Francesco Ambrosio

What Is the Best Anchoring Group for a Dye in a Dye-Sensitized Solar Cell?

We developed a computational procedure to screen many different anchoring groups used or usable to connect a dye to the semiconducting surface in a dye-sensitized solar cell. The procedure leads to a clear identification of the anchoring groups that bind strongly to the surface and facilitate the electron injection at the same time, providing clear-cut indications for the design of new dyes. The complicated interplay of factors that determine the final results (preferred adsorption mode, the anchors effect on the dyes electronic structure, and dye-semiconductor coupling) is illustrated through a few examples showing how chemical intuition can often be misleading in this problem.

Redox levels in aqueous solution: Effect of van der Waals interactions and hybrid functionals

We investigate redox levels in aqueous solution using a combination of ab initio molecular dynamics (MD) simulations and thermodynamic integration methods. The molecular dynamics are performed with both the semilocal Perdew-Burke-Ernzerhof functional and a nonlocal functional (rVV10) accounting for van der Waals (vdW) interactions. The band edges are determined through three different schemes, namely, from the energy of the highest occupied and of the lowest unoccupied Kohn-Sham states, from total-energy differences, and from a linear extrapolation of the density of states. It is shown that the latter does not depend on the system size while the former two are subject to significant finite-size effects. For the redox levels, we provide a formulation in analogy to the definition of charge transition levels for defects in crystalline materials. We consider the H(+)/H2 level defining the standard hydrogen electrode, the OH(-)/OH(∗) level corresponding to the oxidation of the hydroxyl ion, and the H2O/OH(∗) level for the dehydrogenation of water. In spite of the large structural modifications induced in liquid water, vdW interactions do not lead to any significant structural effect on the calculated band gap and band edges. The effect on the redox levels is also small since the solvation properties of ionic species are little affected by vdW interactions. Since the electronic properties are not significantly affected by the underlying structural properties, it is justified to perform hybrid functional calculations on the configurations of our MD simulations. The redox levels calculated as a function of the fraction α of Fock exchange are found to remain constant, reproducing a general behavior previously observed for charge transition levels of defects. Comparison with experimental values shows very good agreement. At variance, the band edges and the band gap evolve linearly with α. For α ≃ 0.40, we achieve a band gap, band-edge positions, and redox levels in overall good agreement with experiment.

Structural, Dynamical, and Electronic Properties of Liquid Water: A Hybrid Functional Study

We study structural, dynamical, and electronic properties of liquid water through ab initio molecular dynamics (MD) simulations based on a hybrid functional which includes nonlocal van der Waals (vdW) interactions. The water dimer, the water hexamer, and two phases of ice are studied as benchmark cases. The hydrogen-bond energy depends on the balance between Fock exchange and vdW interactions. Moreover, the energetic competition between extended and compact structural motifs is found to be well described by theory provided vdW interactions are accounted for. Applied to the hydrogen-bond network of liquid water, the dispersion interactions favor more compact structural motifs, bring the density closer to the experimental value, and improve the agreement with experimental observables such as radial distribution functions. The description of the self-diffusion coefficient is also found to improve upon the combined consideration of Fock exchange and vdW interactions. The band gap and the band edges are found to agree with experiment within 0.1 eV.

Singlet fission in linear chains of molecules

We develop a model configuration interaction Hamiltonian to study the electronic structure of a chain of molecules undergoing singlet fission. We first consider models for dimer and trimer and then we use a matrix partitioning technique to build models of arbitrary size able to describe the relevant electronic structure for singlet fission in linear aggregates. We find that the multi-excitonic state (ME) is stabilized at short inter-monomer distance and the extent of this stabilization depends upon the size of orbital coupling between neighboring monomers. We also find that the coupling between ME states located on different molecules is extremely small leading to bandwidths in the order of ∼10 meV. This observation suggests that multi-exciton states are extremely localized by electron-phonon coupling and that singlet fission involves the transition between a relatively delocalized Frenkel exciton and a strongly localized multi-exciton state. We adopt the methodology commonly used to study non-radiative transitions to describe the singlet fission dynamics in these aggregates and we discuss the limit of validity of the approach. The results indicate that the phenomenology of singlet fission in molecular crystals is different in many important ways from what is observed in isolated dimers.

