Intermolecular Transfer Integrals Research Articles

A Domino Protocol toward High-performance Unsymmetrical Dibenzo[d,d']thieno[2,3-b;4,5-b']dithiophenes Semiconductors.

Unsymmetric organic semiconductors have many advantages such as good solubility, rich intermolecular interactions for potential various optoelectronic applications. However, their synthesis is more challenging due to intricate structures thus normally suffering tedious synthesis. Herein, we report a trisulfur radical anion (S3⋅-) triggered domino thienannulation strategy for the synthesis of dibenzo[d,d']thieno[2,3-b;4,5-b']dithiophenes (DBTDTs) using readily available 1-halo-2-ethynylbenzenes as starting materials. This domino protocol features no metal catalyst and the formation of six C-S and one C-C bonds in a one-pot reaction. Mechanistic study revealed a unique domino radical anion pathway. Single crystal structure analysis of unsymmetric DBTDT shows that its unique unsymmetric structure endows rich and multiple weak S⋅⋅⋅S interactions between molecules, which enables the large intermolecular transfer integrals of 86 meV and efficient charge transport performance with a carrier mobility of 1.52 cm2 V-1 s-1. This study provides a facile and highly efficient synthetic strategy for more high-performance unsymmetric organic semiconductors.

A Domino Protocol toward High‐performance Unsymmetrical Dibenzo[d,d′]thieno[2,3‐b;4,5‐b′]dithiophenes Semiconductors

AbstractUnsymmetric organic semiconductors have many advantages such as good solubility, rich intermolecular interactions for potential various optoelectronic applications. However, their synthesis is more challenging due to intricate structures thus normally suffering tedious synthesis. Herein, we report a trisulfur radical anion (S3⋅−) triggered domino thienannulation strategy for the synthesis of dibenzo[d,d′]thieno[2,3‐b;4,5‐b′]dithiophenes (DBTDTs) using readily available 1‐halo‐2‐ethynylbenzenes as starting materials. This domino protocol features no metal catalyst and the formation of six C−S and one C−C bonds in a one‐pot reaction. Mechanistic study revealed a unique domino radical anion pathway. Single crystal structure analysis of unsymmetric DBTDT shows that its unique unsymmetric structure endows rich and multiple weak S⋅⋅⋅S interactions between molecules, which enables the large intermolecular transfer integrals of 86 meV and efficient charge transport performance with a carrier mobility of 1.52 cm2 V−1 s−1. This study provides a facile and highly efficient synthetic strategy for more high‐performance unsymmetric organic semiconductors.

Theoretical Study of Charge Mobility Properties of Complexes Si(DPP)(CH3)2 and Si(Bzimpy)(CH3)2

The direction of organic electronics research is attracting more and more interest from the scientific community. One of the indicators of such interest is the appearance of commercially available products with screens based on organic compounds. Therefore, conducting experimental and theoretical research in this area is an urgent task. Pentacoordination neutral complexes of silicon are poorly studied from the point of view of application in organic electronics, as well as six-coordination analogues. We present data on the calculation of reorganization energies, intermolecular transfer integrals, transfer rates and charge mobility for the optimized structures of pentacoordinated silicon complexes Si(DPP)(CH3)2 and Si(bzimpy)(CH3)2. We have applied Marcus-Hush model for calculation of charge mobilities. The Si(DPP)(CH3)2 structure contains one diphenylpyridine (DPP = 2,6-diphenylperidine) ligand. The Si(bzimpy)(CH3)2 structure contains one benzimidazole (bzimpy = 2,6-bis(benzimidazole-2'-il)pyridine) ligand. Computational data were obtained using the B3LYP hybrid functional and the basis set 6-31G*. All calculations were performed using Gaussian09 program package. The charge mobility data obtained for Si(DPP)(CH3)2 and Si(bzimpy)(CH3)2 pentacoordinated silicon complexes were compared with their six-coordinate counterparts Si(DPP)2 and Si(bzimpy)2 for which experimental data on charge mobilities become available last years. Comparison with six-coordination analogues of complexes showed that penta-coordination complexes Si(DPP)(CH3)2 and Si(bzimpy)(CH3)2 have much higher mobility of electrons, while Si(bzimpy)(CH3)2 also has higher hole mobility. We suppose this could be related to different symmetry of the pentacoordinated and hexacoordinted complexes. It is shown that the mobility of holes is much higher in the complex Si(bzimpy)(CH3)2 than in Si(DPP)(CH3)2.

