Capturing energy from the sun

Capturing energy from the sun

ROMA – They say renewable energy is the future.
That is why Professor Molibeli Taele, a renowned National University of Lesotho (NUL) scientist, is developing photovoltaic (PV) solar cells to make that dream a reality.

If you know his works, you understand why he has been honored by a prestigious world scientific body, the Third World Academy of Sciences, as among the top ten “up-and-coming scientists,” selected from all over the world.
He says he is developing and testing solar cells made from rare earth metals, with a very strange sounding name — Cadmium-Tellurium (CdTe) solar cells.

As he works on this strange topic, he likes to repeat his favourite saying which says: “Some things are more fun in the dark… Photovoltaics isn’t one of them.” With photovoltaics, there must be light and that light must be from the sun.
Professor Taele and his team’s job (his is collaborating with the University of Darhum) is to develop a material that can capture that light to the best extent possible.

And he is doing exactly that.
Now, please relax, and don’t worry about the science jargon even as we explore the unfamiliar territory of the photovoltaics.
If you do, you will learn at the feet of Professor Taele.

“We seek to increase the efficiency of solar cells,” he says as he narrate the story of capturing energy from the sun.
“Our job is to get as much energy out of the sun’s rays as possible and that is not easy.”
Just how difficult is it?

“Of all the energy that reaches your normal laboratory scale solar cell, only 21 percent is, on average, converted into usable energy.”
Many have lower efficiency.
So their task was: could they improve efficiency of their cells?
They believed that by using a certain method of solar cell fabrication, they could increase their efficiency.
What was the method?

You will soon know.
But let’s start here so we don’t get lost — remember we are in the middle of a rather tricky territory by this point.
“We selected cadmium and tellurium, both of which are rare earth metals, because, they are both comparatively low-cost,” Professor said.
“It is true that cadmium can be toxic but the toxicity is largely reduced when it is in a form of a compound, which is how we use it.”
And then there is the fabrication process — the fabrication of a solar cell.
They start with glass, ordinary glass that we and you know.
But that is modified a bit.
You see, glass reflects some light.

So it is smeared on its outer surface with what they call anti-reflective coating (ARC).
The plan is clear, ARC reduces reflection and increases energy absorption.
On the inner surface, they deposit what they call Transparent Conductive Oxide (TCO).
No worries, we will tell you what that means.
It is an electrode.

An electrode is a material that conducts electricity and forms a link between metal and non-metal parts of a circuit.
Note that the TCO remains transparent because it has to allow light to pass through, even as it conducts electricity.
Then another substance, cadmium sulphide, is placed over the TCO.

Cadmium sulphide is said to release electrons as it receives energy from light.
Those electrons will generate electricity (remember electricity is the movement of electrons).
Then comes another layer, in the form of our material of interest.
It is Cadmium-Tellurium (CdTe) compound.
This is an absorber.

What does that mean?
The CdTe film acts as the primary photo-conversion layer and absorbs most visible light.
The CdTe, and TCO layers form an electric field that converts light absorbed in the CdTe layer into current and voltage.
“It creates positive charges,” Professor Taele says. “We call those charges holes.”
You still remember those electrons that were released?
Do you?

They enter the circuit as electricity and after transferring energy, they get back into the system through the positive charges (recall this, unlike charges attract).
So the electrons are sort of “absorbed” though the “holes,” only to be released once more if there is more energy from the sun.
Now you get the point.
Just when you thought we must be done already, they now heat (aneal) this last layer with cadmium chloride.
Without going into further details, just know that they then scratch (finishing touches), and heat again and then overlay the whole thing with either gold or aluminium as anodes.
Then our solar cell is done.

Complicated isn’t it?
Those are the heights Professor Taele is willing to climb to get you a clean bill of energy.
Did they improve the efficiency?
Yes. Now they went a few notches to 17 percent, up from 16.5 percent.
Is it a small improvement?
It does sound small.

But it isn’t because when small things come together, they make a big thing.
“We believe we have succeeded because we used a method called “closed space sublimation,” instead of “vapour deposition”, when depositing terrarium over cadmium,” Professor Taele says.
Never mind what those depositions are.
It does not matter anymore.
Just be happy that this hardworking NUL professor is doing this amazing work for you!

Own Correspondent

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