Research Progress on Photosensitizers for DSSC

The Effect of Conjugated Nitrile Structures as Acceptor Moieties on the Photovoltaic Properties of Dye-Sensitized Solar Cells: DFT and TD-DFT Investigation

A major challenge in improving the overall efficiency of dye-sensitized solar cells is improving the optoelectronic properties of small molecule acceptors. This work primarily investigated the effects of conjugation in nitriles incorporated as acceptor moieties into a newly designed series of D-A-A dyes. Density functional theory was employed to specifically study how single-double and single-triple conjugation in nitriles alters the optical and electronic properties of these dyes. The Cy-4c dye with a highly conjugated nitrile unit attained the smallest band gap (1.80 eV), even smaller than that of the strong cyanacrylic anchor group (2.07 eV). The dyes lacking conjugation in nitrile groups did not contribute to the LUMO, while LUMOs extended from donors to conjugated nitrile components, facilitating intramolecular charge transfer and causing a strong bind to the film surface. Density of state analysis revealed a considerable impact of conjugated nitrile on the electronic properties of dyes through an effective contribution in the LUMO, exceeding the role of the well-known strong 2,1,3-benzothiadiazole acceptor unit. The excited state properties and the absorption spectra were investigated using time-dependent density functional theory (TD-DFT). Conjugation in the nitrile unit caused the absorption band to broaden, strengthen, and shift toward the near-infrared region. The proposed dyes also showed optimum photovoltaic properties; all dyes possess high light-harvesting efficiency (LHE) values, specifically 96% for the dyes Cy-3b and Cy-4c, which had the most conjugated nitrile moieties. The dyes with higher degrees of conjugation had longer excitation lifetime values, which promote charge transfer by causing steady charge recombination at the interface. These findings may provide new insights into the structure of conjugated nitriles and their function as acceptor moieties in DSSCS, which may lead to the development of extremely effective photosensitizers for solar cells.

Enhancing Efficiency of Dye Sensitized Solar Cells by Coinage Metal Doping of Cyanidin-Silver Trimer Hybrids at TiO2 Support Based on Theoretical Study

Identification of a natural-based sensitizer with optimal stability and efficiency for dye-sensitized solar cell (DSSC) application remains a challenging task. Previously, we proposed a new class of sensitizers based on bio-nano hybrids. These systems composed of natural cyanidin dyes interacting with silver nanoclusters (NCs) have demonstrated enhanced opto-electronic and photovoltaic properties. In this study, we explore the doping of silver nanocluster within a cyanidin-Ag3 hybrid employing Density Functional Theory (DFT) and its time-dependent counterpart (TDDFT). Specifically, we investigate the influence of coinage metal atoms (Au and Cu) on the properties of the cyanidin-Ag3 system. Our findings suggest that cyanidin-Ag2Au and cyanidin-AgAuCu emerge as the most promising candidates for improved light harvesting efficiency, increased two-photon absorption, and strong coupling to the TiO2 surface. These theoretical predictions suggest the viability of replacing larger silver NCs with heterometallic trimers such as Ag2Au or AgAuCu, presenting new avenues for utilizing bio-nano hybrids at the surface for DSSC application.

Screening potential dye sensitizers for water splitting photocatalysts using a genetic algorithm

Addressing the global fossil energy crisis necessitates the efficient utilization of sustainable energy sources. Hydrogen, a green fuel, can be generated using sunlight, water, and a photocatalyst. Employing sensitizers holds promise for enhancing photocatalyst performance, enabling high rates of hydrogen evolution through increased visible light absorption. However, sifting through millions of diverse molecules to identify suitable dyes for specific photocatalysts poses a significant challenge. In this study, we integrate genetic algorithm and geometry-frequency-noncovalent extended tight binding methods to efficiently screen 2.6 million potential sensitizers with a D-π-A-π-AA structure within a short timeframe. Subsequently, these optimized sensitizers are rigorously reassessed by using DFT/TDDFT methods, elucidating why they may serve as superior dyes compared to the reference dye WS5F, particularly in terms of light absorption, driving force, binding energy, etc. Additionally, our methodology uncovers molecular motifs of particular interest, including the furan π-bridge and the double cyano anchoring acceptor, which are prevalent in the most promising set of molecules. The developed genetic algorithm workflow and dye design principles can be extended to various compelling projects, such as dye-sensitized solar cells, organic photovoltaics, photo-induced redox reactions, pharmaceuticals, and beyond.

