This paper reports on fabrication of the first free-standing single-crystal organic transistor¹, a type of device that exhibits the best performance among other known organic transistors. It is necessary to mention that transistors are the basic building blocks of electronic circuits, and, therefore, good organic transistors are necessary to build the foundation for organic electronics—one of the modern research areas promising low-cost, large-area, and possibly flexible electronic components.

Although transistors based on polycrystalline and amorphous organic semiconductors have been around for a couple of decades, the most outstanding problem that remained is the problem of defects in these materials, present at critical surfaces and interfaces at rather high densities. This is an important problem, because defects such as grain boundaries and chemical impurities slow down electrons and holes flowing through organic semiconductor by trapping and scattering these charges and, therefore, decrease the charge carrier mobility. This ultimately results in a poorer performance of electronic devices, compared to the ideal (hypothetical) device, in which defects are completely eliminated. Worse performance means difficulties in commercialization. For this reason, the work of scientists in this area on improving mobility of organic transistors is extremely important. Besides the applied interest, there is a fundamental aspect of the problem: impurities and structural disorder mask the intrinsic properties of electrical conduction, and, therefore, polycrystalline or amorphous devices cannot be used to study the intrinsic physics of charge transport in organic semiconductors.

To improve the performance there has been a continuous effort to minimize the density of defects in organic transistors by various approaches. We used monolithic single crystals of organic materials, characterized by unprecedented chemical purity and structural order. Contrary to the transistors that use polycrystalline films, our devices are completely free from grain boundaries. Due to these factors, the devices reported in our 2003 Applied Physics Letters¹ turned out to be the cleanest system, with characteristics not affected by defects. It has been shown later that mobility of charge carriers in these devices can be very high (for an organic semiconductor)²—an order of magnitude higher than the record mobility reported for the state-of-the-art polycrystalline thin-film transistors. This ultimately allowed us to use these structures to study intrinsic physics of charge transport in organic materials and to demonstrate for the first time a band-like charge conduction^3,4 and Hall effect⁵—phenomena that are important for fundamental understanding of these semiconductors.

As a result of the research on organic transistors performed globally during the last decade, it became clear that scientists are able to push the mobility of charge carriers in these materials higher and higher, so that this important metric has been steadily increasing year after year. Such encouraging progress is due to the improvements constantly introduced by the scientists into organic materials and devices. Naturally, one would ask a question: What is the fundamental limit of performance of organic transistors in terms of mobility? In other words, it would be interesting to know how far we can go with these semiconductors.

Professor Vitaly Podzorov's most-cited paper with 156 cites to date: Sundar VC, et al., "Elastomeric transistor stamps: reversible probing of charge transport in organic crystals," Science 303(5664): 1644-6, 12 March 2004. Professor Vitaly Podzorov's paper(s) represented in the Research Front map with 79 cites to date: Podzorov V, Pudalov VM, Gershenson ME, "Field-effect transistors on rubrene single crystals with paralene gate insulator," Applied Physics Letters 82(11): 1739-41, 17 March 2003. Source: Essential Science Indicators.

An ultimate performance would correspond to a device in which disorder and impurities are completely eliminated. This was the primary motivation for our work: single crystals are believed to be the purest available materials with the lowest density of defects, and, therefore, they are promising to reveal the ultimate performance limits of organic semiconductors. Such studies, however, could have become possible only if the right techniques for single-crystal device fabrication were invented, so that high-quality transistor structures could be prepared at the surface of organic crystals without introducing defects.

This part of work—fabrication of single-crystal transistors—was one of the most challenging tasks for many research groups, including ours, who have chosen the experimental approach based on single crystals. The challenge was to fully realize and overcome the fact that organic semiconductors (and especially single crystals) are incompatible with fabrication techniques typically used in conventional semiconductor technology, such as, for example, sputtering or photolithography. These processes inevitably introduce a very high density of surface defects, if applied directly to organic crystals, which in turn dramatically lower the device performance. New fabrication approaches had to be introduced for preparing transistor structures at vulnerable surfaces of organic crystals. Our 2003 report in Applied Physics Letters¹ turned out to be the first work to overcome the fabrication bottleneck of single-crystal organic transistors. More techniques for the fabrication of devices of this type appeared later^6,7. Single-crystal transistors have demonstrated that mobility of charge carriers in organic semiconductors can be much higher than previously thought; they’ve become a workhorse for the studies of intrinsic physics of organic semiconductors.

I think that the field of organic semiconductors is a very interesting area, where much more fundamental and applied science of a high quality can be generated. What is specifically encouraging is that some organic electronic products have made it into the early stages of commercialization—OLED is one bright example; other devices (photovoltaic cells and transistors) are close to being commercialized. Therefore, it is clear that organic electronics will be around for quite a while, and it will definitely require more systematic research efforts both on applied and fundamental frontiers in order to better understand and improve the existing products and invent new ones. Some of the especially hot areas that I think will keep gaining momentum for years to come are photovoltaics, photonics, and devices for biotechnology. In all these areas, organic semiconductors and hybrid (organic-inorganic) technologies will be very promising players.

Practical applications of organic transistors in general are numerous, since they are one of the basic elements of electronic circuits and can be used in active matrix organic light-emitting displays, smart cards and RF-id tags (see, e.g., ^8,9,10). However, the best use of the single-crystal devices specifically is in research on fundamental properties of organic semiconductors. Using our devices one might be able to understand fundamental limits of transistor performance and learn some useful device physics. Because single-crystal transistors are free from numerous non-intrinsic phenomena, related to the interaction of charge carriers with defects, it is typically much easier to interpret the data obtained with these devices and to come up with new ideas of device architecture with novel functionalities. For instance, by looking at light-induced effects in single-crystal transistors, we have recently discovered a general phenomenon which is also relevant to polycrystalline devices, where this effect has been obscured by defects. In principle, this effect can be used for optical recording and storage of information¹¹; in addition, it provided a viable explanation of the origin of ubiquitous effects of parameter drift, frequently observed in amorphous and polycrystalline transistors.

Furthermore, in parallel with the fundamental studies, the applied research on single-crystal devices is advancing as well. There is a set of recent 2006-2007 papers, in which researchers have demonstrated high-performance, mechanically flexible transistors using ultra-thin bendable organic single crystals¹² and suggested a new technique that enables fabrication of large arrays of those devices on flexible substrates¹³. These results indicate that single-crystal transistors might eventually become very useful for purely applied purposes.