30 September 2010

What I'm Doing or: What I'm Not Doing, Part 1

So, being that we came all this way more than two years ago in the name of me doing something with myself, people back home ask from time to time the perfectly reasonable question: 'What exactly are you doing with yourself?'

I've effectively dodged this question many times, but it's true that I should be answering it about now. There couldn't be a better day to give it a go, really, as I've been doing a whole lot of nothing. I had the best intention of reading academic papers, maybe doing some writing, but...you know how these things go.

So if you're keen, come on a journey with me, won't you?

You won't? Ahem.

You over there, will you?

Oh, excellent! Let's!

Whatcha doin'?
I am doing my PhD in Photovoltaic Engineering at the University of New South Wales. I came here with the intention of doing a Masters by Research degree but 'upgraded' at the end of 2009 to extend my candidacy. This means more is expected of me at the end, but I needed the extra time anyway.

Get jazzed on lingo, turkey
Photovoltaics (PV) is the technical name for solar cells, that is, devices that convert light into electricity with the aim of using that electricity as a power source. This only makes sense if we're converting sunlight into electricity, so some pedants say 'solar photovoltaics', unnecessarily in my opinion. 'Solar power' can refer to many different types of technology, but 'solar cells' are a specific thing. A 'solar panel' is typically a bunch of solar cells wired up into the kind of thing you see on rooftops, like this:

The cells are the individual square-shaped things in the white frames in that image.

So you can say I'm working on PV, solar cells, solar panels, solar power, solar energy...any of that stuff, if you like.

Solar cells: beneficial or witchcraft?
Solar cells work something like the chemical batteries that we're all familiar with. Roughly speaking, a chemical battery is a source of electrical potential because energy has been stored in it in the form of a chemical reaction that cannot occur until the battery terminals are connected via a conductive path.

Just like that, a solar cell has a negative and positive terminal, but unlike a battery, no energy is stored within a solar cell. Instead, a solar cell is made of certain materials in which electrons can be freed up when illuminated. However, like a battery, a solar cell has a positive terminal into which electrons flow and negative terminal that they flow out of. So a solar cell sitting in the sun will have a voltage, that is, an electrical potential, across its terminals, like a fresh chemical battery.

The upshot of the story is that a decent solar cell has to be designed to convert sunlight into electricity in an efficient way. The main design problem in solar cells is that the sun is a broadband source–composed of a whole range of colours, or wavelengths, of light–but there has to be some range of wavelengths over which the solar cell cannot absorb. This isn't for mystical reasons. Instead, this is because in order to be able to establish an electrical potential, we have to be able to excite electrons by giving them some amount of excess energy, but they have to be able to 'stay excited' until we use that energy in the way we'd like.

The Scottish Analogy
Think of it like this: let's say that it's 1611 and I want to raise the curtain of the Globe Theatre stage at the beginning of this new play Macbeth. I can do this in one of two ways: one, I can have some guys pull on a rope that's attached across a pulley to the curtain. They pull, the curtain goes up. But those guys are unreliable drunks and I don't want to be screwed if they don't show up at the right time. Instead, I can attach the non-curtain end of the rope to some weights and lift the weights to an appropriate height. When the curtain needs to go up, I can drop the weights, and zoom! Up the curtain goes, smoothly and on time.

Problem is, if I want the assembly to be ready at curtain time, I have to lift the weights up ahead of time and stick them on a shelf. Otherwise the weights will fall, as they tend to. So there's a certain threshold energy, or amount of work, that we have to do in order to store the energy required to lift the curtain in the form of a raised weight. It's the height of the shelf times the weight of the sandbags, or whatever they are.

But here's the tricky part: the shelf has to be the right height. If the shelf's too low, there won't be enough stored energy to raise the curtain. If the shelf's too high, it will raise the curtain, but we'll waste a lot of energy getting the sandbags up there in the first place.

That's a pretty dumb example, but it illustrates the point. There is a certain threshold energy that electrons must be given in order to become freed up for conduction, that is, in order to do work in our circuit. Quantum mechanics tells us two important things here:
1) the energy of a given amount of light is proportional to the wavelength of that light (the shorter the wavelength, the higher the energy) and
2) the energy in light is delivered in discrete energy 'packets' called photons.

