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Tuneable Plastic Lighting with Carbon Nanotube Polymer Nanocomposites: An Interview with Professor David Carroll

Interview By Will Soutter

Corresponding author: Professor David Carroll, carroldl@wfu.edu
Director, Center for Nanotechnology and Molecular Materials, Wake Forest University

In this Thought Leader interview, David Carroll, Director of the Center for Nanotechnology and Molecular Materials at Wake Forest University, talks to Will Soutter from AzoNano about their new FIPEL lighting technology, how it will compete in the lighting market, and the limits of nanomanufacturing.

WS: Please give us an overview of the new lighting technology you have developed.

DC: We have developed a new lighting device to rival fluorescent and OLED lighting, which involves a new type of polymer, and also a new type of material structure. We have a number of publications about the technology, the first of which came out recently in Organic Electronics. Basically the device uses charge generation layers and field concentrators inside a polymer, to create an AC-driven polymer electroluminescent device. We call this FIPEL (field-induced polymer electroluminescence), and it is a mixture of new materials and new architectures, so it’s actually more than just one innovation here.

WS: How long have you been working on the FIPEL technology?

DC: We built the first prototypes about 10 years ago, in 2003. They weren’t very bright, but the concept was there. In the intervening period, various other groups have worked on their own versions which operate on the same principle, but those have been fairly low performaning devices too, in general.

About four years ago, we patented a new polymer which emits white light, and it turns out that this polymer is ideal for use in this architecture. About a year ago, we discovered a set of charge injection materials which can be blended with the polymer, that make it light up much brighter, and it all came together from there. So there have been a few stages, but I'd say the technology as it stands now as 1-2 years old.

In the recent publication, the device we had gave about 100-200 cd/m2 - not very bright, but that was a proof of concept. More recent models we've made have been regularly getting between 10,000 and 20,000 cd/m2, so they are as bright, if not brighter than most white-emitting OLEDs.

WS: You mentioned that FIPEL is a mixture of novel material and novel structure - what does the nanostructure of the material contribute to its properties, compared to the bulk properties of the material itself?

DC: Well the material itself is pretty important. Something that is demonstrated in one of the forthcoming publications is the effect of carbon nanotubes acting as nanoantennae. They have quite a drastic effect, not only for charge injection, which is expected because of their high aspect ratio, but also on things like the triplet states inside the matrix, which drives up the efficiency of the device quite radically.

As for the structure of the material, there is a degree of crystallization that occurs around these nanotubes which affects how they work, so the way that the nanotubes are inserted into the matrix is extremely important, to try and control this effect. We are not the first group to put carbon nanotubes into a field-induced device, but we are the first to do it this particular way, and that is crucial to the efficacy of the material.

The nanotubes play multiple roles in this device - they are not just for charge injection, like most people think. Other groups have used them in plasma-type displays, where the nanotubes provide better plasma, because of the lower charge injection barriers. In our case, they do far more than just that - they have a really important effect on the whole matrix.

Professor Carroll explains why the FIPEL technology is such an exciting development.

WS: The technology seems to be in a fairly advanced stage of development - how far away is the technology from use in commercial products?

DC: We should be beginning alpha-stage production within the next year. There is some development still to do - we are currently working on finding the best way to manufacture the material inexpensively, and putting together the first assembly line.

WS: How will FIPEL devices compete with existing lighting products?

DC: In terms of commercial competiveness, FIBEL will certainly compete with LEDs on cost, and the color temperature and color rendering are also significantly better than LEDs, although they are not as bright.

What’s really important about our device is the ability to dial in any color you can imagine, because of the way the polymer is designed. We have red, green and blue emitters, and we can blend them together in any proportion. Those RGB emitters also transfer a certain amount of energy over to phosphorescent dyes, and the proportions of the dyes can also be tuned - so we have six different parameters that we can use to tune in any colour that we want.

In another of the forthcoming publications, we literally stepped through the CIE color index - (0.34, 0.31), (0.34, 0.32), (0.34, 0.33), etc., we were amazed at the level of fine adjustment we could achieve.

This has some really interesting implications. For example, we know can take the photopic response of the human eye is, so we can tune in a color which overlays that curve as perfectly as possible - we can build a lamp which is exactly attuned to your eye. That gives FIPEL an instant competitive edge over both LEDs and compact fluorescents.

If you consider the bulb alone, we can't quite match the efficiency of LEDs, but it is still a very efficient device. A full study on the efficiencies will be in the next publication that comes out, but the typical numbers are above what is typically reported for OLEDs, and that is considered quite high. This means that FIPEL is definitely competitive with fluorescent lamps, on color, efficiency, and on cost.

In fact, cost is one of the main advantages that FIPEL has. A typical flat panel fluorescent for an office building would currently be about US$200 - the FIPEL equivalent would be about $50. We expect the cost of ownership over the lifetime of the product to be much less as well. FIPEL devices  with typical encapsulants would have lifetimes of 20,000 to 50,000 hours, and some devices have shown lifetimes much longer than that.

The FIPEL material is also much more environmentally friendly - no toxic substances like mercury are used, and there are no rare earth elements.

WS: How will FIPEL compete in other applications, such as display screens?

