Single-crystal organic charge-transfer interfaces probed using Schottky-gated heterostructures

Visualizing the interfacial-layer-based epitaxial growth process toward organic core-shell architectures

Organic heterostructures (OHTs) with the desired geometry organization on micro/nanoscale have undergone rapid progress in nanoscience and nanotechnology. However, it is a significant challenge to elucidate the epitaxial-growth process for various OHTs composed of organic units with a lattice mismatching ratio of > 3%, which is unimaginable for inorganic heterostructures. Herein, we have demonstrated a vivid visualization of the morphology evolution of epitaxial-growth based on a doped interfacial-layer, which facilitates the comprehensive understanding of the hierarchical self-assembly of core-shell OHT with precise spatial configuration. Significantly, the barcoded OHT with periodic shells obviously illustrate the shell epitaxial-growth from tips to center parts along the seeded rods for forming the core-shell OHT. Furthermore, the diameter, length, and number of periodic shells were modulated by finely tuning the stoichiometric ratio, crystalline time, and temperature, respectively. This epitaxial-growth process could be generalized to organic systems with facile chemical/structural compatibility for forming the desired OHTs.

Organic Bilayer Heterostructures with Built-In Exciton Conversion for 2D Photonic Encryption

Organic multilayer heterostructures with accurate spatial organization demonstrate strong light-matter interaction from excitonic responses and efficient carrier transfer across heterojunction interfaces, which are considered as promising candidates toward advanced optoelectronics. However, the precise regulation of the heterojunction surface area for finely adjusting exciton conversion and energy transfer is still formidable. Herein, organic bilayer heterostructures (OBHs) with controlled face-to-face heterojunction via a stepwise seeded growth strategy, which is favorable for efficient exciton propagation and conversion of optical interconnects are designed and synthesized. Notably, the relative position and overlap length ratio of component microwires (LDSA /LBPEA = 0.39-1.15) in OBHs are accurately regulated by modulating the crystallization time of seeded crystals, resulting into a tailored heterojunction surface area (R = Loverlap /LBPEA = 37.6%-65.3%). These as-prepared OBHs present the excitation position-dependent waveguide behaviors for optical outcoupling characteristics with tunable emission colors and intensities, which are applied into two-dimensional (2D) photonic barcodes. This strategy opens a versatile avenue to purposely design OBHs with tailored heterojunctions for efficient energy transfer and exciton conversion, facilitating the application possibilities of advanced integrated optoelectronics.

High-efficiency blue-emission crystalline organic light-emitting diodes sensitized by "hot exciton" fluorescent nanoaggregates

Sensitizing fluorescent materials is an effective way to maximally use excitons and obtain high-efficiency blue organic light-emitting diodes (OLEDs). However, it is a persistent challenge for present amorphous thin-film OLEDs to improve photon emission under low driving voltage, severely impeding the development of OLED technology. Here, we propose a novel OLED architecture consisting of a crystalline host matrix (CHM) and embedded "hot exciton" nanoaggregates (HENAs), which effectively sensitize blue dopant (D) emission. Owing to the advantages of the crystalline thin-film route, the device exhibits largely enhanced blue photon output [Commission International de L'Eclairage coordinates of (0.15, 0.17)], with a low turn-on/operation voltage of 2.5 V (at 1 cd/m²)/3.3 V (at 1000 cd/m²), an extremely low Joule heat loss ratio (7.8% at 1000 cd/m²), and a maximum external quantum efficiency (EQE) up to 9.14%. These areal photon output features have outperformed the present amorphous thin-film blue OLEDs with high EQE, demonstrating that the CHM-HENA-D OLED is promising for future OLEDs.

