Time-dependent density functional theory investigations on the excited states of Ru(II)-dye-sensitized TiO2 nanoparticles: the role of sensitizer protonation

The dynamics of light-induced interfacial charge transfer of different dyes in dye-sensitized solar cells studied by ab initio molecular dynamics

The charge-transport dynamics at the dye-TiO₂ interface plays a vital role for the resulting power conversion efficiency (PCE) of dye sensitized solar cells (DSSCs). In this work, we have investigated the charge-exchange dynamics for a series of organic dyes, of different complexity, and a small model of the semiconductor substrate TiO₂. The dyes studied involve L1, D35 and LEG4, all well-known organic dyes commonly used in DSSCs. The computational studies have been based on ab initio molecular dynamics (aiMD) simulations, from which structural snapshots have been collected. Estimates of the charge-transfer rate constants of the central exchange processes in the systems have been computed. All dyes show similar properties, and differences are mainly of quantitative character. The processes studied were the electron injection from the photoexcited dye, the hole transfer from TiO₂ to the dye and the recombination loss from TiO₂ to the dye. It is notable that the electronic coupling/transfer rates differ significantly between the snapshot configurations harvested from the aiMD simulations. The differences are significant and indicate that a single geometrically optimized conformation normally obtained from static quantum-chemistry calculations may provide arbitrary results. Both protonated and deprotonated dye systems were studied. The differences mainly appear in the rate constant of recombination loss between the protonated and the deprotonated dyes, where recombination losses take place at significantly higher rates. The inclusion of lithium ions close to the deprotonated dye carboxylate anchoring group mitigates recombination in a similar way as when protons are retained at the carboxylate group. This may give insight into the performance-enchancing effects of added salts of polarizing cations to the DSSC electrolyte. In addition, solvent effects can retard charge recombination by about two orders of magnitude, which demonstrates that the presence of a solvent will increase the lifetime of injected electrons and thus contribute to a higher PCE of DSSCs. It is also notable that no simple correlation can be identified between high/low transfer rate constants and specific structural arrangements in terms of atom-atom distances, angles or dihedral arrangements of dye sub-units.

Effects of co-adsorption on interfacial charge transfer in a quantum dot@dye composite

The sensitive electronic environment at the quantum dot (QD)-dye interface becomes a roadblock to enhancing the energy conversion efficiency of dye-functionalized quantum dots (QDs). Energy alignments and electronic couplings are the critical factors governing the directions and rates of different charge transfer pathways at the interface, which are tunable by changing the specific linkage groups that connect a dye to the QD surface. The variation of specific anchors changes the binding configurations of a dye on the QD surface. In addition, the presence of a co-adsorbent changes the dipole-dipole and electronic interactions between a QD and a dye, resulting in different electronic environments at the interface. In the present work, we performed density functional theory (DFT)-based calculations to study the different binding configurations of N719 dye on the surface of a Cd₃₃Se₃₃ QD with a co-adsorbent D131 dye. The results revealed that the electronic couplings for electron transfer were greater than for hole transfer when the structure involved isocyanate groups as anchors. Such strong electronic couplings significantly stabilize the occupied states of the dye, pushing them deep inside the valence band of the QD and making hole transfer in these structures thermodynamically unfavourable. When carboxylates were involved as anchors, the electronic couplings for hole transfer were comparable to electron transfer, implying efficient charge separation at the QD-dye interface and reduced electron-hole recombination within the QD. We also found that the electronic couplings for electron transfer were larger than those for back electron transfer, suggesting efficient charge separation in photoexcited QDs. Overall, the current computational study reveals some fundamental aspects of the relationship between the interfacial charge transfer for QD@dye composites and their morphologies which benefit the design of QD-based nanomaterials for photovoltaic applications.

Real Space-Real Time Evolution of Excitonic States Based on the Bethe-Salpeter Equation Method

We introduce a method for constructing localized excitations and simulating the real time dynamics of excitons at the Many-Body Perturbation Theory Bethe-Salpeter Equation level. We track, on the femto-seconds scale, electron injection from a photoexcited dye into a semiconducting slab. From the time-dependent many-body wave function we compute the spatial evolution of the electron and of the hole; full electron injection is attained within 5 fs. Time-resolved analysis of the electron density and electron-hole interaction energy hints at a two-step charge transfer mechanism through an intermediary partially injected state. We adopt the Von-Neumann entropy for analyzing how the electron and hole entangle. We find that the excitation of the dye-semiconductor model may be represented by a four-level system and register a decrease in entanglement upon electron injection. At full injection, the electron and the hole exhibit only a small degree of entanglement indicative of pure electron and hole states.

