Organic Semiconductor Molecules Research Articles

Preparation of Highly-Ordered Tetrabenzorporphyrin Films Controlled by Supramolecular Assemblies and Precursor Approach

The control of the orientation and packing structures of small organic semiconductor molecules is the important issue for the improvement of charge carrier mobilities of organic field effect transistors. Tetrabenzoporphyrin (BP) is one of the promising p-type organic semiconductors but solubility in organic solvents is very low. We have developed a precursor approach, where a spun-cast film of soluble precursor, tetra-bicyclo[2.2.2]octadiene-fused porphyrin (CP), was heated to obtain BP crystalline film by in-situ retro-Diels–Alder reaction. However, the obtained BP film is a polycrystalline film and the field-effect transistors (FETs) hole mobilities were 10-1~10-2 cm2V-1s-1[1]. Recently, we reported the effect of alkyl substituents at 5,15-positions of BP for the control of the molecular arrangement. The best performance of OFET hole mobility of 5,15-disubstituted BP was 4.1 cm2V-1s-1 [2, 3].A supramolecular strategy has attracted attentions as the effective method for the control of molecular arrangement in films. We applied the combination of supramolecular assembling method and the precursor approach to prepare well-ordered BP films. 5,15-Bis(p-dodecylamidephenyl)-CP was deposited on the substrate by drop-casting, spin-coating or dip-coating method, followed by the heating to 200 ºC for 10 min to give the corresponding 5,15-bis(p-dodecylamidephenyl)-BP films. The film morphologies of the BP were compared with 5,15-bis(p-dodecyloxyphenyl)-BP, which doesn’t include amide moieties for hydrogen-bonding, using UV-vis absorption spectroscopy, atomic force microscopy (AFM), X-ray diffraction (XRD) analysis, and p-polarized multiple-angle incidence resolution spectrometry (p-MAIRS) to clarify the effect of amide groups. OFET performance of the BPs will be also discussed to clarify the effect of intermolecular hydrogen bonding interaction.

Synthesis and Characterization of Octacyano-Cu-Phthalocyanine.

Octacyano-metal-substituted phthalocyanine MPc(CN)8 is a promising n-type stable organic semiconductor material with eight cyano groups, including a strong electron-withdrawing group at its molecular terminals. However, most MPc(CN)8 have not been thoroughly investigated. Therefore, CuPc(CN)8 was synthesized in this study and its crystal structure, chemical and electronic states, thermal stability, and electrical properties were investigated. This article discusses the various properties of CuPc(CN)8, as compared to those of CuPc and FePc(CN)8. The previously reported FePc(CN)8 is an organic semiconductor molecule with a molecular structure similar to that of CuPc(CN)8. X-ray diffraction (XRD) measurements revealed that CuPc(CN)8 has a crystalline structure in the P1̅ space group. The crystal structure forms an in-plane network parallel to the molecular plane through multiple hydrogen bonds by the cyano groups at the molecular terminals. Interestingly, the crystal structure, especially the molecular stacking, of CuPc(CN)8 differs from that of FePc(CN)8. The absorption edge observed in the ultraviolet-visible spectrum of CuPc(CN)8 shifted to a longer wavelength than that of CuPc, which was attributed to the energy gap of CuPc(CN)8 being smaller than that of CuPc owing to the influence of the cyano groups at the molecular terminals, according to the molecular orbital calculation results using density functional theory. Ultraviolet photoelectron spectroscopy measurements confirmed that CuPc(CN)8 had a stronger n-type character than CuPc because of the orbital energy stabilization by the cyano groups. Thermogravimetry/differential thermal analysis measurements revealed that the thermal stability of CuPc(CN)8 was significantly higher than that of FePc(CN)8. CuPc(CN)8 exhibited photoconduction upon visible-light irradiation, and its electrical conductivity was higher than that of CuPc, which was attributed to a reduction in the electron injection barrier at the electrode interfaces.

