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Reconfigurable Topography for Rapid Solution Processing of Transparent Conductors Haosheng Wu, Monisha Menon, Evan Gates, Aditya Balasubramanian, and Christopher J. Bettinger* Transparent conducting electrodes (TCE) are an integral component of optoelectronic devices used in technologies ranging from clean energy production to consumer electronics.[1] Indium tin oxides (ITO) are commonly used in many applications despite many challenges associated with the high cost of raw materials, brittle mechanical properties, and processing steps that often require high vacuum and elevated temperatures. These potential drawbacks motivate the pursuit of alternative materials to replace ITO for use as next-generation TCE. The ideal material for TCE is cost-effective, permissive of integration with flexible substrates, and compatible with mild processing strategies. Towards this end, many new materials and structures are currently being explored as next-generation TCE including metallic nanowires,[2–5] carbon nanotubes,[6,7] graphene,[8–10] self-assembled nanomeshes,[11] and polymernanoparticle composites.[12,13] Current efforts are motivated by the need to maximize optical transmittance while maintaining sufficient electrical conductivity for the prospective application. Materials processing and fabrication strategies are also important considerations in the economical production of TCE. Lithographic methods produce TCE with ordered structures that generally afford both higher performance and increased cost of production compared to bulk deposition processes that generate TCE with randomly oriented substituents absent of short-range order. This tradeoff is evident in the fabrication of high performance TCE using ordered metallic nanostructures utilizing well-defined templates.[14–18] Methods for solution-processing of metallic nanowires into random arrays are
Prof. C. J. Bettinger Department of Materials Science and Engineering Department of Biomedical Engineering Carnegie Mellon University Pittsburgh, PA 15213, USA E-mail: [email protected] Prof. C. J. Bettinger McGowan Institute of Regenerative Medicine 450 Technology Drive Suite 300, Pittsburgh, PA 15219, USA H. Wu, M. Menon, A. Balasubramanian Department of Materials Science and Engineering Carnegie Mellon University Pittsburgh, PA 15213, USA E. Gates Department of Mechanical Engineering Carnegie Mellon University Pittsburgh, PA 15213, USA
DOI: 10.1002/adma.201302377 706
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amenable to fabrication of large area TCE.[19–23] Furthermore, solution-processing of nanomaterials is compatible with largevolume manufacturing techniques such as roll-to-roll processing.[8] Hybrid approaches have emerged as potential strategies to fabricate organic and inorganic nanomaterials into structures for use in TCE.[24,25] The ideal materials processing strategy will combine the high performance of ordered materials with the cost-effectiveness of solution-processed randomly oriented structures. A hybrid strategy for TCE fabrication based on transfer printing of ordered silver nanowire (AgNW) networks is described in this Communication. The key component of this solution-based approach is AgNW templating using donor substrates with reconfigurable topography. TCE based on AgNW arrays with areas ≈1 cm2 can be rapidly integrated with flexible substrates. Flexible TCE processed in this manner exhibit performance metrics comparable to ITO layers 200 nm in thickness while obviating processing steps that employ extreme temperatures, high vacuum, or photolithography.[26] Bi-layer substrates under uniaxial compression exhibit micron-scale features that can impart mechanical flexibility in functional electronic devices including interconnects,[27,28] transistors[29–31] and solar cells.[32,33] Wrinkled elastomeric substrates can also serve as templates to guide the assembly of nanomaterials.[34,35] Bi-layer substrates are produced by forming thin rigid oxide membranes on pre-strained elastomeric substrates using ultraviolet ozone (UVO) (Figure 1).[36] Reconfigurable topographic substrates are formed by alternating between the compressed state (ε0 = 0%) which induces the formation of highly anisotropic micron-scale wrinkles and the strained state (ε0 = +40%) which abolishes the structures in a reversible manner (Figure S1, Supporting Information).[37] Reconfigurable topographic substrates template AgNW deposited from solution (Figure 1b-i). AgNW register and align with troughs of the grating features in aqueous environments prior to evaporation of the solvent (Figure S2). The rapid positioning of AgNW is attributed to radial outward flow in combination with the high specific gravity of silver (ρAg = 10.49 g cm−3).