Synthesis and Photovoltaic Performance of β-Amino-Substituted Porphyrin Derivatives

New β-amino-substituted porphyrin derivatives bearing carboxy groups were synthesized and their performance as sensitizers in dye-sensitized solar cells (DSSC) was evaluated. The new compounds were obtained in good yields (63-74%) through nucleophilic aromatic substitution reactions with 3-sulfanyl- and 4-sulfanylbenzoic acids. Although the electrochemical studies indicated suitable HOMO and LUMO energy levels for use in DSSC, the devices fabricated with these compounds revealed a low power conversion efficiency (PCE) that is primarily due to the low open-circuit voltage (Voc) and short-circuit current density (Jsc) values.

Molecular Engineering of Photosensitizers for Solid-State Dye-Sensitized Solar Cells: Recent Developments and Perspectives

Dye-sensitized solar cells (DSSCs) are a feasible alternative to traditional silicon-based solar cells because of their low cost, eco-friendliness, flexibility, and acceptable device efficiency. In recent years, solid-state DSSCs (ss-DSSCs) have garnered much interest as they can overcome the leakage and evaporation issues of liquid electrolyte systems. However, the poor morphology of solid electrolytes and their interface with photoanodes can minimize the device performance. The photosensitizer/dye is a critical component of ss-DSSCs and plays a vital role in the device's overall performance. In this review, we summarize recent developments and performance of photosensitizers, including mono- and co-sensitization of ruthenium, porphyrin, and metal-free organic dyes under 1 sun and ambient/artificial light conditions. We also discuss the various requirements that efficient photosensitizers should satisfy and provide an overview of their historical development over the years.

Predicting the dye-sensitized solar cell performance of novel linear carbon chain-based dyes: insights from DFT simulations

In this paper, we employ density functional theory (DFT) simulations to predict the energy conversion efficiency of a novel class of organic dyes based on linear carbon chain (LCC) linkers for application in dye-sensitized solar cells (DSSCs). We investigate the role of the anchoring group, which serves as a bridge connecting the linker and the surface. Specifically, we compare the performance of cyanoacrylic acid, dyes PY-4N and PY-3N, with that of phosphonate derivatives, dyes PY-4NP and PY-3NP, wherein the carboxylic group of the cyanoacrylic moiety is replaced with phosphonic acid. The observed variations in the UV/VIS absorption spectra have a slight impact on the light harvesting efficiency (LHE). Based on the empirical parameters we have taken into account, the electron injection efficiency (Φinj) and electron collection efficiency (ηcoll) values do not impact the short-circuit current density (JSC) values of all the studied dyes. The open-circuit voltage (Voc) is theoretically predicted using the improved normal model (INM) method. Among the dyes, PY-4N and PY-3N demonstrate the highest Voc values. This can be attributed to a more favorable recombination rate value, which is related to the energy gap between the HOMO level of the dyes and the conduction band minimum (CBM) of the surface. Dyes PY-4N and PY-3N are predicted to demonstrate remarkably high photoelectric conversion efficiency (PCE) values of approximately 21.79% and 16.52%, respectively, and therefore, they are expected to be potential candidates as organic dyes for applications in DSSCs. It is worth noting that PY-4NP and PY-3NP exhibit strong adsorption energy on the surface and interesting PCE values of 11.66% and 8.29%, respectively. This opens up possibilities for their application in DSSCs either as standalone sensitizers or as co-sensitizers alongside metal-free organic dyes or organic-inorganic perovskite solar cells.

Stability Modification of Dye-sensitized Solar Cells by Ruthenium Dyes Embedded on Eggshell Membranes

Dye-sensitized solar cells (DSSCs) have been one of the most promising technologies to convert sunlight into electricity repeatedly based on the mechanism that dyes inject/accept electron into the metal oxides/from redox mediator. Specifically, N719 ([RuL2(NCS)2], L: 4,4'-dicarboxy-2,2'-bipyridine), immobilized on TiO2 through the interaction between its ligands (-COO- and -NCS) and the oxygen on the TiO2 surface, has been used as a conventional DSSC dye with high voltage. Nevertheless, -NCS ligands have been removed from Ru2+ in N719 due to UV irradiation and exchanged with H2O or OH- in electrolyte, resulting in voltage drop. In this work, we developed the first DSSC using the N719-adsorbed Eggshell (ESM)-TiO2 composite to maintain the immobilization of N719 on TiO2 through electrostatic interaction between the protein of ESM and N719. The DSSC using the composite maintained the voltage even after 12 h light irradiation, although the voltage of DSSC without ESM dropped drastically. It means that the ESM contributed to stable photovoltaic performances of DSSCs through the protection of NCS ligands of N719.

