Mapping Energy Levels for Organic Heterojunctions

An organic semiconductor thin film is a solid‐state matter comprising one or more molecules. For applications in electronics and photonics, several distinct functional organic thin films are stacked together to create a variety of devices such as organic light‐emitting diodes and organic solar cells. The energy levels at these thin‐film junctions dictate various electronic processes such as the charge transport across these junctions, the exciton dissociation rates at donor–acceptor molecular interfaces, and the charge trapping during exciton formation in a host–dopant system. These electronic processes are vital to a device's performance and functionality. To uncover a general scientific principle in governing the interface energy levels, highest occupied molecular orbitals, and vacuum level dipoles, herein a comprehensive experimental research is conducted on several dozens of organic–organic heterojunctions representative of various device applications. It is found that the experimental data map on interface energy levels, after correcting variables such as molecular orientation‐dependent ionization energies, consists of three distinct regions depending on interface fundamental physical parameters such as Fermi energy, work function, highest occupied molecular orbitals, and lowest unoccupied molecular orbitals. This general energy map provides a master guide in selection of new materials for fabricating future generations of organic semiconductor devices.     

Scalable Fabrication of Highly Crystalline Organic Semiconductor Thin Film by Channel‐Restricted Screen Printing toward the Low‐Cost Fabrication of High‐Performance Transistor Arrays

Control over the morphology and crystallinity of small‐molecule organic semiconductor (OSC) films is of key importance to enable high‐performance organic optoelectronic devices. However, such control remains particularly challenging for solution‐processed OSC devices because of the complex crystallization kinetics of small‐molecule OSC materials in the dynamic flow of inks. Here, a simple yet effective channel‐restricted screen‐printing method is reported, which uses small‐molecule OSCs/insulating polymer to yield large‐grained small‐molecule OSC thin‐film arrays with good crystallization and preferred orientation. The use of cross‐linked organic polymer banks produces a confinement effect to trigger the outward convective flow at two sides of the channel by the fast solvent evaporation, which imparts the transport of small‐molecule OSC solutes and promotes the growth of small‐molecule OSC crystals parallel to the channel. The small‐molecule OSC thin‐film array produced by screen printing exhibits excellent performance characteristics with an average mobility of 7.94 cm² V^?1 s^?1 and a maximum mobility of 12.10 cm² V^?1 s^?1, which are on par with its single crystal. Finally, screen printing can be carried out using a flexible substrate, with good performance. These demonstrations bring this robust screen‐printing method closer to industrial application and expand its applicability to various flexible electronics.     

Controlling Molecular Doping in Organic Semiconductors

The field of organic electronics thrives on the hope of enabling low‐cost, solution‐processed electronic devices with mechanical, optoelectronic, and chemical properties not available from inorganic semiconductors. A key to the success of these aspirations is the ability to controllably dope organic semiconductors with high spatial resolution. Here, recent progress in molecular doping of organic semiconductors is summarized, with an emphasis on solution‐processed p‐type doped polymeric semiconductors. Highlighted topics include how solution‐processing techniques can control the distribution, diffusion, and density of dopants within the organic semiconductor, and, in turn, affect the electronic properties of the material. Research in these areas has recently intensified, thanks to advances in chemical synthesis, improved understanding of charged states in organic materials, and a focus on relating fabrication techniques to morphology. Significant disorder in these systems, along with complex interactions between doping and film morphology, is often responsible for charge trapping and low doping efficiency. However, the strong coupling between doping, solubility, and morphology can be harnessed to control crystallinity, create doping gradients, and pattern polymers. These breakthroughs suggest a role for molecular doping not only in device function but also in fabrication—applications beyond those directly analogous to inorganic doping.     

