Asymmetric Pentagonal Metal Meshes for Flexible Transparent

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Asymmetric pentagonal metal meshes for flexible transparent electrodes and heaters Daniel Lordan, Micheal Burke, Mary Manning, Alfonso Martin, Andreas Amann, Dan O' Connell, Richard Murphy, Colin Lyons, and Aidan J. Quinn ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12995 • Publication Date (Web): 12 Jan 2017 Downloaded from http://pubs.acs.org on January 13, 2017

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ACS Applied Materials & Interfaces

Asymmetric pentagonal metal meshes for flexible transparent electrodes and heaters Daniel Lordan†, Micheal Burke†, Mary Manning, Alfonso Martin, Andreas Amann, Dan O’ Connell, Richard Murphy, Colin Lyons and Aidan J. Quinn* Tyndall National Institute, University College Cork, Lee Maltings Complex, Dyke Parade, Cork, Ireland *Corresponding Author. E-mail: [email protected] †Both authors contributed equally to this work.

KEYWORDS Metal mesh, transparent flexible electrode, pentagon, asymmetric, transparent heaters, polyethylene terephthalate, PET

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ABSTRACT: Metal meshes have emerged as an important class of flexible transparent electrodes. We report on the characteristics of a new class of asymmetric meshes, tiled using a recently-discovered family of pentagons. Micron-scale meshes were fabricated on flexible polyethylene terephthalate substrates via optical lithography, metal evaporation (Ti 10 nm, Pt 50 nm) and lift-off. Three different designs were assessed, each with the same tessellation pattern and linewidth (5 µm), but with different sizes of the fundamental pentagonal unit. Good mechanical stability was observed for both tensile strain and compressive strain. After 1,000 bending cycles, devices subjected to tensile strain showed fractional resistance increases in the range 8% to 17% while devices subjected to compressive strain showed fractional resistance increases in the range 0% to 7%. The performance of the pentagonal metal mesh devices as visible transparent heaters via Joule heating was also assessed. Rapid response times (~ 15 seconds) at low bias voltage (≤ 5V) and good thermal resistance characteristics (213-258 oC cm2/W) were found using measured thermal imaging data. De-icing of an ice-bearing glass coupon on top of the transparent heater was also successfully demonstrated.

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1. INTRODUCTION Transparent conductive electrodes are used in a variety of applications such as thin film solar cells1, liquid crystal displays,2 touch panel displays3 and inorganic/organic light emitting diodes.4 The current market is dominated by indium-doped tin oxide (ITO) due to its high optical transparency and low sheet resistance.5 ITO films (700 nm thick) deposited on both rigid and flexible substrates yielded optical transparency, values between 78% and 85% (averaged over the visible electromagnetic spectrum, not including the substrate contribution), with corresponding sheet resistance, Rsheet, values between 6 Ω/sq and 9 Ω/sq.6 However, the brittle nature of ITO, coupled with the rising cost of indium will likely impede its use for future flexible optoelectronic devices.7-8 These shortcomings have resulted in the investigation of a large number of alternative materials and architectures for flexible transparent electrodes. Potential candidates to date include graphene,9-10 carbon nanotubes,11-12 conductive polymers13-14 and metal nanowire networks.15-16 Monolayer graphene has high intrinsic optical transparency of ~ 97.7% (measured transparency values between 97.1-97.5% reported) and good mechanical properties.10,

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However, monolayer graphene’s intrinsic sheet resistance of ~ 6 k Ω/sq is too large for use as a transparent electrode and development of stable adsorbate doping strategies has proved challenging. Graphene’s sensitivity to ambient adsorbates, as well as process residue from large area transfer of graphene deposited using chemical vapor deposition (CVD) also present significant barriers to commercial adoption.23-24 The large range of sheet resistance values reported for high-quality monolayer graphene (Rsheet ~ 125-1,200 Ω/sq) reflects these challenges.10, 19-20 Carbon nanotubes incorporated into conductive polymer support matrices possess adequate mechanical flexibility and have the potential for low cost fabrication.25-27 Sheet resistance values 3



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in the range of 50-500 Ω/sq have been reported along with transparency values between 63% and 87%,11,

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depending on nanotube quality (length, diameter and chirality distributions),

concentration, doping level, and intra-tube junction resistance, as well as polymer thickness.4, 2930

