Annotation of the results obtained in 2020
Calculations of charge transport in single crystals of thiophene-phenylene co-oligomers (TFSO) were carried out. Using the hopping model of charge transport, we estimated the mobility of electrons and holes in three TPSOs for which electroluminescence (EL) was detected - DHex-TTPTT, DH-TTP(F)TT, TMS-PTTP-TMS (DHex is dihexyl, TMS is trimethylsilyl, T is thiophene, P is phenylene, P(F) is perfluorinated phenylene). The reorganization energies and the charge transfer integrals in the crystals of these compounds were calculated. It is shown that the fluorination of DHex-TTPTT leads to a decrease in the reorganization energy for both hole transfer and electron transfer, which should increase the charge mobility within the hopping model. On the other hand, the change in the crystal packing of DHex-TTPTT as a result of fluorination decreases the charge transfer integrals for electrons. As a result, the calculated hole mobility in DH-TTP (F) TT is higher than that in DH-TTPTT, and the electron mobility is lower. The equilibrium geometries, the energy levels of the frontier orbitals, the optical gap width, and the oscillator strength of the S0-S1 transition were calculated for three compounds with a condensed conjugated nucleus based on benzothieno[3,2-b][1]-benzothiophene (BTBT) and tetrathienoacene (TTA), studied in the project: 2T-BTBT-2T, DHex-P-TTA and DHex-T-TTA. All three studied compounds exhibit sufficiently high energies of the HOMO orbitals, which facilitates efficient injection and transport of holes, as well as significant oscillator strengths of the S0-S1 transition, which leads to efficient absorption and emission of light. An original effective synthesis procedure was developed and new BTBT derivatives were obtained: 2,7-bis(4-octylphenyl)[1]benzothieno[3,2-b][1]benzothienothiophene (DOct-P-BTBT), 2,7-bis(4-dodecylphenyl)[1]benzothieno[3,2-b][1]benzothienothiophene (DDodec-P-BTBT) 2,7-bis(5'-hexyl-2,2'-bithien-5-yl)[1]benzothieno[3,2-b][1]benzothienothiophene (DHex-2T-BTBT). For the first time, TPCO was synthesized, consisting of seven aromatic fragments TMS-PTTP(F)TTP-TMS. All the new compounds obtained were isolated in an individual state and their molecular structure was proved by NMR spectroscopy, GPC and elemental analysis. The study of the optical properties of the synthesized molecules in solutions has been carried out. Two-dimensional (2D) films (i.e., no more than a few molecular layers thick) were grown from DOct-P-BTBT, DDoc-P-BTBT, DH-P-TTA and DH-T-TTA with lateral dimensions from 300 um to 1 mm, the optimal conditions for 2D crystallization were revealed. The surface topography and thickness of 2D films were investigated by atomic force microscopy (AFM); the grown films demonstrated molecular smoothness with a root mean square roughness in the range of 0.3–0.5 nm. For all compounds, the thinnest films were found, which were classified as monolayers with a thickness corresponding to the length of the molecule (2.1 - 3.7 nm). Almost all 2D films showed signs of single crystals, as evidenced by the faceting of the domains of 2D films and the anisotropy of photoluminescence (PL). The method of vapor physical transport was used to grow 3D single crystals from TMS-PTTP(F)TTP-TMS, TMS-TTPTT-TMS, TMS-P4TP-TMS, TMS-PTTP(F), and TMS-P-TTA-P-TMS. The grown single crystals were in the form of rectangular plates with lateral dimensions of 1–3 mm or flat needles 0.5 mm wide and 1.5 mm long. The PL properties of 3D crystals (P(F)TPTP(F), P(F)TP(F)TP(F), and TMS-P4TP-TMS were studied by the integrating sphere method: PL spectra were obtained and the PL quantum yield of 3D crystals was measured. P(F)TPTP(F) and P(F)TP(F)TP(F), the PL quantum yield reached 49% and 59%, respectively, and in TMS-P4TP-TMS crystals it was 6% (corrected for PL reabsorption 16 %). A technique has been developed for estimation of the quantum yield (CV) of PL of 2D crystals using confocal PL microscopy. The technique is based on the use of a reference sample - a film of a solid solution of a phosphor with known properties. The PL spectra of 2D DDec-P-BTBT films of various thicknesses were obtained. It was shown that the PL spectra of 2D DDec-P-BTBT films experience a strong bathochromic shift (~ 0.3 eV) relative to the spectra in solutions, which indicates a strong intermolecular interaction in 2D layers. Unipolar organic field-effect transistors (OFET) with an active layer of 2D films (2D