Charge Transfer Material Research Articles

Tuning optoelectronic properties of indandione-based D-A materials by malononitrile group acceptors: A DFT and TD-DFT approach.

Indandione-based materials are promising candidates for organic electronics, offering high electron mobility and tunable optoelectronic properties. In this study, we explore the optoelectronic and photovoltaic properties of indandione-based donor-acceptor (D-A) materials, specifically (R1) and (R2), by introducing malononitrile group acceptors into their molecular structure. These strong electron-withdrawing acceptors are designed to enhance charge transfer and overall material performance. The designed molecules (DM1-DM4) exhibit a low optical band gap of approximately 1.77eV, significantly lower than the reference materials (R1 and R2) at around 2.24eV in a solvent environment. Among the designed molecules, DM4 stands out with superior photovoltaic parameters, including a narrow optical band gap (1.77eV), higher electron affinity (3.49eV), an extended excited state lifetime (10.0ns) owing to its low electron and hole reorganization energies (λe ~ 0.13eV and λh ~ 0.24eV), and improved short-circuit current density (Jsc) of ~ 15.73mA/cm2. Notably, DM4 achieves a power conversion efficiency (PCE) of ~ 18.5%, making it an excellent candidate for device applications. A comprehensive computational investigation was carried out on indandione-based D-A materials with malononitrile group acceptors (DM1-DM4) using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods, as implemented in Gaussian 16 software. We examined the electronic and optical properties of the proposed molecules through frontier molecular orbital (FMO) analysis, UV-Vis absorption spectra, density of states (DOS), exciton binding energy (Eb), and transition density matrix (TDM) analysis, utilizing GaussView 6.0 and Multiwfn 3.8 software. The photovoltaic parameters and power conversion efficiency (PCE) were evaluated using the Scharber and Alharbi models.

Hybrid Local and Charge-Transfer Material with Ultralong Room Temperature Phosphorescence for Efficient Organic Afterglow Light-Emitting Diodes.

Organic ultralong room temperature phosphorescence (OURTP) materials capable of combining various emission behaviors for diversified optoelectronic properties and applications have recently gained a vigorous development, but it remains a forbidden challenge in designing OURTP molecules with hybrid local and charge-transfer (HLCT) feature, possibly due to the elevated difficulties in simultaneously meeting the stringent requirements of both HLCT and OURTP emitters. Here, through introducing multiple heteroatoms into one-dimensional fused ring of coumarin with moderate charge transfer perturbation in donor-π-acceptor architecture, we demonstrate a HLCT-featured OURTP molecule showing both promoted fluorescence with a quantum yield of 77% in solution and long-lived OURTP with a lifetime of 251 ms in conventional host material used in electroluminescent device. Thus, efficient OURTP organic light-emitting diodes (OLEDs) were fabricated, exhibiting bright electroluminescence with an exciton utilization efficiency of 85% and yellow OURTP lasting over 2 s for afterglow. Impressively, the HLCT OURTP-OLEDs can be further optimized to reach an unprecedented total external quantum efficiency (EQE) of ~12% and OURTP EQE up to 3.11%, representing the highest performance among the reported OURTP-OLEDs. These impressive results highlight the significance to fuse HLCT and OURTP together in enriching OURTP materials and improving the afterglow OLED performances.

Regulation of the light color and electroluninescence of rigid orthogonally linked hybrid local and charge-transfer (HLCT) material: Intramolecular charge-transfer and intermolecular excimer

Regulation of the light color and electroluninescence of rigid orthogonally linked hybrid local and charge-transfer (HLCT) material: Intramolecular charge-transfer and intermolecular excimer

Highly Luminescent and Semiconducting Supramolecular Organic Charge Transfer Complex Generated via H‐Bonding Interaction Pathway

AbstractThe construction of a charge transfer (CT) organic material with important electrical properties is demonstrated where the charge transfer state is generated via delocalization through H‐bonding interaction pathway. The reaction of simple organic molecule pyromellitic acid with orthophenylene diamine in 1:2 ratios gives a colorless cocrystal 1’. The rare transition phenomena of a colorless cocrystal 1’ to an orange charge transfer state 1 take place in methanol water media over several days. The charge transfer state is isolated as orange single crystals of 1 which show semiconducting as well as fluorescent properties. The X‐ray diffraction analysis of 1 demonstrates the formation of charge separated extended H‐bonded network which is supposed to be solely responsible for the delocalization of charge in the system. This is the first example of formation of donor–acceptor‐based charge transfer material constructed by just tuning the H‐bond distance of a colorless cocrystalline state with non‐chalcogen and non‐halogenated molecules.

