Organic Single Crystals: An Essential Step to New Physics and Higher Performances of Optoelectronic Devices

Abstract

Organic Electronics is one of the most active areas of interdisciplinary research, targeting implementation of organic semiconductors in electronic applications ranging from low-cost, light-weight, flexible and wearable electronics, to light-emitting diodes, sensors, and solar cells.1 In order to provide a stronger fundamental support to this thriving applications-driven field, a deeper understanding of the basic principles governing the operation of organic materials and devices is necessary. Such understanding would help to design better semiconductors, improve device performance, and invent novel device concepts and prototypes. However, this task is very challenging because of the complexity and enormous variety of organic-molecule semiconductors, as well as the multitude of factors and microscopic phenomena that define their electronic properties. For instance, one of the important fundamental challenges in organic electronics is gaining a deeper understanding of charge carrier transport in organic semiconductors, including the structure–property relationship. Charge conduction in these materials frequently occurs in a regime at the border between band-like coherent motion of delocalized carriers in extended states, and an incoherent hopping through localized states. Many intrinsic factors are competing to define the dominant transport mechanism, including the strength of intermolecular interactions governed by the molecular structure and crystal packing (the transfer integrals); carrier self-localization due to formation of polarons; electron–phonon coupling; scattering; and off-diagonal thermal disorder (see, e.g.,2-4). Depending on the interplay between these processes, one obtains a system with band-like or hopping charge transport. Besides these intrinsic factors, a significant role in practical devices is played by the static disorder (chemical impurities and structural defects) that leads to trap states capable of carrier immobilization at various time scales. An important aspect of organic semiconductors is that π–π interactions favoring a band transport are rather weak, thus resulting in narrow bands (a few hundred meV)5 and intrinsically low carrier mobilities, μ ≈ 0.1–20 cm2 V−1 s−1 (see, e.g.,6), small in comparison with inorganic semiconductors, where μ can reach a few thousand cm2 V−1 s−1.7 It is relatively easy for extrinsic disorder (defects and impurities) to destroy the narrow bands in organic semiconductors and completely dominate the charge transport. Therefore, an important practical and fundamental task is to get rid of static disorder and experimentally access the intrinsic transport regime. This can be achieved in devices built upon molecular crystals, where many types of disorder (for instance, grain boundaries) are completely eliminated or minimized (see, e.g.,7), For this reason, single crystals have been key players in the development of (both inorganic and organic) semiconductor science and technology. The interest in organic single crystals has been further boosted by the development of both solution-growth processes and molecular structures specifically tailored for achieving enhanced solubilities. The synergy between these features allows for the growth of single crystals (usually of very small size, i.e., a few hundred microns, yet effective in terms of electronic performances), of both p- or n-type, in some cases even directly on patterned electrodes.8-15 Among the various organic electronic devices that can be realized based on single crystals, the organic field effect transistor (OFET) occupies a predominant role. Thanks to the continuing development of single-crystalline OFETs, we now have a variety of approaches resulting in high-performance devices, ranging from free-standing single-crystal transistors with contacts/dielectric/gate structure deposited at their surfaces, to thin-film crystalline layers laminated or grown on top of an insulating substrate with a bottom gate.16-23 Different methods have individual advantages and drawbacks, but together they represent a powerful complementary toolbox for the fabrication and studies of single-crystal OFETs based on various organic semiconductors and dielectric materials. Characterization methods have also seen a rather unprecedented development that, along with conventional FET measurements, now include Hall effect studies,24-27 photoconductivity analyses,28-32 as well as electron-spin resonance in gated single-crystal transistors.33, 34 Such unprecedented progress in materials and device development has led to a number of foundational results obtained with organic single-crystals—spanning from fundamental properties to novel applications—during the last dozen of years; these are briefly mentioned below with some representative references. These achievements include, but are not limited to, (a) obtaining record high mobilities in organic semiconductors by using single-crystal OFETs based on discrete vapor grown crystals or solution grown crystalline films (with μ in the range 1–20 cm2 V−1 s−1) for both holes7, 18, 21, 35 and electrons;27, 36 (b) reproducible observation of a band-like transport and intrinsic mobility anisotropy in a number of systems,24-27, 35-38 signifying that charge delocalization in van der Waals crystals is indeed possible, and the transport regime not dominated by static disorder can be achieved; (c) the discovery of new types of conducting and even metallic interfaces;39-41 (d) observation of a long-range (1–10 μm) diffusion of mobile triplet excitons in high-quality organic crystals, as well as non-linear regimes in photoconductivity with non-trivial set of power exponents (1, 1/2, 1/3, and 1/4) due to singlet fission, triplet fusion and interaction of mobile excitons with charge carriers;28-31 (e) realization of ionic-liquid gated single-crystal devices;42, 43 (f) realization of solid-state ionizing radiation sensors based on solution-grown organic crystals;44-48 (g) development of flexible single-crystal devices suitable for the studies of electro-mechanical properties of organic semiconductors;22, 49, 50 (h) realization of extremely high current density in organic light-emitting transistors;51 (i) study of electrical magnetochiral anisotropy in a bulk chiral molecular conductor;52 (j) elucidation of the role of dimensionality on charge transport;53 and (k) physical limitation of charge carrier mobility values in molecular semiconductors.54 The fundamental aspects of charge transport in crystalline organic semiconductors are reviewed in the Feature Article by Fratini et al., in which the way charge transport is intrinsically limited by the presence of large thermal mole­cular motions—a direct consequence of the weak van der Waals inter-molecular