Exciton Delocalization and Polarizability in Perylenetetracarboxylic Diimide Probed Using Electroabsorption and Fluorescence Spectroscopies.

Perylenetetracarboxylic diimide (PTCDI) is an n-type organic semiconductor molecule that has been widely utilized in numerous applications such as photocatalysis and field-effect transistors. Polarizability and dipole moment, which are inherent properties of molecules, are important parameters that determine their responses to external electric and optical fields, physical properties, and reactivity. These parameters are fundamentally important for the design of innovative materials. In this study, the effects of external electric fields on absorption and fluorescence spectra were investigated to obtain the PTCDI parameters. The PTCDI substituted by an octyl group (N,N'-Dioctyl-3,4,9,10-perylenedicarboximide) dispersed in a polymethyl methacrylate (PMMA) matrix was studied in this work. The features of vibronic progression in the absorption spectrum were analogous to those observed in solution. The red shift of the absorption band caused by the Stark effect was mainly observed in the presence of an external electric field. Changes in parameters such as the dipole moment and polarizability between the ground and the Franck-Condon excited states of the PTCDI monomer were determined. The fluorescence spectrum shows a contribution from a broad fluorescence band at wavelengths longer than the monomer fluorescence band. This broad fluorescence is ascribed to the excimer-like fluorescence of PTCDI. The effects of the electric field on the fluorescence spectrum, known as the Stark fluorescence or electrofluorescence spectrum, were measured. Fluorescence quenching is observed in the presence of an external electric field. The change in the polarizability of the monomer fluorescence band is in good agreement with that of the electroabsorption spectrum. A larger change in the polarizability was observed for the excimer-like fluorescence band than that for the monomer band. This result is consistent with exciton delocalization between PTCDI molecules in the excimer-like state.

Oxygen-Induced Lattice Strain for High-Performance Organic Transistors with Enhanced Stability.

Integrating the merits of low cost, flexibility, and large-area processing, organic semiconductors (OSCs) are promising candidates for the next-generation electronic materials.The mobility and stability are the key figures of merit for its practical application. However, it is greatly challenging to improve the mobility and stability simultaneously owing to the weak interactions and poor electronic coupling between OSCs molecules. Here, an oxygen-induced lattice strain (OILS) strategy is developed to achieve OSCs with both high mobility and high stability. Utilizing the strategy, the maximum mobility of dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) organic field-effect transistor (OFET) rises to 15.3cm2 V-1 s-1 and the contact resistance lowers to 25.5Ωcm. Remarkably, the thermal stability of DNTT is much improved, and a record saturated power density of ≈3.4×104 Wcm-2 is obtained. Both the experiments and theoretical calculations demonstrate that the lattice compressive strain induced by oxygen is responsible for their high performance and stability. Furthermore, the universality of the strategy is manifested in both n-type and p-type small OSCs. This work provides a novel strategy to improve both the mobility and the stability of OSCs, paving the way for the practical applications of organic devices.

Humidity/Oxygen-Insensitive Organic Synaptic Transistors Based on Optical Radical Effect.

For most organic synaptic transistors based on the charge trapping effect, different atmosphere conditions lead to significantly different device performance. Some devices even lose the synaptic responses under vacuum or inert atmosphere. The stable device performance of these organic synaptic transistors under varied working environments with different humidity and oxygen levels can be a challenge. Herein, a moisture- and oxygen-insensitive organic synaptic device based on the organic semiconductor and photoinitiator molecules is reported. Unlike the widely reported charge trapping effect, the photoinduced free radical is utilized to realize the photosynaptic performance. The resulting synaptic transistor displays typical excitatory postsynaptic current, paired-pulse facilitation, learning, and forgetting behaviors. Furthermore, the device exhibits decent and stable photosynaptic performances under high humidity and vacuum conditions. This type of organic synaptic device also demonstrates high potential in ultraviolet B perception based on its environmental stability and broad ultraviolet detection capability. Finally, the contrast-enhanced capability of the device is successfully validated by the single-layer-perceptron/double-layer network based Modified National Institute of Standards and Technology pattern recognition. This work could have important implications for the development of next-generation environment-stable organic synaptic devices and systems.

