A Solid-State Intrinsically Stretchable Polymer Solar Cell - ACS

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A Solid-State Intrinsically Stretchable Polymer Solar Cell Lu Li, Jiajie Liang, Huier Gao, Ying Li, Xiaofan Niu, Xiaodan Zhu, Yan Xiong, and Qibing Pei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12908 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 28, 2017

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ACS Applied Materials & Interfaces

A Solid-State Intrinsically Stretchable Polymer Solar Cell

Lu Li1,2*, Jiajie Liang2,, Huier Gao2, Ying Li1, Xiaofan Niu2, Xiaodan Zhu2, Yan Xiong2, Qibing Pei2* 1

Research Institute for New Materials Science, Chongqing University of Arts and Sciences, Chongqing, China 402160 2

Department of Materials Sciences and Engineering, California NanoSystems

Institute, Henry Samuli School of Engineering and Applied Science, University of California, Los Angeles, California 90095 * Author to whom correspondence should be addressed, Email: [email protected], [email protected]

Abstract: An organic solar cell based on a bulk heterojunction (BHJ) of a conjugated polymer and a methanofullerene PC61BM or PC71BM exhibits a complex morphology that controls both its photovoltaic and mechanical compliance (flexibility, and stretchability).

Here

the

donor-acceptor

blend

of

poly(thieno[3,4-b]-thiophene/benzodithiophene) (PTB7) and PC71BM containing a small amount of diiodooctane (DIO) in the spin-casting solution is reported to exhibit elastic deformability. The blend comprises nanometer-size, nanocrystalline grains that are relatively uniformly distributed. Large external deformation is accommodated by relative sliding between the grains. Re-orientation of the nanocrystallites and the global re-orientation of the PTB7 polymer chain were observed along the stretching 1

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direction up to 100% strain, which was reversible as the blend was allowed to relax to 0% strain. The polymer solar cell based on PTB7:PC71BM:DIO with such reversible morphological changes exhibited rubbery elasticity at room temperature. The device could be stretched up to 100 % strain, and power-conversion efficiency shows slight increase up to 30% strain and a global increase of power generation as the photoactive area increases with strain. Solar cells were fabricated employing a layer of the PTB7:PC71BM:DIO blend sandwiched between a pair of stretchable transparent electrodes each comprising a stack of a silver nanowire percolation network and a single wall carbon nanotube network embedded in the surface of a poly(urethane acylate) elastomer film. The solar cells were semitransparent, and could be stretched like a rubbery film by as much as 100% strain. The measured power-conversion efficiency was 3.48%, which was increased to 3.67% after one cycle of stretching to 50% strain and lowered to 2.99% after 100 stretching cycles. The total power generation from the cells was significantly increased thanks to the expanded active area as the cells were stretched.

Keywords: Polymer solar cells, stretchable, intrinsic, solid-state, semitransparent

Introduction Advancements in power conversion efficiency continue to be made in organic solar cells (OPV) pushing this technology closer to the photovoltaic markets.1-2 It is likely that the initial market introduction will involve the unique attributes such as flexibility, 2


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elasticity and ductility, for which the OPVs were originally envisioned.3-5 As an extension of the field of flexible polymer solar cells, stretchable polymer solar cells could not only be deployed at significantly reduced installation cost but also in many new locations where conventional rigid photovoltaic panels cannot be applied such as building-integrated structures with curved surface, portable or wearable electronics that could be collapsed into small volumes, moving parts of machinery and robots, and etc.6-7 The first stretchable polymer solar cell was reported by Lipomi et al. in 2011.8 In this device, an organic active layer was coated on an uniaxial stretched poly(dimethyl siloxane) (PDMS) substrate. Release of the tension produced buckles in the active layer that accommodated subsequent cycles of strain. An eutectic gallium-indium liquid metal was used as the top contact—which would limit the practical deployment of the OPV. Later, Kaltenbrunner et al. demonstrated an OPV on an ultrathin (1.4 µm) polyester foil capable of accommodating bending radii >10 µm.9 This solar cell was stretchable when mounted to an elastic substrate, but showed poor stability under repeated compression: -27% reduction in efficiency after 22 cycles of 50% compression. Pursuing intrinsically stretchable solar cell of which the active layer permits truly elastic deformation are more compelling.10 Many aspects of the BHJ systems comprising conjugated polymers and PCBM could determine the deformability of the active layer and resulting solar cell devices, such as chemical structure (e.g., the presence of the fused or isolated rings in the main chain,11 length and composition of pendant groups,12 size and intermolecular forces within 3