Advances and prospects of porphyrin derivatives in the energy field

At present, porphyrin is developing rapidly in the fields of medicine, energy, catalysts, etc. More and more reports on its application are being published. This paper mainly takes the ingenious utilization of porphyrin derivatives in perovskite solar cells, dye-sensitized solar cells, and lithium batteries as the background to review the design idea of functional materials based on the porphyrin structural unit in the energy sector. In addition, the modification and improvement strategies of porphyrin are presented by visually showing the molecular structures or the design synthesis routes of its functional materials. Finally, we provide some insights into the development of novel energy storage materials based on porphyrin frameworks.

Weak noncovalent interactions in two positional isomers of acrylonitrile derivatives: inputs from PIXEL energy, Hirshfeld surface and QTAIM analyses

A single crystal X-ray diffraction analysis was performed on two positional isomers (m-tolyl and p-tolyl) of acrylonitrile derivatives, namely, (Z)-3-(4-(pyridin-2-yl) phenyl)-2-(m-tolyl) acrylonitrile (1) and (Z)-3-(4-(pyridin-2-yl)phenyl)-2-(p-tolyl) acrylonitrile (2). Compound 1 crystallized in the monoclinic P2₁/n space group with two crystallographically independent molecules. Compound 2 also possesses two crystallographically independent molecules and crystallized in the triclinic P-1 space group. The Hirshfeld surface analysis revealed that, in both isomers, intermolecular H⋅⋅⋅H/C/N contacts contribute significantly to the crystal packing. More than 40% of the contribution arises from intermolecular C-H⋅⋅⋅C(π) contacts. In both compounds, the relative contribution of these contacts is comparable, indicating that the positional isomeric effects are marginal. The structures in which these isomers are arranged in the solid state are very similar, and the lattice energies are also comparable between the isomers. The Coulomb-London-Pauli-PIXEL (CLP-PIXEL) energy analysis identified the energetically significant dimers. The strength of the intra- and intermolecular interactions was evaluated using the quantum theory of atoms in molecules approach. The UV-Vis absorbance in three different solvents (chloroform, ethanol, and ethyl acetate) for isomers 1 and 2 are very similar. This result is in good agreement with the time-dependent density-functional theory (TD-DFT) calculations.

Biosynthesis, characterization and optimization of TiO2 nanoparticles by novel marine halophilic Halomonas sp. RAM2: application of natural dye-sensitized solar cells

BACKGROUND

Metal oxide nanoparticles (NPs) are becoming valuable due to their novel applications. The green synthesis of TiO₂ NPs is more popular as a flexible and eco-friendly method compared to traditional chemical synthesis methods. TiO₂ NPs are the most commonly used semiconductor in dye-sensitized solar cells (DSSCs).

RESULTS

The biogenic TiO₂ NPs were produced extracellularly by the marine halophilic bacterium Halomonas sp. RAM2. Response surface methodology (RSM) was used to optimize the biosynthesis process, resulting in a starting TiO₂ concentration of 0.031 M and a pH of 5 for 92 min (⁓15 nm). TiO₂ NPs were well-characterized after the calcination process at different temperatures of 500, 600, 700 and 800 °C. Anatase TiO₂ NPs (calcined at 500 °C) with a smaller surface area and a wider bandgap were nominated for use in natural dye-sensitized solar cells (NDSSCs). The natural dye used as a photosensitizer is a mixture of three carotenoids extracted from the marine bacterium Kocuria sp. RAM1. NDSSCs were evaluated under standard illumination. After optimization of the counter electrode, NDSSC_Bio(10) (10 layers) demonstrated the highest photoelectric conversion efficiency (η) of 0.44%, which was almost as good as NDSSC_P25 (0.55%).

CONCLUSION

The obtained results confirmed the successful green synthesis of TiO₂ NPs and suggested a novel use in combination with bacterial carotenoids in DSSC fabrication, which represents an initial step for further efficiency enhancement studies.

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).

Graphene loaded TiO2 submicron spheres scattering layer for efficient dye-sensitized solar cell

