The USA’s National Renewable Energy Laboratory (NREL) has announced a new record of 31.1 percent solar conversion efficiency with its latest two-junction solar cell, edging out the previous record of 30.8 percent set by Alta Devices in March this year.
The record-breaking solar cell was made by sandwiching a gallium arsenide cell base between a gallium indium phosphide cell facing. When combined, the materials allowed for an enhanced flow of photons within the cell structure, leading to increased energy output. The researchers then added an anti-reflective coating to the front of the cell, with a highly reflective gold layer to the rear wall to ensure maximum light absorption.
The project is part of the U.S. Department of Energy Foundation Program to Advance Cell Efficiency (F-PACE), which intends producing a multijunction concentrator cell with a power conversion efficiency of 48 percent.
According to the NREL, the internal chemistry at play inside dual-junction solar cells is the key to improving efficiency levels.
“Historically, scientists have bumped up the performance of multijunction cells by gradually improving the material quality and the internal electrical properties of the junctions,” NREL scientist Myles Steiner said.
But internal optics plays an under-appreciated role in high-quality cells that use materials from the third and fifth columns of the periodic tables – the III-V cells.
“The scientific goal of this project is to understand and harness the internal optics.”
The advantage of two-junction gallium arsenide/ gallium indium phosphate (GaInP) solar cells over their single junction counterparts is their ability to recycle or reabsorb photons through a process known as luminescent coupling – photons that would otherwise be scattered and lost into the substrate of the cell material.
The NREL found that under a one-sun illumination, the solar cell achieved significant increases in both the open-circuit voltage and the short-circuit current.
The 31.1 percent mark is just the latest in a steady improvement in GaAs technology. Alta Devices beat the NREL’s record of 25.7 percent in 2010, before a setting a string of its own: 26.4 in 2010; 28.8 percent in 2012; and finally, 30.8 in 2013.
The NREL does not expect its latest 31.1 percent milestone to last – the lab predicts its new cell will perform well when concentrating lenses are added to focus sunlight onto the cell.
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