Conductive polymers such as poly(3,4-ethylenedioxythiophene):polystyrenesulfonic acid (PEDOT:PSS) are commercially available in aqueous dispersions and allow cost-effective fabrication by coating or printing methods.31-32 Sheet resistance values Rsheet ~750 Ω/sq and a transparency of 96% have been reported for 100 nm thick PEDOT:PSS films.33 The conductive properties of these films can be improved by the addition of high boiling point solvents (Rsheet ~ 65-176 Ω/sq, T ~ 80-88%)31 or acids (Rsheet ~ 39 Ω/sq, T ~ 80%).34 Despite the high transparency and mechanical flexibility,35 polymer films often suffer from unstable sheet resistance due to thermal and environmental stresses.36 Metal nanowire networks also allow fabrication of transparent conductive electrodes using solution-based processes. Sheet resistance values between 6.5-38.7 Ω/sq and transparency values between 85-91% have been reported.15, 37 However, metal nanowire network films have a high surface roughness and high fractional light scatter (i.e. haze) when large diameters are employed.38-39 Mesh-patterned metal films have emerged as promising candidates for the transparent conductive electrode market.40 The transparency and sheet resistance can be controlled by varying mesh geometry, linewidth, metal thickness as well as employing metals with different resistivity values. The use of linewidths ≤ 5 µm is advantageous due to being undetectable by the naked eye.41-42 This attribute allows the potential use of metal meshes for applications that require clear visibility. 4


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Recent research efforts on ultra-thin metal meshes have focused on symmetrical geometries such as squares40, 43 and hexagons44-45 along with patterning techniques such as UV lithography,41 nanoimprint lithography46 and novel techniques such as rolling mask lithography.47 Ghosh et al. reported square metal meshes (~ 50 nm thick Ni, 20 µm linewidth) on a 2 nm layer of Ni on fused silica substrates patterned by UV lithography with values of Rsheet ~ 28 Ω/sq and T ~ 77%.40 Hexagonal Cu

metal meshes (~ 62 nm Cu, 1 µm linewidth) patterned using UV

lithography have been reported by Kim et al. with values of Rsheet ~ 6.2 Ω/sq and T ~ 91% when an aluminium doped zinc oxide (~ 75 nm thick) capping layer was applied.44 Rolith Inc. have fabricated square Al metal meshes (~ 300 nm to 500 nm thick) with sub-micron linewidths (~ 300 nm) using a novel “rolling mask” lithography method, yielding devices with low sheet resistance (~ 3.5 Ω/sq), high transparency (~ 96%) and low haze (4-5%).48 While extensive work on symmetrical patterns for metal meshes have been reported, there have been very few reports on asymmetric metal meshes, such as grain boundary lithography. To our knowledge, there have been no reports on uniform asymmetric designs. A new class of asymmetric pentagons which can tile a 2D plane was reported late in 2015.49 The newly discovered asymmetric pentagon’s unit cell consists of a 12 pentagon array and is non-unique. The use of asymmetric metal mesh geometries may have the potential ability to distribute forces when strained leading to improved mechanical stability of these devices. This is particularly important for the integration of flexible optoelectronic devices on deformed or uneven surfaces. Recently, a promising application of metal meshes as a replacement candidate material for ITO in visible transparent heater technology has been proposed.5,

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Visible transparent heaters are

used for the de-icing and defrosting of automotive windows, advertisement boards and aviation 5



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displays, which require visual transparency in cold environments.51-53 Besides visual transparency, low sheet resistance is required to achieve a high steady-state “saturation” temperature at acceptable operating voltages. Here we report on the electrical characteristics of asymmetric metal meshes, tessellated using a new class of pentagons.49 Three different designs were assessed, each with the same tessellation pattern and linewidth (5 µm), but with different sizes of the fundamental pentagonal unit. Mechanical stability was assessed for both tensile strain and compressive strain. We also report on the performance of the pentagonal metal mesh devices as visible transparent heaters.

2. EXPERIMENTAL SECTION Fabrication of Pt metal mesh. The metal mesh devices were patterned on 125 µm thick heatstabilised polyethylene terephthalate (PET) substrate “Melinex” (Dupont Teijin UK, item # ST504). A 70 mm x 70 mm sized piece was used for processing. Hexamethyldisilazane (HMDS) was spun on the substrate using a Laurell WS400 spinner at 3,000 revolutions per minute (RPM) for 50 s to promote resist adhesion. LOR3A (positive resist) was spun on the substrate at 3,000 RPM for 50 s to produce a nominal thickness of ~ 300 nm followed by baking on a hotplate at 150 oC for 3 minutes. Again HMDS was applied at 3,000 RPM for 50 s followed by S1805 at 3,000 RPM for 50 s to produce desired thickness of ~ 450 nm. This was then baked at 115 oC for 2 minutes on a hotplate. The substrate was then placed in a Karl Suss MA1006 mask aligner and the wafer was exposed to a dark-field chrome mask (Compugraphics) by ultraviolet (UV) radiation for 3.5 s (exposure dose ~ 35 mJ/cm2). The patterns were developed using MF319 developer for 45 s and immediately placed in deionized water (DI) water to stop the reaction. The substrate was then placed in a Temescal FC2000 electron-beam evaporator system. Prior to 6


