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.

Organic single crystals are an established part of the emerging field of organic optoelectronics, because they provide an ideal platform for the studies of the intrinsic physical properties of organic semiconductors. As organic crystals have low melting temperatures and high vapor pressures and are soluble in numerous organic solvents, both solution and gas-phase methods can be used for crystal growth. The nature of the individual molecules and the interactions between molecules determine which growth method is preferred for particular materials. Organic semiconductors with very low decomposition or melting temperatures can be grown from solutions, whereas semiconductors with high vapor pressures can be grown using physical vapor transport methods. High-quality crystals can be obtained using both methods. Crystal growth and crystal engineering of multicomponent organic compounds are emerging fields that can provide a variety of new materials with different physical properties. The growth of large crystals from the melt by zone melting, the Bridgman, or the Czochralski methods has been used to produce stable materials used in wafer manufacturing or large scintillator detectors. In this article, single-crystal growth methods for organic semiconductors are discussed with the aim of preparing high-quality specimens for determination of the basic properties of organic semiconductors.

High-mobility organic field-effect transistors (OFETs) are the basic units for a variety of high-performance electronic applications. Here, we review recent progress in controlling molecular packing and crystal growth in high-mobility, small molecular organic FETs. Strategies to tune molecular packing of organic semiconductors and their impact on charge transport are described. Methods for the controlled growth of single-crystal organic semiconductors required for large-area device construction are reviewed. Furthermore, the advantages, limitations, and potential of these methods are also discussed.

Recent advances in understanding electronic charge transport in organic semiconductors are motivated by the fast growth of organic electronics. In particular, organic single crystals provide an ideal test bed for systematic studies of charge transport, with rapid progress in single-crystal-based field-effect transistors in the past few years. Charge densities induced in crystals by the field-effect have been in the low limit regime (1010 cm–2 to 1013 cm–2) mainly due to the difficulties of boosting gate dielectric capacitance. Consequently, the transport physics of organic crystals in the high-charge-density regime has not been systematically explored. With the emergence of the electrolyte gating technique, ultrahigh charge densities (1013 cm–2 to 1015 cm–2) can be achieved. In this article, we first discuss the general methodologies of applying electrolyte gating to organic crystals. We then review several recent research highlights, including the maximization of charge density and improvement of carrier mobility, enhanced understanding of the mobility-charge density relationship, and observations of ambipolar transport and a novel conductivity peak that occurs only at high charge densities. These recent achievements are extremely important for ongoing efforts to realize novel transport behavior in organic crystals, such as superconductivity and the insulator-to-metal transition.

Research on organic thin-film transistors tends to focus on improvements in device performance, but very little is understood about the ultimate limits of these devices, the microscopic physical mechanisms responsible for their limitations, and, more generally, the intrinsic transport properties of organic semiconductors. These topics are now being investigated through the study of transport in organic transistors realized using molecular single crystals of unprecedented chemical purity and structural quality. These studies are elucidating detailed microscopic aspects of the physics of organic semiconductors and corresponding devices and have also led to unforeseen high values for carrier mobility in these materials. Here, we discuss developments in this area and present a brief outlook on future goals that have come into experimental reach.

The aim of this article is to briefly review the progress made over the past few years in the theoretical description of the intrinsic charge-transport properties of organic molecular crystals. We first discuss the state-of-the-art methodologies used in the derivation of the electronic coupling and electron-phonon coupling constants. We illustrate the application of these techniques to two classes of semiconductors of interest for crystal-based organic electronics: crystals consisting of a single molecular building block, such as oligoacenes and their derivatives, and bimolecular crystals consisting of donor and acceptor compounds. After a brief overview of recent developments in the polaron modeling of the electronic and electrical properties of these systems, we examine the impact that the interplay between electronic interactions and various electron-phonon mechanisms has on the temperature dependence of the charge-carrier mobility.

This article provides an overview of the basic aspects of the structure and dynamics of excitons in molecular crystals that give rise to their unique spectroscopic behavior. The two different types of optically accessible excitons, charge-transfer and Frenkel, are described and their different properties discussed. Particular attention is paid to the spin properties of Frenkel excitons (i.e., singlet and triplet) and also to their coupling to intramolecular vibrations. Experimental challenges in the study of molecular crystal optical properties are also reviewed, including their high optical density, complex refractive index behavior, and issues with sample crystallinity and chemical purity. Once created, excitons in molecular crystals can exhibit interesting dynamical behavior, including diffusion over large length scales and ionization into electron–hole pairs. Exciton–exciton interactions are also important, ranging from fusion or annihilation (two excitons combine into one exciton) to fission (one exciton splits into two excitons). The long-range diffusion and exciton fission effects have particular relevance for the design of organic photovoltaic materials.

Comparative silk research has begun to provide us with valuable insights into a class of biopolymers that have evolved an enormous range of material performance based on highly adapted structure-property interactions. As outlined in this article, such insights cover the analysis of biological and bioinspired spinning technologies, the discovery of a novel type of melt spinning at ambient temperatures, and an advanced predictive modeling framework that uses ab initio calculations. Importantly, the huge diversity and extensive range of material properties found in natural silks is providing a fertile field for discoveries that could change the polymer paradigm and our approach to using proteins as structural materials. For example, highly relevant to both sustainability and engineering properties is the role of water in silk processing and function, as this article will explore in some detail.