DFT and TD-DFT Investigation of a Charge Transfer Surface Resonance Raman Model of N3 Dye Bound to a Small TiO2 Nanoparticle

Raman spectroscopy is an important method for studying the configuration of Ru bipyridyl dyes on TiO₂. We studied the [Ru(II)(4,4′-COOH-2,2′-bpy)2(NCS)2)] dye (N3) adsorbed on a (TiO₂)5 nanoparticle using Density Functional Theory, DFT, to optimize the geometry of the complex and to simulate normal Raman scattering, NRS, for the isolated N3 and the N3–(TiO₂)5 complex. Two configurations of N3 are found on the surface both anchored with a carboxylate bridging bidentate linkage but one with the two NCS ligands directed away from the surface and one with one NSC tilted away and the other NCS interacting with the surface. Both configurations also had another –COOH group hydrogen bonded to a Ti-O dangling bond. These configurations can be distinguished from each other by Raman bands at 2104 and 2165 cm⁻¹. The former configuration has more intense Normal Raman Scattering, NRS, on TiO₂ surfaces and was studied with Time-Dependent Density Functional Theory, TD-DFT, frequency-dependent Raman simulations. Pre-resonance Raman spectra were simulated for a Metal to Ligand Charge Transfer, MLCT, excited state and for a long-distance CT transition from N3 directly to (TiO₂)5. Enhancement factors for the MLCT and long-distance CT processes are around 1 × 103 and 2 × 102, respectively. A Herzberg–Teller intensity borrowing mechanism is implicated in the latter and provides a possible mechanism for the photo-injection of electrons to titania surfaces.

An electron rich indaceno [2,1-b:6,5-b′] dithiophene derivative as a high intramolecular charge transfer material in dye sensitized solar cells

The synthesis, characterisation and photovoltaic performance of an indacenodithiophene (IDT)-based organic dye in DSSCs has been presented.

Critical Role of Protons for Emission Quenching of Indoline Dyes in Solution and on Semiconductor Surfaces

By combining time-correlated single photon counting (TCSPC) measurements, density functional theory (DFT), and time-dependent DFT (TD-DFT) calculations, we herein investigate the role of protons, in solutions and on semiconductor surfaces, for the emission quenching of indoline dyes. We show that the rhodanine acceptor moieties, and in particular the carbonyl oxygens, undergo protonation, leading to nonradiative excited-state deactivation. The presence of the carboxylic acid anchoring group, close to the rhodanine moiety, further facilitates the emission quenching, by establishing stable H-bond complexes with carboxylic acid quenchers, with high association constants, in both ground and excited states. This complexation favors the proton transfer process, at a low quencher concentration, in two ways: bringing close to the rhodanine unit the quencher and assisting the proton release from the acid by a partial-concerted proton donation from the close-by carboxylic group to the deprotonated acid. Esterification of the carboxylic group, indeed, inhibits the ground-state complex formation with carboxylic acids and thus the quenching at a low quencher concentration. However, the rhodanine moiety in the ester form can still be the source of emission quenching through dynamic quenching mechanism with higher concentrations of protic solvents or carboxylic acids. Investigating this quenching process on mesoporous ZrO₂, for solar cell applications, also reveals the sensitivity of the adsorbed excited rhodanine dyes toward adsorbed protons on surfaces. This has been confirmed by using an organic base to remove surface protons and utilizing cynao-acrylic dye as a reference dye. Our study highlights the impact of selecting such acceptor group in the structural design of organic dyes for solar cell applications and the overlooked role of protons to quench the excited state for such chemical structures.

Double Linker Triphenylamine Dyes for Dye-Sensitized Solar Cells

Most organic dyes synthesized for dye-sensitized solar cells (DSC) use a single linker group to bind to the metal oxide photo-anode. Here we describe the synthesis and testing of two new triphenylamine dyes containing either two carboxylic acids 5-[2-(4-diphenylamino-phenyl)-vinyl]-isophthalic acid (10) or two cyanoacrylic acids (2Z, 2′Z)-3, 3′-(5-((E)-4-(diphenylamino) styryl)-1, 3-phenylene) bis (2-cyanoacrylic acid) (8) as linker groups. Full characterization data are reported for these dyes and their synthetic intermediates. DSC devices have been prepared from these new dyes either by passive or fast dyeing and the dyes have also been tested in co-sensitized DSC devices leading to a PCE (η = 5.4%) for the double cyanoacrylate linker dye (8) co-sensitized with D149. The dye:TiO2 surface interactions and dye excitations are interpreted using three modelling methods: density functional theory (at 0 K); molecular dynamics (at 298 K); time dependent density functional theory. The modelling results show the preferred orientation of both dyes on an anatase (1 0 1) TiO2 surface to be horizontal, and both the simulated and experimental absorption spectra of the dye molecules indicate a red shifted band for (8) compared to (10). This is in line with broader light harvesting and Jsc for (8) compared to (10).

