Computational Study of Cresyl Violet Covalently Attached to the Silane Coupling Agents: Application to TiO2-Based Photocatalysts and Dye-Sensitized Solar Cells

The covalent attachment of photosensitizing dyes to TiO₂ using silane coupling agents (SCAs) is a promising strategy for enhancing the photocatalytic activity of TiO₂-based photocatalysts and the photovoltaic conversion of dye-sensitized solar cells (DSSCs). This approach can control the geometry and orientation of the photosensitizing dye on the TiO₂ surface. In this study, a density functional theory (DFT) and time-dependent DFT (TD-DFT) investigation was carried out on cresyl violet (CV) covalently attached to SCAs with a terminal oxirane group (OTES–Cn) to reveal the influence of OTES–Cn on the geometry of the photosensitizing dyes. The potential of CV covalently attached to OTES–Cn (CV–OTES–Cn) to act as a photosensitizing dye was also analyzed. The hydroxyl group formed by the epoxy-opening reaction between CV and OTES–Cn strongly influenced the geometry of CV–OTES–Cn, which was attributed to a CH–O interaction. Additionally, TD-DFT, frontier molecular orbital and molecular electrostatic potential calculations revealed that CV–OTES–Cn has excellent optical properties and electron injection ability. In particular, the characteristics of the unbent conformation of CV–OTES–Cn are expected to contribute significantly to the photocurrent in TiO₂-based photocatalysts and DSSCs. These findings enhance the understanding of the covalent attachment strategy using SCAs and contribute to improving TiO₂-based photocatalysts and DSSCs.

Effects of physical orientation of dye molecules and molecular orbitals on performance of solid-state dye sensitized solar cells

Performance of Dye-sensitized devices depends on the photon absorption and carrier injection properties of the sensitizer (dye). The orientation of the dye molecule affects the photon absorption cross-section, injection efficiency and carrier transport. These effects are studied, using three variants of cyanine dyes in n-TiO₂/Dye/p-CuSCN heterojunction. The results show correlation of dye-molecule's orientation on the short-circuit-photocurrent (I_sc). The open-circuit-voltage (V_oc) is also subjective. The orientation of the dye molecule influence the photon-harvesting efficiency and obstruct the hole-conductor penetrating onto the working-electrode. Additionally, Cumulative effects of e-e, e-h, spin-coupling and HOMO/LUMO distribution are identified.

Investigations of the charge transfer phenomenon at the hybrid dye/BiVO4 interface under visible radiation

Photocatalytic hybrid systems were realized by associating bismuth vanadate (BiVO₄) nanostructured thin films with anchored organic and metal-organic complex molecules. The chosen dyes are based on indoline and azo-based moieties. Optical and photoinduced charge transfer features were investigated experimentally and analysed theoretically through the electron band alignment on the organic/inorganic interface. Quantum calculations were carried out for the studied hybrid systems by using DFT and semi-empirical approaches. The calculations were performed by implementing a cluster model applied for the nanostructures and hybrid systems. The electronic density peculiarities point out efficient charge transfer for D149 based hybrids compared to azo-based systems. The electron distribution in hybrid systems inferred from the computational analysis and their experimental probing using Kelvin Force Microscopy (KFM) maps the way to understanding the photoinduced charge transfer occurring at the interfaces between organic dyes and an inorganic photocatalyst. The presented approach helps to predict suitable photoactive hybrid materials leading to efficient photocatalytic devices.

Simple diphenylamine based D–π–A type sensitizers/co-sensitizers for DSSCs: a comprehensive study on the impact of anchoring groups

Three new diphenylamine based metal-free dyes were designed and developed as efficient sensitizers as well as co-sensitizers along with N3 in DSSCs. Their detailed photophysical, electrochemical, theoretical and photovoltaic properties have been explored.

Enhancement of intramolecular charge transfer strength in diphenylamine substituted symmetric 1,3,4-oxadiazole derivatives

The intramolecular charge transfer characteristic of two diphenylamine substituted symmetric 1,3,4-oxadiazole derivatives (DPAOXD and DPAOXDBEN) was studied through a combination of experimental techniques and theoretical calculations. Significant enhancement of intramolecular charge transfer strength has been found in both these compounds through molecular structure modification. The experimental result found only a small red shift in the absorption spectra (∼15 nm) but a very large red shift in the emission spectra (∼114 nm for DPAOXD and ∼140 nm for DPAOXDBEN) with increasing solvent polarity, indicating a large extent charge transfer occurred in their excited state. The increase of molecular dipole moment from the ground state to the charge transfer excited state is calculated to be 22.10 D in DPAOXD and 26.67 D in DPAOXDBEN, respectively. Theoretical calculations present clear evidence that electrons transfer from the terminal diphenylamine to the bi-1,3,4-oxadiazole rings in DPAOXD, and the two 1,3,4-oxadiazole rings and central benzene ring in DPAOXDBEN. As compared to the methoxy group, the substitution by a diphenylamine group could increase both the transferred charge and distance, which could substantially strengthen the charge transfer character. Further introduction of a central benzene ring in DPAOXDBEN could further increase the transferred distance, and then the charge transfer strength. These findings could provide good guidance for the design of molecules with high intramolecular charge transfer characteristics.

