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Watts fit to print: Developing flexible, organic solar cells

By Shane Huntington

Polymer chemist Dr David Jones and materials scientist Dr Scott Watkins discuss the latest in flexible electronics -- the printable organic solar cell.

SHANE HUNTINGTON
I'm Dr Shane Huntington. Thanks for joining us. Over 100 years ago Albert Einstein published the paper that explained the photoelectric effect, the process that allows light to be converted to electricity by certain materials. Fast forward to today and silicon based solar cells have spread across houses and other buildings in cities around the world. Today these cells not only provide large amounts of power, they also impact electricity prices and base load distribution. But silicon based photovoltaics have their limitations and researchers are turning to alternative organic based systems.While the principles of silicon and organic solar cells are the same, their components, production methods and final properties are as different and some of their potential applications. To bring us up to speed on organic photovoltaics, we are joined in this episode by Dr David Jones from the Department of Chemistry at the University of Melbourne and Dr Scott Watkins, Stream Leader of Organic Photovoltaics at the Commonwealth Scientific and Industrial Research Organisation, the CSIRO's Materials Science and Engineering Division. Both of our guests are members of the Victorian Organic Solar Cell Consortium. Welcome to Up Close, David and Scott.

DAVID JONES
Good morning Shane.

SCOTT WATKINS
Hi.

SHANE HUNTINGTON
David, I might start with you. Solar cells as well all know collect light and convert it to electricity. What are the main components that are required in a solar cell to make this function work?

DAVID JONES
Basically you need an ability absorb light and once you absorb light you need to be able to take that light and convert it into positive and negative charges, then collect those at the electrodes. In this case we're using organic materials, polymers or small molecules to serve those functions, so we have organic polymers which can conduct positive charge or negative charge.

SHANE HUNTINGTON
Most of our listeners would be very well aware of the standard battery which has a positive and a negative terminal and some stuff in the middle that allows you to create a voltage across those two terminals. How does this compare with what you would find in a solar cell and how does that work?

DAVID JONES
The battery is effectively a stored chemical charge, so you basically store a potential for generating electricity in some form of chemical form, whether it's in nickel or whether it's in nickel hydride or whether its mercury type batteries, or it's a good old fashioned lead acid battery where you're actually using the charging process to store electricity. In a solar cell we're basically saying that when we are absorbing light, we want to take every photon of light and we want to generate an electron from it. Therefore in this case we're using the absorbed light to drive the process of generating a positive and negative charge out of the solar cell.So as in batteries and you can actually print batteries these days as well, then you're basically printing layers or generating layers of structure, where you're actually going to store a chemical form of the electricity which you can then regenerate. Whereas for a solar cell you're basically printing or making a structure where when it absorbs light, you can get that light to separate charges and drive that process.

SHANE HUNTINGTON
Scott, let me ask you about what's existing at the moment. We can't drive around a modern city of the world these days without seeing large quantities of solar cells on rooftops. What are these solar cells made of?

SCOTT WATKINS
So these solar cells that people mainly see are made from silicon and it's a very highly purified form of silicon. These devices only work well when the silicon is that level of purity and that has enabled a huge range of applications as you say. But while they are widespread solar power, still only contributes about one per cent of the world's electricity, so there's a lot of room to grow in a lot of different areas.

SHANE HUNTINGTON
When we look at all these cells - and it doesn't matter I think whether you look at the cell in a home calculator or something larger on a rooftop or on a building - they all have that very square configuration that we're very used to, in a planar configuration so they're fixed in terms of their shape and their structure. You're talking about something different though. You're looking at the possibility of making solar cells flexible. How do you go about making these heavy solar cells that we see on rooftops currently and how does that compare with this need for flexibility?

SCOTT WATKINS
Well flexibility can be important in a lot of applications but also just conformability, the ability to wrap something to a certain shape and slight curves in it. And what's key to it is that the organic solar cells that we're making are made from very thin layers of material so the materials that David described, they're actually pretty close to insulators when you really look at them. So they only work when the layers are very, very thin and that being very thin enables them to therefore be made into devices that can be conformed or shaped. And that just opens up these different applications for solar beyond those rigid square shaped things that you're talking about. So beyond just large scale power generation into providing power on demand in off grid applications and a lot of other things.

