Modulating triphenylamine-based organic dyes for their potential application in dye-sensitized solar cells: a first principle theoretical study

Exploring the dual capabilities of BNQDs: a comprehensive study on enhancing photoelectric performance and photoluminescence via ligand functionalization

CONTEXT

Boron nitride quantum dots (BNQDs) are emerging as promising multifunctional nanomaterials for renewable energy and optoelectronics owing to their versatile properties. However, rational design principles to tailor their photoelectric and photoluminescent capabilities remain scarce. This study employs density functional theory (DFT) to provide fundamental insights into using urea, thiourea, and PPD ligands to modulate the bandgap, charge transfer dynamics, and recombination processes of BNQDs. Modeling explains that incorporating specific ligands enables visible light absorption, spatial charge separation, continuous photocatalytic cycling, and high quantum yields in BNQDs. The structure-property relationships established pave the way for targeted synthesis of high-performance BNQD photocatalysts and light emitters.

METHODS

This investigation utilized density functional theory (DFT) with the B3LYP functional and 6-31G(d,p) basis set to optimize the geometries of pristine and ligand-functionalized boron nitride quantum dots (BNQDs). The absorption spectra were generated using time-dependent DFT (TDDFT). A Ti38O76 cluster modeled the TiO2 substrate. The cpcm solvation model in Gaussian 09 defined the toluene solvent. Cohesive energies, charge transfer lengths, recombination rates, and conversion efficiencies were calculated to establish structure-property relationships. Multiwfn analyzed the charge densities. The modeling provides insights into tuning BNQD photocatalytic and photoluminescent properties using specific ligands.

Computational insights into bis-N,N-dimethylaniline based D-π-A photosensitizers bearing divergent-type of π-linkers for DSSCs

In this study, theoretically designed D-π-A derivatives containing different π-subunits as linkers were investigated to enlighten their potential applicability in photovoltaic applications. For this aim, we first focused on clarifying the effect of tailored π-linker scaffolds on the frontier orbital energies of the investigated photosensitizers. In the concomitant step, global descriptors, TiO₂ adsorption, maximum absorbance wavelength, light-harvesting efficiency (LHE), energy conversion efficiency (η), short circuit current density (J_SC), open circuit photovoltage (V_OC), fill factor (FF), and reorganization energy (λ_e, λ_h, λ_T) values, electron density differentiation maps (EDDM), transition density matrices (TDM), fragmental contributions on electron-hole overlap were investigated in detail. Based on the trend of the calculated properties, 2,3-dimethylthieno [3,4-b]pyrazine (D-Ɛ₃-δ_n-A; n = 1-3) and 5-isobutyl-10,11-dimethyl-10,11-dihydro-5H-pyrrolo [3,4-e]thieno [2',3':4,5]pyrrolo [3,2-g]thieno [3,2-b]indole (D-Ɛ₆-δ_n-A; n = 1-3) bearing molecules were identified as the best-suited and improved dye candidates for DSSC applications. Following the prediction of photovoltaic properties for the pristine dye molecules, our consecutive efforts have contributed to a similar calculation protocol comprising DFT and subsequent TD-DFT computations for the D-Ɛ_n-δ_n-A@Ti₅O₁₀ clusters to elucidate the interaction of the investigated photosensitizers with the semiconductor layer (TiO₂).

Tuning the charge transfer and optoelectronic properties of tetrathiafulvalene based organic dye-sensitized solar cells: a theoretical approach

Here, we have designed a series of dyes following the donor-π-acceptor (D-π-A) architecture by incorporating tetrathiafulvalene (TTF) as the donor unit and phthalazine (PTZ), diketopyrrolopyrrole (DPP) and quinoxaline (QNX) as the acceptor units, along with the thiophene unit as a π-bridge. The designed dyes have been designated as TTF-PTZ, TTF-DPP and TTF-QNX respectively. We have used cyanoacrylic acid as the anchoring group for the dyes TTF-PTZ and TTF-DPP, while for the third dye, TTF-QNX, we used a carboxylic group. The structural, electronic and photochemical properties of the designed dyes are investigated under the regime of density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. In this regard, the dihedral angle, energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), energy difference between the HOMO and LUMO (Δ H-L values), partial density of states (PDOS), ground state oxidation potential (GSOP), excited state oxidation potential (ESOP), ionization potential (IP), electron affinity (EA), molecular electrostatic potential surface (MEPS) analysis, reorganization energy (λ), electronic coupling matrix element (V), charge transfer rate (k CT), hopping mobility (μ hop), absorption spectra, exciton binding energy (EBE) and electron density difference (EDD) of the designed dyes are calculated. This study reveals that the dyes TTF-DPP-4 and TTF-DPP-6' exhibit the lowest Δ H-L values. The study also reveals that the attachment of the -NH2 group at the donor unit and the -NO2 and -CF3 groups at the acceptor units lower the Δ H-L values of all of the designed dyes. We have also observed that the GSOP of all the designed dyes lie below the redox potential of the I-/I3 - electrolyte couple. However, the ESOP of the TTF-PTZ and TTF-QNX groups of dyes, along with the most of the dyes belonging to the TTF-DPP group, lie above the conduction band of the TiO2 semiconducting surface. Moreover, the total reorganization energy (λ tot) values are low for the TTF-DPP and TTF-QNX groups of dyes, which confirm the better electron-hole separation efficiency in these groups of dyes. Furthermore, the absorption properties of the designed dyes indicate that the TTF-DPP groups of dyes possess the maximum absorption wavelength (λ max) values and attachment of the -CH3 group at the donor part increases the electron density of the dyes, which in turn results into the maximum red-shift. Therefore, the study reveals that the designed dyes are likely to exhibit facile charge transport. Moreover, the electronic properties of the dye-TiO2 clusters strengthen the performance of the dyes compared to those of the isolated dyes. Hence, our study provides good recommendations for the further design of dyes to enhance the performance of dye-sensitized solar cells (DSSCs).

