A solar cell’s ability to convert sunlight to electric current is limited by the band gaps of the semiconductors from which it is made. For example, semiconductors with wide band gaps respond to shorter wavelengths with higher energies (lower left). A semiconductor with an intermediate band has multiple band gaps and can respond to a range of energies (lower right). |
Solar cells are made from semiconductors whose ability to respond to light
is determined by their band gaps (energy gaps). Different colors have different
energies, and no single semiconductor has a band gap that can respond to
sunlight’s full range, from low-energy infrared through visible light to
high-energy ultraviolet.
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, 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 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.
Kin Man Yu and Wladek Walukiewicz have long been leaders in multiband solar cell technology. |
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.
At top, a test device of the new multiband solar cell was arranged to block current from the intermediate band; this allowed a wide range of wavelengths found in the solar spectrum to stimulate current that flowed from both conduction and valence bands (electrons and holes, respectively). In a comparison device, at bottom, the current from the intermediate band was not blocked, and it interfered with current from the conduction band, limiting the device’s response. |
“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.
