEYWORDS: interfacial fracture energy, surface energy, work of adhesion, buffer layer, organic solar cell

1

ACS Paragon Plus Environment


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ABSTRACT Here, we demonstrate the co-optimization of the interfacial fracture energy and power conversion efficiency (PCE) of poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl2′,1′,3′-benzothiadiazole)] (PCDTBT)-based organic solar cells (OSCs) by surface treatments of the buffer layer. The investigated surface treatments of the buffer layer simultaneously changed the crack path and interfacial fracture energy of OSCs under mechanical stress and the work function of the buffer layer. To investigate the effects of surface treatments, the work of adhesion values were calculated and matched with the experimental results based on the Owens-Wendt model. Subsequently, we fabricated OSCs on surface-treated buffer layers. In particular, ZnO layers treated with poly [(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9– dioctylfluorene)] (PFN) simultaneously satisfied the high mechanical reliability and PCE of OSCs by achieving high work of adhesion and optimized work function.

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INTRODUCTION Organic solar cells (OSCs) based on polymer-fullerene bulk heterojunction (BHJ) structures have been highlighted as potential power source candidates for wearable electronics because of their outstanding advantages including solution-based processing1, light weight2, and strong resistance to deformation.3-4 Although many efforts have been made to achieve both high power conversion efficiency (PCE)5

and stability under illumination6-7, BHJ layers are known to have poor mechanical

reliability due to the low interfacial fracture energy, Gc, between the BHJ and buffer layers8-9. The associated weak adhesion can cause mechanical failure, even during fabrication processes on flexible substrates. In addition, it can induce large deformations, such as delamination and cracking, upon bending, folding, and stretching of the devices10. Since this weak mechanical robustness critically limits the reliable integration of BHJ layers into wearable electronics, many researchers have attempted to enhance the weak adhesion and cohesion of P3HT based BHJ layers by controlling the molecular weight11-12, film stacking order of buffer and active layers13, mixing ratio between donor and acceptor8, and processing methods and annealing conditions8-9, 13. However, these studies have focused on controlling active layers to improve the cohesion energy of within BHJ layers. Therefore, it has been challenging to co-optimize the interfacial fracture energy and PCE simultaneously. Also, because most P3HT based BHJ layers have weak cohesion, it is difficult to conduct a quantitative study on interfacial fracture energy between a buffer layer and an active layer. Energy level adjustments between the organic active layer and metal electrodes are often required through surface treatments of buffer layers, which can improve carrier extraction and device performance14. Interfacial engineering has typically been performed by incorporating 3



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interface dipoles that can provide delicate control of the energy level on buffer layers15. In particular,

poly

[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9–

dioctylfluorene)] (PFN)16, ethoxylated polyethylenimine (PEIE)17, and UV-ozone (UVO)18 treatments have been widely adopted for the modification of buffer layers such as poly(3,4ethylenedioxythiophene):poly(styrenesulfonate)

(PEDOT:PSS)

and

zinc

oxide

(ZnO).

Meanwhile, such modification of buffer layers on a molecular level affects the interfacial fracture energy between the buffer and photoactive layers, because the surface energy of the layers may change. Hence, proper surface treatments should be carefully applied to simultaneously optimize efficiency and Gc. In this study, we demonstrate the co-optimization of both the Gc and PCE by an appropriate surface treatment that adjusts the surface energy. Specifically, relatively hydrophobic buffer layers exhibit much stronger Gc to a poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT): [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) layer than hydrophilic surfaces by forming strong molecular attraction, because the surface energy of the active layer has a high dispersion component. It should be noted that this interfacial phenomenon can be explained by not only the polar but also the dispersion contribution to surface energy. To clarify the effect of surface energies on Gc, we measured the surface energy of each buffer layer in terms of dispersion and polar components. Moreover, the work of adhesion, Wa, was calculated based on the Owens-Wendt model and matched with the experimental results. Consequently, the theoretical predictions could be matched with the experimentally measured trend of Gc. We fabricated OSCs to investigate the surface treatment effects on PCE in order to provide a guideline for interlayer selection to achieve efficient and

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reliable OSCs. Here, we demonstrate that utilizing PFN-treated ZnO as the buffer layer is the optimal method to simultaneously satisfy the high mechanical reliability and PCE of OSCs.

