A hot buzzword in the materials science communities these days is nano-engineering – that is, exploring unique physics at the nanoscale (10^-9 meters or less) to improve device performance. The idea is that there are fundamentally different physics operating material behavior at this small scale. This means that engineering designs can exploit this physics using certain structures, interfaces, etc. controlled at the nanoscale level that will lead to qualitatively different performance compared to design control at the micrometer or millimeter level, for example.
A new paper in Advanced Materials demonstrates how these unique properties at the nanoscale will be helpful for designing next generation solar cells (or other electronics). Barbero et al designed an experimental process in which a liquid polymer containing a very small concentration of single walled carbon nanotubes (SWNT) is placed in a mold (yellow object below) that creates nanopillars made of small bundles of nanotube networks (blue object below).This is particularly exciting because nanotubes have a host of beneficial properties as an optimal property for new electronic devices. They have terrific tensile strength, which means they can undergo great stress without breaking. They also have exceptional electrical mobility along their one-dimensional axis, making them ideal for charge transfer and transport which are crucial for solar cell performance.
The main obstacle in creating nanotube-based devices has been determining an experimental method to distribute the nanotubes evenly in a network that takes advantage of their fast charge transport along the tube axis. Nanotubes strongly interact with each other due to van der Waals interactions, which make the nanotubes want to bundle together rather than connect along the tube axes. Because of this, just randomly coating a surface with nanotubes will lead to clumps of nanotubes, known as aggregation, that severely limits conductivity and charge transport.
Barbero’s method provides a way to guide the nanotubes into the pillars in such a way that they elongate and connect in sparse networks without clumping. The authors explain that viscous forces in the liquid polymer create a drag force to pull the nanotubes into the pillars. The unique nanoscale of the pillars then only lets a few nanotubes into each pillar, preventing aggregation and promoting their orientation along the pillar axis (although more research will be required to examine their exact orientation using polarized Raman spectroscopy). It is likely that, as the pillar radius increases, the conductivity should decrease as more nanotubes can begin to aggregate similar to bulk behavior (this is a perfect future experiment!). Comparing conductivity measurements across the thickness of the nanopillar structure vs a random network (shown below), Barbero found 1-8 orders of magntiude higher conductivity with the pillars, depending on the additional polymer used (P3HT or polystyrene).This is a great first step in finding clevely constructed nanodevices to optimize charge transport for a host of electronic devices. The combination of these higher conductivities with the promising optical absorption properties of CNTs makes this an excellent foundation for CNT-polymer hybrid solar cells.
Barbero, D., Boulanger, N., Ramstedt, M., & Yu, J. (2014). Nano-Engineering of SWNT Networks for Enhanced Charge Transport at Ultralow Nanotube Loading Advanced Materials DOI: 10.1002/adma.201305843