New growth technique could bring space-age solar cells down to Earth

Scientists at the US National Renewable Energy Laboratory (NREL) have achieved a technological breakthrough for solar cells previously thought impossible. By integrating an aluminum source into their hydride vapor phase epitaxy (HVPE) reactor, the scientists have managed to grow the semiconductors aluminum indium phosphide (AlInP) and aluminum gallium indium phosphide (AlGaInP) for the first time using the HVPE technique.

"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 Kevin Schulte, a scientist in NREL's Materials Applications & Performance Center and lead author of a new paper on the research in ACS Applied Energy Materials. "That's one of the reasons a lot of the III-V industry has gone with metalorganic vapor phase epitaxy (MOVPE), which is the dominant III-V growth technique. This innovation changes things."

III-V solar cells – so named because of the position the materials fall on the periodic table – are commonly used in space applications. Notable for high efficiency, these types of cells are too expensive for terrestrial use, but researchers are developing techniques to reduce their costs.

One method pioneered at NREL relies on a new growth technique called dynamic hydride vapor phase epitaxy (D-HVPE). Traditional HVPE, which for decades was considered the best technique for producing light-emitting diodes and photodetectors for the telecommunications industry, fell out of favor in the 1980s with the emergence of MOVPE. Both processes involve depositing chemical vapors onto a substrate, but MOVPE came to be preferred because of its ability to form abrupt heterointerfaces between two different semiconductor materials, a place where HVPE traditionally struggled. That has now changed with the advent of D-HVPE.

The earlier version of HVPE used a single chamber for depositing a single chemical on a substrate, which was then removed. The growth chemistry was then swapped for another, and the substrate returned to the chamber for the next chemical application. In contrast, D-HVPE relies on a multi-chamber reactor: the substrate moves back and forth between chambers, greatly reducing the time to make a solar cell.

A single-junction solar cell that takes an hour or two to make using MOVPE can potentially be produced in under a minute by D-HVPE. Despite these advances, however, MOVPE still had another advantage: the ability to deposit wide-bandgap, aluminum-containing materials that offer the highest solar cell efficiencies. HVPE has long struggled with growing these materials due to difficulties with the chemical nature of the usual aluminum-containing precursor, aluminum monochloride.

The researchers always planned on introducing aluminum into D-HVPE, but first focused their efforts on validating the growth technique. "We've tried to move the technology forward in steps instead of trying to do it all at once," Schulte said. "We validated that we can grow high-quality materials. We validated that we can grow more complex devices. The next step now for the technology to move forward is aluminum."

Schulte's co-authors include three scientists from a North Carolina company called Kyma Technologies. These scientists developed a method for producing a unique aluminum-containing molecule that could be flowed into the D-HVPE chamber.

The method is based on an aluminum trichloride generator, which is heated to 400°C to generate aluminum trichloride from solid aluminum and hydrogen chloride gas. Aluminum trichloride is much more stable in the HVPE reactor environment than the monochloride form. The other components – gallium chloride and indium chloride – were vaporized at 800°C. The three elements were combined and deposited on a substrate at 650°C.

Using D-HVPE, the NREL scientists were previously able to make solar cells from gallium arsenide (GaAs) and gallium indium phosphide (GaInP). In these cells, the GaInP is used as the ‘window layer’, which passivates the front surface and permits sunlight to reach the GaAs absorber layer below, where the photons are converted to electricity. This window layer must be as transparent as possible, but GaInP is not as transparent as the aluminum indium phosphide (AlInP) used in MOVPE-grown solar cells.

The current efficiency record for MOVPE-grown GaAs solar cells that incorporate AlInP window layers is 29.1%. With only GaInP, the maximum efficiency for HVPE-grown solar cells is estimated to be 27%.

Now that aluminum has been added to the mix of D-HVPE, the scientists said they should be able to reach parity with solar cells made via MOVPE.

"The HVPE process is a cheaper process," said Ptak, a senior scientist in NREL's National Center for Photovoltaics. "Now we've shown a pathway to the same efficiency that's the same as the other guys, but with a cheaper technique. Before, we were somewhat less efficient but cheaper. Now there's the possibility of being exactly as efficient and cheaper."

This story is adapted from material from the National Renewable Energy Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.