Organic single crystals or crystalline micro/nanostructures: Preparation and field-effect transistor applications

Organic single crystals hold great promise for the development of organic semiconductor materials, because they could reveal the intrinsic electronic properties of these materials, providing high-performance electronic devices and probing the structure-property relationships. This article reviews the preparation methods for organic single crystals or crystalline micro/nanostructures, including vapor phase growth methods and solution-processed methods, and summarizes a few methods employed in the fabrication of field-effect transistors along with dozens of examples concerning both small molecules and polymers with high field-effect performance.     

The Emergence of Organic Single‐Crystal Electronics

Organic semiconducting single crystals are perfect for both fundamental and application‐oriented research due to the advantages of free grain boundaries, few defects, and minimal traps and impurities, as well as their low‐temperature processability, high flexibility, and low cost. Carrier mobilities of greater than 10 cm² V^?1 s^?1 in some organic single crystals indicate a promising application in electronic devices. The progress made, including the molecular structures and fabrication technologies of organic single crystals, is introduced and organic single‐crystal electronic devices, including field‐effect transistors, phototransistors, p‐n heterojunctions, and circuits, are summarized. Organic two‐dimensional single crystals, cocrystals, and large single crystals, together with some potential applications, are introduced. A state‐of‐the‐art overview of organic single‐crystal electronics, with their challenges and prospects, is also provided.     

Alignment and patterning of organic single crystals for field-effect transistors

Organic field-effect transistors are of great importance to electronic devices. With the emergence of various preparation techniques for organic semiconductor materials, the device performance has been improved remarkably. Among all of the organic materials, single crystals are potentially promising for high performances due to high purity and well-ordered molecular arrangement. Based on organic single crystals, alignment and patterning techniques are essential for practical industrial application of electronic devices. In this review, recently developed methods for crystal alignment and patterning are described.     

Orientation-independent charge transport in single spherulites from solution-processed organic semiconductors

Due to the rapidity of morphological development during deposition, solution-processed organic semiconductor thin films exist in semicrystalline or polycrystalline states, incorporating a high degree of local variations in molecular orientation compared to their single-crystal counterparts. Spherulites, a common crystalline superstructure found in these systems, for example, incorporate a large distribution of molecular orientations about the radial axis to maintain their space-filling growth habit. Here, we aim to determine how this distribution of molecular orientations influences charge transport by fabricating arrays of devices on single spherulites. Given that the orientation distribution that is present about the radial axis mandates the presence of low-angle grain boundaries within single spherulites, we find intraspherulitic charge transport to be independent of the general direction of π-stacking; organic field-effect transistors exhibit comparable mobilities regardless of how their channels are oriented with respect to the general π-stacking direction.     

Weak epitaxy growth of metal-free phthalocyanine on p-sexiphenyl monolayer and double-layer films

Weak epitaxy growth (WEG) can afford high-mobility thin films of disk-like organic semiconductor of which mobility is up to the level of the corresponding single crystals. We investigated the WEG behavior and mechanism of planar phthalocyanine in the model system of metal-free phthalocyanine (H2Pc) grown on p-sexiphenyl (p-6P) ultrathin films (monolayers and double layers). Highly oriented H2Pc films with molecules standing up exhibited two kinds of different in-plane orientations, i.e., three sets of in-plane orientations and only one set of in-plane orientation, on p-6P monolayer and double-layer films, respectively. The surface geometrical channels of p-6P substrate dominated the oriented nucleation and growth of H2Pc film. Consequently, the H2Pc film showed incommensurate and commensurate epitaxy on p-6P ultrathin films.     

