Progress in organic single-crystal field-effect transistors
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ntroduction The development of organic field-effect transistors (FETs) based on conjugated molecules or polymers is driven by applications in the field of plastic electronics,1–7 in which molecular semiconductors are used to produce large-area, low-cost, flexible electronic devices. These devices also provide an ideal setting for the development of a basic understanding of the microscopic physical mechanisms associated with charge transport in organic semiconductors. For instance, it is not well understood why some molecules lead to charge-carrier mobilities that are much larger than those for others. When compared to inorganic semiconductors such as silicon or III–V compounds, it seems clear that technology based on organic semiconductors would vastly benefit from a much more systematic, basic understanding of the electronic properties of these materials. Investigating the intrinsic transport properties of organic semiconductors and their interfaces, which determine the transistor performance, requires materials of the highest quality to minimize extrinsic effects. Research groups have been pursuing the study of FETs based on single crystals of organic conjugated molecules,8–18 which are now setting benchmarks for the performance of organic FETs. Single-crystal devices have led to observations of new physical phenomena and to the exploration of molecular materials that are pushing the limits of organic electronics beyond what had been initially foreseen. Here we provide a short introduction to the field, with special focus on the interplay between the transport properties of organic
semiconductors and the physics of organic devices. In doing so, we also discuss the microscopic mechanisms that determine charge-carrier motion in these systems.
Fabrication of organic single-crystal transistors A variety of techniques are used to realize FETs19 based on organic single crystals. Crystals can be grown directly on substrates, for instance, by letting a drop-casted solution containing molecules evaporate,20 or by seeding (vapor-phase) crystal growth at controlled locations.21 The most common fabrication technique, however, separates crystal growth and transistor assembly.19 A common strategy relies on manual lamination (see Figure 1) of organic crystals grown from vapor phase onto a substrate, in which the gate, source, and drain contacts are fabricated prior to lamination.8,10,11 One can choose between a solid conducting substrate (acting as a gate) coated with a dielectric of choice,8,10,11,18,19 or elastomer stamps22,23 covered with a metal layer, molded to form the source and drain electrodes, and a recessed gate (so that air or vacuum acts as a dielectric). These latter devices show the largest mobility values and highest quality, as manifested by the observation of a band-like temperature dependence of the carrier mobility24 (see below for more details on band-like transport). Techniques of this type have been applied to a broad variety of different molecular crystals to investigate both p- and n-channel devices23–27 (se
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