RESULTS AND DISCUSSION Adhesion specimen preparation and test. As shown in Figure 1a, PCDTBT and PC71BM were employed in the active layer of OSCs as the donor and acceptor, respectively. To investigate the dependency of Gc on the surface energies, adhesive fracture between the buffer and active layers is required. Among high-performance donor material polymers, PCDTBT is a representative amorphous polymer with stronger cohesion than adhesion to adjacent layers by interfering with the phase separation.19-20 In addition, we employed PEDOT:PSS and ZnO, which are hydrophilic and relatively hydrophobic materials, respectively, as buffer layers to investigate the effect of surface energies on Gc. Figure 1b shows that PCDTBT and PC71BM are hydrophobic, with high CAs of more than 90°. We also confirmed that ZnO is relatively hydrophobic (CA: 55°) compared to the hydrophilic PEDOT:PSS (CA: 14°).

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Figure 1. a) Chemical structures of active materials poly[N-9′-heptadecanyl-2,7-carbazole-alt5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] and [6,6]-phenyl-C71-butyric acid methyl ester (PCDTBT and PC71BM) and buffer materials poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) and zinc oxide (PEDOT:PSS and ZnO); b) Contact angles (CAs) of water on active and buffer materials; c) Photograph of specimens for double cantilever beam (DCB) testing; d) Schematic of DCB specimen layers of Ag/active layer/buffer layer/indium tin oxide (ITO)/glass; e) Schematic of DCB specimens for measuring the interfacial fracture energy.

The Gc values at the interfaces between active and buffer layers were characterized using double cantilever beam (DCB) tests. The DCB test was adopted because it has been shown to be useful for the measurement of Gc for various materials, including graphene21, nanoparticle thin films22, and fuel cells.23 To fabricate specimens for DCB tests, a 1 inch × 1 inch indium tin oxide (ITO)/glass substrate was cut into three pieces (Figure 1c), and each piece was sandwiched by a dummy

glass

substrate

using

epoxy.

The

specimens

were

composed

of

Ag/PCDTBT:PC71BM/buffer layers, as shown in figure 1d. Figure 1e shows a schematic illustration of DCB testing. By pulling the specimen, multiple loading/crack-growth/unloading cycles were performed to measure the crack length, a, and Gc (J/m2), of PCDTBT on the buffer 6


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layers. The measured Gc is defined as the macroscopic work of fracture per unit area to separate two layers.24

Interfacial fracture energies of PCDTBT:PC71BM layers with different buffer layers. Typically, appropriate buffer layers between the photoactive layer and electrodes improve the performance of OSCs by preventing recombination and assisting carrier transportation25-26. PEDOT:PSS and ZnO are well characterized and representative buffer materials for normal and inverted solar cells.27-30 Figure 2a summarizes the measured Gc values for PCDTBT:PC71BM layers on the two buffer layers above. It was observed that the Gc of ZnO is approximately twice as high as that of PEDOT:PSS. Not only was Gc higher, the delamination path was also different; in the case of PEDOT:PSS, the interfacial fracture was observed between PEDOT:PSS and the active layer (Figure 2b). For the ZnO sample, on the other hand, the fracture occurred between the active layer and the Ag electrode (Figure 2c).

Figure 2. a) Interfacial fracture energies on PEDOT:PSS and ZnO buffer layers. The delamination path changes depending on the use of b) PEDOT:PSS and c) ZnO buffer layers.

