The Role of the π Linker in Donor-π-Acceptor Organic Dyes for High-Performance Sensitized Solar Cells

Investigation of the Geometric and Spectroscopic Properties of Four Twisted Triphenylpyridinium Donor-Acceptor Dyes

The geometric and spectroscopic properties of four cationic N-aryl-2,4,6-triphenylpyridinium-based donor-acceptor dyes─1-[4-(9H-carbazol-9-yl)phenyl]-2,4,6-triphenylpyridinium, 1-[4-(N,N-diphenylamino)phenyl]-2,4,6-triphenylpyridinium, 1-(9-phenyl-9H-carbazol-3-yl)-2,4,6-triphenylpyridinium, and 1-(9-ethyl-9H-carbazol-3-yl)-2,4,6-triphenylpyridinium─are reported. The four dyes exhibited a twisted, quasi-perpendicular geometry about the central donor-acceptor bond, shown by X-ray crystallography and supported by Raman spectroscopy and DFT calculations. The electronic absorption spectra show weak charge transfer (CT) transitions at about 400 nm (ε ∼ 3000 L mol^-1 cm^-1). Time dependent (TD) DFT supported the nature of the CT transition, displaying an 89-97% shift in electron density from the donor to the acceptor upon electronic excitation. Excited state geometry calculations revealed significant geometry changes upon electronic excitation. Enhancement of vibrational modes attributable to this transition was also recognized in the resonance Raman spectra. Emission spectroscopies showed two distinct emission bands. The lower energy band, resulting from radiative decay of the CT excited state, exhibited large anomalous Stokes shifts of ∼9000 cm^-1. Much of the Stokes shift was a consequence of geometry changes between the ground and excited states. This was confirmed by variable temperature emission studies, with Stokes shifts reducing by up to 3000 cm^-1 upon cooling from 293 to 80 K. Additionally, a high energy aggregation induced emission band was present for two of the dyes, resulting from the inhibition of excited state geometry reorganization and supported by solid-state emission spectra. These phenomena exemplify the importance of geometry in short range donor-acceptor dyes such as these.

Vacancy Engineering in Transition Metal Sulfide and Oxide Catalysts for Hydrodeoxygenation of Lignin-Derived Oxygenates

The catalytic hydrodeoxygenation (HDO) of lignin has long been a hot research topic and vacancy engineering is a new means to develop more efficient catalysts for this process. Oxygen vacancies and sulfur vacancies are both widely used in HDO. Based on the current research status of vacancies in the field of lignin-derived oxygenates, this Minireview discusses in detail design methods for vacancy engineering, including surface activation, synergistic modification, and morphology control. Moreover, it is clarified that in the HDO reaction, vacancies can act as acidic sites, promote substrate adsorption, and regulate product distribution, whereas for the catalysts, vacancies can enhance stability and reducibility, improve metal dispersion, and improve redox capacity. Finally, the characterization of vacancies is summarized and strategies are proposed to address the current deficiencies in this field.

Computational study of 4,4′-dimethoxy triphenylamine donor linked with low band gap π-spacers by single and double bonds for DSSC applications

In this work, ten metal-free organic dyes based on the 4,4′-dimethoxy triphenylamine donor and cyanoacrylic acid acceptor were designed and computationally studied for their potential in DSSC.

Theoretical design of thiazolothiazole-based organic dyes with different electron donors for dye-sensitized solar cells

In this study, we have designed four novel organic donor-π-acceptor dyes (D1, D2, D3, D4), used for dye sensitized solar cells (DSSCs). The electron acceptor (anchoring) group was 2-cyanoacrylic for all dyes whereas the electron donor unit varied (coumarin, indoline, carbazole, triphenylamine) and the influence was investigated. These dyes, based on thiazolothiazole as π-spacer, were studied by density functional theory (DFT) and its extensible time dependant DFT (TDDFT) approaches to shed light on how the π-conjugation order influence the performance of the dyes in the DSSCs. The theoretical results have shown that the LUMO and HOMO energy levels of these dyes can be ensuring positive effect on the process of electron injection and dye regeneration. The trend of the calculated HOMO-LUMO gaps nicely compares with the spectral data. Key parameters in close connection with the short-circuit current density (Jsc), including light harvesting efficiency (LHE), injection driving force (ΔGinject.) and total reorganization energy (λtotal), were discussed. The calculated results of these dyes reveal that dye D2, with indoline as electron donor group, can be used as a potential sensitizer for TiO2 nanocrystalline solar cells due to its best electronic and optical properties and good photovoltaic parameters.

Theoretical investigation of new thiazolothiazole-based D-π-A organic dyes for efficient dye-sensitized solar cell

Novel ten organic donor-π-acceptor dyes (D-π-A), used for dye-sensitized solar cells (DSSCs), based on thiazolothiazole were studied by density functional theory (DFT) and time dependant DFT (TDDFT) approaches to shed light on how the π-conjugation order influence the performance of the dyes. The electron acceptor (anchoring) group was 2-cyanoacrylic for all dyes whereas the electron-donor unit varied and the influence was investigated. The theoretical results have shown that TDDFT calculations using the Coulomb attenuating method CAM-B3LYP with the polarized split-valence 6-31G (d,p) basis sets and the polarizable continuum model (PCM) were reasonably capable of predicting the excitation energies, the absorption and the emission spectra of the molecules. The LUMO and HOMO energy levels of these dyes can ensure a positive effect on the process of electron injection and dye regeneration. The trend of the calculated HOMO-LUMO gaps nicely compares with the spectral data. Key parameters in close connection with the short-circuit current density (Jsc), including light-harvesting efficiency (LHE), injection driving force (ΔG(inject)) and total reorganization energy (λtotal), were discussed. In addition, the estimated values of open-circuit photovoltage (Voc) for these dyes were presented. The calculated results of these dyes reveal that the D6 dye can be used as a potential sensitizer for TiO2 nanocrystalline solar cells due to its best electronic and optical properties and good photovoltaic parameters.