[38] Additional alignment is ensured as the contact line recedes in a mechanism that has been previously observed.[35] The application of uniaxial strain recovers the flat substrate and exposes the patterned AgNW networks in preparation for transfer printing (Figure 1b-iii). Thermal release tape provides a suitable intermediate material to transfer AgNW from reconfigurable donor substrates to flexible polyethylene terephthalate (PET) target substrates (Figure 1b-iv; Figure S2).[39–42] Furthermore, this procedure can be repeated to produce more complex patterns on PET substrates such as orthogonal lattices (Figure 2). The characteristic wavelength λ0 and amplitude A0 of anisotropic
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, 26, 706–711
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COMMUNICATION Figure 1. Solution Processing and Transfer Printing of Silver Nanowire Networks Using Donor Substrates with Reconfigurable Topography. a) Donor substrates are fabricated using (a-i) pre-strained bi-layer composites consisting of silicone films with (a-ii) thin silicon oxide membranes formed by exposure to ultraviolet ozone. a-iii) Releasing the pre-strain leads to formation of highly anisotropic grating microstructure arrays. Reapplying the prestrain abolishes the features. b) Topographic substrates are used to (b-i) pattern AgNW deposited from solution. b-ii) Grating features align AgNW (See Supplementary Information). b-iii) AgNW structures are then accessed by reapplying uniaxial strain, (b-iv) loaded onto a transfer substrate, and (b-v) deposited on a target substrate. b-vi) This process can be repeated to pattern orthogonal arrays of AgNW structures into lattice geometries. c) Representative SEM micrographs of topographic substrates with wavelengths of λ0 ≈ 40 μm are shown in (c-i) cross-section and (c-ii) top-down perspective. d) The amplitude A0 and wavelength λ0 of the features are positively correlated and can be controlled by altering the thickness of the silicon oxide membrane t.
sinusoid-like grating features are governed by intrinsic and extrinsic properties of the substrate and the rigid membrane as the follows: Bt g0 8 0 = √ , A0 = t −1 gc gc
(1)
where t is the thickness of the rigid membrane, ε0 is the prestrain, and εc is the critical strain to induce buckling where εc is estimated by: 1 gc = 4
3E s (1 − 4 μm) (Figure S5). This intermediate step improves the transmittance of the final AgNW network. The formation of a lattice of AgNW structures through a sequential two-step patterning strategy is also critical in achieving TCE with high electrical conductivity. Spontaneous wrinkle formation produces grating arrays with a non-negligible defect density that effectively eliminates long-range order in the resulting features on the substrate. “Y-shaped junction” defects, the convergence of two adjacent ridge features into one ridge,[46] form during the strain relaxation of PDMS-SiO2 bi-layers (Figure S6). These defects produce discontinuities in the primary AgNW network and ultimately reduce the electrical conductivity of film in the direction parallel to the predominant orientation of the AgNW structures. An additional transfer step can deposit a secondary AgNW network that is perpendicular to the primary AgNW network. The secondary network dramatically increases the conductivity in the film by ensuring a percolating network. Omitting the deposition of the secondary orthogonal AgNW network produces insulating films if the smallest dimension is >1 mm (data not shown). Therefore, the secondary orthogonal AgNW network is essential for fabricating AgNW lattices for use in practical TCE with areas >1 cm2. The physical properties of TCE composed of AgNW lattices exhibit several notable trends when fabricated using transfer printing with donor substrates that have reconfigurable topography. The center-to-center lattice spacing between parallel
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AgNW structures transferred onto PET target substrates λ is 10–30% larger than λ0 (Figure 2). This can be attributed to the slight strain-dependence of λ0 in this system. This observed behavior represents a departure from previous work in other bi-layer substrates which the wavelength λ0 is largely independent of the applied tensile strain,[29,30] yet lies in close agreement with composites created by exposing PDMS substrates to UVO.