SHANE HUNTINGTON
Why is it that the silicon cells are so fixed? Can we not make them in such a way that they are flexible?

SCOTT WATKINS
There are efforts going on using very, very small amounts of silicon but silicon actually doesn't absorb a lot of light so you need a fairly thick layer to be able to capture all of the light, whereas the organic materials, they're similar to dyes that people use to make clothing and things like that. They do have very high extinction coefficient -- they absorb a lot of light. That means, so you can use very, very thin layers of these materials to absorb that light.

SHANE HUNTINGTON
So what would one of these cells actually look like if you were to hold one in your hand?

SCOTT WATKINS
Well a piece of plastic with a coating on the top. I mean if you look at potato chip packets for example, the sort of feel and texture is something that people might be familiar with. We can make them potentially in different colours. The more black they are the better they will work but if there is a need to trade off appearance for a particular aesthetic outcome then you can vary that colour to change the appearance.

SHANE HUNTINGTON
David, I'm going to follow up on that aspect of them being black because I wanted to get on to the dyes themselves. What is that trade-off between colour and how efficient they are?

DAVID JONES
Following up from what Scott said, I mean basically you can have a solar cell whatever colour you want as long as it's black, because if you want to absorb the most wavelength of light to convert into electricity, you want to absorb all the visible. So that would be the ultimate colour for a solar cell, but we can use alternative coloured plastics to make different colour but we then get a slightly less efficient solar cell.

SHANE HUNTINGTON
Now when we talk about these particular dyes, once you put this material out into sunlight or I assume many forms of light, not just sunlight, what is actually occurring with the dye that allows you to generate electricity?

DAVID JONES
Well the dye actually absorbs light and it goes into an excited state, so that's a rather technical term to say it's got energy bound into it. Then that goes to an interface, a join between the two materials I mentioned before and that's where you get a separation of positive and negative charges. So that plastic then has to be able to conduct those charges to the electrodes, so basically the energetics of that is what we're trying to control and design into our materials. Then that also controls the colour, but as Scott said these have normally very, very high absorption of light and breadth of absorption of light is what we design into them.

SHANE HUNTINGTON
Is there a limit in terms of looking at the thickness versus the type of energy you're getting out, so do these things have to be a certain thickness to optimise that output?

DAVID JONES
Most of the devices we've been printing or making in the laboratory tend to be about 100 to 200 nanometres thick. And basically that's an optimum of absorbing of the light and also then the transport of the charge through the material.

SHANE HUNTINGTON
So when we talk about a few hundred nanometres how many atoms of the dye are we talking? Is that just about 1000 atoms or something is it?

DAVID JONES
Again because [we're] polymers it's probably about that, yeah and so we're talking about a fraction of the thickness of a human hair so these are very, very thin layers of organic material.

SHANE HUNTINGTON
A fraction of what you would experience if you were looking at silicon substrates?

DAVID JONES
Absolutely, yeah.

SHANE HUNTINGTON
I'm Shane Huntington and you're listening to Up Close. In this episode we're talking about new generation printable organic solar cells with material scientists David Jones and Scott Watkins. Scott, one of the important aspects of the project, it's taking the process of printing these cells from the laboratory environment to that that you would find in an industrial suite of activities. How do you go about this and how do the two processes differ?

SCOTT WATKINS
Yeah, well it's been a really exciting journey that and I think there's a lot of groups around the world that are looking at aspects of organic solar cells and that's providing huge insight into how they work and new materials and things. But the majority of that research is really limited to devices about the size of your fingernail and a few years ago that's what we were doing too. But we embarked upon this project and this collaboration between the University of Melbourne, Monash University and CSIRO where researchers from across the three institutions came together. We set up the program in such a way that one arm of the program was this really applied process where we were looking at the issues about translating the materials that we have now to this larger scale.So initially we evaluated a few different printing techniques to see which ones would give us the uniformity and the thicknesses that we needed. Then we trialled those on a small scale so for about 18 months we were making devices about 10 centimetres wide. Then after proving that technique we had the confidence to go and invest in larger scale equipment that's now closer to what most people would regard as pilot scale, that can do up to about 30 centimetres wide and can do about five times faster. So that process of understanding how we can control the layer thicknesses that David talked about and the way that the two materials arrange within the film, the internal microstructure of the film, we have a lot of knowledge about how that works on these tiny fingernail size devices.But replicating that by using a different coating technique and doing it on a larger scale is what has been the focus of the research. And it's been really, really exciting to be able to take that and now we can hold things in our hands, take them outside, plug them into motors and turn them around and things like that. So it's been a great journey.

