|Fig. 1: Structure of carbon nanotubes based solar cells.|
Burning coal and natural gas are currently the most effective methods to produce electricity. However, these methods generate a significant amount of carbon dioxide into the atmosphere. Solar energy is the most abundant source of clean energy and the development of solar cells is therefore the most promising solution for reducing global greenhouse gas emissions. About 90% of the solar cells currently on the market are based on silicon. However, prices for both single-crystal and polycrystalline silicon are very high due to the high temperature processing. Solution processed organic photovoltaic solar cells exhibit advantages such as low cost, ready availability, light weight, flexibility and stretchability.  The efficiency of organic bulk heterojunction solar cells is approaching 8% if polythiophene is used as electron donor and fullerene derivative as electron acceptor.  However, the charge transport in these materials depends strongly on fullerene hopping transport.
Single-walled carbon nanotubes (SWNTs) can be used an alternative to fullerene derivatives as an active layer in hybrid photovoltaic system shown in Figure 1 due to the following advantages: 
The mobility of SWNTs is extraordinarily high due to its ballistic conduction in the one-dimensional structure. The high mobility of SWNTs allows more efficient transport in SWNTs once the charges are generated from polymer/nanotube junction.
The high aspect ratio of SWNTs allows a better transport diffusion pathway and more conductive percolation networks than fullerene does. Therefore, the close circuit current density is improved.
SWNTs can improve the crystallinity of polythiophenes on the side of SWNTs. The crystalline structure of the polymer is an more efficient intermedium for the interchain transport. 
The major problem of using SWNTs as a active layer of solar cells is due to the presence of metallic SWNTs. The metallic SWNTs can act as a charge carrier recombination center, which causes inefficient charge separation. Recently, separation of semiconducting SWNTs by polythiophene (rr-P3ATs) has been demonstrated. This is a high yield, high throughput and low cost process comparing to other sorting methods.  Since rr-P3ATs are themselves are the most common polymers used for solar cells, there is no need to remove them before use. The polymer wrapped individual SWNTs supramolecular structure can provide extremely large surface area for charge separation and exciton dissociation for hybrid solar cells.
© Huiliang Wang. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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