Density Functional Theory (DFT) Study of Coumarin-based Dyes Adsorbed on TiO2 Nanoclusters—Applications to Dye-Sensitized Solar Cells

Coumarin-based dyes have been successfully used in dye-sensitized solar cells, leading to photovoltaic conversion efficiencies of up to about 8%. Given the need to better understand the behavior of the dye adsorbed on the TiO2 nanoparticle, we report results of density functional theory (DFT) and time-dependent DFT (TD-DFT) studies of several coumarin-based dyes, as well as complex systems consisting of the dye bound to a TiO2 cluster. We provide the electronic structure and simulated UV-Vis spectra of the dyes alone and adsorbed to the cluster and discuss the matching with the solar spectrum. We display the energy level diagrams and the electron density of the key molecular orbitals and analyze the electron transfer from the dye to the oxide. Finally, we compare our theoretical results with the experimental data available and discuss the key issues that influence the device performance.

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abstract: Reference LPI-ARTICLE-2006-008doi:10.1002/adma.200502540View record in Web of Science Record created on 2006-05-03, modified on 2016-08-08

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title: Advancing beyond current generation dye-sensitized solar cells
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title: Coumarin dyes containing low-band-gap chromophores for dye-sensitised solar cells
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title: A coumarin-derivative dye sensitized nanocrystalline TiO2 solar cell having a high solar-energy conversion efficiency up to 5.6%
abstract: It is found that newly synthesized coumarin derivatives work 
as highly efficient photosensitizers for dye-sensitized nanocrystalline 
TiO2 solar cells producing a 5.6% solar-light-to-electricity 
conversion efficiency, the highest efficiency so far among organic 
dye-sensitized solar cells, with a short-circuit current density of 13.8 mA 
cm−2, an open-circuit photovoltage of 0.63 V, and a fill 
factor of 0.63 under standard AM 1.5 irradiation (100 mW 
cm−2).

title: Electron transport in coumarin-dye-sensitized nanocrystalline TiO2 electrodes.
abstract: We investigated electron transport kinetics in terms of electron diffusion coefficient (D) and electron lifetime (tau) in coumarin-dye-sensitized nanocrystalline TiO2 electrodes by intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS). We found that the values of tau for coumarin-dye-sensitized TiO2 electrodes were much shorter than that for an electrode coated with a Ru complex (N719 dye), suggesting that the back-electron-transfer process corresponding to recombination between conduction-band electrons in the TiO2 and I3- ions in the electrolyte occurs more easily in coumarin-dye-sensitized solar cells. In addition, the values of tau depended on the kind of coumarin dye, each of which has a different number of thiophene moieties, suggesting that the molecular structure of the adsorbed dyes also affects the kinetics of electron transport in the TiO2 electrodes.

title: A High‐Light‐Harvesting‐Efficiency Coumarin Dye for Stable Dye‐Sensitized Solar Cells
abstract: Dye-sensitized solar cells (DSSCs) have been studied extensively as potential alternatives to conventional inorganic solid solar cells, by using wide-bandgap nanocrystalline TiO2 sensitized with ruthenium polypyridine complexes or metal-free organic dyes as photoelectrodes. Through molecular design, ruthenium complexes have achieved power-conversion efficiencies of over 11 %, while metal-free organic dyes have reached ca. 9 % power-conversion efficiency under AM 1.5 (AM: air mass) simulated solar light of 100 mW cm (1 sun). Several ruthenium polypyridyl complexes have shown their ability to withstand thermal or light-soaking stress tests for at least 1000 h while retaining an efficiency above 7 %, whereas for organic-dye-based DSSCs the longterm stability, which is the critical requirement for practical applications, so far remains a serious problem. Organic dyes are also promising for applications in DSSCs in that they have much higher molar extinction coefficients than those for ruthenium polypyridine complexes, which are favorable for light-harvesting efficiency (LHE) and hence photocurrent generation. Among the organic dye sensitizers tested in DSSCs, coumarin dyes are strong candidates because of their good photoelectric conversion properties. However, one of their drawbacks is that a high concentration of 4-tert-butylpyridine (TBP) is usually required for a high power-conversion efficiency. Under continuous light soaking of 1 sun for a short period of one day, the photovoltaic performance was observed to drop dramatically because of the dissolution of the dye into electrolyte containing 0.5 M or more TBP. Therefore, it still remains a great challenge to acquire a DSSC based on a metal-free organic dye with high efficiency that is stable in the long term. In this paper, we report a new coumarin dye, 2-cyano-3-{5′-[1-cyano-2-(1,1,6,6tetramethyl-10-oxo-2,3,5,6-tetrahydro-1H,4H,10H-11-oxa-3aaza-benzo[de]anthracen-9-yl)-vinyl]-[2,2′]bithiophenyl-5-yl}acrylic acid (NKX-2883), shown in Figure 1, for use in DSSCs. These DSSCs exhibited LHE values of near unity, incident photon-to-electron conversion efficiency (IPCE) over a wide spectral region on transparent TiO2 films of only 6 lm thickness, and maintained ca. 6 % power-conversion efficiency under continuous light soaking of 1 sun at 50–55 °C for 1000 h. Figure 2a shows the UV-vis absorption spectrum for NKX2883 in an ethanol solution. NKX-2883 exhibited two p–p* electron-transition peaks (426 and 552 nm) in the visible region. Compared to NKX-2677 (2-cyano-3-[5’-(1,1,6,6-tetramethyl-10-oxo-2,3,5,6-tetrahydro-1H,4H,10H-11-oxa-3a-azabenzo[de]anthracen-9-yl)-[2,2’]bithiophenyl-5-yl]acrylic acid), one of the best organic dyes for DSSCs reported previously, the introduction of one more CN group into the molecular frame decreases the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), thus extending the maximum absorption from 511 to 552 nm. This red-shift may favor light harvesting and hence photocurrent generation in DSSCs, as will be discussed below. The 552 nm peak showed a broad feature with a full width at half-maximum absorbance of ca. 110 nm, comparable to that for ruthenium polypyridyl complexes, contributing broadly to the high LHE. The molar extinction coefficient (e) of NKX-2883 in ethanol was determined to be 9.74 × 10 dm mol cm at 552 nm, which is about seven times larger than that of N3 (cis-di(thiocyanate)-bis(2,2’-bipyridyl-4,4’-dicarboxylic acid); e= 1.42 × 10 dm mol cm at 532 nm), and 60 % larger than that for NKX-2677 (e= 6.43 × 10 dm mol cm at 511 nm). The LUMO (–0.69 V vs the normal hydrogen electrode (NHE)) of NKX-2883 is more negative than the conduction-band edge of TiO2 (–0.5 V vs NHE), [23] ensuring that electron injection from the excited dye to the conduction band of TiO2 is thermodynamically favorable. The dye-loaded films were obtained by dipping the TiO2 film in dye solutions with different concentrations: 0.02, 0.1, 0.3, and 1.0 mM. The normalized UV-vis absorption spectra for dye-loaded films are plotted in Figure 2b. It is evident that the spectrum becomes slightly broader with an increasing conC O M M U N IC A TI O N