Growth direction dependent separate-channel charge transport in the organic weak charge-transfer co-crystal of anthracene-DTTCNQ

Co-crystallization is an efficient way of molecular crystal engineering to tune the electronic properties of organic semiconductors. In this work, we synthesized anthracene-4,8-bis(dicyanomethylene)4,8-dihydrobenzo[1,2-b:4,5-b']-dithiophene (DTTCNQ) single crystals as a template to study the crystal growth direction dependent charge transport properties and attempted to elucidate the mechanism by proposing a separate-channel charge transport model. Single-crystal anthracene-DTTCNQ field-effect transistors showed that ambipolar transport properties could be observed in all crystal growth directions. Furthermore, upon changing the measured crystal directions, the electronic properties experienced a weak change from n-type dominated ambipolar, balanced ambipolar, to p-type dominated ambipolar properties. The theoretical calculations at density functional theory (DFT) and higher theory levels suggested that the anthracene-DTTCNQ co-crystal motif was a weak charge-transfer complex, in line with the experiment. Furthermore, the detailed theoretical analysis also indicated that electron or hole transport properties originated from separated channels formed by DTTCNQ or anthracene molecules. We thus proposed a novel separate-channel transport mechanism to support additional theoretical analysis and calculations. The joint experimental and theoretical efforts in this work suggest that the engineering of co-crystallization of weak charge-transfer complexes can be a practical approach for achieving tuneable ambipolar charge transport properties by the rational choice of co-crystal formers.

Vapor-Phase Living Assembly of π-Conjugated Organic Semiconductors

In contrast to well-studied amphiphilic block copolymers (BCPs) and π-stacked dyes, living assembly of hydrophobic π-conjugated materials has not yet been explored to date. Using a microspacing physical vapor transport (PVT) technique, the prefabricated microrods of organic semiconductors involving 9,10-dicyanoanthracene (DCA, A) or its binary alloy (B) can act as seeds to initiate living homoepitaxial growth from their ends, giving elongated microrods with controlled length. Red-green-red tricolor fluorescent microrod heterostructures with low dispersity are further realized by living heteroepitaxial growth of B microrod blocks on A seed microrod tips. Upon varying the growth sequence of each block, reverse triblock microrods are also accessible. Such a seed-induced living growth is applicable to triblock microrod heterostructures of more binary combinations as well as even more complex penta- and hepta-block heterostructures comprising A and B. By virtue of a convenient vapor-phase growth method, the present work demonstrates the generality of living assembly of π-conjugated materials.

Rational design of high-quality 2D/3D perovskite heterostructure crystals for record-performance polarization-sensitive photodetection

Polarization-sensitive photodetection is central to optics applications and has been successfully demonstrated in photodetectors of two-dimensional (2D) materials, such as layered hybrid perovskites; however, achieving high polarization sensitivity in such a photodetector remains extremely challenging. Here, for the first time, we demonstrate a high-performance polarization-sensitive photodetector using single-crystalline 2D/3D perovskite heterostructure, namely, (4-AMP)(MA)₂Pb₃Br₁₀/MAPbBr₃ (MA = methylammonium; 4-AMP = 4-(aminomethyl)piperidinium), which exhibits ultrahigh polarization sensitivity up to 17.6 under self-driven mode. To our knowledge, such a high polarization selectivity has surpassed all of the reported perovskite-based devices, and is comparable to, or even better than, the traditional inorganic heterostructure-based photodetectors. Further studies reveal that the built-in electric field formed at the junction can spatially separate the photogenerated electrons and holes, reducing their recombination rate and thus enhancing the performance for polarization-sensitive photodetection. This work provides a new source of polarization-sensitive materials and insights into designing novel optoelectronic devices.

Sub-5 nm single crystalline organic p-n heterojunctions

The cornerstones of emerging high-performance organic photovoltaic devices are bulk heterojunctions, which usually contain both structure disorders and bicontinuous interpenetrating grain boundaries with interfacial defects. This feature complicates fundamental understanding of their working mechanism. Highly-ordered crystalline organic p–n heterojunctions with well-defined interface and tailored layer thickness, are highly desirable to understand the nature of organic heterojunctions. However, direct growth of such a crystalline organic p–n heterojunction remains a huge challenge. In this work, we report a design rationale to fabricate monolayer molecular crystals based p–n heterojunctions. In an organic field-effect transistor configuration, we achieved a well-balanced ambipolar charge transport, comparable to single component monolayer molecular crystals devices, demonstrating the high-quality interface in the heterojunctions. In an organic solar cell device based on the p–n junction, we show the device exhibits gate-tunable open-circuit voltage up to 1.04 V, a record-high value in organic single crystalline photovoltaics.

