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ABSTRACT: Since dye-sensitized solar cells (DSSCs) appeared as a promising inexpensive alternative to the traditional silicon-based solar cells, DSSCs have attracted a considerable amount of experimental and theoretical interest. In contrast with silicon-based solar cells, DSSCs use different components for the light-harvesting and transport functions, which allow researchers to fine-tune each material and, under ideal conditions, to optimize their overall performance in assembled devices. Because of the variety of elementary components present in these cells and their multiple possible combinations, this task presents experimental challenges. The photoconversion efficiencies obtained up to this point are still low, despite the significant experimental efforts spent in their optimization. The development of a low-cost and efficient computational protocol that could qualitatively (or even quantitatively) identify the promising semiconductors, dyes, and electrolytes, as well as their assembly, could save substantial experimental time and resources. In this Account, we describe our computational approach that allows us to understand and predict the different elementary mechanisms involved in DSSC working principles. We use this computational framework to propose an in silico route for the ab initio design of these materials. Our approach relies on a unique density functional theory (DFT) based model, which allows for an accurate and balanced treatment of electronic and spectroscopic properties in different phases (such as gas, solution, or interfaces) and avoids or minimizes spurious computational effects. Using this tool, we reproduced and predicted the properties of the isolated components of the DSSC assemblies. We accessed the microscopic measurable characteristics of the cells such as the short circuit current (J(sc)) or the open circuit voltage (V(oc)), which define the overall photoconversion efficiency of the cell. The absence of empirical or material-related parameters in our approach should allow for its wide application to the optimization of existing devices or the design of new ones.
Accounts of Chemical Research 04/2012; 45(8):1268-77. · 21.64 Impact Factor
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ABSTRACT: We investigate the efficiency of several partial atomic charge models (Mulliken, Hirshfeld, Bader, Natural, Merz-Kollman and ChelpG) for investigating the through-space charge-transfer in push-pull organic compounds with Time-Dependent Density Functional Theory approaches. The results of these models are compared to benchmark values obtained by determining the difference of total densities between the ground and excited states. Both model push-pull oligomers and two classes of "real-life" organic dyes (indoline and diketopyrrolopyrrole) used as sensitisers in solar cell applications have been considered. Though the difference of dipole moments between the ground and excited states is reproduced by most approaches, no atomic charge model is fully satisfactory for reproducing the distance and amount of charge transferred that are provided by the density picture. Overall, the partitioning schemes fitting the electrostatic potential (e.g. Merz-Kollman) stand as the most consistent compromises in the framework of simulating through-space charge-transfer, whereas the other models tend to yield qualitatively inconsistent values.
Physical Chemistry Chemical Physics 03/2012; 14(16):5383-8. · 3.57 Impact Factor
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ABSTRACT: We present a combined experimental and computational approach to study Zn1–xCdxO nanowires (NWs) and their integration in light-emitting diode (LED) structures. Self-standing Zn1–xCdxO NWs have been electrodeposited on fluorine-doped tin oxide and p-GaN substrates. The electrochemical behavior has been studied, and the reaction mechanism is discussed. Low-dimensional Zn1–xCdxO structures have been obtained for CdCl2 concentrations in the deposition bath lower than 6 μM whereas at higher concentration it is admixed with crystallized CdO and the aspect ratio of the wires is decreased. According to scanning electron microscopy observations, the Zn1–xCdxO NWs have a higher aspect ratio (>30) than pure ZnO NWs (20) grown in similar conditions. Analyses show that the ZnO is doped with cadmium incorporated within ZnO NWs and that Cd doping increases with increasing Cd(II) content in the deposition bath. X-ray diffraction studies show increased lattice parameters in Cd-alloyed ZnO NWs. Photoluminescence studies on pure ZnO and Zn1–xCdxO NWs show the near band-edge emission red shifted by 3–7 nm as a function of Cd(II) concentration (4 or 8 μM in the electrolyte). The structural and optical properties of the prepared Zn1–xCdxO materials have been interpreted using density functional theory (DFT) to computationally simulate the effect of Cd substitution for Zn in the ZnO lattice. DFT calculations show that the crystal lattice parameters increase with the partial replacement of Zn atoms by Cd and that the band gap enlargement is due to the increased lattice parameters. We demonstrate the possibility to tailor the electroluminescence emission wavelength by cadmium doping in ZnO nanowires integrated in Zn1–xCdxO NWs/p-GaN heterojunction based LED structures. Reported results are of great interest for the research on band gap engineering of low-dimensional zinc oxide by doping/alloying NWs and for wavelength-tunable LED applications.
07/2011;
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ABSTRACT: A step-by-step theoretical protocol based on density functional theory (DFT) and time-dependent DFT at both the molecular and periodic levels is proposed for the design of dye-sensitized solar cell (DSSC) devices including dyes and electrolyte additives. This computational tool is tested with a fused polycyclic pyridinium derivative as a novel dye prototype. First, the UV-vis spectrum of this dye alone is computed, and then the electronic structure of the system with the dye adsorbed on an oxide semiconductor surface is evaluated. The influence of the electrolyte part of the DSSC is investigated by explicitly taking into account the electrolyte molecules co-adsorbed with the dye on the surface. We find that tert-butylpyridine (TBP) reduces the electron injection by a factor of 2, while lithium ion increases this injection by a factor of 2.4. Our stepwise protocol is successfully validated by experimental measurements, which establish that TBP divides the electronic injection by 1.6 whereas Li(+) multiplies this injection by 1.8. This procedure should be useful for molecular engineering in the field of DSSCs, not only as a complement to experimental approaches but also for improving them in terms of time and resource consumption.
