Southerly by David Haywood

9

What on earth is a Grätzel solar cell, and why is it so important?

This is a transcript of an episode of Public Address Science which was originally broadcast on Radio Live, 21st April 2007, 2 pm - 3 pm.

You can listen to the original audio version of the programme by clicking on the 'Play the audio for this post' link at the top of this page or the 'Audio' button at the bottom of this page.

* * *

Theme music...

Voiceover:

Solar energy... in theory, it should be the answer to all our energy problems. Properly managed, more than enough solar energy falls on the roofs of New Zealand houses to provide all our domestic electricity needs.

So why don't we make use of it?

One problem is that -- fairly obviously -- the sun only shines during the day, which means that storage batteries [or similar] are required to provide energy for use at night. The other problem is that the solar cells used to generate electricity from sunlight are incredibly expensive.

That's because the raw silicon ingots used for solar cell manufacture require production technology that is astonishingly high-tech, enormously energy-intensive, and therefore mind-bogglingly costly. As a result, a 60 watt solar panel (enough to power a single dim incandescent light bulb) will set you back by around $850 dollars.

And why would you pay that sort of money when you get virtually unlimited electricity out of the power lines at a fraction of the cost and effort?

But the price of solar generated electricity is actually coming down. For each doubling of production capacity in the factories that manufacture solar cells, the price has fallen by around 20 per cent. In recent years, that's equated to a price drop of around 5 per cent per annum.

So do we all just have to wait for a few decades until we can afford those shiny solar-electricity panels on our roof?

Well, not necessarily. A new type of solar cell technology has emerged which looks set to change everything. Grätzel solar cells seem likely to slash the cost of solar generated electricity. They've actually been around since the early 1990s, but it's only comparatively recently that scientists have been able to get them work in a reliable

manner.

One of the research teams at the forefront of Grätzel solar cell technology is the Nanomaterials Research Centre at Massey University. I talked to Dr Wayne Campbell about what the future might hold for solar energy.

I asked him to start by explaining how conventional silicon solar cells are made.

Dr Wayne Campbell:

The common silicon solar cell [which] you can buy is basically made from pure silicon ingots. It's... sliced up into little slices, and then doped with a n-type or a p-type dopant to make the actual solar cell. It forms what they call a p-n junction. When light shines over that junction you get electron transfer.

Interviewer:

So putting it in very simplistic terms: the energy in the photons of sunlight knocks loose electrons from the doped silicon material, which produces an electric current. In contrast, how do Grätzel cells work -- and how are they made?

Dr Wayne Campbell:

It's a photoelectochemical cell. It works completely differently really. In a simple sense you have a dye, which absorbs light [and] excites an electron up to a higher energy part of the molecule. From there that energy transfers to a semi-conductor -- in this case it's usually Titanium dioxide -- and from there it's collected on a transparent conducting surface. It's basically photoexcitation followed by charge separation... and then you get the loop of the electron back to the dye again.

Interviewer:

So the energy from the sunlight is first absorbed into a dye, and then there's a second step where the energy is transferred from the dye into a semiconductor material. And, in this case, the semi-conductor material is titanium dioxide, which is presumably cheaper to produce than the silicon crystals in conventional solar cells?

Dr Wayne Campbell:

It's a very thin layer, so it's

very cheap.

Interviewer:

And is the manufacture of Grätzel cells a simpler production process than for conventional silicon cells?

Dr Wayne Campbell:

Yeah, basically [either] it's screen printing, or simple pyrolysis, or plasma deposition.

Interviewer:

So I know that your research group have been collaborating with Professor Grätzel who invented these Grätzel cells in Switzerland. What aspect of the technology are you actually looking at?

Dr Wayne Campbell:

Our main area [is] not so much developing the cell anymore -- it's just developing a better dye for these cells. The current dyes that are used are quite expensive because they're Ruthenium-based. So they're based on a fairly rare metal which would have limited supplies if it was used in large quantities.

Interviewer:

So you've had quite a bit of success with your research. What are the advantages of the new Grätzel cell dyes that your team has developed.

Dr Wayne Campbell:

It's a lot cheaper to make than the Ruthenium dyes -- basically because it doesn't have any rare metals.

It's based on a chlorophyll molecule, which is the porphyrin haem group in blood (the red molecule in blood). [So] there's no reason for [the Grätzel solar cells] to be toxic -- or anything like that -- [when they are disposed of] afterwards either.

Interviewer:

So basing your dyes on chlorophyll, the chemical that plants use to absorb sunlight, and blood, an energy carrier in animals, that's really a case of science imitating nature. What efficiency are you getting out of the Grätzel solar cells?

Dr Wayne Campbell:

The best for the Grätzel cell was with the Ruthenium dye -- and that's quoted at 10.1 per cent.

Interviewer:

Okay, and with the new dye that you've developed?

Dr Wayne Campbell:

The latest report from Grätzel's lab is for 7.1 per

cent, so we're quite happy with that. [And] it seems to be very reproducible, [whereas] some of these other dyes don't always seem to be reproducible.

The dye itself hasn't actually been optimised properly in the cell either. [Grätzel's laboratory tested it with] the electrolytes and stuff that they normally would use with their Ruthenium dyes.

By modifying things like [the electrolytes] you can actually get a lot more performance out of the cell. We expect even better than 7 per cent, definitely.

Interviewer:

So you've got good efficiency -- but not quite as good as normal silicon solar cells, which are around the 9 to 15 per cent range, but of course your Grätzel cells would end up being much cheaper, wouldn't they? Do you have any feel for the cost reduction?

Dr Wayne Campbell:

Probably it's going to be [about] one-tenth the cost -- but we don't have any exact figures really, at the moment.

Interviewer:

Wow, that would be a significant cost reduction compared to the comparatively slow rate that the price of conventional solar cells is dropping. So even if the Grätzel cells stay at their current efficiency you're still cutting something like four-fifths off the price on a per watt basis (in comparison to silicon-based cells).

Dr Wayne Campbell:

Yeah... [the Grätzel cells will only be] a fraction of the [current] cost.

* * *

Voiceover:

Despite a somewhat lower efficiency, the much lower cost of Grätzel solar cells is certain to bring about a dramatic sea-change in the amount of solar-derived electricity in our society's energy mix.

Dr Campbell expects to see Grätzel cells based on the more expensive Ruthenium dyes in shops within the next few years. Although it may take a while longer before cells with the cheaper chlorophyll-based dyes make an appearance.

Either way, the Grätzel solar cells are another important part of the jigsaw of technologies that will be needed to ensure a secure energy future.

Theme music...

* * *

Further information on Grätzel solar cells:

9 responses to this post

Post your response…

Please sign in using your Public Address credentials…

Login

You may also create an account or retrieve your password.