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High optical transparent two-dimensional electronic conducting system and process for generating same

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High optical transparent two-dimensional electronic conducting system and process for generating same


Hybrid transparent conducting materials are disclosed with combine a polycrystalline film and conductive nanostructures, in which the polycrystalline film is “percolation doped” with the conductive nanostructures. The polycrystalline film preferably is a single atomic layer thickness of polycrystalline graphene, and conductive nanostructures preferably are silver nanowires.
Related Terms: Graphene Optic Graph Optical Crystallin

Browse recent Purdue Research Foundation patents - West Lafayette, IN, US
USPTO Applicaton #: #20140014171 - Class: 136256 (USPTO) -
Batteries: Thermoelectric And Photoelectric > Photoelectric >Cells >Contact, Coating, Or Surface Geometry

Inventors: Muhammad Ashraful Alam, Ruiyi Chen, Suprem R. Das, David B. Janes, Changwook Jeong

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The Patent Description & Claims data below is from USPTO Patent Application 20140014171, High optical transparent two-dimensional electronic conducting system and process for generating same.

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This invention was made with government support under DE-SC0001085 awarded by Department of Energy. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present disclosure relates, generally, to transparent conducting materials (“TCMs”) and, more particularly, to hybrid TCMs including a polycrystalline film that is “percolation doped” with conductive nanostructures.

B. Description of the Related Art

Transparent conducting electrodes (TCEs) require high transparency and low sheet resistance for applications in photovoltaics, photodetectors, flat panel displays, touch screen devices and imagers. Indium tin oxide (ITO), or other transparent conductive oxides, have typically been used, and provide a baseline sheet resistance (RS) vs. transparency (T) relationship. However, ITO is relatively expensive (due to limited abundance of indium), brittle, unstable, inflexible. It increases in brittleness with aging and is chemically unstable under acid/base conditions. ITO transparency drops rapidly for wavelengths above 1000 nm, so it has poor transmittance in the near infrared. Furthermore, metallic-ion diffusion from ITO into thin barrier layers may result in parasitic leakage. These and other problems make ITO-based technologies non-ideal for applications such as thin film photovoltaics (“PVs”), flexible electronics, touch-screen displays, light emitting diodes, and the like.

A suitable replacement for ITO is desired therefore. However, since resistivity and transmittance are often fundamentally constrained by the intrinsic properties of a material, developing TCMs with both low sheet resistance (e.g., RS<10Ω/□) and high transmittance (e.g., T>90%) has been a persistent challenge. Various alternative TCMs to ITO have been explored, including, by way of example, networks of carbon nanotubes (“CNTs”) and networks of metal nanowires (“NWs”). In networks of silver nanowires (AgNWs nanonet) and single-wall carbon nanotubes (SWCNTs nanonet), for NW or CNT densities corresponding to 85-95% transparency (T), conduction is typically dominated by percolation through junctions with relative large tube-tube/NW-NW contact resistance (RNW-NW), resulting in a rapid increase in baseline sheet resistance (RS) (k Ω/□−G Ω/□, depending on the NW/NT) as T increases. Networks of only metallic nanowires exhibit sheet resistance of the order of kilo-ohm/□ and more. Approaches involving welding of the nanowires, thermal annealing under pressure, or electroplating decrease RS by improving RNW-NW, but it has been challenging to reduce overall RS below ≈30Ω/□, especially for broadband T at 90%. Moreover, micrometer-sized holes within the network add series resistance to devices that rely on vertical current transport such as LEDs and solar cells. Composite transparent conducting electrodes (TCEs) employing silver NWs with another conducting polymer such as PEDOT:PSS and a combination of TiO2 nanoparticles with PEDOT:PSS have recently been demonstrated with sheet resistances of 12Ω/□ at average T of 86% over wavelengths 350-800 nm and 15Ω/□ at T550 nm of 83% respectively. The conducting polymer and TiO2 nanoparticle primarily reduce the tube-tube contact resistance.

Other alternative TCMs to ITO have been explored, including chemical vapor deposited (“CVD”) polycrystalline graphene (“poly-graphene” or “PG”) films, including single layer graphene (SLG) and few-layer graphene (FLG). “Single-crystal” graphene, such as that obtained by exfoliation from highly ordered pyrolytic graphite (HOPG) crystals, exhibits several interesting physical phenomena, including an RS lower than ITO, at a given optical transparency. Single-layer graphene (SLG) or few-layer graphene provide sufficiently high transparency (≈97% per layer) to be a potential replacement for ITO. However, the exfoliated approach yields samples that are too small for practical applications, and large-area synthesis approaches, including chemical vapor deposition (CVD), typically involving growth on copper foil and subsequent transfer to an arbitrary substrate, produces grain sizes typically ranging from a few micrometers to a few tens of micrometers, depending on the specific growth conditions. The resulting films have relatively high sheet resistance due to small grain sizes and high-resistance grain boundaries (HGBs).

