Sarah Kurtz Principal scientest at NREL discusses the key to high efficieny CPV cells

Interview with Sarah Kurtz principal scientist at NREL, discussing the issue of key to high efficiency cells for concentrated photovoltaics in relation to a talk she will give at the upcoming Concentrated Photovoltaics summit in San Diego this February.

In the build up to the highly anticipated Concentrated Photovoltaics Summit USA, taking place Feb 3-4 2009 in San Diego. Earlier this week CPV Today the event organisers took time out of planning to discuss cell efficiency with Sarah Kurtz Principal scientist at NREL and a speaker at the event in February. At the summit Sarah Kurtz will be discussing the successes of multijuntion cells in revolutionizing the CPV industry, the following discussion provides an exciting sneak preview into this anticipated session.

"The real key to high efficiencies is the near-perfect quality of the single-crystal materials"

CPV follows a complementary approach and uses concentrating optics to focus the light onto small cells. The optics may be designed for low or high concentration.

As the US Department of Energy's National Renewable Energy Laboratory (NREL) explains, the low-concentration concepts use silicon or other low-cost cells; high-concentration optics may use more expensive, higher-efficiency cells. The higher-efficiency cells can reduce the cost per watt if the cost of the small cells is minimal.

According to NREL, multijunction concentrator cells have achieved much higher efficiencies than any other approach. This is not surprising for two reasons:

(1) theoretically, the highest efficiencies may be achieved if multiple semiconductor materials (with a range of bandgaps) are chosen to match the spectral distribution of the sun, and (2) the compound semiconductors used in these cells are direct-gap materials and can be grown with near-perfect quality. The multijunction approach has been described extensively in the literature.

On the same, Sarah Kurtz, Principal Scientist, PhD, NREL, told "Theoretically, one expects that multijunction cells can achieve higher efficiencies than single-junction cells because they can match the band gap of the materials with the different colors of light in the solar spectrum, but the real key to these high efficiencies is the near-perfect quality of the single-crystal materials. Achieving the highest efficiencies requires doing everything right at the same time: near-perfect material quality, matching the band gap to the energy of the photons, and concentrating the sunlight."

Recently, concentrator cells have been reaching increasingly impressive efficiencies, inspiring new interest in the high-efficiency, high-concentration approach.

In August this year, scientists at NREL set a world record in solar cell efficiency with a photovoltaic device that converts 40.8 percent of the light that hits it into electricity. It is acknowledged as the highest confirmed efficiency of any photovoltaic device to date.

The inverted metamorphic triple-junction solar cell was designed, fabricated and independently measured at NREL. The 40.8 percent efficiency was measured under concentrated light of 326 suns. One sun is about the amount of light that typically hits Earth on a sunny day. The new cell is a natural candidate for the space satellite market and for terrestrial concentrated photovoltaic arrays, which use lenses or mirrors to focus sunlight onto the solar cells.

The new solar cell differs significantly from the previous record holder - also based on a NREL design.

Instead of using a germanium wafer as the bottom junction of the device, the new design uses compositions of gallium indium phosphide and gallium indium arsenide to split the solar spectrum into three equal parts that are absorbed by each of the cell's three junctions for higher potential efficiencies.

This is accomplished by growing the solar cell on a gallium arsenide wafer, flipping it over, then removing the wafer. The resulting device is extremely thin and light and represents a new class of solar cells with advantages in performance, design, operation and cost.

About the new design, which uses compositions of gallium indium phosphide and gallium indium arsenide, Sarah said: "In short, this approach, invented by Mark Wanlass, and currently pursued under the leadership of John Geisz, grows the multijunction cell in an inverted configuration."

"The lattice-matched GaInP top cell is grown first, then the lower-band-gap junctions can be grown from alloy compositions with increasing lattice constants, providing great flexibility in the exact design. After the epitaxial growth is completed, the substrate is removed, exposing the top cell. In addition to the greater design flexibility, providing a pathway to higher efficiency, this approach provides the possibility of reducing substrate cost by either reusing the substrate or using a lower cost (still single crystal, but not electrically active) substrate. This approach has the disadvantage of needing to grow graded layers, adding to the cost of the epitaxial growth," explained Sarah, who is who is scheduled to speak in front of a 300+ strong crowd during the CPV Summit USA 2009 (scheduled to take place on February 3-4 in San Diego).

On how 40.8 percent efficiency was measured and also what were the main hurdles in doing so, Sarah said the 40.8 percent efficiency was measured using a flash simulator, tuning the spectrum by adjusting the flash conditions.

"This is a difficult measurement because it is difficult to control (and quantify) the spectrum adequately during the short flash. The uncertainty of this measurement is larger than the difference between this 40.8 percent efficiency and champion efficiencies reported by Spectrolab and Emcore, which are approaching or exceeding 40 percent. A one-sun efficiency is easier to measure because the spectrum of a continuous solar simulator can be measured and tweaked to match the reference spectrum," said Sarah.

Commenting on the same, Sunrgi's founding partner Dr. KRS Murthy, said, "Yes, some companies have reported such efficiencies. We welcome them, however, it will take some time for such efficiencies to be transferred from the laboratory to the manufacturing floor. There are areas for further improvement in cell efficiency, its current carrying capacity, extension of cell saturation and other packaging aspects of the cell."

According to Dr. Murthy, Sunrgi has already achieved about four times the sun concentration compared to other CPV companies in the industry.

"Other improvements will be in cooling, tracking and over all system efficiencies. However, success of the solar PV industry will be pivoted on the cost of the systems, cost of the completely installed solar farms and the speed of installation. Speed of installation is vital to measure up to the ever increasing energy demand from not only in USA, but also in China and India which are the most energy hungry, very high growth markets for energy. Other major energy markets include countries in Europe, Middle East, Australia, South America and also Africa," shared Dr. Murthy.

CPV Summit 2009, San Diego

CPV Summit 2009 is scheduled to take place in San Diego (February 3-4, 2009). For more information, click here:

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