Ambipolar Materials Research Articles

DFT study of co- and homo-oligo(fluoro-substituted aniline)s as ambipolar materials

In this work, a series of co- and homo-oligomers consisting 2-fluoroaniline (FA) and aniline (A) were studied as active p/n-conducting materials for use in the charge storage applications using the density functional theory (DFT) approach. The calculations were performed on the di-, tetra-, hexa-, and octamer derivatives of co-oligo(FA-A), oligo(FA), and oligo(A) in their undoped and both the p- and n-doped phases. The calculated results were used to evaluate the extent of the π-conjugation length, band gap energy, charge carrier injection rate, ionization potential, and electron affinity of co-oligomers with respect to the parent oligomers. A further characterization of the oligomers was performed through the UV–vis spectral characteristics computed by the time-dependent DFT. The results obtained were extrapolated to the infinite chain length to investigate the influence of fluorine substitution on the observed behavior of the corresponding polymers. More satisfactory properties were obtained for the co-polymers in both the p- and n-doped species with respect to poly(FA), reflecting their hole and electron transport facilities for conductivity.

A theoretical investigation of the effect of fluorination and bromination on the optoelectronic properties of tetrathienophenazine derivatives

The electronic and optical properties of crystalline tetrathienophenazine derivatives (l-TTP, m-TTP, t-TTP) and their four fluorinated and brominated derivatives are predicted using density functional theory within the generalized gradient approximation and including the van der Waals weak interactions. We analyze how the position of sulfur atoms and fluorination and bromination modify the charge transport in TTP derivatives. Our results show that the introduction of bromine substituents into TTP derivatives has a stronger effect than fluorination. The introduction of the fluorine atoms improves the hole transport in the l-TTP derivatives, while bromine atoms improve both hole and electron transport of the three TTP derivatives. Our findings suggest a new series of promising ambipolar organic materials.

COMPARISON OF PERFORMANCES OF ORGANIC PHOTOVOLTAIC CELLS USING SubPc AS CENTRAL AMBIPOLAR LAYER IN TERNARY STRUCTURES AND AS ELECTRON ACCEPTOR IN BINARY STRUCTURES

We compare the performances of organic photovoltaic cells (OPVCs) based on binary and ternary planar heterojunctions. The organic active layers are pentathiophene (5T), subphthalocyanine (SubPc) and fullerene (C60). SubPc being an ambipolar material we used it either as electron acceptor in binary OPVCs or as central layer in ternary cells in order to increase the efficiency of OPVCs using 5T as electron donor. So, the different OPVC configurations were 5T/C60, 5T/SubPc and 5T/SubPc/C60. The effect of the different organic layer thicknesses on the device performances was studied. In order to understand the behavior of the different OPVC configurations, we proceeded with a morphological study. The influence of the high roughness of the 5T layer on the OPVCs performances is discussed. The best OPVCs performances are obtained with the binary structure 5T/SubPc. Its maximum efficiency corresponds to an increase of 50% compared to the OPVC based on the couple 5T/C60. External Quantum Efficiency measurements show that both layers participate to the current generation. The efficiency increase is mainly due to the increase of the open circuit voltage ([Formula: see text]). In the case of ternary OPVCs, [Formula: see text] is limited by the band structure of 5T and C60, moreover, the efficiency is also limited by the poor charge collection efficiency of the ternary structure and the series resistance of the three stacked organic layers.

Synthesis of Heterocyclic Core-Expanded Bis-Naphthalene Tetracarboxylic Diimides.

A highly reactive bis-naphthalene tetracarboxylic diimide (bis-NDI) intermediate, TBrDNDI, was designed and synthesized for core-expanded NDIs. Based on this intermediate, we achieved 9- and 11-membered core-expanded bis-NDI derivatives. Through expanding the NDI core and introducing electron-donor or electron-acceptor groups, the frontier energy orbitals, optical and electrical properties of these bis-NDIs can be finely tuned to obtain air-stable ambipolar or n-type materials.

