Start Date: September 2008
Zhenan Bao, Department of Chemical Engineering, Stanford University
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.
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.
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.