OFET) based on DHex-P-BTBT, DDec-T-BTBT, DHex-P-TTA compounds were obtained. The maximum mobility of charge carriers for these 2D OFET was in the range 0.6–1 cm2 / Vc. For the first time, ambipolar OPTs based on 2D organic semiconductors were obtained, which 2D organic light transistors (OLET) have been created from. A 2D DHex-P-TTA sample was studied as an active layer. The experimental setup for characterizing OLET samples was equipped with a fiber spectrometer for studying EL spectra and a specially designed silicon photodetector for measuring the absolute EL intensity. Thin-film 3D samples of OFET and OLET (i.e., with an active layer deposited by vacuum deposition) were made based on the most promising compounds: TMS-P4TP-TMS, TMS-TTPTT-TMS, DHex-P-TTA, DHex-T-TTA. The study of OLET samples consisted of the following stages: measurement of the transfer and output characteristics, obtaining images of operating OLET in the dark during the measurement of the transfer characteristic, measurement of EL spectra and measurement of the absolute EL power. TMS-P4TP-TMS and DHex-P-TTA oligomers, in which ambipolar transport of charge carriers and the highest EL intensity were observed, turned out to be the most successful materials for the creation of OLET. Based on TMS-P4TP-TMS, it was possible to create OLET samples with a controlled position of the EL region in the channel using the voltage at the gate of the transistor. The EL of OLET spectra were obtained on the basis of TMS-P4TP-TMS and TMS-TTPTT-TMS. OLET samples based on TMS-P4TP-TMS showed the most efficient EL in the spectral range 450–650 nm with luminous efficacy 1 lm/W. A generalization of the results of this and previous stages of the project was carried out and the most promising methods and approaches for the creation of energy-efficient OLETs were formulated. Data on the project and the results obtained are presented in electronic media Gazeta.ru and Indicator.ru (https://indicator.ru/chemistry-and-materials/sposob-sozdat-ultratonkie-i-gibkie-ekrany-09-03-2020.htm, https://www.gazeta.ru/science/news/2020/03/10/n_14138839.shtml). The results of the studies carried out are presented in the form of five reports at international conferences and published in three highly rated journals from the quartile Q1 (Dyes and Pigments 185, 108911 (2021); ACS Appl. Mater. Interfaces 12, 9507-9519 (2020); Adv. Opt. Mater., 8, 2000041 (2020)).

 

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Annotation of the results obtained in 2018
According to the project plan, we performed quantum-chemical calculations of the properties of thiophene-phenylene co-oligomers (TPCOs), synthesis of the most promising TPCOs, growth of ultrathin (2D) and bulky (3D) single crystals, study of the photoluminescent properties of the grown crystals, fabrication of unipolar and ambipolar organic field-effect transistors and study of their electrical properties. The initial plan of this stage was expanded - samples of light-emitting transistors were created and their electroluminescence was investigated. Using DFT calculations, the effect of phenyl rings fluorination on the geometry and properties of TPCO with conjugated cores PTPTP and TTPTT (T - thiophene, P - phenylene) was investigated. It was shown that fluorination reduces the energy of the frontier orbitals by up to 0.8 eV. The lowest energies of the frontier orbitals were obtained for the compound with the largest number of fluorine atoms in the conjugate core, P(F)TP(F)TP(F). In addition, fluorination leads to planarization of the molecule, which in turn reduces the optical bandgap, increases the oscillator strength of the first electronic transition, and reduces the reorganization energy. The effect of terminal groups (CF3, CH3, F) on the properties of the investigated TPCO was studied as well. Addition of the CH3 group increases the frontier orbitals energies by ~ 0.15 eV, and addition of F and CF3 groups reduces these energies by ~ 0.4 and ~ 0.1 eV, respectively. An original effective synthetic route for fluorine-containing TPCOs with perfluorobenzene fragments (2.2'-(2,3,5,6-tetrafluoro-1,4-phenylene)bis(5-phenylthiophene) (PTP(F)TP) and 5.5'- (2,3,5,6-tetrafluoro-1,4-phenylene) bis (5'-hexyl-2,2'-bithiophene) (DH-TTP(F)TT)) was developed for the first time. Necessary amounts of TPCOs with four and five conjugated aromatic rings and different end groups were synthesized: trimethylsilyl (TMS-PTPTP-TMS, TMS-PTTP-TMS), trifluoromethyl (CF3-PTTP-CF3), n-hexyl (DH-TTPTT) and n-decyl (DD-PTPTP). All the new obtained compounds were synthesized in an individual state and their molecular structure was proved by NMR spectroscopy, GPC and elemental analysis. For the highest purification yield possible, original high vacuum reactor was developed. It allowed purification of the oligomers with a yield up to 90%. Dozens of samples of plate-shaped single crystals of TMS-PTPTP-TMS, PTP(F4)TP, and 1,4-bis (5-phenylfuran-2-yl) benzene (BPFB) were grown. PTP(F4)TP single crystals were grown for the first time, their crystal structure was determined. The grown single crystals were investigated by the methods of polarization optical microscopy C-DIC (interference reflective microscopy in circularly polarized light) and atomic force microscopy (AFM). The photoluminescent properties of the grown single crystals were investigated, and the external quantum yield of the photoluminescence of crystals was measured by using an integrating sphere. The level of self-doping in TMS-PTPTP-TMS and BPFB single crystals was estimated by two methods – photoluminescence (PL) and photothermal spectroscopy. In the case of BPFB single crystals, the external quantum yield of PL with an increase in the doping level to 0.1% increased from ~ 50 to ~ 75%. From the PL data for BPFB single crystals, it was shown that the diffusion length of singlet excitons reaches 24 nm, which exceeds the known diffusion lengths of singlet excitons for semiconductor oligomer crystals. Ultrathin crystals were grown from solution on substrates from available TPCOs with pi-conjugated core of 5 aromatic rings and with long terminal symmetric alkyl substituents (DD-TTPTT, DD-PTPTP, DH-TTPTT), as well as oligothiophenes DD-5T and DH-5T. These crystals, with lateral sizes ranging from hundreds of microns to more than one millimeter, were studied by C-DIC optical microscopy, AFM and spectral microellipsometry. Monocrystalline monolayer domains were visualized using C-DIC microscopy. Thickness of the crystals was measured by AFM with an accuracy that allows one to reliably detect monolayer domains (their thickness corresponded to the length of one molecule: 3.2–4.0 nm). AFM measurements have shown a high degree of smoothness of the surface of ultrathin crystals (RMS <0.3 Å). Microellipsometry provided a direct confirmation of the assumptions about the size of the domains, which were preliminary made on the basis of C-DIC microscopy and AFM study. Ultrathin films of DH-TTPTT and DH-5T were studied using grazing incidence X-ray diffraction (GIXD) and X-ray reflectometry with an involvement of the DFT data of molecular geometry. It was shown that the films have a crystalline structure, a significant part of which is represented by monolayer single crystals. The most complete data were obtained for DH-TTPTT, for example, X-ray reflection data allowed us to determine the film thickness, which were in accordance with AFM data. The so-called Bragg rods, inherent in two-dimensional crystals, were observed only at certain positions of the surface, relative to its normal dimensions. This indicate a large size of single-crystal domains comparable to the size of the x-ray beam on the sample surface. The width of the Bragg rods corresponds to a coherence length of more than 100 nm. Organic monolayers of such a high structural order have been obtained for the first time. Moreover, the obtained diffraction data for monolayers allows us to pose the problem of complete solving of the crystal structure of monolayers (as for 3D single crystals). The obtained X-ray data on the crystallinity, film thickness, molecular tilt in films fully correspond to the results obtained using optical microscopy, spectral ellipsometry and AFM. A model of a monolayer organic light-emitting transistor has been developed. The one-dimensional and stationary approximation was used. The model is based on the Poisson’s equation, the continuity equations, and the drift-diffusion equations. The non-radiative recombination on traps (Shockley-Reed-Hall) is taken into account in the continuity equations. The efficiency of electroluminescence was calculated by the formula that takes into account the quenching of excitons near the metal. Contact effects were investigated and key parameters that affect the operation of organic light-emitting transistors were identified with the help of the developed model. The effect of nonradiative recombination on charge