Thiophene/phenylene Co-oligomers Research Articles

Supramolecular Transistors with Quantum Interference Effect.

The charge transport through supramolecular junctions exhibits unique quantum interference (QI) effects, which provide an opportunity for the design of supramolecular transistors. Benefiting from the configuration dependence of QI, configuration control of the supramolecular assemblies to demonstrate the QI features is a key but challenging step. In this work, we fabricated the supramolecular transistors and investigated the charge transport through the conducting channel of the individual π-stacked thiophene/phenylene co-oligomers (TPCOs) using the electrochemically gated scanning tunneling microscope break junction technique. We controlled the configuration of the supramolecular channel and switched the QI features between the anti-resonance and resonance states of the supramolecular channels. We observed the supramolecular transistor with its on/off ratio above 103 (∼1300), a high gating efficiency of ∼165 mV/dec, a low off-state leakage current of ∼30 pA, and the channel length scaled down to <2.0 nm. Density functional theory calculations suggested that the QI features in π-stacked TPCOs vary depending on the supramolecular architecture and can be manipulated efficiently by fine-tuning the supramolecular configurations. This work reveals the potential of the supramolecular channels for molecular electronics and provides a fundamental understanding of intermolecular charge transport.

Effect of fluorination on properties of thiophene-phenylene co-oligomers with annulated core: a DFT study

Organic light-emitting transistors are a new type of optoelectronic devices that combine the functionality of OLED and a transistor that controls it. The working layer of these devices requires organic semiconductors that combine high charge mobility with a high photoluminescence quantum yield. One of the promising classes of such materials are thiophene-phenylene co-oligomers, the properties of which can be tuned over a wide range by adding various substituents. In this work, we address the effect of fluorination on the properties of two model thiophene-phenylene co-oligomers with an annulated central fragment, P–TTA–P and P–BTBT–P. It is shown that fluorination of both molecules lowers their frontier orbitals energy levels, and this should enable electron transport in their crystals and films. At the same time, fluorination has a qualitatively different effect on the delocalization of frontier orbitals, the width of the optical gap, the oscillator strength, the exciton binding energy, and the Raman spectrum, which is explained by the difference in the equilibrium geometry of fluorinated molecules. It is expected that the revealed relationships between the structure and properties of the studied compounds will contribute to the rational design of organic semiconductors for efficient light emitting devices.

Effect of Fluorination on Properties of Thiophene–Phenylene Co-Oligomers with Annulated Core: A DFT Study

Effect of Fluorination on Properties of Thiophene–Phenylene Co-Oligomers with Annulated Core: A DFT Study

Novel low-bandgap donor–acceptor thiophene-phenylene co-oligomers for light-emitting semiconductor devices

Thiophene-phenylene co-oligomers (TPCOs) have shown their high potential for organic light-emitting devices because of their high luminescence and efficient charge transport. However, unsubstituted TPCOs have relatively wide optical bandgaps and the high-lying lowest unoccupied molecular orbital (LUMO) energies so that efficient electron transport is a challenge. Electron-withdrawing groups (EWGs) and fluorinated fragments embedded into the TPCO molecule structure could result in the lower LUMO energy and narrower optical bandgap. Here, we report the synthesis of two novel TPCOs series with either phenylene or perfluorinated phenylene central core and end-capped with various EWGs (aldehyde, 2-ethylhexyl cyanoacetate, hexyl rhodanine and dicyanorhodanine) and with long alkyl terminal and side chains increasing the solubility. All the oligomers synthesized were found to be thermally stable and crystalline materials with relatively low LUMO energies (down to −3.50 eV), narrow bandgaps (down to 1.9 eV), and efficient photoluminescence in the green – deep red spectral regions both in solution and solid-state. The TPCOs with 2-ethylhexyl cyanoacetate EWG were crystallized in large-area single-crystal monolayers, which showed strongly polarized photoluminescence and demonstrated their high potential as active layers in solution-processed single-layer organic light-emitting transistors.