The Orbital Nature of Electron Holes in BaFeO3 and Implications for Defect Chemistry

Understanding the nature of electron holes in mixed protonic–electronic conducting perovskites is key to the successful development of novel electrode materials for protonic ceramic fuel and electrolyzer cells (PCFCs). Here, we use density functional theory to investigate the orbital interactions and defect chemistry of BaFeO3−δ─a model material for more complex cation stoichiometries used in PCFCs. The calculations revealed BaFeO3−δ to be a negative charge-transfer material with a dominating d5L ( L = ligand hole) configuration. Chemical bonding analysis using the projected Crystal Orbital Hamilton Population (COHP) further showed that the ligand holes are partially delocalized in pdσ* bonds with an approximately 80% share at the oxygen ions. The defect chemistry of BaFeO3 was explored with respect to oxygen vacancies and protonic defects. The oxygen vacancy formation energy increases with decreasing ligand hole concentration due to a rising Fermi level at which electrons from the removed oxygen are accommodated. This effectively outweighs a concomitant decrease in the Fe–O bond strength evidenced by energy-integrated COHP analysis. The dissociative H2O absorption was studied, where OH– fills an oxygen vacancy and H+ is attached to a regular oxygen ion. The energy of this reaction becomes more negative with decreasing ligand hole concentration. This trend largely reflects a more favorable OH– affinity with decreasing ligand hole concentration.

Pressure-Induced Local Excitation Promotion: New Route toward High-Efficiency Aggregate Emission Based on Multimer Excited State Modulation.

Achieving high-efficiency solid state emission is essential for practical applications of organic luminescent materials. However, intermolecular interactions generally induce formation of multimeric aggregate excited states with deficient emissive ability, making it extremely challenging to enhance emission in aggregated states. Here we demonstrate a novel strategy of continuously regulating multimeric excitation constituents with a high-pressure technique successfully enhancing the emission in a representative organic charge-transfer material, Laurdan (6-lauroyl-N,N-dimethyl-2-naphthylamine). The Laurdan crystal exhibits distinct emission enhancement up to 4.1 GPa accompanied by a shift in the emission color from blue to cyan. Under compression, the π-π interplanar distance in Laurdan multimers is reduced, and intermolecular wave function diffusion is demonstrated to be improved simultaneously, which results in local excitation promotion and thus enhanced emission. Our findings not only provide new insights into multimeric excited state emission modulation but also pave the way for the further design of high-performance aggregated luminophores.

Engineering dual charge transfer material modified ZnxCd1−xS towards highly effective photocatalytic pure water splitting

The MOF-derived Co@NC and Co–Pi as charge transfer materials for Zn0.5Cd0.5S are prepared to achieve directional modulation separation of charge carrier where Co@NC and Co–Pi capture electrons and holes, respectively, realizing pure water splitting.

An electron rich indaceno [2,1-b:6,5-b′] dithiophene derivative as a high intramolecular charge transfer material in dye sensitized solar cells

The synthesis, characterisation and photovoltaic performance of an indacenodithiophene (IDT)-based organic dye in DSSCs has been presented.

Nonionic Sc3N@C80 Dopant for Efficient and Stable Halide Perovskite Photovoltaics

One major challenge in the commercialization of halide perovskite solar cells is the device instability against moisture. Efficient perovskite solar cells usually incorporate the ionic dopants within the charge transfer material to secure their electrical conductivity. Typical ionic dopants are hygroscopic, resulting in significant moisture adsorption and accelerated perovskite degradation. Herein, we report a nonionic dopant, Sc3N@C80, consisting of a hydrophobic fullerene cage that encapsulates the metal salt to achieve a moisture resistive and highly electrically conductive hole transfer layer (HTL). The direct electronic transaction between Sc3N@C80 and spiro-OMeTAD renders a drastically improved conductivity and a lower Fermi-level of the HTL to minimize the Schottky barrier. The hydrophobicity of Sc3N@C80 also decreases the moisture wettability. Perovskite solar cells using the Sc3N@C80-doped HTL exhibit an efficiency surge from 18.15 to 20.77% (champion cell exhibiting 21.09%) and improved device s...