Dye-Sensitized Solar Cells (DSSCs) have attracted great attention due to environmentally friendly low-cost processing, excellent working ability in diffuse light, and potential to meet the power demands of future buildings due the true class of building integrated photovoltaics (BIPV). Nevertheless, DSSCs have relatively low photoconversion efficiency (PCE) due to multiple issues. Several strategies have been employed to enhance its PCE. For instance, bi-layered structure of photoelectrode i.e., mesoporous TiO₂ transparent layer with top scattering layer was introduced which scatter light inside on large angles improves the harvesting ability of photoelectrode thus enhanced PCE. However, scattering layer is composed of aggregated small particles which offer sluggish electron transport due to multiple grain boundaries, consequently, unwanted recombination reaction which leads to poor PCE. This issue has been addressed for transparent layer immensely but ignored for scattering layer. Mostly for scattering layer in previous studies novel structures have been proposed to enhance scattering properties and dye adsorption only. Therefore, in this study for the first time presenting dual functional graphene/TiO₂ scattering layer in which solvent exfoliated graphene is incorporated in TiO₂ submicron spheres which enhanced electron transport properties, while submicron spheres scatter light effectively. Scattering and electron transport characteristics of DSSCs are thoroughly investigated with the function of graphene loading. Electrochemical impedance spectroscopy (EIS) has revealed that diffusion coefficient length and coefficient and conductivity attained maximum value at 0.01 wt%. while other important parameters such as electron lifetime and electron density in conduction band have been improved till 0.020 wt% graphene loading. However, results indicated that with 0.01 w% graphene 33% higher PCE was achieved than without scattering layer and 13% higher than scattering layer without graphene. The depraving in PCE at >0.01 wt% graphene despite of excellent electron transport improvement is attributed to the loss of diffuse reflectance and higher optical absorption by graphene.

Model systems for dye-sensitized solar cells: cyanidin-silver nanocluster hybrids at TiO2 support

Theoretical study of structural, optical, and photovoltaic properties of novel bio-nano hybrids (dye-nanocluster), as well as at TiO2 surface model support is presented in the context of the application for dye-sensitized solar cells (DSSC). A group of anthocyanidin dyes (pelargonidin, cyanidin, delphinidin, peonidin, petunidin, and malvidin) represented by cyanidin covalently bound to silver nanoclusters (NCs) with even or odd number of valence electrons have been investigated using DFT and TDDFT approach. The key role of nanoclusters as acceptors in hybrids cyanidin-NC has been shown. The nanoclusters with an even number of valence electrons are suitable as acceptors in hybrids. The interaction of bio-nano (cyanidin-NC) hybrid with the TiO2 surface model has been investigated in the context of absorption in near-infrared (NIR) and charge separation due to donor and acceptor subunits. Altogether, the theoretical concept serves to identify the key steps in the design of novel solar cells based on bio-nano hybrids at TiO2 surface for DSSC application.

Back to the future: asymmetrical DπA 2,2'-bipyridine ligands for homoleptic copper(i)-based dyes in dye-sensitised solar cells

Metal complexes used as sensitisers in dye-sensitised solar cells (DSCs) are conventionally constructed using a push-pull strategy with electron-releasing and electron-withdrawing (anchoring) ligands. In a new paradigm we have designed new DπA ligands incorporating diarylaminophenyl donor substituents and phosphonic acid anchoring groups. These new ligands function as organic dyes. For two separate classes of DπA ligands with 2,2'-bipyridine metal-binding domains, the DSCs containing the copper(i) complexes [Cu(DπA)₂]⁺ perform better than the push-pull analogues [Cu(DD)(AA)]⁺. Furthermore, we have shown for the first time that the complexes [Cu(DπA)₂]⁺ perform better than the organic DπA dye in DSCs. The synthetic studies and the device performances are rationalised with the aid of density functional theory (DFT) and time-dependent DFT (TD-DFT) studies.

2-((4-(2-Ethylhexyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indol-7-yl)methylene)malononitrile

New small acceptor–donor (A–D) molecules have been recently investigated as a component of organic solar cells. In this research, 2-((4-(2-ethylhexyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indol-7-yl)methylene)malononitrile was prepared from 4-(2-ethylhexyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole in a two-step process via Vilsmeier–Haack formylation and Knoevenagel reaction with malononitrile. The structures of newly synthesized compounds were established by means of elemental analysis, high-resolution mass spectrometry, 1H, 13C NMR, IR and UV–Vis spectroscopy, as well as mass spectrometry.

Solar Energy Materials-Evolution and Niche Applications: A Literature Review

The demand for energy has been a global concern over the years due to the ever increasing population which still generate electricity from non-renewable energy sources. Presently, energy produced worldwide is mostly from fossil fuels, which are non-renewable sources and release harmful by-products that are greenhouses gases. The sun is considered a source of clean, renewable energy, and the most abundant. With silicon being the element most used for the direct conversion of solar energy into electrical energy, solar cells are the technology corresponding to the solution of the problem of energy on our planet. Solar cell fabrication has undergone extensive study over the past several decades and improvement from one generation to another. The first solar cells were studied and grown on silicon wafers, in particular single crystals that formed silicon-based solar cells. With the further development in thin films, dye-sensitized solar cells and organic solar cells have significantly enhanced the efficiency of the cell. The manufacturing cost and efficiency hindered further development of the cell, although consumers still have confidence in the crystalline silicon material, which enjoys a fair share in the market for photovoltaics. This present review work provides niche and prominent features including the benefits and prospects of the first (mono-poly-crystalline silicon), second (amorphous silicon and thin films), and third generation (quantum dots, dye synthesized, polymer, and perovskite) of materials evolution in photovoltaics.