Realizing organic p–n junctions based on ordered crystalline materials with dimensions comparable to the exciton diffusion length of most organic semiconductors remains a challenge. Here, the authors report a strategy to form molecular monolayer crystal-based p–n junctions with thickness below 5 nm.

Balanced Ambipolar Charge Transport in Phenacene/Perylene Heterojunction-Based Organic Field-Effect Transistors

Electronic devices relying on the combination of different conjugated organic materials are considerably appealing for their potential use in many applications such as photovoltaics, light emission, and digital/analog circuitry. In this study, the electrical response of field-effect transistors achieved through the evaporation of picene and PDIF-CN₂ molecules, two well-known organic semiconductors with remarkable charge transport properties, was investigated. With the main goal to get a balanced ambipolar response, various device configurations bearing double-layer, triple-layer, and codeposited active channels were analyzed. The most suitable choices for the layer deposition processes, the related characteristic parameters, and the electrode position were identified to this purpose. In this way, ambipolar organic field-effect transistors exhibiting balanced mobility values exceeding 0.1 cm² V^-1 s^-1 for both electrons and holes were obtained. These experimental results highlight also how the combination between picene and PDIF-CN₂ layers allows tuning the threshold voltages of the p-type response. Scanning Kelvin probe microscopy (SKPM) images acquired on picene/PDIF-CN₂ heterojunctions suggest the presence of an interface dipole between the two organic layers. This feature is related to the partial accumulation of space charge at the interface being enhanced when the electrons are depleted in the underlayer.

Molecular Semiconductors for Logic Operations: Dead-End or Bright Future?

The field of organic electronics has been prolific in the last couple of years, leading to the design and synthesis of several molecular semiconductors presenting a mobility in excess of 10 cm² V^-1 s^-1 . However, it is also started to recently falter, as a result of doubtful mobility extractions and reduced industrial interest. This critical review addresses the community of chemists and materials scientists to share with it a critical analysis of the best performing molecular semiconductors and of the inherent charge transport physics that takes place in them. The goal is to inspire chemists and materials scientists and to give them hope that the field of molecular semiconductors for logic operations is not engaged into a dead end. To the contrary, it offers plenty of research opportunities in materials chemistry.

Self-gating in semiconductor electrocatalysis

The semiconductor-electrolyte interface dominates the behaviours of semiconductor electrocatalysis, which has been modelled as a Schottky-analogue junction according to classical electron transfer theories. However, this model cannot be used to explain the extremely high carrier accumulations in ultrathin semiconductor catalysis observed in our work. Inspired by the recently developed ion-controlled electronics, we revisit the semiconductor-electrolyte interface and unravel a universal self-gating phenomenon through microcell-based in situ electronic/electrochemical measurements to clarify the electronic-conduction modulation of semiconductors during the electrocatalytic reaction. We then demonstrate that the type of semiconductor catalyst strongly correlates with their electrocatalysis; that is, n-type semiconductor catalysts favour cathodic reactions such as the hydrogen evolution reaction, p-type ones prefer anodic reactions such as the oxygen evolution reaction and bipolar ones tend to perform both anodic and cathodic reactions. Our study provides new insight into the electronic origin of the semiconductor-electrolyte interface during electrocatalysis, paving the way for designing high-performance semiconductor catalysts.

Polymer additive controlled morphology for high performance organic thin film transistors

Solution-crystallizable small-molecule organic semiconductors, such as 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene), 5,11-bis(triethylgermylethynyl)anthradithiophene (diF-TEG-ADT), 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT), and N,N'-1H,1H-perfluorobutyl dicyanoperylenecarboxydiimide (PDIF-CN2), demonstrate various practical advantages including high mobility, air stability and solution processibility. In this article, we review various polymer additive based approaches to control the crystal morphology and the resultant charge transport of some bench-mark, high performance, solution crystallizable, small-molecule organic semiconductors. The polymer additives are discussed under the categories of non-conjugated polymers and conjugated polymers. The approaches and structure-performance correlations that we discussed here may be applied far beyond the examples shown in this review and have important implications for high performance organic semiconductors in general.