Epitaxial growth of crystalline thin-films of perfluorinated copper phthalocyanine on the single-crystal copper phthalocyanine

Understanding and controlling the growth structures of molecular heterojunctions are important for the pursuit of highly efficient organic semiconductor devices. In this study, a well-defined molecular heterojunction was constructed by the deposition of an n-type organic semiconductor molecule, perfluorinated copper phthalocyanine (F16CuPc), on single-crystals of a typical p-type material, copper phthalocyanine (CuPc). The heterojunction was characterized using synchrotron radiation surface X-ray diffraction techniques to unveil the epitaxial growth of crystalline thin-films of F16CuPc on the CuPc surface. The inter-lattice relationships between F16CuPc and CuPc deduced from the present results imply that the in-plane orientation of the F16CuPc crystal lattice was predominantly governed by the molecular-level alignments of the π-backbones of the F16CuPc and CuPc molecules at the nucleation center for crystal growth at the interface.

A Calcination-Free Sol-Gel Method to Prepare TiO2 -Based Hybrid Semiconductors for Enhanced Visible Light-Driven Hydrogen Production.

In recent years, the sol-gel method has been extensively utilized to develop efficient and stable organic semiconductor composite titanium dioxide (TiO2 ) photocatalysts. However, the high-temperature calcination requirements of this method consume energy during preparation and degrade encapsulated organic semiconductor molecules, resulting in decreased photocatalytic hydrogen production efficiency. In this study, we found that by selecting an appropriate organic semiconductor molecule, 1,4-naphthalene dicarboxylic acid (NA), high-temperature calcination can be avoided in the sol-gel process, yielding an organic-inorganic hybrid material with stable and effective photocatalytic properties. The uncalcined material displayed a hydrogen production rate of 2920±15 μmol g-1 h-1 , which was approximately twice the maximum production rate observed in the calcined material. Likewise, the specific surface area of the uncalcined material, at 252.84 m2 g-1 , was significantly larger compared to the calcined material. Comprehensive analyses confirmed successful NA and TiO2 doping, while UV-vis and Mott-Schottky tests revealed a reduced energy bandgap (2.1 eV) and expanded light absorption range. Furthermore, the material maintained robust photocatalytic activity after a 40-hour cycle test. Our findings demonstrate that by using NA doping without calcination, excellent hydrogen production performance can be achieved, offering a novel approach for environmentally friendly and energy-saving production of organic semiconductor composite TiO2 materials.

Novel Organic Superbase Dopants for Ultraefficient N-Doping of Organic Semiconductors.

Doping is a powerful technique for engineering the electrical properties of organic semiconductors (OSCs), yet efficient n-doping of OSCs remains a central challenge. Herein, the discovery of two organic superbase dopants, namely P2-t-Bu and P4-t-Bu as ultra-efficient n-dopants for OSCs is reported. Typical n-type semiconductors such as N2200 and PC61 BM are shown to experience a significant increase of conductivity upon doping by the two dopants. In particular, the optimized electrical conductivity of P2-t-Bu-doped PC61 BM reaches a record-high value of 2.64Scm-1 . The polaron generation efficiency of P2-t-Bu-doped in PC61 BM is found to be over 35%, which is 2-3 times higher than that of benchmark n-dopant N-DMBI. In addition, a deprotonation-initiated, nucleophilic-attack-based n-doping mechanism is proposed for the organic superbases, which involves the deprotonation of OSC molecules, the nucleophilic attack of the resulting carbanions on the OSC's π-bonds, and the subsequent n-doping through single electron transfer process between the anionized and neutral OSCs. This work highlights organic superbases as promising n-dopants for OSCs and opens up opportunities to explore and develop highly efficient n-dopants.

