Transparent Conductive Adhesives for Tandem Solar Cells Using

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Transparent Conductive Adhesives for Tandem Solar Cells Using Polymer-Particle Composites Talysa Klein, Benjamin G Lee, Manuel Schnabel, Emily L. Warren, Paul Stradins, Adele Tamboli, and Maikel F.A.M. van Hest ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00175 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 20, 2018

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Transparent Conductive Adhesives for Tandem Solar Cells Using Polymer-Particle Composites Talysa R. Klein*, Benjamin G. Lee, Manuel Schnabel, Emily L. Warren, Paul Stradins, Adele C. Tamboli, Maikel F.A.M. van Hest National Renewable Energy Laboratory, Golden, CO, 80401, USA KEYWORDS: Transparent Conductor, Adhesives, Polymer-Particle Composites, Flexible Microspheres, Lamination, Tandem Solar Cells ABSTRACT: Transparent conductive adhesives (TCAs) can enable conductivity between two substrates, which is useful for a wide range of electronic devices. Here we have developed a TCA composed of polymer-particle blend with Ethylene-Vinyl Acetate as the transparent adhesive and metal-coated flexible Poly(Methyl Methacrylate) microspheres as the conductive particles that can provide conductivity and adhesion regardless of surface texture.

This TCA

layer was designed to be nearly transparent, conductive in only the out of plane direction, and of practical adhesive strength to hold substrates together. Series resistance was measured at 0.3 and 0.8 Ω-cm2 for 8% and 0.2% particle coverage, respectively, while remaining over 92% transparent in both cases. For applications in photovoltaic devices, such as mechanically stacked multi-junction III-V/Si cells, a TCA with 1% particle coverage will have less than 0.5% power loss due to the resistance and less than 1% shading loss to the bottom cell. INTRODUCTION Multi-junction solar cells are heavily studied in the solar industry for their ability to increase efficiencies past the single-junction Shockley-Queisser limits.1 For example, silicon (Si) has a single junction limit around 29.4%2, with the best Si cell currently at 26.7%.3 However, when Si is combined with other cells in multi-junction or tandem solar cell, efficiencies > 35% have been demonstrated without using optical concentration.3 These Si-based tandem devices are 1 ACS Paragon Plus Environment

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constructed in one of two ways. The first is growing or depositing a cell on top of the Si substrate, referred to as monolithically grown tandems.4 Epitaxial growth of III-V or II-VI materials on Si is complicated due to mismatched lattice constants and thermal expansion coefficients, causing less than ideal cell performance.5 Depositing solution-processed thin-film Photovoltaic (PV) devices onto a Si substrate, such as perovskites6 and organic photovoltaics (OPV),7 results in decreased efficiency from non-uniform coatings (both on planar and textured Si). The second method for fabricating tandems is mechanical stacking or bonding independently processed cells together. This method of forming a tandem has the advantage of separate cell processing, resulting in higher overall efficiency.

Figure 1. (a) Schematic of tandem cells in a 4-terminal configuration where each cell is joined to a common glass slide using a transparent adhesive. (b) The schematic of the 2-terminal configuration uses the developed transparent conductive adhesive (TCA) as the connecting interlayer, eliminating the need for intermediate grid lines between the cells. (c, d) The compliant microspheres can deform to bridge uneven, textured, or planar top and bottom substrates, as shown with a representative conductive pathway between cells for one and two textured surfaces. Mechanically-stacked tandem cells currently employ a common transparent substrate (such as glass) and transparent adhesives.8 This approach produces two independent electrical networks, one for the top cells and one for the bottom cells, resulting in a 4-terminal (4T) device, as shown in Figure 1a. 4T tandems produce high efficiencies, as demonstrated for GaAs/Si and GaInP/Si tandems with 1-sun efficiencies of > 32%;9 however, this geometry has complicated wiring for module integration and requires intermediate grids for lateral current collection on both the top and bottom cells, increasing the shading losses to the bottom cells. Therefore, moving to 2-

