Mariachiara Pastore

Aggregation of Organic Dyes on TiO2 in Dye-Sensitized Solar Cells Models: An ab Initio Investigation

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.

Influence of the dye molecular structure on the TiO2 conduction band in dye-sensitized solar cells: disentangling charge transfer and electrostatic effects

We report a thorough theoretical and computational investigation of the effect of dye adsorption on the TiO2 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 TiO2 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–TiO2 interface. We clearly demonstrate that an extensive charge rearrangement accompanies the dye–TiO2 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 TiO2 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.

Energy levels, charge injection, charge recombination and dye regeneration dynamics for donor–acceptor π-conjugated organic dyes in mesoscopic TiO2 sensitized solar cells

Two new D–π–A type organic sensitizers, MP124 and MP-I-50, were synthesized and their electrochemical and spectroscopic properties studied. Efficiencies of DSSC devices utilizing these dyes were also investigated, where sensitization solvent, sensitization time and additive concentration were all varied. Under standard AM 1.5G simulated solar radiation, optimized MP124 devices show an efficiency of 7.45% (Voc = 0.73 V; Jsc = 14.44 mA cm−2; FF = 70%) while optimized MP-I-50 devices show an efficiency of 5.66% (Voc = 0.68 V; Jsc = 12.06 mA cm−2; FF = 69%). Transient absorption spectroscopy studies show that regeneration of dye cations by the red-ox electrolyte was more efficient in MP124cells which is attributed to its higher HOMO energy leading to greater driving force for the regeneration reaction. Transient photovoltage studies showed that electron lifetimes were longer lived in MP124 explaining the higher Voc for these cells compared to MP-I-50cells. DFT and MP2 calculations indicate that this is due to the greater tendency of MP-I-50 to form charge-transfer complexes with the I2 species in the electrolyte, due to the presence of an additional EDOT in its structure compared to MP124. This work highlights the effect that small changes to the sensitizer structure can have on the interfacial charge transfer reactions and ultimately on the device efficiency.

Joint electrical, photophysical and computational studies on D-π-A dye sensitized solar cells: the impacts of dithiophene rigidification

The rigidification of π-conjugated segments represents a feasible tactic towards energy-level engineering of organic D-π-A dyes in mesoscopic titania solar cells. In this work, comparions of four dyes with the di(3-hexylthiophene), dihexyldithienosilole, dihexylcyclopentadithiophene and N-hexyldithienopyrrole linkers have revealed some general influences of π-linker rigidification on the optoelectronic features of titania solar cells employing a cobalt(II/III) redox electrolyte, in terms of energetic and kinetic viewpoints. Compared to a dye with the di(3-hexylthiophene) linker, its three counterparts with rigidified dithiophene blocks present bathochromic and hyperchromic absorptions of solar photons. Transient absorption measurements have shown that the incorporation of Si-, C- and N-bridged dithiophene segments decelerates the dye regeneration kinetics. The rigidification of π-conjugated dithiophene linkers brings forth a general open-circuit photovoltage diminishment, in the range from 60 to 190 mV. Further insightful impedance analyses have disclosed that the open-circuit photovoltage reduction, due to the π-linker alternation from di(3-hexylthiophene) to N-hexyldithienopyrrole, is predominantly caused by an adverse downward displacement of the titania conduction band edge, despite a positive contribution from attenuated charge recombination at the titania/electrolyte interface.

Novel Carbazole-Phenothiazine Dyads for Dye-Sensitized Solar Cells: A Combined Experimental and Theoretical Study

We report a joint experimental and computational work on new organic donor-acceptor dye sensitizers in which a carbazole (CZ) and a phenothiazine (PTZ) units are linked together by an alkyl C6H13, while two different anchoring groups are employed: the cyanoacrylic acid (CS1A, CSORG1) and the rhodanine-3-acetic acid (CS4A, CSORG4). The CZ moiety has multiple roles of (i) acting as an extra-electron donor portion, providing more electron density on the PTZ; (ii) suppressing the back-electron transfer from TiO2 to the electrolyte by forming a compact insulating dye layer; (iii) modulating dye aggregation on the semiconductor surface; and (iv) acting as an antenna, collecting photons and, through long-range energy transfer, redirecting the captured energy to the dye sensitizer. We show that the introduction of the CZ donor remarkably enhances the photovoltaic performances of the rhodanine-based dye, compared to the corresponding simple PTZ dye, with more than a two-fold increase in the overall efficiencies, while it does not bring beneficial effects in the case of the cyanoacrylic-based sensitizer. Based on quantum mechanical calculations and experimental measurements, we show that, in addition to a favored long-range energy transfer, which increases the light absorption in the blue region of the spectrum, the presence of the CZ unit in the CSORG4 dye effectively induces a beneficial aggregation pattern on the semiconductor surface, yielding a broadened and red-shifted light absorption, accounting for the two-fold increase in the generated photocurrent.

