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Instant insight: Nanohighway to solar cells
19 December 2006
Hiroshi Imahori of Kyoto University, Japan, discusses electrophoresis as a means to make molecular highways for organic solar cells.
The shortage of fossil fuels and the degradation of the global environment has focused research attention on solar cells, which can convert sustainable solar energy into electricity. However, the cost of electricity from inorganic solar cells (silicon-based photovoltaics) is presently much higher than that generated by hydroelectric power and nuclear or fossil fuels. Therefore, it is necessary to develop low-cost solar cells with high power conversion efficiencies ). Organic solar cells would be promising candidates if they fulfil their potential, especially as they bear unique advantages over inorganic solar cells as they are flexible, lightweight and colorful. Since the beginning of the 1990s, substantial advances in power conversion efficiency have been made in dye-sensitized solar cells (with up to 11%) and bulk heterojunction solar cells ( up to 6%).1
Porphyrin-fullerene 'nanohighways' for efficient hole and electron transport.
Heterojunction solar cells have two layers, a p-type donor layer and n-type acceptor. Their conduction and valence bands are at different levels, and their bandgaps are different. Light absorbed by the p-type layer generates an excited state. This migrates to the junction and is separated into an electron and a positive 'hole', eventually producing a current.
Electrophoresis is a technique for separating proteins and nucleic acids. This technique can be applied to film deposition on an electrode from colloidal dispersions, known as electrophoretic deposition, an essentially a two-step process. First, particles such as macromolecules and colloids are charged in a polar solution. Application of an electric field forces the charged particles to move toward one of two electrodes. Then, the particles are gradually deposited onto the electrode, leading to the formation of an organic thin film. Thus, it is a fast and economical process that makes it possible to control and fabricate nano- and micro-structures on an electrode surface. We have focused on electrophoretic deposition that allows donor and acceptor molecules to be assembled onto electrodes for molecular photovoltaics. In particular, using weak intermolecular interactions (such as hydrogen bonding or lyophobic interactions) during the deposition process is also crucial for the bottom-up fabrication of donor and acceptor molecules on electrodes.2
References1. H Hoppe and N S Sariciftci, J. Mater. Chem., 2006, 16, 45
2. H Imahori, J. Mater. Chem. 2007, 17, 31, DOI: 10.1039/b609269c
3. S Kang, T Umeyama, M Ueda, Y Matano, H Hotta, K Yoshida, S Isoda, M Shiro and H Imahori, Adv. Mater. 2006, 18, 2549