Molecular Template Growth and Its Applications in Organic Electronics and Optoelectronics

Direct characterization of graphene doping state by in situ photoemission spectroscopy with Ar gas cluster ion beam sputtering

On the basis of an in situ photoemission spectroscopy (PES) system, we propose a novel, direct diagnosis method for the characterization of graphene (Gr) doping states at organic semiconductor (OSC)/electrode interfaces. Our in situ PES system enables ultraviolet/X-ray photoelectron spectroscopy (UPS/XPS) measurements during the OSC growth or removal process. We directly deposit C₆₀ films on three different p-type dopants-gold chloride (AuCl₃), (trifluoromethyl-sulfonyl)imide (TFSI), and nitric acid (HNO₃). We periodically characterize the chemical/electronic state changes of the C₆₀/Gr structures during their aging processes under ambient conditions. Depositing the OSC on the p-type doped Gr also prevents severe degradation of the electrical properties, with almost negligible transition over one month, while the p-type doped Gr without an OSC changes a lot following one month of aging. Our results indicate that the chemical/electronic structures of the Gr layer are completely reflected in the energy level alignments at the C₆₀/Gr interfaces. Therefore, we strongly believe that the variation of energy level alignments at the OSC/graphene interface is a key standard for determining the doping state of graphene after a certain period of aging.

Constructing Nanostructured Donor/Acceptor Bulk Heterojunctions via Interfacial Templates for Efficient Organic Photovoltaics

We demonstrate that a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)/diindenoperylene (PEDOT:PSS/DIP) interfacial bilayer could serve as a structural template to enable the morphological control of bulk heterojunctions (BHJs) by co-evaporation of tetraphenyldibenzoperiflanthene:fullerene (DBP:C₆₀), which greatly improves the device performances. Especially, we show that isolated crystalline domains of C₆₀ can be well-controlled at the nanoscale during the co-evaporation. Photoluminescence spectra indicate the realization of DIP/DBP cascade energy architecture, which significantly facilitates both the energy transfer and photocurrent generation. In addition, with bias-dependent external quantum efficiency analysis, we reveal that such a cascade energy device architecture greatly suppresses the energy recombination in both carrier and exciton transfer, resulting in a high open-circuit voltage and a high fill factor. By carefully optimizing the interfacial and BHJ layers, we achieved a high-performance organic photovoltaic cell with a power conversion efficiency of 5.0 ± 0.3%.

Impact of room temperature on pentacene thin film growth and electronic structure

The electronic structure and transport properties of organic thin films depend critically on the film structure. We have shown by means of ultraviolet photoelectron spectroscopy that the growth of vacuum-sublimed pentacene on highly oriented pyrolytic graphite substrates depends significantly on the room temperature and the substrate cleanness. At T = 291 K, all molecules adopt a flat lying orientation in the contact layer to a clean (vacuum-annealed) substrate, whereas at T = 303 K, some molecules are standing upright. On defect-rich substrates, the thin films are rather disordered for both temperatures and comprise standing and lying molecules.

Small Molecules Derived from Thieno[3,4-c]pyrrole-4,6-dione (TPD) and Their Use in Solution Processed Organic Solar Cells

In this work, microwave synthesis, chemical, optical and electrochemical characterization of three small organic molecules, TPA-TPD, TPA-PT-TPD and TPA-TT-TPD with donor-acceptor structure and their use in organic photovoltaic cells are reported. For the synthesis, 5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione was used as electron withdrawing fragment while the triphenylamine was used as electron donating fragment. Molecular electronic geometry and electronic distribution density were established by density functional theory (DFT) calculations and confirmed by optical and chemical characterization. These molecules were employed as electron-donors in the active layer for manufacturing bulk heterojunction organic solar cells, where [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) was used as electron-acceptor. As cathode, Field's metal (FM), an eutectic alloy (Bi/In/Sn: 32.5%, 51%, and 16.5%, respectively) with a melting point above 62 °C, was easily deposited by drop casting under vacuum-free process and at air atmosphere. Prepared devices based on TPA-TPD:PC71BM (1:4 w/w ratio) presented a large VOC = 0.97 V, with JSC = 7.9 mA/cm², a FF = 0.34, then, a power conversion efficiency (PCE) of 2.6%.