Helical Polycyclic Heteroaromatic as Hole Transport Material for Perovskite Solar Cell: Remarkable Impact of Alkyl Substitution Position

AbstractPolycyclic aromatic hydrocarbons play a critical role in the development of organic semiconductors. This study unravels the impact of alkyl substitution position on the molecular energy level, glass transition temperature, diffusion of external species, and hole transport of a pyrrole‐rich, helical polycyclic heteroaromatic, that is TBPC. Compared to terminal substitution, internal hexyl substitution results in steric repulsion with TBPC, weakening π─π stacking and therefore improving thin film morphology. Internal substitution also reduces energy disorder, lowers reorganization energy, and increases intermolecular transfer integrals, leading to enhanced hole mobility. Notably, the organic semiconductor with internal hexyl substitution (TBPC‐611) exhibits a higher molecular packing density, resulting in a markedly higher glass transition temperature and slower diffusion of external species. Using TBPC‐611 as the hole transport material, this work successfully fabricates perovskite solar cells with an average power conversion efficiency above 24%, showing good photostability and thermostability. These findings contribute new insights to the development of high‐performance organic semiconductors.

Comprehensive AbInitio Investigation of the Phase Diagram of Quasi-One-Dimensional Molecular Solids.

An abinitio investigation of the family of molecular compounds TM_{2}X is conducted, where TM is either TMTSF or TMTTF and X takes centrosymmetric monovalent anions. By deriving the extended Hubbard-type Hamiltonians from first-principles band calculations and evaluating not only the intermolecular transfer integrals but also the Coulomb parameters, we discuss their material dependence in the unified phase diagram. Furthermore, we apply the many-variable variational MonteCarlo method to accurately determine the symmetry-breaking phase transitions, and show the development of the charge and spin orderings. We show that the material-dependent parameter can be taken as the correlation effect, represented by the value of the screened on-site Coulomb interaction U relative to the intrachain transfer integrals, for the comprehensive understanding of the spin and charge ordering in this system.

A study on the luminescence properties of high-performance benzothieno[3,2-b][1]benzothiophene based organic semiconductors

In terms of molecular structures, benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives qualify as the best-performing organic semiconductors for hole transport, but it exhibits poor luminescence properties. For instance, a classical molecule of 2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) only shows photoluminescence quantum yields (PLQYs) of 5.0% in chlorobenzene. Herein, we present three kinds of organic semiconductors based on phenyl-BTBT molecule core, namely, 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene (DPh-BTBT), 2,7-bis(4-ethylphenyl) benzothieno[3,2-b][1] benzothiophene (DBEP-BTBT), and 2,7-bis(4-methoxyphenyl)benzothieno[3,2-b][1]benzothiophene (DBOP-BTBT). Notably, blue fluorescence with relatively high PLQYs of 27%, 36%, and 48% in chlorobenzene was shown. Due to the surface defects and grain boundaries in the thin films, the PLQYs decrease to 3.4% for DPh-BTBT and 7.4% for DBOP-BTBT, respectively. Nevertheless, the DBEP-BTBT film still exhibits an absolute PLQY as high as 12%, which is a rare and precious characteristic in BTBT derivatives. Meanwhile, the differences in photophysical characteristics, crystal structures, and charge transport properties of the three BTBT derivatives were systematically studied. Similar to DPh-BTBT and DBOP-BTBT, the DBEP-BTBT crystals display typical layered herringbone packing structure with multi- and strong intermolecular interactions, which are conductive to form a two-dimensional charge transport network in layered thin films for high mobility. The mobility of DBEP-BTBT in polycrystalline thin film transistors is up to 3.22 cm2V−1s−1 in ambient air, which is close to DPh-BTBT (3.64 cm2V−1s−1) and much higher than that of DBOP-BTBT (0.5 cm2V−1s−1). Moreover, the calculations show that the intermolecular transfer integrals of HOMOs are almost isotropic for six nearest neighbour contacts in DBEP-BTBT and DPh-BTBT crystals, implying their possibility of isotropic hole transport property in single crystals.