title: Thiophene-Functionalized Coumarin Dye for Efficient Dye-Sensitized Solar Cells: Electron Lifetime Improved by Coadsorption of Deoxycholic Acid
abstract: This paper reports a new coumarin dye, 2-cyano-3-(5-{2-[5-(1,1,6,6-tetramethyl-10-oxo-2,3,5,6-tetrahydro-1H,4H,10H-11-oxa-3a-aza-benzo[de]anthracen -9-yl)-thiophen-2-yl]-vinyl}-thiophen-2-yl)-acrylic acid (NKX-2700), and its application in dye-sensitized solar cells (DSSCs). Under illumination of simulated AM1.5G solar light (100 mW cm-2) with an aperture black mask, 5.0% of power conversion efficiency [short-circuit photocurrent density (Jsc) = 12.0 mA cm-2, open-circuit photovoltage (Voc) = 0.59 V, and fill factor (FF) = 0.71] was obtained for NKX-2700 based DSSC, which was significantly improved to 8.2% (Jsc = 15.9 mA cm-2, Voc = 0.69 V, FF = 0.75) upon addition of 120 mM deoxycholic acid (DCA) to the dye solution for TiO2 sensitization. Coadsorption of DCA decreased dye coverage by ∼50% but significantly improved the Jsc by 33%. The breakup of π-stacked aggregates might improve electron injection yield and thus Jsc. Electrochemical impedance data indicate that the electron lifetime was improved by coa...

title: Molecular Designs and Syntheses of Organic Dyes for Dye-Sensitized Solar Cells
abstract: Dye-sensitized solar cells (DSSCs) based on organic dyes adsorbed on nanocrystalline TiO2 electrodes have received considerable attention because of their high incident solar light-to-electricity conversion efficiency and low cost of production. A number of organic dyes have so far been developed, and the relationship between their chemical structures and the photovoltaic performances of DSSCs based on the dyes has been examined. To create high-performance DSSCs, it is important to develop effective organic dye sensitizers, which should be optimized for the chemical structures to possess good light-harvesting features, to provide good electron communication between the dyes and a TiO2 electrode, and to control the molecular arrangements on the TiO2 electrode. The aim of this microreview is to highlight the design and synthesis of organic dyes for DSSCs based on recent work of organic chemists.

title: Molecular Design of Coumarin Dyes for Efficient Dye-Sensitized Solar Cells
abstract: We have developed novel coumarin dyes for use in dye-sensitized nanocrystalline TiO2 solar cells (DSSCs). The absorption spectra of these novel coumarin dyes are red-shifted remarkably in the visible region relative to the spectrum of C343, a conventional coumarin dye. Introduction of a methine unit (−CHCH−) connecting both the cyano (−CN) and carboxyl (−COOH) groups into the coumarin framework expanded the π conjugation in the dye and thus resulted in a wide absorption in the visible region. These novel dyes performed as efficient photosensitizers for DSSCs. The monochromatic incident photon-to-current conversion efficiency (IPCE) from 420 to 600 nm for a DSSC based on NKX-2311 was over 70% with the maximum of 80% at 470 nm, which is almost equal to the efficiency obtained with the N3 dye system. The IPCE performance of DSSCs based on coumarin dyes depended remarkably on the LUMO levels of the dyes, which are estimated from the oxidation potential and 0−0 energy of the dye. The slow charge recombination,...

