I think that paper is really an excellent example of the right paper coming along at the right time. First of all, it has, as authors, a good cast of people, and they made sure that all the basic aspects of the field could be covered in that one review. Secondly, it is not too long, and it’s not as detailed and complex as a typical chemical review paper. Rather it is a review that everyone can really read and understand. We also made sure when we wrote it that we discussed what was really the state of the art at the time, and that we did so in a very accessible manner. I think that’s what ultimately made it so successful. And what I mean by the right paper and the right time, those electroluminescent polymers had been discovered by Richard Friend’s group 10 years earlier, but it took that entire decade for the field to really mature and be ready for the next generation of researchers to come along and carry it forward, which is what has happened.

“What is extremely interesting with these conjugated polymers is that they can be used as transistors—thin-film organic transistors, for instance—but they also have electroluminescent properties.”

What you have to understand is that organic semiconductors, which are based on molecular or oligomeric or macromolecular materials, can have a number of different applications, just as any semiconductor can. What is extremely interesting with these conjugated polymers is that they can be used as transistors—thin-film organic transistors, for instance—but they also have electroluminescent properties. And actually now there are devices that use both aspects at the same time. So they use organic semiconductors as transistors and as light emitters in the same device. Displays, for instance, are one really big potential application and there are a number of these already on the market. If you think of a flexible display made of organic material, than the organic thin-film transistor will drive the organic light-emitting diode, and that is a direct connection between the electroluminescent uses of these organic semiconductors and the transistor applications. There are even new devices (the light-emitting field-effect transistors, LEFETs) in which light emission is obtained in a transistor configuration.

That’s a simple question to answer. Since the mid-1980s, the European Commission of the European Union has had a series of what they refer to as framework programs, a goal of which was to show European scientists that if they started working together across European country borders, they could do a pretty good job. So the European Commission had calls for proposals that were budgeted for something like four to five to six years. I started working with Richard Friend, Andrew Holmes, Carlo Taliani, Bill Salaneck, and some of these other authors in one of these framework programs some 15 years ago. A project we were all involved in, thanks to the European Commission, was the LEDFOS project, which was oriented toward research on light-emitting diodes made of organic semiconductors.

We thought at the end of the project it would be a good legacy of our collaboration to write a review and to describe not only what we had done in the project but the state of the art of the field of electroluminescence in conjugated polymers. So that’s what we did and Nature was keen to get a review on a field that was really emerging very quickly at the time.

Let me a back up a bit to give you the context. What we do in my laboratory is use computational techniques to study the electronic structure and optical properties of these organic materials. The reason why I think our work has been pretty highly cited, in general, is because we have been able to take an integrated approach to the processes taking place in these devices. So we study the injection of charges, the mobility of charges, and so on.

A key step that has allowed us to do that was to learn how to study the interactions between chains of molecules in the material, rather than looking at the behavior of a single chain. If you take these polymer materials and dissolve them in solution, the chains will totally separate from one another, and you can then study them separately, and what you learn is relevant to the very dilute solutions. Indeed, in these solutions, these materials can generate a very strong luminescent signal. But when you want flexible material—the reason we use these thin-film organic semiconductors—the chains aggregate and so are parked adjacent to each other and this very strong luminescence goes away. Since no applications use dilute solutions, what was needed was to understand the interchain interactions taking place in the aggregated materials. So that’s what we started looking at in the late 1990s.

Our first paper on it was published in JACS in 1998 (Cornil J, et al., "Influence of interchain interactions on the absorption and luminescence of conjugated oligomers and polymers: a quantum-chemical characterization," Journal of the American Chemical Society 120: 1289-99, 1998). Once we were able to develop the methodologies to study these interchain interactions we were able to understand, for instance, the quenching of this luminescence in the thin films. Then we could propose strategies for preventing that quenching. That’s what we laid out in that Advanced Materials paper, and that’s why it’s been cited so many times.

The real major bonus that came out of understanding these interchain interactions at a molecular level was that we realized we could also use them to obtain a molecular picture of charge transport in these new materials. That was the next step. We then wrote a third paper, which was published in PNAS and is also highly cited ("Organic semiconductors: A theoretical characterization of the basic parameters governing charge transport," PNAS 99: 5804-9, 2002), reporting that these new methodologies allowed us to understand, really for the first time, at a molecular level, the charge transport in these materials and how it is affected by the interactions between neighboring chains.

One thing we did in that PNAS paper was to describe the influence of what we call "packing": how the chains or the molecules pack together in these materials, and the electronic coupling between adjacent chains or molecules. That PNAS paper really explains the influence of packing on the electronic coupling. And in order for charge transport to be efficient, of course, you need as large an electronic coupling as possible between adjacent molecules. But that paper still provided a pretty static picture of the process. This wasn’t sufficient to predict the mobility of charges in organic thin-film transistors.

From a theoretical standpoint, one thing that’s happened since then is efforts have been increasingly devoted to describing the dynamics that occur in these materials. The key point being that we have to deal with these thin-film organic transistors at 300 Kelvin, at room temperature, and there they have vibrations and they move with respect to one another—intermolecular vibrations—and the dynamics of these vibrations and movements have a significant impact on charge mobility. So there is a great deal of work now trying to understand these dynamics. The goal is eventually to get to the point that we can reliably predict the mobility of charge carriers in these thin-film transistors.

Let’s say you just synthesized a brand-new conjugated organic semiconductor, a new molecular material, and it looks really good on paper. And now the question is what can you expect it from practice: what kind of mobility of charges will you get in that material? Making that leap from theory to reality is the challenge. People are always shooting for the highest possible charge mobility in their materials. Everything works faster and so devices perform better.

But we’re still unable to say with confidence that, given a particular molecular packing in the solid, a particular crystal structure, how will that material perform in reality. Nobody can predict, given the molecular structure or the chemical formula, how this polymer or oligomer will act. That’s a big challenge still. Therefore what we’re heavily involved with is to be able to couple the motion of charges to this description of the inter- and intra-molecular vibrations. A big part of what we’re trying to do is to develop methodologies that make that connection. And if we’re able to do that, then people will be able to go from there to predict charge mobilities.

Well, I have to say I am not fully satisfied yet! That said, one major source of satisfaction is that we are able to work with so many experimentalists who find our theoretical work relevant to their experimental work. These experimentalists consider our work useful because it tries to solve questions that they consider important, and I take satisfaction from that.

Moreover, we are really not trying to look at just one aspect of these questions or issues, but to take an integrated approach. If you think about luminescence and organic light-emitting diodes, there are at least five processes that take place in succession and are necessary for a functioning device: you need good injection of charges into your organic materials; those charges must be mobile; the positive and negative charges must capture one another, so that they form an excited state on the polymer chains that can also move from chain to chain—a process known as exciton migration—and, hopefully, the decay to the ground state will happen with the emission of a photon.

We are looking at all those aspects, from the injection of charges and how to understand the nature of the interface between the polymer or organic material and the electrode, to the ultimate decay to the ground state. We’re very much involved in understanding the process of exciton migration; how the excited state can move through material, and so on. This is what I like very much about our work: we’re really trying to take an integrated approach to all the processes that are relevant for these organic devices.