卟啉类光电功能材料的研究进展

卟啉及其衍生物是一类具有优良的光电性能的有机半导体材料．引起人们广泛的关注。本文对卟啉类光电材料在模拟生物光合作用中心的光致电荷转移和能量转移，有机太阳能电池．分子光电器件。有机电致发光和光存储等领域的研究进展做一简要介绍。     

Transferable Organic Semiconductor Nanosheets for Application in Electronic Devices

A method has been developed to stabilize and transfer nanofilms of functional organic semiconductors. The method is based on crosslinking of their topmost layers by low energy electron irradiation. The films can then be detached from their original substrates and subsequently deposited onto new solid or holey substrates retaining their structural integrity. Grazing incidence X‐ray diffraction, X‐ray specular reflectivity, and UV–Vis spectroscopy measurements reveal that the electron irradiation of ≈50 nm thick pentacene films results in crosslinking of their only topmost ≈5 nm (3–4 monolayers), whereas the deeper pentacene layers preserve their pristine crystallinity. The electronic performance of the transferred pentacene nanosheets in bottom contact field‐effect devices is studied and it is found that they are fully functional and demonstrate superior charge injection properties in comparison to the pentacene films directly grown on the contact structures by vapor deposition. The new approach paves the way to integration of the organic semiconductor nanofilms on substrates unfavorable for their direct growth as well as to their implementation in hybrid devices with unusual geometries, e.g., in devices incorporating free‐standing sheets.     

Universal Strategy for Efficient Electron Injection into Organic Semiconductors Utilizing Hydrogen Bonds

Molecular n‐dopants that can lower the electron injection barrier between organic semiconductors and electrodes are essential in present‐day organic electronics. However, the development of stable molecular n‐dopants remains difficult owing to their low ionization potential, which generally renders them unstable. It is shown that the stable bases widely used in organic synthesis as catalysts can lower the electron injection barrier similar to that in conventional n‐doping in organic optoelectronic devices. In contrast to conventional n‐doping, which is based on the electron transfer from dopants with low ionization potential, the reduction of the injection barrier caused by adding bases is determined by the formation of hydrogen bonds between the hosts and the bases, providing energy‐level‐independent electron injection. The observation of the efficient electron injection induced by hydrogen bonding affords new perspectives on the method for controlling the behavior of electrons unique to organic semiconductors.     

Isotope Substitution Extends the Lifetime of Organic Molecules in Transmission Electron Microscopy

Structural characterisation of individual molecules by high‐resolution transmission electron microscopy (HRTEM) is fundamentally limited by the element and electron energy‐specific interactions of the material with the high energy electron beam. Here, the key mechanisms controlling the interactions between the e‐beam and C–H bonds, present in all organic molecules, are examined, and the low atomic weight of hydrogen—resulting in its facile atomic displacement by the e‐beam—is identified as the principal cause of the instability of individual organic molecules. It is demonstrated theoretically and proven experimentally that exchanging all hydrogen atoms within molecules with the deuterium isotope, and therefore doubling the atomic weight of the lightest atoms in the structure, leads to a more than two‐fold increase in the stability of organic molecules in the e‐beam. Substitution of H for D significantly reduces the amount of kinetic energy transferred from the e‐beam to the atom (main factor contributing to stability) and also increases the barrier for bond dissociation, primarily due to the changes in the zero‐point energy of the C–D vibration (minor factor). The extended lifetime of coronene‐d₁₂, used as a model molecule, enables more precise analysis of the inter‐molecular spacing and more accurate measurement of the molecular orientations.     

Stretchable Organic Semiconductor Devices

Stretchable electronics are essential for the development of intensely packed collapsible and portable electronics, wearable electronics, epidermal and bioimplanted electronics, 3D surface compliable devices, bionics, prosthesis, and robotics. However, most stretchable devices are currently based on inorganic electronics, whose high cost of fabrication and limited processing area make it difficult to produce inexpensive, large‐area devices. Therefore, organic stretchable electronics are highly attractive due to many advantages over their inorganic counterparts, such as their light weight, flexibility, low cost and large‐area solution‐processing, the reproducible semiconductor resources, and the easy tuning of their properties via molecular tailoring. Among them, stretchable organic semiconductor devices have become a hot and fast‐growing research field, in which great advances have been made in recent years. These fantastic advances are summarized here, focusing on stretchable organic field‐effect transistors, light‐emitting devices, solar cells, and memory devices.     