Intermolecular Interactions in Dye-Sensitized Solar Cells: A Computational Modeling Perspective

We present a unified overview of our recent activity on the modeling of relevant intermolecular interactions occurring in dye-sensitized solar cells (DSCs). The DSC is an inherent complex system, whose efficiency is essentially determined by the interrelated phenomena occurring at the multiple molecular-semiconductor-electrolyte heterointerfaces. In this Perspective, we illustrate the basic methodology and selected applications of computational modeling of dye-dye and dye-coadsorbent intermolecular interactions taking place at the dye-sensitized interface. We show that the proposed methodology offers a realistic picture of aggregation phenomena among surface-adsorbed dyes and nicely describes semiconductor surfaces cosensitized by different dyes. The information acquired from this type of studies might constitute the basis for an integrated multiscale computational description of the device functioning, including all of the possible interdependencies among the device constituents, which may further boost the DSCs efficiency. We believe that this direction should be the target of future computational research in the DSC field.

Computational Modeling of Stark Effects in Organic Dye-Sensitized TiO2 Heterointerfaces

We report a computational modeling study, based on DFT and time-dependent DFT techniques, to investigate the origin and the effect of local electric fields on the optical properties of organic dye-sensitized heterointerfaces, examining the case of the indoline D149 sensitizer on TiO2. On the one hand, we give precise information about the anchoring mode of D149 and its orientation with respect to the TiO2 surface, and on the other hand, we provide the computational framework model to interpret the Stark shifts experimentally observed by PIA spectroscopy. Our results show that the presence of oxidized dye molecules induces major spectral changes on the adjacent neutral dyes, which, along with the simulated effect of injected charge into TiO2, provide Stark shifts nicely reproducing the experimental observations.

The lowest singlet states of octatetraene revisited

The two lowest excited singlet states of all-trans-1,3,5,7-octatetraene, 2 (1)A(-)(g) and 1 (1)B(+)(u), are studied by means of high level ab initio methods computing the vertical and adiabatic excitation energies for both states and the vertical emission energy for the 1 (1)A(g)(-)←2 (1)A(-)(g) transition. The results confirm the known assignment of two energies, the 2 (1)A(-)(g) adiabatic excitation energy and the 2 (1)A(-)(g) vertical emission energy, for which well defined experimental values are available, with an excellent agreement between theory and experiment. In the experimental absorption spectrum, the maximum of the band describing the 1 (1)B(+)(u)←1 (1)A(g)(-) excitation is the first peak and it has been assigned to the (0-0) vibrational transition, but in literature it is normally compared with the theoretical vertical excitation energy. This comparison has been questioned in the past, but a conclusive demonstration of its lack of foundation has not been given. The analysis reported here, while confirming the assignment of the highest peak in the experimental spectrum to the (0-0) adiabatic transition, indicates that it cannot be used as a reference for the vertical excitation energy. The theoretical vertical excitation energies for the 2 (1)A(-)(g) and 1 (1)B(+)(u) states are found to be almost degenerate, with a value, ≃ 4.8 eV, higher than that normally accepted in the literature, 4.4 eV. The motivations which have induced in the past other authors to consider this a correct value are discussed and the origin of their feebleness are analyzed.

Energy-level alignment in organic dye-sensitized TiO2 from GW calculations

The electronic energy levels of some representative isolated and oxide-supported organic dyes, relevant for photovoltaic applications, are investigated using many-body perturbation theory within the GW approximation. We consider a set of all-organic dyes (denominated L0, L2, L3, and L4) featuring the same donor and anchor groups and differing for the linker moieties. We first calculate the energy levels of the isolated molecules, thus allowing us to address the effects of the different linker groups, and resulting in good agreement with photo-electron spectroscopic and electrochemical data. We then consider the L0 dye adsorbed on the (101) surface of anatase-TiO2. We find a density of occupied states in agreement with experimental photo-electron data. The HOMO-LUMO energy gap of the L0 dye is found to be reduced by ~1 eV upon adsorption. Our results validate the reliability of GW calculations for photovoltaic applications and point to their potential as a powerful tool for the screening and rational design of new components of electrochemical solar cells.

First-Principles Computational Modeling of Fluorescence Resonance Energy Transfer in Co-Sensitized Dye Solar Cells.

TiO2 cosensitization by different dyes having complementary absorption represents an appealing strategy to obtain panchromatic sensitization in dye-sensitized solar cells. Fluorescence (Föster) resonance energy transfer (FRET) from an energy relay dye to a sensitizing dye, both grafted onto TiO2, was effectively shown to produce additional photocurrent (Hardin et al. J. Am. Chem. Soc.2011, 133, 10662). Here we develop a realistic cosensitization model to provide a precise estimate of the geometrical parameters, which govern the FRET rate. The reliability of our model is fully confirmed by the quantitative reproduction of the experimental spectral shift in the naphtalocyanine absorption band and by the excellent agreement between the experimentally reported FRET rates. Our model provides a realistic picture of the cosensitized TiO2 interface and is capable, at the same time, of predicting the cosensitization mechanism and the associated FRET kinetics based on the sole photophysical characterization of the isolated donor/acceptor partners.