A new five-junction, mechanically stacked solar cell developed at The George Washington University (GWU) in Washington, District of Columbia (US), can capture nearly all of the energy in the solar spectrum. With a 44.5% conversion efficiency, its creators believe the new design has the potential to become the world´s most efficient solar cell.
The cell is designed for concentrated photovoltaics applications. Comprising a commercial, GaAs-based three-junction solar cell stacked on top of GWU´s new GaSb-based two-junction solar cell, the cell has four terminals, two for the top and two for the bottom part of the stack.
“Our cell is microscale; about 650 microns on the side,” describes Dr Matthew Lumb from GWU and the U.S. Naval Research Laboratory´s Optoelectronics and Radiation Effects branch. “In our paper, we built a mini module, which consisted of an achromatic lens positioned above the solar cell on a temperature controlled stage.” The scientists mounted the system on a large two-axis tracker in Durham, North Carolina, to track the sun. They then monitored the power from the sun and the power coming out of their cell.
The significance of using GaSb-based alloys
The researcher says the significance of their cell is in the use of GaSb-based alloys. About 99% of the light in the direct solar spectrum is between 250 nm and 2500 nm, but he notes that conventional, state-of-the-art solar cells for CPV based on GaAs, InP or Ge substrates cannot capture photons all the way to 2500 nm. “The best you can do is about 1800 nm with conventional lattice-matched materials, leaving quite a lot of available photons uncaptured,” says Lumb. “Our approach is able to collect these wavelengths and also avoids the need for difficult growth of lattice-mismatched alloys, so solar cells can be assembled to address the full spectrum range using the highest possible quality materials.”
Three proven components combined for the first time
The work, which is a collaboration between GWU, the U.S. Naval Research Laboratory, the private company Semprius, and the University of Illinois at Urbana-Champaign, has three main components, each of which has been individually demonstrated before, according to Lumb:
- GaSb-based PV devices have previously been developed, primarily for thermophotovoltaics applications, but also some for conventional PV.
- Concentrator PV with microscale solar cells (commercialized by Semprius).
- Mechanical stacking, either using wafer bonding techniques or, in this case, a technique called “transfer printing” (invented by John Rogers at UIUC, who is now at Northwestern University) and a technology Lumb says the GWU team, Semprius and UIUC have been working on for a few years.
“The new aspect of our work is to bring all of these together in a proof-of-concept demo to capture the whole spectrum,” the researcher reports and adds that they have designed and fabricated a GaSb-based solar cell specifically to be integrated into a mechanically stacked micro CPV cell and built a module to test it. “This is the first CPV cell demonstration that is able to use an efficient, multi-junction cell approach to capture all of the solar photons out to 2500 nm.”
44.5% conversion efficiency ï¿½ï¿½" and reaching for the world record
According to the GWU/Naval Laboratory researcher, the current solar cell conversion efficiency world record is 46%, measured under what is known as standard test conditions, which he notes is a truly outstanding result by the Fraunhofer ISE in Germany. “We made outdoor measurements of our cell, mainly because we do not have a solar simulator able to accurately replicate the spectrum all the way out to 2500 nm,” Lumb says. Strictly speaking, he points out, this means their results cannot be compared with the performance of solar cells under standard test conditions because the spectrum in Durham the day of the measurement was not the same as the standard spectrum. “However, we were very pleased with our first effort at the cell, as what the number does show is that it had very high performance, and our concept proves that it is possible to heterogeneously integrate GaSb-based materials to make very high efficiency devices.”
More importantly, though, Lumb projects he and his colleagues will be able to optimize the cell as their research progresses to push the efficiency higher, yet. “The most significant thing is, because we are able to capture photons which the current world record cell cannot (it is limited to about 1800 nm), we can potentially push the efficiency quite a lot higher than the current record,” Lumb says. “Our modeling predictions say that there is a realistic potential to take the current design above 47% efficiency by fine-tuning the device. And by adding more junctions to make a seven-junction cell the GaSb-based materials should support cell efficiencies approaching 55%.”
Another significant aspect Lumb emphasizes about this work is the mini-module´s (cell + lens) efficiency of 41.2%. “There has only ever been one module result higher than this, and we are excited about how far you can push this using the achromatic optics we used coupled with an improved cell.”
Lumb shares that one goal of the their paper was to give clear data on the design of their GaSb-based 2J cell and the problems they solved making it: “We overcame problems with low carrier mobilty materials, difficulties with the tunnel junction, how to process the materials and by model comparisons to the data, gave information about the material quality of the materials we developed,” he says. Lum now hopes that the paper will help other scientists build upon what his team has demonstrated and that other researchers will try to advance these materials for CPV applications. “We also demonstrated that there is no fundamental barrier to integrating more conventional, GaAs-based solar cells with our GaSb-based materials, and that transfer printing is a great technique to do this,” he adds.
Bringing the concept into the real world
Lumb says it is very difficult to predict what the path and the timeline to market would be for this novel solar cell, giving to consider that CPV has struggled to compete with lower cost, lower efficiency technologies for utility scale PV. However, he believes there is a lot of value in taking solar materials to their limit of performance. “There may be new and interesting applications emerging where achieving the absolute maximum energy output per unit area or per kilogram is a game changer,” he says, naming photovoltaics for space as one good example. “There are also new programs funded by the ARPA-E MOSAIC program that are looking at novel approaches to broaden the scope of CPV, including diffuse capture and fixed tilt technologies, which may be game changers.”
Making the design commercially viable
Lumb admits a lot has yet to happen before this potential world-record-breaking design can become commercially viable. He says at the moment, GaSb substrates are a lot more expensive than GaAs substrates, a reality he says is not driven by material scarcity but more by the volume production levels of GaAs-based devices being much higher than GaSb. “Therefore, 150-mm GaSb wafers for the same price as GaAs would really help,” he says.
However, he says the impact of substrate cost for both the GaAs and GaSb materials can be dramatically reduced by substrate recycling: “In the transfer printing process, the active cells are released from their native substrate during processing, so the expensive substrate can be repolished and used again,” Lumb suggests. “This has been demonstrated for GaAs, and demonstrating that for GaSb would be a great thing to do.”
As mentioned above, the main application for this new kind of super-light-harvesting stacked solar cell would be for very-high-performance CPV systems, and, as Lumb suggests, the space market may benefit as well. “However,” he interjects, “I am open minded about where extremely-high-efficiency solar cells might end up ï¿½ï¿½" there may be new and unexpected applications just around the corner.”
Impact on the future of solar energy
Lumb recalls that multi-junction solar cells have been slowly increasing in efficiency for decades. “We were concerned primarily with this research to explore the limits of what is possible with III-V alloys for PV and not worry too much about cost at this stage,” he says. “I am interested in using these materials, combined with heterogeneous integration techniques such as transfer printing, to see just how far we can go with conversion efficiency. I think we have a pathway to realize the ultimate, practically achievable solar cell this way as we can capture the whole spectrum, and by using transfer printing, we have very high degree of flexibility in what materials we can combine together.”
Lumb and his multi-institutional team aim to further improve their GaSb cell, mainly by tweaking the design and improving passivation of the sidewalls of the cell.
The work is reported in the article “GaSb-based Solar Cells for Full Solar Spectrum Energy Harvesting,” published in Advanced Energy Materials.
Written by Sandra Henderson, Research Editor, Solar Novus Today