Ge-free solar cell seizes unconcentrated record
by Andy Extance
US researchers have produced a low-defect solar cell with a conversion efficiency of 33.8 percent under the standard terrestrial spectrum of sunlight, beating the previous record of 32.0 percent.
The work, performed at the National Renewable Energy Lab, Colorado, furthers the triple-junction inverted metamorphic approach that Emcore has used to attain the record for space-based efficiency, partly by replacing the Ge substrate.
In the July 9 issue of Applied Physics Letters, the group describes a cell which uses InGaAs as the 1.0 eV junction instead of Ge, and is grown on a GaAs substrate that can be removed after the epitaxy is otherwise completed.
Talking to compoundsemiconductor.net Mark Wanlass, one of the inventors of the approach, explained, “When you grow the structure it s inverted, then we flip it and mount it to a silicon or glass handle, and in this case we dissolved the GaAs layer "“ although you can keep and extract the materials.”
The InGaAs layer is better than germanium in current matching with the 1.4 eV GaAs and 1.8 eV GaInP junctions used in leading III-V solar cells.
InGaAs is lattice mismatched with the GaAs junction it is adjacent to, but the inverted approach leaves its growth until last, allowing the 1.8 eV and 1.4 eV layers to be grown on top of the GaAs substrate with high crystal quality.
The “metamorphic” growth of graded GaInP, before InGaAs is finally deposited at the top of the epitaxial structure, also minimises the number of dislocations in this final layer, adding to the performance of the cell.
Punching its weight
As well as providing beneficial electronic properties, eliminating Ge lowers the weight and cost of the final device "“ giving “way higher” W/kg performance, according to Wanlass.
NREL s efficiency mark for concentrated solar, 38.9 percent, falls short of Spectrolab's record of 40.7 percent, although the respective degrees of concentration, 81 versus 240 suns, give NREL scope for improvement.
“At high concentration under the terrestrial spectrum we could be looking at 42-43 percent efficiency,” Wanlass said.
The level of performance is offset by the new substrate-removing process, which raises potentially expensive questions of those who would scale it into production.
“A mass production method of getting the substrates off, that's something that's different to what they're used to, but it can be done,” said Wanlass.
Manufacturing costs, including glass, metal and plastic used in final systems, demand a minimum conversion efficiency of 40 percent for III-V solar to be economical in terrestrial applications.
If commercial cells can be pushed further, to 45 or even 50 percent efficiency, the crucial cost-per-watt metric falls dramatically and could lead to widespread deployment.
Wanlass is hopeful that the Ge-free approach will attain this goal, although he admits it will require more junctions.
“I think we can get there with four,” Wanlass said.
“Our technique looks to be the most promising thing on the table right now, in terms of getting to higher efficiencies.”
Author
Andy Extance is a reporter at compoundsemiconductor.net.
