Enrico Ronca

Cation-Induced Band-Gap Tuning in Organohalide Perovskites: Interplay of Spin-Orbit Coupling and Octahedra Tilting

Organohalide lead perovskites have revolutionized the scenario of emerging photovoltaic technologies. The prototype MAPbI3 perovskite (MA = CH3NH3(+)) has dominated the field, despite only harvesting photons above 750 nm (∼1.6 eV). Intensive research efforts are being devoted to find new perovskites with red-shifted absorption onset, along with good charge transport properties. Recently, a new perovskite based on the formamidinium cation ((NH2)2CH(+) = FA) has shown potentially superior properties in terms of band gap and charge transport compared to MAPbI3. The results have been interpreted in terms of the cation size, with the larger FA cation expectedly delivering reduced band-gaps in Pb-based perovskites. To provide a full understanding of the interplay among size, structure, and organic/inorganic interactions in determining the properties of APbI3 perovskites, in view of designing new materials and fully exploiting them for solar cells applications, we report a fully first-principles investigation on APbI3 perovskites with A = Cs(+), MA, and FA. Our results evidence that the tetragonal-to-quasi cubic structural evolution observed when moving from MA to FA is due to the interplay of size effects and enhanced hydrogen bonding between the FA cations and the inorganic matrix altering the covalent/ionic character of Pb-I bonds. Most notably, the observed cation-induced structural variability promotes markedly different electronic and optical properties in the MAPbI3 and FAPbI3 perovskites, mediated by the different spin-orbit coupling, leading to improved charge transport and red-shifted absorption in FAPbI3 and in general in pseudocubic structures. Our theoretical model constitutes the basis for the rationale design of new and more efficient organohalide perovskites for solar cells 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.

First-Principles Investigation of the TiO2/Organohalide Perovskites Interface: The Role of Interfacial Chlorine

We investigate the prototypical interface between organohalide perovskites and TiO2 by first-principles electronic structure calculations. The investigated heterointerface is representative of conventional dye-sensitized solar cells based on a mesoporous TiO2 scaffold and of flat devices in which a compact TiO2 film is used as electron selective layer. We find that the MAPbI3 and MAPbI3-xClx perovskites tend to grow in (110)-oriented films on TiO2, due to the better structural matching between rows of adjacent perovskite surface halides and TiO2 undercoordinated titanium atoms. Interfacial chlorine atoms further stabilize the (110) surface, due to an enhanced binding energy. We find that the stronger interaction of MAPbI3-xClx with TiO2 modifies the interface electronic structure, leading to a stronger interfacial coupling and to a slight TiO2 conduction band energy upshift. Our modeling study may constitute the basis for a further exploitation of perovskite solar cells.

Inherent electronic trap states in TiO2 nanocrystals: effect of saturation and sintering

We report a quantum mechanical investigation on the nature of electronic trap states in realistic models of individual and sintered anatase TiO2 nanocrystals (NCs) of ca. 3 nm diameter. We find unoccupied electronic states of lowest energy to be localized within the central part of the NCs, and to originate from under-coordinated surface Ti atoms lying mainly at the edges between the (100) and (101) facets. These localized states are found at about 0.3–0.4 eV below the fully delocalized conduction band states, in good agreement with both electrochemical and spectro-electrochemical results. The overall Density-Of-States (DOS) below the conduction band (CB) can be accurately fitted to an exponential distribution of states, in agreement with capacitance data. Water molecules adsorbed on the NC surface raise the energy and reduce the number of localized states, thus modifying the DOS. As a possible origin of additional trap states, we further investigated the oriented attachment of two TiO2 NCs at various possible interfaces. For the considered models, we found only minor differences between the DOS of two interacting NCs and those of the individual constituent NCs. Our results point at the presence of inherent trap states even in perfectly stoichiometric and crystalline TiO2 NCs due to the unavoidable presence of under-coordinated surface Ti(IV) ions at the (100) facets.

A Quantitative View of Charge Transfer in the Hydrogen Bond: The Water Dimer Case

The hydrogen bond represents a fundamental intermolecular interaction that binds molecules in vapor and liquid water. A crucial and debated aspect of its electronic structure and chemistry is the charge transfer (CT) accompanying it. Much effort has been devoted, in particular, to the study of the smallest prototype system, the water dimer, but even here results and interpretations differ widely. In this paper, we reassess CT in the water dimer by using charge-displacement analysis. Besides a reliable estimate of the amount of CT (14.6 me) that characterizes the system, our study provides an unambiguous context, and very useful bounds, within which CT effects may be evaluated, crucially including the associated energy stabilization.

