Solar ♦ news digest
manager of the NREL III-V Multi-junction Photovoltaics Group. “There’s no way around that.”
But this year, Friedman’s team succeeded in bending the rules of the solar spectrum and NREL and its industry partner, Solar Junction, won an R&D 100 award from R&D Magazine for a world- record multi-junction solar cell.
The three-layered cell, SJ3, converted 43.5 percent of the energy in sunlight into electrical energy - a rate that has stimulated demand for the cell to be used in concentrator photovoltaic (CPV) arrays for utility-scale energy production.
The record of 43.5 percent efficiency at 415 suns was eclipsed with a 44 percent efficiency at 947 suns. Both records were verified by NREL. This is NREL’s third R&D 100 award for advances in ultra- high-efficiency multi-junction cells.
CPV technology gains efficiency by using low-cost lenses to multiply the sun’s intensity, which scientists refer to as numbers of suns.
Friedman says earlier success with multi-junction cells - layered semiconductors each optimised to capture different wavelengths of light at their junctions - gave NREL a head start.
The SJ3 cells fit into the market for utility-scale CPV projects. They’re designed for application under sunlight concentrated to 1,000 times its normal intensity by low-cost lenses that gather the light and direct it at each cell.
In regions of clear atmosphere and intense sunlight, such as the U.S. desert Southwest, CPV has outstanding potential for lowest-cost solar electricity. There is enough available sunlight in these areas to supply the electrical energy needs of the entire United States many times over.
Bending Material to the Band Gaps on the Solar Spectrum
Sunlight is made up of photons of a wide range of energies from roughly zero to four electron volts (eV). This broad range of energies presents a fundamental challenge to conventional solar cells, which have a single photovoltaic junction with a single characteristic band gap energy.
NREL Principal Scientist Jerry Olson holds examples of the first multi-junction cells that were developed in the 1980s based on his scientific breakthrough
The researchers at NREL knew that if they could replace the 0.67-eV third junction with one better tuned to the solar spectrum, the resulting cell would capture more of the sun’s light throughout the day. But they needed a material that had an atomic structure that matched the lattice of the layer above it - and that also had the ideal band gap.
“We knew from the shape of the solar spectrum and modelling solar cells that what we wanted was a third junction that has a band gap of about 1.0 electron volt, lattice-matched to gallium arsenide,”
January/February 2013
www.compoundsemiconductor.net 171
Conventional cells most efficiently convert those photons that very nearly match the band gap of the semiconductors in the cell. Higher-energy photons give up their excess energy to the solar cell as waste heat, while lower-energy photons are not collected by the solar cell, and their energy is completely lost.
This behaviour sets a fundamental limit on the efficiency of a conventional solar cell. Scientists overcome this limitation by using multi-junction solar cells. Using multiple layers of materials in the cells, they create multiple junctions, each with different band gap energies. Each converts a different energy range of the solar spectrum.
An invention in the mid-1980s by NREL’s Jerry Olson and Sarah Kurtz led to the first practical, commercial multi-junction solar cell, a GaInP/GaAs two-junction cell with 1.85-eV and 1.4-eV bandgaps that was recognised with an R&D 100 award in 1990, and later to the three-junction commercial cell based on GaInP/GaAs/Ge that won an R&D 100 award in 2001.
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