Ultrafast Excited State Relaxation Dynamics of New Fuchsine- a Triphenylmethane Derivative Dye

An all-inclusive investigation of the ultrafast excited state relaxation dynamics of a triphenylmethane derivative molecule, New Fuchsine (NF), using a combined approach of density functional theory (DFT), femtosecond transient absorption spectroscopy (fs-TAS), and photoluminescence spectroscopy is presented in this work. The DFT calculations confirmed the formation of twisted molecular structure in the excited state of NF in ethanol solution with bond rotation of ≈ 86⁰ . TAS measurements of NF solution exhibited ultrafast ground state-recovery pathway via a conical intersection confirming an ultrafast structural reorientation. On the contrary, TAS measurements of NF thin-film exhibited a longer excited-state lifetime suggesting a hindered molecular twisted state formed as an intermediate step. Photophysical kinetic models are proposed to globally fit the fs-TAS data establishing the twisted intramolecular charge transfer (TICT) state mediated ground state recovery for NF in solution and thin film, respectively. Temperature-dependent photoluminescence study of NF film provided a clear insight into the effect of rotational motion of phenyl rings in NF molecules over the TICT mediated emission.

Aromaticity-Photovoltaic Property Relationship of Triphenylamine-Based D-π-A Dyes: Leads from DFT Calculations

D-π-A-based dyes find a wide range of applications in molecular electronics and photovoltaics in general and dye-sensitized solar cells (DSSC) in particular. We speculated whether there exists a relationship between the degree of aromaticity of the π-spacers used in the D-π-A type dyes and their structural, electronic, energetic, photophysical, and intramolecular charge transfer properties. Triphenylamine (TPA) and cyanoacrylic acid (CAA) have been chosen as the donor and acceptor, respectively. In order to carry out the investigation systematically the π-spacers have been logically chosen based on their experimental resonance energies, which follows the order, furan < pyrrole < thiophene < pyridine < benzene. All the properties have been discussed based on the degree of aromaticity of the π-spacers. Geometric properties such as dihedral angles and bond lengths have been discussed extensively. Energy levels of the frontier molecular orbitals, electrochemical properties, namely, ground and excited state oxidation potentials (GSOP/ESOP), and change in Gibbs free energy for electron injection and regeneration (ΔG_inj/ΔG_reg) have also been evaluated. Photophysical properties like wavelength of maximum absorption (λ_max), oscillator strength (f), light harvesting efficiency (LHE), and intramolecular charge transfer properties, viz., charge transfer distance (D_CT), fraction of charge transferred (q_CT), and change in dipole moment (μ_CT) have been assessed. The adsorption characteristics of dye with (TiO₂)₉ nanocluster have been studied along with their optical properties. Results reveal that the nature of the relationship between the aforementioned properties and the extent of aromaticity of the π-spacers is inherently multifaceted. It thus turns out that it is highly difficult to quantify the relationship. These properties of D-π₁-π₂-A molecules can be regarded to be arising from two groups, namely, π-spacers with lower and higher resonance energies. This results in a natural trade-off in selection of competing properties. The qualitative aromaticity photovoltaic property relationship thus obtained may serve as a guide to tailor-design various properties of D-π-A type dyes for application in the intramolecular charge transfer devices.

Molecular engineering of anchoring groups for designing efficient triazatruxene-based organic dye-sensitized solar cells

Triazatruxene with designed anchoring groups provides better photovoltaic activities.

Excited state C–N bond dissociation and cyclization of tri-aryl amine-based OLED materials: a theoretical investigation

Chemical degradation of TAA occurs through the conical intersection present between ground and first excited singlet state with C–N bond dissociation, which channels the excited molecules to dissociate and form radical fragments or cyclized products.