A microscopic and macroscopic investigation of the adsorption of N719 dye on ZnO nanopowders (ZNP) and ZnO nanorods (ZNR) for dye sensitized solar cells using statistical physics treatment and DFT simulation

In this paper, three adsorption isotherms of N719 dye on two different adsorbents, ZnO nanopowder and ZnO nanorods, at three different thicknesses have been fitted using a monolayer model with three types of receptor sites treated by statistical physics. The model involved parameters are: three coefficients (n 1, n 2 and n 3) indicating the numbers of adsorbed dye molecules per site, three parameters (N m1, N m2 and N m3) indicating the receptor site densities and three adsorption energies ((-ε 1), (-ε 2) and (-ε 3)). The evolution of these parameters in relation with thickness of ZnO was discussed. The pore size distribution (PSD) of ZnO nanopowder and ZnO nanorods as a function of the thickness has been studied using the chosen adequate model. The molecular electrostatic potential (MEP) has been investigated to optimize the different adsorbed geometries of the complex N719 dye@ZnO. The intermolecular interactions between the N719 dye and the ZnO surface have been studied by using the quantum theory of atoms in molecules (AIM) and reduced density gradient RDG. The results of the MEP, topological AIM and RDG are in agreement with the results of statistical physics.

Effects of interfacial adsorption configurations on dye-sensitized solar cell performance at the stoichiometric and defective TiO2 anatase (101) surfaces: a theoretical investigation

Interfacial adsorption configuration plays a crucial role in influencing the photovoltaic performance of dye-sensitized solar cells (DSSCs), and thus, theoretical investigations are needed to further understand the impacts of different absorption configurations on stoichiometric and defective TiO2(101) surfaces on the short-circuit photocurrent density (JSC) and open-circuit voltage (VOC) of DSSCs. Herein, calculations of isolated dyes and dye/TiO2 systems were performed on the donor-π bridge-acceptor (D-π-A) type porphyrin sensitizers bearing different donor moieties and an α-cyanoacrylic acid anchoring group (T1-3), using DFT and TD-DFT methods. And, for the first time, comparative analysis of interfacial electron transfer (IET) and density of states (DOS) were carried out on dye/TiO2 systems with stoichiometric and defective surfaces to provide further insight into the electronic factors influencing the efficiency of DSSCs, which can well explain the experimental variation trends of JSC and VOC values. It turned out that attachment via the carboxyl and cynao groups in a tridentate binding mode can result in more efficient IET rates and an upshifted conduction band in comparison with those of the bidentate attachment. More interestingly, we found that the adsorption configuration on defective surfaces containing an O2c vacancy induced more upshifted CBM and relatively fast IET, especially for the bonding mode through two O atoms of the carboxyl group.

Resonance State Method for Electron Injection in Dye Sensitized Solar Cells

Herein, the ab initio method is applied to examine metastable molecular excited states on a solid surface using resonance state theory and Green's function. A formula for the complex energy correction that determines the decay rate is presented; the configuration interaction effect together with major molecule-surface interactions are considered in more detail as compared to previous studies. Furthermore, the lifetimes of the excited states of Ru-terpyridine dyes adsorbed on an anatase surface are calculated, and the effects of the molecular structure and adsorption mode on the electron injection rate are studied. Also, the adsorption structures and relative stabilities of a series of Ru-terpyridine dyes-including the black dye-are reported. An implicit solvation model is necessary to reliably calculate the alignment between the photoabsorption spectrum and the conduction band density of states, governing the injection rate. Finally, some of the factors that limit the injection ability of dyes are discussed.

Probing the interaction between 2,2'-bithiophene-5-carboxylic acid and TiO2 by photoelectron spectroscopy: A joint experimental and theoretical study

The interaction between 2,2'-bithiophene-5-carboxylic acid (PT2) sublimed under ultra-high vacuum conditions and anatase (101) and rutile (110) TiO₂ single crystal surfaces is investigated by studying the electronic spectral density near the Fermi level with synchrotron-based spectroscopy. The experimental results are compared to density functional theory calculations of the isolated PT2 molecule and of the molecule adsorbed on an anatase TiO₂ (101) cluster. The relative concentrations of Ti, C, and S atoms indicate that the adsorbed molecule remains intact upon deposition, which is typical of a Stranski-Krastanov growth mode. The analysis of the O1s spectrum suggests a predominant bidentate geometry of the adsorption with both rutile and anatase surfaces, as supported by previous theoretical simulations. It is also theoretically and experimentally demonstrated that the PT2 adsorption causes the appearance of new electronic states in the gap near the TiO₂ valence band. A pinning effect of the LUMO level of the dye is also theoretically predicted.