The actual electronic band structure of a rubrene single crystal

A proper understanding on the charge mobility in organic materials is one of the key factors to realize highly functionalized organic semiconductor devices. So far, however, although a number of studies have proposed the carrier transport mechanism of rubrene single crystal to be band-like, there are disagreements between the results reported in these papers. Here, we show that the actual dispersion widths of the electronic bands formed by the highest occupied molecular orbital are much smaller than those reported in the literature, and that the disagreements originate from the diffraction effect of photoelectron and the vibrations of molecules. The present result indicates that the electronic bands would not be the main channel for hole mobility in case of rubrene single crystal and the necessity to consider a more complex picture like molecular vibrations mediated carrier transport. These findings open an avenue for a thorough insight on how to realize organic semiconductor devices with high carrier mobility.

Highly Ordered Semiconducting Polymer Arrays for Sensitive Photodetectors

Semiconducting conjugated polymers possess attractive optoelectronic properties and low-cost solution processability and are inherently mechanically flexible. However, the device performance is susceptible to the fabrication methods because of the relatively weak intermolecular interaction of the polymers and their inherent conformational and energetic disorder. An efficient fabrication technique for large-scale integration of high-quality polymer architectures is essential for realizing high-performance optoelectronic devices. Here, we report an efficient method for fabrication of polymer nanowire arrays with a precise position, a smooth surface, a homogeneous size, high crystallinity, and ordered molecular packing. The controllable dewetting dynamics on a template with asymmetric wettability permits the formation of discrete capillary bridges, resulting in the ordered molecular packing arising from unidirectional recession of the three-phase contact line. The high quality of nanowire architectures is evidenced by the morphological characteristics and hybrid edge-on and face-on molecular packing with high crystallinity. On the basis of these high-quality nanowire arrays, photodetectors with a responsivity of 84.7 A W^-1 and detectivity of >10¹² Jones are realized. Our results provide a platform for integration of high-quality polymer architectures for use in high-performance optoelectronic devices.

Fully rubbery integrated electronics from high effective mobility intrinsically stretchable semiconductors

An intrinsically stretchable rubbery semiconductor with high mobility is critical to the realization of high-performance stretchable electronics and integrated devices for many applications where large mechanical deformation or stretching is involved. Here, we report fully rubbery integrated electronics from a rubbery semiconductor with a high effective mobility, obtained by introducing metallic carbon nanotubes into a rubbery semiconductor composite. This enhancement in effective carrier mobility is enabled by providing fast paths and, therefore, a shortened carrier transport distance. Transistors and their arrays fully based on intrinsically stretchable electronic materials were developed, and they retained electrical performances without substantial loss when subjected to 50% stretching. Fully rubbery integrated electronics and logic gates were developed, and they also functioned reliably upon mechanical stretching. A rubbery active matrix based elastic tactile sensing skin to map physical touch was demonstrated to illustrate one of the applications.

Highly Oriented Liquid Crystal Semiconductor for Organic Field-Effect Transistors

We report a mesogenic compound which introduces nematic liquid crystal (LC) ordering into the benzothienobenzothiophene (BTBT) family of LCs, creating a new class of LC semiconducting materials which respond in a facile way to anisotropic surfaces, and can, thereby, be effectively processed into highly oriented monodomains. Measurement on these domains of the electrical conductivity, with in situ monitoring of domain quality and orientation using LC birefringence textures in electroded cells, brings a new era of precision and reliability to the determination of anisotropic carrier mobility in LC semiconductors.