title: Theoretical investigation of the excited states of coumarin dyes for dye-sensitized solar cells.
abstract: Using time-dependent density functional theory (TD-DFT), configuration interaction single (CIS) method, and approximate coupled cluster singles and doubles (CC2) method, we investigated the absorption spectra of coumarin derivative dyes (C343, NKX-2388, NKX-2311, NKX-2586, and NKX-2677), which have been synthesized for efficient dye-sensitized solar cells. The CC2 calculations are found in good agreement with the experimental results except for the smallest coumarin dye (C343). TD-DFT underestimates the vertical excitation energy of the larger coumarin dyes (NKX-2586 and -2677). Solvents (methanol) are found to induce a red shift of the vertical excitation energies, and their effects on the molecular geometry and the electronic structure are examined in detail. The deprotonated form of coumarin is also investigated, where a blue shift of the vertical excitation energies is observed.

title: A TD-DFT investigation of UV spectra of pyranoidic dyes: a NCM vs PCM comparison
abstract: We report the calculation of the UV spectra of more than 40 substituted coumarins and chromones in solution. In particular, two solvent models are compared, the multipolar expansion model (NCM) developed in Nancy and the PCM scheme developed in Pisa and Napoli. All calculations are carried out at the NCM-TDPBE0/6-311G(2d,2p)//NCM-B3LYP/6-311G(2d,2p) and at the PCM-TDDFT/6-311+G(2d,2p)//PCM-DFT/6-311G(2d,2p) levels of theory. For most of the studied coumarin and chromone derivatives, there are at least two allowed excited states corresponding to transitions presenting a strong oscillator strength in the UV region. For the NCM treatment of substituted pyranones, the required accuracy for the design of molecules has been reached (we found a accuracy on the predictive data of 7 nm/0.08 eV for λ max ( 1 ) and 9 nm/0.18 eV for λ max ( 2 ) ). Compared to PCM values, the NCM formalism provides λ max ( 1 ) predictions with equivalent accuracy and reliability. For λ max ( 2 ) , the importance of diffuse functions is clearly demonstrated.

title: Molecular design of coumarin dyes with high efficiency in dye-sensitized solar cells
abstract: Density functional theory (DFT) and time-dependent DFT calculation have been applied to study the coumarin dyes which act as photosensitizer in dye-sensitized solar cells. The absorption spectra of coumarin dyes in ethanol can be well reproduced with theoretical method. The outstanding performance of NKX-2311 roots in higher light harvesting efficiency and more efficient electron injection efficiency into conduction band of TiO 2 electrode according to our computation. Absorption peaks and HOMO energy levels of dyes can be employed as the two effective theoretical parameters for the judgement of their solar to energy efficiency. New coumarin dyes which probably possess higher efficiency were designed referring to our conclusions.

title: Coumarin derivatives for dye sensitized solar cells: a TD-DFT study.
abstract: Time dependent density functional theory (TD-DFT) calculations have been carried out to study the electronic structure and the optical properties of five coumarin based dyes: C343, NKX-2311, NKX-2586, NKX-2753 and NKX-2593. We have found out that the position and width of the first band in the electronic absorption spectra, the absorption threshold and the LUMO energy with respect to the conduction band edge are key parameters in order to establish some criteria that allow evaluating the efficiency of coumarin derivatives as sensitizers in Dye Sensitized Solar Cells (DSSC). Those criteria predict the efficiency ordering for the coumarin series in good agreement with the experimental evidence. Presumably, they might be used in the design of new efficient organic based DSSC.

title: Computational study of titanium (IV) complexes with organic chromophores
abstract: A computational study of small titanium complexes with the chromophores catechol, alizarin, and coumarin 343 is presented. Employing density functional theory (DFT), the ground-state geometries, energies, and harmonic frequencies of the different compounds are calculated. Furthermore, time-dependent DFT and the configuration interaction singles (CIS) method are used to determine excitation energies and excited-state gradients. Based on these results, the character of the excited states as well as electronic-vibrational coupling strengths are analyzed, and the implications for electron-transfer reactions at dye–semiconductor interfaces are discussed.

title: THE INHOMOGENEOUS ELECTRON GAS.
abstract: This paper deals with the ground state of an interacting electron gas in an external potential $v(\mathrm{r})$. It is proved that there exists a universal functional of the density, $F[n(\mathrm{r})]$, independent of $v(\mathrm{r})$, such that the expression $E\ensuremath{\equiv}\ensuremath{\int}v(\mathrm{r})n(\mathrm{r})d\mathrm{r}+F[n(\mathrm{r})]$ has as its minimum value the correct ground-state energy associated with $v(\mathrm{r})$. The functional $F[n(\mathrm{r})]$ is then discussed for two situations: (1) $n(\mathrm{r})={n}_{0}+\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{n}(\mathrm{r})$, $\frac{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{n}}{{n}_{0}}\ensuremath{\ll}1$, and (2) $n(\mathrm{r})=\ensuremath{\phi}(\frac{\mathrm{r}}{{r}_{0}})$ with $\ensuremath{\phi}$ arbitrary and ${r}_{0}\ensuremath{\rightarrow}\ensuremath{\infty}$. In both cases $F$ can be expressed entirely in terms of the correlation energy and linear and higher order electronic polarizabilities of a uniform electron gas. This approach also sheds some light on generalized Thomas-Fermi methods and their limitations. Some new extensions of these methods are presented.

