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US researchers develop aluminum GaAs solar cells with new epitaxial growth technique

Researchers at the U.S. National Renewable Energy Laboratory have grown aluminum indium phosphide and aluminum gallium indium phosphide semiconductor materials with a new hydride vapor phase epitaxy reactor for the first time. They claim that the new semiconductors could help to produce cheaper, more efficient gallium arsenide solar cells.

A group of scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) is developing gallium arsenide (GaAs) solar cells by adding aluminum-containing molecules to the dynamic hydride vapor phase epitaxy (D-HVPE) process, which the research institute unveiled two years ago.

The scientists claim to have grown aluminum indium phosphide (AlInP) and aluminum gallium indium phosphide (AlGaInP) semiconductors for the first time with their new epitaxial growth technique, after announcing new advances for the method in August. “There’s a decent body of literature that suggests that people would never be able to grow these compounds with hydride vapor phase epitaxy,” said NREL researcher Kevin Schulte.

The aluminum trichloride used for the manufacturing process is produced from solid aluminum and hydrogen chloride gas through an aluminum trichloride generator, which was heated to 400 degrees Celsius. Gallium chloride and indium chloride were then vaporized at 800 degrees Celsius and three elements were combined and deposited on a substrate at 650 degrees Celsius.

More transparent

The addition of aluminum to the D-HVPE could be decisive in reaching the same efficiency levels as the metalorganic vapor phase epitaxy (MOVPE), which is the dominant process in the III-V solar cell manufacturing industry. The latter is known to provide better results in forming abrupt heterointerfaces between two different semiconductor materials.

AlInP manufactured with this process has the advantage of being more transparent than gallium indium phosphide (GaInP), which has also been used by the NREL scientists to produce GaAs solar cells. This increased transparency may be particularly useful, as the layer made with this material is used to passivate the cell’s front surface and allow sunlight to reach the GaAs absorber layer below.

Efficiency target

With the addition of aluminum, GaAs solar cells made under the D-HVPE process should now reach parity with III-V solar cells made via MOVPE, the scientists said. Currently, the highest conversion efficiency for a GaAs solar cells produced with AlInP via MOPVE is 29.1%, while that of similar cells grown based on GaInP via D-HVPE is around 27%. “Before, we were somewhat less efficient but cheaper. Now there’s the possibility of being exactly as efficient and cheaper,” said research co-author Aaron Ptak.

The findings were published in the paper Growth of AlGaAs, AlInP, and AlGaInP by Hydride Vapor Phase Epitaxy, in ACS Applied Energy Materials.

Gallium arsenide (GaAs) and other III-V materials – named after the groups in the periodic table that they belong to – are among the best known in terms of their efficiency potential for solar cells. However, cost has thus far limited them to niche applications to power satellites and drones.

Source: pv magazine