Influences of Structural Modification of Naphthalenediimides with Benzothiazole on Organic Field-Effect Transistor and Non-Fullerene Perovskite Solar Cell Characteristics.

Developing air-stable high-performance small organic molecule-based n-type and ambipolar organic field-effect transistors (OFETs) is very important and highly desirable. In this investigation, we designed and synthesized two naphthalenediimide (NDI) derivatives (NDI-BTH1 and NDI-BTH2) and found that introduction of 2-(benzo[d]thiazol-2-yl) acetonitrile groups at the NDI core position gave the lowest unoccupied molecular orbital (LUMO; -4.326 eV) and displayed strong electron affinities, suggesting that NDI-BTH1 might be a promising electron-transporting material (i.e., n-type semiconductor), whereas NDI-BTH2 bearing bis(benzo[d]thiazol-2-yl)methane at the NDI core with a LUMO of -4.243 eV was demonstrated to be an ambipolar material. OFETs based on NDI-BTH1 and NDI-BTH2 have been fabricated, and the electron mobilities of NDI-BTH1 and NDI-BTH2 are 14.00 × 10-5 and 8.64 × 10-4 cm2/V·s, respectively, and the hole mobility of NDI-BTH2 is 1.68 × 10-4 cm2/V·s. Moreover, a difference in NDI-core substituent moieties significantly alters the UV-vis absorption and cyclic voltammetry properties. Thus, we further successfully employed NDI-BTH1 and NDI-BTH2 as electron transport layer (ETL) materials in inverted perovskite solar cells (PSCs). The PSC performance exhibits that NDI-BTH2 as the ETL material gave higher power conversion efficiency as compared to NDI-BTH1, that is, NDI-BTH2 produces 15.4%, while NDI-BTH1 gives 13.7%. The PSC performance is comparable with the results obtained from OFETs. We presume that improvement in solar cell efficiency of NDI-BTH2-based PSCs is due to the well-matched LUMO of NDI-BTH2 toward the conduction band of the perovskite layer, which in turn increase electron extraction and transportation.

High-triplet-energy Bipolar Host Materials Based on Phosphine Oxide Derivatives for Efficient Sky-blue Thermally Activated Delayed Fluorescence Organic Light-emitting Diodes with Reduced Roll-off

We designed and synthesized two new ambipolar host materials, namely CzPO and Cz3PO. Combining CzPO and Cz3PO with CzTRZ2 as the emitter resulted in improved maximum external quantum efficiencies o...

2,3‐Di(thiophen‐2‐yl)quinoxaline Amine Derivatives: Yellow‐Blue Fluorescent Materials for Applications in Organic Electronics

AbstractIn this work, we have designed and synthesized new donor–acceptor (D–A) type, 2,3‐di(thiophen‐2‐yl)quinoxaline based amine derivatives (2–10). These compounds contain diaryl/heterocyclic amine segment and quinoxaline core as an electron donor and acceptor respectively. The synthesized molecules were fully characterized by standard spectroscopic techniques. Absorption spectra of all compounds showed intramolecular charge transfer (ICT) transition in the range of 390 to 461 nm. Dyes emit in yellow–blue region with emission maxima within 465 to 566 nm on excitation at their respective ICT absorption maxima. Moreover, compound 2, 3 and 9 exhibit aggregation induced emission in THF:water system due to the formation of nanoaggregates. Electrochemical properties of 2–10 were studied by cyclic voltammetry. The HOMO and LUMO energy level were found in the range of –4.90 to –5.70 eV and –3.10 to –3.36 eV respectively, with an electrochemical band gap (Eg) from 1.72 to 2.48 eV. Molecules showed good thermal stability. Theoretical studies were also carried out by using DFT and TD–DFT calculation. Based on photophysical and electrochemical properties these derivatives could be used as solid state emitter and ambipolar charge transport materials in optoelectronic devices.