Intermolecular Charge Transfer in H- and J-Aggregates of Donor–Acceptor–Donor Chromophores: The Curious Case of Bithiophene-DPP

A vibronic exciton model is developed to describe low-energy electronic excitations in slip-stack aggregates of donor–acceptor–donor (DAD) chromophores with substantial overlap between the neighboring donor and acceptor fragments. In such stacks, J- and H-aggregate behavior is driven by intermolecular charge transfer (ICT) and not Coulomb coupling as is assumed in the Kasha model. In-phase (out-of-phase) intermolecular charge transfer integrals result in J-aggregate (H-aggregate) behavior, as unambiguously determined by the vibronic spectral signatures. Interestingly, both J- and H-aggregates are red-shifted, in contrast to the predictions of the Kasha model. Simulated spectral line shapes agree well with recent experiments on two derivatives of the bithiophene diketopyrrolopyrrole chromophore, 2T-DPP-2T, with different terminal groups.

Theoretical study of charge transport properties of a Si(DPP)2 complex

This article provides a study of the calculations of charge transport, reorganization energy, intermolecular transfer integral and charge mobility for the optimized structure of the new neutral hexacoordinated silicon complex Si(DPP)2. The main calculated and experimental geometric data on the optimized structure are also presented. The complex has a distorted octahedral geometry with short Si-N bond lengths averaging: calculated - 1.958365 Å and experimental - 1.9111 Å. The Si(DPP)2 structure contains two diphenylpyridine (DPP = 2,6-diphenylpyridine ligand) ligands. Optimization and computational calculations of the necessary data of charge transport were performed using a B3LYP functional and a 6-31G* basis set. Geometrical parameters for structure optimization were obtained from recent, known, experimental data. The obtained theoretical data on the charge transport of the Si(DPP)2 structure were compared with the known experimental data. Based on a comparison of the theoretical and experimental values for the charge transport properties of the Si(DPP)2 compound, it was shown that the method we used for calculating the charge mobility gives relatively close values. This work shows that computational methods can help in the further study of future structures, as well as predict the future parameters of as yet unexplored silicon complexes.

Integrating Unexpected High Charge-Carrier Mobility and Low-Threshold Lasing Action in an Organic Semiconductor.

Integrating high charge-carrier mobility and low-threshold lasing action in an organic semiconductor is crucial for the realization of an electrically pumped laser, but remains a great challenge. Herein, we present an organic semiconductor, named as 2,7-di(2-naphthyl)-9H-fluorene (LD-2), which shows an unexpected high charge-carrier mobility of 2.7 cm2 V-1 s-1 and low-threshold lasing characteristic of 9.43 μJ cm-2 and 9.93 μJ cm-2 and high-quality factor (Q) of 2131 and 1684 at emission peaks of 420 and 443 nm, respectively. Detailed theoretical calculations and photophysical data analysis demonstrate that a large intermolecular transfer integral of 10.36-45.16 meV together with a fast radiative transition rate of 8.0×108 s-1 are responsible for the achievement of the superior integrated optoelectronic properties in the LD-2 crystal. These optoelectronic performances of LD-2 are among the highest reported low-threshold lasing organic semiconductors with efficient charge transport, suggesting its promise for research of electrically pumped organic lasers (EPOLs).

