Small-molecule Organic Semiconductors Research Articles

The charge transport properties of dicyanomethylene-functionalised violanthrone derivatives

The study of organic small molecule semiconductor materials as active components of organic electronic devices continues to attract considerable attention due to the range of advantages these molecules can offer. Here, we report the synthesis of three dicyanomethylene-functionalised violanthrone derivatives (3a, 3b and 3c) featuring different alkyl substituents. It is found that the introduction of the electron-deficient dicyanomethylene groups significantly improves the optical absorption compared to their previously reported precursors 2a–c. All compounds are p-type semiconductors with low HOMO–LUMO gaps (≈1.25 eV). The hole mobilities, measured from fabricated organic field-effect transistors, range from 3.6 × 10−6 to 1.0 × 10−2 cm2 V−1 s−1. We found that the compounds featuring linear alkyl chains (3b and 3c) displayed a higher mobility compared to the one with branched alkyl chains, 3a. This could be the result of the more highly disordered packing arrangement of this molecule in the solid state, induced by the branched side chains that hinder the formation of π–π stacking interactions. The influence of dicyanomethylene groups on the charge transport properties was most clearly observed in compound 3b which has a 60-fold improvement in mobility compared to 2b. This study demonstrates that the choice of the solubilising group has a profound effect on the hole mobility on these organic semiconductors.

The efficient method for searching stable structures in herringbone-phase organic semiconductors using density functional theory

We present the herringbone structure search (HSS) method to predict the crystal structures of organic semiconductors from molecular structural formulas. The charge transport efficiency of small-molecule organic semiconductors is governed by molecular packing, often in a two-dimensional herringbone structure. This method predicts crystal structures within the herringbone framework using the density functional theory. As a demonstration, we successfully applied it to promising molecules, predicting crystal structures that agreed well with experimental data. This method offers an efficient approach for in silico screening of new organic molecules, aiding the development of high-performance organic semiconductors.

Friction Force Mapping of Molecular Ordering and Mesoscopic Phase Transformations in Layered-Crystalline Organic Semiconductor Films.

It is critical to understand molecular ordering processes in small-molecule organic semiconductor (OSC) films in optimizing electronic device applications, although it is difficult to observe and investigate the ordering characteristics at a mesoscopic or device scale. Here, we report that friction force microscopy (FFM) allows visualizing the ordering transformation process from a thermodynamically metastable phase to a stable phase at a mesoscopic scale. We utilized 2-octyl-benzothieno[3,2-b]naphtho[2,3-b]thiophene (2-C8-BTNT) as a typical highly layered-crystalline OSC. We found that the friction force between an AFM tip and spin-coated OSC films significantly depends on whether local film states are in metastable monolayer phase or stable bilayer-type herringbone (b-LHB) phase that exhibits high carrier mobility. The formation of the stable b-LHB phase leads to lower friction than the metastable monolayer phase, clearly visualizing the molecular order. Force map (Fmap) analysis indicates that the lower friction in the b-LHB phase should be associated with the reduction of interfacial adhesion force. Notably, the observed results demonstrate that the spin-coated thin film changes from continuous film with the monolayer phase to rugged microcrystal grains with the b-LHB phase when left at ambient conditions. By contrast, an appropriate post-thermal annealing process facilitates the phase transformation without inducing such morphological changes. The technique provides a unique and effective tool for revealing the relationship between processing conditions and device performance in polycrystalline OSC films.