Structural Rearrangement of Organic Semiconductor Molecules with an Asymmetric Shape in Thin Films

2-Decyl-7-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C10) exhibits excellent performances as an active layer in organic thin-film transistors, and its performances are greatly influenced by the molecular packing, i.e., the crystalline polymorphs. This compound has the so-called thin-film phase in a vapor-deposited film, which is different from the single-crystal structure (the bulk phase). In this work, thin films of Ph-BTBT-C10 are prepared by spin coating, and the effect of aging on the film structure is investigated by high-resolution infrared (IR) spectroscopy in combination with X-ray diffraction. The results show that the as-spun film has the same thin-film phase as the vapor-deposited film and that the thin-film phase is rapidly converted to the bulk phase by aging at room temperature. In addition, the spectral analysis also reveals conformational changes in the decyl chain during the crystalline transition process. This work highlights the importance of using IR spectroscopy with a high-wavenumber resolution for structural analysis of molecular thin films.

Disordered Phase-Assisted Growth of Organic Semiconductor Crystals on Self-Assembled Monolayer Templates.

Achieving high mobility and bias stability is a challenging obstacle in the advancement of organic thin-film transistors (OTFTs). To this end, the fabrication of high-quality organic semiconductor (OSC) thin films is critical for OTFTs. Self-assembled monolayers (SAMs) have been used as growth templates for high-crystalline OSC thin films. Despite significant research progress in the growth of OSC on SAMs, a detailed understanding of the growth mechanism of the OSC thin films on a SAM template is lacking, which has limited its use. In this study, the effects of the structure (thickness and molecular packing) of SAM on the nucleation and growth behavior of the OSC thin films were investigated. We found that disordered SAM molecules assisted in the surface diffusion of the OSC molecules and resulted in a small nucleation density and large grain size of the OSC thin films. Moreover, a thick SAM with disordered SAM molecules on the top was found to be beneficial for the high mobility and bias stability of the OTFTs.

Universally Exhaustive Generation of Molecular Structures and Prediction of Their Electronic States Using Machine Learning for N-type Organic Transistor Materials.

We have proposed a new method for the exploration of organic functional molecules, using an exhaustive molecular generator combined without combinatorial explosion and electronic state predicted by machine learning and adapted for developing n-type organic semiconductor molecules for field-effect transistors. Our method first enumerates skeletal structures as much as possible and next generates fused ring structures using substitution operations for atomic nodes and bond edges. We have succeeded in generating more than 4.8 million molecules. We calculated the electron affinity (EA) of about 51 thousand molecules with DFT calculation and trained the graph neural networks to estimate EA values of generated molecules. Finally, we obtained the 727 thousand molecules as candidates that satisfy EA values over 3 eV. The number of these possible candidate molecules is far beyond what we have been able to propose based on our knowledge and experience in synthetic chemistry, indicating a wide diversity of organic molecules.

Machine learning based charge mobility prediction for organic semiconductors.

Transfer integral is a crucial parameter that determines the charge mobility of organic semiconductors, and it is very sensitive to molecular packing motifs. The quantum chemical calculation of transfer integrals for all the molecular pairs in organic materials is usually an unaffordable task; fortunately, it can be accelerated by the data-driven machine learning method now. In this work, we develop machine learning models based on artificial neutral networks to predict transfer integrals accurately and efficiently for four typical organic semiconductor molecules: quadruple thiophene (QT), pentacene, rubrene, and dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT). We test various forms of features and labels and evaluate the accuracy of different models. With the implementation of a data augmentation scheme, we have achieved a very high accuracy with the determination coefficient of 0.97 and mean absolute error of 4.5meV for QT, and similar accuracy for the other three molecules. We apply these models to studying charge transport in organic crystals with dynamic disorders at 300K and obtain the charge mobility and anisotropy in perfect agreement with the brutal force quantum chemical calculation. If more molecular packings representing the amorphous phase of organic solids are supplemented to the dataset, the current models can be refined to study charge transport in organic thin films with polymorphs and static disorders.

