Ph.D. Student Taha Ayari measures the photovoltaic performance of the InGaN solar cells with a solar simulator. (Image courtesy of Ougazzaden laboratory)

A team of semiconductor researchers based in France has used a boron nitride separation layer to grow indium gallium nitride (InGaN) solar cells that were then lifted off their original sapphire substrate and placed onto a glass substrate. By combining the InGaN cells with photovoltaic (PV) cells made from materials such as silicon or gallium arsenide, the new lift-off technique could facilitate fabrication of higher efficiency hybrid PV devices able to capture a broader spectrum of light. Such hybrid structures could theoretically boost solar cell efficiency as high as 30 percent for an InGaN/Si tandem device.

This technique is the third major application for the hexagonal boron nitride lift-off technique, which was developed by a team of researchers from the Georgia Institute of Technology, the French National Center for Scientific Research (CNRS), and Institut Lafayette in Metz, France. Earlier applications targeted sensors and light-emitting diodes (LEDs).

Putting these structures together with photovoltaic cells made of silicon or a III-V material, would enable them to cover the visible spectrum with the silicon and utilize the blue and UV light with indium gallium nitride to gather light more efficiently. The boron nitride layer doesn’t impact the quality of the indium gallium nitride grown on it, so the researchers were able to lift off the InGaN solar cells without cracking them.

The technique could lead to production of solar cells with improved efficiency and lower cost for a broad range of terrestrial and space applications. “This demonstration of transferred InGaN-based solar cells on foreign substrates, while increasing performance, represents a major advance toward lightweight, low cost, and high efficiency photovoltaic applications,” the researchers wrote in their paper.

Using this technique, it is possible to process InGaN solar cells and put a dielectric layer on the bottom that will collect only the short wavelengths. The longer wavelengths can pass through it into the bottom cell. So, by using this approach each surface can be optimized separately.

The researchers began the process by growing monolayers of boron nitride on two-inch sapphire wafers using an MOVPE process at approximately 1300 degrees Celsius. The boron nitride surface coating is only a few nanometers thick and produces crystalline structures that have strong planar surface connections, but weak vertical connections.

The InGaN attaches to the boron nitride with weak van der Waals forces, allowing the solar cells to be grown across the wafer and removed without damage. The InGaN structures are then placed onto the glass substrate with a backside reflector for enhanced performance. So far, the cells have been removed from the sapphire manually, but the researchers believe the transfer process could be automated on a large scale to drive down the cost. Beyond demonstrating placement atop an existing PV structure, the researchers hope to increase the amount of indium in their lift-off devices to boost light absorption and increase the number of quantum wells from five to 40 or 50.