Pyrene-Like HOMO Governs Polaron Delocalization in Model Graphitic Strips: A Combined Experimental and Computational Analysis

Polycyclic aromatic hydrocarbons (PAHs) with large graphitic cores have attracted significant attention as charge-transfer materials for photovoltaic and molecular electronics applications. In this work, we probe the redox and optoelectronic properties of novel hexabenzo[a,c,fg,j,l,op]tetracene (HBT) and its methoxylated analogue MeOHBT to seek whether long HBT-based graphitic strips can be viable candidates as efficient charge-transfer material. Our data reveal that despite the presence of eight electron-rich methoxy groups in MeOHBT the redox/optoelectronic properties of these two HBTs are very similar, an unusual finding in comparison with other smaller PAHs and poly-p-phenylene wires. Precise crystal structures of neutral HBTs and their cation radicals in comparison with carefully benchmarked DFT calculations revealed that the polaron in both HBT+• and MeOHBT+• is mainly localized at the central pyrene, with a minor spillover to peripheral biphenyl moieties, and mirrors the distribution of pyrene-like HOMO of neutral HBTs. Finally, aided by the DFT calculations, we show that redox properties of long HBT-based graphitic strips are nearly invariant to both substitution and length, with the polaron delocalization limited largely to only one pyrene moiety. This finding suggests that this class of wires could be promising candidate materials for long-range charge transfer studies.

From Molecules to Devices: A DFT/TD-DFT Study of Dipole Moment and Internal Reorganization Energies in Optoelectronically Active Aryl Azo Chromophores

The present paper elicits a very useful, computational exploration of molecular architectonics of benzothiazole scaffold. The investigation elucidates hopping transport phenomenon in donor–acceptor ensembles lying within a same molecule, comprising an azo group as the core entity. In depth electronic and charge transfer behavior of certain substituted aryl azo benzothiazoles (organic π conjugated) considering notions of density functional theory (DFT) and time-dependent density functional theory (TD-DFT) has been investigated. Moreover, the effect of structural variation in aryl azo moiety (−CF3, −SCH3) in presence/absence of solvent has been examined. Also, the impact of disparate solvents namely, polar protic, polar aprotic, and nonpolar solvents has been deduced. Interestingly, results indicate that (E)-2-((4-(trifluoromethyl)phenyl)diazenyl)benzo[d]thiazole (BAF) and (E)-2-(phenyldiazenyl)benzo[d]thiazole (BAB) have affirmed to be the promising candidates for the organic charge transfer material in or...

Theoretical investigation on the electronic structures and spectroscopic properties as well as the features as dyes in dye-sensitized solar cells of quinonoid containing Re(I) complexes

A series of Re(I) model complexes with the bidentate NˆO type ligand (NˆO = 2,5-bis(4ʹ-(isopropyl)-anilino)-1,4-benzoquinone) involving quinone group, which are expressed as [Re2(CO)6(NˆO)L2] (L = 4-dimethylaminopyridine) 1, [Re(CO)3(NˆO)L] (L = 4-dimethylaminopyridine a1; 4-carboxylicpyridine a2; ethynyl b1; phenol b2), and [Re(CO)2(NˆO)L] (L = 4,4ʹ-dicarboxyl-2,2ʹ-bipyridine (NˆN)) c1, have been investigated by means of the density functional theory (DFT) in combination with time-dependent DFT (TDDFT) to explore their electronic structures and spectroscopic properties as well as the features as dyes in dye-sensitized solar cells (DSSCs). The geometry structure optimization results in the ground state at PBE1PBE/[LanL2DZ, 6-311G(d)] level revealed that Re(I) in all the model complexes exhibited the distorted octahedral coordination conformation. The frontier molecular orbitals of the complexes were effectively tuned on the energy level and energy gaps through introducing the ligands or ancillary group with different π-donating ability. TD/DFT calculations uncovered that all the mononuclear complexes displayed absorptions and emissions both in the visible light region. The comprehensive results including ionization potential (IP) and electron affinity (EA) along with the reorganization energy (λ) properties indicated that a2 and c1 had the advantage to act as the charge transfer material. The calculated light harvest efficiency (LHE) and the negative difference of free energy (−ΔGinject) value of ∼0.5 and 0.7 eV, respectively for a2 and c1 suggested that the two complexes were promising candidates as dyes in DSSCs.