Polarity Switching in Organic Electronic Devices via Terminal Substitution of Active-Layer Molecules

Organic electronic devices often suffer from poor charge injection limiting their performance. Specifically, high-performance electronic devices usually need Ohmic contacts, but it is not easy to realize them in junctions of the organic semiconductor with the electrodes because of the contact problems. In this work, polarity switching in organic field-effect transistors (OFETs) that is independent of the electrode work function is demonstrated─the switching of charge injection from the p-type to ambipolar and to the n-type─via modification of the donor/acceptor character of the molecular terminal substituents. By using three thiophene–phenylene co-oligomers with the same conjugated core, similar crystal packings, but different terminal substituents (methyl, trimethylsilyl, and trifluoromethyl), the polarity switching in both thin-film and single-crystal OFETs is demonstrated. The ultraviolet photoelectron spectroscopy studies and electronic structure calculations justify a definitive role of the interface dipole stemming from the terminal groups in controlling the heights of charge injection barriers and hence in charge injection into the OFET active layer. The results obtained are expected to facilitate rational design of organic semiconductors for high-performance electronic devices.

Optically pumped lasing in a single crystal cavity of thiophene/phenylene co-oligomers grown via improved crystal growth methods in solution

Single crystals of thiophene/phenylene co-oligomers (TPCOs) are promising organic laser media. Owing to their poor solubility in solvents, single crystals of TPCOs have conventionally been grown using sublimation processes. However, the morphology of the grown crystals is not ideal for lasing by the self-cavity effect. In this report, single crystals of 5,5‴-bis(4-biphenylyl)-2,2′:5′,2″:5″,2‴-quaterthiophene (BP4T), which is one of the longest TPCOs, are grown through improved methods in solution to achieve lasing with a well-shaped single crystal cavity. The high quality factors (5.9 × 103) and group refractive indexes (>3.5) suggest that the obtained crystals are superior organic laser media.

Conduction band electronic states of ultrathin thiophene-phenylene co-oligomer and substituted biphenyl films on the surface of layer-by-layer grown ZnO

The results of studying the electronic states of the conduction band and interface potential barrier during the formation of ultrathin films of thiophene-phenylene co-oligomer CH3-phenylene-thiophene-thiophene-phenylene -CH3(CH3-PTTP-CH3) on the surface of ZnO and films of biphenyl tetracarboxylic dianhydride (BPDA) on the ZnO surface are presented. A 100 nm thick ZnO layer was prepared by atomic layer deposition (ALD). Organic CH3-PTTP-CH3 films and BPDA films up to 8 nm thick were formed by thermal vacuum deposition. During film deposition, the electronic characteristics of the surface were studied using total current spectroscopy (TCS) in the energy range from 5 eV to 20 eV above EF. In this energy range, the structure of the maxima of the unoccupied electronic states of CH3-PTTP-CH3 and BPDA films was determined. As a result of the CH3-PTTP-CH3 film deposition, a decrease in the work function to 4.0 eV was found, compared with the value of the work function of 4.2 eV measured from the ALD ZnO-substrate. This corresponds to the transfer of a negative charge from the CH3-PTTP-CH3 film to the substrate. The charge transfer at the interface between the BPDA film and the ALD ZnO-substrate occurs in the opposite direction, since a 4.7 eV increase of the work function was registered during the formation of this interface. The CH3-PTTP-CH3 and BPDA films studied and the layer-by-layer grown ZnO film represent a continuous coating on sufficiently large surface areas of the order of 10 μmx10 μm. The roughness of the ZnO surface does not exceed 4 nm, and the surface roughness of CH3-PTTP-CH3 and BPDA films was 10-15 nm. Keywords: thiophene-phenylene co-oligomers, biphenyl tetracarboxylic dianhydride, ultrathin films, ZnO, atomic layer deposition method, electronic properties, low-energy electron spectroscopy, interface potential barrier.

Luminescent 2D single crystals of thiophene–phenylene co-oligomers for field-effect devices

Photo/electroluminescent and semiconductor properties of macroscopic 2D single crystals are presented, and the impact of phenylene insertion is investigated.

Unoccupied Electron States of Ultrathin Films of Thiophene–Phenylene Cooligomers on the Surface of Polycrystalline Gold

Unoccupied electronic states in the energy range from 5 to 20 eV above the Fermi level have been studied in ultrathin films of dimethyl-substituted thiophene–phenylene cooligomers CH3-phenylene–thiophene–thiophene–phenylene–CH3 (CH3–PTTP–CH3) on polycrystalline gold surfaces of two types: the ex situ Au layer thermally deposited in a special chamber and the in situ Au surface prepared inside an analytical chamber. The film structure is studied by the X-ray diffraction (XRD) method. The formation of a superposition of the amorphous phase and the crystalline phase with period 3.8 nm is discussed. The energy positions of the maxima of the unoccupied electronic states and the character of formation of the boundary potential barrier have been studied by the total current spectroscopy (TCS). The structures of the FSTCS maxima of the 5–7-nm-thick CH3–PTTP–CH3 films are not different when using various types of Au substrates and the ZnO semiconductor surface prepared by atomic layer deposition (ALD). As a CH3–PTTP–CH3 layer is deposited on the ex situ Au and in situ Au surfaces, the electron work function increases insignificantly (by ~0.1 eV) as the coating thickness increases to 5–7 nm. At such thicknesses of the CH3–PTTP–CH3 films, the electron work function is 4.7 ± 0.1 eV in the case of the ex situ Au substrate and 4.9 ± 0.1 eV in the case of the in situ Au substrate. A possible influence of the processes of physicochemical interaction at the boundary between the film and the substrate on the formation of the boundary potential barrier in the structures under study is discussed.