Organic Semiconductor Single Crystals for Electronics and Photonics

Organic semiconducting single crystals (OSSCs) are ideal candidates for the construction of high-performance optoelectronic devices/circuits and a great platform for fundamental research due to their long-range order, absence of grain boundaries, and extremely low defect density. Impressive improvements have recently been made in organic optoelectronics: the charge-carrier mobility is now over 10 cm² V^-1 s^-1 and the fluorescence efficiency reaches 90% for many OSSCs. Moreover, high mobility and strong emission can be integrated into a single OSSC, for example, showing a mobility of up to 34 cm² V^-1 s^-1 and a photoluminescence yield of 41.2%. These achievements are attributed to the rational design and synthesis of organic semiconductors as well as improvements in preparation technology for crystals, which accelerate the application of OSSCs in devices and circuits, such as organic field-effect transistors, organic photodetectors, organic photovoltaics, organic light-emitting diodes, organic light-emitting transistors, and even electrically pumped organic lasers. In this context, an overview of these fantastic advancements in terms of the fundamental insights into developing high-performance organic semiconductors, efficient strategies for yielding desirable high-quality OSSCs, and their applications in optoelectronic devices and circuits is presented. Finally, an overview of the development of OSSCs along with current challenges and future research directions is provided.

Organic 2D Optoelectronic Crystals: Charge Transport, Emerging Functions, and Their Design Perspective

2D organic semiconductor crystals are emerging as a fascinating platform with regard to their applications in organic field-effect transistors (OFETs), attributed to their enhanced charge transport efficiency and their new optoelectronic functions, based on their unique morphological features. Advances in material processing techniques have not only enabled easy fabrication of few-monolayered 2D nanostructures but also facilitated exploration of the interesting properties induced by characteristic 2D morphologies. However, to date, only a limited number of representative organic semiconductors have been utilized in organic 2D optoelectronics. Therefore, in order to further spur this research, an intuitive crystal engineering principle for realizing organic 2D crystals is required. In this regard, here, not only the important implications of applying 2D structures to OFET devices are discussed but also a crystal engineering protocol is provided that first predicts molecular arrangements depending on the molecular factors, which is followed by realizing 2D supramolecular synthon networks for different molecular packing motifs. It is expected that 2D organic semiconductor crystals developed by this approach will pave a promising way toward next-generation organic 2D optoelectronics.

Hole Mobility Modulation in Single-Crystal Metal Phthalocyanines by Changing the Metal-π/π-π Interactions

Weak intermolecular interactions in organic semiconducting molecular crystals play an important role in determining molecular packing and electronic properties. Single crystals of metal-free and metal phthalocyanines were synthesized to investigate how the coordination of the central metal atom affects their molecular packing and resultant electronic properties. Single-crystal field-effect transistors were made and showed a hole mobility order of ZnPc>MnPc>FePc>CoPc>CuPc>H₂ Pc>NiPc. Density functional theory (DFT) and 1D polaron transport theory reach a good agreement in reproducing the experimentally measured trend for hole mobility. Additional detail analysis at the DFT level suggests the metal atom coordination into H₂ Pc planes can tune the hole mobility via adjusting the intermolecular distances along the shortest axis with closest parallel π stackings.

Band-Like Carrier Transport at the Single-Crystal Contact Interfaces between 2,5-Difluoro-7,7,8,8-tetracyanoquinodimethane and Electron Donors

Some heterojunction interfaces formed with molecular solids show metal-like transport behavior. In order to clarify the requirement, interfaces are fabricated by lamination of single-crystal electron-accepting 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane (F₂TCNQ) and electron-donating molecules with a wide range of ionization potentials. Carrier injection between the acceptor and donor crystals leads to highly conducting interfaces, some of which exhibited band-like charge transport behaviors. Combinations with weak donors also resulted in interfaces with band-like transport properties. Accordingly, band-like conduction was achieved for interfaces where the donor and acceptor crystals do not have well-matched band energies. The results indicate that the wide range of candidates have great potential for modification of the electronic structure of organic crystals. The present method is expected to enable control of the electronic properties of the interface.

Organic semiconductor crystals

A comprehensive overview of organic semiconductor crystals is provided, including the physicochemical features, the control of crystallization and the device physics.