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terminal (2T) devices where the cells are monolithically connected in one network may streamline the manufacturing process and decrease shading losses, as shown in Figure 1b. For 2T devices, the interlayer needs to be: (1) transparent in the wavelength range that the bottom cell absorbs, (2) possess sufficient conductivity in the out of plane direction to transfer current between the cells with minimal series resistance losses, and (3) have adhesive strength and durability to hold the cells together over the lifetime of the module. Interlayers formed by wafer bonding satisfy these three criteria, and have been used to fabricate >30% efficient 2T IIIV/Si tandem cells.10,11 However, wafer bonding requires extremely clean, polished surfaces, resulting in a process that is costly, with low yields, and is incompatible with textured silicon.12 For inexpensive, 2T tandem cells, transparent conductive adhesive (TCA) materials must be developed that are compliant and compatible with micron-scale textured silicon. Research developing TCAs has primarily been focused on identifying replacements for transparent conductive oxides (TCOs). TCOs are used in a variety of applications including flat panel displays,13 flexible electronics,14,15 solar cells,16 and many more. The combination of low sheet resistances (5-20Ω/☐) with high transparencies (>80%) is why TCOs are widely used.17 Using vacuum deposition, TCO films can be grown on almost any substrate, but require large capital costs, and many use expensive materials, such as indium.17 For applications in solar devices, the need for solution processed, low cost TCOs is driven by rapidly decreasing cost per watt goals. TCO replacement materials such as carbon nanotubes,14 metal nanowires,15 conductive polymers,18 and graphenes14 have been studied for use as transparent conductive electrodes (TCEs)17 in PV devices. Additionally, various particle-polymer blends using these materials combined with transparent polymers, such as commercial epoxies,19 silicones,20 poly(methyl methacrylate) (PMMA),21 and polyurethanes,22 have been developed. For this application, TCEs have been focused on high in plane (lateral) conductivity of the films. While each of these interlayers has many promising aspects, few of them can be thick enough to bridge uneven or textured surfaces while retaining transparency and high conductivity in the out-ofplane direction, which is most relevant for the applications in tandem solar cells. In this paper, we discuss a TCA for mechanically stacked tandem cells using polymer-particle composite of transparent adhesives and metal-coated flexible microspheres. As shown in Figure 1c, the TCA can be used between one smooth and one textured, or two smooth or textured surfaces as shown in Figure 1d. The metal-coated conductive polymer microspheres provide 3 ACS Paragon Plus Environment


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flexibility to substrate texture and deform to provide greater contact area between the substrates, while the polymer provides adhesive strength and transparency. The TCA is designed to be highly conductive in the out-of-plane direction with little to no conductivity laterally within the film. This study provides experimental results for transparency and series resistance for various particle loadings of this TCA.

Methods The TCA was synthesized by combining ethylene-vinyl acetate (EVA) resin pellets (DuPont, ELVAX PV1400, 33% vinyl acetate) and toluene in a 1:5 ratio by volume and stirring on a hot plate at 100°C for approximately 2 hours, or until the EVA was fully dissolved. In a separate vial, a measured weight of this EVA: toluene solution and a measured weight of silver-coated, flexible poly(methyl methacrylate) (PMMA) microspheres (Cospheric, PMPMS-AG, diameter range of 45-53µm, silver coating thickness 250nm) were thoroughly mixed on a stirring hot plate at 100°C, uncovered, for 30 minutes. The weight ratio of EVA solution and silver-coated PMMA microspheres was varied to create diverse percent microsphere coverages for testing. While toluene can easily dissolve PMMA, the metal coating of the spheres protects them from being dissolved in the solvent. The liquid TCA was then loaded into a syringe and dispensed on the bottom substrate in an amount to completely cover the desired bottom substrate. The top substrate was positioned by hand on top of the TCA/bottom substrate before pressing the sample in a hot press for various durations, temperatures, and pressures.