Integrating Unexpected High Charge‐Carrier Mobility and Low‐Threshold Lasing Action in an Organic Semiconductor

AbstractIntegrating high charge‐carrier mobility and low‐threshold lasing action in an organic semiconductor is crucial for the realization of an electrically pumped laser, but remains a great challenge. Herein, we present an organic semiconductor, named as 2,7‐di(2‐naphthyl)‐9H‐fluorene (LD‐2), which shows an unexpected high charge‐carrier mobility of 2.7 cm2 V−1 s−1 and low‐threshold lasing characteristic of 9.43 μJ cm−2 and 9.93 μJ cm−2 and high‐quality factor (Q) of 2131 and 1684 at emission peaks of 420 and 443 nm, respectively. Detailed theoretical calculations and photophysical data analysis demonstrate that a large intermolecular transfer integral of 10.36–45.16 meV together with a fast radiative transition rate of 8.0×108 s−1 are responsible for the achievement of the superior integrated optoelectronic properties in the LD‐2 crystal. These optoelectronic performances of LD‐2 are among the highest reported low‐threshold lasing organic semiconductors with efficient charge transport, suggesting its promise for research of electrically pumped organic lasers (EPOLs).

A general charge transport picture for organic semiconductors with nonlocal electron-phonon couplings

The nonlocal electron-phonon couplings in organic semiconductors responsible for the fluctuation of intermolecular transfer integrals has been the center of interest recently. Several irreconcilable scenarios coexist for the description of the nonlocal electron-phonon coupling, such as phonon-assisted transport, transient localization, and band-like transport. Through a nearly exact numerical study for the carrier mobility of the Holstein-Peierls model using the matrix product states approach, we locate the phonon-assisted transport, transient localization and band-like regimes as a function of the transfer integral (V) and the nonlocal electron-phonon couplings (ΔV), and their distinct transport behaviors are analyzed by carrier mobility, mean free path, optical conductivity and one-particle spectral function. We also identify an “intermediate regime” where none of the established pictures applies, and the generally perceived hopping regime is found to be at a very limited end in the proposed regime paradigm.

Interface electronic structure and valence band dispersion of bis(1,2,5-thiadiazolo)-p-quinobis(1,3-dithiole) on polycrystalline Au electrodes

Bis(1,2,5-thiadiazolo)-p-quinobis(1,3-dithiole) (BTQBT) is a promising material for application in vertical organic transistor devices because of considerable energy dispersion in its valence band (VB). In this study, the electronic structure at the interface of BTQBT on polycrystalline Au electrodes was elucidated by photoelectron spectroscopy (PES). The excitation photon energy-dependence of the PES corroborated a widely dispersing VB of the solid-state BTQBT on Au. An intermolecular transfer integral of 75 meV and the hole effective mass of 4.3m 0 were derived from the present results.

Analysis of External and Internal Disorder to Understand Band-Like Transport in n-Type Organic Semiconductors.

Charge transport in organic semiconductors is notoriously extremely sensitive to the presence of disorder, both internal and external (i.e., related to interactions with the dielectric layer), especially for n-type materials. Internal dynamic disorder stems from large thermal fluctuations both in intermolecular transfer integrals and (molecular) site energies in weakly interacting van der Waals solids and sources transient localization of the charge carriers. The molecular vibrations that drive transient localization typically operate at low-frequency (<a-few-hundred cm-1 ), which makes it difficult to assess them experimentally. Hitherto, this has prevented the identification of clear molecular design rules to control and reduce dynamic disorder. In addition, the disorder can also be external, being controlled by the gate insulator dielectric properties. Here a comprehensive study of charge transport in two closely related n-type molecular organic semiconductors using a combination of temperature-dependent inelastic neutron scattering and photoelectron spectroscopy corroborated by electrical measurements, theory, and simulations is reported. Unambiguous evidence that ad hoc molecular design enables the electron charge carriers to be freed from both internal and external disorder to ultimately reach band-like electron transport is provided.

Effect of Alkyl Chain Length on Charge Transport Property of Anthracene-Based Organic Semiconductors.