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evaporation, the chamber was pumped down to ~ 5 x 10-7 Torr. A 10 nm Ti adhesion layer was evaporated (at ~ 0.3 nm/s) followed by 50 nm Pt (~ 0.1 nm/s). Lift-off of the metal-capped photoresist was achieved by placing the wafer in R1165 Resist Remover at 90 oC followed by a DI water rinse and blow drying with nitrogen. Individual mesh devices were of size 7 mm x 11 mm (total die size of 12 mm x 15 mm) with two macro electrodes (7 mm x 2mm) for twoterminal resistance measurements. Four smaller mesh devices of size 2 mm x 2 mm (same linewidth and open area) were utilised for four-terminal sheet resistance measurements. Characterisation. Initial sheet resistance (Rsheet) values of the Pt mesh devices were evaluated from four-terminal current-voltage measurements performed at room temperature under ambient conditions using an Agilent E5270B parameter analyser interfaced to a LakeShore Desert TTPX probe station (10 mV – 200 mV bias voltage range). The same setup was used for two-terminal resistance measurements. Transparency and fractional light scatter (i.e. haze) data were measured using a UV-vis spectrophotometer (PerkinElmer Lambda 950) over a wavelength range of 400 nm – 800 nm). Quoted transparency values were taken at a wavelength of 550 nm. An integrating sphere setup was utilized to measure the transmitted and scattered light of the mesh devices to evaluate device haze. To test the mechanical stability of the mesh devices, they were manually flexed over a known radius of curvature (~ 3.8 mm) in air. The two-terminal resistance was measured periodically every 200 cycles (up to 1,000 bending cycles). Optical microscopy images were taken using a Leica DMRB microscope in transmission mode at 5x, 10x and 50x magnifications. To test the viability of the pentagonal mesh device for use as a transparent heater, a thin mist of water was sprayed to a 1 cm x 1cm glass coupon (1.2 mm thick) which was subsequently held above liquid nitrogen vapor. This process was repeated several times until an ice layer of 7



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thickness ~0.5 mm was observed. The glass substrate was then placed on top of the mesh device which was connected to a power supply (Aim – TTi EX752M) and a constant bias voltage was then applied. Thermal images and temperature vs time plots were obtained using a FLIR ONE Thermal Imager (120 x 160 pixels resolution, working distance ~ 4.5 cm) interfaced to an Android smartphone. Data analysis was performed using the FLIR Tools software. The accuracy of the FLIR One temperature readings in comparison to a temperature probe (IKA Werke ETSD4) is given in Table S3 in the supporting information.

3. RESULTS AND DISCUSSION Devices with a fixed linewidth of 5 µm and metal thickness (Ti ~ 10 nm, Pt ~ 50 nm) were fabricated on PET as described in the Experimental Section for substrate sizes up to 4” (photo in Figure S2). This approach could potentially be scaled up for manufacturing using roll-to-roll photolithography or nanoimprint lithography. The pentagonal metal mesh devices were based on targeting a lower transmission threshold of 70% for the mesh itself i.e. larger metal area coverage. The transparency of a metal mesh structure, Tmesh, can be approximated from the geometric design as: Tmesh ≈ 1 - Ametal ⁄Atotal (Equation 1), where Ametal is the area within the unit cell covered by metal and Atotal is the total unit cell area. A 10x optical microscopy image in transmission mode of the newly discovered asymmetric pentagon design is shown in Figure 1a. For this particular pentagon, the (non-unique) unit cell consists of an array of twelve pentagons (one example shaded in grey in Figure 1a). Following equation 1, the expected intrinsic transparency of the pentagonal mesh, Tmesh , was estimated as: Tmesh ≈ (1 - 1.36 w/d) (Equation 2), 8


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where w is the linewidth of the mesh and d is the length of the smallest side (w

Asymmetric Pentagonal Metal Meshes for Flexible Transparent

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