Monolayer-to-thin-film transition in supramolecular assemblies: the role of topological protection

The ability to control the transition from a two-dimensional (2D) monolayer to the three-dimensional (3D) molecular structure in the growth of organic layers on surfaces is essential for the production of functional thin films and devices. This has, however, proved to be extremely challenging, starting from the currently limited ability to attain a molecular scale characterization of this transition. Here, through innovative application of low-dose electron diffraction and aberration-corrected transmission electron microscopy (acTEM), combined with scanning tunneling microscopy (STM), we reveal the structural changes occurring as film thickness is increased from monolayer to tens of nanometers for supramolecular assembly of two prototypical benzenecarboxylic acids - terephthalic acid (TPA) and trimesic acid (TMA) - on graphene. The intermolecular hydrogen bonding in these molecules is similar and both form well-ordered monolayers on graphene, but their structural transitions with film thickness are very different. While the structure of TPA thin films varies continuously towards the 3D lattice, TMA retains its planar monolayer structure up to a critical thickness, after which a transition to a polycrystalline film occurs. These distinctive structural evolutions can be rationalized in terms of the topological differences in the 3D crystallography of the two molecules. The templated 2D structure of TPA can smoothly map to its 3D structure through continuous molecular tilting within the unit cell, whilst the 3D structure of TMA is topologically distinct from its 2D form, so that only an abrupt transition is possible. The concept of topological protection of the 2D structure gives a new tool for the molecular design of nanostructured films.

Using Femtosecond Laser Irradiation to Enhance the Vertical Electrical Properties and Tailor the Morphology of a Conducting Polymer Blend Film

We report femtosecond infrared laser-induced selective tailoring of carrier transport as well as surface morphology on a conducting polymer blend thin film. Maximal 2.4 times enhancement on vertical current transport in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester, was achieved by this irradiation. The laser irradiation induced a photo expansion without deteriorating its molecular structure and the film morphology could be customized in the micron scale by adjusting the laser writing parameters. In the photoexpanded region, the face-on populations were about 2.2 times larger in comparison with the pristine region, which was a major contributor to the enhanced carrier transport.

Charge transport in organic nanocrystal diodes based on rolled-up robust nanomembrane contacts

The investigation of charge transport in organic nanocrystals is essential to understand nanoscale physical properties of organic systems and the development of novel organic nanodevices. In this work, we fabricate organic nanocrystal diodes contacted by rolled-up robust nanomembranes. The organic nanocrystals consist of vanadyl phthalocyanine and copper hexadecafluorophthalocyanine heterojunctions. The temperature dependent charge transport through organic nanocrystals was investigated to reveal the transport properties of ohmic and space-charge-limited current under different conditions, for instance, temperature and bias.

Templating effects of tetrakis(thiadiazole)porphyrazine on the structure and optical properties of copper phthalocyanine thin films

Molecular templating is an attractive method to improve the crystallinity and control the molecular orientations of organic thin films. Here, we report on the templating effects of an organic [Formula: see text]-type semiconductor, tetrakis(thiadiazole)porphyrazine (H2TTDPz), on the structure and optical absorption of a [Formula: see text]-type semiconductor, copper phthalocyanine (CuPc). X-ray diffraction measurements for the double layer thin films, CuPc/H2TTDPz, indicate a face-on orientation of CuPc, which is replicating the structure of the H2TTDPz thin films, even though the CuPc thin films usually form edge-on-type thin films. The optical absorption measurements show new low-energy transitions in the templated CuPc films.

Frontiers of solvent-free functional molecular liquids

The breakthrough of functional molecular liquids (FMLs) in cutting-edge research and their fundamental liquid features on the basis of molecular architectures are highlighted in this Feature Article.

Electrochemically Organized Isolated Fullerene-Rich Thin Films with Optical Limiting Properties

Electrochemical assembly was applied directly to determine the aggregation of nanoclusters in isolated fullerene-rich (54-63 wt %) thin films. The electroactive reactions were achieved using electroactive carbazole and pyrene, which led to distinguishable nanoparticle-like and irregular cluster formations. These films, with amorphous and transparent states, showed good photoactivity and significant optical limiting response with an excellent threshold of 63 mJ cm(-2). This work thus paves a way to assemble highly isolated (or monodispersed) building blocks into thin films at the molecular level with control of the nanostructural formations through important molecular design.