Triple Acceptors in a Polymeric Architecture for Balanced Ambipolar Transistors and High‐Gain Inverters

The exploration of novel molecular architectures is crucial for the design of high‐performance ambipolar polymer semiconductors. Here, a “triple‐acceptors architecture” strategy to design the ambipolar polymer DPP‐2T‐DPP‐TBT is introduced. The utilization of this architecture enables DPP‐2T‐DPP‐TBT to achieve deep‐lying highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels of ?5.38/?4.19 eV, and strong intermolecular interactions, which are favorable for hole/electron injection and intermolecular hopping through π‐stacking. All these factors result in excellent ambipolar transport characteristics and promising applications in complementary‐like circuits for DPP‐2T‐DPP‐TBT under ambient conditions with high hole/electron mobilities and a gain value of up to 3.01/3.84 cm² V^?1 s^?1 and 171, respectively, which are among the best performances in ambipolar polymer organic thin‐film transistors and associated complementary‐like circuits, especially in top‐gate device configuration with low‐cost glass as substrates. These results demonstrate that the “triple‐acceptors architecture” strategy is an effective way for designing high‐performance ambipolar polymer semiconductors.     

Direct Imaging of the Induced‐Fit Effect in Molecular Self‐Assembly

Molecular recognition is a crucial driving force for molecular self‐assembly. In many cases molecules arrange in the lowest energy configuration following a lock‐and‐key principle. When molecular flexibility comes into play, the induced‐fit effect may govern the self‐assembly. Here, the self‐assembly of dicyanovinyl‐hexathiophene (DCV6T) molecules, a prototype specie for highly efficient organic solar cells, on Au(111) by using low‐temperature scanning tunneling microscopy and atomic force microscopy is investigated. DCV6T molecules assemble on the surface forming either islands or chains. In the islands the molecules are straight—the lowest energy configuration in gas phase—and expose the dicyano moieties to form hydrogen bonds with neighbor molecules. In contrast, the structure of DCV6T molecules in the chain assemblies deviates significantly from their gas‐phase analogues. The seemingly energetically unfavorable bent geometry is enforced by hydrogen‐bonding intermolecular interactions. Density functional theory calculations of molecular dimers quantitatively demonstrate that the deformation of individual molecules optimizes the intermolecular bonding structure. The intermolecular bonding energy thus drives the chain structure formation, which is an expression of the induced‐fit effect.     

Effects of crystal structures and intermolecular interactions on charge transport properties of organic semiconductors

In this study, the effects of the packing configuration and intermolecular interaction on the transport properties are investigated based on density functional theory. Molecular design from the standpoint of a quantum-chemical view is helpful to engender favorable molecular packing motifs. The transfer integral along the orientation with π–π overlap is much larger than other directions without π–π overlap, and the mobility along this orientation is higher than that along other directions. The intermolecular interaction analyses demonstrate that hydrogen bonds play a crucial role with strong electrostatic interactions in charge transfer. There will be a synergistic relationship when the π–π stacking and intermolecular interaction coexist in the same direction. It turns out that intermolecular interactions are responsible for charge transport, while π–π stacking interactions dominate donor–acceptor transport. Incorporating the understanding of the molecular packing motifs and intermolecular interactions into the design of organic semiconductors can assist in the development of novel materials.     

