The achievement, announced at the IEEE Photovoltaic Specialists Conference in Tampa, Florida, is a world record for a two junction cell measured under one sun illumination, beating the 30.8% mark achieved by Alta Devices’ two-junction cell two months ago.
The U.S. Department of Energy‘s Foundation Program to Advance Cell Efficiency (F-PACE) is aiming for a 48% efficient concentrator cell a cell in which efficiency is bolstered by the presence of mirrors to magnify sunlight.
The starting point for the program, part of the Energy Department’s SunShot Initiative, was a 25.7% efficient single junction gallium arsenide cell, a mark bettered by Alta Devices to 26.4% in 2010 and 28.8% last year, before the two junction record hit in April.
An NREL press release announcing the achievement stated the record-breaking cell is made of a gallium indium phosphide cell atop a gallium arsenide cell, has an area of about 0.25cm2 and was measured under the AM1.5 global spectrum at 1,000 W/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.
Focusing on internal optics
"Historically, scientists have bumped up the performance of multi junction cells by gradually improving material quality and the internal electrical properties of the junctions by optimizing variables such as bandgaps and the layer thicknesses," said NREL scientist Myles Steiner. "But internal optics play an underappreciated role in high-quality cells that use materials from the third and fifth columns of periodic tables the III-V cells. The scientific goal of this project is to understand and harness the internal optics."
The NREL announcement added: ‘When an electron-hole pair recombines, a photon can be produced, and if that photon escapes the cell, luminescence is observed that is the mechanism by which light-emitting diodes work. In traditional single-junction gallium-arsenide cells, however, most of the photons are simply absorbed in the cell’s substrate and 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, generating additional current luminescent coupling.
‘NREL researchers improved the cell’s efficiency by enhancing the photon recycling in the lower, gallium-arsenide 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.’