Paper TC+EM+EN+TF-WeA3
Modeling and Characterization of Ag Nanowire-Based Transparent Conductors: Towards Optimization of Electrical and Optical Properties
Wednesday, October 30, 2013, 2:40 pm, Room 102 B
| Session: |
Transparent Conductors and Photovoltaics |
| Presenter: |
S. Narayanan, Carnegie Mellon University |
| Authors: |
S. Narayanan, Carnegie Mellon University
C. Treacy, Carnegie Mellon University
M.R. Bockstaller, Carnegie Mellon University
L.M. Porter, Carnegie Mellon University
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| Correspondent: |
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Many contemporary devices, including displays, solar cells and LEDs, employ transparent conducting films. The traditional materials for transparent conductors are transparent, conductive metal oxides, primarily tin-doped indium oxide (ITO). However, due to the increased cost of indium and other challenges with ITO, significant effort has been devoted to develop alternatives that are cheaper, flexible, and compatible with a variety of substrates. One alternative is based on random networks of solution-processed silver nanowires (Ag-NWs). While being comparable to ITO in their electrical and optical properties, the high variability in films of Ag-NWs fabricated by solution-processing is a major concern for scalability and reproducibility. The variability in NW coverage can be attributed to instability of the NWs in solution, which can be addressed by the use of polymer additives and modified solution chemistries. For example, we found that composites of Ag-NWs and poly(ethylene dioxythiphene):poly(styrene sulfonate) (PEDOT:PSS) can be deposited more uniformly and reproducibly than films comprised of Ag-NWs only. Ag-NW films that were spun-cast from solution showed bulk-like electrical resistivities (~2-50 Ω/sq) while being highly transparent (~70-90%). The films show a variability in NW coverage of ~15%, owing to aggregation. Spun-cast films of the Ag-NW/PEDOT:PSS composites show similar transmittances and resistivities but with significantly reduced variability in NW coverage of <5%. Composites with higher aspect ratios (smaller NW diameters) also show similar resistivities at transmittance values approximately 5% higher, thus showing great potential for use as transparent conductors. Additionally, by obtaining similar resistivities at lower Ag-NW coverage densities, the composites effect a lowering of the threshold for percolative conduction. In order to understand how the processing and physical conditions affect the electrical and optical properties of these NW networks, we have also modeled NW-networks under conditions similar to those of the experimental system. Both simulation and experiment show that the percolation threshold of these networked conductors can be shifted towards lower NW densities via parameters such as the nature of the NW dispersion, the composition of the network, and the NW geometry. In this presentation, we discuss results how each of these parameters affects the electrical and optical properties of Ag-NW networks, including their reproducibility, on the way towards achieving optimized characteristics for their use in devices.