title: Self-Consistent Equations Including Exchange and Correlation Effects
abstract: From a theory of Hohenberg and Kohn, approximation methods for treating an inhomogeneous system of interacting electrons are developed. These methods are exact for systems of slowly varying or high density. For the ground state, they lead to self-consistent equations analogous to the Hartree and Hartree-Fock equations, respectively. In these equations the exchange and correlation portions of the chemical potential of a uniform electron gas appear as additional effective potentials. (The exchange portion of our effective potential differs from that due to Slater by a factor of $\frac{2}{3}$.) Electronic systems at finite temperatures and in magnetic fields are also treated by similar methods. An appendix deals with a further correction for systems with short-wavelength density oscillations.

title: Density-functional exchange-energy approximation with correct asymptotic behavior.
abstract: Current gradient-corrected density-functional approximations for the exchange energies of atomic and molecular systems fail to reproduce the correct 1/r asymptotic behavior of the exchange-energy density. Here we report a gradient-corrected exchange-energy functional with the proper asymptotic limit. Our functional, containing only one parameter, fits the exact Hartree-Fock exchange energies of a wide variety of atomic systems with remarkable accuracy, surpassing the performance of previous functionals containing two parameters or more.

title: Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density.
abstract: A correlation-energy formula due to Colle and Salvetti [ Acta 37, 329 (1975)], in which the correlation energy density is expressed in terms of the electron density and a Laplacian of the second-order Hartree-Fock density matrix, is restated as a formula involving the density and local kinetic-energy density. On insertion of gradient expansions for the local kinetic-energy density, density-functional formulas for the correlation energy and correlation potential are then obtained. Through numerical calculations on a number of atoms, positive ions, and molecules, of both open- and closed-shell type, it is demonstrated that these formulas, like the original Colle-Salvetti formulas, give correlation energies within a few percent.

title: Ab initio effective core potentials for molecular calculations. Potentials for K to Au including the outermost core orbitals
abstract: Ab initio effective core potentials (ECP’s) have been generated to replace the innermost core electron for third‐row (K–Au), fourth‐row (Rb–Ag), and fifth‐row (Cs–Au) atoms. The outermost core orbitals—corresponding to the ns2np6 configuration for the three rows here—are not replaced by the ECP but are treated on an equal footing with the nd, (n+1)s and (n+1)p valence orbitals. These ECP’s have been derived for use in molecular calculations where these outer core orbitals need to be treated explicitly rather than to be replaced by an ECP. The ECP’s for the forth and fifth rows also incorporate the mass–velocity and Darwin relativistic effects into the potentials. Analytic fits to the potentials are presented for use in multicenter integral evaluation. Gaussian orbital valence basis sets are developed for the (3s, 3p, 3d, 4s, 4p), (4s, 4p, 4d, 5s, 5p), and (5s, 5p, 5d, 6s, 6p) ortibals of the three respective rows.

title: Density-functional thermochemistry. III. The role of exact exchange
abstract: Despite the remarkable thermochemical accuracy of Kohn–Sham density‐functional theories with gradient corrections for exchange‐correlation [see, for example, A. D. Becke,  (1992)], we believe that further improvements are unlikely unless exact‐exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange‐correlation functional containing local‐spin‐density, gradient, and exact‐exchange terms is tested on 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total atomic energies of first‐ and second‐row systems. This functional performs significantly better than previous functionals with gradient corrections only, and fits experimental atomization energies with an impressively small average absolute deviation of 2.4 kcal/mol.

title: Optimization of Gaussian-type basis sets for local spin density functional calculations. Part I. Boron through neon, optimization technique and validation
abstract: Gaussian-type orbital and auxiliary basis sets have been optimized for local spin density functional calculations. This first paper deals with the atoms boron through neon. Subsequent papers will provide a list through xenon. The basis sets have been tested for their ability to give equilibrium geometries, bond dissociation energies, hydrogenation energies, and dipole moments. These results indicate that the present optimization technique yields reliable basis sets for molecular calculations.  Gaussian basis sets, density functional theory, boron–neon, geometries, energies of reactions.

title: Molecular excitation energies to high-lying bound states from time-dependent density-functional response theory: Characterization and correction of the time-dependent local density approximation ionization threshold
abstract: This paper presents an evaluation of the performance of time-dependent density-functional response theory (TD-DFRT) for the calculation of high-lying bound electronic excitation energies of molecules. TD-DFRT excitation energies are reported for a large number of states for each of four molecules: N2, CO, CH2O, and C2H4. In contrast to the good results obtained for low-lying states within the time-dependent local density approximation (TDLDA), there is a marked deterioration of the results for high-lying bound states. This is manifested as a collapse of the states above the TDLDA ionization threshold, which is at ??HOMOLDA (the negative of the highest occupied molecular orbital energy in the LDA). The ??HOMOLDA is much lower than the true ionization potential because the LDA exchange-correlation potential has the wrong asymptotic behavior. For this reason, the excitation energies were also calculated using the asymptotically correct potential of van Leeuwen and Baerends (LB94) in the self-consistent field step. This was found to correct the collapse of the high-lying states that was observed with the LDA. Nevertheless, further improvement of the functional is desirable. For low-lying states the asymptotic behavior of the exchange-correlation potential is not critical and the LDA potential does remarkably well. We propose criteria delineating for which states the TDLDA can be expected to be used without serious impact from the incorrect asymptotic behavior of the LDA potential

