Small-molecule ambipolar transistors

S-Heterocyclic s-Indacenodifluorene: Synthesis, Properties, and Thermally Tunable Ambipolar Field-Effect Mobility

Design of semiconducting materials with facile control of charge transport and the nature of charge carriers is essential for realizing niche applications with organic electronics. Described herein are the synthesis, crystal structure, and analysis of the electronic properties of a four-stage redox amphoteric S-heterocyclic s-indacenodifluorene 6, including the study of its ambipolar charge carrier mobility (µh and µe) in organic field-effect transistor (OFET) devices. Despite being electron-rich, our investigation revealed reversible reduction potentials for 6 in the cyclic voltammetry, which is attributed to the recovery of locally aromatic thiophene and cyclopentadienyl anion units upon electron injection for the two antiaromatic S-heterocyclic as-indacene units in accordance with the Glidewell-Lloyd rule of aromaticity. In line with this, we observed an interesting thermal tunability of the nature of charge carriers from p-type to balanced ambipolar to n-type charge transport with reasonable semiconductor mobility in all regimes of transport. This behavior is correlated with the modification of the transport levels upon annealing of the semiconductor and possible increase in the extent of π-electron delocalization with increasing temperature. This proof-of-concept tunability of the nature of charge transport indicates the efficacy of our molecular design.

Molecular Design and Alignment for Ambipolar SCLC Mobility in Self-Assembled Columnar Discogens

The future of next-generation electronics relies on low-cost organic semiconductors that are tailored to simultaneously provide all requisite optoelectronic properties, focusing greatly on ambipolar charge-transport and solution processability. In this regard, room-temperature discotic liquid crystals (DLCs) are potential candidates, where quasi-1D self-assembly affords a charge-transport channel along their columnar axis. This work shows a molecular design strategy by utilizing anthraquinone as the primary motif, surrounded by ester functionalized tri-alkoxy phenyl units to develop room-temperature DLCs (1.1-1.3). Here, the polar ester functionality stabilizes the columnar mesophase over a wide range through the involvement of dipole-dipole interaction along with the π-π stacking. Throughout the entire mesophase transition, reported compounds 1.1-1.3 exhibit a highly ordered 2D columnar oblique (Colob) self-assembly. Space charge limited current (SCLC) experiments reveal balanced ambipolar charge transport, with the maximum hole and electron mobilities of 5.04 and 4.93 cm2 V-1 s-1, respectively. From the conoscopic results, their propensity to align in a highly homeotropic fashion is demonstrated. It is further justified by the azimuthal plot corresponding to the (11) peak of grazing incidence small angle X-ray scattering (GISAXS), denoting the crucial role of the design and alignment for efficient movement of charge carriers in the material.

Effect of Molecular Structure on the B3LYP-Computed HOMO-LUMO Gap: A Structure -Property Relationship Using Atomic Signatures

Compounds possessing a small highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap (E gap) are highly desirable due to their instability and reactivity, making them useful for a wide range of applications. However, the search for new organic compounds with a low E gap is an expensive endeavor due to the exponentially increasing pool of virtual compounds. Accordingly, in this study, atomic Signatures were utilized as molecular descriptors to investigate the correlation between the molecular structure and the B3LYP-computed E gap, thus aiding in the development of a quantitative structure-property relationship (QSPR). An easy-to-use robust model was constructed using forward-stepping multilinear regression with leave-one-out cross validation, resulting in a regression coefficient (r 2) of 0.86 and a predictability (q 2) of 0.76. The use of atomic Signatures as molecular descriptors successfully inferred correlations between different structural motifs and E gap. The atomic fragments containing π-bonds in various aromatic compounds were found to be the most significant atomic Signatures, explaining nearly 50% of the variance in the data, with regression coefficients that decreased E gap. This is attributed to π-electron delocalization, making this molecular fragment a reactive site in a molecule. Finally, an external test set was used to further evaluate the model's predictive performance. The developed QSPR can be utilized as a reliable initial screening tool to identify potential candidates possessing low E gap values.

Molecular orientation of dielectric layers at indigo/dielectric interfaces impacts the ordering of indigo films in organic field-effect transistors

Organic multilayer systems, which are stacked layers of different organic materials, are used in various organic electronic devices such as organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs). In particular, OFETs are promising as key components in flexible electronic devices. In this study, we investigated how the inclusion of an insulating tetratetracontane (TTC) interlayer in ambipolar indigo-based OFETs can be used to alter the crystallinity and electrical properties of the indigo charge transport layer. We find that the inclusion of a 20-nm-thick TTC film thermally annealed at a low temperature of 70 °C acts to significantly increase the ambipolar electrical transport of the indigo layer. X-ray diffraction, atomic force microscopy, and vibrational sum frequency generation measurements showed that annealing the TTC film significantly improved its ordering. The electronic sum-frequency generation spectra of TTC/indigo bilayers show that this improved ordering of TTC films promotes the growth of crystalline indigo films that exhibit charge mobilities in OFET that are nearly an order of magnitude larger than those measured for devices grown on unannealed TTC layers. Furthermore, using vibrational sum-frequency generation spectroscopy, we found that pre-annealing the TTC layer prior to indigo deposition can suppress the formation of defects within the TTC layer during indigo film growth, which also contributes to enhanced charge transport. Our results highlight the importance of controlling the molecular ordering within the interlayer contacts in OFET structures to achieve an enhanced performance.