Designing Push-Pull Porphyrins for Efficient Dye-Sensitized Solar Cells

Over the past decade, tremendous effort has been made to improve the light-harvesting ability of push-pull porphyrin dyes. Despite notable success achieved in this direction, push-pull porphyrin dyes still suffer from a poor light-harvesting efficiency owing to the lack of absorption between the Soret and Q-bands. To tackle this issue, here we design a series of push-pull porphyrin dyes with anchoring groups either at meso- or β-position using calculations based on first-principles time-dependent density functional theory. In contrast to the common perception, we find that porphyrin dyes bearing an electron-donor at the meso-position and an electron-acceptor at the β-position produce an additional extended band between the Soret and Q-bands appearing at around 500 nm due to S₀ → S₃ excitation, leading to a much higher light-harvesting performances compared to meso- and β-disubstituted ones. In addition, changing the π-conjugated linker at the acceptor site from ethylene linker (C═C) to acetylene linker (C≡C) further improves the light-harvesting ability of meso-β-porphyrin dyes, making them promising candidates for dye-sensitized solar cell application.

Unravelling the Pathway Complexity in Conformationally Flexible N-Centered Triarylamine Trisamides

Two families of C3 -symmetrical triarylamine-trisamides comprising a triphenylamine- or a tri(pyrid-2-yl)amine core are presented. Both families self-assemble in apolar solvents via cooperative hydrogen-bonding interactions into helical supramolecular polymers as evidenced by a combination of spectroscopic measurements, and corroborated by DFT calculations. The introduction of a stereocenter in the side chains biases the helical sense of the supramolecular polymers formed. Compared to other C3 -symmetrical compounds, a much richer self-assembly landscape is observed. Temperature-dependent spectroscopy measurements highlight the presence of two self-assembled states of opposite handedness. One state is formed at high temperature from a molecularly dissolved solution via a nucleation-elongation mechanism. The second state is formed below room temperature through a sharp transition from the first assembled state. The change in helicity is proposed to be related to a conformational switch of the triarylamine core due to an equilibrium between a 3:0 and a 2:1 conformation. Thus, within a limited temperature window, a small conformational twist results in an assembled state of opposite helicity.

The Effect of Donor Group Rigidification on the Electronic and Optical Properties of Arylamine-Based Metal-Free Dyes for Dye-Sensitized Solar Cells: A Computational Study

One of the most significant aspects in the development of dye-sensitized solar cells is the exploration and design of high-efficiency and low-cost dyes. This paper reports the theoretical design of various triphenylamine analogues, wherein the central nitrogen moiety establishes an sp(2)-hybridization, which endows a significant participation in the charge-transfer properties. Density functional theory (DFT) and time-dependent DFT methodologies were utilized to investigate the geometry, electronic structure, photochemical properties, and electrochemical properties of these dyes. Different exchange-correlation functionals were initially evaluated to establish a proper methodology for calculating the excited-state energy of the reference dye, known as DIA3. Consequently, TD-LC-ωPBE with a damping parameter of 0.175 Bohr(-1) best correlates with the experimental value. Four new dyes, namely, Dhk1, Dhk2, Dhk3, and Dhk4, were designed by modifying the rigidity of the donor moiety. According to the results, altering the type and position of binding in the donor group leads to distinct planarity of the dyes, which significantly affects their properties. The designed Dhk4 dye showed more red-shifted and broadened absorption spectra owing to the enhanced coplanarity between its donor and π-bridge moiety, which brings an advantage for its potential use as sensitizer for photovoltaic applications.

A theoretical description of charge reorganization energies in molecular organic P-type semiconductors

Charge transport properties of materials composed of small organic molecules are important for numerous optoelectronic applications. A material's ability to transport charges is considerably influenced by the charge reorganization energies of the composing molecules. Hence, predictions about charge-transport properties of organic materials deserve reliable statements about these charge reorganization energies. However, using density functional theory which is mostly used for the predictions, the computed reorganization energies depend strongly on the chosen functional. To gain insight, a benchmark of various density functionals for the accurate calculation of charge reorganization energies is presented. A correlation between the charge reorganization energies and the ionization potentials is found which suggests applying IP-tuning to obtain reliable values for charge reorganization energies. According to benchmark investigations with IP-EOM-CCSD single-point calculations, the tuned functionals provide indeed more reliable charge reorganization energies. Among the standard functionals, ωB97X-D and SOGGA11X yield accurate charge reorganization energies in comparison with IP-EOM-CCSD values. © 2016 Wiley Periodicals, Inc.

Second order hyperpolarizability of triphenylamine based organic sensitizers: a first principle theoretical study

Designed metal-free dyes have been investigated by Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) to evaluate the ground state and excited state geometries of triphenylamine-based organic sensitizers.

Molecular engineering of quinoxaline dyes toward more efficient sensitizers for dye-sensitized solar cells

N-annulated perylene-containing quinoxaline sensitizer (NIQ4) displays remarkable performance in light harvesting, electron injection, and dye regeneration.