title: Quantum Calculation of Molecular Energies and Energy Gradients in Solution by a Conductor Solvent Model
abstract: A new implementation of the conductor-like screening solvation model (COSMO) in the GAUSSIAN94 package is presented. It allows Hartree−Fock (HF), density functional (DF) and post-HF energy, and HF and DF gradient calculations:  the cavities are modeled on the molecular shape, using recently optimized parameters, and both electrostatic and nonelectrostatic contributions to energies and gradients are considered. The calculated solvation energies for 19 neutral molecules in water are found in very good agreement with experimental data; the solvent-induced geometry relaxation is studied for some closed and open shell molecules, at HF and DF levels. The computational times are very satisfying:  the self-consistent energy evaluation needs a time 15−30% longer than the corresponding procedure in vacuo, whereas the calculation of energy gradients is only 25% longer than in vacuo for medium size molecules.

title: Quantum mechanical continuum solvation models.
abstract: 6.2.2. Definition of Effective Properties 3064 6.3. Response Properties to Magnetic Fields 3066 6.3.1. Nuclear Shielding 3066 6.3.2.  Coupling 3067 6.3.3. EPR Parameters 3068 6.4. Properties of Chiral Systems 3069 6.4.1. Electronic Circular Dichroism (ECD) 3069 6.4.2. Optical Rotation (OR) 3069 6.4.3. VCD and VROA 3070 7. Continuum and Discrete Models 3071 7.1. Continuum Methods within MD and MC Simulations 3072

title: Effect of deprotonation on absorption and emission spectra of Ru(II)-bpy complexes functionalized with carboxyl groups.
abstract: Changes in the ground and excited state electronic structure of the [Ru(bpy)(3)](2+) (bpy = 2,2'-bipyridine) complex induced by functionalization of bpy ligands with carboxyl and methyl groups in their protonated and deprotonated forms are studied experimentally using absorption and emission spectroscopy and theoretically using density functional theory (DFT) and time dependent DFT (TDDFT). The introduction of the carboxyl groups shifts the metal-to-ligand-charge-transfer (MLCT) absorption and emission bands to lower energies in functionalized complexes. Our calculations show that this red-shift is due to the stabilization of the lowest unoccupied orbitals localized on the substituted ligands, while the energies of the highest occupied orbitals localized on the Ru-center are not significantly affected. Consistent with previously observed trends in optical spectra of related Ru(II) complexes, deprotonation of the carboxyl groups results in a blue shift in the absorption and phosphorescence spectra. The effect originates from interplay of positive and negative solvatochromism in the protonated and deprotonated complexes, respectively. This results in more delocalized character of the electron transition orbitals in the deprotonated species and a strong destabilization of the three lowest unoccupied orbitals localized on the substituted and unsubstituted ligands, all of which contribute to the lowest-energy optical transitions. We also found that owing to the complexity of the excited state potential energy surfaces, the calculated lowest triplet excited state can be either weakly optically allowed (3)MLCT or optically forbidden Ru (3)d-d transition depending on the initial wavefunction guess used in TDDFT calculations.

title: The sun's total and spectral irradiance for solar energy applications and solar radiation models
abstract: Using the most recent composite time series of total solar irradiance spaceborne measurements, a solar constant value of 1366.1 W m−2 is confirmed, and simple quadratic expressions are proposed to predict its daily value from the Zurich sunspot number, the MgII index, or the 10.7 cm radio flux index. Whenever these three indices are available on a daily basis (since 1978), it is possible to predict the sun’s irradiance within 0.1% on average, as accurately as current measurements. Based on this value of the solar constant, an improved approximation of the extraterrestrial solar spectrum from 0 to 1000 μm is proposed. It is obtained by dividing the spectrum into nine bands and selecting representative (and recent) spectra, as well as appropriate scaling coefficients for each band. Comparisons with frequently used spectra are discussed, confirming previous findings of the literature. This synthetic and composite spectrum is proposed at 0.5-nm intervals in the UV (280–400 nm), 1-nm intervals between 0–280 and 400–1705 nm, 5-nm intervals between 1705 and 4000 nm, and progressively larger intervals beyond 4 μm, for a total of 2460 wavelengths.

title: Calculation of the photocurrent-potential characteristic for regenerative, sensitized semiconductor electrodes
abstract: The steady state anodic photocurrent for sensitized semiconductor electrodes, where the sensitizer is regenerated by a redox electrolyte, has been modeled taking into account the rate of light absorption by the sensitizer S, the rate of electron injection from the excited state S∗ of the sensitizer to the conduction band of the semiconductor, the rate of decay of S∗ (radiative and non-radiative) and the rate of reductive regeneration of the sensitizer by the redox electrolyte. In this model the rate of recombination between the conduction band electron and the oxidized sensitizer, S+, and the reactions between S∗ and the redox couple have been assumed to be negligible. The rate constant for injection, kinj, the injection efficiency, φinj, the photocurrent density, JP, and the steady state concentrations of S∗ and S+, have been calculated as a function of light intensity, Helmholtz potential, λmax and halfwidth, ΔE0.5, of the sensitizer absorption spectrum, and the semiconductor band gap and electron affinity both for monochromatic light and for AM1.5 sunlight simulated by radiation from a 5600K black body. For the calculation of kinj as a function of Helmholtz potential, the Gurney -Gerischer-Marcus (GGM) model has been used. Allowance for the distribution of electrode potential between the semiconductor and the electrolyte has been introduced in principle. The steady state concentrations of S∗ and S+ were used in the Nernst equation to calculate the S ∗ S + quasi-reversible potential. It is shown that the short-circuit current density of the cell is a maximum for a sensitizer with a λmax of about 1550 nm. Inter-relationships between variables involving the sensitizer have been used to show that only four sensitizer parameters are needed when considering the effects of changing the sensitizer. These parameters are the reorganization energies and the standard reduction potentials for the couples S + S ∗ and S + S . For a related series of sensitizers, such as obtained by changing a substituent group, only the two standard reduction potentials are required to predict the effects of changing the sensitizer.

