Patent Application
20080072961
The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and
The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.
Carbon nanotubes have demonstrated a significantly enhanced conductivity and robustness compared to the metallic materials used in the known art. A photovoltaic cell comprising a plurality of carbon nanostructures, i.e., carbon nanotubes, carbon nanowires, or carbon fibers covered by first and second oxides and a sensitized material, e.g., a dye, provides for improved efficiency by reducing the recombination rate of electrons and holes, and improving the transport of the photoexcited electrons through the first and second oxides. The structure described herein accomplishes five advantages over known art. First, the long nanostructures provide the required porosity/surface area needed for sufficient loading of sensitizer, e.g., dye, molecules, whether organic or inorganic, for optical absorption, and serve as an extended electrode (cathode) for electrons. Second, the first oxide layer immediately over the carbon nanostructure provides the required band alignment for separation of photo-excited electrons and holes. Third, the second oxide layer formed over the first oxide layer has a conduction band lying in between that of the first oxide layer and the LUMO (lowest unoccupied molecular orbital) of the dye molecules. The conduction and valence bands of the second oxide layer are concomitantly higher than that of the conduction and valence bands of the first oxide layer, thereby acting as a buffer layer between electrons in the first oxide layer and holes in the electrolyte or in the dye molecules. This prevents recombination of the electrons and holes without affecting the open circuit voltage due to its band alignment with the first oxide layer, second oxide layer, and the dye molecules. Fourth, the nanostructure/first oxide/second oxide structure provides a much shorter transport path (approximately tens of nanometers) for electrons over convention photovoltaic cells, thus increasing the charge extraction rate and further reducing electron-hole recombination. Fifth, the carbon nanostructure may serve as an internal mirror that not only reflects residual photons not absorbed by the dye molecules, and therefore may enhance the optical absorption of the dye molecules due to surface plasma effect, thus increasing the effective light absorption efficiency of the dye molecules.
Referring to
Carbon nanostructures 16, having a length of 1 to 100 micrometers, but preferably approximately 10 micrometers, are grown on either the optional oxide layer 14 or the transparent conducting layer 12 generally in a non-uniform manner as is known in the industry from a plurality of catalysts such as Cobalt Aluminum nanoparticles formed overlaying the transparent conducting oxide 12 or the optional oxide layer 14. While only a few carbon nanostructures 16 are shown in
A first layer 18 is formed to cover the carbon nanostructure 16, and a second layer 20 is formed to cover the first layer 18. Both of the first and second layers of 18, 20 are preferably formed by a dry atomic layer deposition, chemical vapor deposition, or solution processes through chemical route. The choice of the material for the first and second layers is interrelated. The second layer must have a conduction band higher than the conduction band of the first layer. The first layer 18 may comprise, for example, titanium oxide, niobium oxide, magnesium oxide, hafnium oxide, zirconium oxide, strontium oxide, barium oxide, molybdenum oxide, tungsten oxide, cobalt oxide, nickel oxide, or certain combinations of these oxides. The second layer 20 may comprise, for example, zinc oxide, or Mg1-xZnxOy, vanadium oxide, copper oxide, iron oxide, or combinations of these materials. Alternatively, layer 20 may also comprise semiconductors such as CdS, ZnS, ZnSe, ZnTe, CdTe, CdSe, PbS, GaAs, AlAs, GaN, AlN, InP, InSb, or a combination of these II-VI and III-V semiconductors. The thickness of the first layer 18 preferably comprises a thickness of between 10.0 and 100.0 nanometers, and the thickness of the second layer 20 preferably comprises a thickness of approximately 1.0 to 10.0 nanometers.
A dye layer 22 is formed over the second layer 20, preferably by, but not limited to, immersing the structure 10 in a solution of dye complexes such as ruthenium complexes where one of the ligands is typically 4,4′-dicarboxy-2,2′-bipyridyl. The time of immersion can vary from a few minutes to a few days depending on temperature and solution concentration.
An electrolyte material 24 is formed over the dye layer 22. The electrolyte material preferably comprises a layer of a polymer material since it is a condensed material and not subject to leaking. Alternatively, the electrolyte material 24 may comprise, for example, a gel comprising a mixture of iodine and organic material, or a solution comprising iodine salt and organic solvent. An electrode 26 is formed over a catalyst layer 24. The catalyst 24 comprises Pt, carbon, or a combination of the two.
Referring to
While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.