Quinoxaline based amines as blue-orange emitters: Effect of modulating donor system on optoelectrochemical and theoretical properties

A series of eight novel donor‒acceptor based quinoxaline‒amine derivatives 2–9 were prepared by modulating donor species on quinoxaline core with Buchwald‒Hartwig coupling amination reaction. The synthesized molecules were fully characterized and studied for impact of D−A interaction on optoelectrochemical properties of derivatives. Absorption spectra of 2−9 display intramolecular charge transfer (ICT) transitions in the range of 377–456 nm. Dyes 2–9 show positive solvatochromism in emission and emit in blue‒orange region with emission maxima 472–592 nm on excitation at their respective ICT maxima in toluene, chloroform, DCM and neat solid film. Herein, dyes 2, 3 and 7 which possess intense emission in solid state were further studied for Aggregation Induced Emission (AIE) effect. The HOMO and LUMO energy level for compound 2–9 obtained by CV were found in the range of −5.23 to −5.83 eV and −3.55 to −3.74 eV. Theoretical studies of molecules were also carried out by using TD‒DFT calculations. The comparable HOMO and LUMO energy level of 2−9 with reported ambipolar materials and efficient solid state emission make synthesized compounds potential candidate for solid state emissive, ambipolar materials in organic electronics.

Rational Design of Low-Band Gap Star-Shaped Molecules With 2,4,6-Triphenyl-1,3,5-triazine as Core and Diketopyrrolopyrrole Derivatives as Arms for Organic Solar Cells Applications.

A series of D–A novel star-shaped molecules with 2,4,6-triphenyl-1,3,5-triazine (TPTA) as core, diketopyrrolo[3,4-c]pyrrole (DPP) derivatives as arms, and triphenylamine (TPA) derivatives as end groups have been systematically investigated for organic solar cells (OSCs) applications. The electronic, optical, and charge transport properties were studied using density functional theory (DFT) and time-dependent DFT (TD-DFT) approaches. The parameters such as energetic driving force ΔEL−L, adiabatic ionization potential AIP, and adiabatic electron affinity AEA were also calculated at the same level. The calculated results show that the introduction of different groups to the side of DPP backbones in the star-shaped molecules can tune the frontier molecular orbitals (FMOs) energy of the designed molecules. The designed molecules can provide match well with those of typical acceptors PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) and PC71BM ([6,6]-phenyl-C71-butyric acid methyl ester). Additionally, the absorption wavelengths of the designed molecules show bathochromic shifts compared with that of the original molecule, respectively. The introduction of different groups can extend the absorption spectrum toward longer wavelengths, which is beneficial to harvest more sunlight. The calculated reorganization energies suggest that the designed molecules are expected to be the promising candidates for ambipolar charge transport materials except molecule with benzo[c]isothiazole group can be used as hole and electron transport material. Moreover, the different substituent groups do not significantly affect the stability of the designed molecules.

Seebeck Tensor Analysis of (p × n)-type Transverse Thermoelectric Materials

ABSTRACTSingle-leg (p × n)-type transverse thermoelectrics (TTE) are reviewed as an alternative to conventional or “longitudinal” double-leg thermoelectrics for applications at room temperature and below. As the name suggests, this unique behavior of (p × n)-type transverse thermoelectrics results from choosing ambipolar anisotropic materials that have a Seebeck tensor with orthogonal p- and n-type Seebeck coefficients, leading to transverse relation between net heat and net electrical current. One feature of such materials is that they can operate near intrinsic doping and, therefore will not suffer from dopant freeze-out, opening the possibility of new cryogenic operation for solid state cooling. In this work, a Seebeck tensor analysis of thermoelectric materials is presented. To compare the performance of transverse thermoelectric materials, a transverse power factor PF⊥ is introduced. Materials searches based on these simple criteria reveal that over 1/4 of the database of about 48,000 inorganic materials could potentially function as (p × n)-type TTE’s, demonstrating the underappreciated prevalence of this class of materials.

New types of organic semiconductors based on diketopyrrolopyrroles and 2,1,3-benzochalcogenadiazoles: a computational study.