SHANE HUNTINGTON
I wanted to ask you, you refer to plugging them into motors and so forth. What sort of energy conversion are you getting there and how many watts can you generate from something the size of, I guess you're talking an A4 page?

SCOTT WATKINS
Yeah, so in the devices that we're making at the moment we're sort of in the range of 10 to 20 watts per square metre in what we can do in the lab now and we can on very small scale devices approach numbers that are probably closer to the 80 watts per square metre with some of the new materials we're developing. We will eventually move them into the printing process, but right now those sort of 10 to 20 watts can power a lot of different things. We're really looking to match that power output with some particular applications as part of the journey to making them more efficient.

SHANE HUNTINGTON
David, let's talk a little bit about the printing process itself. How do you go about printing these sorts of sheets? It sounds like there are multiple layers, there's a very, very specific requirement for thickness. I can't imagine this is your average home printer or even your 3D printer?

DAVID JONES
In fact the printers we're using are commercial printers you can buy off the shelf and that's one of the advantages of using an industrially relevant printing technology. But we are getting down to the limits of thickness with some of these coating on printing techniques, but they are multi-layer structures and so we have to print three or four layers to get the completed solar cell. And we have to print them on top of each other and going back this A4 sheet or A4 module, we also need to remember this is a continuous printing process so this is an A4 sheet cut from a roll of printed organic solar cells. So the A4 sheet is simply the size that we've decided to use to test our printing process and then evaluate the modules of printed solar cells.

SHANE HUNTINGTON
Scott, when we think of metres per minute of printing we start to envision the old printing press for newspapers and the like. Is that the type of printing we're doing or is it very different in these circumstances?

SCOTT WATKINS
That is a valid comparison because we are using processes, as David mentioned, that have been established industrially to coat materials down onto different substrate. So the type of processes that we're using, so reverse gravure or slot dye coating are used to coat these type of things like newspapers or coatings for various plastics. Also screen printing, it's exactly how you used to print t-shirts, so these are really established processes and the question was raised a little bit earlier about 3D printing and the analogy and I say there that what we're doing is 3D printing, but two of our dimensions are much larger than the third one.So our device has a large area but very, very thin. But the philosophy behind 3D printing and why it's so attractive is that it enables you to print on demand and make variations to your structure of your thing and that's the same with printed organic solar cell. So we can change the electrodes that we're printing to deliver different currents or different voltages. We can change the shape or the pattern of the device on demand so that it takes away that need to have this long production run that's just doing exactly the same thing every time, to enable you to make cells that match particular applications or particular needs. That's that same philosophy behind 3D printing, about enabling you to make variations very easily and very simply on a per device scenario.

SHANE HUNTINGTON
When people look at the average magazine that's printed in some of these sorts of printing presses it looks smooth, but on the type of scale you're talking about I can imagine there is quite a variability in that printing on the surface. Do these processes require a higher level of precision in order to produce the materials you're talking about, or are the materials able to have quite a variation in thickness in them and still work?

SCOTT WATKINS
We do need to control the thickness around them and that's really why we have chosen the coating techniques, the printing techniques that we have, because they are capable using existing equipment of giving the range of thicknesses that we need and the range of uniformity that we need.

SHANE HUNTINGTON
David, in terms of the organic polymers that you need to use are there specific requirements that these organic polymers need to meet to be usable in a solar cell, or can you just use any old organic polymer? What are the precursors for these materials?