[35] Grating structures with larger λ0 and A0 can accommodate more AgNW per area on donor substrates. AgNW structures exhibit increasing line width and film thicknesses as λ0 and A0 both increase. The edges of the transferred AgNW structures contain a small fraction of AgNW that are randomly oriented due to insufficient hydrodynamic focusing or strong adhesion to the SiO2 surface. AgNW networks prepared on target substrates with λ = 35 μm have a sheet resistance Rs of 29 Ω −1 and 88.6% transparency (wavelength of incident light λhv = 550 nm), which are comparable to high-end ITO coatings (sheet resistance of Rs,ITO ≈ 10–50 Ω −1; specular transmittance of TITO ≈ 85%) (Figure 3).[22] AgNW networks (lattice spacing λ = 65 μm) with the lowest sheet resistances (2.9 ± 1.3 Ω −1) are composed of AgNW structures organized into multilayers. Fused AgNW multilayers form bundles that reduce the sheet resistance and the local transmittance. However, high transmittances are retained due to the preservation of micron-scale lattices. The observed value for corresponding spectral transmittances (74.5 ± 1.5%) approaches the calculated fractional area of AgNW coverage on the film. The highest measured value of spectral transmittance for TCE prepared from AgNW networks in this study is 91.3 ± 1.1% for films with narrow lattice spacing (λ = 25 μm). These films are insulating due to discontinuous AgNW networks that are in the sub-percolation threshold regime. Reconfigurable transfer substrates with lattice spacing of λ < 36 μm contain features that are discontinuous. This in turn prevents
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Adv. Mater. 2014, 26, 706–711
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COMMUNICATION Figure 3. Optical and Electrical Properties of Silver Nanowire Networks. a) The voltage-current characteristics of AgNW networks indicate a strong dependence on λ. AgNW networks prepared using donor substrates with reconfigurable grating microstructures exhibit reduced sheet resistances as λ increases from 36 to 65 μm. Samples with lattice spacing λ = 25 μm were insulating (data not shown). b) These values are stable at currents from 0.1 to 1 mA. c) The normalized specular transmittances for the AgNW networks are shown for visible wavelengths. d) The sheet resistance and specular transmittance (λhv = 550 nm) are shown for AgNW networks.
the formation of percolating AgNW structures on the target substrate thereby rendering the eventual TCE to be electronically insulating. This is the primary mechanism for the formation of insulating networks at small values of λ. Increasing the surface density of AgNW networks (# AgNW per area) occurs as λ increases because the larger corresponding A0 can template more AgNW for a given substrate area. This increase will compensate the larger AgNW pitch due to increased λ. Increasing the surface density of AgNW produces a well-characterized trade-off between conductivity and transmittance that is also observed in disordered AgNW networks.[22,47] The rapid alignment of the AgNW into the grooves of the anisotropic features on reconfigurable transfer substrates can be attributed to a net outward flow in the droplet during evaporation of the solvent and a high specific gravity of silver. The net radial outward flow during evaporation of a droplet is a welldocumented effect that occurs due to contact line pinning.[48] This radial flow aligns the AgNW parallel to the features while the high specific gravity anchors them into the groove. Long AgNW (Length ≈ 25 μm, Diameter ≈ 90 nm, AgNW25) can also be patterned within grooves more efficiently than silver nanoparticles (AgNP, Figure S7) or short nanowires (Length ≈ 10 μm, Diameter ≈ 40 nm, AgNW10, Figure S8). Selective sequestration and alignment of AgNW within the grooves of reconfigurable topography is essential for achieving high figures of merit in the eventual TCE network. Networks formed from AgNW10 exhibit a much smaller fraction of pristine surface compared to networks formed using AgNW25. However,
Adv. Mater. 2014, 26, 706–711
the former exhibit slightly higher optical transmittances due to smaller diameter of AgNW10 nanowires, which reduces scattering and absorption of incident light. Networks with shorter nanowires require a higher surface density to create percolating networks. Networks formed from AgNW10 and AgNP using reconfigurable transfer substrates were largely insulating across a wide range of wavelengths. TCE fabricated using AgNW10 and lattice spacings >40 μm were electrically conductive (Figure S9). The sheet resistance was significantly higher compared to TCE networks prepared using AgNW25 with comparable lattice spacings. AgNW networks composed of patterned structures formed using transfer substrates with reconfigurable topography exhibit substantially higher optical transmittances for a given sheet resistance compared to networks composed of randomly orientated AgNW networks (Figure 3d and Figure S10). TCE based on randomly aligned AgNW from solutions of 0.1 mg mL−1 exhibit characteristic sheet resistance of 2.7 ± 1.4 Ω −1 at optical transmittances of 58.3 ± 4.8% (λhv = 550 nm). The ratio of DC conductivity to optical conductivity can be used as a lumped figure-of-merit (FoM, F o M = FopF DC ) to sum(8 hv ) marize the electrical conductivity of thin metallic transparent films as established by the following relationship: 188.5 Fop (8 hv ) −2 T (8 hv ) = 1 + F DC Rs