Probing single-molecule electron-hole transfer dynamics at a molecule-NiO semiconductor nanocrystalline interface

Interfacial charge transfer dynamics in dye-sensitized NiO nanoparticles are being investigated for photocathodes in p-type dye-sensitized solar cells. In the photoreaction, after fast electron transfer from NiO to a molecule, the recombination of the hole in the nanoparticles with the electron in a reduced molecule plays an important role in the charge separation process and solar energy harvesting. Nevertheless, knowledge of the interfacial charge recombination (CR) rate and its mechanism is still limited due to the complex photoinduced electron and hole dynamics and lack of characterization of the inhomogeneity of the dynamics. Here, we report our work on probing interfacial charge recombination dynamics in Zn(ii)-5,10,15,20-tetra(3-carboxyphenyl)porphyrin (m-ZnTCPP) dye-sensitized NiO nanoparticles by correlating single-molecule fluorescence blinking dynamics with charge transfer dynamics using single-molecule photon-stamping spectroscopy. The correlated analyses of single-molecule fluorescence intensity, lifetime, and blinking reveal the intrinsic distribution and temporal fluctuation of interfacial charge transfer reactivity, which are closely related to site-specific molecular interactions and dynamics.

A detailed experimental and theoretical investigation of the role of cyano groups in the π-bridged acceptor of sensitizers for use in dye-sensitized solar cells (DSCs)

Porphyrin sensitizers LX1, LX2 and LX3 were synthesized for use in DSCs and their electronic properties with TiO₂ were investigated in detail using theoretical calculations.

Ultrafast photoelectron migration in dye-sensitized solar cells: Influence of the binding mode and many-body interactions

In the present contribution, the ultrafast photoinduced electron migration dynamics at the interface between an alizarin dye and an anatase TiO₂ thin film is investigated from first principles. Comparison between a time-dependent many-electron configuration interaction ansatz and a single active electron approach sheds light on the importance of many-body effects, stemming from uniquely defined initial conditions prior to photoexcitation. Particular emphasis is put on understanding the influence of the binding mode on the migration process. The dynamics is analyzed on the basis of a recently introduced toolset in the form of electron yields, electronic fluxes, and flux densities, to reveal microscopic details of the electron migration mechanism. From the many-body perspective, insight into the nature of electron-electron and hole-hole interactions during the charge transfer process is obtained. The present results reveal that the single active electron approach yields quantitatively and phenomenologically similar results as the many-electron ansatz. Furthermore, the charge migration processes in the dye-TiO₂ model clusters with different binding modes exhibit similar mechanistic pathways but on largely different time scales.

Assessing the effects of surface-bound humic acid on the phototoxicity of anatase and rutile TiO₂ nanoparticles in vitro

In this study, the cytotoxicity of two different crystal phases of TiO2 nanoparticles, with surface modification by humic acid (HA), to Escherichia coli, was assessed. The physicochemical properties of TiO2 nanoparticles were thoroughly characterized. Three different initial concentrations, namely 50, 100, and 200 ppm, of HA were used for synthesis of HA coated TiO2 nanoparticles (denoted as A/RHA50, A/RHA100, and A/RHA200, respectively). Results indicate that rutile (LC50 (concentration that causes 50% mortality compared the control group)=6.5) was more toxic than anatase (LC50=278.8) under simulated sunlight (SSL) irradiation, possibly due to an extremely narrow band gap. It is noted that HA coating increased the toxicity of anatase, but decreased that of rutile. Additionally, AHA50 and RHA50 had the biggest differences compared to uncoated anatase and rutile with LC50 of 201.9 and 21.6, respectively. We then investigated the formation of reactive oxygen species (ROS) by TiO2 nanoparticles in terms of hydroxyl radicals (·OH) and superoxide anions (O2(·-)). Data suggested that O2(·-) was the main ROS that accounted for the higher toxicity of rutile upon SSL irradiation. We also observed that HA coating decreased the generation of ·OH and O2(·-) on rutile, but increased O2(·-) formation on anatase. Results from TEM analysis also indicated that HA coated rutile tended to be attached to the surface of E. coli more than anatase.

Impact of neutral and anion anchoring groups on the photovoltaic performance of triphenylamine sensitizers for dye-sensitized solar cells

Triphenylamine-based sensitizer with a carboxylate anchoring group (COO⁻) was synthesized and compared with neutral dye (COOH) for DSSC application.