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The delamination interfaces were examined by analyzing the detached surfaces with Raman spectroscopy, as shown in Figure S1. For the PEDOT:PSS buffer layer (Figure S1a and b), the Raman spectra of the delaminated surfaces at the active layer/PEDOT:PSS interfaces were similar to those for PCDTBT31 and PEDOT:PSS32. The data for the ZnO buffer layer (Figure S1c and d) also indicate that delamination clearly occurred at the interfaces between Ag and the active layer, because the Raman spectra showed a high resemblance to the spectra of Ag33 and PCDTBT31.

Controlling surface energies of buffer layers. To understand the factors that cause Gc and delamination path changes, we first investigated the surface roughness of the buffer layers and morphological changes in PCDTBT:PC71BM on the buffer layers (Figure S2). Both buffer layer surfaces had similar surface roughness values (PEDOT:PSS: 2.14 nm and ZnO: 2.47 nm). Also, it was confirmed using time-of-flight secondary ion mass spectrometry (TOF-SIMS) data that the active layer was homogeneously distributed, regardless of the buffer layer, without vertical phase separation (Figure S2c and d). These analyses suggest that the variations in Gc and delamination paths did not result from changes in roughness or morphology. To examine the effects of surface energies on Gc, the ZnO surface was transformed to a relatively hydrophobic surface by PFN treatment and to a relatively hydrophilic surface by UVO and PEIE treatments. These treatments are widely used to align energy levels and enhance charge collection and extraction for high-performance OSCs17, 34-36. Figure 3a shows the CAs of ZnO surfaces modified by the above treatments. The CA of 54.89° for bare ZnO was changed to 62.15°, 14.84°, and 10.98° by PFN, UVO, and PEIE treatments, respectively. In addition, Gc varied significantly depending on the treatment, as shown in Figure 3b. While the PFN treatment 8


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increased Gc to 1.66 J/m2, the hydrophilically modified surfaces exhibited decreased Gc values, similar to that of PEDOT:PSS (Figure 2a). Interestingly, the delamination path also switched from the electrode to the ZnO side when the ZnO surface was hydrophilically treated. Figure 3c presents the Gc and CA values for ZnO, PEDOT:PSS, and treated ZnO surfaces, implying that the wettability between two materials significantly affects Gc.

Figure 3. a) CAs of water of ZnO surfaces treated with poly [(9,9-bis(3'-(N,Ndimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)] (PFN), UV-ozone (UVO), and ethoxylated polyethylenimine (PEIE). The inset images on the right and left are water droplets on bare ZnO and PEIE-treated ZnO, respectively; b) Interfacial fracture energy of ZnO specimens treated with PFN, UVO, and PEIE; c) Interfacial fracture energy comparison according to CAs.

Enhancing mechanism of surface treatments. To understand how the surface energies of two facing surfaces affect Gc, we calculated the work of adhesion, Wa,37-38 defined as the required energy to separate a unit area of the interface to create new surfaces. It is known that Gc is a result of Wa and a function φ that characterizes the temperature and rate-dependent viscoelastic effect including plastic energy dissipation of materials.38-39 Previous studies revealed their multiplicative relationship as follows:38-39 Gc = Wa + (Wa × φ)

(1)

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However, the actual behavior is much more complicated than Equation 1, because the viscoelastic effect of the interface cannot easily be separated from the deformation of the bulk material during crack propagation.39 The viscoelasticities of PEIE and PFN are also not considered here because of the very thin thicknesses of around 7 nm as shown in Figure S3 and faster displacement rate at crack tip.40 To calculate Wa, we used the formula derived from the Owens-Wendt model:40 Wa = 2 (γ1d γ2d)1/2 + 2(γ1p γ2p)1/2

(2)

In Equation 2, the surface energy γ (including γ1 and γ2) is a sum of polar γp and dispersion (nonpolar) γd components. The γp component results from permanent intermolecular forces by permanent and induced dipoles and hydrogen bonding, whereas the γd component is mainly caused by instantaneous dipoles. Table 1 shows the measured CAs on various buffer and active layers and corresponding surface energies. For measurement of the wetting parameters, water and diiodomethane (DM) were used with both polar γLp and dispersion γLd components (water: γLp = 50.30 and γLd = 22.85 mJ/m2; DM: γLp = 2.30 and γLd = 48.50 mJ/m2). From the measured angles, θ, the polar γp and dispersion γd components were calculated using Equation 3, derived from Young’s equation and the Owen-Wendt model:41