(3)
where T(λhv) and σop(λhv) is the wavelength-dependent transmittance and optical conductivity of the film (λhv = 550 nm) and
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Rs and σDC represent the sheet resistance and DC conductivity of the film, respectively. The calculated FoM values range from 104 to 414 for AgNW networks formed from AgNW25 with lattice spacings ranging from 36 and 65 μm, respectively (Figure S11). FoM is positively correlated with λ because increasing the thickness of AgNW bundles increases σDC while the σop(λhv) remains largely constant due to the preservation of lattice geometries. Furthermore, the transition from the percolation regime into the bulk-like regime within patterned AgNW structures occurs between lattice spacings of λ = 36 μm and λ = 45 μm where the thickness of the AgNW structures exceeds a critical value of tmin that defines the threshold between the percolation regime to the bulk regime in AgNW. This observation is in close agreement with the detailed studies by De et al.[21,49] Achieving lattice spacing of λ > 100 μm may further improve the FoM by virtue of enhancing the transmittance by increasing the area fraction of the pristine PET substrate. The increased susceptibility to Y-shaped junction defects could be offset by reducing defect formation during spontaneous wrinkling. A larger λ and a smaller defect density may be achieved by using alternate materials during the preparation of reconfigurable substrates. The performance of AgNW networks prepared using this fabrication strategy could be further increased by fabricating composites using additives such as graphene, nanotubes, conducting polymers, and other electronically active carbon nanomaterials.[2] The value of FoM presently achieved in AgNW networks using reconfigurable topography compares favorably with alternative TCE materials such as carbon nanotube films (FoM = 25)[50] and industry standards including ITO (FoM = 400–800).[21] It should be noted that the films were characterized by measured the specular transmittance which can be up to 10% lower than the diffusive transmittance according to Hu et al.[22] Diffusive transmittances of 85–90% and a sheet resistances
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Reconfigurable Topography for Rapid Solution Processing of Transparent Conductors Haosheng Wu, Monisha Menon, Evan Gates, Aditya Balasubramanian, and Christopher J. Bettinger* Transparent conducting electrodes (TCE) are an integral component of optoelectronic devices used in technologies ranging from clean energy production to consumer electronics.[1] Indium tin oxides (ITO) are commonly used in many applications despite many challenges associated with the high cost of raw materials, brittle mechanical properties, and processing steps that often require high vacuum and elevated temperatures. These potential drawbacks motivate the pursuit of alternative materials to replace ITO for use as next-generation TCE. The ideal material for TCE is cost-effective, permissive of integration with flexible substrates, and compatible with mild processing strategies. Towards this end, many new materials and structures are currently being explored as next-generation TCE including metallic nanowires,[2–5] carbon nanotubes,[6,7] graphene,[8–10] self-assembled nanomeshes,[11] and polymernanoparticle composites.[12,13] Current efforts are motivated by the need to maximize optical transmittance while maintaining sufficient electrical conductivity for the prospective application. Materials processing and fabrication strategies are also important considerations in the economical production of TCE. Lithographic methods produce TCE with ordered structures that generally afford both higher performance and increased cost of production compared to bulk deposition processes that generate TCE with randomly oriented substituents absent of short-range order. This tradeoff is evident in the fabrication of high performance TCE using ordered metallic nanostructures utilizing well-defined templates.[14–18] Methods for solution-processing of metallic nanowires into random arrays are