traps (Shockley-Reed-Hall recombination) was studied. It was shown that with increasing concentration of traps, the efficiency of electroluminescence decreases in the voltage region that corresponds to ambipolar conductivity. The effect of the quenching of excitons near the contacts of the drain and source was investigated. It was shown that due to this effect, the efficiency is greatly reduced in the ranges of gate voltages corresponding to unipolar conduction modes (electron or hole). To simulate contact effects, an analytical formula was derived for the transfer characteristics of a unipolar organic field-effect transistor with space charge limited current under the drain and source contacts in the saturation mode. Transistors with polycrystalline and monocrystalline active layers were fabricated from the oligomers under study. The ambipolar conductivity was found for 5-rings TPCOs (TMS-PTPTP-TMS and PTP(F)TP) in transistors with polycrystalline layers. The results indicate the high potential of these oligomers to create effective OLETs. Unipolar n-channel transistors with an electron mobility of more than 0.1 cm2 / (Vs), a threshold voltage of about 10 V and a minimum contribution of contact effects were created on the basis of CF3-PTTP-CF3 single crystals. An original approach is proposed to reduce the contact resistance by using an intermediate layer (between the electrode and the active layer), consisting of a polycrystalline organic semiconductor with energies of boundary orbitals different from those of the active layer. The use of this approach for transistors based on TMS-PTTP-TMS single crystals with ambipolar conductivity significantly increased the current in the ambipolar mode: current at the minimum of the transfer characteristic is only 2-3 times lower than at the boundaries of the ambipolar conductivity range. For the first time, light-emitting transistors based on TMS-PTPTP-TMS were developed, their electroluminescence was investigated. A light-emitting transistor with an active layer of a furan-containing oligomer deposited from a solution was demonstrated for the first time. The results of the research were presented at three international conferences in more than 10 presentations and published in two journal papers from the Q1 quartile (J. Mater. Chem. C, DOI: 10.1039/c8tc04151b, Synth. Met. 2018, v.246, 254).

 

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Annotation of the results obtained in 2019
An original effective synthesis method of new thiophene-phenylene co-oligomers (TPCO) containing perfluorobenzene fragments, including with alkyl groups for the growth of ultrathin films from solution was developed. 2,2'-(2,3,5,6-tetrafluoro-1,4-phenylene)bis[5-(4-hexylphenyl) thiophene] (Hex-PTPFTP-Hex), 2,2'-(1,4-phenylene)bis[5-(2,3,4,5,6-pentafluorophenyl)thiophene] (PFTPTPF) and 2,2'-(2,3,5,6-tetrafluoro-1,4-phenylene)bis[5-(2,3,4,5,6-pentafluorophenyl)thiophene] (PFTPFTPF) were synthesized for the first time. The synthesis scheme was based on reactions of organometallic synthesis under the conditions of Kumada and the interaction of organolithium derivatives with perfluorobenzenes. Using the reaction of organometallic synthesis under Suzuki conditions, a BTBT derivative was obtained with a conjugated P-BTBT-P fragment and hexyl terminal substituents (Hex-P-BTBT-P-Hex), and also TPСO containing five aromatic fragments and terminal trimethylsilyl groups (TMS -TTPTT-TMS). All obtained new compounds were isolated in an individual state and their molecular structure was proved by NMR spectroscopy, GPC and elemental analysis. For the obtained oligomers, deep purification was carried out by column chromatography or high vacuum sublimation, which allowed reducing the concentration of self-dopants to the level of 30–200 ppm depending on the molecular structure of the oligomer. For new TPCOs, optical absorption and photoluminescence (PL) spectra were studied, and the quantum yield of PL in a solution was measured. The quantum yield of PL in a solution for TPCO containing perfluorobenzene fragments exceeded 80%. Quantum chemistry methods were used to calculate the equilibrium geometry, energy levels of frontier orbitals, the optical gap, the oscillator strength of the S0-S1 transition, the direction of the dipole moment of this transition, and the reorganization energy for compounds with P-BTBT-P, T-BTBT-T, and BTBT conjugated cores. It was found that the addition of phenyl and thiophene