Hierarchically assembled microspheres consisting of nanosheets of highly exposed (001)-facets TiO2 for dye-sensitized solar cells

Mono-dispersed 2D nanosheets of 3D microspheres nanostructure was synthesized. It shows highly exposed (001)-facets (ca. 82%) and high surface area (112.2 m² g⁻¹). 11.13%-efficient DSSC is obtained with TiO₂ microspheres working electrode.

The Role of Porphyrin-Free-Base in the Electronic Structures and Related Properties of N-Fused Carbazole-Zinc Porphyrin Dye Sensitizers

Dye sensitizers can significantly affect power conversion efficiency of dye-sensitized solar cells (DSSCs). Porphyrin-based dyes are promising sensitizers due to their performances in DSSCs. Here, based upon a N-fused carbazole-zinc porphyrin-free-base porphyrin triad containing an ethynyl-linkage (coded as DTBC), the novel porphyrin dyes named DTBC-MP and DTBC-TP were designed by varying the porphyrin-free-base units in the π conjugation of DTBC in order to study the effect of porphyrin-free-base in the modification of electronic structures and related properties. The calculated results indicate that, the extension of the conjugate bridge with the porphyrin-free-base unit results in elevation of the highest occupied molecular orbital (HOMO) energies, decrease of the lowest unoccupied molecular orbital (LUMO) energies, reduction of the HOMO-LUMO gap, red-shift of the absorption bands, and enhancement of the absorbance. The free energy changes demonstrate that introducing more porphyrin-free-base units in the conjugate bridge induces a faster rate of electron injection. The transition properties and molecular orbital characters suggest that the different transition properties might lead to a different electron injection mechanism. In terms of electronic structure, absorption spectra, light harvesting capability, and free energy changes, the designed DTBC-TP is a promising candidate dye sensitizer for DSSCs.

Approach to multifunctional device platform with epitaxial graphene on transition metal oxide

Heterostructures consisting of two-dimensional materials have shown new physical phenomena, novel electronic and optical properties, and new device concepts not observed in bulk material systems or purely three dimensional heterostructures. These new effects originated mostly from the van der Waals interaction between the different layers. Here we report that a new optical and electronic device platform can be provided by heterostructures of 2D graphene with a metal oxide (TiO₂). Our novel direct synthesis of graphene/TiO₂ heterostructure is achieved by C₆₀ deposition on transition Ti metal surface using a molecular beam epitaxy approach and O₂ intercalation method, which is compatible with wafer scale growth of heterostructures. As-grown heterostructures exhibit inherent photosensitivity in the visible light spectrum with high photo responsivity. The photo sensitivity is 25 times higher than that of reported graphene photo detectors. The improved responsivity is attributed to optical transitions between O 2p orbitals in the valence band of TiO₂ and C 2p orbitals in the conduction band of graphene enabled by Coulomb interactions at the interface. In addition, this heterostructure provides a platform for realization of bottom gated graphene field effect devices with graphene and TiO₂ playing the roles of channel and gate dielectric layers, respectively.

Organic-free Anatase TiO₂ Paste for Efficient Plastic Dye-Sensitized Solar Cells and Low Temperature Processed Perovskite Solar Cells

Recently, the synthesis of fine TiO2 paste with organic-free binder emerged as an indispensable technique for plastic photovoltaics due to the low temperature processing requirement. In this study, pure anatase TiO2 nanoparticles and organic-free TiO2-sol were successfully synthesized individually in organic-free solution. By mixing the pure anatase TiO2 with the newly developed TiO2-sol binder, mechanically robust and well-interconnected TiO2 films were prepared via UV-irradiation at low temperature for applications in plastic dye-sensitized solar cells (p-DSSCs). The structural, electrical, and photovoltaic properties of the films as well as the devices were investigated by various techniques. The dye-loading amount of the obtained film is 2.6 times that of the P25 electrodes. As revealed by electrochemical impedance spectroscopy results, the film derived from the as-prepared anatase TiO2 paste (A-TiO2) exhibits much smaller charge transport resistance and lower electron recombination rate than the P25 film, while the introduction of TiO2-sol into the paste can further remarkably decrease the resistance of the produced film (AS-TiO2). The p-DSSCs employing AS-TiO2 photoanode yield a high efficiency up to 7.51%, which is 86% higher than the P25 reference cells and also 31% higher than the A-TiO2 cell. As a proof of concept, the newly developed AS-TiO2 paste was also applied to low temperature processed perovskite solar cells (PSCs), and a promising high efficiency up to 9.95% was achieved.