Origin of low electron–hole recombination rate in metal halide perovskites

To address the slow recombination of photogenerated charges in tetragonal CH3NH3PbI3, the evolution of extra electrons and holes is simulated through advanced ab initio molecular dynamics. We show that the localization of the charge carriers and their hopping from one polaronic state to another occur on a subpicosecond time scale. The localization, attended by weak bond contractions and elongations in the inorganic sublattice, is induced by thermal vibrations and only moderately perturbed by the disordered field generated by the organic cations. The simultaneous simulation of extra electrons and holes shows that they preferentially localize in spatially distinct regions. As determined by the overlap between the electron and hole wave functions, the probability of radiative bimolecular recombination is lowered by two orders of magnitude compared with that of optical generation. The separate polaronic localization of electrons and holes emerges as the key feature for achieving exceptional photovoltaic properties.

Oxygen defects in amorphous Al2O3: A hybrid functional study

The electronic properties of the oxygen vacancy and interstitial in amorphous Al2O3 are studied via ab initio molecular dynamics simulations and hybrid functional calculations. Our results indicate that these defects do not occur in amorphous Al2O3, due to structural rearrangements which assimilate the defect structure and cause a delocalization of the associated defect levels. The imbalance of oxygen leads to a nonstoichiometric compound in which the oxygen occurs in the form of O2– ions. Intrinsic oxygen defects are found to be unable to trap excess electrons. For low Fermi energies, the formation of peroxy linkages is found to be favored leading to the capture of holes. The relative +2/0 defect levels occur at 2.5 eV from the valence band.

Electronic Levels of Excess Electrons in Liquid Water

We provide a consistent description of the electronic levels associated with localized and delocalized excess electrons in liquid water by combining hybrid-functional molecular dynamics simulations with a grand canonical formulation of solutes in aqueous solution. The excess electron localizes in a cavity with an average radius of 1.8 Å and a majority coordination of five water molecules. The vertical binding energy, the optical s-p transitions, and the adiabatic redox level are found to agree closely with their experimental counterparts. The energy level associated with electron delocalization V0 is inferred to lie at -0.97 eV with respect to the vacuum level.

Hole diffusion across leaky amorphous TiO2 coating layers for catalytic water splitting at photoanodes

We investigate the mechanism of hole diffusion across leaky amorphous TiO2 (am-TiO2) layers. Through ab initio molecular dynamics simulations, we construct an atomistic model of am-TiO2 consistent with the experimental characterization. We first demonstrate that the oxygen vacancies do not occur in am-TiO2, as they can be assimilated by the amorphous structure upon structural rearrangement. Hence, their role in hole diffusion is ruled out. In contrast, O–O peroxy linkages are formed in pristine am-TiO2 upon injection of excess holes, with an associated defect level lying at 1.25 ± 0.15 eV above the valence band of the material. We show that such linkages can provide a viable mechanism for hole diffusion in am-TiO2, as illustrated by a diffusion path of 1.2 nm with energy barriers lower than 0.5 eV in our atomistic model of am-TiO2.

Note: Assessment of the SCAN+rVV10 functional for the structure of liquid water

The performance of the SCAN+rVV10 functional in modeling the structural properties of liquid water is studied through constant-volume ab initio molecular dynamics simulations with both classical and quantum nuclei. The radial distribution functions are found to be slightly overstructured with respect to experiment, but overall similar to those achieved with the bare SCAN and the rVV10 functionals. From the pressures calculated during the dynamics, it is inferred that the SCAN+rVV10 functional leads to a noticeable overestimation of the density of liquid water.

Absolute Energy Levels of Liquid Water

We determine the ionization potential (IP) and the electron affinity (EA) of liquid water together with the absolute redox level of the standard hydrogen electrode (SHE) by combining advanced electronic-structure calculations, ab initio molecular dynamics simulations, thermodynamic integration, and potential alignment at the water-vacuum interface. The calculated SHE level lies at 4.56 eV below the vacuum level, close to the experimental reference of 4.44 eV inferred by Trasatti. The band edges are determined through a hybrid functional designed to reproduce the band gap achieved with highly accurate GW calculations. Our analysis yields IP = 9.7 eV and EA = 0.8 eV, consistent with both photoemission spectra of liquid water and thermodynamical data for the hydrated electron.