Automated Scanning Dielectric Microscopy Toolbox for Operando Nanoscale Electrical Characterization of Electrolyte‐Gated Organic Transistors

AbstractElectrolyte‐gated organic transistors (EGOTs) leveraging organic semiconductors' electronic and ionic transport characteristics are the key enablers for many biosensing and bioelectronic applications that can selectively sense, record, and monitor different biological and biochemical processes at the nanoscale and translate them into macroscopic electrical signals. Understanding such transduction mechanisms requires multiscale characterization tools to comprehensively probe local electrical properties and link them with device behavior across various bias points. Here, an automated scanning dielectric microscopy toolbox is demonstrated that performs operando in‐liquid scanning dielectric microscopy measurements on functional EGOTs and carries out extensive data analysis to unravel the evolution of local electrical properties in minute detail. This paper emphasizes critical experimental considerations permitting standardized, accurate, and reproducible data acquisition. The developed approach is validated with EGOTs based on blends of organic small molecule semiconductor and insulating polymer that work as accumulation‐mode field‐effect transistors. Furthermore, the degradation of local electrical characteristics at high gate voltages is probed, which is apparently driven by the destruction of local crystalline order due to undesirable electrochemical swelling of the organic semiconducting material near the source electrode edge. The developed approach paves the way for systematic probing of EGOT‐based technologies for targeted optimization and fundamental understanding.

37‐3: Donor‐Acceptor Alignment and Charge Separation in Small Molecule Organic Semiconductors

Understanding complex molecular interactions in OLED materials such as donor‐acceptor charge transfer are crucial for device efficiency. Utilizing simulation tools as a virtual microscope, we investigate how fundamental molecular properties, especially Coulomb interactions, impact charge transfer dynamics and overall device performance. Our findings reveal that molecular packing and microelectrostatic environments significantly influence charge transfer binding energy, underscoring the importance of molecular arrangement in material design. These insights, difficult to obtain experimentally, are pivotal in formulating design guidelines for high‐performance organic compounds, thereby advancing our understanding of OLED molecular interactions and aiding in the development of more efficient and durable devices.

Organic Electronics in Flexible Devices: Engineering Strategy, Synthesis, And Application

In recent years, flexible electronics have made significant strides, especially in organic light-emitting diodes, photovoltaics, thin-film transistors, integrated circuits, sensors, and memories. This study investigates the pivotal role of organic electronics in shaping flexible and wearable technology. It explores key aspects such as conductive polymers, small molecule organic semiconductors, and the use of organic dyes and pigments in displays, alongside synthesis techniques like polymerization and vapor deposition. These methods highlight advances in flexibility engineering and integration into wearable devices, enabling innovations like textile-integrated electronics and ultra-thin, skin-like interfaces. The discussion also covers synthesis and fabrication strategies, emphasizing cost-effective, solution-processing techniques for manufacturing electrodes, interconnects, and metal contacts on various flexible substrates. Advanced materials like graphene polymer composites, carbon-nanotube composites, and polymer-ceramic composites are investigated for enhancing stretchability and flexibility in device architectures. Furthermore, the paper examines the evolution of flexible devices, focusing on design strategies for flexibility and stretchability, innovative engineering polymers, composites, and device architectures, including 3D integration. Integration techniques in wearable devices, such as textile integration and skin-like electronics, are discussed, showcasing their transformative potential in applications like health monitoring devices, smart clothing, and wearable displays. Finally, the paper proposed solutions to address challenges such as durability, stability, scalability, and manufacturing efficiency, emphasizing the ongoing quest for improved materials and manufacturing processes. The conclusion underscores the transformative potential of organic electronics, envisioning a new era of wearable and flexible technology seamlessly integrated into daily life, revolutionizing our interactions with electronic devices.

Machine learning-assisted chemical space generation of small molecule organic semiconductors for efficient photodetectors

Organic photodetectors (OPDs), the emerging next-generation candidates for light sensing applications, could compensate deficiencies of the traditional inorganic photodetectors. To get OPDs of highly improved performance, and broader applications, machine learning (ML) technique is employed. A chemical space of 20,000 organic semiconductors (OSCs) by RDKit via BRICKS (Breaking Retrosynthetically Interesting Chemical Substructures) method was achieved, from the selected building blocks of the OSC data obtained from the literature, contributing to big data. Out of these, top twenty OSCs were screened based on synthetic accessibility score, chemical similarity analysis, and structural behavior understanding.

A Novel Non-Fullerene D-A Interface with Two Asymmetrical Electron Acceptors Facilitates Charge and Energy Transfer for Effective Carbon Dioxide Reduction.