(Actually, it was out of these claims, developed by Max Planck and Albert Einstein, that quantum mechanics was born! So it's wrong to say 'quantum mechanics tells us two important things'. I should've said 'two important things tell us quantum mechanics'.)

Anyway, we can say because of points (1) and (2) that there's some wavelength cutoff for solar cells. Beyond some wavelength, light of a certain colour won't be able to excite electrons across the 'energy gap' under normal conditions, and this light will just pass through the material. This actually explains the entire phenomenon of absorption of light: this energy gap is a natural characteristic of all materials (crudely speaking) and materials that we call 'transparent' simply have an energy gap too big for electrons to be excited by photons that we can see, which range from about 0.4-0.7 millionths of a meter in wavelength. There are many more photons that we can't see, of course. The long-wavelength ones range from radio waves (which can be meters long) through microwaves to the infrared (a few millionths of a meter). The short-wavelength ones go from the UV up through X-rays to gamma and cosmic rays. The latter are so-called 'ionising radiation' because they tend to be high-energy enough to knock electrons out of their atomic orbits, at which point disturbing things start to happen to your DNA.

This graph shows the distribution of energy (per unit area per unit time) in the solar spectrum as seen from the earth: you'll see that the majority of sunlight is visible (400-700 nm) and peaks in the yellow (~500nm). Actually, again, I've got it backward. We call this region the 'visible' precisely because it's the colour of the sun! Our eyeballs evolved to absorb and our brains evolved to process the information brought to us by sunlight! If our sun were a different colour, our eyes would be sensitive to that instead. But that's such a massive what-if that I should stop right now.

Size matters
So how big does this energy gap need to be for an efficient solar cell? Clearly, the bigger the gap, the fewer photons we absorb overall, since the sun only pumps out a limited amount. If you assume that every photon over the threshold excites one electron, you get fewer electrons for a bigger gap, which means that your current output (electrons per second) suffers. Your voltage will be big, but your current small.

Likewise, if your gap is very small, you'll definitely absorb more photons and produce a high current, but your voltage will be very small, because (for a reason I'll discuss later) the electrons that you've excited don't wind up with the energy of the photons that created them, but instead the threshold energy, and the average electron energy determines the voltage.

The output power of your solar cell–the number we really care about in the end–is defined as the voltage V times the current I. So as we adjust the energy gap, V and I change, and the power peaks at some 'medium' value of the energy gap. This value turns out to be very close to the natural energy gap of crystalline silicon (Si), the most well-understood material on the planet, the basis for microelectronics and the second-most plentiful element in the earth's crust. Lucky for us!

The solar spectrum above shows the cutoff wavelength of Si, which is at approximately 1100nm, or a tenth of the thickness of a human hair. Everything to the left can be absorbed by Si, while everything to the right passes right through it.

And so it is that nearly all solar cells in use are made of thin slices of giant Si crystals. While Si is not an ideal material for solar cells from the point of view of physics, in terms of economy, the environment, ease of processing, and so on, it can't be beat. Many have tried, none have succeeded.

The catch
Since the first Si solar cells were demonstrated in the mid-20th century and their potential was realised, a tremendous amount of work has gone into improving the devices, to the point where they convert sunlight into electricity nearly as well as one could hope. The theoretical limit on the efficiency of a conventional Si cell is only 29%, and UNSW holds the world record with its 25% efficient PERL cell. However, this is a small-area device, and is far too expensive to put into solar panels like the ones above. Those cells are typically 18 or 19% efficient.

So the question is this: we can always spend more money and make the traditional Si solar cell more and more efficient, but if our aim is to change our energy systems to rely on solar power, we need it to be cost-effective, meaning that we have high efficiencies for low costs (in relative terms). What are the fundamental reasons that Si cells are limited to 29% efficiency, and is it possible to design a device that relies on Si or a similarly abundant material that boasts a drastically higher efficiency?

We'll find out in Part 2!