Semiconductive Single Molecular Bilayers Realized Using Geometrical Frustration

A unique solution‐based technology to manufacture self‐assembled ultrathin organic‐semiconductor layers with ultrauniform single‐molecular‐bilayer thickness over an area as large as wafer scale is developed. A novel concept is adopted in this technique, based upon the idea of geometrical frustration, which can effectively suppress the interlayer stacking (or multilayer crystallization) while maintaining the assembly of the intralayer, which originates from the strong intermolecular interactions between π‐conjugated molecules. For this purpose, a mixed solution of extended π‐conjugated frameworks substituted asymmetrically by alkyl chains of variable lengths (i.e., (πCore)‐C_n's) is utilized for the solution process. A simple blade‐coating with a solution containing two (πCore)‐C_n's with different alkyl chain lengths is effective to provide single molecular bilayers (SMBs) composed of a pair of polar monomolecular layers, which is analogical to the cell membranes of living organisms. It is demonstrated that the chain‐length disorder does not perturb the in‐plane crystalline order, but acts effectively as a geometrical frustration to inhibit multilayer crystallization. The uniformity, stability, and size scale are unprecedented, as produced by other conventional self‐assembly processes. The obtained SMBs also exhibit efficient 2D carrier transport as organic thin‐film transistors. This finding should open a new route to SMB‐based ultrathin superflexible electronics.     

Nanopatched Graphene with Molecular Self‐Assembly Toward Graphene–Organic Hybrid Soft Electronics

Increasing the mechanical durability of large‐area polycrystalline single‐atom‐thick materials is a necessary step toward the development of practical and reliable soft electronics based on these materials. Here, it is shown that the surface assembly of organosilane by weak epitaxy forms nanometer‐thick organic patches on a monolayer graphene surface and dramatically increases the material's resistance to harsh postprocessing environments, thereby increasing the number of ways in which graphene can be processed. The nanopatched graphene with the improved mechanical durability enables stable operation when used as transparent electrodes of wearable strain sensors. Also, the nanopatched graphene applied as an electrode modulates the molecular orientation of deposited organic semiconductor layers, and yields favorable nominal charge injection for organic transistors. These results demonstrate the potential for use of self‐assembled organic nanopatches in graphene‐based soft electronics.     

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.     

A New Architecture for Fibrous Organic Transistors Based on a Double‐Stranded Assembly of Electrode Microfibers for Electronic Textile Applications

Herein, a unique device architecture is proposed for fibrous organic transistors based on a double‐stranded assembly of electrode microfibers for electronic textile applications. A key feature of this work is that the semiconductor channel of the fiber transistor comprises a twist assembly of the source and drain electrode microfibers that are coated by an organic semiconductor. This architecture not only allows the channel dimension of the device to be readily controlled by varying the thickness of the semiconductor layer and the twisted length of the two electrode microfibers, but also passivates the device without affecting interconnections with other electrical components. It is found that the control of crystalline nanostructure of the semiconductor layer is critical for improving both the production yield of the device and the charge‐carrier transport in the device. The resulting fibrous organic transistors show a high output current of over ?5 mA at a low operation voltage of ?1.3 V and a good on/off current ratio of 10⁵. The device performance is maintained after repeated bending deformation and washing with a strong detergent solution. Application of the fibrous organic transistors to switch current‐driven LED devices and detection of electrocardiography signals from a human body are demonstrated.     

Epitaxially Grown Strained Pentacene Thin Film on Graphene Membrane

Organic‐graphene system has emerged as a new platform for various applications such as flexible organic photovoltaics and organic light emitting diodes. Due to its important implication in charge transport, the study and reliable control of molecular packing structures at the graphene–molecule interface are of great importance for successful incorporation of graphene in related organic devices. Here, an ideal membrane of suspended graphene as a molecular assembly template is utilized to investigate thin‐film epitaxial behaviors. Using transmission electron microscopy, two distinct molecular packing structures of pentacene on graphene are found. One observed packing structure is similar to the well‐known bulk‐phase, which adapts a face‐on molecular orientation on graphene substrate. On the other hand, a rare polymorph of pentacene crystal, which shows significant strain along the c‐axis, is identified. In particular, the strained film exhibits a specific molecular orientation and a strong azimuthal correlation with underlying graphene. Through ab initio electronic structure calculations, including van der Waals interactions, the unusual polymorph is attributed to the strong graphene–pentacene interaction. The observed strained organic film growth on graphene demonstrates the possibility to tune molecular packing via graphene–molecule interactions.     

