Frontier Molecular Energy Levels Research Articles

A new homologous series of semi-conducting liquid crystals based on phenyl-anthracene: synthesis and effect of the alkyloxy terminal chain on charge transport and photoconductive properties.

π-Conjugated systems are the key to the charge transport properties of organic semiconductors. Four multifunctional mesogenic materials were designed and synthesised from 2-amino anthracene and para-hydroxybenzaldehyde. Each material contains a central, rigid phenyl-anthracene core and one flexible alkyloxy chain with different lengths based on the concept of combining liquid crystal (LC) materials featuring facile processability, highly ordered alignment, and effective charge transport. (E)-N-(Anthracen-2-yl)-1-(4-(alkyloxy)phenyl) methanimine (n-OPIA) with n = 8, 10, 12, and 16 was chemically characterized by nuclear magnetic resonance spectroscopy and high-resolution mass spectrometry. Their thermal behaviours were performed by means of differential scanning calorimetry and polarised optical microscopy and showed differences in mesomorphic behaviours with respect to the alkyloxy chain length. Concerning their frontier molecular energy levels, they were studied by optical spectroscopy and cyclic voltammetry and compared to values obtained via ab initio density-functional theory (DFT) calculations. LC field-effect transistors (LC-FETs) using a solvent vapour enhanced drop casting based n-OPIA material showed efficient electrical parameters and interesting photoconductive properties, indicating their potential applications in multifunctional integrated devices. As the alkyloxy chain length of the n-OPIA compounds increased, the hole mobility, on/off current ratio, and responsivity decreased. The 8-OPIA homologue exhibits a mobility one order of magnitude larger than the previously studied homologue, namely 10-OPIA.

N-Annulated Perylene Diimide Non-Fullerene Acceptors for Organic Photovoltaics

This work covers the development of non-fullerene acceptors for use in organic photovoltaics built using the N-annulated perylene diimide dye. The classic perylene diimide dye has been extensively used to construct non-fullerene acceptors, leading to device power conversion efficiencies of over 10%. Strong visible light absorption and deep frontier molecular energy levels have made such materials (both molecular and polymeric) near ideal for pairing with narrow-gap conjugated polymers in bulk-heterojunction active layers. The N-annulation of the dye provides an extra site for side-chain engineering and alters the electronic structure of the polycyclic aromatic core. In addition, N-annulation allows for selective bromination of the perylene core, leading to building blocks that are useful for the construction of large molecular frameworks using the atom-economical direct heteroarylation cross-coupling method. Herein, we detail a series of molecules developed by our team that are based on the N-annulated perylene diimide in the form of dimers with different cores (both electron-rich and electron-deficient); dimers with varied side chains; tetramers with varying geometries; and large, asymmetric molecules with internal energy cascades. The use of these molecules as non-fullerene acceptors in organic photovoltaic devices (binary and ternary blends, outdoor and indoor light applications, and spin-coated vs. slot-die-coated photoactive layers) is presented.

The Energy Level Conundrum of Organic Semiconductors in Solar Cells.

The frontier molecular energy levels of organic semiconductors are decisive for their fundamental function and efficiency in optoelectronics. However, the precise determination of these energy levels and their variation when using different techniques makes it hard to compare and establish design rules. In this work, the energy levels of 33 organic semiconductors via cyclic voltammetry (CV), density functional theory, ultraviolet photoelectron spectroscopy, and low-energy inverse photoelectron spectroscopy are determined. Solar cells are fabricated to obtain key device parameters and relate them to the significant differences in the energy levels and offsets obtained from different methods. In contrast to CV, the photovoltaic gap measured using photoelectron spectroscopy (PES) correlates well with the experimental device VOC . It is demonstrated that high-performing systems such as PM6:Y6 and WF3F:Y6, which are previously reported to have negligible ionization energy (IE) offsets (ΔIE), possess sizable ΔIE of ≈0.5eV, determined by PES. Using various D-A blends, it is demonstrated that ΔIE plays a key role in charge generation. In contrast to earlier reports, it is shown that a vanishing ΔIE is detrimental to device performance. Overall, these findings establish a solid base for reliably evaluating material energetics and interpreting property-performance relationships in organic solar cells.