3,4-Diaminopyridine-2,5-dicarbonitrile

Pyridines fused with heterocyclic rings are of great interest as both photovoltaic materials and biologically active compounds. The most convenient precursors for these compounds are pyridine-2,3-diamines. In this communication, 3,4-diaminopyridine-2,5-dicarbonitrile was synthesized by the reaction of 2,5-dibromo-3,4-diaminopyridine with copper cyanide; the best yield of the target compound was achieved by heating the reaction mixture in N,N-dimethylformamide at 120 °C for 6 h. The structure of the newly synthesized compound was established by means of elemental analysis, high resolution mass-spectrometry, 1H, 13C NMR, IR, UV spectroscopy and mass-spectrometry.

Can domain-based local pair natural orbitals approaches accurately predict phosphorescence energies?

Since the discovery of the peculiar conducting and optical properties of aromatics, many efforts have been made to characterize and predict their phosphorescence. This physical process is exploited in modern Organic Emitting Light Diodes (OLEDs), and it is also one of the processes decreasing the efficiency of Dye-sensitized solar cells (DSSCs). Herein, we propose a computational strategy for the accurate calculation of singlet-triplet gaps of aromatic compounds, which provides results that are in excellent agreement with available experimental data. Our approach relies on the domain-based local pair natural orbital (DLPNO) variant of the "gold standard" CCSD(T) method. The convergence of our results with respect to the key technical parameters of the calculation, such as the basis set used, the approximations employed in the perturbative triples correction, and the dimension of the PNOs space, was thoroughly discussed.

Effect of the Ancillary Ligand on the Performance of Heteroleptic Cu(I) Diimine Complexes as Dyes in Dye-Sensitized Solar Cells

A series of heteroleptic Cu(I) diimine complexes with different ancillary ligands and 6,6'-dimethyl-2,2'-bipyridine-4,4'-dibenzoic acid (dbda) as the anchoring ligand were self-assembled on TiO₂ surfaces and used as dyes for dye-sensitized solar cells (DSSCs). The binding to the TiO₂ surface was studied by hard X-ray photoelectron spectroscopy for a bromine-containing complex, confirming the complex formation. The performance of all complexes was assessed and rationalized on the basis of their respective ancillary ligand. The DSSC photocurrent-voltage characteristics, incident photon-to-current conversion efficiency (IPCE) spectra, and calculated lowest unoccupied molecular orbital (LUMO) distributions collectively show a push-pull structural dye design, in which the ancillary ligand exhibits an electron-donating effect that can lead to improved solar cell performance. By analyzing the optical properties of the dyes and their solar cell performance, we can conclude that the presence of ancillary ligands with bulky substituents protects the Cu(I) metal center from solvent coordination constituting a critical factor in the design of efficient Cu(I)-based dyes. Moreover, we have identified some components in the I^-/I₃ ^--based electrolyte that causes dissociation of the ancillary ligand, i.e., TiO₂ photoelectrode bleaching. Finally, the detailed studies on one of the dyes revealed an electrolyte-dye interaction, leading to a dramatic change of the dye properties when adsorbed on the TiO₂ surface.

Dual Application of Waste Grape Skin for Photosensitizers and Counter Electrodes of Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs), a powerful system to convert solar energy into electrical energy, suffer from the high cost of the Pt counter electrode and photosensitizer. In this study, the dual application of waste grape skin is realized by employing the grape skin and its extract as the carbon source of the carbon-based counter electrode and photosensitizer, respectively. The ultraviolet–visible absorption and Fourier transform infrared spectroscopy verify the strong binding between the dye molecules (anthocyanins) in the extract and the TiO₂ nanostructure on the photoanode, contributing to a high open-circuit voltage (V_OC) value of 0.48 V for the assembled DSSC device. Moreover, the waste grape skin was subjected to pyrolysis and KOH activation and the resultant KOH-activated grape skin-derived carbon (KA-GSDC) possesses a large surface area (620.79 m² g⁻¹) and hierarchical porous structure, leading to a high short circuit current density (J_SC) value of 1.52 mA cm⁻². Additionally, the electrochemical impedance spectroscopy reveals the efficient electron transfer between the electrocatalyst and the redox couples and the slow recombination of electrolytic cations and the photo-induced electrons in the conduction band of TiO₂. These merits endow the DSSC with a high photovoltaic efficiency of 0.48%, which is 33% higher than that of a common Pt-based DSSC (0.36%). The efficiency is also competitive, compared with some congeneric DSSCs based on other natural dyes and Pt counter electrode. The result confirms the feasibility of achieving the high-value application of waste grape skin in DSSCs.