Converting carbon dioxide (CO2) into high-value chemicals using solar energy remains a formidable challenge. In this study, the CSC@PM6:IDT6CN-M:IDT8CN-M non-fullerene small-molecule organic semiconductor is designed with highly efficient electron donor-acceptor (D-A) interface for photocatalytic reduction of CO2. Atomic Force Microscope and Transmission Electron Microscope images confirmed the formation of an interpenetrating fibrillar network after combination of donor and acceptor. The CO yield from the CSC@PM6:IDT6CN-M:IDT8CN-M reached 1346µmolg-1h-1, surpassing those of numerous reported inorganic photocatalysts. The D-A structure effectively facilitated charge separation to enable electrons transfer from the PM6 to IDT6CN-M:IDT8CN-M. Meanwhile, attributing to the dipole moments of the strong intermolecular interactions between IDT6CN-M and IDT8CN-M, the intermolecular forces are enhanced, and laminar stacking and π-π stacking are strengthened, thereby reinforcing energy transfer between acceptor molecules and significantly enhanced charge separation. Moreover, the strong internal electric field in the D-A interface enhanced the excited state lifetime of PM6:IDT6CN-M:IDT8CN-M. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis demonstrated that carboxylate (COOH*) is the predominant intermediate during CO2 reduction, and possible pathways of CO2 reduction to CO are deduced. This study presents a novel approach for designing materials with D-A interface to achieve high photocatalytic activity.

LiTFSI-Free Hole Transport Materials for Robust Perovskite Solar Cells and Modules with High Efficiencies.

Lithium bis(trifluoromethane) sulfonamide (LiTFSI) and oxygen-doped organic semiconductors have been frequently used to achieve record power conversion efficiencies of perovskite solar cells (PSCs). However, this conventional doping process is time-consuming and leads to poor device stability due to the incorporation of Li ions. Herein, aiming to accelerate the doping process and remove the Li ions, we report an alternative p-doping process by mixing a new small-molecule organic semiconductor, N2,N2,N7,N7-tetrakis (4-methoxyphenyl)-9-(4-(octyloxy) phenyl)-9H carbazole-2,7-diamine (labeled OH44) and its preoxidized form OH44+(TFSI-). With this method, a champion efficiency of 21.8% has been achieved for small-area PSCs, which is superior to the state-of-the-art EH44 and comparable with LiTFSI and oxygen-doped spiro-OMeTAD. Moreover, the stability of OH44-based PSCs is improved compared with those of EH44, maintaining more than 85% of its initial efficiency after aging in an ambient condition without encapsulation for 1000 h. In addition, we achieved efficiencies of 14.7 and 12.6% for the solar modules measured with a metal mask of 12.0 and 48.0 cm2, respectively, which demonstrated the scalability of this method.

Influence of Excess Conjugated Wrapping Polymer in Semiconducting Single-Walled Carbon Nanotube Dispersions