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crystallites13) and microstructural order (e.g., the addition of methanofullerene,14 intercalation of methanofullerenes between side chains of polymers,15 effect of processing

conditions,14

presence

of

plasticizing

additives,16

makeup

of

methanofullerene phase17). One key determinant of the mechanical properties in the active layer of an organic solar cell is the morphology of the donor-acceptor BHJ, which also plays a key role in the power conversion efficiencies of organic solar cells. While morphological control for electronic and photonic properties such as charge-transport properties and charge transfer for photoluminescent quenching has been extensively investigated,18-19 it has not been employed as an effective means to impart mechanical stretchability. 16 We report the development of a thin-film polymer solar cell that appears and can be deformed like an elastomer film through morphological control. The solar cell employs a pair of stretchable transparent electrodes each comprised of a stack of a silver nanowire (AgNW) percolation network and a single wall carbon nanotube (SWNT) network embedded in the surface of an elastomeric film. A layer of a photovoltaic

bulk

heterojunction

blend

of

poly(thieno[3,4-b]-thiophene/benzodithiophene) and [6,6]-phenyl-C71-butyric acid methyl ester is sandwiched between the pair of transparent electrodes. By the addition of DIO, the phase separation in the blend leads to nanometer-size grains, with the power conversion efficiency improved from 1.79% to 3.48%. The polymer OPV exhibits rubbery elasticity at room temperature, a power conversion efficiency (PCE) of 3.48% which is enhanced to 3.67% after one cycle of stretching to 50% strain. The 4


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device could be stretched by up to 100 % strain.

Results and discussion Figure 1(a) illustrates the general architecture of the elastomeric polymer solar cells. The photovoltaic active layer is comprised of a donor polymer, poly(3-hexylthiophene) (P3HT) or poly(thieno[3,4-b]-thiophene/benzodithiophene) (PTB7), an acceptor molecule, phenyl-C61-butyric acid methyl ester (PCBM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). DIO is also added in certain donor-acceptor blend solutions to modify the morphology of the resulting blends. The pair of transparent electrodes were each made up of a percolation network of single-walled carbon nanotubes (SWNT) and a percolation network of silver nanowires (AgNWs) stacked together and inlaid in the surface layer of a poly(urethane acylate) (PUA) elastomer film. A thin layer of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) and ethoxylated polyethylenimine (PEIE) were inserted on the anode and cathode interfaces to enhance transport of holes and electrons, respectively. The sandwiched structure of the OPV, PUA-AgNW/SWNT/ PEDOT:PSS/ Active materials/ PEIE/ SWNT/ AgNW-PUA, was fabricated by an all-solution based process. The SWNT/AgNW-PUA composite electrode was fabricated by successively depositing SWNT, AgNWs, urethane acrylate monomer on a glass release substrate. The coatings were heated to cure the monomer layer to form a crosslinked PUA film which was then separated from the release substrate.20 In the resulting SWNT/AgNW-PUA composite electrode, the SWNT layer was in the outer layer of 5