Solar energy conversion using first row d-block metal coordination compound sensitizers and redox mediators

The use of renewable energy is essential for the future of the Earth, and solar photons are the ultimate source of energy to satisfy the ever-increasing global energy demands. Photoconversion using dye-sensitized solar cells (DSCs) is becoming an established technology to contribute to the sustainable energy market, and among state-of-the art DSCs are those which rely on ruthenium(ii) sensitizers and the triiodide/iodide (I3 -/I-) redox mediator. Ruthenium is a critical raw material, and in this review, we focus on the use of coordination complexes of the more abundant first row d-block metals, in particular copper, iron and zinc, as dyes in DSCs. A major challenge in these DSCs is an enhancement of their photoconversion efficiencies (PCEs) which currently lag significantly behind those containing ruthenium-based dyes. The redox mediator in a DSC is responsible for regenerating the ground state of the dye. Although the I3 -/I- couple has become an established redox shuttle, it has disadvantages: its redox potential limits the values of the open-circuit voltage (V OC) in the DSC and its use creates a corrosive chemical environment within the DSC which impacts upon the long-term stability of the cells. First row d-block metal coordination compounds, especially those containing cobalt, and copper, have come to the fore in the development of alternative redox mediators and we detail the progress in this field over the last decade, with particular attention to Cu2+/Cu+ redox mediators which, when coupled with appropriate dyes, have achieved V OC values in excess of 1000 mV. We also draw attention to aspects of the recyclability of DSCs.

Through structural isomerism: positional effect of alkyne functionality on molecular optical properties

Literature studies on the effects of alkyne functionality in manipulating the optical properties of donor-π-acceptor-type molecular scaffolds have been scarce compared to those on the alkene functional group. Here, two structurally isomeric donor-acceptor (D-A) dyes were synthesized to study the positional effect of alkyne functionality (triple bond) on their optical, electrochemical and charge generation properties in order to design efficient dyes for possible application in dye sensitized solar cells (DSSCs). These dyes, named CAPC and PACC, contain carbazole and cyanoacrylic acid as the donor and acceptor units, respectively, and the π-conjugation length within the molecules was controlled by the introduction of an alkyne group. The D-π-A design was followed in designing CAPC with the alkyne serving as the π-spacer, while in PACC, alkyne was placed on the donor, which was directly in conjugation with the acceptor. This rendered equal conjugation lengths within the designed dyes. With the help of photophysical characterizations, it was concluded that CAPC featured better characteristics for a DSSC dye than PACC. Our conclusions were further supported by the results of transient absorption spectroscopy, electrochemical analysis, fluorescence lifetime studies and density functional theory.

Accurate predictions of the electronic excited states of BODIPY based dye sensitizers using spin-component-scaled double-hybrid functionals: a TD-DFT benchmark study

The vertical excitation energies of 13 BODIPY based dye sensitizers are benchmarked by means of TD-DFT, using 36 functionals from different DFT rungs. Most TD-DFT results were found to overestimate the excitation energies, and show mean absolute error (MAE) values in the range 0.2-0.5 eV. The dispersion-corrected, spin-component-scaled, double-hybrid (DSD) functionals DSD-BLYP and DSD-PBEP86 were found to have the smallest MAE values of 0.083 eV and 0.106 eV, respectively, which is close to the range of average errors found in the more expensive coupled-cluster methods. Moreover, DSD-BLYP and DSD-PBEP86 functionals show excellent consistency and quality of results (standard deviation = 0.048 eV and 0.069 eV respectively). However, the range separated hybrid (RSH) and the range separated double hybrid (RSDH) functionals were found to provide the best predictability (linear determination coefficient R 2 > 0.97 eV).

Molecular engineering of ruthenium-based photosensitizers with superior photovoltaic performance in DSSCs: novel N-alkyl 2-phenylindole-based ancillary ligands

In this work, we report the design and successful synthesis of two new heteroleptic polypyridyl Ru(ii) complexes (SD-5 and SD-6), by incorporating hetero-aromatic electron-donating N-alkyl-2-phenylindole moieties into the ancillary ligand.