Crucial Role of Nuclear Dynamics for Electron Injection in a Dye-Semiconductor Complex

We investigate the electron injection from a terrylene-based chromophore to the TiO2 semiconductor bridged by a recently proposed phenyl-amide-phenyl molecular rectifier. The mechanism of electron transfer is studied by means of quantum dynamics simulations using an extended Hückel Hamiltonian. It is found that the inclusion of the nuclear motion is necessary to observe the photoinduced electron transfer. In particular, the fluctuations of the dihedral angle between the terrylene and the phenyl ring modulate the localization and thus the electronic coupling between the donor and acceptor states involved in the injection process. The electron propagation shows characteristic oscillatory features that correlate with interatomic distance fluctuations in the bridge, which are associated with the vibrational modes driving the process. The understanding of such effects is important for the design of functional dyes with optimal injection and rectification properties.

Optical properties of a conjugated-polymer-sensitised solar cell: the effect of interfacial structure

Dye-sensitised solar cells (DSSCs) have sparked considerable interest over two decades. Recently, a method of polymer-wire sensitisation was demonstrated; the polymer is suggested to form a hole transport pathway (wire) following initial charge separation. We predict the optical properties of this polymer in various interfacial configurations, including the effects of chain length and attachment to {100} or {101} TiO2 facets. Contrary to most DSSCs, the {100} facet model best describes the experimental spectrum, predicting a relative thickness of 5.7 ± 0.2 μm, although {101} attachment, if implemented, may improve collection efficiency. Long chains are optimal, and stable attachment sites show minimal differences to absorbance in the major solar emission (visible) band. Combinations of {100}, {101}, and pseudo-bulk TiO2 models in three-parameter fits to experiment confirm the relative importance of the {100} facet.

Dye chemistry with time-dependent density functional theory

In this perspective, we present an overview of the determination of excited-state properties of "real-life" dyes, and notably of their optical absorption and emission spectra, performed during the last decade with time-dependent density functional theory (TD-DFT). We discuss the results obtained with both vertical and adiabatic (vibronic) approximations, choosing relevant examples for several series of dyes. These examples include reproducing absorption wavelengths of numerous families of coloured molecules, understanding the specific band shape of amino-anthraquinones, optimising the properties of dyes used in solar cells, mimicking the fluorescence wavelengths of fluorescent brighteners and BODIPY dyes, studying optically active biomolecules and photo-induced proton transfer, as well as improving the properties of photochromes.

Quantitative structure–property relationship modeling of ruthenium sensitizers for solar cells applications: novel tools for designing promising candidates

To date, the most common way of screening new potential sensitizers for dye sensitized solar cells is via the traditional time and money consuming trial and error approach.

An overview of selected current approaches to the characterization of aqueous inorganic clusters

This Perspective highlights several modern characterization techniques used to identify nanoscale inorganic clusters in the context of multiple case studies.

Highly Efficient SnO2/MgO Composite Film-Based Dye-Sensitized Solar Cells Sensitized with N719 and D358 Dyes

SnO 2/ MgO composite film-based dye-sensitized solar cells (DSCs), sensitized with both N719 dye and metal-free D358 dye, employing [Formula: see text] redox couple-based liquid electrolyte, show superior performance to those sensitized with only D358 dye, and N719 dye. A significant improvement in the power conversion efficiency was attained by co-sensitizing the N719-based DSCs with metal-free D358 dye when compared to those obtained for DSCs with individual dyes. As confirmed by UV-visible absorption spectra, N719 dye adsorption is more prominent than that of D358 dye when sensitizing the SnO 2/ MgO composite film with the two dyes, D358 and N719. However, N719 and D358 dyes, when used alone, are prone to form aggregates on the SnO 2/ MgO composite film, when N719 dye is used together with D358, the latter effectively suppresses the aggregation of N719 dye on the SnO 2/ MgO composite film, thereby enhancing the power conversion efficiency of the DSCs. Hence, the corresponding power conversion efficiency of the SnO 2/ MgO composite film-based DSCs can be significantly improved by sensitizing with both N719 and D358 dyes. The reported power conversion efficiencies for the SnO 2/ MgO composite film-based DSCs, sensitized with, (a) D358 dye, (b) N719 dye, and (c) both N719 dye and D358 dye, are 6.37%, 7.43% and 8.60% respectively, under AM 1.5 illumination.