A comprehensive computational study is performed on model compounds based on 2,1,3-benzochalcogenadiazoles and diketopyrrolopyrroles of A-π-A'-π-A architecture (A and A' represent 2,1,3-benzochalcogenadiazoles and diketopyrrolopyrroles, respectively, and π is the bridging unit between them including thiophene, furan, and selenophene) for their utility as organic semiconductors. The compounds were found to possess planar geometry, which is a desired property for organic semiconductors. The electronic properties, including adiabatic and vertical electron affinity (EA), adiabatic and vertical ionization potential (IP), reorganization energy (λ), hole injection barrier and electron injection barrier, transfer integral, and charge mobility, were calculated. The electron affinity is higher in the case of thiophene/selenophene as the linker for a given terminal benzochalcogenadiazole than the corresponding compounds with furan as a linker, while the ionization potential is lowest for compounds having selenophene as the linker with a given terminal benzochalcogenadiazole moiety than the compounds having furan or thiophene as a linker. The hole injection barrier in these compounds is lower than the electron injection barrier, which facilitates the hole injection from the metal electrode, while hole reorganization energy is found to be larger than the electron reorganization energy. The compounds possess hole mobilities in the range of 2.50-4.91cm2/Vs and electron mobilities in a similar range of 4.58-9.68cm2/Vs. This study reveals that compounds based on a combination of diketopyrrolopyrrole and 2,1,3-benzochalcogenadiazole units would exhibit hole transporting properties and act as potential ambipolar materials.

Improving optoelectronic and charge transport properties of D-π-D type diketopyrrolopyrrole-pyrene derivatives as multifunctional materials for organic solar cell applications.

A series of novel diketopyrrolopyrrole-pyrene-based molecules were designed for small molecule based organic solar cell (SMOSC) applications. Their electronic and charge transfer properties were investigated by applying the PBE0/6-31G(d,p) method. The absorption spectra were simulated using the TD-PBE0/6-31G(d,p) method. The results showed that the frontier molecular orbital (FMO) energy levels, reorganization energy, the energetic driving force, and absorption spectra can be tuned by the introduction of different aromatic heterocyclic groups to the side of diketopyrrolopyrrole fragments' backbones. Additionally, the designed molecules possess suitable FMOs to match those of typical acceptors PC61BM and PC71BM. Meanwhile, the designed molecules can act as good ambipolar charge transport materials in SMOSC applications. Meanwhile, the electron and hole reorganization energies of the designed molecules are smaller than those of the typical electron and hole transport materials, respectively. Moreover, the differences between electron and hole reorganization energies do not exceed 0.046 eV. Our results suggest that the designed molecules can act as promising candidates for donor and ambipolar charge transport materials in SMOSC applications.

Controlling Ambipolar Transport and Voltage Inversion in Solution-Processed Thin-Film Devices through Polymer Blending

Ambipolar semiconductors are attracting a great interest as building blocks for photovoltaics and logic applications. Field-effect transistors built on solution-processable ambipolar materials hold strong promise for the engineering of large-area low-cost logic circuits with a reduced number of devices components. Such devices still suffer from a number of obstacles including the challenging processing, the low Ion/Ioff, the unbalanced mobility, and the low gain in complementary metal–oxide–semiconductor (CMOS)-like circuits. Here, we demonstrate that the simple approach of blending commercially available n- and p-type polymers such as P(NDI2OD-T2), P3HT, PCD-TPT, PDVT-8, and IIDDT-C3 can yield high-performing ambipolar field-effect transistors with balanced mobilities and Ion/Ioff > 107. Each single component was studied separately and upon blending by means of electrical characterization, ambient ultraviolet photoelectron spectroscopy, atomic force microscopy, and grazing incidence wide angle X-ray scattering to unravel the correlation between the morphology/structure of the semiconducting films and their functions. Blends of n- and p-type semiconductors were used to fabricate CMOS-like inverter circuits with state-of-the-art gains over 160 in the case of P(NDI2OD-T2) blended with PDVT-8. Significantly, our blending approach was successful in producing semiconducting films with balanced mobilities for each of the four tested semiconductor blends, although the films displayed different structural and morphological features. Our strategy, which relies on establishing a correlation between ambipolar performances, film morphology, molecular structure, and blending ratio, is extremely efficient and versatile; thus it could be applied to a wide range of polymers or solution processable small molecules.

