COLIN KLINGER, YOGESHWARI PATEL, and HENK W. CH. POSTMA
Solar cells have great potential as an alternative energy source because of the enormous amount of available energy and its distributed nature that may enable a distributed power generation grid . However, for solar energy to be cost-effective on a utility scale, the price of purchase, installation, operation and maintenance over the lifetime of a solar panel per kWh generated must compare favorably to current power generation technology, which for fossil-fuel based generation is 0.03-0.05$/kWh . Improvements are being made to solar cells to 1) increase the efficiency, and 2) lower the price. For instance, solar concentrators are being developed that focus solar light reflecting off a large mirror on a solar cell with a smaller surface area. Multi-junction devices are being developed that use junctions between materials with different band gaps to capture a greater number of photons and limit loss of excess photon energy when the excited high-energy electron relaxes to the Fermi level.
Carbon Nanotube Solar Cells. © Klinger C, Patel Y, and Postma HWC. PLoS ONE 7,5 (2012); doi:10.1371/journal. pone.0037806. Licensed under the Creative Commons Attribution 3.0 Unported License, http://creativecommons. org/licenses/by/3.0/.
Gratzel cells , also known as Dye-Sensitized Solar Cells (DSSCs), offer a particularly interesting path to cost-effective solar power. By sacrificing some efficiency but offering a greater reduction in cost, the total price per kWh can be reduced considerably. While this initial argument for DSSCs is very compelling, it is worth noting that the current state-of-the art DSSCs have efficiencies that rival their solid-state counterparts - . Another advantage of DSSCs is that they operate well in low-light and overcast conditions. DSSCs typically consist of a transparent semiconducting film on conducting glass that functions as a photo-active electrode (figure 1a, top). A glass plate is coated with Pt and acts as the counter electrode (figure 1a, bottom). Light-sensitive dye molecules are adsorbed on a semiconducting material on another slide and the assembly is immersed in an electrolyte, typically iodide-triiodide (I/I3). An incoming photon with energy hv excites an electron from the dye into the conduction band of the semiconductor and it migrates to the bottom electrode. The electrolyte reduces the dye, creating triiodide (3I – ^I3- + 2e). The electrons follow the external circuit through the load to the counter electrode. The triiodide migrates through the electrolyte to the Pt electrode and gets reduced, thereby completing the circuit. The transparent semiconductor is typically made of nanoporous TiO2. Using a nanoporous material significantly increases the surface area available for dye molecules but at the same time limits the electron migration rate. Different transparent semiconductors are being studied with higher mobility, such as nanowire-based electrodes , . The liquid electrolyte is not very stable at the wide range of temperatures solar cells typically are exposed to, so high-mobility solids are being investigated as well , , culminating recently in a record 12% conversion efficiency . Various dyes have been used in DSSCs, ranging from metal-free organic dyes  through highly efficient Ru-based organic dyes such as ‘N3 dye’ ,  and ‘black dye’ - to engineered semiconductor quantum dots with a very high extinction coefficient . C60 has been shown to work as a ‘dye’ as well , . Carbon nanotubes (CNTs) , , offer a potentially cheaper and easier alternative to these materials. They are photo active, highly conductive, strong, and chemically inert. Carbon nanotubes can be synthesized in multiple ways such as chemical vapor deposition or laser ablation. The natural ratio of as-synthesized carbon nanotubes is 2/3 semiconducting to 1/3 metallic.
FIGURE 1: Carbon nanotube solar cells; comparison to Dye-Sensitized Solar Cells (DSSC), construction, and energeticts. a) DSSC. b) Carbon Nanotube Solar Cell, CNSC. c) Layout of a CNSC. The top and bottom glass slides are covered in carbon nanotube films which are electrically connected by the iodide-triiodide electrolyte that is contained by the silicone separator. The top film is the photoactive electrode, while the bottom electrode is the counter electrode. The inset is an Atomic Force Micrograph of the height of a 2×2 m section of a carbon nanotube film. d) Band diagram of the CNSC.
Here, we present proof-of-concept solar cells that are entirely made of carbon nanotubes, carbon-nanotube-based solar cells (CNSCs, figure 1b). They are a variation on the DSSC, and potentially offer many advantages beyond DSSCs. 1) No Dye. As these cells use semiconducting CNTs for photo conversion, they do not rely on dyes, which may bleach, severely limiting the useful life of DSSCs. 2) No Pt. Pt is often used as counter electrodes and their use in DSSCs represent an undesirable reliance on noble metals which may inhibit the use of DSSCs on a large, i. e. utility, scale. In addition, Pt has been reported to degrade due to the contact with the electrolyte . Carbon nanotubes, in contrast, are chemically inert, and indeed show promising characteristics as counter electrodes -. 3)
No In. As the carbon nanotube film itself is a transparent conductor, the use of a conducting coating made of, e. g. InSnO, is not required, eliminating the need for the exceedingly rare Indium. 4) The application of carbon nanotubes to the glass slides is a low temperature spray-coating process. In addition, these CNSCs multiply the advantages offered by DSSCs over single and multi-junction solar cells that require high-grade semiconductors and clean-room manufacturing. The use of low-grade materials and resulting projected significant reduction in cost of manufacturing potentially offsets the limited efficiency of these cells when relating the energy produced per dollar spent in manufacturing and installation.
In addition to CNT-only cells, we report on effiency improvement strategies, using different assembly techniques and using graphite (gra – phenium) counter electrodes. Graphite has no band gap, is extremely pliable, robust, and provides the ability to shrink the distance between it and the active semiconducting electrode. The cost, relative abundance, ease of introduction into the cell, and lack of need for spray deposition render graphite an attractive counter electrode material.