Investigation of the influence of coadsorbent dye upon the interfacial structure of dye-sensitized solar cells

The interface between Ru(tcterpy)(NCS)3TBA2 [black dye (BD); tcterpy = 4,4',4″-tricarboxy-2,2':6',2″-terpyridine, NCS = thiocyanato, TBA = tetrabutylammonium cation] and nanocrystalline TiO2, as found in dye-sensitized solar cells, is investigated by soft-X-ray synchrotron radiation and compared with the adsorption structure of cis-Ru(Hdcbpy)2(NCS)2TBA2 (N719; dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) on TiO2 to elucidate the relationship between the adsorption mode of BD and the photocurrent with and without coadsorbed indoline dye D131. The depth profile is characterized with X-ray photoelectron spectroscopy and S K-edge X-ray absorption fine structure using synchrotron radiation. Both datasets indicate that one of the isothiocyanate groups of BD interacts with TiO2 via its S atom when the dye is adsorbed from a single-component solution. In contrast, the interaction is slightly suppressed when D131 is coadsorbed, indicated by the fact that the presence of D131 changes the adsorption mode of BD. Based upon these results, the number of BD dye molecules interacting with the substrate is shown to decrease by 10% when D131 is coadsorbed, and the dissociation is shown to be related to the short-circuit photocurrent in the 600-800 nm region. The design of a procedure to promote the preferential adsorption of D131 therefore leads to an improvement of the short-circuit current and conversion efficiency.

Thiocyanate-free cyclometalated ruthenium(II) sensitizers for DSSC: a combined experimental and theoretical investigation

In an effort to bring out efficient thiocyanate-free dyes for dye sensitized solar cells (DSSC) we have designed, synthesized and characterized four novel cyclometalated ruthenium(II) dyes (M1 to M4) with superior photochemical properties. All dyes contain terpyridyl ligands (TPY) with carboxylic acids as anchoring groups and cyclometalated ligand (TPY-C) with substituents for fine tuning the electronic properties. We obtain a broad absorption band which extends up to 725 nm due to metal to ligand charge transfer (MLCT) when donating groups are used, which slightly blue-shifts when a withdrawing group is used. In addition to the CT, small HOMO-LUMO gaps are obtained from electrochemical measurements which indicate characteristics of an ideal sensitizer. All four dyes were used as sensitizers for DSSC and photoelectrochemical measurements were carried out. Reasonably good efficiency (7.1%) has been achieved for . We have carried out periodic-DFT studies of these dyes adsorbed on the (TiO2)38 cluster. They revealed that, in bidentate bridging mode the dyes preferably bind with the help of two carboxylic groups onto the TiO2. To the best of our knowledge we are the first to do DFT studies of thiocyanate free cyclometalated ruthenium(ii) dyes tethered to TiO2.

DFT/TDDFT study of the adsorption of N3 and N719 dyes on ZnO(101̅0) surfaces

ZnO has attracted a great deal of research as a potential replacement of TiO2 for dye-sensitized solar cells (DSSCs), owing to the unique combination of interesting electronic properties (i.e., high electron mobility) and structural richness. Here, we present a DFT/TDDFT study about the interaction of the prototypical N3 and N719 Ru(II) sensitizers on ZnO models to understand some of the atomistic details that are crucial to the dye/semiconductor interaction. We pay particular attention to the adsorption mode of the sensitizer and to the effect of the complexation on the electronic structure of the dye. The sensitizers are predicted to strongly interact with the ZnO surface. In particular, the interaction is strengthened when three dye carboxylic groups are involved in the adsorption. Moreover, if the anchoring group bears a proton, the adsorption is predicted to be dissociative. The charge density donation from the dye to the semiconductor raises the valence and conduction band edges of the latter, in such a way that the optical gap of ZnO widens. Proton transfer from the dye to the semiconductor balances the charge donation effect and restores the electronic levels of the noninteracting fragments. The impact of dye/semiconductor interaction on the adsorbed dye optical properties is then discussed.

Adsorption of organic dyes on TiO2 surfaces in dye-sensitized solar cells: interplay of theory and experiment

First-principles computer simulations can contribute to a deeper understanding of the dye/semiconductor interface lying at the heart of Dye-sensitized Solar Cells (DSCs). Here, we present the results of simulation of dye adsorption onto TiO(2) surfaces, and of their implications for the functioning of the corresponding solar cells. We propose an integrated strategy which combines FT-IR measurements with DFT calculations to individuate the energetically favorable TiO(2) adsorption mode of acetic acid, as a meaningful model for realistic organic dyes. Although we found a sizable variability in the relative stability of the considered adsorption modes with the model system and the method, a bridged bidentate structure was found to closely match the FT-IR frequency pattern, also being calculated as the most stable adsorption mode by calculations in solution. This adsorption mode was found to be the most stable binding also for realistic organic dyes bearing cyanoacrylic anchoring groups, while for a rhodanine-3-acetic acid anchoring group, an undissociated monodentate adsorption mode was found to be of comparable stability. The structural differences induced by the different anchoring groups were related to the different electron injection/recombination with oxidized dye properties which were experimentally assessed for the two classes of dyes. A stronger coupling and a possibly faster electron injection were also calculated for the bridged bidentate mode. We then investigated the adsorption mode and I(2) binding of prototype organic dyes. Car-Parrinello molecular dynamics and geometry optimizations were performed for two coumarin dyes differing by the length of the π-bridge separating the donor and acceptor moieties. We related the decreasing distance of the carbonylic oxygen from the titania to an increased I(2) concentration in proximity of the oxide surface, which might account for the different observed photovoltaic performances. The interplay between theory/simulation and experiments appears to be the key to further DSCs progress, both concerning the design of new dye sensitizers and their interaction with the semiconductor and with the solution environment and/or an electrolyte upon adsorption onto the semiconductor.