γL(1 + cos θ) = 2 (γd γLd)1/2 + 2(γp γLp)1/2

(3)

The work of adhesion is determined by the corresponding interactions between the γp and γd components of the buffer and the active layers. The surface energies in Table 1 show that the PCDTBT material of the photoactive layer had a noticeably higher dispersion fraction compared to the polar term. The active layer had a low polar term of 0.39 mJ/m2, which means that it interacts strongly with layers that have high dispersion surface energies. Therefore, even though 10


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hydrophilic layers such as PEDOT:PSS and PEIE-treated ZnO have higher surface energies than un-treated ZnO and PFN-treated ZnO, the relatively high γp components of hydrophilic surfaces minimally contribute to the Wa a to the active layer.

Table 1. Contact angles of active and various buffer layers by water (H2O) and diiodomethane (DM), as well as calculated total surface energies and in polar and dispersion terms from the measured contact angles. Surface energy [mJ/m2] γs γsd

Contact angles [°] H 2O DM

γsp

PCDTBT

91.10 ± 0.43

33.89 ± 0.69

43.14

42.75

0.39

PEDOT:PSS

10.35 ± 0.37

26.79 ± 0.82

72.76

30.03

42.73

ZnO

54.89 ± 3.14

18.46 ± 0.23

54.17

39.27

14.90

27.65 ± 1.07

26.32 ± 0.43

66.34

31.76

34.58

69.58 ± 2.24

17.8 ± 0.70

49.47

43.27

6.20

14.02 ± 0.38

28.24 ± 1.18

71.17

29.80

41.37

79.90 ± 0.76

25.22 ± 0.75

40.95

37.27

3.68

PEIE-treated ZnO PFN-treated ZnO UVO-treated ZnO Ag

Figure 4a shows the calculated values for Wa to the active layer for various buffer layers. For the hydrophilic PEDOT:PSS as well as PEIE- and UVO-treated ZnO surfaces, Wa was approximately 79-81 mJ/m2, but Wa increased to 86-89 mJ/m2 for the hydrophobic ZnO and PFN-treated ZnO surfaces. Indeed, the trend of the Wa values matched that of the Gc values that were experimentally measured, as shown in Figure 4b. For the hydrophobic surfaces, delamination occurred at the interface between the active layer and Ag, where Wa is 82.23 mJ/m2. Thus, the actual Gc at the interface between the active and hydrophobic buffer layers was expected to be higher than the measured Gc.

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Figure 4. a) Calculated work of adhesion of PCDTBT to various buffer layers such as PEDOT:PSS, ZnO, PEIE-treated ZnO, PFN-treated ZnO, and UVO-treated ZnO; b) Interfacial fracture energy as a function of the work of adhesion; Schematic mechanism describing the effect of c) UVO- and PEIE-treated ZnO surfaces and d) untreated ZnO and PFN-treated ZnO surfaces on interfacial fracture energies and crack paths.

The Wa between two layers allows prediction of where the crack paths would develop; first, when surfaces become hydrophilic by UVO or PEIE treatment, the molecular interactions between the treated surface and active layer becomes weak due to the low γd as opposed to the high γd of PCDTBT. As a result, the delamination occurs at the buffer/active interface (Figure 4c). However, in untreated or PFN-treated ZnO surfaces, the relatively strong adhesion of the buffer layer to the active layer causes delamination to occur between the active and Ag layers (Figure 4d).