While these potential ITO replacements each resolve several practical issues associated with ITO, their respective RS−T curves are not significantly different from that of ITO (as shown in FIG. 9). To achieve technologically relevant sheet resistance values (e.g., RS<20Ω/□), the density of a network of CNTs or NWs must significantly exceed the percolation threshold. These high densities of CNTs or NWs, however, reduce the transmittance of such TCMs considerably. Moreover, even with low RS, vertical current collection in PV cells is compromised by current crowding at the small-area interface between a network of CNTs or NWs and the bulk emitter layer. Meanwhile, experimental data suggests that there is a fundamental limitation to the sheet resistance and transmittance of pure poly-graphene films, making it difficult for poly-graphene to compete successfully with ITO.

It is therefore desired to produce an alternative to ITO that simultaneously exhibits high transparency and a technologically relevant sheet resistance value (e.g., RS<20Ω/□).

SUMMARY

OF THE INVENTION

According to one aspect of the invention, a hybrid transparent conducting material (TCM) comprises a granular polycrystalline film and a layer, on the granular polycrystalline film, comprising a plurality of randomly dispersed conductive nanostructures. The conductive nanostructures are in contact with, or adjacent, the polycrystalline film. The granular polycrystalline film preferably is an atomic monolayer, and the granular polycrystalline film preferably is a polycrystalline graphene film. Most preferably, the polycrystalline film is an atomic monolayer of polycrystalline graphene.

The conductive nanostructures preferably are metallic nanowires, more preferably silver nanowires. In one embodiment, each of the conductive nanostructures has a length greater than 1 μm and a cross-sectional dimension of less than 1 μm. Preferably, average length of the conductive nanostructures is greater than average grain diameter in the granular polycrystalline film, and average distance between the conductive nanostructures is greater than average length of the conductive nanostructures.

In one aspect of the invention, density of the plurality of conductive nanostructures randomly dispersed in the granular polycrystalline film is below a percolation threshold. Preferably, density of the plurality of conductive nanostructures randomly dispersed on the polycrystalline film is at most sixty percent of the percolation threshold. In some embodiments, the conductive nanostructures do not form a percolating network for charge carriers in the polycrystalline film.

In the hybrid TCM of the invention, preferably the granular polycrystalline film and the nanowire layer separately each have a sheet resistance of 20 ohms per square or greater. The hybrid TCM has a sheet resistance below twenty ohms per square. The hybrid TCM of the invention preferably has a transmittance above ninety percent for solar radiation. Preferably the transparent electrode has a sheet resistance below twenty ohms per square and a transmittance above ninety percent for solar radiation.

In the hybrid TCM, the number of conductive nanostructures preferably is less than one half the number of grains in the granular polycrystalline film, more preferably the number of conductive nanostructures is less than one fourth the number of grains in the granular polycrystalline film.

In some embodiments, the transparent electrode may comprise a plurality of stacked layers, where each of the plurality of stacked layers comprises polycrystalline graphene that is percolation doped with metallic nanowires. The transparent electrode may have a sheet resistance below twenty ohms per square and a transmittance above ninety percent for solar radiation.

A photovoltaic cell is provided according to the invention which comprises a transparent electrode comprising polycrystalline graphene that is percolation doped with metallic nanowires, wherein the metallic nanowires do not form a percolation network for charge carriers across the transparent electrode. Because the nanowires by themselves have poor electrical contact they do not provide good percolation transport apart from the combination with the graphene. The metallic nanowires are at sufficiently low density that they do not form a percolation network for charge carriers across the transparent electrode. The photovoltaic cell can comprise a transparent electrode comprising a plurality of stacked layers, with each of the plurality of stacked layers comprising polycrystalline graphene that is percolation doped with metallic nanowires.

A liquid crystal display according to the invention comprises a transparent electrode comprising polycrystalline graphene that is percolation doped with metallic nanowires.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing advantages and features of the invention will become apparent upon reference to the following detailed description and the accompanying drawings, of which:

FIG. 1A illustrates one embodiment of a poly-graphene microstructure having uniform square grains (“square”).

FIG. 1B illustrates one embodiment of a poly-graphene microstructure having uniform hexagonal grains (“hex1”).

FIG. 1C illustrates one embodiment of a poly-graphene microstructure having perturbed hexagonal grains with Gaussian size distribution (“‘hex2”).

FIG. 1D illustrates one embodiment of a poly-graphene microstructure having random grains with normal size distribution (“rand1”).

FIG. 1E illustrates one embodiment of a poly-graphene microstructure having random grains with log-normal size distribution (“rand2”).