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the PUA film, and the AgNW layer underneath the SWNT layer. The composite transparent electrode exhibited a high transparency, low sheet resistance, low surface roughness, and elastomeric stretchability, as shown in Figures S1-S3. A thin layer of PEDOT:PSS was spin-coated onto the composite electrode to enhance hole collection. This PEDOT:PSS layer also served to protect the composite electrode from solvent attack in the subsequent coating of the photovoltaic blend. The resulting film is designated as (PUA-AgNW/SWNT/ PEDOT:PSS/ Active materials) to indicate that the SWNT layer is in electrical contact with the PEDOT:PSS layer. The film was laminated with a second SWNT/AgNW-PUA film pre-coated with a thin layer of PEIE to complete the device fabrication. The PEIE layer was introduced as an electron-collecting layer.21 The as-fabricated OPV sandwich, 150 µm in total thickness, appeared like a piece of plastic film, with a colored region stemming from the photovoltaic blend layer (Figure 1 (b)) The donor-acceptor blend of P3HT:PCBM was first examined as the active materials in the laminated bulk heterojunction OPV. The blend was annealed to attain large-size crystalline domains for high OPV PCE, which is commonly practiced for this bulk heterojunction blend.

22

PCE of the OPV is 1.79 % under one sun solar

illuminator (Figure 1d). Note that the efficiency is lower than the conventional device based on ITO/glass and evaporated metal cathode.23 One cause for the lowered PCE is that both electrodes in the present OPVs are transparent, leading to lower absorption efficiency than the conventional devices using a reflective metallic cathode. The present OPV using the P3HT:PCBM blend is semitransparent, and is flexible like a 6


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neat PUA film, which is not unexpected as greater than 99% of the materials in the OPV is comprised of PUA in the substrates. The OPV can be mechanically stretched by as much as 7% without showing crack formation or tearing by the naked eye. However, the photovoltaic response diminishes with strain. The PCE decreases to 0.43 % when the OPV film is stretched by 10% strain, and recovers to 1.00 % after the stretched film is allowed to relax. Stretching beyond 10% caused terminal, unrecoverable failure. Figure 1 (c) and Figure S4 (a) respectively show the tapping and phase mode atomic force microscopy (AFM) images for the P3HT:PCBM blend film. The blend film shows a rough surface and noodle-like morphology with a typical feature size of 10-20 nm. This morphology with a large interfacial area is good for efficient charge generation, but not favorable for large-strain stretchability. Figure S5 shows the optical microscope image of the P3HT:PCBM blend film before and at 10% strain. At 10% strain, major cracks on the surface can be observed. Lipomi et al. also reported that a thin P3HT layer transferred onto polydimethylsiloxane (PDMS) is deformable, whereas the P3HT:PCBM blend is not, and develops large cracks at 10% strains, leading to loss of photovoltaic response in the device.24 In the present OPV, the PEDOT:PSS and PEIE layer could survive larger strains in the laminated architecture shown in Figure 1(a). Polymers containing these layers had been shown capable of repeated stretching by 40% for 100 cycles. The PTB7:PC71BM blend is another widely investigated photovoltaic blend with reported PCE now exceeding 8%.25 The use of DIO in the coating solution has a 7



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profound impact on the blend morphology and the PCE of the resulting OPV (Figures 1e-1h).26 Similar results were obtained in the laminated OPVs: using DIO significantly improved the PCE, from 1.79% when no DIO was used to 3.45% when 3% by weight of DIO was admixed in the blend solution before spin casting. The PCE of 3.45% is lower than the 8% obtained for OPVs that use an evaporated metal electrode which enhances the absorption efficiency. The laminated OPVs did not have a reflective electrode and were semitransparent. The resulting OPV appears like a piece of semitransparent plastic sheet, similar to the photograph shown in Figure 1 (b). The absorption spectrum of the photovoltaic PTB7:PC71BM blend, 100 nm thick as optimized for the PCE of the OPV, exhibits a peak at 680 nm and negligible absorption beyond 760 nm, the band gap of the photosensitizing PTB7 polymer. The transmittance of the entire OPV sandwich is around 45% between 400-700 nm, and higher than 70 % between 750 nm and 1100 nm. The stretchability of the laminated OPVs was also affected by the addition of DIO. The OPV without DIO behaved like the OPV based on P3HT:PCBM. The AFM images in Figure 1 (e) and Figure S4 (b) clearly show that there are large domains (about 100-200 nm in diameter) in the blend film without DIO, which is not favorable for exciton migration to the donor/acceptor interface nor for mechanical strain. At 10% strain, the PCE diminished to only 0.3%, and most of the degradation was irreversible. On the other hand, the PTB7:PC71BM blend containing 3 wt% DIO shows a morphology with much smaller grains and uniform distribution, indicative of improved miscibility between PTB7 and PC71BM. The OPV with DIO could be 8


