In the early days, the 1970s, we were laying out the initial discoveries and then building up the framework of the field. The big thing in the early 1980s was the theoretical work on solitons etc., and then the synthesis of a range of new materials that really began to lead to the promise that we would have soluble, processable polymers that would still have the optical properties of metals and semiconductors. At the beginning of the 1990s, a couple of things happened. First, the field matured to the point that we knew enough about polymers to synthesize stable polymers with specific energy gaps. Around 1990 this led to the discovery of LEDs, the application that was discovered in Cambridge by Richard Friend and his collaborators and that created one focus of much of the work in the 1990s. A second focus was the work on soluble polyaniline. There had been a lot of discussion suggesting that organic polymer metals would never be stable and never be processable. In the early 1990s, Cao, Smith, and I published a series of papers showing that it could be done.

That was a follow-up on the original LED work done at Cambridge by Friend and his collaborators. That work opened the field of LEDs, but the efficiencies of those LEDS were very low. In effect, you had to almost dark-adapt your eye to see them. The efficiencies were in the neighborhood of 1/1000ths of a percent. Still, that was the beginning of the LED field. But very quickly after that Braun and I were able to get the efficiency up to one percent, which is what this paper was about. It demonstrated that these LEDs might be something that could be commercially interesting. So Friend deserves the credit for the initial discovery without any doubt, but our paper demonstrated that the efficiency could be high enough to be of technological relevance.

Well, we reported approximately one percent in that early paper. I think efficiencies have improved up to 8 to 10 percent in polymers. And that is in serious displays that we are making at UNIAX, the company we started here in Santa Barbara, where we’re actually processing a whole display. There are some reports in the literature with polymers and particularly with small molecules of even higher efficiency. So it’s gone up by at least a factor of ten in the past decade.

Yes and no. It’s probably enough to get initial products out. For example, the efficiency we have now with green emitting polymers—where the eye is most sensitive—are good enough such that if used in the display of a cellular telephone, they would save energy and give more talk time between charges. But they’re not yet good enough in all colors and that’s an ongoing problem. We—and that’s the global we—are pushing toward full-color displays. So far, efficiencies and lifetimes are not good enough in the blue and red for that application.

It’s a critical step. It’s a conducting polymer that has electrical conductivities approaching those of conventional metals. We use it for making electrodes. Take LEDs, for example. Polyaniline is a transparent electrode. So it points out one of interesting features of these polymer metals in that they have properties that are different from regular metals in many ways. One is they are not shiny and silvery in the visible part of the spectrum. The plasma frequency is in the infrared. So they look shiny and reflective in the infrared but in the visible spectrum they’re basically transparent. So we use polyaniline to make a bi-layer electrode, with indium-tin-oxide, or ITO, which is a well-known transparent conductor and which can be put down on glass, for example, or plastic. Then we put a layer of polyaniline from solution on top of that to make the bi-layer. That is pretty much the way all these LEDs are made now. The wonderful thing is you can put this metallic polymer easily onto the surface of ITO, and make this bi-layer electrode because you’re just casting the film from solution. The effect is you stabilize the ITO, which otherwise has a tendency to change its work function. This makes the device much more efficient.

If you plotted it, it would look like a hockey stick. We’re just taking off now. A lot of hard work has been done on the synthesis of new materials and somehow proving the industrial viability. Let me use the LED as an example again. The first device lasted only minutes. While it immediately attracted some attention in the scientific community, there was a lot of skepticism about commercial applications, and rightly so. People felt these materials would never be pure enough to lead to commercially interesting products. Why is that? Think about semiconductor technology. The breakthrough in the 1950s was to make ultra, ultra pure silicon. We’re not talking about one percent, but parts-per-billion purity in conventional semiconductors. Now if you think about polymerization reactions and polymers, it’s a wonderfully creative area to design and synthesize new polymers, but it’s a lot like cooking. You’re making these things in a soup, with catalysts and this and that, and the idea that you would end up with something sufficiently pure to be used in semiconductor devices was hard for people to accept. So I think only when we began at UNIAX to show lifetimes of many thousands of hours did that become believable. That kind of lifetime is now being produced in many labs around the world. And you know how science is: once you know that something can be done you get there a little bit faster. I’m hopeful that progress will be even more rapid from now on in.

Sometimes things are obvious after the fact. But each one of these things was in a way a surprise. The LED itself was a surprise. With hindsight maybe it shouldn’t have been, because light emission from semiconductors was known, but it came out of nowhere in this field. The early polymers had not been luminescent. The idea of very high-efficiency solar cells is incredible. The discovery of lasers and laser action was a real surprise. There had been research and publications that suggested this was never going to happen because of concentration quenching, which is a well-known phenomenon in small molecules. All of those things were a surprise. The last one, in a technology sense, was a surprise even to me; that we were able as a community to get the purity of these materials up to a level which is sufficient to make electrical devices from polymers that will work and last for years. That was far from obvious.

There are many. One opportunity I see goes back to the beginning of the research in polymers in metallic forms. We have not even begun to approach the limits to the intrinsic properties of these materials. We are making metals from these polymers but they are relatively poor metals. They’re very disordered. If you cast a film from solution of a high-molecular-weight polymer what you get is a tangled spaghetti. What you really want, if you want very high performance, is more like spaghetti in the box before you cook it. You want aligned chains that are chain extended. The theoretical work tells us that when you do that, you will have metals which have electrical conductivities significantly higher than copper or anything else and have a strength greater than steel. This is really high performance and something worth going for. The first steps have been taken. In case of polyacetyline, even in the 1980s we were able to demonstrate conductivity approaching that of copper with a strength comparable to steel, and we were nowhere near the intrinsic limit. This is a very interesting goal of polymer science: manipulating polymer chains into forms that are most useful. We need some help in that from the polymer community but I think it can be done.

As far as other things, we’ll see the development of these electronic devices in a variety of directions. One thing I’m also very interested in right now and we’re beginning to work toward is biology. We’re using these polymers as elements for biosensors. Several groups in the country and around the world are working on that. The work falls on the boundary between physics, chemistry, and biology. And that’s where you often find new things. There have also been some reports in the literature around the idea that you might be able to use metallic polymers for the repair of nerve damage. If that were the case, it would obviously have a major impact. That’s something else I’d like to look into. There’s a lot of vitality in this field, and lots of new ideas still emerging from the simple fact that what you have here are materials with the electronic and optical properties of metals and semiconductors, while retaining the processing advantages and mechanical advantages of polymers.

Dr. Alan J. Heeger
Institute of Polymers and Organic Solids
University of California, Santa Barbara
Santa Barbara, CA, USA