Single-walled carbon nanotubes (SWNTs) are promising nanomaterials for incorporation into organic electronic devices (OEDs), with the potential to fabricate flexible devices while exploiting inexpensive solution-processing techniques. As-synthesized SWNTs are insoluble and comprised of a mixture of metallic and semiconducting SWNTs (sc-SWNTs), necessitating dispersal and purification before integration into OEDs. Refinement of conjugated polymer extraction techniques has allowed for the isolation of sc-SWNTs from metallic in a reproducible and scalable manner. The availability of highly-pure sc-SWNT materials has facilitated the production of thin-film transistors (TFTs) with very high charge carrier mobilities, outperforming organic small molecule and polymer semiconductors. However, the realization of commercial OED applications of polymer-sorted sc-SWNTs have not yet been achieved, partially due to the prohibitive time and materials costs associated with purifying sc-SWNTs.Current protocols for dispersing sc-SWNTs with conjugated polymers involve three broad steps: (1) dispersion of bulk SWNT material, (2) removal of non-dispersed carbonaceous materials, and (3) removal of excess polymer through filtration or centrifugation. The final step of removal of excess polymer is time-consuming and wasteful, but viewed as necessary for preparing high-performing TFTs, as the conjugated polymer has much lower performance compared to SWNTs.In this study we performed the first systematic investigation of the effect of excess polymer on SWNT TFT performance. Three SWNT concentrations were investigated, with varying ratios of excess polymer added to each. TFT device performance was monitored using several metrics, including: mobility, threshold voltage, on/off ratios and hysteresis. Characterization of large numbers of replicate TFT devices determined that below a threshold amount of excess polymer the presence of excess polymer did not have a negative impact on device performance. Detailed analysis of the sc-SWNT films through Raman spectroscopy and atomic force microscopy (AFM) confirmed that a simple rinsing step was sufficient to remove all the unbound conjugated polymer from the substrate surface without affecting the sc-SWNT network. The volume of solvent required for the rinsing step was substantially lower than that required for filtration or centrifugation steps. Furthermore, at higher SWNT concentrations the excess polymer prevented nanotube bundling, resulting in moderate improvements in both mobility and on/off ratios. Our results were reproducible for two different conjugated polymer sc-SWNT systems, demonstrating the versatility of this procedure.

TCAD simulation of organic field-effect transistors based on spray-coated small molecule organic semiconductor with an insulating polymer blend

TCAD simulation of organic field-effect transistors based on spray-coated small molecule organic semiconductor with an insulating polymer blend

Nanocrystal Array Engineering and Optoelectronic Applications of Organic Small-Molecule Semiconductors.

Organic small-molecule semiconductor materials have attracted extensive attention because of their excellent properties. Due to the randomness of crystal orientation and growth location, however, the preparation of continuous and highly ordered organic small-molecule semiconductor nanocrystal arrays still face more challenges. Compared to organic macromolecules, organic small molecules exhibit better crystallinity, and therefore, they exhibit better semiconductor performance. The formation of organic small-molecule crystals relies heavily on weak interactions such as hydrogen bonds, van der Waals forces, and π-π interactions, which are very sensitive to external stimuli such as mechanical forces, high temperatures, and organic solvents. Therefore, nanocrystal array engineering is more flexible than that of the inorganic materials. In addition, nanocrystal array engineering is a key step towards practical application. To resolve this problem, many conventional nanocrystal array preparation methods have been developed, such as spin coating, etc. In this review, the typical and recent progress of nanocrystal array engineering are summarized. It is the typical and recent innovations that the array of nanocrystal array engineering can be patterned on the substrate through top-down, bottom-up, self-assembly, and crystallization methods, and it can also be patterned by constructing a series of microscopic structures. Finally, various multifunctional and emerging applications based on organic small-molecule semiconductor nanocrystal arrays are introduced.

Highly Ordered Small Molecule Organic Semiconductor Thin-Films Enabling Complex, High-Performance Multi-Junction Devices.

Organic semiconductors have opened up many new electronic applications, enabled by properties like flexibility, low-cost manufacturing, and biocompatibility, as well as improved ecological sustainability due to low energy use during manufacturing. Most current devices are made of highly disordered thin-films, leading to poor transport properties and, ultimately, reduced device performance as well. Here, we discuss techniques to prepare highly ordered thin-films of organic semiconductors to realize fast and highly efficient devices as well as novel device types. We discuss the various methods that can be implemented to achieve such highly ordered layers compatible with standard semiconductor manufacturing processes and suitable for complex devices. A special focus is put on approaches utilizing thermal treatment of amorphous layers of small molecules to create crystalline thin-films. This technique has first been demonstrated for rubrene─an organic semiconductor with excellent transport properties─and extended to some other molecular structures. We discuss recent experiments that show that these highly ordered layers show excellent lateral and vertical mobilities and can be electrically doped to achieve high n- and p-type conductivities. With these achievements, it is possible to integrate these highly ordered layers into specialized devices, such as high-frequency diodes or completely new device principles for organics, e.g., bipolar transistors.