DAVID JONES
Well this is one of the design aspects and these are very, very complicated specialised plastics or polymers which we spent a lot of time designing and optimising. But effectively the main thing is that the area we've gone over before, they've got to absorb light and they've got to be able to transport charge. But we need to be able to make them into inks because obviously we're doing a printing technique and therefore all the specialty polymers that we look at have to be formulated as inks for a printing process. And once we've done that and we've spent a lot of time optimising these inks, then we can put them into the various printing processes, whether its screen printing or whether its spray painting or whether it's other forms of printing technology.And having said that the process of designing molecules means that we can also say okay, in the future if we go back to looking at paints where they went from oil based paints to emulsion or water based paints, this is a process that we probably need to do for our printing process as well. Having the printing program and the materials design program very, very closely linked, we can then sit down and say these are the things we need to change to be able to change that formulation, that ink from an oil based to a water based ink, for the same printing processes. But change one thing with these molecules and they stop working, so this is a very, very complicated transformation process and development process that we'll need to go through.

SHANE HUNTINGTON
Are these low temperature printing techniques or are they at quite high temperatures?

DAVID JONES
These are all room temperature.

SHANE HUNTINGTON
Scott, in terms of the electrodes that go with the films, are they printed in the same way or is that a completely different process?

SCOTT WATKINS
Some of them are. At the moment we're taking what we can source commercially available which is indium tin oxide on the plastic as the front electrode. The indium tin oxide is the same as what's used in your LCD displays on your current phones and things. That's coated onto the plastic that we buy in and that's the front electrode. The back electrode, we do experiment with a few different things. We can vacuum evaporate down things like aluminium but in the printer devices we're actually printing down a conducting polymer and then printing on top of that a grid of silver. So these are things that are established that have been known for a long time. The conducting polymers were actually developed in photography and they've been used in a range of applications from antistatics through to other forms of organic electronics like organic light emitting diodes.And then the silver grid on the top is just to help with the charge collection. So there is a range of choices there and there's also research going on, some here and some in other parts of the world about using more exotic things like silver nanowires or carbon nanotubes or graphing to replace some of these electrodes, because the ITO is one of the more expensive parts of the cell at the moment. So there's a considerable motivation to replace that, but the basic answer is there's a variety of different materials available. Some of them can be printed and that adds to this versatility of the technology as a whole.

SHANE HUNTINGTON
As people try and get their heads around the idea of these super thin layers and they are super thin, a few hundred nanometres, the next question would be how long does it take to actually produce one of these types of cells from go to whoa? What sort of time are we talking about?

SCOTT WATKINS
Yeah, well we're trying to make that as quick as possible so some of the printing partners we work with, so Innovia Security who produce the Australian and other country's banknotes; they print the banknotes for example at 400 metres a minute. Now we would love to be able to do that and a lot of their advice to us is that things will work better when we go faster. At the moment we're probably more like the 10 metres per minute would be our sort of higher end speeds at the moment, but even that allows you to produce a lot of solar cells in a very short period of time.

SHANE HUNTINGTON
I mean 10 metres per minute, how does that compare to the production of silicon based solar cells? I would imagine that would be more a metres per day scenario?

SCOTT WATKINS
Yeah, the silicon production is a batch process. You have to purify the ingots of silicon and then cut that up, then deposit it down and dope it, scribe it and things like that so it's harder to do a direct metre per minute. But it's a complicated process. I mean it's established and it's out there and there's factories doing it 24 hours a day, so we can't dismiss it as being too slow because it does work. But this is a very, very fast process that we're developing for the printed organic solar cells.

SHANE HUNTINGTON
I'm Shane Huntington and my guests today are materials scientists Scott Watkins and David Jones. We're talking about printing flexible electronics here on Up Close. Scott, you're using a particular process and particular types of organic polymers that you are generating. Are there competing technologies that are around in the world at the moment that are producing these sorts of non-silicon based photovoltaic systems as well?