title: Ultrafast electron injection from metal polypyridyl complexes to metal-oxide nanocrystalline thin films
abstract: Interfacial electron transfer (ET) between molecular adsorbates and semiconductor nanoparticles has been a subject of intense recent interest. In this paper, we review our recent work in understanding ultrafast photoinduced electron injection from ruthenium and rhenium polypyridyl complexes to metal oxide nanoparticles. Electron injection rates were measured using femtosecond IR spectroscopy, which provided a direct probe of adsorbate vibrational spectra and IR absorption of injected electrons in semiconductor. The consequence of competition between ultrafast electron injection and intramolecular relaxation in these transition metal complexes was carefully examined. A two-state injection model was proposed to account for biphasic kinetics, allowing a comparison of injection rate in different systems. The components of the chromophore(donor)–bridge–nanoparticle (acceptor) complexes were systematically varied to examine their effect on ET rate. The observed trends were discussed by considering the change in the strength of electronic coupling and density of accepting states in semiconductor and compared with Marcus theory of interfacial electron transfer.

title: Parameters influencing the efficiency of electron injection in dye-sensitized solar cells.
abstract: In this paper we focus upon the electron injection dynamics in complete nanocrystalline titanium dioxide dye-sensitized solar cells (DSSCs) employing the ruthenium bipyridyl sensitizer dye N719. Electron injection dynamics and quantum yields are studied by time-resolved single photon counting, and the results are correlated with device performance. In typical DSSC devices, electron injection kinetics were found to proceed from the N719 triplet state with a half-time of 200 +/- 60 ps and quantum yield of 84 +/- 5%. We find that these injection dynamics are independent of presence of iodide/triiodide redox couple and of the pH of the peptization step used in the synthesis of the TiO(2) nanoparticles. They are furthermore found to be only weakly dependent upon the application of electrical bias to the device. In contrast, we find these dynamics to be strongly dependent upon the concentration of tert-butylpyridine (tBP) and lithium cations in the electrolyte. This dependence is correlated with shifts of the TiO(2) conduction band energetics as a function of tBP and Li(+) concentration, from which we conclude that a 100 meV shift in band edge results in an approximately 2-fold retardation of injection dynamics. We find that the electron injection quantum yield determined from these transient emission data as a function of tBP and Li(+) concentration shows a linear correlation with device short circuit density J(sc). We thus conclude that the relative energetics of the dye excited state versus the titanium dioxide acceptor state is a key determinant of the dynamics of electron injection in DSSC, and that variations in these energetics, and therefore in the kinetics and efficiency of electron injection, impact directly upon device photovoltaic efficiency. Finally, we discuss these results in terms of singlet versus triplet electron injection pathways and the concept of minimization of kinetic redundancy.

title: Effect of Tetrahydroquinoline Dyes Structure on the Performance of Organic Dye-Sensitized Solar Cells
abstract: Eleven novel donor acceptor π-conjugated (D-π-A) organic dyes have been engineered and synthesized as sensitizers for the application in dye-sensitized solar cells (DSSCs). The electron-donating moieties are substituted tetrahydroquinoline, and the electron-withdrawing parts are cyanoacrylic acid group or cyanovinylphosphonic acid group. Different lengths of thiophene-containing conjugation moieties (thienyl, thienylvinyl, and dithieno[3,2-b;2‘,3‘-d]thienyl) are introduced to the molecules and serve as electron spacers. Detailed investigation on the relationship between the dye structure, photophysical and photoelectrochemical properties, and performance of DSSCs is described here. The bathochromic shift and increase of the molar extinction coefficient of the absorption spectrum are achieved by introduction of larger conjugation moiety. Even small structural changes of dyes result in significant changes in redox energies and adsorption manner of the dyes on TiO2 surface, affecting dramatically the perform...