Are Coarse-Grained Structures as Good as Atomistic Ones for Calculating the Electronic Properties of Organic Semiconductors?

The quality of amorphous molecular morphologies obtained with a recently introduced coarse-grained model, representing molecules in terms of connected anisotropic beads (Phys. Chem. Chem. Phys. 2019, 21, 26195), is benchmarked against reference atomistic data. Typical small-molecule organic semiconductors in their pristine and doped forms are chosen as a challenging and technologically relevant case study for our comparison, which includes both structural features and the resulting electronic properties, such as charge carrier energy levels, energetic disorder, and intermolecular charge transfer couplings. Our analysis shows that our accurate coarse-grained model leads to molecular glasses that are very similar to native atomistic samples, with the discrepancy being further reduced upon back-mapping. The electronic properties computed for back-mapped morphologies are almost indistinguishable from the atomistic reference, especially for multibranched poly(hetero)cyclic hydrocarbons usually employed as organic semiconductors. This study provides a proof of principle for highly accurate large-scale simulations of complex molecular systems at a reduced computational cost.

Sensitive photodetection below silicon bandgap using quinoid-capped organic semiconductors.

High-sensitivity organic photodetectors (OPDs) with strong near-infrared (NIR) photoresponse have attracted enormous attention due to potential applications in emerging technologies. However, few organic semiconductors have been reported with photoelectric response beyond ~1.1 μm, the detection limit of silicon detectors. Here, we extend the absorption of organic small-molecule semiconductors to below silicon bandgap, and even to 0.77 eV, through introducing the newly designed quinoid-terminals with high Mulliken-electronegativity (5.62 eV). The fabricated photodiode-type NIR OPDs exhibit detectivity (D*) over 1012 Jones in 0.41 to 1.2 μm under zero bias with a maximum of 2.9×1012 Jones at 1.02 μm, which is the highest D* for reported OPDs in photovoltaic-mode with response spectra beyond 1.1 μm. The high D* in 0.9 to 1.2 μm is comparable to those of commercial InGaAs photodetectors, despite the detection limit of our OPDs is shorter than InGaAs (~1.7 μm). A spectrometer prototype with a wide measurable region (0.4 to 1.25 μm) and NIR imaging under 1.2-μm illumination are demonstrated successfully in OPDs.

3D-Printed Organic Conjugated Trimer for Visible-Light-Driven Photocatalytic Applications.

Small molecule organic semiconductors (SMOSs) have emerged as a new class of photocatalysts that exhibit visible light absorption, tunable bandgap, good dispersion, and solubility. However, the recovery and reusability of such SMOSs in consecutive photocatalytic reactions is challenging. This work concerns a 3D-printed hierarchical porous structure based on an organic conjugated trimer, named EBE. Upon manufacturing, the photophysical and chemical properties of the organic semiconductor are maintained. The 3D-printed EBE photocatalyst shows a longer lifetime (11.7 ns) compared to the powder-state EBE (1.4 ns). This result indicates a microenvironment effect of the solvent (acetone), a better dispersion of the catalyst in the sample, and reduced intermolecular π-π stacking, which results in improved separation of the photogenerated charge carriers. As a proof-of-concept, the photocatalytic activity of the 3D-printed EBE catalyst is evaluated for water treatment and hydrogen production under sun-like irradiation. The resulting degradation efficiencies and hydrogen generation rates are higher than those reported for the state-of-the-art 3D-printed photocatalytic structures based on inorganic semiconductors. The photocatalytic mechanism is further investigated, and the results suggest that hydroxyl radicals (HO⋅) are the main reactive radicals responsible for the degradation of organic pollutants. Moreover, the recyclability of the EBE-3D photocatalyst is demonstrated in up to 5 uses. Overall, these results indicate the great potential of this 3D-printed organic conjugated trimer for photocatalytic applications.

Recent Research Progress of Organic Small-Molecule Semiconductors with High Electron Mobilities.