Growth of Highly Oriented Ultrathin Crystalline Organic Microstripes: Effect of Alkyl Chain Length

The growth of organic semiconductor with controllable morphology is a crucial issue for achieving high-performance devices. Here we present the systematic study of the effect of the alkyl chain attached to the functional entity on controlling the growth of oriented microcrystals by dip-coating. Alkylated DTBDT-based molecules with variable chain lengths from n-butyl to n-dodecyl formed into one-dimensional micro- or nanostripe crystals at different pulling speeds. The alignment and ordering are significantly varied with alkyl chain length, as is the transistor performance. Highly uniform oriented and higher-molecular-order crystalline stripes with improved field-effect mobility can be achieved with an alkyl-chain length of around 6. We attribute this effect to the alkyl-chain-length-dependent packing, solubility, and self-assembly behavior.

Epitaxial Templating of C60 with a Molecular Monolayer

Commensurate epitaxial monolayers of truxenone on Cu (111) were employed to template the growth of monolayer and bilayer C60. Through the combination of STM imaging and LEED analysis we have demonstrated that C60 forms a commensurate 8 × 8 overlayer on truxenone/Cu (111). Bilayers of C60 retain the 8 × 8 periodicity of templated monolayers and although Kagome lattice arrangements are observed these are explained with combinations of 8 × 8 symmetry.

Growth and annealing kinetics of α-sexithiophene and fullerene C60mixed films

Thin films of α-sexithiophene (6T) and C60mixtures deposited on nSiO substrates at 303 and 373 K were investigated in real time andin situduring the film growth using X-ray diffraction. The mixtures are observed to contain the well known 6T low-temperature crystal phase and the β phase, which usually coexist in pure 6T films. The addition of C60modifies the structure to almost purely β-phase-dominated films if the substrate is at 303 K. In contrast, at 373 K the low-temperature crystal phase of 6T dominates the film growth of the mixtures. Post-growth annealing experiments up to 373 K on equimolar mixtures and pure 6T films were also performed and followed in real time with X-ray diffraction. Annealing of pure 6T films results in a strong increase of film ordering, whereas annealing of equimolar 6T:C60mixed films does not induce any significant changes in the film structure. These results lend further support to theories about the important influence of C60on the growth behaviour and structure formation process of 6T in mixtures of the two materials.

Highly-oriented molecular arrangements and enhanced magnetic interactions in thin films of CoTTDPz using PTCDA templates

In the present work, the templating effect of thin layers of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) on the growth of cobalt tetrakis(thiadiazole)porphyrazine (CoTTDPz) thin films was examined. X-ray diffraction and optical absorption spectra indicate that while CoTTDPz forms amorphous thin films on the bare substrates, it forms crystalline thin films on the PTCDA templates, in which the molecular planes of CoTTDPz are considered to be parallel to the substrates. Magnetic measurements reveal a significantly enhanced antiferromagnetic interaction of CoTTDPz in the templated thin films, with values reaching over 13 K. The ability to generate crystalline films and to control their orientation using molecular templates is an important strategy in the fields of organic electronics and spintronics in order to tailor the physical properties of organic thin films to suit their intended application.

Tailoring the Dielectric Layer Structure for Enhanced Performance of Organic Field-Effect Transistors: The Use of a Sandwiched Polar Dielectric Layer

To investigate the origins of hydroxyl groups in a polymeric dielectric and its applications in organic field-effect transistors (OFETs), a polar polymer layer was inserted between two polymethyl methacrylate (PMMA) dielectric layers, and its effect on the performance as an organic field-effect transistor (OFET) was studied. The OFETs with a sandwiched dielectric layer of poly(vinyl alcohol) (PVA) or poly(4-vinylphenol) (PVP) containing hydroxyl groups had shown enhanced characteristics compared to those with only PMMA layers. The field-effect mobility had been raised more than 10 times in n-type devices (three times in the p-type one), and the threshold voltage had been lowered almost eight times in p-type devices (two times in the n-type). The on-off ratio of two kinds of devices had been enhanced by almost two orders of magnitude. This was attributed to the orientation of hydroxyl groups from disordered to perpendicular to the substrate under gate-applied voltage bias, and additional charges would be induced by this polarization at the interface between the semiconductor and dielectrics, contributing to the accumulation of charge transfer.