Journal of the American Chemical Society 05/2011; 133(20):8005-13. · 9.91 Impact Factor
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ABSTRACT: We, herein, explore the potential use of new anchoring groups for the development of hybrid organic–inorganic materials and more specifically for the chemisorption of dyes in dye-sensitized solar cells. The structural and the electronic properties of four different compounds (the 1,2-benzenediole-, catechol-, the pentane-2,4-dione (acetylacetone), and the corresponding thio derivatives) adsorbed on a {100} clean ZnO surface were investigated by the means of density functional theory in a periodic framework. Subsequent—harmonic—infrared (IR) spectral calculations of the adsorbed and isolated systems pointed out that the adsorption process may be followed by IR techniques. From our analysis, all anchoring groups seem to be suitable as anchoring groups in hybrid devices both from a structural and electronic point of view, although additional requirements may be important for specific applications. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
International Journal of Quantum Chemistry 05/2011; 112(9):2062 - 2071. · 1.36 Impact Factor
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ABSTRACT: Acetylacetone (acacH) adsorption on ZnO (10-10) surface has been studied by a theoretical periodic approach using density functional theory. Two dissociative adsorption modes were investigated and compared to the most stable adsorption mode of formic acid. Acetylacetone appears as a suitable anchoring group for hybrid materials, with adsorption energies of the same order of magnitude as formic acid. IR spectra of the acac/ZnO systems were computed in order to determine the spectral signature of adsorption and, possibly, of each adsorption mode to follow the coordination of acac on ZnO at the experimental level. The results have been compared to Fourier transform infrared (attenuated total reflection-IR) experimental spectra. The present investigation points out the interest of acetylacetone as an anchoring group for the development of new ZnO-based functionalized hybrid layers for corrosion protection, light emitting diodes, photocatalytic systems, and dye-sensitized solar cells.
Langmuir 02/2011; 27(7):3442-50. · 4.19 Impact Factor
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ABSTRACT: We have investigated the role of electrolyte composition, in terms of solvent and additive, on the open-circuit voltage (V(oc)) of ZnO-based dye-sensitized solar cells (DSSCs) using a combined experimental and theoretical approach. Calculations based on density functional theory (DFT) have been performed in order to describe the geometries and adsorption energies of various adsorbed solvents (nitromethane, acetonitrile and dimethylformamide) and p-tert-butylpyridine (TBP) (modeled by methylpyridine) on the ZnO (100) surface using a periodic approach. The densities of states (DOS) have been calculated and the energy position of the conduction band edge (CBE) has been evaluated for the different molecules adsorbed. The effect of the electrolyte composition on the standard redox potential of the iodide/triiodide redox couple has been experimentally determined. These two data values (CBE and standard redox potential) allowed us to determine the dependence of V(oc) on the electrolyte composition. The variations determined using this method were in good agreement with the measured V(oc) for cells made of electrodeposited ZnO films sensitized using D149 (indoline) dye. As in the case of TiO(2)-based cells, a correlation of V(oc) with the donor number of the adsorbed species was found. The present study clearly points out that both the CBE energy and the redox potential variation are important for explaining the experimentally observed changes in the V(oc) of DSSCs.
Physical Chemistry Chemical Physics 11/2010; 12(44):14710-9. · 3.57 Impact Factor
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ABSTRACT: Time dependent density functional theory (TD-DFT) in conjunction with a hybrid exchange correlation functional (PBE0) were applied to characterize the photophysical behavior of the 8-hydroxyquinoline-5-sulfonic acid (8-HQS) in solution as a function of the pH. In particular, absorption and emission spectra of each species as well as their relative stability in the first excited state were computed. From these calculations it is possible to directly derive quantities otherwise hardly experimentally accessible such as excited state acidic dissociation constants (pK(a)*) and corresponding distribution diagrams at the excited state. These two latter quantities were determined by first principles from the relative stabilities of the species at the excited state computed at the TD-DFT level. Consequently, the evolution of the absorption and emission spectral properties of 8-HQS as a function of the pH could be fully simulated from first principles. Finally, insights on energetics and the mechanism of the phototautomerization reaction supposed to be responsible for the absence of fluorescence of the 8-HQS molecule were derived from the calculations.
The Journal of Physical Chemistry A 05/2010; 114(18):5932-9. · 2.95 Impact Factor
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ABSTRACT: The ground and excited state properties of three indoline dyes, namely D102, D131 and D149, especially designed for dye sensitized solar cell (DSC) applications have been studied by the means of density functional theory (DFT) and time-dependent DFT (TD-DFT) and compared with experimental absorption and fluorescence spectra. By comparison, insight on the behavior of a related dye (D205) is also given. Beside the good agreement between the computed and experimental spectra further proving the accuracy of the method used, that is, a hybrid (here PBE0) exchange correlation functional in conjunction with a polarizable continuum solvent (PCM) model and a medium size basis set, the calculations allow new insights into the electronic structure of this family of indolines, and in particular the electronic and geometrical structure at the first excited state. Possible insights on the optimization of dyes for photovoltaics applications, as well as warnings on the extrapolation of isolated dye properties towards cells behavior are given.
Physical Chemistry Chemical Physics 12/2009; 11(47):11276-84. · 3.57 Impact Factor