High-mobility field-effect transistors from large-area solution-grown aligned C60 single crystals

Field-effect transistors based on single crystals of organic semiconductors have the highest reported charge carrier mobility among organic materials, demonstrating great potential of organic semiconductors for electronic applications. However, single-crystal devices are difficult to fabricate. One of the biggest challenges is to prepare dense arrays of single crystals over large-area substrates with controlled alignment. Here, we describe a solution processing method to grow large arrays of aligned C(60) single crystals. Our well-aligned C(60) single-crystal needles and ribbons show electron mobility as high as 11 cm(2)V(-1)s(-1) (average mobility: 5.2 ± 2.1 cm(2)V(-1)s(-1) from needles; 3.0 ± 0.87 cm(2)V(-1)s(-1) from ribbons). This observed mobility is ~8-fold higher than the maximum reported mobility for solution-grown n-channel organic materials (1.5 cm(2)V(-1)s(-1)) and is ~2-fold higher than the highest mobility of any n-channel organic material (~6 cm(2)V(-1)s(-1)). Furthermore, our deposition method is scalable to a 100 mm wafer substrate, with around 50% of the wafer surface covered by aligned crystals. Hence, our method facilitates the fabrication of large amounts of high-quality semiconductor crystals for fundamental studies, and with substantial improvement on the surface coverage of crystals, this method might be suitable for large-area applications based on single crystals of organic semiconductors.     

Self-assembled submicron-height terrace structures of pentacene single crystals formed in liquid crystal cells

A flat-surface single crystal structure of pentacene organic semiconductor was formed, with a submicron-height terrace structure, in liquid crystal solvent cells; the formation mechanism is discussed. By cooling the pentacene solution in a heated cell until supersaturated, a variety of segregated crystal morphology was observed, including dendrite, lozenge and needle-like crystals. Segregation of lozenge crystals was promoted by the appropriate pentacene concentration combined with the rubbing process of polyimide alignment layers, and the crystal morphology was examined in detailed. As a result, based on terrace-structural growth, low-profile flat-surface crystal morphology was found in addition to conventional pyramidal morphology. The molecular alignment of the flat pentacene crystal was confirmed by anisotropy of the microscopic Raman scattering intensity of the polarized incident light used for excitation. The self-assembly of flat thin single crystal plates, whose maximum size reach 150 µm approximately, may be applicable to practical electronic devices such as organic transistors.     

Engineered growth of organic crystalline films using liquid crystal solvents

Thin films of organic molecular crystals have drawn widespread attention for their scientifically interesting and potentially useful electronic, photonic, and chemical properties. However, because their properties are extremely sensitive to structural imperfections, domain size, and crystallographic orientation, preparation of high-quality thin films with controlled microstructural organization under technologically favorable conditions has long been a bottleneck toward practical applications and better controlled fundamental studies. Here a technique is introduced combining atmospheric pressure vapor-phase deposition with solution-phase growth in a thin layer of thermotropic liquid crystal solvent. The method produces relatively large crystals, enables control over crystallographic orientation and growth habit, and involves mild processing conditions compatible with a variety of substrates and organic materials. Results are presented for the organic semiconductor tetracene, along with a discussion of film growth and alignment mechanisms.     

Electroless silver plating of the surface of organic semiconductors

The integration of nanoscale processes and devices demands fabrication routes involving rapid, cost-effective steps, preferably carried out under ambient conditions. The realization of the metal/organic semiconductor interface is one of the most demanding steps of device fabrication, since it requires mechanical and/or thermal treatments which increment costs and are often harmful in respect to the active layer. Here, we provide a microscopic analysis of a room temperature, electroless process aimed at the deposition of a nanostructured metallic silver layer with controlled coverage atop the surface of single crystals and thin films of organic semiconductors. This process relies on the reaction of aqueous AgF solutions with the nonwettable crystalline surface of donor-type organic semiconductors. It is observed that the formation of a uniform layer of silver nanoparticles can be accomplished within 20 min contact time. The electrical characterization of two-terminal devices performed before and after the aforementioned treatment shows that the metal deposition process is associated with a redox reaction causing the p-doping of the semiconductor.     