interactions—is discussed. Thermal motions in molecular crystals cause substantial fluctuation of the excitonic coupling between neighboring molecules, and Aragó et al. discuss their effects on the exciton dynamics. The extent of charge carrier wavefunction localization induced by dynamic disorder can be probed spectroscopically in small molecule semiconductors, and thus provide a new insight into the nature of shallow charge traps in these materials, as discussed by Meneau et al. The large conductivity often observed at the interface of two different organic semiconductors, originating from transfer of charge between the constituent materials, has been investigated in detail by Krupskaya et al. through a systematic study of single crystal interfaces. A study of the charge-transport properties of pure anthradithiophene isomers suggests that the benefit of isomer purity is not consistent, as discussed by Hallani et al.; while the study of stimulated emission properties in two polymorphs of a dicyanodistyrylbenzene derivative by Varghese et al. emphasizes the significance of intermolecular interactions in controlling the optical properties, as well as light amplification processes, of organic conjugated materials. The impact of polymorphism on the crystal's physical properties is discussed in the Feature Article by Jones et al., together with its effects on organic electronic devices. The role of polymorph transformation in core-chlorinated naphthalene diimides and its impact on the organic transistor performance is investigated by Purdum et al. As field effect transistors are overall the more appealing building blocks in electronics, the Feature Article by Pfattner et al. reviews how organic FETs can be tailor-designed to perform fundamental studies while offering a wide spectrum of potential applications. The performance of environmentally stable, solution-processed n-type OFETs, comparable to commercial amorphous Si transistors, is reported by Yi et al., while Niazi et al. discuss the role of contact-induced nucleation in the fabrication of organic crystalline FETs in large-area solution processing. Among its multiple advantages, solution processing has allowed exploration of the fabrication of organic single crystal FETs on flexible substrates, as described del Pozo et al. The Feature Article by Fraboni et al. reports on the recent application of organic single crystals grown from solution as ionizing radiation sensors (both scintillators and solid-state), and reviews the latest developments in the field. As fabrication and positioning of micro- and nano crystals grows in importance in a wide range of technological applications, there is an increasing requirement for development of a shared technological platform able to process materials from solutions at a large area, as described by Gentili et al. This Special Issue of Advanced Functional Materials contains a number of original contributions complemented by comprehensive feature articles, dealing with various aspects of organic single-crystalline or highly crystalline thin-film devices, and mostly reflects the invited talks recently presented at the symposium on “Organic Semiconducting Single Crystals: From Fundamentals to Advanced Devices” at the European MRS Spring Meeting 2015. The collection of these papers thus reflects some of the most recent, important and rapidly developing directions within the sector of organic electronics that focuses on fundamental materials science and electronic properties of organic single crystals. Beatrice Fraboni received an MPhil in micro­electronics from the University of Cambridge, UK, and a PhD in Physics from the University of Bologna, Italy. In 2000 she joined the Faculty of Physics at the University of Bologna where she is presently a professor in condensed matter physics. Her research activity focuses on the characterization of the electrical transport properties of organic and inorganic semiconducting materials and of advanced electronic devices, in particular on the study of radiation effects on solid state radiation detectors. She presently co­ordinates a European Project (www.iflexis.eu) on the development of flexible and printed organic radiation detectors. Alessandro Fraleoni Morgera received his PhD in industrial chemistry, with a focus on semiconducting polymers, from the University of Bologna, Italy, in 2002. Until 2007, he worked at the Department of Industrial Chemistry of the University of Bologna in the field of organic optoelectronics and bioelectronics. In 2008, he moved to Elettra-Sincrotrone Trieste, the Italian synchrotron light source located in Trieste, Italy, to work on organic electronics. Since 2013, he is a senior researcher at the Department of Engineering and Architecture at the University of Trieste, where he continues to pursue research in growth and characterization of organic semiconducting single crystals. Yves Henri Geerts was born in Brussels in 1967. He accomplished his diploma studies with Jean-Pierre Sauvage at the Université Louis Pasteur in Strasbourg, France. In 1993, he obtained his PhD degree from the Université Libre de Bruxelles (ULB), Belgium. After postdoctoral stays with Klaus Müllen at the Max Planck Institute for Polymer Research (MPIP) in Mainz, Germany, and Richard Schrock at MIT in Boston, USA, he accepted a FNRS position at ULB, in 1997. He was appointed professor at the same university in 1999. His current research focuses on the synthesis, self-assembly and processing of molecular semiconductors. Alberto Morpurgo is a full professor at the University of Geneva, Switzerland. He received his Laurea degree from the University of Genova (Italy) and his PhD in Physics from the University of Groningen (the Netherlands). Before moving to Geneva, he worked at Delft University and Stanford University. He is an expert in nanoelectronics and on organic semiconductors. The current activity of his research group is mainly focused on the study of 2D materials and their interfaces, and heavily exploits field-effect techniques—including ionic liquid gating—to control the electronic state of these systems. Vitaly Podzorov is an associate professor at Rutgers University, New Jersey, USA. He received his Masters' degree in Physics from Moscow Institute of Physics and Technology in 1995. In 1995–1997, he worked at Lebedev Institute of Physics in Moscow on optical spectroscopy of inorganic semiconductors. In 2002, he received his PhD in condensed matter physics from Rutgers University, where he studied strongly-correlated multiferroic oxides. His current research interests include charge carrier transport and optical properties of highly ordered organic semiconductors; molecular self-assembly; electric field-effect in layered inorganic materials and strongly-correlated oxides; and photophysics of hybrid (organo–inorganic) perovskites.