Organic electronics has made great progress in the past decades, which is inseparable from the innovative development of organic electronic devices and the diversity of organic semiconductor materials. It is worth mentioning that both of these great advances are inextricably linked to the development of organic high-performance semiconductor materials, especially the representative n-type organic small-molecule semiconductor materials with high electron mobilities. The n-type organic small molecules have the advantages of simple synthesis process, strong intermolecular stacking, tunable molecular structure, and easy to functionalize structures. Furthermore, the n-type semiconductor is a remarkable and important component for constructing complementary logic circuits and p-n heterojunction structures. Therefore, n-type organic semiconductors play an extremely important role in the field of organic electronic materials and are the basis for the industrialization of organic electronic functional devices. This review focuses on the modification strategies of organic small molecules with high electron mobility at molecular level, and discusses in detail the applications of n-type small-molecule semiconductor materials with high mobility in organic field-effect transistors, organic light-emitting transistors, organic photodetectors, and gas sensors.

Applying of C8-BTBT-Based EGOFETs at Different pH Values of the Electrolyte

Electrolyte-gated organic field-effect transistors (EGOFETs) is a popular platform for numerous sensing and biosensing applications in aqueous media. In this work, the variation of electrical characteristics of EGOFETs based on small-molecule organic semiconductor C8-BTBT and polystyrene blend in water solutions at different pH values was investigated. A positive shift of the threshold voltage with near-Nernstian pH sensitivity was demonstrated in the pH range from 4.9 to 2.8, while no measurable pH dependence in the range from 4.9 to 8.6 pH was registered. These results indicate chemical doping of the molecules of organic semiconductors by protons from the electrolyte in the acidic region. In order to check the applicability of the EGOFETs in a flow mode, a flow chamber was designed and assembled. The preliminary results obtained in the flow mode measurements showed a fast response to pH variation.

Insights into the kinetics and self-assembly order of small-molecule organic semiconductor/quantum dot blends during blade coating.

Organic-inorganic nanocomposite films formed from blends of small-molecule organic semiconductors and colloidal quantum dots are attractive candidates for high efficiency, low-cost solar energy harvesting devices. Understanding and controlling the self-assembly of the resulting organic-inorganic nanocomposite films is crucial in optimising device performance, not only at a lab-scale but for large-scale, high-throughput printing and coating methods. Here, in situ grazing incidence X-ray scattering (GIXS) gives direct insights into how small-molecule organic semiconductors and colloidal quantum dots self-assemble during blade coating. Results show that for two blends separated only by a small difference in the structure of the small molecule forming the organic phase, crystallisation may proceed down two distinct routes. It either occurs spontaneously or is mediated by the formation of quantum dot aggregates. Irrespective of the initial crystallisation route, the small-molecule crystallisation acts to exclude the quantum dot inclusions from the growing crystalline matrix phase. These results provide important fundamental understanding of structure formation in nanocomposite films of organic small molecules and colloidal quantum dots prepared via solution processing routes. It highlights the fundamental difference to structural evolution which can be made by seemingly small changes in system composition. It provides routes for the structural design and optimisation of solution-processed nanocomposites that are compatible with the large-scale deposition manufacturing techniques that are crucial in driving their wider adoption in energy harvesting applications.

Two multifunctional benzoquinone derivatives as small molecule organic semiconductors for bulk heterojunction and perovskite solar cells

Two novel quinone derivatives (NN3 and NN4) were synthesized in this work and they were characterized to be used as small organic semiconductor molecules in different types of photovoltaic applications. To make accessible compounds, three simple steps were followed to prepare NN3 and NN4 compounds. Furthermore, energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were determined for the computationally optimized models of the investigated compounds. The obtained optical and electrochemical results of this work indicated that NN3 and NN4 compounds were good candidates for application in the fields of bulk heterojunction (BHJ) and perovskite solar cells. Indeed, investigating new energy resources has been seen an important topic of research for producing clean energies and portable storage systems.