有机单晶场效应晶体管

有机单晶场效应晶体管的研究对于探索电子的本质特性具有十分重要的意义。近几年来,不管是在制备技术还是在器件性能的研究方面,有机单晶场效应晶体管均取得了很大的进步,并由此引起了社会的广泛关注,成为场效应晶体管领域的一个重要研究方向。本文主要介绍了有机单晶的生长方法、有机场效应器件的各种制备技术、器件的迁移率及其影响因素,并对有机单晶场效应晶体管的发展前景和面临的一些问题作了简要的讨论。     

有机单晶场效应晶体管

有机单晶场效应晶体管的研究对于探索电子的本质特性具有十分重要的意义。近几年来,不管是在制备技术还是在器件性能的研究方面,有机单晶场效应晶体管均取得了很大的进步,并由此引起了社会的广泛关注,成为场效应晶体管领域的一个重要研究方向。本文主要介绍了有机单晶的生长方法、有机场效应器件的各种制备技术、器件的迁移率及其影响因素,并对有机单晶场效应晶体管的发展前景和面临的一些问题作了简要的讨论。     

Perovskite Single Crystals with Self-Cleaning Surface for Efficient Photovoltaics

Metal halide perovskite single crystals are promising for diverse optoelectronic applications. As a universal issue of solution-grown perovskite single crystals, surface contamination causes adverse effect on material properties and device performance. Herein, learning from the self-cleaning effect of lotus leaf, we address the surface contamination issue by introducing an amphiphilic long-chain organic amine into the perovskite crystal growth solution. Self-assembly of CTAC provides a hydrophobic crystal surface, inducing spontaneous removal of residual growth solution, which results in clean surface and better optoelectronic properties of perovskite single crystals. An impressive efficiency of 23.4 % is obtained, setting a new record for FA_xMA_1-xPbI₃ single-crystal perovskite solar cells (PSCs). Moreover, our strategy also applies to perovskite single crystals with different morphology and composition, which may contribute to improvement of other single-crystal perovskite optoelectronic devices.     

MFI型沸石晶体的择优定向生长

首次采用"双模板剂"法在SiO2-Na2O-正丙胺-溴化N-乙基-六亚甲基四胺(EtHMTA^+)-NaF-H2O体系中合成得到了b轴择优取向的ZSM-5(MFI型)晶体,并用扫描电镜(SEM)和XRD进行表征。择优取向程度与所用的两种模板剂的比例有关,当正丙胺:EtHMTA^+比为0.7:0.3时,ZSM-5晶体择优取向最明显。     

Advanced surface modification of indium tin oxide for improved charge injection in organic devices

A new method is described for surface modification of ITO with an electroactive organic monolayer. This procedure was done to enhance hole injection in an electronic device and involves sequential formation of a monolayer of a pi-conjugated organic semiconductor on the indium tin oxide (ITO) surface followed by doping with a strong electron acceptor. The semiconductor monolayer is covalently bound to the ITO, which ensures strong adhesion and interface stability; reduction of the hole injection barrier in these devices is accomplished by formation of a charge-transfer complex by doping within the monolayer. This gives rise to very high current densities in simple single layer devices and double layer light emitting devices compared to those with untreated ITO anodes.     

Crystallization Mechanism of 9,9-Diphenyl-dibenzosilole from Solids

Organic semiconductor (OSC) crystals have great potential to be applied in many fields, as they can be flexibly designed according to the demands and show an outstanding device performance. However, OSCs with the capacity of solid-state crystallization (SSC) are developing too slowly to meet demands in productions and applications, due to their difficulties in molecular design and synthesis, unclear mechanism and high dependence on experimental conditions. In this work, in order to solve the problems, we synthesized an organic semiconductor capable of SSC at room temperature by adjusting the relationship between conjugated groups and functional groups. The thermodynamic and kinetic properties have been studied to discover the model of film SSC. Moreover, it can be purposefully controlled to prepare the high-quality crystals, and their corresponding organic electronic devices were further fabricated and discussed.     

Ultrathin organic semiconductor films — Soft matter effect

The growth of organic semiconductor thin films has been a crucial issue in organic electronics, especially the growth at the early stages. The thin-film phase has been found to be a common phenomenon in many organic semiconductor thin films, which is closely related with the weak van der Waals interaction between organic molecules, the long-range interaction between organic molecules and the substrate, as well as the soft matter characteristics of ultrathin films. The growth behavior and soft matter characteristics of the thin-film phase have great effects on thin film morphology and structure, for example, the formation and coalescence of grain boundaries, which further influences the performance of organic electronic devices. The understanding of thin-film phase and its intrinsic quality is necessary for fabricating large-size, highly ordered, continuous and defect-free ultrathin films. This review will focus on the growth behavior of organic ultrathin films, i.e., the level of the first several molecular layers, and provide an overview of the soft matter characteristics.     