Anthracene, a simple planar building block for organic semiconductors, shows strong intermolecular interactions and exhibits strong blue fluorescence. Thus, its derivatives have a great potential to integrate considerable charge carrier mobility and strong emission within a molecule. Here, we systematically studied the influence of alkyl chain length on the crystal structures, thermal properties, photophysical characteristics, electrochemical behaviors, and mobilities for a series of 2,6-di(4-alkyl-phenyl)anthracenes (Cn-Ph-Ants, where n represents the alkyl chain length). Among them, Cn-Ph-Ants (n = 0, 1, 2, and 3) display similar layered herringbone (LHB) packing motifs, which facilitate two-dimensional charge transport and thereby enables high-performance organic field-effect transistors (OFETs). All Cn-Ph-Ants exhibit similar work functions and show strong blue fluorescence with photoluminescence quantum yields (PLQY) of approximately 40% in toluene. In addition, the absolute powder PLQYs of C0-, C2-, C3-, C4-, and C6-Ph-Ants are 24.6, 8.2, 5.7, 10.9, and 8.6%, respectively. Note that the alkyl chain length shows a significant effect on the charge mobilities of Cn-Ph-Ants. Our newly synthesized C1-, C3-, and C4-Ph-Ants show hole mobilities of up to 2.40, 1.34, and 1.00 cm2 V-1 s-1, respectively, with mobilities of 3.40, 1.57, and 0.82 cm2 V-1 s-1 for C0-, C2-, and C6-Ph-Ants, indicating an increasing tendency of mobility with shorter alkyl chain length. This feature is related to the microstructures of the thin films, which reveal the enhanced film order, crystallinity, and grain size with a decrease in the alkyl chain length. Moreover, we theoretically analyze the intermolecular transfer integrals of HOMOs, which increase at T-shaped contacts as the alkyl chain length decreases, which improves the intermolecular charge transport properties, leading to the increases in mobility. Interestingly, the anisotropy of the transfer integral tends to decrease upon substitution with longer alkyl chains, suggesting that alkyl chain adjustments may facilitate isotropic charge transport property in 2,6-alkylated anthracenes.

Steric effect of halogen substitution in an unsymmetrical benzothienobenzothiophene organic semiconductor

Abstract We report a crystal structure analysis of an unsymmetrical organic semiconductor 2-bromo-7-butyl[1]benzothieno[3,2-b][1]benzothiophene (Br-BTBT-C4) and its transistor characteristics. Unlike the parent compound 2-butyl-[1]benzothieno[3,2-b][1]benzothiophene (BTBT-C4) forming a bilayer-type structure composed of head-to-head herringbone packings, Br-BTBT-C4 forms a dense head-to-tail herringbone packing arranged so as to avoid the steric hindrance between bromine atoms. The intermolecular transfer integrals in Br-BTBT-C4 suggest its advantage for carrier transport; however, the transistor characteristics of Br-BTBT-C4 and BTBT-C4 were found to be almost the same. This is because BTBT-C4 has a higher crystallinity than Br-BTBT-C4 in vacuum deposited thin films.

Method for Fast Estimation of Lattice Distortion Energy in Organic Semiconductors

The efficient operation of organic electronic devices requires a high mobility of charge carriers in their active layers. According to modern concepts, the charge mobility in the best organic semiconductors is limited by dynamic disorder, i.e., fluctuations of intermolecular charge transfer integrals caused by the nonlocal electron–phonon coupling and thermal motion of molecules. However, the estimate of nonlocal electron–phonon coupling currently requires time- and resource-consuming methods, which complicates the search for high-mobility organic semiconductors among numerous candidates. In this work, a method has been proposed to rapidly estimate the lattice distortion energy, which is the main characteristic of the nonlocal electron–phonon coupling, by comparing the reorganization energies of molecules and molecular dimers. The determined lattice distortion energies are in good agreement with the values previously obtained by other methods. Furthermore, the proposed method has allowed addressing the effect of intermolecular delocalization on the nonlocal electron–phonon coupling. The results obtained indicate that the proposed approach is promising for the efficient search for organic semiconductors with a weak nonlocal electron–phonon coupling and a high charge mobility.