Stark effect in perovskite/TiO2 solar cells: evidence of local interfacial order

To unveil the mechanisms controlling photovoltaic conversion in high-performing perovskite-based mesostructured solar cells, we focus on the key role played by the mesoporous oxide/perovskite interface. We employ several spectroscopic techniques to design a complete scenario and corroborate our results with first principle density functional theory calculations. In particular Stark spectroscopy, a powerful tool allowing interface-sensitive analysis is employed to prove the existence of oriented permanent dipoles, consistent with the hypothesis of an ordered perovskite layer, close to the oxide surface. The existence of a structural order, promoted by specific local interactions, could be one of the decisive reasons for highly efficient carriers transport within perovskite films.

Highly reactive {001} facets of TiO2-based composites: synthesis, formation mechanism and characterization

Titanium dioxide (TiO2) is one of the most widely investigated metal oxides due to its extraordinary surface, electronic and catalytic properties. However, the large band gap of TiO2 and massive recombination of photogenerated electron-hole pairs limit its photocatalytic and photovoltaic efficiency. Therefore, increasing research attention is now being directed towards engineering the surface structure of TiO2 at the most fundamental and atomic level namely morphological control of {001} facets in the range of microscale and nanoscale to fine-tune its physicochemical properties, which could ultimately lead to the optimization of its selectivity and reactivity. The synthesis of {001}-faceted TiO2 is currently one of the most active interdisciplinary research areas and demonstrations of catalytic enhancement are abundant. Modifications such as metal and non-metal doping have also been extensively studied to extend its band gap to the visible light region. This steady progress has demonstrated that TiO2-based composites with {001} facets are playing and will continue to play an indispensable role in the environmental remediation and in the search for clean and renewable energy technologies. This review encompasses the state-of-the-art research activities and latest advancements in the design of highly reactive {001} facet-dominated TiO2via various strategies, including hydrothermal/solvothermal, high temperature gas phase reactions and non-hydrolytic alcoholysis methods. The stabilization of {001} facets using fluorine-containing species and fluorine-free capping agents is also critically discussed in this review. To overcome the large band gap of TiO2 and rapid recombination of photogenerated charge carriers, modifications are carried out to manipulate its electronic band structure, including transition metal doping, noble metal doping, non-metal doping and incorporating graphene as a two-dimensional (2D) catalyst support. The advancements made in these aspects are thoroughly examined, with additional insights related to the charge transfer events for each strategy of the modified-TiO2 composites. Finally, we offer a summary and some invigorating perspectives on the major challenges and new research directions for future exploitation in this emerging frontier, which we hope will advance us to rationally harness the outstanding structural and electronic properties of {001} facets for various environmental and energy-related applications.

Modeling opto-electronic properties of a dye molecule in proximity of a semiconductor nanoparticle

A general methodology is presented to model the opto-electronic properties of a dye molecule in the presence of a semiconductor nanoparticle (NP), a model system for the architecture of dye-sensitized solar cells. The method is applied to the L0 organic dye solvated with acetonitrile in the neighborhood of a TiO2 NP. The total reaction potential due to the polarization of the solvent and the metal oxide is calculated by extending the polarizable continuum model integral equation formalism. The ground state energy is computed by using density functional theory (DFT) while the vertical electronic excitations are obtained by time-dependent DFT in a state-specific corrected linear response scheme. We calculate the excited state oxidation potential (ESOP) for the protonated and deprotonated forms of the L0 dye at different distances and configurations with respect to the NP surface. The stronger renormalizations of the ESOP values due to the presence of the TiO2 nanostructure are found for the protonated dye, reaching a maximum of about -0.15 eV. The role of protonation effect is discussed in terms of the atomic Löwdin charges of the oxidized and reduced species. On the other hand, we observed a weak effect on the L0 optical excitation gap due to the polarization response of the NP.