Surface Reconstructions in Organic Crystals: Simulations of the Effect of Temperature and Defectivity on Bulk and (001) Surfaces of 2,2':6',2″-Ternaphthalene

2,2':6',2″-Ternaphthalene (NNN) is a novel, blue-emitting material, suitable for preparation of organic light-emitting diodes. Its crystal structure has been solved recently, but its thermal behavior and surface properties have not yet been explored, partly due to the difficulty in obtaining high quality crystals. In the present study we use classical molecular dynamics to investigate the thermal behavior of bulk and (001) surfaces of NNN. Our bulk simulations indicate the occurrence of a phase transition at about 600 K. The transition is facilitated by the presence of a free (001) surface, since a reconstruction leading to a very similar structure occurs around 550 K in our surface models. This holds for both ideal and defective surface models, containing a small number of vacancies (one or two missing molecules in the outermost layer). In all cases, the process is triggered by thermal motion and involves the reorientation of the molecules with respect to the (001) plane. Both the bulk and surface phases share the monoclinic space group P21/a with a herringbone disposition of molecules. These findings and their implications for the use of NNN in organic electronics are discussed.

Probing the chemical structure of monolayer covalent-organic frameworks grown via Schiff-base condensation reactions

XPS and STM analysis shows that covalent links formed within a surface-supported Schiff-base 2D-COF consist of a mixture of imine and hemiaminal groups.

Rhodium-catalyzed oxidative coupling of N-acyl anilines with alkynes using an acylamino moiety as the traceless directing group

A rhodium-catalyzed oxidative annulation of N-acyl anilines with alkynes was developed by using the acylamino group as a traceless directing group for the first time.

Heteroatom-connected ferrocenyl substituted naphthalimides

A family of heteroatom (oxygen, sulphur and nitrogen) connected ferrocenyl naphthalimides 3a–3f were designed and synthesized by the nucleophilic aromatic substitution and Buchwald coupling reactions.

A two excited state model to explain the peculiar photobehaviour of a flexible quadrupolar D–π–D anthracene derivative

The excited state deactivation of the investigated compound involves two distinct fluorescent states. ¹A* → ¹B* transition is accompanied by structural motions, with the ¹B* state exhibiting a certain ICT character.

Synthesis, photophysical, electrochemical, and DSSC application of novel donor–acceptor triazole bridged dendrimers with a triphenylamine core and benzoheterazole as a surface unit

Triazole bridged novel donor–acceptor dendrimers were synthesized via click chemistry. Lower generation dendrimers when used as additives exhibit better current-generating capacity and power conversion efficiency in DSSCs.

Molecular self-assembly on two-dimensional atomic crystals: insights from molecular dynamics simulations

van der Waals (vdW) epitaxy of ultrathin organic films on two-dimensional (2D) atomic crystals has become a sovereign area because of their unique advantages in organic electronic devices. However, the dynamic mechanism of the self-assembly remains elusive. Here, we visualize the nanoscale self-assembly of organic molecules on graphene and boron nitride monolayer from a disordered state to a 2D lattice via molecular dynamics simulation for the first time. It is revealed that the assembly toward 2D ordered structures is essentially the minimization of the molecule-molecule interaction, that is, the vdW interaction in nonpolar systems and the vdW and Coulomb interactions in polar systems that are the decisive factors for the formation of the 2D ordering. The role of the substrate is mainly governing the array orientation of the adsorbates. The mechanisms unveiled here are generally applicable to a broad class of organic thin films via vdW epitaxy.

Crystal structure of bis-[(phenyl-methanamine-κN)(phthalocyaninato-κ(4) N)zinc] phenyl-methan-amine tris-olvate

The asymmetric unit of the title compound, 2[Zn(C32H16N8)(C7H9N)]·3C7H9N, comprises two independent complex mol-ecules and three benzyl-amine solvent mol-ecules. Each complex mol-ecule features a penta-coordinated Zn(2+) ion within a square-pyramidal geometry, whereby the N5 donor set is defined by four atoms of the phthalocyaninate dianion (PC) and an N-bound benzyl-amine mol-ecule; it is the relative orientations of the latter that differentiate between the independent complex mol-ecules. The uncoordinated benzyl-amine mol-ecules display different conformations in the structure, with syn-Car-Car-Cm-N (ar = aromatic, m = methyl-ene) torsion angles spanning the range -28.7 (10) to 35.1 (14)°. In the crystal, N-H⋯N and N-H⋯π inter-actions lead to supra-molecular layers in the ab plane. The layers have a zigzag topology, have the coordinating and non-coordinating benzyl-amine mol-ecules directed to the inside, and present the essentially flat PC resides to the outside. This arrangement enables adjacent layers to associate via π-π inter-actions [inter-centroid distance between pyrrolyl and fused-benzene rings = 3.593 (2) Å] so that a three-dimensional architecture is formed.