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stretched by as much as 100%. The PCE actually showed a slight increase with stretching up to 50% strain. At strains exceeding 30%, the PCE gradually diminished, but remained at a fairly high value of 2.77% at 100% strain. By comparing the morphologies of the PTB7:PC71BM to the PTB7:PC71BM:DIO blends, we speculate that the large-strain stretchability of the PTB7:PCBM71:DIO blend may be attributed to the small grain size of the phase separated domains. Unlike P3HT which is semicrystalline, PTB7 is largely amorphous in the blend, allowing the small phase grains to undergo relative motion in order to accommodate large deformations. DIO has a boiling point of 167-169 oC at 6 mm Hg which is much higher than that of chlorobenzene. Upon the vaporization of chlorobenze during the spin casting of the photovoltaic blend, the residual DIO would create free volumes between grains in the solid-state blend. Whether or not the DIO vaporized away or remained, the nanometer-size grains can slide against each other to accommodate large external deformations. A natural analogue for this nanograin morphology is beach sand: the grains can slide against each other no matter whether the sand is dry or submerged in water. In the nanograin photovoltaic blend, the sliding deformation does not significantly alter the inter-grain charge transport, and thus the photovoltaic property remains largely intact.

9



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(a)

(b)

(c)

(d)

(e)

(f)

10


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(h) (g) Figure 1. Morphological characteristics of donor-acceptor blends and corresponding photovoltaic performance with stretching. (a) Schematic illustration of the sandwich structure of an elastomeric OPV. (b) Optical photograph of the semitransparent OPV placed on a white paper with printed UCLA logo as the background, and a US quarter for size reference. Tapping mode AFM images of a P3HT:PCBM (c), PTB7:PC71BM (e), and PTB7:PC71BM:DIO (g) blend film. All imaged areas are 1 µm × 1 µm. J-V characteristics of the OPV based on P3HT:PCBM (d), PTB7:PC71BM (f), and PTB7:PC71BM:DIO (h). The photovoltaic PTB7:PC71BM:DIO blend film was further imaged by tapping mode AFM to investigate the morphology-stretchability relationship. The fresh blend film shows a uniform morphology indicative of good miscibility between PTB7 and PC71BM (Figure 2a), and the formation of an interpenetrating donor-acceptor network can be observed. The donor and acceptor grains within the blend film are randomly oriented. This morphology is consistent with the transmission electron microscopic images obtained by Yu et al.27 At 50% stretching strain, the grains exhibits a slight preferential alignment perpendicular to the axis of strain as shown in 11



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Figure 2(b). The alignment is preserved after the film has been allowed to relax to 0% strain (Figure 2c). The phase images of the film are shown in Figures 2(d)-(f) for the freshly made, 50% strained and relaxed film, respectively. A similar trend is observed in the surface height profile: a randomly orientated phase profile for the as-fabricated film and intermittent buckles formed at 50 % strain. The vertically aligned features, perpendicular to strain direction, become more pronounced as the blend film is relaxed to 0% strain. The average surface roughness measured was 0.77 nm before stretching and increased to 1.7 nm after a stretching cycle, consistent with this morphological change. We note that in the stretchable OPV devices, the PTB7PC71BM blend was sandwiched between a pair of composite electrodes. Barring from substantial delamination, the buckling of the blend layer upon stretchable cycle may not take place. Fresh film

50% strain

Recovery

(a)

(b)

(c)

12


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(d)

(e)

(f)