Impact of Spin–Orbit Coupling on Photocurrent Generation in Ruthenium Dye-Sensitized Solar Cells

Relativistic TDDFT calculations have been performed employing a novel computational approach to evaluate the impact of spin-orbit coupling (SOC) in the optical and photovoltaic properties of panchromatic Ru(II) dyes for dye-sensitized solar cells (DSCs). The employed computational setup accurately reproduces the optical properties of the investigated dyes, allowing an assessment of the factors responsible for the varying SOC with the dye metal-ligand environment. While for the prototypical panchromatic black dye sensitizer a negligible SOC effect is found, the SOC-induced spectral broadening calculated for the recently reported DX1 dye partly enhances the light-harvesting efficiency and consequently the photocurrent generation in DSCs based on this dye.

Modeling the effect of ionic additives on the optical and electronic properties of a dye-sensitized TiO2 heterointerface: absorption, charge injection and aggregation

We present a joint experimental and theoretical study with the aim of investigating the individual effects of Li-TFSI and EMIM-TFSI additives on the optical and charge transfer properties of D102 dye in solution as well as at the dye-sensitized TiO2 interface. Experimental results show that while the spectral shifts are negligible for the dye in solution, when moving to the TiO2 film in air the addition of both lithium and EMIM salts clearly gives rise to a slight red-shift with the appearance of a lower-energy shoulder in the absorption spectrum. Computational modelling confirms the weak tendency to have stable dye/additive complexes in solution and predicts appreciable spectral red-shifts as a consequence of the interaction of the dye with Li+ and EMIM+ cations. We also predict a strong effect of the additives on the electronic coupling between the dyes LUMO and the TiO2 conduction band states, reflecting on the calculated injection rates. Further, by modeling the formation and the optical response of selected dye/additive aggregate models, we find a general broadening of the absorption band, accounting for the experimentally observed lower-energy shoulder in the D102 absorption spectrum recorded on TiO2 films where Li and EMIM salts are added.

Density Relaxation in Time-Dependent Density Functional Theory: Combining Relaxed Density Natural Orbitals and Multireference Perturbation Theories for an Improved Description of Excited States.

Making use of the recently developed excited state charge displacement analysis [E. Ronca et al., J. Chem. Phys. 140, 054110 (2014)], suited to quantitatively characterize the charge fluxes coming along an electronic excitation, we investigate the role of the density relaxation effects in the overall description of electronically excited states of different nature, namely, valence, ionic, and charge transfer (CT), considering a large set of prototypical small and medium-sized molecular systems. By comparing the response densities provided by time-dependent density functional theory (TDDFT) and the corresponding relaxed densities obtained by applying the Z-vector postlinear-response approach [N. C. Handy and H. F. Schaefer, J. Chem. Phys. 81, 5031 (1984)] with those obtained by highly correlated state-of-the-art wave function calculations, we show that the inclusion of the relaxation effects is imperative to get an accurate description of the considered excited states. We also examine what happens at the quality of the response function when an increasing amount of Hartree-Fock (HF) exchange is included in the functional, showing that the usually improved excitation energies in the case of CT states are not always the consequence of an improved description of their overall properties. Remarkably, we find that the relaxation of the response densities is always able to reproduce, independently of the extent of HF exchange in the functional, the benchmark wave function densities. Finally, we propose a novel and computationally convenient strategy, based on the use of the natural orbitals derived from the relaxed TDDFT density to build zero-order wave function for multireference perturbation theory calculations. For a significant set of different excited states, the proposed approach provided accurate excitation energies, comparable to those obtained by computationally demanding ab initio calculations.

Charge-displacement analysis for excited states

We extend the Charge-Displacement (CD) analysis, already successfully employed to describe the nature of intermolecular interactions [L. Belpassi et al., J. Am. Chem. Soc. 132, 13046 (2010)] and various types of controversial chemical bonds [L. Belpassi et al., J. Am. Chem. Soc. 130, 1048 (2008); N. Salvi et al., Chem. Eur. J. 16, 7231 (2010)], to study the charge fluxes accompanying electron excitations, and in particular the all-important charge-transfer (CT) phenomena. We demonstrate the usefulness of the new approach through applications to exemplary excitations in a series of molecules, encompassing various typical situations from valence, to Rydberg, to CT excitations. The CD functions defined along various spatial directions provide a detailed and insightful quantitative picture of the electron displacements taking place.

Photoinduced Energy Shift in Quantum-Dot-Sensitized TiO2: A First-Principles Analysis

We investigate the photoinduced dipole (PID) phenomenon, which holds enormous potential for the optimization of quantum dot-sensitized solar cells (QDSSCs), by means of first-principles electronic structure calculations. We demonstrate that the sensitization of the TiO2 substrate with core/shell QDs produces almost no changes in the ground state but decisively improves the performance upon photoexcitation. In particular, the maximum attainable VOC is predicted to increase by ∼25 meV due to two additive effects: (i) the displacement of the photoexcited hole away from the TiO2 surface and (ii) the interfacial electrostatic interaction established between the TiO2-injected electrons and the holes residing in the QD core. We believe that this work, explaining the mechanisms by which PID cells deliver better efficiencies, paves the way for the design of new QDSSCs with improved efficiencies.