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.

Synthesis of Novel Derivatives of Carbazole-Thiophene, Their Electronic Properties, and Computational Studies

A series of carbazole-thiophene dimers,P1–P9, were synthesized using Suzuki-Miyaura and Ullmann coupling reactions. InP1–P9, carbazole-thiophenes were linked at the N-9 position for different core groups via biphenyl, dimethylbiphenyl, and phenyl. Electronic properties were evaluated by UV-Vis, cyclic voltammogram, and theoretical calculations. Particularly, the effects of conjugation connectivity on photophysical and electrochemical properties, as well as the correlation between carbazole-thiophene and the core, were studied. Carbazole connecting with thiophenes at the 3,6-positions and the phenyl group as a core group leads to increased stabilization of HOMO and LUMO energy levels where the bandgap (ΔE) is significantly reduced.

Effect of Substituents in Catechol Dye Sensitizers on Photovoltaic Performance of Type II Dye-Sensitized Solar Cells

In order to provide a direction in molecular design of catechol (Cat) dyes for type II dye-sensitized solar cells (DSSCs), the dye-to-TiO2 charge-transfer (DTCT) characteristics of Cat dyes with various substituents and their photovoltaic performance in DSSCs are investigated. The Cat dyes with electron-donating or moderately electron-withdrawing substituents exhibit a broad absorption band corresponding to DTCT upon binding to TiO2 films, whereas those with strongly electron-withdrawing substituents exhibit weak DTCT. This study indicates that the introduction of a moderately electron-withdrawing substituent on the Cat moiety leads to not only an increase in the DTCT efficiency, but also the retardation of back electron transfer. This results in favorable conditions for the type II electron-injection pathway from the ground state of the Cat dye to the conduction band of the TiO2 electrode by the photoexcitation of DTCT bands.

Development of a functionally separated D–π-A fluorescent dye with a pyrazyl group as an electron-accepting group for dye-sensitized solar cells

A functionally separated D–π-A dye OUK-3 with a pyrazyl group as an electron-accepting group and a carboxyl group as an anchoring group has been newly developed as a photosensitizer for dye-sensitized solar cells.

Acid-assisted hydrothermal synthesis of nanocrystalline TiO2 from titanate nanotubes: influence of acids on the photodegradation of gaseous toluene

In order to efficiently remove volatile organic compounds (VOCs) from indoor air, one-dimensional titanate nanotubes (TiNTs) were hydrothermally treated to prepare TiO2 nanocrystals with different crystalline phases, shapes and sizes. The influences of various acids such as CH3COOH, HNO3, HCl, HF and H2SO4 used in the treatment were separately compared to optimize the performance of the TiO2 nanocrystals. Compared with the strong and corrosive inorganic acids, CH3COOH was not only safer and more environmentally friendly, but also more efficient in promoting the photocatalytic activity of the obtained TiO2. It was observed that the anatase TiO2 synthesized in 15 mol/L CH3COOH solution exhibited the highest photodegradation rate of gaseous toluene (94%), exceeding that of P25 (44%) by a factor of more than two. The improved photocatalytic activity was attributed to the small crystallite size and surface modification by CH3COOH. The influence of relative humidity (20%-80%) on the performance of TiO2 nanocrystals was also studied. The anatase TiO2 synthesized in 15 mol/L CH3COOH solution was more tolerant to moisture than the other TiO2 nanocrystals and P25.

Properties of chromophores determining recombination at the TiO2-dye-electrolyte interface

Different classes of chromophores have been developed for the dye solar cell (DSC), including as the two main classes organometallic dyes, with Ru-based complexes and Zn-porphyrin complexes, and metal-free dyes. They result in different recombination behavior by electron transfer at the titania/dye/electrolyte interface, where the dye molecule plays a pivotal role. We present an overview of the main factors that control recombination depending on dye structural and electronic properties: the substituent's nature, the size and nature of the π bridge, the type of absorption onto the titania surface, and the structure of the donor electron moiety or anchoring group. Different recombination mechanisms arise, including direct recombination to redox electrolytes and through a dye cation intermediate.

Photophysical and electrochemical properties, and molecular structures of organic dyes for dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) based on organic dyes adsorbed on oxide semiconductor electrodes, such as TiO(2), ZnO, or NiO, which have emerged as a new generation of sustainable photovoltaic devices, have attracted much attention from chemists, physicists, and engineers because of enormous scientific interest in not only their construction and operational principles, but also in their high incident-solar-light-to-electricity conversion efficiency and low cost of production. To develop high-performance DSSCs, it is important to create efficient organic dye sensitizers, which should be optimized for the photophysical and electrochemical properties of the dyes themselves, with molecular structures that provide good light-harvesting features, good electron communication between the dye and semiconductor electrode and between the dye and electrolyte, and to control the molecular orientation and arrangement of the dyes on a semiconductor surface. The aim of this Review is not to make a list of a number of organic dye sensitizers developed so far, but to provide a new direction in the epoch-making molecular design of organic dyes for high photovoltaic performance and long-term stability of DSSCs, based on the accumulated knowledge of their photophysical and electrochemical properties, and molecular structures of the organic dye sensitizers developed so far.