Assessment of time-dependent density functionals for the electronic excitation energies of organic dyes used in DSSCs

The absorption spectra modeled as the vertical excitation energies of 13 dye sensitizers used in dye-sensitized solar cells (DSSCs) are benchmarked by means of time-dependent (TD)-DFT, using 36 functionals from different DFT rungs.

Toward Eco-Friendly Dye-Sensitized Solar Cells (DSSCs): Natural Dyes and Aqueous Electrolytes

Due to their low cost, facile fabrication, and high-power conversion efficiency (PCE), dye-sensitized solar cells (DSSCs) have attracted much attention. Ruthenium (Ru) complex dyes and organic solvent-based electrolytes are typically used in high-efficiency DSSCs. However, Ru dyes are expensive and require a complex synthesis process. Organic solvents are toxic, environmentally hazardous, and explosive, and can cause leakage problems due to their low surface tension. This review summarizes and discusses previous works to replace them with natural dyes and water-based electrolytes to fabricate low-cost, safe, biocompatible, and environmentally friendly DSSCs. Although the performance of “eco-friendly DSSCs” remains less than 1%, continuous efforts to improve the PCE can accelerate the development of more practical devices, such as designing novel redox couples and photosensitizers, interfacial engineering of photoanodes and electrolytes, and biomimetic approaches inspired by natural systems.

Recent Investigations on Thiocyanate-Free Ruthenium(II) 2,2'-Bipyridyl Complexes for Dye-Sensitized Solar Cells

Three decades ago, dye-sensitized solar cells (DSSCs) emerged as a method for harnessing the energy of the sun and for converting it into electricity. Since then, a lot of work has been devoted to create better global photovoltaic efficiencies and long term stability. Among photosensitizers for DSSCs, thiocyanate-free ruthenium(II) complexes have gained increasing interest due to their better stability compared to conventional thiocyanate-based complexes, such as benchmark dyes N719 and Z907. In this mini-review, two classes of thiocyanate-free Ru(II) complexes are presented: (a) bis-bipyridyl compounds bearing an ancillary cyclometalating bidentate ligand; (b) bipyridyl compounds bearing non-cyclometalating ancillary ligands. The coverage, mainly from 2014 up to now, is not exhaustive, but illustrates the most recent design strategies and photovoltaic properties of these two families of ruthenium(II) dyes.

Dye-sensitized solar cells based on Fe N-heterocyclic carbene photosensitizers with improved rod-like push-pull functionality

A new generation of octahedral iron(ii)–N-heterocyclic carbene (NHC) complexes, employing different tridentate C^N^C ligands, has been designed and synthesized as earth-abundant photosensitizers for dye sensitized solar cells (DSSCs) and related solar energy conversion applications. This work introduces a linearly aligned push–pull design principle that reaches from the ligand having nitrogen-based electron donors, over the Fe(ii) centre, to the ligand having an electron withdrawing carboxylic acid anchor group. A combination of spectroscopy, electrochemistry, and quantum chemical calculations demonstrate the improved molecular excited state properties in terms of a broader absorption spectrum compared to the reference complex, as well as directional charge-transfer displacement of the lowest excited state towards the semiconductor substrate in accordance with the push–pull design. Prototype DSSCs based on one of the new Fe NHC photosensitizers demonstrate a power conversion efficiency exceeding 1% already for a basic DSSC set-up using only the I⁻/I₃⁻ redox mediator and standard operating conditions, outcompeting the corresponding DSSC based on the homoleptic reference complex. Transient photovoltage measurements confirmed that adding the co-sensitizer chenodeoxycholic acid helped in improving the efficiency by increasing the electron lifetime in TiO₂. Time-resolved spectroscopy revealed spectral signatures for successful ultrafast (<100 fs) interfacial electron injection from the heteroleptic dyes to TiO₂. However, an ultrafast recombination process results in undesirable fast charge recombination from TiO₂ back to the oxidized dye, leaving only 5–10% of the initially excited dyes available to contribute to a current in the DSSC. On slower timescales, time-resolved spectroscopy also found that the recombination dynamics (longer than 40 μs) were significantly slower than the regeneration of the oxidized dye by the redox mediator (6–8 μs). Therefore it is the ultrafast recombination down to fs-timescales, between the oxidized dye and the injected electron, that remains as one of the main bottlenecks to be targeted for achieving further improved solar energy conversion efficiencies in future work.

Iron-based photosensitizers for dye-sensitized solar cells with a rod-like push–pull design. Solar cell performance was limited by ultrafast (sub-ps) recombination, but yielded better performance than the homoleptic parent photosensitizer.

