Thursday, February 25, 2010

Method for Forming Nanostructured Solar Cells with Active Region Energy Wells

Philips Electronics inventors James C. Kim (San Jose, CA) and Sungsoo Yi (Sunnyvale, CA) have developed the means to produce multi-junction solar cells with graded energy wells. The active region of the solar cell is nanostructured. The nanostructures are formed from a material that comprises a III-V compound semiconductor and an element that alters the band gap of the III-V compound semiconductor.   The method involves doping both the energy wells and the barriers to influence electron-hole recombination rate by affecting piezoelectric field effect due to polarization charges at an interface between energy wells and adjacent barriers.

For example, the III-V compound semiconductor could be gallium nitride (GaN). As an example, the "band gap altering element" could be indium (In). The concentration of the indium in the active region is non-uniform such that the active region has a number of energy wells, separated by barriers. The energy wells may be "graded", by which it is meant that the energy wells have a different band gap from one another, generally increasing or decreasing from one well to another monotonically.  The nanostructured solar cells are detailed in U.S. Patent Application 20100047957.

The energy wells are "graded", by which it is meant that band gap of each energy well progressively decreases moving away from the window. Thus, the energy wells that are closer to the window absorb photons that have energy that is at least as high as the band gap, but do not absorb photons having less energy. However, energy wells that are further from the window are able to absorb photons having less energy

The active region comprises nanostructures. The nanostructures may be nanocolumns, nanowires, nanorods,  nanotubes, etc. The nanostructures are formed from a material that comprises a III-V compound semiconductor and an element that alters the band gap of the III-V compound semiconductor.  The III-V compound semiconductor could be gallium nitride (GaN), which has a band gap that is about 3.4 eV. As an example, the "band gap altering element" could be indium (In). When In is incorporated into GaN, and replaces the Ga, the band gap of the resulting InGaN is lower than the band gap of GaN. The more In that is incorporated (and therefore the more Ga that is replaced), the lower the band gap of InGaN. If all of the Ga is replaced by In, resulting in InN, the band gap is about 0.7 eV. A significant amount of In can be incorporated into nanostructures of InGaN, with the nanostructures being substantially free of defects and strain. 

Solar-cell technology is currently poised to make significant progress in mass adoption due in part to the looming shortage of traditional energy sources, e.g. crude oil and natural gas, and to the increased awareness of "green-technology" benefits. Solar-cell technology, though capturing "free" energy from the sun, has been expensive with per-watt ownership cost ($/W) far exceeding $/W offered by electric utilities. Recently at $5/W, the pay-off period for a solar panel is as much as 50% of its lifespan, due largely to the expense of the semiconductor material used. The persistently high $/W figure has led to ideas on cost reduction, an example of which is concentrated photovoltaics (CPV).

In CPV, the sun's energy is concentrated hundreds of times onto a solar cell. CPV, however, requires the solar cell to be very thermally robust due to high heat resulting from the concentration. Fortunately, the advent of new materials with more robust thermal properties and band gap energies better suited to the sun's spectrum has made solar technology attractive again through CPV.

Until recently, silicon (Si) has been at the core of solar-cell technology. However, efficiency for the best single-junction-Si-based cells only reaches about 22%. Recently, multi-junction solar cell designs have achieved efficiencies that far surpass single junction devices. In the multi-junction design, typically, each junction is formed of a different material. For example, III-V compound semiconductors (e.g., InGaAs, InGaP) and group IV materials (e.g., Ge), have been used together to make multi-junction solar cells. These multi-junction solar cells typically use a different material for each junction. Laboratory efficiencies as high as 40.7% in a three junction design using the three mentioned semiconductors have been claimed.

However, these III-V compound-semiconductor-based solar cells are more expensive than single junction Si devices due to material cost as well as manufacturing complexity. Therefore, these devices have been excluded in the traditional solar-panel business where sheer size requirement meant prohibitive cost. Space and other niche applications have, however, sustained the specialized interests in the more expensive but more efficient and robust solar cells based on the III-V compound semiconductors. 

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