DC: A wide range of applications are potentially viable lighting in a commercial, office, or domestic setting, and displays. The tunable colors are a huge benefit in the display market, because FIPEL devices give fantastic rich colors, and we can modify them to give a deeper blue, or a purer red, for example. The other big advantage in displays, particularly compared to OLEDs, is the very low current draw - even on quite big devices, the current drawn is around 2 mA, which is hardly anything.

The material is also very tolerant to manufacturing faults, which is important in larger displays. Because of the low current, you don’t get runaway current effects. So I do see FIPEL as a very very good competitor in that market.

The only issue with display applications is that our devices are AC-driven. Integrating an AC-driven device into an active matrix display may be quite challenging. On the other hand, using AC does give you a very fast response time, which is always desirable in displays - we are operating at 40,000 Hz.

The other issue that we may have is differing efficiencies between colors. The devices in general are very efficient, but that still has to be shown “in situ”, in the circuits they will eventually be working in. Measuring efficiency is also not quite as simple as with OLED, because of the AC power. Coupling power into this capacitive device will inevitably lead to resonances associated with the circuit, which makes it a little bit more difficult to work with. I don’t think FIPEL will replace OLED in displays tomorrow, but I think there is potential, because of the colors, brightness, and other advantages, for it to become a viable competitor in the future.

WS: How will FIPEL perform in some of the more niche lighting applications that were suggested when OLED started taking off, for example full-wall lighting?

DC: White lighting in general never really took off with OLEDs. I think a lot of people massively underestimated the difficulties in working with it on a large scale, caused by the number of layers in the devices, the high currents involved, etc.

The devices that we built have just 3 or 4 layers, depending on the exact archtecture - a fraction of the number needed with OLEDs. Most importantly, we induce a polarization current in the device, which means that there isn’t a lot of current that flows through interfaces - that reduces a lot of the damage typically caused by the current, and the device will not have to go through nearly so much of the strain that OLEDs have to withstand, even at very high brightness.

So with FIPEL, you can consider doing something like whole-wall lighting installations, although realistically that will always be a high-end architectural design gimmick rather than a commonplace thing. But there are a lot of cool applications like that which you can think about with this technology. We have had a lot of interest from architects, because of the flexibility of the platform - you can make it in any shape that you want, because it is plastic.

David Carroll in the lab with graduate student Greg Smith and their new FIPEL lighting technology. Image credit: Wake Forest University

WS: Do you anticipate any issues with scaling up production of the technology?

DC: Not really - the existing fabrication method should scale up well. The main challenge we have currently is trying to integrate printing into the manufacturing line. FIPEL uses nanocomposites -  nanotubes with things attached to them, and a blend of a polymer with small molecules, which have a tendency to attach themselves to the nanotube. These can be quite challenging materials to work with, when you are talking about simple manufacturing processes.

There is a lot we still don't know - and not just us, everybody working in commercialization of nanotechnology. Something that seems relatively simple, like designing a spray nozzle for a nanotube printing system which prevents aggregation, is actually quite difficult, and that's not something that we can do well yet.

There is so much that we need to do as an industry on these basic fronts. The nanocomposites being developed work so extraordinarily well, and yet we have had trouble using them, because we don’t have good manufacturing routes to them. That is always going to be the big question mark working with nanomaterials - they perform very well, and there is a huge market out there that they would take by storm, but working with nano is hard, and there is a long way still to go.

WS: What other projects can we expect to see from your group in the future?

DC: In general, our research group tries to look at the areas that other people are not working in. We’re not going to try and build the best OLED, because there are lots of people in that field who are better at it than we are. 

The other project we had recently that garnered a lot of attention was “Power Felt” - a fabric that generates power from the heat of the human body. We have made some more advances in that - we have come up with a new construct for that technology which has been widely tested at this point, and that’s getting pretty good.

What you are going to see coming out next from our group are nano-inks of Copper Zinc Tin Sulfide (CZTS). This is the material that will replace silicon in photovoltaics. Nanoformulation is a great way to make this stuff without hydrazine, and without all the expensive and dangerous processing, and I think this is going to be the next generation of solar cells.

Silicon as a PV material doesn’t work very well. Consumers are buying into silicon PV because there are lots of tax credits and subsidies associated with it. If it were a really great technology, that wouldn’t be necessary - people would just buy it. CZTS is going to be that great - I’m convinced of it. You’re going to see some really exciting stuff around that from us, probably in the summer of next year.

About the Carroll Research Group

The Carroll Research Group focuses on “Quantum Matrix Composites.”  These materials are based on spatially correlated arrays of quantum-functional nanomaterials, within an electroactive host.  Local symmetries, degrees of freedom, and length scales lead to a rich assortment of interactions within the nanodot or nanowire architecture.  The group specifically seeks to understand the quantum-cooperative behavior that results.

The group also explores how these materials might lead to the development of technologies in:

  • Power: photovoltaics, PV/T, lighting and display systems, piezo-thermoelectrics.
  • Medicine: cancer therapeutics, sensor technology, and biological-electronic interfaces (Bionics).
Date Added: Dec 7, 2012 | Updated: Jun 18, 2013
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