Figure 2. The morphology characteristics of the photovoltaic blend. Height mode (a)-(c) and phase mode (d-e) AFM images of a PTB7:PC71BM:DIO blend film unstretched (a, d), stretched at 50 % strain (b, e) and relaxed to 0% strain (c, f). All imaged areas are 1 µm × 1 µm, and the height scale bar is 10 nm (a) or 20 nm (b, c). The insets are 3D topographical images each 1 µm × 1 µm area. To further understand the morphology-stretchability relationship, PeakForce Quantitative NanoMechanical (QNM) mapping was used to probe local forces during atomic force microscopic imaging

28-29

. Local elastic moduli were derived from the

unloading portion of the force vs. deformation curves using Derjauin-Muller-Toporov (DMT) model 30. The surface Young’s modulus distribution of PTB7: PC71BM blend films obtained by the PeakForce QNM mapping is displayed in Figure 3. The phase separation forms large size grains with modulus values around 12±1.0 GPa. The continuous phase (in darker color) is 7.0±0.8 GPa. Neat PTB7 and neat PC71BM have reported Young’s modulus of ~1.1 GPa and ~3.1 GPa, respectively.14,31 Considering the imaging results by TEM and phase mode AFM, it is obvious that the large grains are made of PC71BM, and the significantly higher modulus compared to neat PC71BM 13



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is probably due to filler effect by PTB7 in the PC71BM domain.12,16 The PeakForce QNM imaging shows much finer grains in the blend containing DIO. The Young’s modulus is 9.5±0.5 GPa in the bright regime and 5.5±0.5 GPa in the dark areas, both being significantly lower than corresponding values without DIO. The stretched photovoltaic blend film on soft substrate could not be imaged by this method, as force-distance curve was dominated by stress-strain behavior of the soft substrate whereas the bend film is orders of magnitude stiffer. Still, the modulus mapping shown in Figure 3 is consistent with the beach sand morphology for the DIO-containing blend.

Figure 3.

Young’s modulus map of (a) PTB7: PC71BM (1:1.5) and (b) PTB7:

PC71BM : DIO thin films spin cast on glass precoated with PEDOT:PSS. Conjugated polymer have the property that optical transition dipole moment (pi-pi*) can be aligned along the polymer backbone, and thus the uniaxially aligned polymer film results in anisotropic optoelectronic properties.32-33 UV-visible absorption spectra of the blended films were measured with linearly polarized light parallel or 14


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perpendicular to the strain direction. The results before, being stretched to various strains, and after relaxed to 0% strain are summarized in Table 1.

Figure S7

displays the representative spectra for the films before stretching and after one full cycle of stretching to 100% strain (relaxed back to 0% strain). The absorption peak at 460 nm is largely due to PC71BM and that at 680 nm is due to PTB7. Apparently, the 100% stretching leads to large plastic, unrecoverable reorientation of the both PC71BM and PTB7 in the blend without DIO, which is quite characteristic for conjugated polymers. 14, 17 The dichroic ratios measured under large strains (see Table 1 left columns) are relatively small considering the stretch ratios thanks to crack formation in the blend films. On the other hand, the PC71BM:PTB7:DIO films show larger dichroic ratios for both the PC71BM and PTB7 absorption peaks. No crack formation could be observed The high dichroic ratios are indicative of molecular re-orientation of PC71BM and PTB7. Further interesting is the complete nearly complete recovery of the dichroic ratios to 1 when the blend films are allowed to relax to 0% strain, a strong indication of elastic deformation of both PC71BM and PTB7. The most plausible explanation is that both PC71BM and PTB7 are crystalline: the nanocrystallites re-orient with stretching, leading to high measured dichroic ratio. When the blend is relaxed, the domain relaxes and the nanocrystallites return a random orientation. Again, this morphological change is consistent with the beach sand model. We note that in another stretchable conjugated polymer blend system comprising a soluble alkyloxy phenyl substituted poly(1,4-phenylenevinylene) (SuperYellow) and an ionically conductive solid electrolyte, both the conjugated 15