Charge transfer from and to manganese phthalocyanine: bulk materials and interfaces

Manganese phthalocyanine (MnPc) is a member of the family of transition-metal phthalocyanines, which combines interesting electronic behavior in the fields of organic and molecular electronics with local magnetic moments. MnPc is characterized by hybrid states between the Mn 3d orbitals and the π orbitals of the ligand very close to the Fermi level. This causes particular physical properties, different from those of the other phthalocyanines, such as a rather small ionization potential, a small band gap and a large electron affinity. These can be exploited to prepare particular compounds and interfaces with appropriate partners, which are characterized by a charge transfer from or to MnPc. We summarize recent spectroscopic and theoretical results that have been achieved in this regard.

Preparation of Single-Crystalline Heterojunctions for Organic Electronics

Organic single-crystalline heterojunctions are composed of different single crystals interfaced together. The intrinsic highly ordered heterostructure in these multicomponent solids holds the capacity for multifunctions, as well as superior charge-transporting properties, promising high-performance electronic applications such as ambipolar transistors and solar cells. However, this kind of heterojunction is not easily available and the preparation methods need to be developed. Recent advances in the efficient strategies that have emerged in yielding high-quality single-crystalline heterojunctions are highlighted here. The advantages and limitations of each strategy are also discussed. The obtained single-crystalline heterojunctions have started to exhibit rich physical properties, including metallic conduction, photovoltaic effects, and so on. Further structural optimization of the heterojunctions to accommodate the electronic device configuration is necessary to significantly advance this research direction.

Fast-Response Photonic Device Based on Organic-Crystal Heterojunctions Assembled into a Vertical-Yet-Open Asymmetric Architecture

Crystalline dioctyl-3,4,9,10-perylenedicarboximide nanowires and 6,13-bis(triisopropylsilylethynyl) pentacene microplates are integrated into a vertical-yet-open asymmetrical heterojunction for the realization of a high-performance organic photovoltaic detector, which shows fast photoresponse, ultrahigh signal-to-noise ratio, and high sensitivity to weak light.

2D coherent charge transport in highly ordered conducting polymers doped by solid state diffusion

Doping is one of the most important methods to control charge carrier concentration in semiconductors. Ideally, the introduction of dopants should not perturb the ordered microstructure of the semiconducting host. In some systems, such as modulation-doped inorganic semiconductors or molecular charge transfer crystals, this can be achieved by spatially separating the dopants from the charge transport pathways. However, in conducting polymers, dopants tend to be randomly distributed within the conjugated polymer, and as a result the transport properties are strongly affected by the resulting structural and electronic disorder. Here, we show that in the highly ordered lamellar microstructure of a regioregular thiophene-based conjugated polymer, a small-molecule p-type dopant can be incorporated by solid state diffusion into the layers of solubilizing side chains without disrupting the conjugated layers. In contrast to more disordered systems, this allows us to observe coherent, free-electron-like charge transport properties, including a nearly ideal Hall effect in a wide temperature range, a positive magnetoconductance due to weak localization and the Pauli paramagnetic spin susceptibility.

A Dewetting-Induced Assembly Strategy for Precisely Patterning Organic Single Crystals in OFETs

Simple methods for patterning single crystals are critical to fully realize their applications in electronics. However, traditional vapor and solution methods are deficient in terms of crystals with random spatial and quality distributions. In this work, we report a dewetting-induced assembly strategy for obtaining large-scale and highly oriented organic crystal arrays. We also demonstrate that organic field-effect transistors (OFETs) fabricated from patterned n-alkyl-substituted tetrachloroperylene diimide (R-4ClPDI) single crystals can reach a maximum mobility of 0.65 cm(2) V(-1) s(-1) for C8-4ClPDI in ambient conditions. This technique constitutes a facile method for fabricating OFETs with high performances for large-scale electronics applications.