rings to the BTBT core narrows the optical gap and significantly increases the strength of the S0 – S1 transition oscillator, which is consistent with experimental data. Ultrathin crystalline films were grown from BTBT-based oligomers: Dec-P-BTBT-P-Dec, Dec-T-BTBT-T-Dec, Hex-P-BTBT-P-Hex. For Dec-P-BTBT-P-Dec, crystalline domains were visualized by interference microscopy, and the films were shown to have molecular smoothness and to consist of one or few molecular layers. Results were obtained on the structural characteristics of ultrathin Dec-TTPTT-Dec and Hex-TTPTT-Hex films using surface-enhanced Raman spectroscopy (SERS). The high sensitivity of SERS revealed the orientation of monolayer single-crystal domains with optical spatial resolution. The SERS microscopy revealed the presence of a submonolayer, an amorphous material between crystalline domains, which is practically inaccessible to optical or common atomic force microscopy. The results are presented in journal paper [J. Phys. Chem. C 123, 27242 (2019), https://doi.org/10.1021/acs.jpcc.9b08083]. Studies of PL spectroscopy revealed that the quantum yield of PL of 3D crystals of Dec-P-BTBT-P-Dec, Dec-T-BTBT-T-Dec and Hex-P-BTBT-P-Hex is higher at a higher concentration of molecular self-dopants, and it exceeds 30% for Dec-PTPTP-Dec crystals. Using confocal PL microscopy, PL maps of 2D organic semiconductors were first obtained. The polarization anisotropy of the PL of monolayers was found, which allowed us to conclude that they are single crystals. The model of a monolayer organic light emitting transistor was further developed: the model takes into account quenching of excitons by charges and its effect on the efficiency and external quantum efficiency of organic light emitting transistors. The model was supplemented by the diffusion equation for excitons, in which quenching of excitons by free electrons and holes is taken into account. Using this model, the influence of the process of quenching of excitons by free charge carriers on the efficiency of a light-emitting transistor is studied. Quantum-chemical calculations of work function modifier molecules were carried out. As such molecules, two TPCOs with a conjugated PTTP core (P - phenyl, T - thiophene) and with electron-acceptor terminal groups: F-PTTP-F and CF3-PTTP-CF3 were studied. For these molecules, the static dipole moments of the end groups and half of the molecule were calculated. The relative efficiency of the studied modifiers of the electrode work function was compared. The approach proposed at the previous project stage for the efficient injection of electrons and holes, which consists in using polycrystalline organic semiconductor sub-electrode layers, was further developed. New materials were found for electrode interlayers to improve the injection of holes into the active layer. Monolayer transistors based on Hex-5T-Hex, Hex-TTPTT-Hex, Dec-5T-Dec, and Dec-TTPTT-Dec compounds were fabricated, the maximum charge mobility for which exceeded 0.2 cm2/Vs. It was shown that the mobility values in monolayer transistors are not inferior to those obtained on 3D samples of the same or similar compounds. The results were published in [ACS Appl. Mater. Interfaces 11, 6315 (2019), https://doi.org/10.1021/acsami.8b20700]. For monolayer transistors based on Dec-P-BTBT-P-Dec, a charge mobility of about 1 cm2/Vs was demonstrated. The charge mobility for the ultrathin transistor based on Dec-P-BTBT-P-Dec was significantly higher and exceeded 7 cm2/Vs, which corresponds to the best achievements in the field of organic field-effect transistors. Samples of light-emitting organic transistors based on fluorinated TPCO were fabricated. The tendency of a nonmonotonic dependence of electron mobility with an increase in the number of fluorine atoms in the oligomer molecule, which is consistent with theoretical calculations, was revealed. For the first time, electroluminescence of field-effect transistors based on 2D organic semiconductors was shown. A 2D Dec-P-BTBT-P-Dec single crystal with lateral sizes of more than 1 mm and a thickness of about 15 nm was used as the active layer. The results of studies of this stage of the project (http://sunhen.phys.msu.ru/en/nauka/proekty) were presented in the form of more than 10 reports at international conferences and published in two articles from quartile Q1 (ACS Appl. Mater. Interfaces and J. Phys. Chem. C).

 

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