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polymer domain and the ionic domains are amorphous Stretching does not induce any collective re-orientation and the dichroic ratio remains around 1 at high strains.34 Table 1. Dichroic ratio determined from the polarized UV-vis absorption spectra of PTB7: PC71BM and PTB7: PC71BM:DIO blend films at 680 nm and 460 nm, respectively. Dichroic Ratio (DR) Strain

PTB7:PC71BM 460 nm

PTB7:PC71BM:DIO 680 nm

Stretched

Relaxed

0%

1.00

--

1.00

25%

1.10

1.03

50%

1.20

75% 100%

460 nm

Stretched Relaxed

680 nm

Stretched

Relaxed

Stretched

Relaxed

--

1.00

--

1.00

--

1.23

1.02

1.33

1.04

1.73

1.02

1.06

1.52

1.11

1.35

1.01

1.84

1.01

1.16

1.20

1.61

1.32

1.36

1.03

1.89

1.01

1.45

1.42

1.74

1.72

1.79

1.01

2.09

1.01

The current density-voltage (J-V) characteristics and PCE of the OPV at various stains are shown in Figure 4(a), with specific parameters listed in Table 2. The Isc is 0.424 mA for the un-stretched OPV and is increased to 0.760 mA when stretched to 50% strain, mostly due to the expansion of the active area. When the OPV is allowed to relax back to 0% strain, the Isc becomes 0.492 mA which is still higher than the fresh device before the stretching-relaxation cycle. The Jsc fluctuates with strain, while the fill factor (FF) shows a gradual decrease at higher strains. The Voc increases with strain, as is the maximum power (Pmax). After a full cycle of stretching-relaxation, the Jsc, Voc, and PCE are all higher than those of the fresh device. Figure 4(b)-(d) respectively show the J-V characteristics and PCE of the OPV after certain stretching-relaxing cycles between 0 % and 50 % strain. The PCE increases after the

16


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first cycle of stretching-relaxing, and then turns to gradual decrease with further stretching cycles. After 100 cycles, the PCE remains a fairly high value of 2.99 %. The active area of the OPV is expanded during stretching, the power generation of the OPV is increased with stretching, even though the PCE is slightly reduced at large strains (Table 2). The fresh OPV based on the PTB7:PC71BM:DIO blend produces 0.130 mW power at 1 sun. The value is increased to 0.158 mW at 25% strain and 0.164 mW at 50% and 100% strains. Figure 4(e) shows the OPV at 50% and 100% strains. The active area is deformed fairly uniformly up to 100 % strain. The deformation was completely reversible and repeatable: no visual sign of tearing, delamination, or crack formation was observed during repeated stretching and relaxation. Figure 4(d) inset shows the optical photograph of the surface of the active layer after 100 cycles of stretching to 50% strain. The OPV surface after the stretching cycles remains smooth and flat, and does not show any buckling patterns in the imaged areas of 250 µm × 250 µm.

(b)

(a)

17



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(d)

(c)

(e) Figure 4. Device characterization of stretchable solar cells. Output power and PCE (a) of an OPV at specified strains. J-V characteristics (b) and PCE (c), (d) of the OPV after specified stretching-relaxing cycles between 0 % and 50 % strains. Inset: Optical photograph of the solar cell after 100 cycles of stretching-relaxing between 0 % and 50 % strains. (e) Optical photographs of the OPV clamped on a vice, stretched to 0 %, 50 % and 100 % strains, respectively. Table 2. Photovoltaic parameters for elastomeric OPVs a) with different stretching strains.