Probing the Influence of Alkyne Substitution on the Electronic and Magnetic Properties of Diindeno[1,2-b;1',2'-i]anthracenes

To further the ability to manipulate the properties of open-shell molecules, logical and incremental modifications to molecular structure are key steps that provide fine-tuning of established diradicaloid scaffolds. We report the synthesis of an electronically "pure" diradicaloid based on a 2,6-anthroquinoidal core where the once necessary ethynyl "wings" are removed. Through the simplification of the overall electronic structure, the singlet-triplet energy gap increases by 0.3-0.4 kcal mol-1 in the reported diradicaloids while avoiding significant disruption to their optoelectronic properties.

Highly Soluble Ambipolar anti-Perylene-3,4 : 9,10-bis(benzimidazole)s Stabilize a Room-Temperature Columnar Hexagonal Phase

A new series of highly soluble perylene anti-bis(4,5-dialkoxybenzimidazole)s bearing branched flexible chains stabilizing room temperature columnar hexagonal phase and with balanced ambipolar charge carrier mobility is reported for the first time. Only the anti isomer was successfully separated and characterized. These compounds have a high extinction coefficient, small optical band gap and wide absorption range, thus making them a promising class of ambipolar organic semiconductors capable of self-organizing.

Ambipolar Charge Transport in Organic Semiconductors: How Intramolecular Reorganization Energy Is Controlled by Diradical Character

The charged forms of π-conjugated chromophores are relevant in the field of organic electronics as charge carriers in optoelectronic devices, but also as energy storage substrates in organic batteries. In this context, intramolecular reorganization energy plays an important role in controlling material efficiency. In this work, we investigate how the diradical character influences the reorganization energies of holes and electrons by considering a library of diradicaloid chromophores. We determine the reorganization energies with the four-point adiabatic potential method using quantum-chemical calculations at density functional theory (DFT) level. To assess the role of diradical character, we compare the results obtained, assuming both closed-shell and open-shell representations of the neutral species. The study shows how the diradical character impacts the geometrical and electronic structure of neutral species, which in turn control the magnitude of reorganization energies for both charge carriers. Based on computed geometries of neutral and charged species, we propose a simple scheme to rationalize the small, computed reorganization energies for both n-type and p-type charge transport. The study is supplemented with the calculation of intermolecular electronic couplings governing charge transport for selected diradicals, further supporting the ambipolar character of the investigated diradicals.

Ambipolar Nickel Dithiolene Complex Semiconductors: From One- to Two-Dimensional Electronic Structures Based upon Alkoxy Chain Lengths

Air-stable single-component ambipolar organic semiconductors that conduct both holes and electrons are highly desired but have been rarely realized. Neutral nickel bis(dithiolene) complexes are promising candidates that fulfill the stringent electronic requirements of shallow HOMO levels and deep LUMO levels, which can reduce the carrier injection barrier to overcome the work function of gold electrodes and ensure air stability. However, most nickel bis(dithiolene) analogs that have been characterized as ambipolar semiconductors have twisted molecular structures that hinder the effective intermolecular interactions required for carrier conduction. To address this issue, we synthesized planar alkoxy-substituted nickel bis(dithiolene) analogs that facilitate dense packing with effective intermolecular interactions. Remarkably, changing the methoxy substituents to ethoxy or propoxy groups led to a dramatic change in the packing mode, from one-dimensional to herringbone-like, while maintaining effective intermolecular interactions. These materials overcome the usual trade-off between crystallinity and solubility; they are highly crystalline, even in their film forms, and are highly soluble in organic solvents. They are therefore readily solution-processable to form semiconducting layers with well-defined and well-ordered structures in field-effect transistors. Devices based on these compounds exhibited efficient ambipolar characteristics, even after several months of exposure to air, achieving high carrier mobilities of up to 10^-2 cm² V^-1 s^-1 and large on/off ratios of up to 10⁵, which are the top-class performances achieved for a single-component ambipolar semiconductor material driven in air.

Isomeric dibenzooctazethrene diradicals for high-performance air-stable organic field-effect transistors

Realizing both high-performance and air-stability is key to advancing singlet-diradical-based semiconductors to practical applications and realizing their material potential associated with their open-shell nature. Here a concise synthetic route toward two stable dibenzooctazethrene isomers, DBOZ1 and DBOZ2, was demonstrated. In the crystalline phase, DBOZ2 exhibits two-dimensional brick wall packing with a high degree of intermolecular electronic coupling, leading to a record-breaking hole mobility of 3.5 cm2 V-1 s-1 for singlet diradical transistors, while retaining good device stability in the ambient air.

Medium Diradical Character, Small Hole and Electron Reorganization Energies and Ambipolar Transistors in Difluorenoheteroles

Four difluorenoheteroles having a central quinoidal core with the heteroring varying as furan, thiophene, its dioxide derivative and pyrrole have shown to be medium character diradicals. Solid-state structures, optical, photophysical, magnetic, and electrochemical properties have been discussed in terms of diradical character, variation of aromatic character and captodative effects (electron affinity). Organic field-effect transistors (OFETs) have been prepared, showing balanced hole and electron mobilities of the order of 10^-3  cm²  V^-1  s^-1 or ambipolar charge transport which is first inferred from their redox amphoterism. Quantum chemical calculations show that the electrical behavior is originated from the medium diradical character which produces similar reorganization energies for hole and electron transports. The vision of a diradical as simultaneously bearing pseudo-hole and pseudo-electron defects might justify the reduced values of reorganization energies for both regimes. Structure-function relationships between diradical and ambipolar electrical behavior are revealed.