Large Modulation of Charge Carrier Mobility in Doped Nanoporous Organic Transistors

Molecular doping of organic electronics has shown promise to sensitively modulate important device metrics. One critical challenge is the disruption of structure order upon doping of highly crystalline organic semiconductors, which significantly reduces the charge carrier mobility. This paper demonstrates a new method to achieve large modulation of charge carrier mobility via channel doping without disrupting the molecular ordering. Central to the method is the introduction of nanopores into the organic semiconductor thin films via a simple and robust templated meniscus‐guided coating method. Using this method, the charge carrier mobility of C₈‐benzothieno[3,2‐b]benzothiophene transistors is boosted by almost sevenfold. This paper further demonstrates enhanced electron transport by close to an order of magnitude in a diketopyrrolopyrrole‐based donor–acceptor polymer. Combining spectroscopic measurements, density functional theory calculations, and electrical characterizations, the doping mechanism is identified as partial‐charge‐transfer induced trap filling. The nanopores serve to enhance the dopant/organic semiconductor charge transfer reaction by exposing the π‐electrons to the pore wall.     

Enhancing Light Emission in Interface Engineered Spin‐OLEDs through Spin‐Polarized Injection at High Voltages

The quest for a spin‐polarized organic light‐emitting diode (spin‐OLED) is a common goal in the emerging fields of molecular electronics and spintronics. In this device, two ferromagnetic (FM) electrodes are used to enhance the electroluminescence intensity of the OLED through a magnetic control of the spin polarization of the injected carriers. The major difficulty is that the driving voltage of an OLED device exceeds a few volts, while spin injection in organic materials is only efficient at low voltages. The fabrication of a spin‐OLED that uses a conjugated polymer as bipolar spin collector layer and ferromagnetic electrodes is reported here. Through a careful engineering of the organic/inorganic interfaces, it is succeeded in obtaining a light‐emitting device showing spin‐valve effects at high voltages (up to 14 V). This allows the detection of a magneto‐electroluminescence (MEL) enhancement on the order of a 2.4% at 9 V for the antiparallel (AP) configuration of the magnetic electrodes. This observation provides evidence for the long‐standing fundamental issue of injecting spins from magnetic electrodes into the frontier levels of a molecular semiconductor. The finding opens the way for the design of multifunctional devices coupling the light and the spin degrees of freedom.     

Fluorination Triggered New Small Molecule Donor Materials for Efficient As‐Cast Organic Solar Cells

Solution‐processable small molecules (SMs) have attracted intense attention due to their definite molecular structures, less batch‐to‐batch variation, and easier structure control. Herein, two new SM donors based on substituted isatin unit (DI3T, DI3T‐2F) are synthesized and applied as electron donors with the mixture of PC₇₁BM to construct organic photovoltaics. As a result, 5,6‐difluoro isatin derivative (DI3T‐2F) obtains a power conversion efficiency of 7.80% by a simple solution spin‐coating fabrication process without any additives, solvent, or thermal annealing process. More intuitively, due to stronger intermolecular interaction and higher hole mobility after the incorporation of fluorine atoms in end units, the devices present good tolerance to active layer thickness. The results indicate that DI3T‐2F shows promising potential for large‐scale printing processes and flexible application of efficient small molecule organic solar cells.     

Cysteine–Ag Cluster Hydrogel Confirmed by Experimental and Numerical Studies

The native cysteine (Cys)‐Ag₃ cluster hydrogel is approved for the first time by both experimental and theoretical studies. From the detailed molecular structure and energy information, three factors are found to ensure the self‐assembly of Cys and Ag₃, and result in the hydrogel. First, the Ag–S bonds make Cys and Ag₃ form Cys‐Ag₃‐Cys monomer. Second, intermolecular hydrogen bonds between carboxyl groups of adjacent monomer push them self‐assembled. Third, more monomer precisely self‐assemble to produce the –[Cys‐Ag₃‐Cys]_n multimer, e.g., a single molecular chain with the left‐handed helix conformation, via a benign thermodynamic process. These multimers entangle together to form micro‐network to trap water and produce hydorgel in situ. The hydrogen bonds of hydrogel are sensitive to thermal and proton stimuli, and the hydrogel presents lysosome targeting properties via fluorescent imaging with biocompatibility.