A comprehensive theoretical study on the electronic absorption spectra of a representative group of organic dyes (L0, D4, D5, C217, and JK2) employed in dye-sensitized solar cell devices is reported. A benchmark evaluation on different time-dependent density functional theory (TDDFT) approaches with respect to highlevel correlated coupled cluster (CC) and multireference perturbation theory (MRPT) benchmark calculations is performed in the gas phase. The benchmark results indicate that TDDFT calculations using the hybrid MPW1K and the long-range correct CAM-B3LYP functionals represent a valuable tool of comparable accuracy to that of the much more computationally demanding ab initio methods. Thus, the problem of the comparison between the calculated excitation energies and the measured absorption maximum wavelengths has been addressed employing the MPW1K functional and including the solvation effects by a polarizable continuum model. The present results show that taking into account the chemical and physical phenomena occurring in solution (i.e., protonation/deprotonation of the carboxylic function and the explicit solute-solvent interactions) is of crucial importance for a meaningful comparison between the calculated and the experimental absorption spectra. Our investigation paves the way to the reliable computational design and predictive screening of organic dye sensitizers, even before their synthesis, in analogy to what has been achieved for transition-metal complexes.

A density functional theory (DFT), time-dependent DFT, and ab initio second order Møller-Plesset perturbation theory study of the aggregation of the metal free indoline D102 and D149 dyes on extended TiO(2) models is reported. By selecting the relevant dimeric arrangements on the TiO(2) surface and evaluating, at the same time, the associated spectroscopic response, an almost quantitative description of the extremely different aggregation behavior of the two dyes is provided. Nicely reproducing the experimental evidence, the present results predict strong aggregation interactions and a sizable red-shift of the absorption band in the case of D102, while negligible effects for D149. Our results open the possibility of computationally screening the various aggregation patterns and predicting the corresponding optical response, thus paving the way to an effective molecular engineering of further enhanced sensitizers for solar cell applications.

The n-electron valence state perturbation theory (NEVPT) is a form of multireference perturbation theory which is based on a zero order reference wavefunction of CAS-CI type (complete active space configuration interaction) and which is characterized by the utilization of correction functions (zero order wavefunctions external to the CAS) of multireference nature, obtained through the diagonalization of a suitable two-electron model Hamiltonian (Dyall's Hamiltonian) in some well defined determinant spaces. A review of the NEVPT approach is presented, starting from the original second order state-specific formulation, going through the quasidegenerate multi-state extension and arriving at the recent implementations of the third order in the energy and of the internally contracted configuration interaction. The chief properties of NEVPT - size consistence and absence of intruder states - are analyzed. Finally, an application concerning the calculation of the vertical spectrum of the biologically important free base porphin molecule, is presented

We report a thorough theoretical and computational investigation of the effect of dye adsorption on the TiO 2 conduction band energy in dye-sensitized solar cells that is aimed at assessing the origin of the shifts induced by surface adsorbed species in the position of the TiO 2 conduction band. We thus investigate a series of working dye sensitizers and prototypical surface adsorbers and apply an innovative approach to disentangle electrostatic and charge-transfer effects occurring at the crucial dye-TiO 2 interface. We clearly demonstrate that an extensive charge rearrangement accompanies the dye-TiO 2 interaction, which amounts to transfer of up to 0.3-0.4 electrons from the dyes bound in a dissociative mode to the semiconductor. Molecular monodentate adsorption leads to a much smaller CT. We also find that the amount of CT is modulated by the dye donor groups, with the coumarin dyes showing a stronger CT. A subtle modulation of the semiconductor conduction band edge energy is found by varying the nature of the dye, in line with the experimental data from the literature obtained by capacitance and open circuit voltage measurements. We then decompose the total conduction band shift into contributions directly related to the sensitizer properties, considering the effect of the electric field generated by the dye on the semiconductor conduction band. This effect, which amounts to ca. 40% of the total shift, shows a linear correlation with the TiO 2 conduction band shifts. A direct correlation between the dye dipole and the observed conduction band shift is retrieved only for dyes of similar structure and dimensions. We finally found a near-exact proportionality between the amount of charge transfer and the residual contribution to the conduction band shift, which may be as large as 60% of the total shift. The present findings constitute the basis for obtaining a deeper understanding of the crucial interactions taking place at the dye-semiconductor interface, and establish new design rules for dyes with improved DSC functionality. Broader context The effect of surface-adsorbed species on the TiO 2 conduction band energy is a highly debated issue in the eld of dye-sensitized solar cells. The possible modulation of the position of the TiO 2 conduction band appears to be a viable way to obtain higher cell open circuit voltages, and thus higher solar energy conversion efficiency. By applying rst principles computational modeling, we investigate a series of working dyes and co-adsorbers to disentangle electrostatic and charge-transfer effects occurring at the dye-TiO 2 interface. We clearly demonstrate that an extensive charge rearrangement accompanies the dye-TiO 2 interaction. A subtle modulation of the TiO 2 conduction band is found by varying the nature of the dye, in line with available experimental data. Such conduction band shis are decomposed into contributions directly related to the sensitizer properties. A linear correlation is found between the dye electrostatic potential and the conduction band shi, wh...

Theoretical and computational modeling is a powerful tool to investigate and characterize the structural, electronic, and optical properties of the main components of dye-sensitized solar cells (DSCs). In this article we focus on the description of the ground and excited state properties of both standalone and TiO 2 -adsorbed metallorganic and fully organic dyes, relevant to modeling the dye→semiconductor electron injection process, which is the primary charge generation step in DSCs. By reviewing previous data from our laboratory, integrated with new calculations, we wish to critically address the potential and limitations of current DFT and TDDFT computational methods to model DSCs. While ruthenium dyes are accurately described by standard DFT approaches, for highly conjugated organic dyes, characterized by strong charge transfer excited states, specifically tailored exchange-correlation functionals are needed. For ruthenium dye/ semiconductor interfaces, a strategy is presented, which accurately describes the electronic and optical properties and the alignment of ground and excited state levels at the same time, allowing us to discuss the coupling and the energetics of the excited states underlying the ultrafast electron injection. For donor−acceptor organic dyes, a simple picture based on the dye lowest unoccupied molecular orbital (LUMO) broadening accounts for the different interfacial electronic coupling exhibited by dyes with different anchoring groups. The explored DFT/TDDFT methods, however, are not capable to deliver at the same time a balanced description of the dye@TiO 2 excited states and of the alignment of the dye excited states with the semiconductor manifold of unoccupied states. This represents a challenge which should be addressed by next generation DFT or post-DFT methods.

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.

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.

Nanostructured dye-sensitized solar cells (DSSCs) are promising photovoltaic devices because of their low cost and transparency. Ruthenium polypyridine complexes have long been considered as lead sensitizers for DSSCs, allowing them to reach up to 11% conversion efficiency. However, ruthenium suffers from serious drawbacks potentially limiting its widespread applicability, mainly related to its potential toxicity and scarcity. This has motivated continuous research efforts to develop valuable alternatives from cheap earth-abundant metals, and among them, iron is particularly attractive. Making iron complexes applicable in DSSCs is highly challenging due to an ultrafast deactivation of the metal-ligand charge-transfer (MLCT) states into metal-centered (MC) states, leading to inefficient injection into TiO 2 . In this review, we present our latest developments in the field using Fe(II)-based photosensitizers bearing N-heterocyclic carbene (NHC) ligands, and their use in DSSCs. Special attention is paid to synthesis, photophysical, electrochemical, and computational characterization.