Comment on “Cooperative Behaviors in Amplified Emission from Single Microcrystals of Thiophene/Phenylene Co‐Oligomers toward Organic Polariton Laser”

AbstractYanagi et al. recently reviewed their progress in the field of amplified emission of thiophene/phenylene co‐oligomers (TPCO) single crystals (Adv. Opt. Mater. 2019, 7, 1900136), which are a promising platform for highly efficient organic light‐emitting devices. In this comment, the most intriguing experimental data from this paper are naturally and easily explained as an effect of unintentional impurities or molecular self‐dopants that appear in the course of chemical synthesis of TPCO.

Fluorinated Thiophene-Phenylene Co-Oligomers for Optoelectronic Devices.

Organic optoelectronics requires materials combining bright luminescence and efficient ambipolar charge transport. Thiophene-phenylene co-oligomers (TPCOs) are promising highly emissive materials with decent charge-carrier mobility; however, they typically show poor electron injection in devices, which is usually assigned to high energies of their lowest unoccupied molecular orbitals (LUMOs). A widely used approach to lower the frontier orbitals energy levels of a conjugated molecule is its fluorination. In this study, we synthesized three new fluorinated derivatives of one of the most popular TPCOs, 2,2'-(1,4-phenylene)bis[5-phenylthiophene] (PTPTP) and studied them by cyclic voltammetry, absorption, photoluminescence, and Raman spectroscopies. The obtained data reveal a positive effect of fluorination on the optoelectronic properties of PTPTP: LUMO levels are finely tuned, and photoluminescence quantum yield and absorbance are increased. We then grew crystals from fluorinated PTPTPs, resolved their structures, and showed that fluorination dramatically affects the packing motif and facilitates π-stacking. Finally, we fabricated thin-film organic field-effect transistors (OFETs) and demonstrated a strong impact of fluorination on charge injection/transport for both types of charge carriers, namely, electrons and holes. Specifically, balanced ambipolar charge transport and electroluminescence were observed only in the OFET active channel based on the partially fluorinated PTPTP. The obtained results can be extended to other families of conjugated oligomers and highlight the efficiency of fluorination for rational design of organic semiconductors for optoelectronic devices.

Незаполненные электронные состояния ультратонких пленок тиофен-фенилен со-олигомеров на поверхности поликристаллического золота

The results of studying unoccupied electronic states in the energy range from 5 eV to 20 eV above the Fermi level of ultrathin films of dimethyl-substituted thiophene-phenylene co-oligomers CH3-phenylene-thiophene-thiophene-phenylene-CH3 (CH3-PTTP-CH3) are presented. The films were deposited on two types of surface of polycrystalline Au: ex situ Au layer thermally deposited in a separate chamber, and on the in situ Au surface prepared inside the analytical chamber. The structure of the films was studied by X-ray diffraction (XRD). The formation of a superposition of the amorphous and crystalline phases with a period of 3.8 nm is discussed. Investigations of the energy positioning of the maxima of unoccupied electronic states and of the process of the formation of the potential boundary barrier were carried out by the method of total current spectroscopy (TCS). The maxima of the fine structure of the total current spectra (FSTCS) of the CH3-PTTP-CH3 film 5–7 nm thick did not differ when using different types of Au substrates and the surface of the ZnO semiconductor prepared by the atomic layer deposition (ALD) method. When the CH3-PTTP-CH3 layer was deposited both on the ex situ Au surface and on the in situ Au surface, a slight (about 0.1 eV) increase in the electronic work function was observed with an increase in the coating thickness to 5–7 nm. At such CH3-PTTP-CH3 film thicknesses, the electron work function was determined as 4.7 ± 0.1 eV for the ex situ Au substrate and 4.9 ± 0.1 eV for the in situ Au substrate. The possible influence of the processes of physicochemical interaction at the film – substrate interface on the formation of the potential boundary barrier in the structures under study is discussed.