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Figure 2. The contact area between a silver-coated microsphere and a glass surfaces is shown in a schematic (a) and a microscope image looking through the glass slide (b). The sphere is in an EVA matrix between the glass slide and a silver-coated as-sawn silicon wafer showing the deformed contact area to the top substrate (circled in red). The silver-coated, flexible PMMA microspheres were selected as the conductive element for their ability to deform and bridge uneven, textured surfaces. Figure 2a presents a schematic of how the diameter and flexibility of the spheres bridges the gap between two independently grown cells and allows for increased contact area for individual spheres when deformed. The microspheres deform where they are in contact with a surface, such as glass, as shown by the microscope image in Figure 2b. The transparent adhesive matrix used was EVA, a common low-cost, low-temperature encapsulation material used for photovoltaic modules;23 however, other transparent adhesives, such as cyanoacrylate, polydimethylsiloxane (PDMS), polyvinyl alcohol (PVA), and polyvinyl butyral (PVB) have also been used successfully. 24 Commercially available EVA is commonly sold in either extruded sheets for lamination or pellets for extruding into sheets.23 EVA can be modified using additives and surface primers to increase the adhesive properties and chemical resistance,23 but the EVA used in this study was used additive-free for simplicity. The TCA mixtures were made with varying silver microsphere concentrations and evaluated for transparency and series resistance between the top and bottom surfaces. Figure 3 shows a schematic of samples made in two configurations: (I) glass/TCA/glass and (II) silver-coated glass or silicon/TCA/patterned silver-coated glass. Configuration I samples were used to determine the optical transmission using a Cary 5000 UV/Vis/NIR transmission/reflection spectrometer, as shown in Figure 3a,b. Configuration II samples were used to measure the TCA’s projected contribution to the series resistance of a tandem solar cell. In addition, the location of the probes for 4-point resistance measurements is shown in Figure 3(c,d). The series resistance, , was calculated by equation 1,

= = (1) where is the current supplied, the measured voltage, and the contact area, as shown in Figure 3c,d. Varying the sample area allowed for statistical analysis of the data. It was



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determined that the patterned evaporated silver contacts contributed 4.0 ∗ 10 to the total measured resistance, and can be considered negligible.Error! Bookmark not defined.

Figure 3. Schematics of (a) configuration I: glass/TCA/Glass for transparency measurements with (b) examples of 11% and 7% microsphere coverage samples. (c) Configuration II: Ag coated glass/TCA/Ag patterned glass and (d) Ag coated silicon/TCA/Ag patterned glass for vertical conductivity measurements with variable contact area (An) for calculating the series resistance (rc), with approximate thicknesses of interlayers and 4-point probe locations. Assembly of the samples was conducted using a hot press with temperature control set at 120°C (top and bottom) and pressure monitoring. To determine the processing procedure, a series of samples was made with varying pressure from 0.1 to 10 bar and pressing times from 5 to 60 minutes. Also, a series of samples were made with in-situ resistance measurements during hot pressing to determine the evolution of contact resistance during the pressing process; this information is shown in Figure S1. For each TCA sample, the percent area coverage of particles in the film was determined by image processing of optical microscope images of configuration I (glass/TCA/glass) samples.

EXPERIMENTAL RESULTS For tandem solar cells, the TCA needs to be transparent for wavelengths below the band gap of the top cell, which may be as large as 1.9 eV (652 nm), and above the band gap of the bottom 6 ACS Paragon Plus Environment

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cell, which may be as small as 1 eV (1240 nm). The transparency of the TCA comes from the EVA being a transparent medium, allowing the light to pass through the interlayer in the areas without microspheres. Figure 4a shows transmission data from samples including a single piece of glass without a TCA, glass/EVA/glass without microspheres, and glass/TCA/glass (configuration 1) samples with varying particle densities. Additionally, Figure 4b shows the average transmission between 1 and 1.9 eV as a function of particle density in the TCA. From these graphs, it can be seen that the glass slide contributes the majority of the optical losses by reflecting about 8% of the incident light, ~4% from each surface. EVA without microspheres has a transmission of 91.6%, equal to that of bare glass, demonstrating no additional losses are introduced by the EVA layer; this is due to refractive index matching. As the particle density increases, the transmission decreases, as expected, due to absorption and reflection from the metal-coated microspheres. This trend can be estimated by the geometric shading from the microspheres, as shown in Figure 4b. In a tandem solar cell configuration, the transparency loss of the TCA needs to be comparable to, or less than, that of average-sized current collection grid lines on the tops of solar cells (typically 100 µm wide fingers placed 3 mm apart). These current collection grids have shading losses of roughly 3% of the total light incident on the solar cell. Applying this criterion to the TCA, particle area coverage needs to be below 3% in order to provide similar solar cell performance metrics.25 Microscope images of samples with coverages of 11%, 5%, and 0.6% are shown in Figure 4c.