A high performance deep-blue emitter with an anti-parallel dipole design

To increase the fluorescence quantum yield of hybrid local and charge-transfer (HLCT) material, we introduce a symmetric linear D-π-A-π-D structure into molecular design and synthesized a new blue emitter, 2,2'-(2′,3′,5′,6'-tetrafluoro-[1,1':4′,1″-terphenyl]-4,4″-diyl)bis(1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole) (4FBTPI). Organic light-emitting devices employing 4FBTPI as a dopant emitter show good performances, with CIE (Commission Internationale de l’Enclairage) coordinates of (0.15, 0.09), which is very close to NTSC blue light standard (0.14, 0.08), current efficiencies (CE) of 6.03 cd A−1, external quantum efficiencies of 6.94%. This performance is comparable to those of recently reported state-of-the-art deep-blue materials with CIEy < 0.10.

Rabi-like vibrational coherence transfer in a hydrogen-bonded charge transfer material

We report ultrafast spectroscopic measurements of intersite charge transfer in a single crystal of the hydrogen-bonded material quinhydrone showing anticorrelated dynamics of vibrational coherences at 172 and $216\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. To explain these coherent dynamics we derive a density matrix model in the presence of higher order electron-vibration coupling. Given the symmetry of vibrations calculated using density functional theory, the Huang-Rhys parameter of the Raman-active vibration found from spontaneous resonance light scattering measurements, and previously reported nonresonant impulsive stimulated Raman scattering measurements on quinhydrone, we restrict the density matrix model to three levels in the excited state of this material to simulate the observed dynamics with a density matrix approach. The close agreement between the experiment and our theoretical treatment leads us to conclude that the measured behavior corresponds to intermolecular Rabi-like oscillatory coherence transfer. These results provide foundational knowledge into the capability of functional organic materials to support quantum coherent transport of charge and energy as well as shed light on recent experimental and theoretical investigations of room temperature organic ferroelectrics.

Distinguishing between Mechanical and Electrostatic Interaction in Single Pass Multi Frequency Electrostatic Force Microscopy Measurements on a Molecular Material.

Single-pass electrostatic force microscopy is postulated as one of the most advanced techniques in terms of spatial resolution and fastness in data acquisition for the study of electrostatic phenomena at the nanoscale. However, crosstalk anomalies, in which mechanical interactions combine with tip-sample electrostatic forces, are still a major issue to overcome, specifically in soft and biological samples. In this paper we propose a novel method based on bimodal-atomic force microscopy to distinguish mechanical crosstalk from electrostatic images. The method is based in the comparison of bimodal AFM images with electrostatic ones, where pure mechanical interaction can be discerned from a mixture of mechanical and electrostatic interactions. The proposed method is optimized and demonstrated using a supramolecular charge transfer material. Finally, the method is used as a tool to depict different crosstalk levels in tetrathiafulvalene-based (TTF) assemblies, discerning between electrical and mechanical interactions. This kind of observation is important for obtaining accurate descriptions of charge distribution in samples made from organic and molecular layers and materials.

Probing ferroelectric behaviour in charge-transfer organic meta-nitroaniline

Potential ferroelectricity in charge-transfer organic materials is often masked by the intrinsic conductivity. Here, we report the compelling evidence of ferroelectricity in organic π-conjugated meta-nitroaniline (m-NA) crystals as shown by the local electromechanical measurements using the piezoresponse force microscopy (PFM) technique. m-NA is a charge-transfer molecular material with the exceptional optical non-linearity and perceptible conductivity along the crystallographic polar axis. While standard Sawyer-Tower measurements revealed an apparently lossy-dielectric hysteresis, The PFM switching spectroscopy indicated clear ferroelectric behaviour in this technologically important multifunctional material. Further study of the pyroelectric properties in m-NA crystals confirmed their high spontaneous polarization of 18 μC/cm2 at room temperature, comparable to the best known organic ferroelectrics.

Electronic properties of the charge transfer material MnPc/F4TCNQ

We present electronic properties of a charge transfer material consisting of Manganese(ii)Phthalocyanine (MnPc) and 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), investigated by means of photoemission spectroscopy and electron energy-loss spectroscopy, as well as supporting density functional theory calculations. We report the successful formation of a bulk material characterized by a strong interaction of the molecular compounds which affects the optical properties significantly. Our investigations reveal a significant charge transfer, whereas the MnPc molecule is oxidized and F4TCNQ is reduced. The valence band data indicate a full charge transfer between the two partners. The electronic excitation spectrum reveals a relatively small energy gap of MnPc/F4TCNQ of about 0.7 eV, which is related to a charge transfer excitation.

