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NEW EFFICIENCY RECORD
FOR SOLAR CELLS

A longstanding efficiency record for electricity produced by solar cells made from cadmium telluride has been broken by researchers at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL).

The NREL team created cells that convert 16.4 percent of the available sunlight that strikes them into electricity. The previous record was 15.8 percent for a cadmium telluride cell - a record that has stood since 1992.

In recent months the rising cost of fuels and the California power crisis have triggered a greater surge in the installation of solar electric systems for homes and businesses. The expanding solar industry will be able to use these more efficient solar cells to help meet that demand.

At NREL's Solar Energy Research Facility, researcher Xiaonan Li uses a close-space sublimation system to make and study thin film cadmium telluride solar cells.

Cadmium telluride represents one of the most promising technologies for so-called thin film solar cells, says NREL research manager John Benner. In the thin film manufacturing process, layers of differing electricity producing materials are applied sequentially to a glass, plastic or steel backing.

Many experts believe thin film cells are the wave of the future, because thin films use materials that are less expensive than the materials used in earlier, conventional solar panels.

Of the several materials that can be used for thin film panels, cadmium telluride yields higher wattage per square foot, at a lower price per watt of capacity.

Increasing efficiency and lowering costs have been the two most important goals during the more than 20 years the National Renewable Energy Laboratory has been conducting research on improving photovoltaic systems.

The record setting cadmium telluride (CdTe) process is different from previous cells and benefits from new insights in understanding of the operation of these solar cells, says Benner. The cell utilizes new materials that interact chemically with the cadmium telluride to improve adhesion, light collection, and electronic properties.

"This technology offers the prospect of getting a better product to customers," said Benner. "Our industry partners can use this technology in expanding capacity to meet the rapidly mounting demand for photovoltaics." In 2000, the solar photovoltaic industry increased production by 29 percent in the United States and 39 percent worldwide.

The record setting cadmium telluride solar cell is still only about half as efficient as a solar cell that can convert sunlight to electricity at 32 percent efficiency, developed in 1999 by NREL and Spectrolab of Sylmar, California. The Boeing Company acquired Spectrolab in October 2000.

Jerry Olson and Sarah Kurtz test the voltage output of a laboratory multijunction solar cell, which achieved record efficiencies in converting sunlight to electricity and is the basis for the Spectrolab cell.

The highly efficient Spectrolab gallium indium phosphide on gallium arsenide multi-junction design is valuable for powering space satellites, the primary market for this type of solar cell. On Earth it is attractive for use in solar concentrator systems.

The NREL team that produced the record setting cadmium telluride solar cell works within the National Center for Photovoltaics in collaboration with the National CdTe Team that also includes scientists from universities and industry.

The Department of Energy established the National Center for Photovoltaics at NREL in 1996 to provide for coordinated research and development to improve the cost effectiveness, performance and reliability of solar-electric technologies. Sandia National Laboratories also participates in the center.

Earlier this month NREL announced $6 million in awards to 11 universities and five companies for high tech research into non-conventional, photovoltaic technologies for creating electricity from sunlight.

Some of the concepts will explore entirely novel methods, materials, and processes for creating electricity from sunlight. Two contracts, for instance, explore solar cells based on new plastic materials.

Each award has the potential to create a breakthrough that could dramatically reduce the cost of solar electricity.

The awards will support fundamental and exploratory research to increase the amount of electricity produced by photovoltaic cells, reduce the cost of electricity produced, and ensure performance over longer periods of time. The three year awards from the fiscal year 2001 budget range from $160,000 to $500,000.

The University of Arizona, Tucson, for instance, will use its $499,000 award to develop new solar electric devices based on self-assembling, liquid crystalline organic layers that can form self-repairing thin films.

NREL CdTe Team Fabricates 16.4% CdS/CdTe Device

The NREL CdTe team has developed a modified CdTe device structureand fabricated a CdS/CdTe polycrystalline thin-film solar celldemonstrating an NREL-confirmed total-area efficiency of 16.4% (Voc =847.5 mV, Jsc = 25.86 mA/cm2, Fill Factor = 74.45%, Imax = 26.400mA, Vmax = 701.5 mV, Pmax = 18.520 mW, and area = 1.131 cm2).This level of performance is the highest so far achieved by the NREL CdTeteam, surpassing the previous world-record efficiency (15.8%) set in 1992for a thin-film CdTe device by the team at the University of South Florida.

This device uses a modified device structure that incorporates two maindifferences from more conventional CdS/CdTe devices. First, a cadmiumstannate (Cd2SnO4, or CTO) transparent conducting oxide (TCO) layerreplaces the SnO2 TCO film. This CTO film has higher electricalconductivity, higher optical transmittance, and a smoother surface. Second,the modified device structure incorporates a zinc stannate (Zn2SnO4, orZTO) buffer layer between the CTO and the CdS window layers. Wefound that there is interdiffusion between the ZTO and CdS layers, and weexploited this interdiffusion feature to improve device performance andreproducibility.

The integration of the CTO and ZTO layers into CdS/CdTe devices is thesubject of two NREL patents: U.S Patent No. 5,922,142 and U.S PatentNo. 6,169,246.

The NREL CdTe team will continue to improve these technologies andCdTe film quality. The hope is that by the end of 202, the team willdemonstrate a 17% CdS/CdTe device, which is the milestone for CdTedevice R&D in DOE's Five-Year PV Program.

Written by: National Renewable Energy Laboratory


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