Thin Film PV

PV-Insider spoke with Dr. Scanlon to get some details on what could be a potential game changer for the thin film industry, which is now in trouble as PV prices drop.

Dr. David Scanlon, on a full Ramsay Fellowship at University College London and Dr. Aron Walsh, a Royal Society University Research Fellow in Chemistry at the University of Bath, the recipent of a €1m Starting Independent Researcher Grant from the European Research Council collaborated this year on using a common compound instead of the rare and expensive materials now used in making thin film, and just published their findings in Applied Physics Letters.

Q: What made you look into this particular combination?

A: This year Aron and I took a look at this material combination of zinc-tin-phosphide. It’s interesting because it’s the same structure as CIGS, but no one has really done too much work on it.

In the 60s and 70s people actually were studying this material, but back then basically you did the research from just mucking about in a lab and if you didn’t get the results you wanted in a couple of experiments, you moved on to the next material.

The reason CIGS and cadmium telluride have had an amount of work done on them is because they showed promising results from the start, back then. People decided, we’ll work on these, these have got promise. Anything else, we’ll push aside.

What people are finding now, however, is anything that has indium in it, it costs too much to make, anything that has cadmium and tellurium in it, it’s basically toxic... so people are revisiting the materials that they kind of glossed over back then.

Q: So if they looked at it back then: what were the problems?

A: It has a band gap that’s slightly bigger than you’d want for perfect absorption. But what we noticed was that at a certain temperature, it undergoes a transformation to a different structure. And when it undergoes the transformation, its band gap gets smaller.

In the 70s, 80s and 90s people tried a few measurements on it. They heated the material up to make it go disordered, but they weren’t making it as disordered as it could be, no one had more than 30%, so they weren’t making the band gap as small as it could be.

Our calculations show that if you actually heat it higher than people have been heating it, you should be able to induce more disorder, and therefore lower the band gap. And if you can somehow control the different ways you heat it and cool it down again, you can actually make various different levels of disorder, with different band gaps.

And if you can do that you can make a solar cell, which is fully ordered on the top, and then gradually more and more disordered as it goes down, which means you’re absorbing the light at loads of different wavelengths. So you’re absorbing way more light than you would with CIGS or something like that which just absorbs at one wavelength.

The beauty of these materials is they have a direct band gap so you can make thin film layer upon layer with more and more disorder and as they are perfectly lattice matched, so that the exact same size - the ordered cell and the disordered cell - have the same lattice size, so you are not having any strain.

Sometimes when you grow one thin film on top of another thin film, they are not actually the same size and you get strain. Strain isn’t good because it causes unwanted electronic effects. But with this material, because it’s all the same material, just slightly different disorder, you have perfect matching. There’s no lattice mismatch, which is really good for a device.

Q: What does “disordering” do: make a new lattice hole?

By disorder, I mean, not a hole.The material is zinc tin and phosphide. You have zinc sitting in a certain position in your crystal structure: tin sitting in a certain position in your crystal structure. But when you start swapping them, that’s the disorder.

And if you swap enough, you have the same material - it’s still zinc tin phosphide, but it’s so disordered that its crystal structure pattern is changed. And this lowers the band gap. Because every time you put a zinc on a tin site, you create two holes. and every time you put a tin on a zinc site, you create two electrons. So they end up balancing out, but the band gap gets smaller.

Q: I had read in The Engineer that you spent five years making this. Is this accurate?

A: No, no, that was wrong. We don’t make stuff in a lab. We do calculations. We simulate the material. We started doing this investigation in about March of this year. Aron has worked on materials for solar cell research for five years.

We advise experimentalists. We don’t fabricate anything. We guide experiment. In the past you had people who go into a lab, mess around and make something and then try to explain it with an experiment, and try and answer the unanswered questions.

The beauty of the computational approach is that I could run through a load of materials on my computer in a couple of months and then, tell an experimentalist, you should try these materials: see if they work. And you have narrowed down the field from what would take them months to do all the experiments.

Q: If the computer says it’s a go, is it a done deal?

I can’t actually speak for how easy it will be to make these films, or how easy it will be to control the disorder, that is the problem for the person who is going to make it, but if it can be done cheaply then the actual sourcing of the three materials: that’s not going to be a problem, there is not going to be any costing there. So if they apply the kind of material processing ideas they already apply for the other thin film stuff, then it should severely reduce the costs.

Q: Halotechnics’ high temperature molten glass (for CSP storage) also came out of this kind of computational power to search through millions of possibilities before anyone “mucks about” in a lab. But I thought they were the exception. So is this the way materials research is normally done now?

A: A lot of companies have modeling for computer simulations. NREL, where Aron used to work: they have a huge theory section. They have for years been driving their research with theory and experiment.

It’s only in the last thirty years that people have started to believe in what theory could say and what it could do, but now its really coming to the forefront now, where its no longer the junior partner. It’s now also a driving force.

It lends an awful lot to the field the ability to calculate properties with quite a good degree of accuracy and then feed that into an experiment, do more experiments and then feed that back into the calculations. It becomes a really nice feedback loop.

You’ve got information going both ways. There’s no guarantee that your model will work perfectly the first time. In an ideal world.

You need theory to put input in, but you also need results from experiments to make sure you’re going right, and if not to try and tweak what your model is: so its very useful that you have the information coming both ways.

As computing power has got stronger there’s more high performance computing centers all around the world and there’s more and more computational chemists and computational physicists and computational engineers: it's starting to become predictive.

Q: If it works in the lab, this would make ideal thin film. Is it a game changer?

The thing about it is that all three are cheap. And abundant. And not toxic. Phosphide, zinc and tin. As the constituent materials are so much cheaper than anything involving indium and are definitely not toxic like cadmium telluride are, then yeah, its got an advantage: it’s cheap already.