Novel organic semiconductors based on 2-amino-anthracene: Synthesis, charge transport and photo-conductive properties.

Anthracene is considered to be a popular choice as a building block for organic semiconductors. The present work is dedicated to the synthesis and characterization of a novel semiconductor (10-OPIA) possessing mesogenic properties, which allows better control over charge transport in the bulk of a material. A novel anthracene-based molecule is characterized for its potential applications: frontier molecular energy levels are studied by optical spectroscopy and cyclic voltammetry and compared to values obtained via ab initio calculations. Thermophysical and mesogenic properties are investigated by optical microscopy and differential scanning calorimetry. Charge transport properties are characterized by means of an OFET device. It is found that this material can be easily aligned and exhibits a field effect hole mobility of 5.22 × 10-5 cm2 V-1 s-1 and an ON/OFF ratio of 104 in the device prepared by drop casting. Finally, the photoconductive properties of this novel material are addressed in order to investigate its potential applications for organic phototransistors: it exhibits a large photoconductive gain of >100 and a photo-responsivity of >1 A W-1.

Self-Assemblies of Zinc Complexes for Aggregation-Induced Emission Luminogen Precursors.

Positional isomers of zinc–nitrobenzoate complexes possessing pyridine -3-(or-4-) carboxamide are used for a comparative theoretical and experimental study to understand their utility as model complexes to understand the role of metal-to-ligand charge transfer in aggregation-induced emission (AIE). Among the five different model zinc complexes, four of them are non-ionic, and one is an ionic complex. The frontier molecular energy levels of different combinations of the positional isomeric complexes and the absorption maximum were ascertained by density functional theory calculations. The PolyQ value obtained from solid samples of each complex is different. Shifts in the emissions to higher wavelengths than the expected emission for the S1 to S0 transition were observed due to aggregations. The highest value of PolyQ among the complexes was 13.56% observed for emission at 439 nm (λex = 350) of the non-ionic complex, namely, (di-aqua)bis(pyridine-3-carboxamide)di(2-nitrobenzoato)zinc(II) monohydrate. Close resemblance in emission lifetime decay profiles of the solid samples of those complexes and the respective solutions of those complexes in dimethyl sulfoxide with or without water showed a common trend, suggesting aggregation-induced emission in each case. Aggregation-induced emission caused by adding water in dimethyl sulfoxide solution of each complex showed an initial increase without a shift in the emission wavelength followed by a quenching with a shift of the respective emission peak to a short wavelength. Dynamic light scattering studies showed an increase in the average particle sizes upon an increase in the concentration of water. This indicated initial participation of water molecules to form aggregates with the complexes, favoring an increase in the AIE intensity. Aggregation of each complex changes with the concentration of water, and an increase in the concentration of water beyond a characteristic limit causes lowering of the emission intensity to the short wavelength.

Low Energy Loss of 0.57 eV and High Efficiency of 8.80% in Porphyrin-Based BHJ Solar Cells

A new A−π–D−π–A small molecule, denoted as MV72, has been designed and synthesized. The molecule comprises a Zn–porphyrin central donor unit and dicyanovinylene-substituted rhodanine (RhCN) end-capping acceptor units linked by bisthienylenevinylene groups as π-conjugated bridges. The optical and electrochemical properties of MV72 have been investigated as an electron donor for solution-processed organic solar cells (OSCs), and the results were compared to those of MV71, which consists of the same molecular backbone but with different rhodanine end-capping acceptor units. The effects of the end-capping terminal units on the absorption spectra, molecular crystallinity, frontier molecular energy levels, charge transport properties, morphology of active layers, and photovoltaic performance have been investigated. The OSCs based on optimized active layers of MV71/PC71BM and MV72/PC71BM (donor and acceptor weight ratios and solvent vapor annealing) deliver overall power conversion efficiencies (PCEs) of 7.61% a...

Optimized synthesis of π-extended squaraine dyes relevant to organic electronics by direct (hetero)arylation and Sonogashira coupling reactions.