Correlation with power conversion efficiency of organic solar cells. Next, we fabricated OSCs to ascertain how surface treatments such as PFN and PEIE affect their PCE. As shown in 12


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the schematic illustration in Figure 5a, the inverted structures consisted of ITO/treated (or untreated) ZnO/PCDTBT:PC71BM/MoO3/Ag. Figure 5a shows the current density-voltage (J-V) characteristics of OSCs using untreated and PEIE- and PFN-treated ZnO measured under 1,000 W·m-2 air mass 1.5 global (AM 1.5 G) illumination. While devices using untreated ZnO had a PCE of 5.89 %, the PCE values of PCDTBT:PC71BM devices treated with PEIE and PFN were enhanced by approximately 20 % to 6.80 and 7.11 %, respectively. As shown in Table 2, the PCEs of PEIE- and PFN-treated ZnO devices increased due to improved open circuit voltage (Voc) and fill factor (FF) values. PEIE and PFN are commonly used materials to improve the performance of OSCs by modifying the interface of the charge extraction layer (ZnO).16, 42 The origin of the improvement of treated OSCs can be explained with work function matching through the formation of interfacial dipoles at the ZnO/active interfaces. It is well known that PEIE and PFN form interfacial dipoles on ZnO surfaces and reduce the work function.6, 17, 36 As shown in Figure 5b, the work functions of untreated ZnO and PFN- and PEIE-treated ZnO were 4.44, 3.57, and 3.70 eV, respectively. As the interfacial dipoles shifted a vacuum level, the highest occupied molecular orbital (HOMO) level of untreated ZnO changed from 7.51 to 6.3 and 6.57 eV in PFN- and PEIE-treated ZnO, respectively. Therefore, the ZnO treatments improved the performance of the devices through enhanced charge transfer and extraction.

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Figure 5. a) Device structures (shown in the inset) and J-V characteristics of PCDTBT:PC71BM based organic solar cells (OSCs) with untreated and PEIE- or PFN-treated ZnO characterized under AM 1.5G 100 mW/cm2 illumination; ultraviolet photoelectron spectroscopy (UPS) spectra of b) the secondary edge region and c) the highest occupied molecular orbital (HOMO) of untreated and PEIE- or PFN-treated ZnO; d) Schematic illustration of PFN-treated ZnO buffer layer selection to satisfy efficiency and reliability of OSCs. Table 2. Representative photovoltaic performance parameters for PCDTBT:PC71BM OSCs. PSCs

Buffer layer Untreated ZnO

PCDTBT: PC71BM

PEIE-treated ZnO PFN-treated ZnO

PCE [%] 5.45±0.50 (5.89) 6.56±0.18 (6.80) 6.60±0.28 (7.11)

Jsc [mA/cm2] 11.59±0.43 (12.06) 12.04±0.34 (12.58) 11.61±0.49 (12.37)

Voc [V] 0.83±0.01 (0.85) 0.85±0.01 (0.88) 0.89±0.00 (0.89)

FF 0.57±0.04 (0.63) 0.64±0.01 (0.66) 0.64±0.01 (0.66)

Means and standard deviations are obtained from 10 devices. The value in brackets illustrates the best cell performance for each device. Even though PEIE and PFN show similar tendencies in terms of the work function change, only PEIE has a high γp component, since ethylamine groups in both the branches and backbone of PEIE increases its surface energy43. The ethylamine groups of the PFN branch on the other hand 14


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only create bonding with hydroxyl groups on ZnO, while the hydrophobic fluorine-based backbone is exposed44. Figure 5d summarizes the effect of the surface treatments on ZnO-based PCDTBT:PC71BM devices in terms of their mechanical reliability and efficiency. The high work of adhesion and interfacial dipole of the PFN-treated ZnO layer yielded an enhanced PCE of OSCs and competitive mechanical reliability compared to untreated ZnO.

CONCLUSIONS The interfacial fracture energy, delamination path, and power conversion efficiency were significantly affected by the interfacial buffer layers, and it was discovered that high mechanical reliability and efficiency can be obtained through appropriate surface energy control at the interfaces. ZnO surface treated with poly [(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)alt-2,7-(9,9–dioctylfluorene)] (PFN) has a large dispersion component of its surface energy, resulting in strong molecular attraction to the poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-

(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]:[6,6]-phenyl-C71-butyric

acid

methyl

ester

(PCDTBT:PC71BM) layer that also has a large dispersion component. In addition, the PFN treatment improved the performance of devices by optimizing work function through the formation of a interfacial dipole at the buffer/active interfaces. We believe that interfacial engineering via surface treatments will provide the design rules to co-optimize both the mechanical reliability and power conversion efficiency of organic solar cells for future flexible optoelectronic devices.