title: Role of energy level alignment in solar cells sensitized with a metal‐free organic dye: A combined experimental and theoretical approach
abstract: We report results of combined experimental and theoretical studies of dye-sensitized solar cells (DSSCs) using 5-(4-sulfophenylazo)salicylic acid disodium salt, known as Mordant Yellow 10 (MY-10), as TiO2 sensitizer. We focus on a single dye but vary the solvent and the pH of the solution as well as the photoelectrode preparation conditions to determine the conditions for best photovoltaic conversion efficiency. We found experimentally that the efficiency, measured under standard air mass 1.5 global (AM 1.5G) conditions, was higher in solutions of ethanol than of water, but still small (up to 0.174%), although the fill factor (FF) was large (up to 0.73). Of the dyes in ethanol, MY-10 in alkaline solution showed the best matching of the solar spectrum but displayed the lowest efficiency. Density functional theory (DFT) calculations provided the optimized geometry, electronic structure, and electronic spectrum of the dye in fully protonated as well as partially and totally deprotonated forms, in solution. The calculated optical spectra are consistent with the experimental data, with strong absorption in the visible range only for the alkaline dye solution. The low device efficiency is very likely related to the weak optical absorption in the visible range. The much higher photovoltaic conversion efficiency of the DSSCs fabricated using acid or roughly neutral pH solutions, corresponding to the protonated and partially deprotonated forms of MY-10, respectively, is likely caused by the better alignment of the ground state of the dye with the redox level of the electrolyte. The decrease with pH of the dye solution of the short-circuit current was linked to a weaker charge injection from the excited state of the dye to the conduction band of the oxide, which is correlated with the shifting of the excited state of the dye deeper into the CB edge of the semiconductor. The variation of the open-circuit voltage with the pH of the solution was linked to the adjustment of the conduction band edge of TiO2, depending on the number of protons transferred from the dye to the oxide surface. Based on our results, we analyze the relative importance of the main criteria that should be met by a dye to be used in a DSSC.

title: DFT Study of Binding and Electron Transfer from a Metal-Free Dye with Carboxyl, Hydroxyl, and Sulfonic Anchors to a Titanium Dioxide Nanocluster
abstract: We report results of density functional theory (DFT) calculations of a metal-free dye, 5-(4-sulfophenylazo)salicylic acid disodium salt, known as Mordant Yellow 10 (MY-10), used as sensitizer for TiO2 dye-sensitized solar cells (DSSCs). Given the need to better understand the behavior of the dyes adsorbed on the TiO2 nanoparticle, we studied various single and double deprotonated forms of the dye bound to a TiO2 cluster, taking advantage of the presence of the carboxyl, hydroxyl, and sulfonic groups as possible anchors. We discuss various binding configurations to the TiO2 substrate and the charge transfer from the pigment to the oxide by means of DFT calculations. In agreement with other reports, we find that the carboxyl group tends to bind in bidentate bridging configurations. The salicylate uses both the carboxyl and hydroxyl substituent groups for either a tridentate binding to adjacent Ti(IV) ions or a bidentate Ti-O binding together with an O-H-O binding, due to the rotation of the carboxyl group out of the plane of the dye. The sulfonic group prefers a tridentate binding. We analyze the propensity for electron transfer of the various dyes and find that for MY-10, as a function of the anchor group, the DSSC performance decreases in the order hydroxyl

title: Crystal growth: Anatase shows its reactive side.
abstract: Fluorine-containing species can cause titania to crystallize with an unusually large fraction of reactive {001} facets.

title: Stress-driven reconstruction of an oxide surface: the anatase TiO(2)(001)-(1 x 4) surface.
abstract: The recently discovered (1 x 4) reconstruction of the stoichiometric anatase TiO(2)(001) surface is studied via first principles density functional calculations. We propose a new structural model for this reconstruction, which is energetically much more favorable than all the other available models, including the unreconstructed (1 x 1) surface. In this model, rows of TiO(3) species periodically replace rows of surface bridging oxygens of the (1 x 1) surface, thus leading to a relief of the large surface tensile stress present on that surface. The corresponding empty-state charge density distribution correctly describes the experimental scanning tunneling microscopy images.

title: First-Principles Modeling of the Adsorption Geometry and Electronic Structure of Ru(II) Dyes on Extended TiO2 Substrates for Dye-Sensitized Solar Cell Applications
abstract: We report a systematic density functional theory (DFT) computational investigation of Ru(II) sensitizer/TiO2 systems relevant to dye-sensitized solar cells (DSSCs). Focusing on the prototypical N719 and the recently introduced YE05 sensitizers, and considering large slab and cluster models for TiO2, we have systematically studied the influence of the molecular adsorption geometry, counterions, and surface protonation on the electronic structure of the dye/semiconductor systems by means of Car−Parrinello molecular dynamics combined with single-point hybrid functional calculations of the electronic properties. Our results show that the homoleptic N719 and YE05 dyes, both bearing two bipyridine ligands functionalized with four carboxylic groups, adsorb onto the TiO2 surface by exploiting three carboxylic groups. The bulky TBA counterions employed in N719 cause a modest energy down-shift of the TiO2 conduction band, whereas the smaller Na+ counterions, which can access the surface more closely, lead to a larg...

title: Computational modelling of TiO2 surfaces sensitized by organic dyes with different anchoring groups: adsorption modes, electronic structure and implication for electron injection/recombination.
abstract: We present a Density Functional Theory investigation aimed to model the possible adsorption modes to the TiO(2) surface of two representative TPA-based dyes, termed L0 and rh-L0, having the two mostly employed anchoring groups, namely the cyanoacrylic and rhodanine-3-acetic acids respectively. The bidentate coordination with proton transfer to a nearby surface oxygen is found to be the energetically favored anchoring mode for both dyes. The calculations show that the different dye anchoring groups give rise to a very different electronic coupling between the dye and the manifold of unoccupied semiconductor states, thus implying different electron injection mechanisms. The strongly coupled L0 dye possibly shows an adiabatic electron injection mechanism, while a non-adiabatic electron injection can be foreseen for the weakly coupled rh-L0 dye. The different orientation with respect to the TiO(2) surface for the two classes of dyes, implying different distances of the donor group from the oxide surface, together with the different electron injection mechanisms might account for the faster recombination reaction measured for the rhodanine-based dyes.