Theoretical investigation of phenothiazine-triphenylamine-based organic dyes with different π spacers for dye-sensitized solar cells

Three phenothiazine-triphenylamine-based organic dyes (CD-1, CD-2 and CD-3) are designed based on the dye WD-8. The geometries, electronic structures, and electronic absorption spectra of these dyes before and after binding to TiO2 are studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The calculated geometries indicate that these dyes show good steric hindrance effect which is advantage to inhibit the close intermolecular π-π aggregation effectively. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of these dyes could ensure positive effect on the process of electron injection and dye regeneration. The simulated spectra of CD-1∼3 show better absorption than that of WD-8 in the low energy zone. All the calculated results demonstrate that these dyes could be used as potential sensitizers for DSSCs and show better performances than WD-8.

Theoretical investigation of triphenylamine-based sensitizers with different π-spacers for DSSC

The molecular geometries, electronic structures, and absorption spectra of two organic dyes, 3-(5-(4-(IDB)phenyl)thiophene-2-yl)-2-cyanoacrylic acid (IDB-1), and 3-(5-(4-(IDB)styryl) thiophene-2-yl)-2-cyanoacrylic acid (IDB-2), before and after binding to TiO2 cluster were investigated by density functional theory (DFT) and time-dependent DFT (TDDFT) methods to understand the effect of enhanced π-conjugation of organic dye on the energy-to-electricity conversion efficiency (η) of dye-sensitized solar cell (DSSC), where, IDB=10,11-dihydro-5H-dibenzo[b,f]azepin-5-yl. The introduction of vinyl unit into the π-spacer enhances the coplanarity and subsequently red-shifts, intensifies, and broadens the absorption spectrum of IDB-2, resulting in the stronger electronic coupling between dye and TiO2 conduction band, thus the more efficient electron transfer. From the theoretical evaluation of electron injection driving force (D), light-harvesting efficiency (LHE), and shift of TiO2 conduction band (ΔEcb), we successfully interpreted the experimentally observed efficiency difference between IDB-1 and IDB-2. Under this theoretical procedure, several novel D-π-A dyes namely IDB-3, IDB-4, and IDB-5, were designed. Our calculated results reveal that IDB-5 has the improved Jsc and Voc compared with IDB-2 because it performs nicely on the three key parameters (D, LHE, and ΔEcb). This work highlight the importance of using dimethyl-substituted cyclopentadithiophene group as π-spacer in achieving more efficient dyes for DSSC. We hope these discussions can provide fundamental guidelines for the optimization of existing cell efficiency as well as the design of novel high-efficiency organic dyes.

The role of the conjugate bridge in electronic structures and related properties of tetrahydroquinoline for dye sensitized solar cells

To understand the role of the conjugate bridge in modifying the properties of organic dye sensitizers in solar cells, the computations of the geometries and electronic structures for 10 kinds of tetrahydroquinoline dyes were performed using density functional theory (DFT), and the electronic absorption and fluorescence properties were investigated via time dependent DFT. The population analysis, molecular orbital energies, radiative lifetimes, exciton binding energies (EBE), and light harvesting efficiencies (LHE), as well as the free energy changes of electron injection (ΔG^inject ) and dye regeneration ( ΔGdyeregen ) were also addressed. The correlation of charge populations and experimental open-circuit voltage (V_oc) indicates that more charges populated in acceptor groups correspond to larger V_oc. The elongating of conjugate bridge by thiophene units generates the larger oscillator strength, higher LHE, larger absolute value of ΔG^inject, and longer relative radiative lifetime, but it induces the decreasing of EBE and ΔGdyeregen. So the extending of conjugate bridge with thiopene units in organic dye is an effective way to increase the harvest of solar light, and it is also favorable for electron injection due to their larger ΔG^inject. While the inversely correlated relationship between EBE and LHE implies that the dyes with lower EBE produce more efficient light harvesting.

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.

Substituent effect on the π linkers in triphenylamine dyes for sensitized solar cells: a DFT/TDDFT study

A series of metal-free organic donor-π bridge-acceptor dyes are studied computationally using density functional theory (DFT) and time-dependent DFT (TDDFT) approaches to explore their potential performances in dye-sensitized solar cells (DSSCs). Taking triphenylamine (TPA) and cyanoacrylic acid moieties as donor and acceptor units, respectively, the effects of different substituents of the π linkers in the TPA-based dyes on the energy conversion efficiency of the DSSCs are theoretically evaluated through optimized geometries, charge distributions, electronic structures, simulated absorption spectra, and free energies of injection. The results show that the molecular orbital energy levels and electron-injection driving forces of the TPA dyes can be tuned by the introduction of substituents with different electron-withdrawing or -donating abilities. The electron-withdrawing substituent always lowers the energies of both frontier orbitals, while the electron-donating one heightens them simultaneously. The efficiency trend of these TPA derivatives as sensitizers in DSSCs is also predicted by analyzing the light-harvesting efficiencies and the free energies of injection. The following substituents are shown to increase the efficiency of the dye: OMe, OEt, OHe, and OH.