18


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Short Circuit

Power

Short Circuit

Open Circuit

Maximum

Voltage, Voc

Power, Pmax

Current Current, Isc

Conversion Fill Factor

Density,

Efficiency

(mA)

(V)

(mW)

Jsc

(%)

(mA/cm2)a)

Fresh

0.396 ± 0.028

9.9 ± 0.7

0.505 ± 0.042

0.579 ± 0.021

0.130 ± 0.010

2.90 ± 0.58

25% strain

0.496 ± 0.039

10.3 ± 0.8

0.469 ± 0.045

0.592 ± 0.025

0.158 ± 0.012

2.87 ± 0.65

50% strain

0.549 ± 0.044

9.9 ± 0.8

0.423 ± 0.046

0.615 ± 0.025

0.164 ± 0.013

2.58 ± 0.63

100% strain

0.559 ± 0.059

8.60 ± 0.9

0.407 ± 0.049

0.612 ± 0.027

0.164 ± 0.017

2.14 ± 0.63

2.95 ± 0.72 Recovery

0.448 ± 0.044

11.2 ± 1.1

0.435± 0.046

0.598 ± 0.022

0.134 ± 0.013 (3.67) b)

a)

The active area of the unstretched cells is 4 mm2.

b)

Highest values. Enhancement of conductivity and field-effect mobility in the direction of stretching

have been reported in conductive polymer and composite films.

34

However, for

polymer solar cells, the charge carrier transport in the thickness direction, orthogonal 19



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Page 20 of 30

to the stretch plane, is more important than within the stretch plane. The beach sand model and the reversible re-orientation of the nanocrystalline domains as shown by the dichroic measurement indicate that the electrical transport across the film thickness could be affected by stretching. In order to investigate the charge carrier transport in the PTB7:PC71BM blend film, single charge carrier devices were fabricated. The current of the hole and electron-only devices increase with stretching from 0 % to 100 % strain as shown in Figure 5. The current decreases as the sandwich device are allowed to relax, but the fully relaxed device still has higher current than the fresh devices. This improved charge carrier transport is another key factor to enhance the performance of the stretched polymer solar cells.

(a)

(b)

Figure 5. Charge transport properties of single charge carrier devices. Current -voltage characteristics of (a) a hole-only device with a thin film sandwich configuration

of

PUA-AgNW/SWNT/PEDOT:PSS/PTB7:PC71BM:DIO/PEDOT:PSS/SWNT/AgNWPUA and (b) an electron-only device with a thin film sandwich configuration of 20


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PUA-AgNW/SWNT/PEIE/PTB7:PC71BM:DIO/PEIE/SWNT/AgNW-PUA. Conclusions In summary, an intrinsically stretchable OPV has been demonstrated employing the donor-acceptor

blend

of

PTB7:PC71BM

and

DIO.

The

blend

comprised

nanometer-size, nanocrystalline grains that were relatively uniformly distributed. Free volumes between the grains as a result of the use of DIO allowed the relative sliding of the grains to accommodate large, external deformation. Such deformation did not induce re-orientation of the nanocrystallites and the global re-orientation of the PTB7 polymer chain along the stretching direction. This re-orientation was reversible as the blend was allowed to relax to 0% strain.

The polymer OPV based on

PTB7:PC71BM:DIO with such reversible morphological changes exhibited rubbery elasticity at room temperature. The device could be stretched up to 100 % strain, and PCE shows slight increase up to 30% strain and a global increase of power generation as the photoactive area increases with strain. Methods Materials: Silver nanowire/single wall carbon nanotube (AgNW/SWNT) bilayers were deposited by successively coating a layer of functionalized SWNT (Carbon Solutions, Inc.) and a layer of AgNW (synthesized according to a literature procedure27) from respective dispersions on glass substrate at ambient temperature and pressure. The AgNW has an average diameter in the range 25-35 nm and average length between 10-20 µm. A polymer precursor solution containing 100 weight parts urethane acrylate oligomer (UA, Sartomer USA), 20 parts ethoxylated bisphenol A 21



