Cadmium Sulfide Makes Thin-Film Solar Cells Attractive
AS A POWDER, cadmium sulfide (CdS) was used as a pigment almost a hundred years ago to make bright
yellow or, when mixed with selenide, bright orange paint.
Cadmium, like lead, is a heavy metal, and CdS paint was
eventually shown to be poisonous. Like the brilliant white
paint based on lead oxide, CdS paint had to be removed
from walls in order not to poison children.
We continue to use large quantities of lead in car batteries, sealed up so that it creates no biological hazard. Similarly,
a layer of CdS, well encapsulated, improves the efficiency of
many types of thin-film solar cells. These cells are cheaper
than silicon (Si) cells and can be made in automated factories. They typically are 100 times thinner than Si cells, consume less material
and energy during the production process and, unlike silicon, do not need a
high degree of purification.
The most advanced thin-film cells in mass production are based on cadmium telluride (Cd Te). Hundreds of megawatts of Cd Te modules are already
installed on roofs all over the world. Every one of these cells is coated with a
layer of CdS that is a hundred times thinner than a human hair. That layer of
CdS can nearly double the efficiency of a Cd Te module. Without the coating,
Cd Te modules typically show an efficiency of 8 to 10 percent, depending on
the method of production. However, with the thin CdS coating, the Cd Te
module’s efficiency increases to more than 15 percent.
While this phenomenon has been known for almost three decades, no one
was able to figure out why it works. Then, a year ago, when Springer Publishing
asked me to write two books summarizing my CdS research, I reviewed a lot of
my early work, along with cutting-edge CdS/Cd Te research, some not yet published, at the University of Delaware’s Institute of Energy Conversion. Bringing
all this together, I could finally make some sense of why CdS works so well.
Dirty semiconductors (those with a lot of impurities and crystal defects)
have many places where electrons get stuck — we call these
“traps.” When an electron is trapped, it produces an electric
field (a “build-in field”). You cannot measure these fields
from the outside, but they make all the difference. In a silicon
semiconductor, the transition region between two layers
with different impurities is called a pn-junction, and it’s the
basic building block of all transistors. But when accidentally
induced by thin-film impurities, random pn-junctions make
the solar cell “leaky,” allowing electrons to move randomly
instead of flowing smoothly toward the conductor.
The CdS layer plugs this leak, and only CdS can do this.
The explanation for this is a little involved, but, to put it
simply: When some minute amount of copper is incorporated in CdS (we
call this doping), its photoconductivity can be reduced (“quenched”) by a
small electric field, and that limits the further increase of this build-in field long
before it causes the junction to become leaky. Many other compounds can be
quenched as well, but the concentration of these intentionally incorporated
impurities has to be precisely adjusted, to about 100 parts per million (ppm).
If there are too many impurities, the field is limited at much higher values and
can cause some junction leakage; if there are not enough, their influence is
not sufficient. Now, 100 ppm happens to be the natural limit of saturation
of copper in CdS. So if too much copper is supplied during production, the
surplus is just rejected. So, CdS works by automatic self-adjustment. All other
compounds have to be doped artificially to that amount. This is not easy to
achieve over many square feet of solar cells, and it explains the limited success
of materials other than copper-doped CdS.
This process works on many other thin-film solar cells, provided the
“junction” is close enough to the interface with CdS. Copper indium dis-elenide, or CIS, solar cells, containing indium or gallium, are thus improved
in a similar fashion.
By KARL W. BöER
Karl W. Böer is Distinguished Pro-
fessor of Physics and Solar Energy
Emeritus in the Department of
Material Science and Engineering
at the University of Delaware. He
is a founder and Fellow of the
American Solar Energy Society.
For further reading, see K. W.Böer,
(2009) phys. stat. sol. A 206, 2665.
After Copenhagen: Time To Act Locally
Copyright © 2010 by the American Solar Energy Society Inc. All rights reserved.
In the wake of COP 15 in Copenhagen, Bill Becker says, the fate of the planet is in our hands. Becker, executive director of the Presidential Climate Action Project and former
central regional director for the U.S. Department of Energy, attended the Copenhagen
conference. The five-way informal agreement that emerged between China, India, Brazil,
South Africa and the United States, he says, was “halfhearted, with no binding agreements
and no targets.”
“We can’t depend on governments to do the job,” Becker said. “When governments
and legislatures write climate policy, we inevitably get an agreement that serves the
lowest common denominator. This throws the problem back upon us, and on what com-
munities, states, corporations and regions can do.”
Becker believes that the United Nations will never forge a treaty with 193 signatories.
Instead, he now hopes that the largest-emitting nations and regions will forge bilateral
and multilateral agreements to cut carbon pollution.
“We can hope for multinational agreements that are much more concrete, binding and
verifiable than this [Copenhagen] three-pager,” he said. “This informal agreement may set
a useful example, but it’s not a good model in terms of completeness.”
Becker is mildly encouraged by progress at city and state levels. “There’s more renew-
able energy potential in state and city commitments than in anything done at the national
solartoday.org SOLAR TODA Y March 2010 19