Although full-spectrum solar cells have been made, none yet have been suitable for manufacture at a consumer-friendly price. Now Wladek Walukiewicz, who leads the Solar Energy Materials Research Group in the Materials Sciences Division (MSD) at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), and his colleagues have demonstrated a solar cell that not only responds to virtually the entire solar spectrum, it can also readily be made using one of the semiconductor industry’s most common manufacturing techniques.
The new design promises highly efficient solar cells that are practical to produce. The results are reported in a recent issue of Physical Review Letters, available online to subscribers.
How to make a full-spectrum solar cell
“Since no one material is sensitive to all wavelengths, the underlying principle of a successful full-spectrum solar cell is to combine different semiconductors with different energy gaps,” says Walukiewicz.
One way to combine different band gaps is to stack layers of different semiconductors and wire them in series. This is the principle of current high-efficiency solar cell technology that uses three different semiconductor alloys with different energy gaps. In 2002, Walukiewicz and Kin Man Yu of Berkeley Lab’s MSD found that by adjusting the amounts of indium and gallium in the same alloy, indium gallium nitride, each different mixture in effect became a different kind of semiconductor that responded to different wavelengths. By stacking several of the crystalline layers, all closely matched but with different indium content, they made a photovoltaic device that was sensitive to the full solar spectrum.
However, says Walukiewicz, “Even when the different layers are well matched, these structures are still complex – and so is the process of manufacturing them. Another way to make a full-spectrum cell is to make a single alloy with more than one band gap.”
In 2004 Walukiewicz and Yu made an alloy of highly mismatched semiconductors based on a common alloy, zinc (plus manganese) and tellurium. By doping this alloy with oxygen, they added a third distinct energy band between the existing two – thus creating three different band gaps that spanned the solar spectrum. Unfortunately, says Walukiewicz, “to manufacture this alloy is complex and time-consuming, and these solar cells are also expensive to produce in quantity.”
The new solar cell material from Walukiewicz and Yu and their colleagues in Berkeley Lab’s MSD and RoseStreet Labs Energy, working with Sumika Electronics Materials in Phoenix, Arizona, is another multiband semiconductor made from a highly mismatched alloy. In this case the alloy is gallium arsenide nitride, similar in composition to one of the most familiar semiconductors, gallium arsenide. By replacing some of the arsenic atoms with nitrogen, a third, intermediate energy band is created. The good news is that the alloy can be made by metalorganic chemical vapor deposition (MOCVD), one of the most common methods of fabricating compound semiconductors.
How band gaps work
Band gaps arise because semiconductors are insulators at a temperature of absolute zero but inch closer to conductivity as they warm up. To conduct electricity, some of the electrons normally bound to atoms (those in the valence band) must gain enough energy to flow freely – that is, move into the conduction band. The band gap is the energy needed to do this.
When an electron moves into the conduction band it leaves behind a “hole” in the valence band, which also carries charge, just as the electrons in the conduction band; holes are positive instead of negative.
A large band gap means high energy, and thus a wide-band-gap material responds only to the more energetic segments of the solar spectrum, such as ultraviolet light. By introducing a third band, intermediate between the valence band and the conduction band, the same basic semiconductor can respond to lower and middle-energy wavelengths as well.
This is because, in a multiband semiconductor, there is a narrow band gap that responds to low energies between the valence band and the intermediate band. Between the intermediate band and the conduction band is another relatively narrow band gap, one that responds to intermediate energies. And finally, the original wide band gap is still there to take care of high energies.
“The major issue in creating a full-spectrum solar cell is finding the right material,” says Kin Man Yu. “The challenge is to balance the proper composition with the proper doping.”
In solar cells made of some highly mismatched alloys, a third band of electronic states can be created inside the band gap of the host material by replacing atoms of one component with a small amount of oxygen or nitrogen. In so—called II-VI semiconductors (which combine elements from these two groups of Mendeleev’s original periodic table), replacing some group VI atoms with oxygen produces an intermediate band whose width and location can be controlled by varying the amount of oxygen. Walukiewicz and Yu’s original multiband solar cell was a II-VI compound that replaced group VI tellurium atoms with oxygen atoms. Their current solar cell material is a III-V alloy. The intermediate third band is made by replacing some of the group V component’s atoms – arsenic, in this case – with nitrogen atoms.
Finding the right combination of alloys, and determining the right doping levels to put an intermediate band right where it’s needed, is mostly based on theory, using the band anticrossing model developed at Berkeley Lab over the past 10 years.
“We knew that two-percent nitrogen ought to do the job,” says Yu. “We knew where the intermediate band ought to be and what to expect. The challenge was designing the actual device.”
Passing the test
Using their new multiband material as the core of a test cell, the researchers illuminated it with the full spectrum of sunlight to measure how much current was produced by different colors of light. The key to making a multiband cell work is to make sure the intermediate band is isolated from the contacts where current is collected.