Prof. C. J. Bettinger Department of Materials Science and Engineering Department of Biomedical Engineering Carnegie Mellon University Pittsburgh, PA 15213, USA E-mail: [email protected] Prof. C. J. Bettinger McGowan Institute of Regenerative Medicine 450 Technology Drive Suite 300, Pittsburgh, PA 15219, USA H. Wu, M. Menon, A. Balasubramanian Department of Materials Science and Engineering Carnegie Mellon University Pittsburgh, PA 15213, USA E. Gates Department of Mechanical Engineering Carnegie Mellon University Pittsburgh, PA 15213, USA
DOI: 10.1002/adma.201302377 706
wileyonlinelibrary.com
amenable to fabrication of large area TCE.[19–23] Furthermore, solution-processing of nanomaterials is compatible with largevolume manufacturing techniques such as roll-to-roll processing.[8] Hybrid approaches have emerged as potential strategies to fabricate organic and inorganic nanomaterials into structures for use in TCE.[24,25] The ideal materials processing strategy will combine the high performance of ordered materials with the cost-effectiveness of solution-processed randomly oriented structures. A hybrid strategy for TCE fabrication based on transfer printing of ordered silver nanowire (AgNW) networks is described in this Communication. The key component of this solution-based approach is AgNW templating using donor substrates with reconfigurable topography. TCE based on AgNW arrays with areas ≈1 cm2 can be rapidly integrated with flexible substrates. Flexible TCE processed in this manner exhibit performance metrics comparable to ITO layers 200 nm in thickness while obviating processing steps that employ extreme temperatures, high vacuum, or photolithography.[26] Bi-layer substrates under uniaxial compression exhibit micron-scale features that can impart mechanical flexibility in functional electronic devices including interconnects,[27,28] transistors[29–31] and solar cells.[32,33] Wrinkled elastomeric substrates can also serve as templates to guide the assembly of nanomaterials.[34,35] Bi-layer substrates are produced by forming thin rigid oxide membranes on pre-strained elastomeric substrates using ultraviolet ozone (UVO) (Figure 1).[36] Reconfigurable topographic substrates are formed by alternating between the compressed state (ε0 = 0%) which induces the formation of highly anisotropic micron-scale wrinkles and the strained state (ε0 = +40%) which abolishes the structures in a reversible manner (Figure S1, Supporting Information).[37] Reconfigurable topographic substrates template AgNW deposited from solution (Figure 1b-i). AgNW register and align with troughs of the grating features in aqueous environments prior to evaporation of the solvent (Figure S2). The rapid positioning of AgNW is attributed to radial outward flow in combination with the high specific gravity of silver (ρAg = 10.49 g cm−3).[38] Additional alignment is ensured as the contact line recedes in a mechanism that has been previously observed.[35] The application of uniaxial strain recovers the flat substrate and exposes the patterned AgNW networks in preparation for transfer printing (Figure 1b-iii). Thermal release tape provides a suitable intermediate material to transfer AgNW from reconfigurable donor substrates to flexible polyethylene terephthalate (PET) target substrates (Figure 1b-iv; Figure S2).[39–42] Furthermore, this procedure can be repeated to produce more complex patterns on PET substrates such as orthogonal lattices (Figure 2). The characteristic wavelength λ0 and amplitude A0 of anisotropic
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, 26, 706–711
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 1. Solution Processing and Transfer Printing of Silver Nanowire Networks Using Donor Substrates with Reconfigurable Topography. a) Donor substrates are fabricated using (a-i) pre-strained bi-layer composites consisting of silicone films with (a-ii) thin silicon oxide membranes formed by exposure to ultraviolet ozone. a-iii) Releasing the pre-strain leads to formation of highly anisotropic grating microstructure arrays. Reapplying the prestrain abolishes the features. b) Topographic substrates are used to (b-i) pattern AgNW deposited from solution. b-ii) Grating features align AgNW (See Supplementary Information). b-iii) AgNW structures are then accessed by reapplying uniaxial strain, (b-iv) loaded onto a transfer substrate, and (b-v) deposited on a target substrate. b-vi) This process can be repeated to pattern orthogonal arrays of AgNW structures into lattice geometries. c) Representative SEM micrographs of topographic substrates with wavelengths of λ0 ≈ 40 μm are shown in (c-i) cross-section and (c-ii) top-down perspective. d) The amplitude A0 and wavelength λ0 of the features are positively correlated and can be controlled by altering the thickness of the silicon oxide membrane t.