Resonant interactions between discrete phonons in quinhydrone driven by nonlinear electron-phonon coupling

This study reports experimental, computational, and theoretical evidence for a previously unobserved coherent phonon-phonon interaction in an organic solid that can be described by the application of Fano's analysis to a case without the presence of a continuum. Using Raman spectroscopy of the hydrogen-bonded charge-transfer material quinhydrone, two peaks appear near $700{\mathrm{cm}}^{\ensuremath{-}1}$ we assign as phonons whose position and line-shape asymmetry depend on the sample temperature and light scattering excitation energy. Density functional theory calculations find two nearly degenerate phonons possessing frequencies near the values found in experiment that share similar atomic motion out of the aromatic plane of electron donor and acceptor molecules of quinhydrone. Further analytical modeling of the steady-state light scattering process using the Peierls-Hubbard Hamiltonian and time-dependent perturbation theory motivates assignment of the physical origin of the asymmetric features of each peak's line shape to an interaction between two discrete phonons via nonlinear electron-phonon coupling. In the context of analytical model results, characteristics of the experimental spectra upon 2.33 eV excitation of the Raman scattering process are used to qualify the temperature dependence of the magnitude of this coupling in the valence band of quinhydrone. These results broaden the range of phonon-phonon interactions in materials in general while also highlighting the rich physics and fundamental attributes specific to organic solids that may determine their applicability in next generation electronics and photonics technologies.

Intermolecular electron transfer from intramolecular excitation and coherent acoustic phonon generation in a hydrogen-bonded charge-transfer solid.

Organic materials that produce coherent lattice phonon excitations in response to external stimuli may provide next generation solutions in a wide range of applications. However, for these materials to lead to functional devices in technology, a full understanding of the possible driving forces of coherent lattice phonon generation must be attained. To facilitate the achievement of this goal, we have undertaken an optical spectroscopic study of an organic charge-transfer material formed from the ubiquitous reduction-oxidation pair hydroquinone and p-benzoquinone. Upon pumping this material, known as quinhydrone, on its intermolecular charge transfer resonance as well as an intramolecular resonance of p-benzoquinone, we find sub-cm(-1) oscillations whose dispersion with probe energy resembles that of a coherent acoustic phonon that we argue is coherently excited following changes in the electron density of quinhydrone. Using the dynamical information from these ultrafast pump-probe measurements, we find that the fastest process we can resolve does not change whether we pump quinhydrone at either energy. Electron-phonon coupling from both ultrafast coherent vibrational and steady-state resonance Raman spectroscopies allows us to determine that intramolecular electronic excitation of p-benzoquinone also drives the electron transfer process in quinhydrone. These results demonstrate the wide range of electronic excitations of the parent of molecules found in many functional organic materials that can drive coherent lattice phonon excitations useful for applications in electronics, photonics, and information technology.

Conversion of n-Type CuTCNQ into p-Type Nitrogen-Doped CuO and the Implication for Room-Temperature Gas Sensing

Sensors to detect toxic and harmful gases are usually based on metal oxides that are operated at elevated temperature. However, enabling gas detection at room temperature (RT) is a significant ongoing challenge. Here, we address this issue by demonstrating that microrods of semiconducting CuTCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) with nanostructured features can be employed as conductometric gas sensors operating at 50 °C for detection of oxidizing and reducing gases such as NO2 and NH3. The sensor is evaluated at RT and up to 200 °C. It was found that CuTCNQ is transformed into a N-doped CuO material with p-type conductivity when annealed at the maximum temperature. This is the first time that such a transformation, from a semiconducting charge transfer material into a N-doped metal oxide, is detected. It is shown here that both the surface chemistry and the type of majority charge carrier within the sensing layer are critically important for the type of response toward oxidizing and reducing gases. A detailed physical description of the NO2 and NH3 sensing mechanism at CuTCNQ and N-doped CuO is provided to explain the differences in the response. For the N-doped CuO sensor, detection limits of 1 ppm for NO2 and 10 ppm for NH3 are achieved.