Second highest occupied molecular orbital effects on the valence band structure of organic semiconductors

We investigate the electronic states of organic semiconductors with herringbone structures using the multi-orbital tight-binding model for highest and second highest occupied molecular orbitals (HOMOs and SHOMOs) to elucidate the effect of SHOMOs on the valence-band state. We diagonalize the Hamiltonian analytically and derive the valence band width considering hybridization of HOMO and SHOMO. It is shown that the SHOMO effect is enhanced in the condition where the sum of the intermolecular transfer integrals of HOMO-SHOMO in the sublattice and the sum between the sublattices is respectively so large that their product is not negligible compared to the energy difference between HOMO and SHOMO. We also investigate acene, phenacene and thienoacene using density functional theory. It is shown that [1]benzothieno[3,2-b][1]benzothiophene(BTBT) based materials satisfy this condition, and SHOMO contributes significantly. It implies that the origins of the high-mobility of BTBT based materials can be ascribed to the light effective mass due to the contribution of SHOMO.

Electronic Structure, Electron-Phonon Coupling, and Charge Transport in Crystalline Rubrene Under Mechanical Strain

Motivated by the potential for the application of organic semiconductors in flexible electronics, we present a theoretical study aiming at elucidating the interplay between mechanical strain and electronic, vibrational, and charge transport properties of the prototypical high-mobility molecular semiconductor rubrene. Our study considers several factors that can play a role in the electro-mechanical response of a soft, van-der-Waals bonded molecular crystal, such as intermolecular charge transfer integrals, lattice dynamics, and electron phonon coupling. We find that compressive strain leads to an increase in magnitude of charge transfer integrals but also of the energetic disorder hampering the mobility. Charge transport simulations, based on the transient localization framework and fed with first-principles inputs, reveal a remarkably different response to strain applied along different crystal axes, in line with the most recent experiments. The critical interplay between the energetic disorder of intrin...

Polaron properties in 2D organic molecular crystals: directional dependence of non-local electron-phonon coupling.

In organic molecular crystals, the polaronic hopping model for the charge transport assumes that the carrier lies at one or a small number of molecules. Such a kind of localization suffers the influence of the non-local electron-phonon (e-ph) interactions associated with intermolecular lattice vibrations. Here, we developed a model Hamiltonian for numerically describing the role played by the intermolecular e-ph interactions on the stationary and dynamical properties of polarons in a two-dimensional array of molecules. We allow three types of electron hopping mechanisms and, consequently, for the nonlocal e-ph interactions: horizontal, vertical, and diagonal. Remarkably, our findings show that the stable polarons are not formed for isotropic arrangements of the intermolecular transfer integrals, regardless of the strengths of the e-ph interactions. Interestingly, the diagonal channel for the e-ph interactions changes the transport mechanism by sharing the polaronic charge between parallel molecular lines in a breather-like mode.

Transport Properties of Molecular Conductors (BSBS)2XF6 (X = As, Sb, and Ta): Investigation of Intermolecular Transfer Integrals in the Radical-Cationic State of Benzothienobenzothiophene-Type Semiconductors

[1]Benzoseleno[3,2-b]benzoselenophene (BSBS) derivatives are known as high-performance transistor materials. Novel and highly conductive charge-transfer (CT) complexes, (BSBS)2XF6 (X = As and Sb), have been prepared, and their transport properties are investigated. At room temperature, these complexes exhibit conductivities larger than 600 S cm–1. The chemical pressure effect originating from the counter anion size is estimated to be 0.3 kbar. The thermoelectric (TE) power is 15–17 μV K–1, demonstrating that the BSBS complexes have remarkably large transfer integrals of 310 meV, as observed in the highly conductive benzothienobenzothiophene (BTBT) complexes. From both molecular orbital calculations and experiments, it is demonstrated that the BTBT skeleton has the property of forming a very large energy bandwidth. The TE power of metallic CT complexes based on organic transistor molecules is useful for experimentally determining the intermolecular transfer integrals of charged semiconductor molecules.