Tunable charge transfer properties in metal-phthalocyanine heterojunctions

Organic materials such as phthalocyanine-based systems present a great potential for organic device applications due to the possibility of integrating films of different organic materials to create organic heterostructures which combine the electrical capabilities of each material. This opens the possibility to precisely engineer and tune new electrical properties. In particular, similar transition metal phthalocyanines demonstrate hybridization and charge transfer properties which could lead to interesting physical phenomena. Although, when considering device dimensions, a better understanding and control of the tuning of the transport properties still remain in the focus of research. Here, by employing conductive atomic force microscopy techniques, we provide an insight about the nanoscale electrical properties and transport mechanisms of MnPc and fluorinated phthalocyanines such as F16CuPc and F16CoPc. We report a transition from typical diode-like transport mechanisms for pure MnPc thin films to space-charge-limited current transport regime (SCLC) for Pc-based heterostructures. The controlled addition of fluorinated phthalocyanine also provides highly uniform and symmetric-polarized transport characteristics with conductance enhancements up to two orders of magnitude depending on the polarization. We present a method to spatially map the mobility of the MnPc/F16CuPc structures with a nanoscale resolution and provide theoretical calculations to support our experimental findings. This well-controlled nanoscale tuning of the electrical properties for metal transition phthalocyanine junctions stands as key step for future phthalocyanine-based electronic devices, where the low dimension charge transfer, mediated by transition metal atoms could be intrinsically linked to a transfer of magnetic moment or spin.

Alignment and Patterning of Ordered Small-Molecule Organic Semiconductor Micro-/Nanocrystals for Device Applications

Large-area alignment and patterning of small-molecule organic semiconductor micro-/nanocrystals (SMOSNs) at desired locations is a prerequisite for their practical device applications. Recent strategies for alignment and patterning of ordered SMOSNs and their corresponding device applications are highlighted.

In situ fabrication and investigation of nanostructures and nanodevices with a microscope

The widespread availability of nanostructures and nanodevices has placed strict requirements on their comprehensive characterization.

Wafer-Scale Precise Patterning of Organic Single-Crystal Nanowire Arrays via a Photolithography-Assisted Spin-Coating Method

A photolithography-assisted spin-coating approach is developed to produce single-crystal organic nanowire (NW) arrays at designated locations with high precision and high efficiency. This strategy enables the large-scale fabrication of organic NW arrays with nearly the same accuracy, reliability, and flexibility as photolithography. The high mobilities of the organic NWs enable the control of the switch of multicolored light-emitting devices with good stability.

All-polymeric control of nanoferronics

In the search for light and flexible nanoferronics, significant research effort is geared toward discovering the coexisting magnetic and electric orders in crystalline charge-transfer complexes. We report the first example of multiferroicity in centimeter-sized crystalline polymeric charge-transfer superstructures that grow at the liquid-air interface and are controlled by the regioregularity of the polymeric chain. The charge order-driven ferroic mechanism reveals spontaneous and hysteretic polarization and magnetization at the donor-acceptor interface. The charge transfer and ordering in the ferroic assemblies depend critically on the self-organizing and molecular packing of electron donors and acceptors. The invention described here not only represents a new coupling mechanism of magnetic and electric ordering but also creates a new class of emerging all-organic nanoferronics.

Effect of Molecular Stacking on Exciton Diffusion in Crystalline Organic Semiconductors

Exciton diffusion is at the heart of most organic optoelectronic devices' operation, and it is currently the most limiting factor to their achieving high efficiency. It is deeply related to molecular organization, as it depends on intermolecular distances and orbital overlap. However, there is no clear guideline for how to improve exciton diffusion with regard to molecular design and structure. Here, we use single-crystal charge-transfer interfaces to probe favorable exciton diffusion. Photoresponse measurements on interfaces between perylenediimides and rubrene show a higher photocurrent yield (+50%) and extended spectral coverage (+100 nm) when there is increased dimensionality of the percolation network and stronger orbital overlap. This is achieved by very short interstack distances in different directional axes, which favors exciton diffusion by a Dexter mechanism. Even if the core of the molecule shows strong deviation from planarity, the similar electrical resistance of the different systems, planar and nonplanar, shows that electronic transport is not compromised. These results highlight the impact of molecular organization in device performance and the necessity of optimizing it to take full advantage of the materials' properties.