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Figure 4. (a) Transmission measurements on samples with varying microsphere concentrations show reduced transmission with increasing percent area coverages of microspheres in the region of interest (680-1130nm). (b) Average transmission for the region of interest (680-1130nm) for variable percent area coverages of microspheres with calculated geometric shading (dashed line). (c) Example microscope images of 11%, 5%, and 0.6% area coverage of microspheres. Series resistance measurements on samples in configuration II as a function of particle area coverage are shown in Figure 5. Samples were processed at 120°C for 5, 10, 30, or 60 minutes with a pressure of 0.1, 0.5, or 10 bar to determine the optimal time and pressure to press the samples. There is little variation in resistance between samples pressed for different durations, therefore a time of 10 minutes was chosen as the standard pressing time moving forward. Additional information on in-situ measurements during pressing is found in supplementary information 1. Figure 5 demonstrates that as the particle area coverage increases, the series resistance decreases due to additional conductive pathways through the TCA. At larger particle densities, the series resistance converges to around 0.3 , which is a limitation of our measurement system, as explained in further detail in supplemental information 2. Pressure has less of an


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impact; however, when samples are pressed at 10 bar, the spheres and substrates consistently break. Despite the sphere breakage, the samples with 7% coverage have similar average series resistance when pressed at 10 bar (0.72 ) and at 0.5 bar (0.35 ). This means that some of the broken spheres are still capable of making electrical connection between surfaces, thus increasing the acceptable processing window of the TCA. This becomes increasingly important as the percent coverage decreases, since there are fewer spheres to distribute the pressing force over, increasing the likelihood for breaking some spheres.Error! ookmark not defined. [supplemental info 1, and ref. 14]

Figure 5. Dependence of TCA series resistance on percent coverage for configuration II samples with the top substrate as silver patterned glass and the bottom substrate as silver coated glass and as-sawn silicon wafer where indicated, pressed for varied time and pressures as noted, with the corresponding values for shadowing loss, power loss, and average system resistance(shown as grey bar).



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For use of the TCA in tandem devices, the percent power loss and shading are the major figures of merit for evaluating a TCA interlayer. Considering a GaInP/Si tandem device with a top cell current-limited case (15 /),5 power loss percentages were calculated from the TCA’s added series resistance. For example, 1% power loss due to resistance in the TCA corresponds to a series resistance of 1.4 ; thus, Fig. 5 shows that all TCA formulations tested would result in much less than 1% loss due to series resistance. Furthermore, the shading losses are proportional to the geometric shading; 1%, 3%, and 5% shading are highlighted in Figure 5. It can be seen that for the GaInP/Si tandem cell applications, a TCA with between 0.3% and 1% area coverage will have less than 1% shading loss and less than 0.5% power loss. Within this region, we tested a sample using 0.6% area coverage on an as-sawn Si wafer. This sample yields an average series resistance of 0.41 , in line with the Ag-coated glass samples. This data point demonstrates that the TCA can be applied to rough Si surfaces without any additional loss in performance. For other TCA-like materials such as a nano-ITO and polymide particle-polymer blend, the minimum series resistance reported has been 2.3 with 78-80% transmission for wavelengths between 500 and 1000 nm.26 Graphene oxide/PEDOT:PSS gel films at 10 nm thick can reach 0.05 with roughly 88% transmission (500-1000 nm).27 However, like most PEDOT:PSS films, the transparency decreases significantly as the film thickness increases to span uneven or textured surfaces. Similar concepts of using solid (non-flexible) microspheres produced an average series resistance of 0.52 using ITO particles/cemedine gel pressed at 4 bar and 800°C for 10 minutes.15 Unfortunately, the ITO particle/cemedine gel processing temperature and time is significantly higher than what finished solar cells would be able to handle without degradation, and ITO cannot deform to textured surfaces. Comparing the EVA silver-coated microsphere TCA to other TCA-like materials, the series resistance, transparency, and processing condition combination provides both excellent performance and unique applicability to textured Si for tandem cell applications. Focusing on the out-of-plane conductivity and transparency for micron-thick interlayers, the use of nano-scale particles or wires would require multiple layers to span the gap resulting in significant optical and resistive losses. Among the conductive polymers, the most common is PEDOT:PSS, which can be used with or without the addition of nano-scale particles.18 When mixed with d-sorbitol,15 PEDOT:PSS yields an adhesive-like substance.18 However, in micron thick films, the 10 ACS Paragon Plus Environment