title: Formic Acid Adsorption on Dry and Hydrated TiO2 Anatase (101) Surfaces by DFT Calculations
abstract: The adsorption of formic acid and sodium formate on the stoichiometric anatase (101) surface has been studied by means of density functional calculations with a slab geometry. On the clean surface, the most stable adsorption structure for HCOOH is a molecular monodentate configuration, hydrogen bonded to a surface bridging oxygen, while for HCOONa a dissociated bridging bidentate geometry is preferred. The bidentate chelating structure is energetically unstable for both the acid and the salt. On the hydrated surface, both HCOOH and HCOONa preferentially form an inner-sphere adsorption complex. HCOOH maintains a monodentate coordination, but, due to the interaction with a nearby water molecule, it becomes dissociated, while HCOONa again prefers a bridging bidentate structure. The energies for adsorption from an aqueous solution are estimated to be 0.30 and 0.79 eV for HCOOH and HCOONa, respectively.

title: A Combined Experimental and Computational Investigation of Anthracene Based Sensitizers for DSSC: Comparison of Cyanoacrylic and Malonic Acid Electron Withdrawing Groups Binding onto the TiO2 Anatase (101) Surface
abstract: Four anthracene based sensitizers, 3-(anthracene-9-yl)-2-cyanoacrylic acid (M1), 2-cyano-3-(10-methoxyanthracene-9-yl)acrylic acid (M2), 2-(anthracene-9-ylmethylene) malonic acid (M3), and 2-((10-methoxyanthracene-9-yl)methylene)malonic acid (M4) were designed and synthesized to understand the binding modes of anchoring groups ( and ) on the nanocrystalline TiO2(101) surface and on the efficiency of dye-sensitized solar cells (DSSCs). All four sensitizers have been fully characterized using ATR-FTIR, UV−vis, and CV. These sensitizers were tested in DSSCs using 0.05 M I2, 0.5 M 1,2-dimethyl-3-n-propylimidazolium iodide (DMPI), and 0.5 M lithium iodide (LiI) in methoxypropionitrile (MPN) redox electrolyte. The sensitizers having a monocarboxylic acid group, i.e., M1 and M2, have shown marginally higher IPCE and efficiency than M3 and M4 having dicarboxylic acid groups. To have a detailed understanding of this behavior, the adsorption and binding energies to the TiO2 surface of these dyes have been investiga...

title: Characterization of the adsorption of Ru-bpy dyes on mesoporous TiO2 films with UV-Vis, Raman, and FTIR spectroscopies.
abstract: In the present work the adsorption of a new dye, [Ru(dcbpyH(2))(2)(bpy-TPA(2))](PF(6))(2), and the well-known (Bu(4)N)(2)[Ru(dcbpyH)(2)(NCS)(2)] complex on mesoporous anatase films were investigated to clarify the role of the carboxylate groups in the anchoring process of the dyes on the semiconductor surface. For this purpose UV-vis, Raman, resonance Raman, and ATR-FTIR spectroscopies have been used. The results of the Raman experiments at different excitation wavelengths demonstrate that photoinduced charge-transfer processes take place efficiently between the adsorbate and the substrate. Moreover, this is the first time that the Raman spectrum of a Ru-bpy dye (in this case, the dye N719) adsorbed on TiO(2) has been obtained without the resonance condition, only by means of SERS enhancement. The coordination of both complexes on the TiO(2) paste films is proposed to occur via bidentate or bridging linkage.

title: High-Throughput Computational Screening of Chromophores for Dye-Sensitized Solar Cells
abstract: Electron injection from a photoexcited chromophore into semiconductor (TiO2) nanoparticles is one of the key electron transfer processes in dye-sensitized solar cells. We describe our model for calculations of electron injection times, which is based on partitioning the semiconductor–chromophore system into fragments (TiO2 slab with adsorbed chromophore’s anchoring group, and an isolated chromophore), and calculating the imaginary part of the self-energy from the electronic properties of the fragments: density of states of TiO2 slab, TiO2–anchoring group coupling, and chromophore’s wave function coefficients. The electronic properties of the semiconductor and its interface with the chromophore’s anchoring group are reused for all chromophores with the same anchoring group (carboxylic acid in this study), and only a calculation of the isolated chromophore’s lowest unoccupied molecular orbital is required for each chromophore. We use this model to calculate electron injection times for a large set of organi...

title: Heterocyclic azodyes as pigments for dye sensitized solar cells - A combined experimental and theoretical study
abstract: We report the results of a combined experimental and theoretical study of four azodyes, derived from three different heterocyclic systems, which we used as sensitizers for TiO2 dye-sensitized solar cells. Although the efficiencies are relatively low, of up to 0.19%, the fill factors are competitive, with values of up to 0.74. To better understand the large differences in the overall photovoltaic conversion efficiency between the four dyes we performed DFT calculations to determine the energy levels of the ground and excited states of the pigments. We explain the experimental results based on the energy level alignment with respect to the TiO2 substrate and the electrolyte.