“The intermediate band must absorb light, but it acts only as a stepping stone and must not be allowed to conduct charge, or else it basically shorts out the device,” Walukiewicz explains.
The test device had negatively doped semiconductor contacts on the substrate to collect electrons from the conduction band, and positively doped semiconductor contacts on the surface to collect holes from the valence band. Current from the intermediate band was blocked by additional layers on top and bottom.
For comparison purposes, the researchers built a cell that was almost identical but not blocked at the bottom, allowing current to flow directly from the intermediate band to the substrate.
The results of the test showed that light penetrating the blocked device efficiently yielded current from all three energy bands – valence to intermediate, intermediate to conduction, and valence to conduction – and responded strongly to all parts of the spectrum, from infrared with an energy of about 1.1 electron volts (1.1 eV), to over 3.2 eV, well into the ultraviolet.
By comparison, the unblocked device responded well only in the near infrared, declining sharply in the visible part of the spectrum and missing the highest-energy sunlight. Because it was unblocked, the intermediate band had essentially usurped the conduction band, intercepting low-energy electrons from the valence band and shuttling them directly to the contact layer.
Further support for the success of the multiband device and its method of operation came from tests “in reverse” – operating the device as a light emitting diode (LED). At low voltage, the device emitted four peaks in the infrared and visible light regions of the spectrum. Primarily intended as a solar cell material, this performance as an LED may suggest additional possibilities for gallium arsenide nitride, since it is a dilute nitride very similar to the dilute nitride, indium gallium arsenide nitride, used in commercial “vertical cavity surface-emitting lasers” (VCSELs), which have found wide use because of their many advantages over other semiconductor lasers.
With the new, multiband photovoltaic device based on gallium arsenide nitride, the research team has demonstrated a simple solar cell that responds to virtually the entire solar spectrum – and can readily be made using one of the semiconductor industry’s most common manufacturing techniques. The results promise highly efficient solar cells that are practical to produce.
“Engineering the Electronic Band Structure for Multiband Solar Cells,” by Nair Lopez, Lothar Reichertz, Kin Man Yu, Ken Campman, and Wladyslaw Walukiewicz, appears in the 10 January, 2011 Physical Review Letters and is available online to subscribers.
Paul Preuss | EurekAlert!
How protons move through a fuel cell
22.06.2017 | Empa - Eidgenössische Materialprüfungs- und Forschungsanstalt
Fraunhofer IZFP acquires lucrative EU project for increasing nuclear power plant safety
21.06.2017 | Fraunhofer-Institut für Zerstörungsfreie Prüfverfahren IZFP
An international team of scientists has proposed a new multi-disciplinary approach in which an array of new technologies will allow us to map biodiversity and the risks that wildlife is facing at the scale of whole landscapes. The findings are published in Nature Ecology and Evolution. This international research is led by the Kunming Institute of Zoology from China, University of East Anglia, University of Leicester and the Leibniz Institute for Zoo and Wildlife Research.
Using a combination of satellite and ground data, the team proposes that it is now possible to map biodiversity with an accuracy that has not been previously...
Heatwaves in the Arctic, longer periods of vegetation in Europe, severe floods in West Africa – starting in 2021, scientists want to explore the emissions of the greenhouse gas methane with the German-French satellite MERLIN. This is made possible by a new robust laser system of the Fraunhofer Institute for Laser Technology ILT in Aachen, which achieves unprecedented measurement accuracy.
Methane is primarily the result of the decomposition of organic matter. The gas has a 25 times greater warming potential than carbon dioxide, but is not as...
Hydrogen is regarded as the energy source of the future: It is produced with solar power and can be used to generate heat and electricity in fuel cells. Empa researchers have now succeeded in decoding the movement of hydrogen ions in crystals – a key step towards more efficient energy conversion in the hydrogen industry of tomorrow.
As charge carriers, electrons and ions play the leading role in electrochemical energy storage devices and converters such as batteries and fuel cells. Proton...
Scientists from the Excellence Cluster Universe at the Ludwig-Maximilians-Universität Munich have establised "Cosmowebportal", a unique data centre for cosmological simulations located at the Leibniz Supercomputing Centre (LRZ) of the Bavarian Academy of Sciences. The complete results of a series of large hydrodynamical cosmological simulations are available, with data volumes typically exceeding several hundred terabytes. Scientists worldwide can interactively explore these complex simulations via a web interface and directly access the results.
With current telescopes, scientists can observe our Universe’s galaxies and galaxy clusters and their distribution along an invisible cosmic web. From the...
Temperature measurements possible even on the smallest scale / Molecular ruby for use in material sciences, biology, and medicine
Chemists at Johannes Gutenberg University Mainz (JGU) in cooperation with researchers of the German Federal Institute for Materials Research and Testing (BAM)...
19.06.2017 | Event News
13.06.2017 | Event News
13.06.2017 | Event News
23.06.2017 | Physics and Astronomy
23.06.2017 | Physics and Astronomy
23.06.2017 | Information Technology