sinusoid-like grating features are governed by intrinsic and extrinsic properties of the substrate and the rigid membrane as the follows: Bt g0 8 0 = √ , A0 = t −1 gc gc
(1)
where t is the thickness of the rigid membrane, ε0 is the prestrain, and εc is the critical strain to induce buckling where εc is estimated by: 1 gc = 4
3E s (1 − 4 μm) (Figure S5). This intermediate step improves the transmittance of the final AgNW network. The formation of a lattice of AgNW structures through a sequential two-step patterning strategy is also critical in achieving TCE with high electrical conductivity. Spontaneous wrinkle formation produces grating arrays with a non-negligible defect density that effectively eliminates long-range order in the resulting features on the substrate. “Y-shaped junction” defects, the convergence of two adjacent ridge features into one ridge,[46] form during the strain relaxation of PDMS-SiO2 bi-layers (Figure S6). These defects produce discontinuities in the primary AgNW network and ultimately reduce the electrical conductivity of film in the direction parallel to the predominant orientation of the AgNW structures. An additional transfer step can deposit a secondary AgNW network that is perpendicular to the primary AgNW network. The secondary network dramatically increases the conductivity in the film by ensuring a percolating network. Omitting the deposition of the secondary orthogonal AgNW network produces insulating films if the smallest dimension is >1 mm (data not shown). Therefore, the secondary orthogonal AgNW network is essential for fabricating AgNW lattices for use in practical TCE with areas >1 cm2. The physical properties of TCE composed of AgNW lattices exhibit several notable trends when fabricated using transfer printing with donor substrates that have reconfigurable topography. The center-to-center lattice spacing between parallel
708
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AgNW structures transferred onto PET target substrates λ is 10–30% larger than λ0 (Figure 2). This can be attributed to the slight strain-dependence of λ0 in this system. This observed behavior represents a departure from previous work in other bi-layer substrates which the wavelength λ0 is largely independent of the applied tensile strain,[29,30] yet lies in close agreement with composites created by exposing PDMS substrates to UVO.[35] Grating structures with larger λ0 and A0 can accommodate more AgNW per area on donor substrates. AgNW structures exhibit increasing line width and film thicknesses as λ0 and A0 both increase. The edges of the transferred AgNW structures contain a small fraction of AgNW that are randomly oriented due to insufficient hydrodynamic focusing or strong adhesion to the SiO2 surface. AgNW networks prepared on target substrates with λ = 35 μm have a sheet resistance Rs of 29 Ω −1 and 88.6% transparency (wavelength of incident light λhv = 550 nm), which are comparable to high-end ITO coatings (sheet resistance of Rs,ITO ≈ 10–50 Ω −1; specular transmittance of TITO ≈ 85%) (Figure 3).[22] AgNW networks (lattice spacing λ = 65 μm) with the lowest sheet resistances (2.9 ± 1.3 Ω −1) are composed of AgNW structures organized into multilayers. Fused AgNW multilayers form bundles that reduce the sheet resistance and the local transmittance. However, high transmittances are retained due to the preservation of micron-scale lattices. The observed value for corresponding spectral transmittances (74.5 ± 1.5%) approaches the calculated fractional area of AgNW coverage on the film. The highest measured value of spectral transmittance for TCE prepared from AgNW networks in this study is 91.3 ± 1.1% for films with narrow lattice spacing (λ = 25 μm). These films are insulating due to discontinuous AgNW networks that are in the sub-percolation threshold regime. Reconfigurable transfer substrates with lattice spacing of λ < 36 μm contain features that are discontinuous. This in turn prevents
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Adv. Mater. 2014, 26, 706–711
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COMMUNICATION Figure 3. Optical and Electrical Properties of Silver Nanowire Networks. a) The voltage-current characteristics of AgNW networks indicate a strong dependence on λ. AgNW networks prepared using donor substrates with reconfigurable grating microstructures exhibit reduced sheet resistances as λ increases from 36 to 65 μm. Samples with lattice spacing λ = 25 μm were insulating (data not shown). b) These values are stable at currents from 0.1 to 1 mA. c) The normalized specular transmittances for the AgNW networks are shown for visible wavelengths. d) The sheet resistance and specular transmittance (λhv = 550 nm) are shown for AgNW networks.
