Low-Resistance Transparent Conductors Based on Co-Percolation in Hybrid Graphene/Nanowire Films

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Title: Low-Resistance Transparent Conductors Based on Co-Percolation in Hybrid Graphene/Nanowire FilmsNon-technical: The program will investigate a new class of low-resistance transparent conducting electrodes based on assemblies of two-dimensional (2-D) and one-dimensional (1-D) materials, including single-layer graphene and metal nanowires. A well-integrated experimental and modeling effort will investigate electrical conductance and optical transmission properties as a function of material parameters, including nanowire density and graphene grain size. The proposed research will provide new understanding of conduction and contact properties in a materials system which could provide performance superior to indium tin oxide (ITO), using more earth-abundant materials and physical advantages including flexibility. The insights from the proposed work can be applied to advanced photovoltaic devices, transparent circuitry, light emitting diodes, displays or detectors/imagers.Technical: The proposed program will investigate a new class of low-resistance transparent conducting electrodes based on co-percolating conduction in hybrid graphene/nanowire networks. In large-area graphene layers and metal nanowire networks, conduction is via percolation, with high-resistance grain boundaries or nanowire junctions limiting the sheet conductance. Hybrid graphene/nanowire systems can significantly reduce the sheet resistance, via percolation doping, in which metal nanowires bridge the grain boundaries, or, via co-percolation, in which the nanowires provide a second, coupled percolation path so that transport bottleneck of one layer are bridged by the other network. Proof of concept experiments have shown that sheet resistances of 20 Ohms/square can be achieved at optical transmittances of 90% in hybrid networks operating in the co-percolation limit, with stable performance observed over several months. In this program, a well-integrated experimental and modeling effort will address two themes. The first theme will focus on understanding the conduction mechanisms and relationship between structure and electronic/optical properties in hybrid graphene/nanowire networks tuned between percolation doping and co-percolation limits. The second theme will focus on understanding the contact properties between these hybrid networks and "device" layers including materials of interest for organic photovoltaic devices or thin-film transistors. This theme will apply the transparent conducting electrodes to contact structures of interest for photovoltaic and transistor devices in order to study potential performance improvements. The intellectual merits of the program include exploration of the conduction and contact properties in a new class of nanostructured 2D-1D hybrids of graphene and metal nanowires to address the fundamental limits of these materials. The well-integrated experimental and modeling efforts will explore the parameter-space of the composite to provide insights into the nature of the conduction in such coupled systems, contact properties to relevant device layers, and pathways toward improved sheet resistance versus transparency relationships. In addition to standard electrical, optical and structural characterization techniques, the experimental effort will utilize thermo-reflectance imaging to provide information on current pathways and bulk versus junction effects within the networks. The modeling effort builds on unique capabilities for understanding nanoscale electronic transport in disordered systems as well as fundamental models of nanostructured/organic photovoltaic devices. The broader impacts of the program include an integrated education and outreach program aimed at addressing challenges in nanotechnology-related education/training arising from the interdisciplinary nature of many of the projects, and aimed at providing opportunities for participants from under-represented groups. The proposed program includes i) development of modules for a "Nanohub-U" course, accessible worldwide, ii) integrated research and professional development experiences for undergraduates, and iii) a research and technology symposium focused on transparent conducting electrodes.