Controlling Polymorphic Transitions in n-Type Organic Semiconductor Single Crystals by Alkyl Chain Engineering

Control of polymorphic behavior is crucial for designing functional organic semiconductor devices as even a slight structural difference may translate to dramatically different electronic properties. One route to controlling structure is through stimulus-induced polymorph transitions, which allows for switching those electronic properties. However, despite advances in predicting crystal structures, the molecular design characteristics governing the polymorphic transition mechanism remains unknown. Here, we systematically investigate a series of n-type organic semiconductor molecules based on 2-dimensional quinoidal terthiophene with varying alkyl side chain lengths to modulate two distinct polymorph transitions, one cooperative martensitic transition and the other non-cooperative nucleation and growth transition. In the three molecular systems, we observe that shortening the alkyl chain past a critical length suppresses the cooperative polymorph transition by limiting the alkyl chain conformation change. On the other hand, the nucleation and growth transition temperature increases as the side chain length decreases, possibly driven by the increase in the melting point of the alkyl chains. We also found that tuning the alkyl chain length modulates the associated quinoidal to aromatic biradical switching that drives the nucleation and growth transition, suggesting a synergy between the crystal structure and electronic structure. Ultimately depending on the exact mechanism of the polymorph transition, adjusting the alkyl chain length may lead to tuning of the polymorph transition temperature or suppression of the transition altogether. This offers a potential molecular design rule to target a particular transition mechanism based on the desired behavior for the system.

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.

Carbodicarbenes and Striking Redox Transitions of their Conjugate Acids: Influence of NHC versus CAAC as Donor Substituents.

Herein, a new type of carbodicarbene (CDC) comprising two different classes of carbenes is reported; NHC and CAAC as donor substituents and compare the molecular structure and coordination to Au(I)Cl to those of NHC-only and CAAC-only analogues. The conjugate acids of these three CDCs exhibit notable redox properties. Their reactions with [NO][SbF6 ] were investigated. The reduction of the conjugate acid of CAAC-only based CDC with KC8 results in the formation of hydrogen abstracted/eliminated products, which proceed through a neutral radical intermediate, detected by EPR spectroscopy. In contrast, the reduction of conjugate acids of NHC-only and NHC/CAAC based CDCs led to intermolecular reductive (reversible) carbon-carbon sigma bond formation. The resulting relatively elongated carbon-carbon sigma bonds were found to be readily oxidized. They were, thus, demonstrated to be potent reducing agents, underlining their potential utility as organic electron donors and n-dopants in organic semiconductor molecules.

Kraft Lignin: From Pulping Waste to Bio‐Based Dielectric Polymer for Organic Field‐Effect Transistors

AbstractLignin is an abundant biopolymer deriving from industrial pulping processes of lignocellulosic biomass. Despite the huge amount of yearly produced lignin waste, it finds scarce application as a fine material and is usually destined to be combusted in thermochemical plants to feed, with low efficiency, other industrial processes. So far, the use of lignin in materials science is limited by the scarce knowledge of its molecular structure and properties, depending also on its isolation method. However, lignin represents an intriguing feedstock of organic material. Here, the structural and chemical‐physical characteristics of two kraft lignins, L1 and L2, are analyzed. First, several molecular characterization techniques, such as attenuated total reflectance ‐ Fourier transform infrared spectroscopy, elemental analyses, gel permeation chromatography, evolved gas analysis‐mass spectrometry, UV–vis, 31P‐ and 13C‐ nuclear magnetic resonance spectroscopies are applied to get insights into their different structures and their degree of molecular degradation. Then, their efficient application as gate dielectric materials is demonstrated for organic field‐effect transistors, finding the increased capacity of L1 with respect to L2 in triggering functional and efficient devices with both p‐type and n‐type organic semiconductor molecules.

