Self-sorting of Metallic Carbon Nanotubes for High Performance Large Area Low Cost Transparent Electrodes

Research Areas & Activities Solar Energy Advanced Electron Transport Materials for Application in Organic Photovoltaics (OPV) Ultra-High Efficiency Thermophotovoltaic Solar Cells Using Metallic Photonic Crystals as Intermediate Absorber and Emitter Hot Carrier Solar Cell: Implementation of the Ultimate Photovoltaic Converter Photon Enhanced Thermionic Emission (PETE) for Solar Concentrator Systems Self-sorting of Metallic Carbon Nanotubes for High Performance Large Area Low Cost Transparent Electrodes Plasmonic Photovoltaics Nanostructured Materials for High-Efficiency Thin Film Solar Cells Lateral Nanoconcentrator Nanowire Multijunction Photovoltaic Cells Artificial Photosynthesis: Membrane-Supported Assemblies that Use Sunlight to Split Water Molecular Solar Cells Advanced Materials and Devices for Low-Cost and High-Performance Organic Photovoltaic Cells Inorganic Nanocomposite Solar Cells by ALD Nanostructured Silicon-Based Tandem Solar Cells Photosynthetic Bioelectricity Nanostructured Metal-Organic Composite Solar Cells Ordered Bulk Heterojunction Photovoltaic Cells Biomass Energy Hydrogen Advanced Combustion CO₂ Capture CO₂ Storage Advanced Materials & Catalysts Advanced Coal Advanced Transportation Electricity Distribution & Infrastructure Other Renewables Integrated Assessment Advanced Nuclear Energy Geoengineering Exploratory Projects All Activities Analysis Activities Technical Reports Solicitations

Research

Solar Energy

Self-sorting of Metallic Carbon Nanotubes for High Performance Large Area Low Cost Transparent Electrodes

September 2008 - August 2011

Investigators

Zhenan Bao, Department of Chemical Engineering, Stanford University

Objective

This project is investigating the use of carbon nanotubes (CNT) for transparent conducting electrodes in photovoltaic applications. In order to decrease the sheet resistance, both the electronic properties of the CNTs – namely the tube conductivity and tube-to-tube contact resistance – and the sheet morphology have to be controlled. This research aims to reach this target through the development of solution-based processes to simultaneously separate metal from semiconductor nanotubes using specific surface functional groups (such as amino- and phenyl-silanes on SiO₂ substrates) and control the alignment of the CNTs into bundles during spin-coating deposition. CNT doping methods are also being investigated to further improve the sheet conductivity.

Background

The application of carbon nanotubes (CNT) to the fabrication of transparent conductive electrodes (TCE) for photovoltaic applications has been investigated in recent years for their potential to address some of the shortcomings of current Indium-Tin-Oxide (ITO) technologies, namely the high material and fabrication costs, and the brittleness of the deposited layers. CNTs have the prospect to meet low fabrication cost targets based on the low cost of earth-abundant carbon and on their potential to be dispersed in solution for large area coating. Additionally, they have excellent thermal, mechanical, and electrical properties that make them an ideal candidate for TCEs: they are stable at the high temperatures required to process inorganic devices; they have excellent strength and flexibility; and they have an electrical conductivity three orders of magnitude greater than metals like copper.

Despite their potential, state-of-the-art CNT-based TCEs still exhibit poor performance with a sheet resistance more than ten times higher than ITO electrodes at comparable light transmittance (~80%). The key problem in reducing the sheet resistance of CNT-based layers is that as-synthesized CNTs usually contain a mixture of approximately 67% semiconducting CNTs (sc-CNT) and only 33% metallic CNTs (met-CNT), where the difference in electronic properties depends on the chirality of the nanotubes. This implies that two-thirds of the film contributes to the absorption of light while decreasing the conductance. Moreover, tube-to-tube contact resistance between sc- and met-CNTs is three orders of magnitude higher than met-met junctions and two orders of magnitude higher than sc-sc junctions. In order to maximize both transmittance and conductance and reach the efficiency of the best ITOs, it is hence necessary to develop a technology that uses 100% met-CNTs.

Some concurrent research efforts are investigating the selective synthesis of sc- or met-CNTs or bulk separation methods. However, these technologies still present fundamental obstacles that require further development. This research is exploring an alternative strategy based on a self-sorting mechanism to be implemented during the deposition process. In addition to sorting met-CNTs, the technology investigated in this project may ultimately be capable of organizing the CNT network into partially aligned bundles. Controlling the morphology of the CNT layer will permit a further decrease in the sheet resistance of CNT-TCEs by minimizing tube-to-tube junctions. Finally, a CNT doping method is also being explored to increase the film conductivity. It is anticipated that the combination of all these approaches will lead to a CNT electrode with the potential to outperform the best available ITO electrodes.


Figure 1:	A schematic illustration of the sorting method. A CNT solution is spin-coated on functionalized surfaces. Met-CNTs are primarily absorbed on phenyl functionalized surfaces while sc-CNTs are absorbed on amine functionalized surfaces. Surfaces that selectively absorb met-CNTs will be used to fabricate transparent electrodes.

Approach

The self-sorting method investigated in this project is based on the use of a surface functionalized with a variety of chemical groups capable of selectively absorbing arc-discharged met-CNTs during spin-coating. In contrast to other bulk separation methods, the separation of CNTs using this approach is not dependent on the dispersion ability of different chemical groups that would selectively bind to either met- or sc-CNTs, but rather the surface binding groups determine the type of CNTs being absorbed on the surface.

The first task of the project is to identify molecular species with high binding affinity towards met-CNTs. Target molecules include amine and aromatic molecule terminated silanes, for which size, shape, electronic structure, and functionality must be optimized against absorption strength and selectivity towards met-CNTs.

Two methods are being investigated for their ability to control the final morphology of the CNT sheet: spin-assisted and polymer-assisted solution coating.

The spin-assisted solution coating technique allows both the simultaneous self-sorting and deposition of the CNTs, and their morphological organization. In fact, the stretching forces applied to the tubes during spin coating allow them to align and to stretch out, leading to the formation of percolation pathways with lesser concentration of defects. Preliminary results show that roughly 70% of the SWNTs are aligned within +/-10% of an arbitrary axis and that the degree of alignment can be tuned based on spin-assembly conditions. Spin speed, soaking time, and CNT concentration have to be optimized to tune the tube density and achieve a transmittance of ≥80% with low sheet resistance.

For polymer-assisted coating, the met-CNTs absorbed on a surface are collected and deposited. Preliminary observations show that addition of conjugated polymers assists the dissolution of CNTs and the deposition of more uniform films compared to surfactant-based aqueous dispersions. Additionally, the presence of polymers triggers the formation of CNT nanofibers that can decrease the number of junctions and greatly improve the sheet conductivity. The length and diameter of the CNT bundles can be easily tuned and their geometry can be optimized for sheet conductivity and transmittance.

Finally, further sheet resistance reduction is being achieved using novel post-fabrication doping methods. This approach has already shown promising preliminary results and the potential for a multi-fold reduction of sheet resistance.

Issued March 2009