Helical Ullazine-Quinoxaline-Based Polycyclic Aromatic Hydrocarbons.

Polycyclic aromatic azomethine ylides (PAMYs) are powerful building blocks in the bottom-up synthesis of internally nitrogen-containing polycyclic aromatic hydrocarbons (N-PAHs) through 1,3-cycloaddition reactions. In this work, the cycloaddition reaction of PAMYs to asymmetric ortho-quinones is presented, which, in contrast to the addition to symmetric para-quinones, facilitates subsequent condensation reactions and allows the synthesis of three helical N-PAHs with ullazine-quinoxaline (UQ-1-3) backbones. UQ-1 and UQ-2 possess two helical centers; however, single-crystal X-ray analysis together with the computational modeling of UQ-3 elucidate the formation of only the thermodynamically most stable geometry with four helical centers in a (P,P,M,M) configuration. For the series UQ-1-3, the number of redox steps is directly correlated with the number of ullazine or quinoxaline units incorporated into the targeted molecular backbones. A detailed investigation of the spectroscopic and magnetic properties of the radical cation and anion as well as the dication and dianion species by in situ EPR/UV/Vis-NIR spectroelectrochemistry is provided. The excellent optical and redox properties combined with helical geometries render them possibly applicable as chiral emitter or ambipolar charge transport material in organic electronics.

Theoretical characterisation and design of D–π–A star-shaped molecules with triphenylamine as core and diketopyrrolopyrroles as arms for organic solar cells

ABSTRACTA series of novel D–π–A star-shaped molecules with triphenylamine (TPA) fragments as cores, diketopyrrolopyrrole (DPP) fragments as arms, and different conjugate π-bridges those connect TPA and DPP fragments have been designed for small molecules based organic solar cells (OSCs) applications. The optical, electronic, and charge transporting properties have been systematically investigated by means of density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The parameters such as energetic driving force ΔEL-L, adiabatic ionisation potentials (AIP), and adiabatic electron affinities (AEA) were also calculated at the same level to preliminarily evaluate the performance of the OSCs. The results of frontier molecular orbitals (FMOs) show that the designed molecules provide the best match matched energy levels with typical acceptors PCBM, bisPCBM, and PC70BM except for molecule containing benzo[c][1,2,5]thiadiazole unit because of its low energetic driving force ΔEL-L. It turned out that the FMO energy levels, the band gaps, AIP, AEA, and absorption spectrum can be tuned effectively by the introduction of different groups. Additionally, our results suggest that the designed molecules can act as promising candidates for donor materials and ambipolar charge transport materials for OSCs.

Van der Waals Heterostructure Devices with Dynamically Controlled Conduction Polarity and Multifunctionality

AbstractControlling the conduction behavior of 2D materials is an important prerequisite to achieve their electronic and optoelectronic applications. However, most of the reported approaches are aware of the shortcomings of inflexibility and complexity, which limits the possibility of multifunctional integration. Here, taking advantage of van der Waals heterostructure engineering, a simple method to achieve a dynamically controlled binary channel in a semivertical MoTe2/MoS2 field effect transistor is proposed. It is enabled by the high switchability between tunneling and thermal transports through simply changing the sign of voltage bias. In addition, the proposed system allows for multifunctional integration of transistor with on/off ratio >107 and diode with rectification ratio >106. Moreover, the devices show screen capability to negative photoresponse effect that is widely observed in ambipolar materials, hence improving the photodetection reliability and sensitivity. This study broadens the functionalities of van der Waals heterostructures and opens up more possibilities to realize multifunctional devices.

