Friday, December 31, 2010

Graphene Solar Cells Could Cut Production Costs to Less Than 50-Cents Per Watt

Wisconsin Alumni Research Foundation (Madison, WI) and University of Utah Research Foundation (Salt Lake City, UT) researchers have developed graphite-based photovoltaic cells and methods for generating electricity from these cells. Graphite based solar cells earned U.S. Patent 7,858,876 for the two universities. 

According to inventors Max Lagally (Madison, WI), Feng Liu(Salt Lake City, UT), in these photovoltaic cells, spatially separated stacks of graphite, each comprising a plurality of vertically stacked semiconducting graphene sheets, serve as a photovoltaic material bridging electrical contacts. The graphene sheets, or "nanoribbons," have nanoscale-width dimensions such that the band gap of each sheet depends on the width of the sheet.

Thus, by incorporating graphene sheets having different widths, and thereby different band gaps, into the photovoltaic cell, the cell can be designed to absorb efficiently across the solar spectrum. The result is a photovoltaic cell that is efficient and inexpensive to manufacture.

Photovoltaic cells convert sunlight directly into electricity through the interaction of photons and electrons within a photoconducting material. To create a photovoltaic cell, a photoconducting material, commonly silicon, is joined by electrical contacts to form a junction.

Presently, most silicon-based photovoltaic cells are silicon p-n junction devices. Photons striking the cell are absorbed and thus cause the formation of electron-hole pairs; electrons and holes moving in opposite directions across the junction create a current. A grid of these electrical contacts creates an array of cells from which the current is gathered. The DC current produced in the cell depends on the materials involved and the energy and intensity of the radiation incident on the cell.

Photovoltaic cells have been available for a number of years, and it has been predicted that the use of photovoltaics will continue to increase for years to come. The major obstacles to photovoltaic use throughout the world are cell efficiency and cell cost. Presently, the cost per watt for most photovoltaic cells is not low enough for these cells to be competitive with other energy sources.

Currently, the industry standard solar-cell material is crystalline Si. However, bulk Si is unlikely to achieve a cost lower than $1.00/watt because of the materials-intensiveness of the process that is used to produce it. Also, the intrinsic cell efficiency of Si is limited by thermodynamics to less than about 30%.

Solar-cell efficiency is limited by both extrinsic and intrinsic factors. Extrinsic losses, such as loss due to reflection and transparency (small opacity) and incomplete collection of photogenerated carriers due to imperfect contacts and leakage, can be overcome by better design and manufacture of the cell modules. Intrinsic losses, however, must be overcome by the design of cell materials through energy band engineering.

For example, even if all the extrinsic losses can be eliminated, the highest efficiency of an ideal cell made from a single material is about 31%, with an optimal band gap of .about.1.35 eV, because solar photons with an energy smaller than the band gap cannot be adsorbed, while energy dissipation due to thermalization of generated electrons and holes for photons with higher energies can produce heat and thus waste energy. One strategy for improving cell efficiency is to use combinations of materials having multiple band gaps. The highest cell efficiency--close to 40%--has been achieved by multi-junction cells made from III-V and Ge thin films. However, these high-efficiency research cells are too expensive to penetrate the general power market.

More recently, a Schottky barrier cell based on single-walled carbon nanotubes has been proposed. However, inexpensive production of such cells may not be possible.

Thus, new materials are needed to increase cell efficiency and reduce cell cost to reach the goal of a production cost of less than $0.50/watt.

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