Molecular Arrangement Control of [1]Benzothieno[3,2-b][1]benzothiophene (BTBT) via Charge-Assisted Hydrogen Bond

Abstract Organic semiconductors have π-conjugation in the constituent molecules and exhibit optical and electrical properties. Since these properties are significantly affected by the overlap of π-orbitals between adjacent molecules, not only their molecular structures but also their molecular arrangement has been well known as critical; however, control of the molecular arrangement without modifying the electronic character of the constituent molecule has been difficult. In the current work, we report organic salts composed of disulfonic acid with a moiety of a representative organic semiconductor molecule, [1]benzothieno[3,2-b][1]benzothiophene (BTBT), as a functional component, and different types of alkylamines as an arrangement-controlling component via charge-assisted hydrogen bonds. We successfully controlled the molecular arrangement of BTBT moiety by changing alkylamines, without changing the structure of disulfonic acid with the BTBT functional moiety. Depending on the bulkiness of alkylamines, the molecular arrangement of these organic salts changed from an edge-to-face herringbone-type arrangement, where CH/π interactions were dominant similar to the common crystal structure of BTBT, to a novel one-dimensional (1D) slipped parallel-type arrangement for BTBT, without changing the molecular structure of disulfonic acid. In addition, we revealed that the dimensionality of the electronic state and properties of the organic salts also changed according to the molecular arrangement of BTBT moiety.

Lattice-mismatch-free growth of organic heterostructure nanowires from cocrystals to alloys

Organic heterostructure nanowires, such as multiblock, core/shell, branch-like and related compounds, have attracted chemists’ extensive attention because of their novel physicochemical properties. However, owing to the difficulty in solving the lattice mismatch of distinct molecules, the construction of organic heterostructures at large scale remains challenging, which restricts its wide use in future applications. In this work, we define a concept of lattice-mismatch-free for hierarchical self-assembly of organic semiconductor molecules, allowing for the large-scale synthesis of organic heterostructure nanowires composed of the organic alloys and cocrystals. Thus, various types of organic triblock nanowires are prepared in large scale, and the length ratio of different segments of the triblock nanowires can be precisely regulated by changing the stoichiometric ratio of different components. These results pave the way towards fine synthesis of heterostructures in a large scale and facilitate their applications in organic optoelectronics at micro/nanoscale.

Controlled Growth and Self‐Assembly of Multiscale Organic Semiconductor (Adv. Mater. 22/2022)

Multiscale Organic Semiconductors The intrinsic properties and devices performance of multiscale organic semiconductors can be regulated through chemical structure design and controlled self-assembly in a wide scale range from micro-molecule to aggregated structures. In article number 2102811, Ning Wang, Yuliang Li, and Ling Bai review the research progress from molecular level to aggregated structures by self-assembly of multiscale organic semiconductor molecules.

Microscopic Study of Molecular Double Doping

Double doping, in which a single dopant molecule induces two charge carriers in an organic semiconductor (OSC), was recently experimentally observed and promises to enhance the efficiency of molecular doping. Here we present a theoretical investigation of p-type molecular double doping in a CN6-CP:bithiophene-thienothiophene OSC system. Our analysis is based on density functional theory (DFT) calculations for the electronic ground state. In a molecular complex with two OSC oligomers and one CN6-CP dopant molecule, we explicitly demonstrate double integer charge transfer and find the formation of two individual polarons on the OSC molecules and a dianion dopant molecule. We show that the vibrational modes and related infrared absorption spectrum of this complex can be traced back to those of the charged dopant and OSC molecules in their isolated forms. The near-infrared optical absorption spectrum calculated by time-dependent DFT shows features of both typical intramolecular polaron excitations and weak intermolecular charge transfer excitations associated with the doping-induced polaron states.