The Electron-Vibrational Interaction in a Thiophene—Phenylene Cooligomer and Its Relationship to the Raman Spectrum

Electron-vibrational interactions play a key role in limiting charge mobility in organic semi-conductors. This paper reports a theoretical study of the electron—phonon interaction in the 5,5′-diphenyl-2,2′-bitiophene (PTTP) molecule, which belongs to the class of thiophene-phenylene cooligomers, which are of great interest for organic optoelectronics due to their electron-transport and luminescent properties; these results are compared with anthracene, which is a model organic semiconductor. The contributions of various vibrational modes to the reorganization energy of PTTP and the anthracene molecules are revealed and it is shown that these contributions correlate with the intensities of the corresponding bands in the Raman spectrum. In particular, it is found that for the PTTP molecule the so-called I-mode with a frequency of ∼1460 cm−1, which corresponds to collective vibration of atoms of all oligomer units, has the highest intensity in both spectra. These results indicate the promise of Raman spectroscopy for studying electron-vibrational interactions in organic semiconductors. Finally, the mobility of holes in PTTP and anthracene is estimated in the framework of the jump model and the reasons for their difference are analyzed. Based on these results, we propose some ways to reduce the electron-vibrational interaction in thiophene—phenylene cooligomers, which is important for the directional molecular design of organic semiconductors.

Surface-Enhanced Raman Spectroscopy of 2D Organic Semiconductor Crystals

Recently developed ultrathin two-dimensional (2D) organic semiconductor crystals are a promising platform for advanced organic electronic devices. Remarkable quality of such crystals results in charge-carrier mobilities comparable to those of bulk crystals, but their structure and orientation are hard to study because of their extremely small thickness. Here, we applied surface-enhanced Raman spectroscopy (SERS) to investigate the structure of the thinnest 2D single crystals—monolayers, which are based on thiophene-phenylene co-oligomers: 1,4-bis(5′-decyl-2,2′-bithiene-5-yl)benzene and 1,4-bis(5′-hexyl-2,2′-bithiene-5-yl)benzene. Their Raman spectra were calculated as a function of the molecule orientation and SERS microscopy maps were acquired. High sensitivity of SERS allowed us to study monolayer single-crystal domains with the optical spatial resolution. Raman anisotropy was used to probe the orientations of single-crystal domains and the molecule orientation within them. Notably, the SERS microscopy ...

Cooperative Behaviors in Amplified Emission from Single Microcrystals of Thiophene/Phenylene Co‐Oligomers toward Organic Polariton Laser

AbstractAfter successful commercialization of organic light‐emitting diodes (OLEDs), organic semiconductor lasers have become a practical research target for versatile optoelectronic applications. Realization of electrically pumped lasing with molecular materials faces critical issues such as exciton quenching by injected carriers, nonradiative triplet states, and difficulties in introduction of resonators. One of potential routes to overcome them is to utilize polaritonic emission based on cooperative coupling of photons with excitons and/or phonons in molecular crystals. This report overviews cooperative behaviors observed in amplified emissions from single‐crystal thiophene/phenylene co‐oligomers (TPCOs) and discusses their expected potential toward organic polariton lasers. First, amplified emission accompanied with unusual time delay, spectral splitting, and fluence dependence of lasing threshold is presented, suggesting superfluorescence from macroscopically correlated molecular excitons, not based on the conventional stimulated emission process. Second, selectively enhanced Raman scattering under off‐resonant condition is shown, which is interpreted as a result of contribution of coherent vibrations in the TPCO crystals. Then, angle‐resolved spectroscopies demonstrate a stable existence of exciton polaritons at room temperature in microcavity devices with TPCO crystals. It is concluded that all those unique behaviors are ascribed to the TPCO excitons and vibrations having large oscillator strength under the uniaxially oriented molecular alignment.

Impact of terminal substituents on the electronic, vibrational and optical properties of thiophene-phenylene co-oligomers.