Self-assembly of functional molecules into 1D crystalline nanostructures

Self-assembled functional nanoarchitectures are employed as important nanoscale building blocks for advanced materials and smart miniature devices to fulfill the increasing needs of high materials usage efficiency, low energy consumption, and high-performance devices. One-dimensional (1D) crystalline nanostructures, especially molecule-composed crystalline nanostructures, attract significant attention due to their fascinating infusion structure and functionality which enables the easy tailoring of organic molecules with excellent carrier mobility and crystal stability. In this review, we discuss the recent progress of 1D crystalline self-assembled nanostructures of functional molecules, which include both a small molecule-derived and a polymer-based crystalline nanostructure. The basic principles of the molecular structure design and the process engineering of 1D crystalline nanostructures are also discussed. The molecular building blocks, self-assembly structures, and their applications in optical, electrical, and photoelectrical devices are overviewed and we give a brief outlook on crucial issues that need to be addressed in future research endeavors.

Fluorination of metal phthalocyanines: single-crystal growth, efficient N-channel organic field-effect transistors, and structure-property relationships

The fluorination of p-type metal phthalocyanines produces n-type semiconductors, allowing the design of organic electronic circuits that contain inexpensive heterojunctions made from chemically and thermally stable p- and n-type organic semiconductors. For the evaluation of close to intrinsic transport properties, high-quality centimeter-sized single crystals of F16CuPc, F16CoPc and F16ZnPc have been grown. New crystal structures of F16CuPc, F16CoPc and F16ZnPc have been determined. Organic single-crystal field-effect transistors have been fabricated to study the effects of the central metal atom on their charge transport properties. The F16ZnPc has the highest electron mobility (~1.1 cm(2) V(-1) s(-1)). Theoretical calculations indicate that the crystal structure and electronic structure of the central metal atom determine the transport properties of fluorinated metal phthalocyanines.

25th anniversary article: organic electronics marries photochromism: generation of multifunctional interfaces, materials, and devices

Organic semiconductors have garnered significant interest as key components for flexible, low-cost, and large-area electronics. Hitherto, both materials and processing thereof seems to head towards a mature technology which shall ultimately meet expectations and efforts built up over the past years. However, by its own organic electronics cannot compete or complement the silicon-based electronics in integrating multiple functions in a small area unless novel solutions are brought into play. Photochromic molecules are small organic molecules able to undergo reversible photochemical isomerization between (at least) two (meta)stable states which exhibit markedly different properties. They can be embedded as additional component in organic-based materials ready to be exploited in devices such as OLEDs, OFETs, and OLETs. The structurally controlled incorporation of photochromic molecules can be done at various interfaces of a device, including the electrode/semiconductor or dielectric/semiconductor interface, or even as a binary mixture in the active layer, in order to impart a light responsive nature to the device. This can be accomplished by modulating via a light stimulus fundamental physico-chemical properties such as charge injection and transport in the device.

Laser-induced selective crosslinking for scaling the heterointerfacial domain in polymer blends

Organic blends containing heterojunction structures at the interfacial phase have been applied extensively in organic optoelectronic devices to modify charge transfer, separation, and recombination processes. Scaling and controlling the transition domains at the hetero-interface are of crucial importance for deep insights into the involved physics and for architecturing the devices with improved performance. However, it is difficult to recognize and characterize these transition domains directly using the conventional microscopic techniques, in particular when different molecules are dissolved in the same solvent with equal solubility. In this work, we introduce a technique defined as laser-induced selective cross-linking to isolate the interfacial phase from other phases into a directly measurable practicity. Thus, the hetero-domains become visualized and directly measurable. Based on the insolubility of the selectively cross-linked molecules in organic solvents, a lift-off process may remove the uncross-linked or incompletely cross-linked molecules, so that the hetero-domain is more clearly visualized and more precisely measured. A transition domain in a scale of about 200 nm is resolved in the F8BT/PFB blend film between their respectively rich phases after the selective cross-linking of the F8BT molecules by a blue laser. Furthermore, hetero-crosslinking between F8BT and PFB molecules was also resolved by both microscopic and near-field spectroscopic investigations.