Dipole Moment in the Excited State: An Important Property for TADF Hosts

In this issue of Chem, Xu and co-workers report highly efficient blue organic light-emitting diodes with small-efficiency roll-off by reducing the dipole moment of the host materials in the excited state. Their simple but effective molecular design introduces a σ-bridge into ambipolar host materials as a separator of π-conjugation.

A novel approach to ambient energy (thermoelectric, piezoelectric and solar-TPS) harvesting: Realization of a single structured TPS-fusion energy device using MAPbI3

Organic-inorganic lead halide perovskite (MAPbI3) has shown remarkable photovoltaic performance along with unique properties such as ambipolar nature, ferroelectric and ultra-high Seebeck coefficient with low thermal conductivity. The exclusive properties of MAPbI3 enable to use as a fusion energy conversion material for harvesting of ambient energy such as the thermal, mechanical and solar energy. Many researchers have focused on each of these topics separately. This paper demonstrates a flexible single structured TPS-fusion energy device comprising of interdigitated electrodes (IDEs) and MAPbI3 to harvest thermal, mechanical and solar energy. A TPS-device is individually operated as a thermoelectric, piezoelectric, and solar (TPS)-energy harvester by independently applying a thermal gradient, mechanical force and solar energy. The TPS-device as a thermoelectric generator is exhibited the higher thermoelectric output performance in in-plane temperature gradient than out-of-plane temperature gradient. The obtained piezoelectric output values of the TPS-device as a piezoelectric generator under periodic applied pressure are 1.47 V and 0.56 µA. The TPS-based photovoltaic device is generated an open-circuit voltage of 0.77 V and a short-circuit current density of 0.022 mA/cm2. This conceptual demonstration of fusion energy harvesting device using IDE-based structure with light active ambipolar material could introduce innovative approaches in fusion-conversion energy technology.

Comparison of performances of three active layers cascade OPVCs with those obtained from corresponding bi-layers

In this study, organic photovoltaic cells based on planar heterojunctions using small-molecules were fabricated. Two cell configurations were used, planar heterojunctions based on a classical electron donor/electron acceptor couple and planar heterojunctions using three active layers, one donor, one acceptor and a central ambipolar layer. The donor is a thiophene derivative called BSTV, the acceptor is the fullerene while the ambipolar material is a phthalocyanine dye, the SubPc. After studying the performances of cells based on the couples SubPc/C60, BSTV/C60 and BSTV/SubPc, we explore the dependence of the short circuit current and the external quantum efficiency on the SubPc interlayer thickness in cells with three active layers. By comparison with the optimum couple SubPc/C60 the ternary structure BSTV/SubPc/C60 exhibits higher short circuit current but smaller fill factor. Nevertheless, for an optimum interlayer thickness of 12.5 nm, the ternary structure allows an improvement of the cell efficiency of 11% by comparison with the highest result obtained with the best couple. As shown by optical measurements, long range Föster energy transfer from the larger band gap donor, BSTV, to the smaller band gap, SubPc, is possible. Therefore, the increase of Jsc is attributed to two-step exciton dissociation process. Excitons produced in the outer BSTV layer are transferred to the SubPc central layer and then dissociated at the interface.

Recombination Strategy for Processable Ambipolar Electroactive Polymers in Pseudocapacitors

In order to circumvent the contradiction between processability and electrochemical performances, we report a recombination strategy in both unit and segment scales for processable ambipolar electroactive materials starting from parental moieties: classic ambipolar polymer (PEBE) and its soluble analogue (PPBP). On the basis of this strategy, two recombined polymers (PEB-P and PEBE-PBP) are successfully prepared via direct (hetero)arylation copolymerization. Through systematical characterization of properties, both PEB-P and PEBE-PBP not only possess the solubility fostered by PPBP but also exhibit identical electrochemical and optical properties to PEBE, demonstrating the reliability of this recombination strategy. Moreover, film electrodes of these recombined polymers exhibit desirable pseudocapacitance, acceptable stability, and superior processability to PEBE, making them promising candidates for Type III supercapacitors and other multifunctional energy storage devices.