Dynamics of photoconversion processes: the energetic cost of lifetime gain in photosynthetic and photovoltaic systems

The continued development of solar energy conversion technologies relies on an improved understanding of their limitations. In this review, we focus on a comparison of the charge carrier dynamics underlying the function of photovoltaic devices with those of both natural and artificial photosynthetic systems. The solar energy conversion efficiency is determined by the product of the rate of generation of high energy species (charges for solar cells, chemical fuels for photosynthesis) and the energy contained in these species. It is known that the underlying kinetics of the photophysical and charge transfer processes affect the production yield of high energy species. Comparatively little attention has been paid to how these kinetics are linked to the energy contained in the high energy species or the energy lost in driving the forward reactions. Here we review the operational parameters of both photovoltaic and photosynthetic systems to highlight the energy cost of extending the lifetime of charge carriers to levels that enable function. We show a strong correlation between the energy lost within the device and the necessary lifetime gain, even when considering natural photosynthesis alongside artificial systems. From consideration of experimental data across all these systems, the emprical energetic cost of each 10-fold increase in lifetime is 87 meV. This energetic cost of lifetime gain is approx. 50% greater than the 59 meV predicted from a simple kinetic model. Broadly speaking, photovoltaic devices show smaller energy losses compared to photosynthetic devices due to the smaller lifetime gains needed. This is because of faster charge extraction processes in photovoltaic devices compared to the complex multi-electron, multi-proton redox reactions that produce fuels in photosynthetic devices. The result is that in photosynthetic systems, larger energetic costs are paid to overcome unfavorable kinetic competition between the excited state lifetime and the rate of interfacial reactions. We apply this framework to leading examples of photovoltaic and photosynthetic devices to identify kinetic sources of energy loss and identify possible strategies to reduce this energy loss. The kinetic and energetic analyses undertaken are applicable to both photovoltaic and photosynthetic systems allowing for a holistic comparison of both types of solar energy conversion approaches.

Self-Assembled Materials Incorporating Functional Porphyrins and Carbon Nanoplatforms as Building Blocks for Photovoltaic Energy Applications

As a primary goal, this review highlights the role of supramolecular interactions in the assembly of new sustainable materials incorporating functional porphyrins and carbon nanoplatforms as building blocks for photovoltaics advancements.

Copper Dithiocarbamates: Coordination Chemistry and Applications in Materials Science, Biosciences and Beyond

Copper dithiocarbamate complexes have been known for ca. 120 years and find relevance in biology and medicine, especially as anticancer agents and applications in materials science as a single-source precursor (SSPs) to nanoscale copper sulfides. Dithiocarbamates support Cu(I), Cu(II) and Cu(III) and show a rich and diverse coordination chemistry. Homoleptic [Cu(S2CNR2)2] are most common, being known for hundreds of substituents. All contain a Cu(II) centre, being either monomeric (distorted square planar) or dimeric (distorted trigonal bipyramidal) in the solid state, the latter being held together by intermolecular C···S interactions. Their d9 electronic configuration renders them paramagnetic and thus readily detected by electron paramagnetic resonance (EPR) spectroscopy. Reaction with a range of oxidants affords d8 Cu(III) complexes, [Cu(S2CNR2)2][X], in which copper remains in a square-planar geometry, but Cu–S bonds shorten by ca. 0.1 Å. These show a wide range of different structural motifs in the solid-state, varying with changes in anion and dithiocarbamate substituents. Cu(I) complexes, [Cu(S2CNR2)2]−, are (briefly) accessible in an electrochemical cell, and the only stable example is recently reported [Cu(S2CNH2)2][NH4]·H2O. Others readily lose a dithiocarbamate and the d10 centres can either be trapped with other coordinating ligands, especially phosphines, or form clusters with tetrahedral [Cu(μ3-S2CNR2)]4 being most common. Over the past decade, a wide range of Cu(I) dithiocarbamate clusters have been prepared and structurally characterised with nuclearities of 3–28, especially exciting being those with interstitial hydride and/or acetylide co-ligands. A range of mixed-valence Cu(I)–Cu(II) and Cu(II)–Cu(III) complexes are known, many of which show novel physical properties, and one Cu(I)–Cu(II)–Cu(III) species has been reported. Copper dithiocarbamates have been widely used as SSPs to nanoscale copper sulfides, allowing control over the phase, particle size and morphology of nanomaterials, and thus giving access to materials with tuneable physical properties. The identification of copper in a range of neurological diseases and the use of disulfiram as a drug for over 50 years makes understanding of the biological formation and action of [Cu(S2CNEt2)2] especially important. Furthermore, the finding that it and related Cu(II) dithiocarbamates are active anticancer agents has pushed them to the fore in studies of metal-based biomedicines.