FIG. 1F illustrates a simplified one-node model for the uniform square grains of FIG. 1A.

FIG. 2A illustrates a grain size distribution for the poly-graphene microstructures of FIG. 1C.

FIG. 2B illustrates a grain size distribution for the poly-graphene microstructure of FIG. 1D

FIG. 2C illustrates a grain size distribution for the poly-graphene microstructure 1E.

FIG. 3 is a graph of normalized sheet conductance for the poly-graphene microstructures of FIGS. 1A-E as a function of sample length, for three different percentages of high-resistance grain boundaries (“GBs”).

FIG. 4 is a graph of normalized conductivity for the poly-graphene microstructures of FIG. 1A-E as a function of the percentage of high-resistance GBs for a relatively long sample.

FIG. 5 illustrates the percolation of charge carriers through a poly-graphene film including high-resistance and low-resistance GBs.

FIG. 6 illustrates the percolation of charge carriers through a poly-graphene film including high-resistance and low-resistance GBs. where the poly-graphene film has been “percolation doped” with metallic nanowires.

FIG. 7 is a graph of normalized conductivity for pure poly-graphene and for two hybrid TCMs as a function of the percentage of high-resistance GBs, for different NW densities.

FIG. 8A illustrates one embodiment of a poly-graphene sample having perturbed hexagonal grains.

FIG. 8B illustrates a poly-graphene sample percolation doped with metallic NWs.

FIG. 8C illustrates a potential profile of the poly-graphene sample of FIG. 8A.

FIG. 8D illustrates a potential profile of the poly-graphene sample of FIG. 8B.

FIG. 8E is a graph of simulated transmittance as a function of incident light wavelength for a regularized network of NWs for two different NW densities.

FIG. 9 is a graph of transmittance as a function of sheet resistance for illustrative embodiments of the presently disclosed hybrid TCMs as well as various conventional TCMs.

FIG. 10 is a flow chart of the manufacturing method for TCM according to the invention.

FIG. 11a shows SLG transfer process of CVD graphene grown on copper and the fabrication process flow of hybrid films (Hybrid 1).

FIG. 11b shows SLG transfer process of CVD graphene grown on copper and the fabrication process flow of hybrid films (Hybrid 2).

DETAILED DESCRIPTION

OF SPECIFIC EMBODIMENTS

While the concepts of the present disclosure are susceptible to various modifications and alternative forms, specific exemplary embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the concepts of the present disclosure to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives consistent with the present disclosure and appended claims.

References in the specification to “one embodiment,” “an embodiment,” “an illustrative embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases do not necessarily refer to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. Publications and patent documents mentioned in the present specification are incorporated by reference herein in their entirety. In particular, the article “Co-Percolating Graphene-Wrapped Silver Nanowire Network for High Performance, Highly Stable, Transparent Conducting Electrodes” (Chen et al., Advanced Functional Materials, 2013), its Supporting Information, and articles referenced therein all are incorporated in their entirety by reference.

In the drawings, specific arrangements or orderings of schematic elements may be shown for ease of description. However, it should be understood by those skilled in the art that the specific ordering or arrangement of the schematic elements in the drawings is not meant to imply that a particular order or sequence of processing, or separation of processes, is required. Further, the inclusion of a schematic element in a drawing is not meant to imply that such element is required in all embodiments or that the features represented by such element may not be included in or combined with other elements in some embodiments.

The present invention uses a polycrystalline film, preferably a single layer graphene (SLG) film. SLG sheets are produced by chemical vapour deposition, and are a single atom thick layer of graphene which has a structure with grains and grain boundaries (GB). The granular nature of the graphene means that it forms a semi-continuous layer of material. It is weakly conductive.

There is an increasing effort to fundamentally understand the structure of graphene GBs and the impact of GBs on mechanical strength as well as electronic transport. The present inventors have classified GBs broadly into two categories, namely high resistance grain-boundaries (HGBs) and low resistance grain-boundaries (LGBs). Both HGBs and LGBs contribute to the resistance in a SLG film, but it is the HGBs which severely limit the (percolating) electronic transport, as indicated by best reported RS of =≈250-700Ω/□ (and 2-6 kΩ/□ for each layer in typical SLG/FLG). In typical CVD graphene grown on copper with average grain size of ≈1 μm, the present inventors have recently estimated the percentage of HGBs by using percolation theory through HGBs to interpret the resistance of circular transfer length measurement (CTLM) devices.



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stats Patent Info
Application #
US 20140014171 A1
Publish Date
01/16/2014
Document #
13919049
File Date
06/17/2013
USPTO Class
136256
Other USPTO Classes
349140, 428144
International Class
/
Drawings
14


Graphene
Optic
Graph
Optical
Crystallin


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