This study reports on the synthesis and characterization of four molecular π-extended squaraine compounds relevant to the field of organic electronics. The compounds each consist of a bis-indole squaraine core end-capped with indoloquinoxaline units employing three different bridging units, namely thiophene, thiazole, and acetylene. Compound 10 bears a thiophene bridge, 11 consists of a thiophene bridge and fluorinated indoloquinoxaline terminal units, and compounds 12 and 13 are bridged by thiazole and acetylene, respectively. The final compounds are constructed using the atom economical direct (hetero)arylation or the classic Sonogashira carbon-carbon bond formation protocols. Each carbon-carbon bond forming reaction employing thiophene bridges (i.e. synthesis of compounds 10 and 11) has been optimized using the stable and reusable silica supported Pd catalyst, SiliaCat® DPP-Pd, streamlining the synthetic procedure. While compounds 12 and 13 were also accessible using the SiliaCat® DPP-Pd catalyst, the use of Herrmann-Beller and Pd(PPh3)4 catalysts, respectively, lead to improved isolated yields of the final materials. Compounds 10-13 were characterized by thermal gravimetric analysis, cyclic voltammetry, optical absorption spectroscopy, photoluminescence spectroscopy, and each structure was analysed using density functional theory. All compounds exhibit high thermal stability and good solubility in common organic solvents, including in the greener alternative 2-methyl tetrahydrofuran. The reported compounds display stable ambipolar redox behaviour, furthermore, we have demonstrated that the frontier molecular energy levels can be effectively tuned by changing the bridging unit as predicted by density functional theory. Most striking is the drastic optical absorption profile changes observed from this class of materials upon post-deposition film annealing, suggesting molecular rearrangement in the solid-state. The induced changes and fine structure observed upon post-deposition annealing is unique to these π-extended squaraines with nothing like it reported in the literature for related squaraine based materials.

Functionalized thienoacridines: synthesis, optoelectronic, and structural properties

The preparation and characterization of hexylated, brominated, and 5-hexyl-2-thienyl-substituted tetrathienoacridine (TTAc) is described. Comparative electrochemical, optical, computational, and solid-state studies with their thienoanthracene (TTAn) analogs demonstrate that replacement of CH with nitrogen leads to an overall lowering of the frontier molecular energy levels and decrease in the HOMO–LUMO energy gap. While the brominated TTAc and TTAn derivatives are isostructural, as indicated by X-ray diffraction studies, the lower sublimation temperature and thermochromic behaviour exhibited by bromo-TTAc is attributed to an additional dipole−dipole interaction resulting from the dipole moment afforded by carbon/nitrogen substitution in the core aromatic ring. This work represents a fundamental study of heteroatom substitution in 2-D conjugated small molecules and investigates how inclusion of an electron deficient aromatic ring into the molecular framework (i.e., acridine versus anthracene) can influence the physical properties.

Theoretical investigation on the crystal structures and electron transport properties of several nitrogen-rich pentacene derivatives

Exploring and synthesizing new simple n-channel organic semiconductor materials with large electron mobility and high air stability have remained a major challenge and hot issue in the field of organic electronics. In the current work, the electron transport properties of four novel nitrogen-rich pentacene derivatives (PBD1, PBD2, PBD3, and PBD4) with two cyano groups as potential n-channel OFET materials have been investigated at the molecular and crystal levels by means of density functional theory (DFT) calculations coupled with the prediction of crystal structures and the incoherent charge-hopping model. Calculations reveal that the studied compounds, which possess low-lying frontier molecular energy levels, large ionization potentials and electron affinities, are very stable exposed to air. Based on predicted crystal structures, the average electron mobility at room temperature (T = 300 K) for PBD1, PBD2, PBD3, and PBD4 is predicted to be as high as 0.950, 0.558, 0.518, and 1.052 cm²·V⁻¹·s⁻¹, which indicate that these four compounds are more than likely to be promising candidates as n-type OFET materials under favorable device conditions. However, this claim needs experimental verification. In addition, the angular-dependent simulation for electron mobility shows that the electron transport is remarkably anisotropic in these molecular crystals and the maximum μ(e) appears along the crystal axis direction since molecules along this direction exhibit the close face-to-face stacking arrangement with short interplanar distances (~3.6-4.0 Å), which induces large electronic couplings.