In a race for the highest solar cells conversion efficiencies, US-based Alata Devices won back its world record title. On May 27, the company announced that its dual junction solar cell attained a conversion efficiency of 31.6% under one sun, which is the highest efficiency recorded for a multijunction solar cell under the subcategory of two junction non-concentrated cells. The efficiency has been certified by National Renewable Energy Laboratory (NREL), which in turn was the previous world record holder since 2013, when it reached an efficiency of 31.1%. Before that, it was Alta Devices that owned the crown at 30.8% efficiency. The US company emphasizes its latest achievement is the 7th time it holds the title for the highest efficiency since 2010.

Using a second cell

The current record level performance has been obtained by extending the company’s core competence of III-V cells made of Gallium Arsenide (GaAs) single junction technology to dual junction. This is attained by adding Indium Gallium Phosphide (InGaP) as the top cell to the basic GaAs structure. Such a tandem cell configuration enhances the absorption band of the sunlight; especially it captures the high energy photons efficiently. The company also holds the current world record for single junction cells with a conversion efficiency of 28.8% illuminated under non-concentrated light.

Producing single junction cells
Regarding its commercial products, Alta Devices is currently supplying only the single junction GaAs cells. These cells are made by growing a thin layer of GaAs on top of a single crystal GaAs substrate, which seeds the growth process accomplished by Metalorganic Chemical Vapor Deposition (MOCVD). Then, a rear metal is coated by means of Physical Vapor Deposition (PVD). A polymer sheet is attached as the mechanical support followed by the separation GaAs thin film from the seed wafer. This process – also called epitaxial lift off – is accomplished by means of chemical etching. The original GaAs wafer can be reused again and produced thin film is deposited with patterned front metal contacts to make it a complete solar cell.

Powering high-end unplugged devices
Unlike many other PV cell and module manufacturers, which make products for large scale power plants and rooftop systems, Alta Devices is focussing on applications powering the “unplugged world”. These applications include unmanned aerial vehicles, consumer electronics, sensors, automotives – where power though plugging a cord is simply not feasible. That actually makes sense. Because high-efficiency PV technologies, such as GaAs, are not cost competitive for large power generation. But customers for high-end applications, like what Alta Devices is focussing on, are not looking at the lowest price. On top, the solar cells of the company come with the advantage of being lightweight and flexible, which make them a good match for mobile applications. These GaAs thin film PV cells have a thickness of 110 µm, weigh only 170 g/m2, and are bestowed with the ability to bend, so they can cover curved surfaces with a spec of 2 cm radius of curvature. Adding to that, Alta Devices is willing to supply the PV films in custom shapes and size, apart from the standard cell size of 5 x 2 cm.

Outdoors Alta Devices’ cells have a rated output power of 260 W/m2. This translates to a conversion efficiency of 26% in production, which is quite close to the top performance of 28.8%.