On 19 June at the 39th IEEE Photovoltaic Specialists Conference (PVSC 39) in Tampa, FL, scientist Myles Steiner at the US Department of Energy's National Renewable Energy Laboratory (NREL) reported record energy conversion efficiency of 31.1% for a two-junction solar cell under one-sun illumination. The previous record of 30.8% was held by Alta Devices of Sunnyvale, CA, USA.
Made of a gallium indium phosphide (GaInP) cell atop a gallium arsenide (GaAs) cell, the tandem cell has an area of about 0.25 square centimeters and was measured under the AM1.5 global spectrum at 1000W/m2. It was grown inverted – similar to the NREL-developed inverted metamorphic multi-junction (IMM) solar cell – and flipped during processing. The cell was covered on the front with a bilayer anti-reflection coating and on the back with a highly reflective gold contact layer.
The work was done as part of the DOE's Foundation Program to Advance Cell Efficiency (F-PACE), a project of the DOE's SunShot Initiative that aims to lower the cost of solar energy to a point at which it is competitive with other sources including fossil fuels.
At the beginning of the F-PACE project, which aims to produce a 48%-efficient concentrator cell, NREL's best single-junction GaAs solar cell was 25.7% efficient. This has been improved upon by others over the years: Alta set a series of records, increasing the GaAs record from 26.4% in 2010 to 28.8% in 2012. Alta's then-record two-junction 30.8%-efficient cell was achieved just two months ago.
The new record may not last long either, but "it brings us one step closer to the 48% milestone," says NREL principal scientist Sarah Kurtz, who leads the F-PACE project in NREL's National Center for Photovoltaics. "This joint project with the University of California, Berkeley and Spectrolab has provided us the opportunity to look at these near-perfect cells in different ways. Myles Steiner, John Geisz, Iván García and the III-V multi-junction PV group have implemented new approaches providing a substantial improvement over NREL's previous results," she adds.
"Historically, scientists have bumped up the performance of multi-junction cells by gradually improving the material quality and the internal electrical properties of the junctions – and by optimizing variables such as the bandgaps and the layer thicknesses," says NREL scientist Myles Steiner. But internal optics plays an underappreciated role in high-quality cells that use materials from the third and fifth columns of the periodic tables, he adds. "The scientific goal of this project is to understand and harness the internal optics."
When an electron-hole pair recombines, a photon can be produced, and if that photon escapes the cell, luminescence is observed (i.e. the mechanism by which light-emitting diodes work). In traditional single-junction GaAs cells, however, most of the photons are simply absorbed in the cell's substrate and are lost. With a more optimal cell design, the photons can be re-absorbed within the solar cell to create new electron-hole pairs, leading to an increase in voltage and conversion efficiency. In a multi-junction cell, the photons can also couple to a lower-bandgap junction (i.e. luminescent coupling), generating additional current.
The NREL researchers improved the cell's efficiency by enhancing the photon recycling in the lower GaAs junction by using a gold back contact to reflect photons back into the cell, and by allowing a significant fraction of the luminescence from the upper GaInP junction to couple into the GaAs junction. Both the open-circuit voltage and the short-circuit current were increased.
Silicon solar cells now dominate the PV market, but researchers see opportunities for new materials. High-efficiency concentrator cells bolstered by lenses that magnify the power of the sun are attracting interest from utilities, says NREL, because the modules have demonstrated efficiencies well over 30%. Also, there may be commercial opportunities for one-sun or low-concentration III-V cells if growth rates can be increased and costs reduced.
The same cell should work well when lenses are added to multiply the sun's power. "We expect to observe similar enhancements of the solar cell characteristics when measured under concentrated illumination," Steiner concludes.