SCOTT WATKINS
Certainly. Silicon even as itself can be used in very thin films and there are - so we've got amorphous silicon is an example and that sees a performance drop compared with the crystalline silicon, but again it matches some applications. Then there are other inorganic semiconductors that are used in solar cells and some of them can be very thin film, so things like cadmium telluride or composite materials that are abbreviated CIGS like copper indium gallium sulphide. These types of inorganic materials can be deposited down as well. Now what's different with them is that the silicon or the cad telluride or the CIGS materials, they each have a sort of defined band gap, the amount of light that the device absorbs, the range of wavelengths that it absorbs. They're defined by the nature of the materials.For organic materials we can change that fairly easily just by making very subtle changes to the organic chemistry of the polymers or the materials that we're using in the devices, so we can vary that band gap. And that has two effects; it changes the colour that the devices appear and it also changes the energy of the light that they appear which feeds directly into the voltage that the cell gives out. So by changing that property of the polymer we can change the voltage that the cell gives and again match that to particular applications. There's also dye-sensitised solar cells which is another type of organic solar cell and these use a layer of titania there that has some dye molecules absorbed on it. The processing there is sometimes related more to what's going on inside chlorophyll in plants and things, but again it's using organic molecules to absorb the light.So all of these technologies, the dye sensitised solar cells, the other thin film inorganic technologies, they all have advantages in that they can adopt different shapes or device configurations and different output voltages. So again it's not really about them competing with each other but more about meeting different market needs or application needs and working together to fill this big need for more electricity that we have.

SHANE HUNTINGTON
In terms of the cells that you're producing, obviously if they look a certain colour that implies that they reflect that colour and not absorb it. But overall what's the range of wavelengths can these particular types of cells absorb in order to go through the process of producing electricity, beyond the range of human vision?

SCOTT WATKINS
Yeah, well again with organic solar cells because you can tune the wavelengths that you can absorb you can vary that entirely, so you can make devices and we're mainly focussed on making devices that absorb through the visible range and out into the infrared so from the UV that starts through the visible out to the infrared. But it is possible to make materials that perhaps have an absorbance mainly out in the infrared, so that would open up the possibility of a solar cell that is fairly transparent to the eye but is still producing a significant amount of power because it's absorbing the light that we can't see.

SHANE HUNTINGTON
So you could go for a scenario where you coat windows for example with these materials and have them transparent for all of us humans but generating electricity the entire time?

SCOTT WATKINS
Certainly and that's an application that there are people around the world working on. We haven't had a direct focus on that ourselves, but it is something that is possible with those materials.

SHANE HUNTINGTON
Scott, most of the people who have existing silicon cells on their roof would hope that they would last a very, very long time, especially if they bought them early when they were quite expensive. It's likely they...

SCOTT WATKINS
Me included.

SHANE HUNTINGTON
You included?

SCOTT WATKINS
Yeah.

SHANE HUNTINGTON
It's likely they will last for quite an extended period. How does that compare to what you expect from these sorts of cells which sound to be a lot more flexible and just less robust in a sense of the traditional cell?

SCOTT WATKINS
Look, that's very true. The long term durability will not compete directly with silicon and I think that's an important to emphasise here, that this technology is not about a direct competition with silicon for a long time. We're about complementing that, about moving it into other applications, doing things where it's too expensive or silicon doesn't provide the right match. So the lifetime of the devices that we make, if we make them on glass we can see several years now. When we make them on plastics we've got devices that have been outside testing for more than six months now and are still performing pretty well, but that's an ongoing test so we can't give you the final answer yet.It's again about not only matching the absolute performance of the cells in terms of how much power they produce but matching that lifetime. You can envisage situations where you have a frame or electrics and just every so often you're replacing this thin sheet that goes on the top sort of thing. So that can some way mitigate the lifetime issues there, but other applications it's just about directly matching the lifetime of the cell with the lifetime of the application that you're putting it into.

SHANE HUNTINGTON
I want to talk about applications a bit because you guys have this incredible advantage of very low mass or as we commonly say low weight. I can imagine scenarios where for space vehicles and so forth where weight or mass is at an absolute premium these would be extraordinarily valuable. But there must be a range of applications you're currently considering as your first areas to move into. What are they and why are your cells more appropriate for those applications?