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Methods Polymer and ZnO precursor solution preparation. The PTB7 and PC71BM were obtained from 1-Material (Quebec, Canada) and Nano-C (Westwood, Massachusetts), respectively. PCDTBT:PC71BM at weight ratios of 1:1 and 1:4 was dissolved at 35 mg/mL in dichlorobenzene (DCB) at 60 °C. This solution was stirred for 12 h in a nitrogen environment. The ZnO precursor solution was prepared with 1.00 g of zinc acetate dehydrate (99 %; Samchun, Daejeon, Korea) and 0.28 g of ethanolamine (99.5 %; Aldrich, St. Louis, Missouri) in 10 mL of 2-methoxyethanol stirred for 12 h in air. Sample preparation. Samples for adhesion testing were fabricated on indium tin oxide (ITO)-deposited glass substrates with 10 ohm/sq of sheet resistance. After O2 plasma treatment of the cleaned ITO substrates, a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Clevios Al4083, Heraeus, Germany) layer was spun onto the substrates at 3000 rpm for 30 s, which was subsequently annealed at 140 °C for 10 min. A layer of ZnO was spun onto the substrates at 5000 rpm for 30 s and annealed at 200 °C for 20 min. Subsequently, PCDTBT:PC71BM was spun at 1100 rpm for 30 s on the PEDOT:PSS layer. This last layer was annealed at 70 °C for 15 min without an additional drying process. Lastly, 150 nm of silver was deposited on the devices by thermal evaporation. To estimate the surface characteristics of the layers, we conducted contact angle (CA) measurement using deionized water on buffer layers and active materials. Double Cantilever Beam (DCB) testing. Specimens were fabricated by cutting a 8.4 mm × 25.4 mm rectangular beam from an Ag/active layer/PEDOT:PSS (or ZnO)/ITO/glass device. Afterwards, dummy glass substrates coated with 1-µm-thick 353ND epoxy (Epo-Tek 353ND consisting of bisphenol F and imidazole; Epoxy Technology, Billerica, Massachusetts) were 16


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attached to make DCB sandwiched structure specimens, as shown in figure 1c. Then, the epoxy was cured for 2 h at 125 °C in a convection oven. The interfacial fracture energy was measured using a high-precision micromechanical test system (Delaminator Adhesion Test System, DTS Company, USA). The sandwiched thin layers between the glass substrates were carefully loaded and unloaded at a constant displacement rate of 0.5 µm/s while simultaneously recording the load-versus-displacement curve. /

/

= =

− 0.64ℎ

1 + 0.64

(3) (4)

where du/dP is the elastic compliance, u is the total displacement of the beam ends, P is the applied load, B is the sample width, h is the half-height of the substrate, E' is the plane-strain modulus of the beam, and Pc is the critical load where the load of the load-displacement curve starts to decrease. Film analysis. The surface morphology of organic films was observed using atomic force microscopy (AFM, XE-100 of Park Systems, Suwon, Korea) in non-contact mode under ambient conditions. Raman spectra were analyzed using a micro-Raman system (JY Horiba LabRam HR spectrometer, Japan).

ASSOCIATED CONTENT Supporting Information Surface characterization by using Raman spectroscopy and AFM

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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS I.L. and J.N. contributed equally to this work. This research was supported by the Basic Science Research Program (2015R1A1A1A05001115, 2015R1A2A2A01006689, 2015M1A2A2057509) and Wearable Platform Materials Technology Center (2016R1A5A1009926) funded by the National Research Foundation under the Ministry of Science, ICT and Future Planning.

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