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transparency is greatly reduced, the adhesive strength is limited, and the fact that the material is water-based may result in detrimental moisture within the modules.18 Similarly, graphene drastically reduces in transparency with increased thickness.27

CONCLUSIONS Using a microparticle-polymer blend of an EVA transparent adhesive and silver-coated compliant PMMA microspheres, a TCA material was demonstrated. This study demonstrated that the adhesive is capable of spanning micron-sized gaps between two independent substrates, textured or smooth, while providing electrical connection between the substrates. The transmission of the TCA is described by geometric shading, and can provide transparency better than average solar cell electrical grids. Conductivity was evaluated to determine the optimal processing conditions, which were found to be 10 minutes at 0.5 bar in a hot press at 120°C for consistent series resistance measurements and adhesion strength. When pressed at higher pressures (10 bar), the PMMA microspheres ruptured, but the electrical connection remained intact, with only slightly higher resistance due to the incomplete pathways through the TCA. The series resistance of the TCA ranges between 0.3 and 0.8 , depending on the particle loading of microspheres. In contrast with most transparent conductors, this TCA material enables low resistance contacts in the out of plane direction, while minimizing lateral current transport. This TCA material can enable new applications, including tandem cell interconnections, where such a geometrically selective contact is required. For application in tandem GaInP/Si devices where the top cell is current limiting, a TCA with between 0.3% and 1% area coverage will have less than 1% shading loss and less than 0.5% power loss due to the resistance of the interlayer. While this work focuses on III-V/Si tandems, this TCA could easily be implemented for various other uses that require transparency, flexibility, and conductivity only in the out of plane direction.

AUTHOR INFORMATION Corresponding Author * [email protected]



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Author Contributions M.F.A.M.v.H., A.C.T, P.S., B.G.L, and T.R.K. conceived of the idea. M.F.A.M.v.H., A.C.T, P.S., B.G.L, M.S., E.L.W. and T.R.K help design, discuss details, and analyze data from the experiments. B.G.L. provided the silver-coated microspheres. T.R.K. implemented the idea, making the TCA and conducting all experimental work. A.C.T, M.F.A.M.v.H and P.S. supervised this project. T.R.K. wrote the manuscript with inputs and discussion from all authors.

ACKNOWLEDGMENT Funding for this work at NREL was provided by DOE through EERE contract SETP DEEE00030299 and under Contract No. DE- AC36-08GO28308. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world- wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.

ASSOCIATED CONTENT Information and figures detailing internal series resistance and in-situ measurements

ABBREVIATIONS TCA, transparent conductive adhesive; EVA, ethyl-vinyl acetate; PMMA, poly(methyl methacrylate); PV, photovoltaic; OPV, organic photovoltaics; 4T, 4-terminal; 2T, 2-terminal; ITO, indium tin oxide; PEDOT:PSS, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate); PDMS, polydimethylsiloxane; PVA, polyvinyl alcohol; PVB, polyvinyl butyral; . REFERENCES

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Transparent Conductive Adhesives for Tandem Solar Cells Using

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