Please tell us a little about your research and educational background.

I am a physical chemist by education. Optical spectroscopy of organic solid state was my initial research activity in the late sixties. The main interest at that time was to understand the nature and the energies of the electronic excitations of large aromatic molecules like benzene, naphthalene, and so on. That was of great importance in order to gauge the quantum chemical theories that were under development at that time. Spectroscopic studies of single crystals at low temperature simplified the analysis, and therefore the assignment of the electronic excitation was possible. My thesis was about benzene single crystals, which I grew and polished for spectroscopic measurements.

“In order to make the organic solid feasible for studying in a convenient form it was essential to prepare thin films.”

At that time the academic community working in the field of organic solids was quite small. But it was soon clear to experimentalists that organic solids made by conjugated molecules had peculiar semiconducting properties such as photoconductivity, which, by the way, was discovered by an Italian scientist, Pochettino, in the early 1900s. Yet that small group of researchers established the foundations of the field, which later developed into two wide related areas: the discovery of metallicity and eventually superconductivity in doped organic solids, and the development of devices based on organic semiconductors.

The discovery that conjugated polymers had similar properties developed in that context and the entire field expanded enormously. Still, the basic physics proved to be the same even though it has taken a lengthy debate. In order to make the organic solid feasible for studying in a convenient form it was essential to prepare thin films. That was also the requirement for the development of devices. We established one of the first ultra-high vacuum facilities in the early nineties and began to study the relations between structure, morphology, and the electronic structure of large conjugated molecules.

  What interested you in working in this field?

The ability to make thin films opened the possibility of studying the basic properties of organic solids without the lengthy and painful process of growing single crystals. At the same time it allowed us to explore the properties of organics by making devices like thin-film transistors. I was very much interested in making devices to prove that organics could play a role in electronics. In the process we learned that by growing rigid rod molecules in a high vacuum we could obtain a large degree of crystalline ordering which depends on the growth condition. This was a great advance that we made in 1991 by studying the growth of sexythiophene (T6) and later it proved to be very similar in other popular conjugated rigid rod molecules.

Our first study of the thin-film transistor made by T6 was published in 1992 in the first volume of a short-lived journal (Advanced Materials for Optics and Electronics) and was entitled "Instability in electrical performance of organic semiconductor devices" indicating the limitations of the device rather than its novelty. It was one of the first reports on organic thin-film transistors but its reference was negligible because of the remoteness of the journal and, possibly, for the understatement of its title—two aspects that researchers should not underestimate when they write papers. Eventually though the merit emerges, as in my case.

Another property that particularly interested me was related to the non-linear optical susceptibility of thin films made by large conjugated molecules, which was expected because of the large polarizabilities of the electrons in large conjugated molecules. There was indeed a great interest in those years in non-linear optical properties (NLO) since it was expected that organic solids could give sufficiently large NLO properties for the development of ultra-fast optical switches. We studied several molecular solids, including fullerenes C₆₀ and C₇₀, in collaboration with Francois Kajzar of CEA in Paris. The response proved not to be sufficient to make an efficient optical switch but it was a great fun.

In the early nineties the establishment of European Union-funded projects opened up the collaboration among different European countries, and we had a great interaction especially with Cambridge, Mons, and Linkoping. It was then when we got into organic light-emitting diodes (OLED) and we could study some basic device properties, such as polarized electroluminescence. The interest in the basic aspects of OLEDs is still alive now when we explore the effect of spin-polarized injection into the devices.

Most of the organic light-emitting devices made from the beginning were constituted by aluminum quinoline (AlQ3): an interesting molecule with good electron transport properties that was developed originally for xerography. Nevertheless the molecular structure allowed, in principle, the formation of two different isomers, i.e., molecules with the same composition but with a different structure. The study of the structure-properties relationship stirred up our attention.

It turned out that we discovered two new crystalline structures and analyzed their optical properties by absorption, emission, and Raman spectroscopies. The photoluminescence of AlQ3 polymorph is very rich. The overall effect of the symmetry of the molecule with its nearly equivalent intermolecular contacts is to determine an amorphous nature of the film, which in turn may be responsible for its good stability and therefore the enduring success of its application to real OLED devices. In fact many of the OLED devices available in the market are made by AlQ3 nowadays.

  How has the field—and your own work—advanced since the 2000 paper?

I think that the field of organic thin films has changed dramatically in recent years. First of all, I see that more and more attention is being devoted to the study of molecular solids made by well-defined molecules with the aim to have a better understanding of the relation between the solid-state ordering and the electrical and optical properties. At some stage it looked as if there were two different approaches to devices: molecules and polymers. I think that the notion that polymers are disordered organic solids and should be studied by that particular characteristic is gaining popularity. There is nothing special in polymers apart from their mechanical stability and their complexity. The first is very useful and the second should be put under control.

At present it is important to use ordered solids made by well-defined molecules in order to get large mobility. And large mobility is essential for the development of electronics based on organic solids. There are expectations in several areas for the development of large-area, cheap, and disposable electronics like, for instance, radio frequency tagging (RFID) to be used as intelligent tags in consumer products to trace their safe distribution.

There is another important avenue that organic semiconductors may lead to and this is associated to the intrinsic properties of carbon-based molecules. Scattering of spins is negligible and therefore organic semiconductors are the semiconductors of choice for spin transport. Five years ago we discovered the new field of Organic Spintronics by combining organic semiconductors with ferromagnetic metals like manganites into a spin valve device where the electrical properties depend on the magnetic field. This area has grown in the past five years into a well-developed field and in September 2007, we held the first "Organic Spintronics" workshop in Bologna.

  What practical applications for organic thin-film transistors have come into being or are expected to do so?

To my knowledge a commercial organic thin-film transistor device is not available on the market yet. The prospective is good but two goals must be achieved before it happens. Mobility should be improved to at least 1 cm² V^-1 s^-1, and, secondly, we should find a thin-film growth process giving reproducible ordered and stable structures. There are already several candidates, and the stability issue may not be as severe as in the case of OLED. Once these goals are achieved the application of RFID devices may be in our hands.

Another area where I foresee a great development is in the fabrication of large-area thin-film transistor panels for the active guiding of OLED flat-panel displays integrating the guiding electronics and the active optoelectronics into a common platform. Very-large-area photodetectors may also become a reality. But the most striking development may yet to be foreseen since we should bear in mind that one of the characteristics of disruptive technologies is just this: we do not know what it may be good for!

Dr. Carlo Taliani
CNR-Institute of Molecular Spectroscopy
Bologna, Italy