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dimethacrylate

(EBA,

Sartomer

USA),

Page 22 of 30

and

one

part

2,2-dimethoxy-2-phenylacetophenone (Sigma-Aldrich) was coated on the as-prepared AgNW/SWNT bilayer on glass substrate. The precursor coating cured on a Dymax ultraviolet curing conveyor equipped with a 2.5 W/cm2 Fusion 300S type ‘H’ ultraviolet curing bulb and then peeled off as a free-standing composite electrode. The resulting composite electrode, denoted SWNT/AgNW-PUA was separated form the glass substrate. Device fabrication: SWNT/AgNW-PUA composite stretchable electrode with sheet resistance of 10 ohm/sq was used to fabricate stretchable OPV devices. The electrode was cleaned by sequential 30 min treatments with detergent followed by deionized

water

in

an

ultrasonic

bath.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (Clevios VP Al 4083 from H. C. Starck Inc.) was spin-coated on a composite electrode at 4000 rpm for 60 s, followed by drying in a vacuum chamber for 24 h to remove residual water.

Onto

the

coated

electrode,

a

solution

containing

thieno[3,4-b]-thiophene/benzodithiophene (PTB7, 1-material Chemscitech Inc.) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM, Sigma-Aldrich) dissolved in chlorobenzene and 1,8-diiodoctane (DIO) (97:3 vol%) as the mixture solvent was spin-coated at 900 rpm for 60 seconds. The weight ratio of PTB7:PC71BM was 1:1.5, and the blend film was 140 nm thickness measured by Dektak 6 Surface Profilometer. On a second composite electrode, a solution of ethoxylated polyethylenimine (PEIE, Mw=75,000 g/mol, Sigma-Aldrich) dissolved in 2-methoxyethanol to 0.4 wt% 22


Page 23 of 30

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concentration was spin-coated at 5000 rpm for 60 s. The PEIE coating was then dried in a vacuum chamber at ambient temperature. The thickness of the PEIE layer was around

5

nm.

Finally,

the

as-prepared

PEIE/composite

electrode

and

PTB7:PC71BM/PEDOT:PSS/composite electrode were laminated with the active materials facing each other, and heated to 100 C for 3 min to afford a strong adheson. The lamination was done in a glovebox protected with nitrogen, with oxygen and moisture levels below 0.1 ppm. The photosensitive area of the freshly prepared devices was 0.12 cm2. Characterization: Stretching and relaxing tests were performed on a motorized linear stage with a computerized controller (Zaber Technologies). A Keithley 2000 digital multimeter was used to monitor resistance changes. Strain and resistance data were recorded via a custom-made LabView code. All measurements were carried out at room temperature in the glovebox. Transmittance spectra were recorded on a Shimadzu UV-1700 spectrophotometer. AFM was performed on a Veeco Multimode Atomic Force Microscope. Current density-voltage (J-V) characteristics were measured with a Keithley 2400 Semiconductor Characterization System. The photovoltaic performance was measured under an air mass of a 1.5 solar illumination at 100 mW/cm2 (1 sun). Supporting Information Transmittance and absorption spectra; Stretchablity testing of composite electrode; AFM measurement of the composite electrode; Optical photograph of a stretchable OPV and AFM images of photovltaic active material blend; Morphology of the P3HT:PCBM blend film; Absorption spectra of blend PTB7:PC71BM:DIO film under 23



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Page 24 of 30

different strains; Polarized UV-vis absorbance spectra of blend PTB7:PC71BM:DIO film; AFM measurement of PEIE; Microscopic images of PEDOT:PSS surface Acknowledgements The work reported here was supported by Air Force Office of Scientific Research (Grant # FA9550-12-1-0074); Open Foundation of State Key Laboratory of Electronic Thin Films and Integrated Devices (KFJJ201507), National Natural Science Foundation of China (51503022, U1663229),Basic and Frontier Research Program of Chongqing

Municipality

(cstc2015jcyjA50036,

cstc2016jcyjys0006,

cstc2016jcyjA0367, KJ1601112), Natural Science Foundation of Yongchuan District (Ycstc,2015nc4001), the research project for Chongqing University of Arts and Sciences (Grant No. z2016xc16)

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Table of Contents

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A Solid-State Intrinsically Stretchable Polymer Solar Cell - ACS

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