SCOTT WATKINS
Yeah, so really we're looking at four things about these cells that we can vary. One is the cost, one is their absolute efficiency, one is their lifetime and one is their aesthetics, their appearance. Trading those four things off against each other can meet different application needs, so some examples of things where in the short term we could trade some of those things off might be integrating them into small consumer devices where you need small amounts of power just to continually trickle charge a battery. So that might be something like a case where you have your laptop or tablet inside and you're trickle charging a battery; that can mean that you put the case outside and an hour's sunlight give you an hour extra battery, that sort of thing.Other applications in completely off grid situations might include solar powered lights and these are particularly relevant in developing countries that don't have grid connected, where they might use kerosene lamps and so a lightweight panel that's connected to a thin battery and a light, they could sit out inside and again that sort of number of an hour in the sunlight would give you an hour of light out of your device in the evening. So those sorts of things and maybe things like water purification as well again in developing countries, are short term applications that could actually provide significant benefit to the users and a product that doesn't exist at the moment. But for us developing the technology can provide some applications that help us get something into the market and look at things like manufacturing yields, how many devices can we make that work per 100 devices sort of thing and how do we run our process for a whole day, things like that.So it can really accelerate the development of the process and the manufacturing conditions that we use, whilst still providing applications that can start actually being used by people.

SHANE HUNTINGTON
When we look at the entire electronics industry at the moment Scott, it's pretty clear that pretty much everything is rigid, whether it's your television, your phone display, the board in your computer. Whatever it is it's all based around that rigidity of the silicon industry. Are you looking at other areas outside of solar cells given this sort of technology that would move some of those things from being rigid to being flexible?

SCOTT WATKINS
Yeah, precisely and organic solar cells is one example of the broader area of organic electronics and the reverse technology, the light emitting devices so organic light emitting diodes is actually a more mature technology. You can already find these OLED screens, organic light emitting diode screens in many high end mobile phones now, particularly all of Samsung's ones. They're again very, very thin devices that are very efficient and give great performance. Now at the moment they're still mainly on glass but they have this same potential to move to flexible and so the processes and the understanding that we're developing on the printed solar cells can also be applied to all of these other areas of organic electronics. So like the lighting, like transistors, like the logic circuits, all of those things that go together to make up these devices that as you say are currently rigid.There is the potential to change them all to something that is at least conformable and possibly flexible as well.

SHANE HUNTINGTON
How far away do you think we are from seeing some of these solar cells, these flexible solar cells in the marketplace?

SCOTT WATKINS
Well they answer we've been saying to that is we're ready to go in some applications now. There are applications that have short term needs so another example would be in advertising type structures where perhaps some sort of advertising display might give you a signal as you go past it or make a sound or something like that, where the lifetime and the power requirements, their short lifetime and the power's not critical. It's just about making an impact. Again that would give us the opportunity to highlight a few of the advantages of organic solar cells, like the way they look, the way they conform, the way they perform under low light conditions and give us a product that we could practice on making.So we could do those sorts of things now. More realistically the other applications that we spoke about earlier, within the next year or two we see the possibility for doing that but that's highly reliant on us getting some partners who can take these actually out into products now.

SHANE HUNTINGTON
David, everyone knows what a silicon based solar cell looks like on a roof, but what would one of your cells look like if we get to the point where they're used in housing to generate electricity on the rooftop?

DAVID JONES
Well this is a question which really goes to our purchase of our printers as well. One of the printers that we've recently commissioned is designed to be able to print on plastic sheets or glass sheets or steel sheets. Of course one of our industrial partners is BlueScope Steel and they have independently a very, very large program funded by the Australian Renewable Energy Agency to look at the new technologies of solar cells coming through and their application to steel roofing. As we may be able to print directly onto steel roofing this is then a very, very positive advantage for a local roofing manufacturing company. So we should be able to print on pretty much every substrate including steel and of course if you get to the steel substrates, then your performance has to match the current performance parameters of a steel roof.BlueScope Steel guarantees their roof for 20 years or so. Therefore we won't be putting our solar cells directly onto their roof until they can guarantee that our solar cells will last as long as their roof.

SHANE HUNTINGTON
Presumably that also means you'd be able to print these onto cars?

DAVID JONES
Also, yes and again because we're looking at all industrially relevant technologies of coating or printing we can also spray paint the components of solar cells onto different substrates. And of course that would then directly relate to spray painting cars and industrial lines.

SHANE HUNTINGTON
Dr David Jones from the Department of Chemistry at the University of Melbourne and Dr Scott Watkins, Stream Leader of Organic Photovoltaics at CSIRO Materials Science and Engineering, both from the Victorian Organic Solar Cell Consortium. Thank you for being our guests in Up Close today and talking with us about printable solar cells.

DAVID JONES
Thank you.

SCOTT WATKINS
Thanks Shane.

University of Melbourne