Solar panels now grace the roofs of more than 2 million Australian homes. But when it comes to solar cell efficiency records, the numbers aren’t so clear-cut.
These days, the best silicon solar cells operate at 26.7 per cent efficiency.
Hang on. Only 26.7 per cent? That seems pretty low, especially when you find out that’s under ideal lab conditions.
But don’t diss the efforts of chemists and physicists — 26.7 per cent, and the tiny efficiency gains that led to it, is a big deal.
Put simply, there’s a limit to how much of the sun’s energy can be converted to electricity by solar systems.
In the case of your standard rooftop silicon panels, efficiency tops out at around 32 per cent.
(And that’s a theoretical figure. Out in the real world, silicon panels are around 20 per cent efficient.)
So why does this 32 per cent limit exist, and can we work around it?
First, we need to understand the nuts and bolts of how solar cells, which make up panels, generate electricity from sunlight.
Let’s go with silicon solar cells, given they’re the most familiar.
These cells are made of a silicon wafer that’s “doped” with small amounts of other elements, so electrons flow around a circuit in a particular direction to give us electricity.
Electrons in silicon atoms usually hang out in what’s called a “valance band”. While there, electrons aren’t free to move around, so the silicon acts like an insulator.
But if an electron gains enough energy, it can jump into a higher energy “conduction band” where it’s mobile and — voila! — able to produce electrical current.
The amount of energy needed to bounce between the valance and conduction bands is called the “band gap”.
In solar cells, sunlight in the form of photons provides the energy kick electrons need to traverse the band gap.
And for silicon, that band gap is 1.1 electron volts.
Named after the physicists who calculated it in 1961, a material’s maximum efficiency is called the Shockley-Queisser limit.
It’s a fairly complex calculation that takes into account a bunch of factors. A big one is that not all photons are created equal when it comes to energy.
The sun spits out a wide spectrum of photons, from ultraviolet through to infrared.
Visible light only makes up part of the spectrum of sunlight that falls on Earth. (Wikimedia Commons: Fulvio314)
But redder photons carry less energy than their bluer counterparts, said Andrew Tilley, a chemist at the University of Melbourne’s Bio21 Institute.
“So light with energy below the band gap passes through the silicon, unabsorbed.”
That ends up being a large proportion, too — around 20 per cent of sunlight falling on a solar cell simply does not contain enough energy to provide that 1.1-electron-volt boost.
Photons that aren’t energetic enough provide a large chunk of wasted energy. This is called the transmission loss.
And then there are heaps of photons that are tooenergetic.
Take, for instance, an orange photon with 2 electron volts hitting a silicon solar cell. It kicks up an electron from the valance band to the conduction band, but traversing the band gap only requires 1.1 electron volts.
This leaves 0.9 electron volts of energy left over, which manifests itself as waste heat or, in fancy physics terms, thermalisation loss.
“It’s a massive challenge,” Dr Tilley said.
In the case of silicon solar cells, thermalisation and transmission account for about 35 and 20 per cent, respectively, of efficiency loss.
The remaining 15 per cent or so is energy lost due to other quirks of optics and thermodynamics.
Of course not. Physicists and chemists are finding ways to capture energy that would usually be lost in transmission and thermalisation and turn it into electricity.
Let’s see how we can get around the big thermalisation problem. One way is to stack layers of solar cells, with each absorbing a different part of the spectrum.
The key is to use old tried-and-tested silicon with another semiconducting material to create a “stacked” solar cell.
Stacked solar cells have been used for years, but generally only in solar-powered devices where space is a premium, like satellites and spacecraft.
While Opportunity’s solar panels were pretty great in terms of efficiency, in the end, they were no match for the Martian dust.(Supplied: NASA/JPL-Caltech/Cornell University/Arizona State University)
But they were made of gallium-arsenide which, even now, costs up to $300 per watt — about 100 times more expensive than silicon panels.
A new and quite promising candidate is a group of materials called perovskites, which have “shot from obscurity to being awesome in a short period”, said Niraj Lal, a visiting fellow at the Australian National University.
The beauty of perovskites — aside from the fact they are cheap and made from plentiful materials such as lead — is that their band gap can be “tuned” depending on their chemical make-up.
Last year, Oxford researchers stacked a perovskite capable of catching high-energy, blue photons on top of a silicon cell, which then caught lower-energy photons towards the red end of the spectrum, to create a stacked or “tandem” solar cell that was 28 per cent efficient.
“When you start combining these cells, then you can really go past the Shockley-Queisser limit,”
“Just in the past year, they’ve cracked records to be better than silicon alone.”
Then there are tricky and complicated methods that take high-energy photons and manipulate them so solar panels can use them without producing as much waste heat.
There are also efforts to catch and manipulate two low-energy photons to create one high-energy photon. This is called photon upconversion.
Other materials, like zinc telluride, can be designed to have a mid-band-gap stepping stone of sorts. These are called intermediate band materials, Dr Tilley said.
“The idea is electrons can use a low-energy photon to get halfway between the valance to conductance bands, then another to get them to conductance.”
While Dr Lal thinks tandem perovskite-silicon solar cells will likely be the next phase of commercial solar cells, there are still quite a few kinks to iron out.
Perovskites are often made of toxic materials like lead and they degrade faster than silicon, especially if they get water on them.
Silicon solar cells tend to come with a guarantee that after 25 years of use, they’ll still operate at 80 per cent of their initial efficiency.
“The challenge is how to make perovskites stable, so they last in a harsh climate like Australia’s,” Dr Lal said.
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