the formation of percolating AgNW structures on the target substrate thereby rendering the eventual TCE to be electronically insulating. This is the primary mechanism for the formation of insulating networks at small values of λ. Increasing the surface density of AgNW networks (# AgNW per area) occurs as λ increases because the larger corresponding A0 can template more AgNW for a given substrate area. This increase will compensate the larger AgNW pitch due to increased λ. Increasing the surface density of AgNW produces a well-characterized trade-off between conductivity and transmittance that is also observed in disordered AgNW networks.[22,47] The rapid alignment of the AgNW into the grooves of the anisotropic features on reconfigurable transfer substrates can be attributed to a net outward flow in the droplet during evaporation of the solvent and a high specific gravity of silver. The net radial outward flow during evaporation of a droplet is a welldocumented effect that occurs due to contact line pinning.[48] This radial flow aligns the AgNW parallel to the features while the high specific gravity anchors them into the groove. Long AgNW (Length ≈ 25 μm, Diameter ≈ 90 nm, AgNW25) can also be patterned within grooves more efficiently than silver nanoparticles (AgNP, Figure S7) or short nanowires (Length ≈ 10 μm, Diameter ≈ 40 nm, AgNW10, Figure S8). Selective sequestration and alignment of AgNW within the grooves of reconfigurable topography is essential for achieving high figures of merit in the eventual TCE network. Networks formed from AgNW10 exhibit a much smaller fraction of pristine surface compared to networks formed using AgNW25. However,
Adv. Mater. 2014, 26, 706–711
the former exhibit slightly higher optical transmittances due to smaller diameter of AgNW10 nanowires, which reduces scattering and absorption of incident light. Networks with shorter nanowires require a higher surface density to create percolating networks. Networks formed from AgNW10 and AgNP using reconfigurable transfer substrates were largely insulating across a wide range of wavelengths. TCE fabricated using AgNW10 and lattice spacings >40 μm were electrically conductive (Figure S9). The sheet resistance was significantly higher compared to TCE networks prepared using AgNW25 with comparable lattice spacings. AgNW networks composed of patterned structures formed using transfer substrates with reconfigurable topography exhibit substantially higher optical transmittances for a given sheet resistance compared to networks composed of randomly orientated AgNW networks (Figure 3d and Figure S10). TCE based on randomly aligned AgNW from solutions of 0.1 mg mL−1 exhibit characteristic sheet resistance of 2.7 ± 1.4 Ω −1 at optical transmittances of 58.3 ± 4.8% (λhv = 550 nm). The ratio of DC conductivity to optical conductivity can be used as a lumped figure-of-merit (FoM, F o M = FopF DC ) to sum(8 hv ) marize the electrical conductivity of thin metallic transparent films as established by the following relationship: 188.5 Fop (8 hv ) −2 T (8 hv ) = 1 + F DC Rs
(3)
where T(λhv) and σop(λhv) is the wavelength-dependent transmittance and optical conductivity of the film (λhv = 550 nm) and
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Rs and σDC represent the sheet resistance and DC conductivity of the film, respectively. The calculated FoM values range from 104 to 414 for AgNW networks formed from AgNW25 with lattice spacings ranging from 36 and 65 μm, respectively (Figure S11). FoM is positively correlated with λ because increasing the thickness of AgNW bundles increases σDC while the σop(λhv) remains largely constant due to the preservation of lattice geometries. Furthermore, the transition from the percolation regime into the bulk-like regime within patterned AgNW structures occurs between lattice spacings of λ = 36 μm and λ = 45 μm where the thickness of the AgNW structures exceeds a critical value of tmin that defines the threshold between the percolation regime to the bulk regime in AgNW. This observation is in close agreement with the detailed studies by De et al.[21,49] Achieving lattice spacing of λ > 100 μm may further improve the FoM by virtue of enhancing the transmittance by increasing the area fraction of the pristine PET substrate. The increased susceptibility to Y-shaped junction defects could be offset by reducing defect formation during spontaneous wrinkling. A larger λ and a smaller defect density may be achieved by using alternate materials during the preparation of reconfigurable substrates. The performance of AgNW networks prepared using this fabrication strategy could be further increased by fabricating composites using additives such as graphene, nanotubes, conducting polymers, and other electronically active carbon nanomaterials.[2] The value of FoM presently achieved in AgNW networks using reconfigurable topography compares favorably with alternative TCE materials such as carbon nanotube films (FoM = 25)[50] and industry standards including ITO (FoM = 400–800).[21] It should be noted that the films were characterized by measured the specular transmittance which can be up to 10% lower than the diffusive transmittance according to Hu et al.[22] Diffusive transmittances of 85–90% and a sheet resistances