Owing to the combination of efficient charge transport and bright luminescence, thiophene-phenylene co-oligomers (TPCOs) are promising materials for organic light-emitting devices such as diodes, transistors and lasers. The synthetic flexibility of TPCOs enables facile tuning of their properties. In this study, we address the effect of various electron-donating and electron-withdrawing symmetric terminal substituents (fluorine, methyl, trifluoromethyl, methoxy, tert-butyl, and trimethylsilyl) on frontier orbitals, charge distribution, static polarizabilities, molecular vibrations, bandgaps and photoluminescence quantum yields of 5,5'-diphenyl-2,2'-bithiophene (PTTP). By combining DFT calculations with cyclic voltammetry and absorption, photoluminescence, and Raman spectroscopies, we show that symmetric terminal substitution tunes the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energies of TPCOs within a range of ∼0.7 eV, shifts the frequencies of the vibrational modes associated with the phenyl rings, changes the photoluminescence quantum yield by about two-fold and slightly changes the bandgap by ∼0.1 eV. We demonstrate that these effects are governed by two factors: the Hammet constant of the substituents and their involvement in the π-conjugation/hyperconjugation described by the effective conjugation length of the substituted oligomer. A detailed picture underlying the effect of the terminal substituents on the electronic, vibrational and optical properties of TPCOs is presented. Overall, the unraveled relationships between the structure and the properties of the substituted PTTPs should facilitate a rational design of π-conjugated (co-)oligomers for efficient organic optoelectronic devices.

Molecular Self‐Doping Controls Luminescence of Pure Organic Single Crystals

AbstractOrganic optoelectronics calls for materials combining bright luminescence and efficient charge transport. The former is readily achieved in isolated molecules, while the latter requires strong molecular aggregation, which usually quenches luminescence. This hurdle is generally resolved by doping the host material with highly luminescent molecules collecting the excitation energy from the host. Here, a novel concept of molecular self‐doping is introduced in which a higher luminescent dopant emerges as a minute‐amount byproduct during the host material synthesis. As a one‐stage process, self‐doping is more advantageous than widely used external doping. The concept is proved on thiophene–phenylene cooligomers (TPCO) consisting of four (host) and six (dopant) conjugated rings. It is shown that &lt;1% self‐doping doubles the photoluminescence in the TPCO single crystals, while not affecting much their charge transport properties. The Monte‐Carlo modeling of photoluminescence dynamics reveals that host–dopant energy transfer is controlled by both excitonic transport in the host and host–dopant Förster resonant energy transfer. The self‐doping concept is further broadened to a variety of conjugated oligomers synthesized via Suzuki, Kumada, and Stille crosscoupling reactions. It is concluded that self‐doping combined with improved excitonic transport and host–dopant energy transfer is a promising route to highly luminescent semiconducting organic single crystals for optoelectronics.

Organic crystal transistors with organic/inorganic hybrid electrodes

ABSTRACTWe improved carrier injection of organic field-effect transistors (OFETs) by utilizing organic/inorganic hybrid electrodes. One of thiophene/phenylene co-oligomers (TPCOs) was employed as an organic semiconductor crystal layer and as a part of hybrid electrodes with Ag. As the volume ratio of Ag:TPCO changed from 1:0 to 1:3, the carrier mobility was enhanced up to ∼10−1 cm2/Vs even after exposure to the atmosphere. This value was 1.5 times larger than that of the OFET with electrodes made of only Ag, which suggests that the introduction of the hybrid electrodes improved the carrier injection from the electrode into the organic crystal.

Organic Nanowire Lasers with Epitaxially Grown Crystals of Semiconducting Oligomers

AbstractEpitaxially grown nanowires of semiconducting thiophene/phenylene co‐oligomers (TPCOs) were prepared by mask‐shadowing deposition on a cleaved (001) surface of a potassium chloride (KCl) crystal. Under optical pumping, single nanowires of both unsubstituted (p‐type) and cyano‐substituted (n‐type) TPCOs exhibited axial Fabry–Pérot (F‐P) lasing above a threshold fluence which decreased to 11–15 μJ cm−2 with increasing nanowire lengths. Their lying molecular orientation and stacking along the nanowire axis in both TPCOs are suitable for effective light propagation with transverse‐electric mode resulting in better lasing performances compared to two‐dimensional platelet crystals of TPCOs.

Enhanced performance of organic solar cells based on thiophene/phenylene co-oligomers

We investigated solar cells in which various thiophene/phenylene co-oligomers (TPCOs) were used as the major photoelectric-conversion component and obtained enhanced performance. These simple planar heterojunction solar cells consisted of TPCO as the p-type semiconductor and fullerenes as the n-type semiconductor. The variation in the highest occupied molecular orbital levels of the TPCOs caused systematic modulation of the open-circuit voltage (VOC) without incurring a large change in the short-circuit current. We obtained a maximum power-conversion efficiency (PCE) of 3.0%, with VOC reaching 1.0V. This PCE exceeds that obtained for a device containing zinc phthalocyanine, a conventional p-type photoelectric-conversion material.