Precise Control Over Kinetics of Molecular Assembly: Production of Particles with Tunable Sizes and Crystalline Forms

It has been long‐pursued but remains a challenge to precisely manipulate the molecular assembly process to obtain desired functional structures. Reported here is the control over the assembly of solute molecules, by a programmed recrystallization of solvent crystal grains, to form micro/nanoparticles with tunable sizes and crystalline forms. A quantitative correlation between the protocol of recrystallization temperature and the assembly kinetics results in precise control over the size of assembled particles, ranging from single‐atom catalysts, pure drug nanoparticles, to sub‐millimeter organic‐semiconductor single crystals. The extensive regulation of the assembly rates leads to the unique and powerful capability of tuning the stacking of molecules, involving the formation of single crystals of notoriously crystallization‐resistant molecules and amorphous structures of molecules with a very high propensity to crystallize, which endows it with wide‐ranging applications.     

Selective adsorption to particular crystal faces of ZnO

We examine the hypothesis that selective adsorption to a particular face of ZnO is responsible for the ability of small organic molecules to control the aspect ratio of ZnO crystals during hydrothermal synthesis. Large, single crystals of ZnO were prepared such that the vast majority of a surface consisted of a single crystal plane, as shown by atomic force microscopy, and the adsorption to a single crystal plane was determined by attenuated total reflectance spectroscopy. The results show that citrate strongly and selectively adsorbs to the (0001) face. Similarly, results show that ethylenediamine selectively adsorbs to the (1010) face. Each of these results separately shows a correlation between selective adsorption to and growth of large areas of a particular face, and thus, each result is consistent with the proposed hypothesis.     

Mechanically compliant single crystals of a stable organic radical

Mechanically compliant organic crystals are the foundation of the development of future flexible, light-weight single-crystal electronics, and this requires reversibly deformable crystalline organic materials with permanent magnetism. Here, we report and characterize the first instance of a plastically bendable single crystal of a permanent organic radical, 4-(4′-cyano-2′,3′,4′,5′-tetrafluorophenyl)-1,2,3,5-dithiadiazolyl. The weak interactions between the radicals render single crystals of the β phase of this material exceedingly soft, and the S–N interactions facilitate plastic bending. EPR imaging of a bent single crystal reveals the effect of deformation on the three-dimensional spin density of the crystal. The unusual mechanical compliance of this material opens prospects for exploration into flexible crystals of other stable organic radicals towards the development of flexible light-weight organic magnetoresistance devices based on weak, non-hydrogen-bonded interactions in molecular crystals.

Mechanically soft crystals are interesting candidates for single crystal electronics. Here, crystals of a stable dithiadiazolyl radical are shown to be plastically bendable and display a change in their spin density in response to mechanical force.     

Control of all‐conjugated block copolymer crystallization via thermal and solvent annealing

Control of the crystallization of conjugated polymers is of critical importance to the performance of organic electronics, such as organic photovoltaic devices, due to the effect on charge separation and transport, particularly for all‐polymer devices. The block copolymer poly(3‐dodecylthiophene)‐block‐poly(9,9‐dioctylfluorene) (P3DDT‐b‐PF), which has matched crystallization temperatures for each block, is used to study the effects of processing history on resulting crystallization. For longer annealing times and rapid quenching to room temperature, P3DDT crystals are preferred whereas for shorter annealing times and slower quenching, PF crystals are preferred. Both crystal forms are evidenced for long annealing time and slow quenching. Additionally, for room temperature annealing in the presence of a chloroform vapor, PF crystals are found in the PF β phase with the predominant crystal peak oriented perpendicular to the thermally annealed case. These results will provide guidance for optimizing annealing strategies for future donor/acceptor block copolymer photovoltaic devices. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 900–906