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Organic optoelectronics is an emerging field that exploits the unique properties of conjugated organic materials to develop new applications that require a combination of performance, low cost, light weight, and processability. For instance, disposable or wearable electronics, light-emitting diodes, smart tags, sensors, and solar cells all fall into this active area of research. Single crystals of conjugated organic molecules are, undoubtedly, the materials with the highest degree of order and purity among the variety of different forms of organic semiconductors. Electronic devices comprising these materials, such as single-crystal transistors and photoconductors developed during the last decade, are by far the best performers in terms of the fundamental parameters such as charge-carrier mobility, exciton diffusivity, concentration of defects, and operational stability. Extremely low density of defects and the resultant remarkable electrical characteristics of some of the organic single-crystal devices allow experimental access to the intrinsic charge transport properties not dominated by charge scattering and trapping. This enables basic studies of the physics of organic semiconductors, including examining the intrinsic structure-property relationship, thus providing a test bed for charge and energy transport theories. The goal of this issue of MRS Bulletin is to provide a broad overview of the state of the art of the field of organic semiconductor single-crystal materials, devices, and theory.
We present comprehensive study of ultrafast charge carrier dynamics in a variety of organic molecular crystals. In all samples, we observed sub-picosecond charge photogeneration and band-like transport, characterized by (i) an increase in charge carrier mobility as the temperature decreases in a wide temperature range of at least 20 K – 300 K and (ii) mobility anisotropy in the a-b plane of the crystals. The temperature dependence of the decay dynamics of the transient photoconductivity reveals the presence of shallow trapping sites in herring-bone-type-structured crystals (such as pentacene (Pc), tetracene (Tc), and rubrene (Rub)), while such traps are apparently absent in “brick-wall”-type crystals (such as functionalized pentacene (FPc)). We also report on the measurements of the charge carrier mobility anisotropy in the a-b plane of two types of FPc single crystals. Anisotropies of approximately 3.5 and 11.6 were obtained in the crystals characterized by crystal structures favoring two-dimensional and one-dimensional charge transport, respectively, consistent with the degree of π-overlap along different directions in the crystals.
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