Effective prediction of SnO2 conduction band edge potential: The key role of surface oxygen vacancies.

Several theoretical studies at different levels of theory have attempted to calculate the absolute position of the SnO2 conduction band, whose knowledge is key for its effective application in optoelectronic devices such us, for example, perovskite solar cells. However, the predicted band edges fall outside the experimentally measured range. In this work, we introduce a computational scheme designed to calculate the conduction band minimum values of SnO2, yielding results aligned with experiments. Our analysis points out the fundamental role of encompassing surface oxygen vacancies to properly describe the electronic profile of this material. We explore the impact of both bridge and in-plane oxygen vacancy defects on the structural and electronic properties of SnO2, explaining from an atomistic perspective the experimental observables. The results underscore the importance of simulating both types of defects to accurately predict SnO2 features and provide new fundamental insights that can guide future studies concerning design and optimization of SnO2-based materials and functional interfaces.

Exotic hexagonal NaCl atom-thin layer on methylammonium lead iodide perovskite: new hints for perovskite solar cells from first-principles calculations.

Alkali halides are simple inorganic compounds extensively used as surface modifiers in optoelectronic devices. In perovskite solar cells (PSCs), they act as interlayers between the light absorber material and the charge selective layers improving their contact quality. They introduce surface dipoles that enable the fine tuning of the relative band alignment and passivate surface defects, a well-known drawback of hybrid organic-inorganic perovskites, that is responsible for most of the issues hampering the long-term performances. Reducing the thickness of such salt-based insulating layer might be beneficial in terms of charge transfer between the perovskite and the electron/hole transport layers. In this context, here we apply density functional theory (DFT) to characterize the structure and the electronic features of atom-thin layers of NaCl adsorbed on the methylammonium lead iodide (MAPI) perovskite. We analyze two different models of MAPI surface terminations and find unexpected structural reconstructions arising at the interface. Unexpectedly, we find an exotic honeycomb-like structuring of the salt, also recently observed in experiments on a diamond substrate. We also investigate how the salt affects the perovskite electronic properties that are key to control the charge dynamics at the interface. Moreover, we also assess the salt ability to improve the defect tolerance of the perovskite surface. With these results, we derive new hints regarding the potential benefits of using an atom-thin layer of alkali halides in PSCs.

Structure and dynamics of Li1.24K0.76CO3 molten carbonate electrolyte from molecular simulations with explicit polarization.

Molten carbonate electrolysis cells represent a key technology for harnessing surplus energy from renewable sources and converting it into gaseous energy carriers. To optimize their efficiency, a comprehensive understanding of each step in the operational process is essential. Here, we focus on the electrolyte of choice in molten carbonate cells: the Li1.24K0.76CO3 melt. Utilizing molecular dynamics with explicit polarization, we demonstrate that the structure of this molten mixture is characterized by a dense network of lithium-carbonate complexes, with K+ ions loosely embedded within this network. This structural insight enables us to rationalize from an atomistic perspective the conductivity trends observed experimentally in molten carbonates. Moreover, our work highlights the importance of including polarization for the simulations of dense liquid carbonates. It also acts as a foundational step towards more advanced theoretical studies for elucidating the role of the electrolyte in these devices.

Antimony-Bismuth Alloying: The Key to a Major Boost in the Efficiency of Lead-Free Perovskite-Inspired Photovoltaics.

The perovskite-inspired Cu2 AgBiI6 (CABI) material has been gaining increasing momentum as photovoltaic (PV) absorber due to its low toxicity, intrinsic air stability, direct bandgap, and a high absorption coefficient in the range of 105  cm-1 . However, the power conversion efficiency (PCE) of existing CABI-based PVs is still seriously constrained by the presence of both intrinsic and surface defects. Herein, antimony (III) (Sb3+ ) is introduced into the octahedral lattice sites of the CABI structure, leading to CABI-Sb with larger crystalline domains than CABI. The alloying of Sb3+ with bismuth (III) (Bi3+ ) induces changes in the local structural symmetry that dramatically increase the formation energy of intrinsic defects. Light-intensity dependence and electron impedance spectroscopic studies show reduced trap-assisted recombination in the CABI-Sb PV devices. CABI-Sb solar cells feature a nearly 40% PCE enhancement (from 1.31% to 1.82%) with respect to the CABI devices mainly due to improvement in short-circuit current density. This work will promote future compositional design studies to enhance the intrinsic defect tolerance of next-generation wide-bandgap absorbers for high-performance and stable PVs.

First-principles design of nanostructured electrode materials for Na-ion batteries: challenges and perspectives.

Post-lithium batteries are emerging as viable solutions for sustainable energy transition. Effective deployment in the market calls for great research efforts in the identification of novel component materials and the assessment of related working principles. Computational modelling can be a key player in boosting innovation and development by enabling rational strategies for the design of appropriately tuned materials with optimized activity towards battery operating processes. By gaining access to the structural and electronic features of functional electrodes, state-of-the-art DFT methods can unveil the subtle structure-property relationship that affects the uptake, transport, and storage efficiency. Hereby, we aim at reviewing the research status of theoretical advances in the field of Na-ion batteries (NIBs) and illustrating to what extent atomistic insights into sodiation/desodiation mechanisms of nanostructured materials can assist the development of effective anodes and cathodes for stable and highly performing devices. Thanks to increasing computer power and fruitful cooperation between theory and experiments, the route for effective design methodologies is being paved and will feed the upcoming developments in NIB technology.

Challenges of modeling nanostructured materials for photocatalytic water splitting.

Understanding the water splitting mechanism in photocatalysis is a rewarding goal as it will allow producing clean fuel for a sustainable life in the future. However, identifying the photocatalytic mechanisms by modeling photoactive nanoparticles requires sophisticated computational techniques based on multiscale modeling. In this review, we will survey the strengths and drawbacks of currently available theoretical methods at different length and accuracy scales. Understanding the surface-active site through Density Functional Theory (DFT) using new, more accurate exchange-correlation functionals plays a key role for surface engineering. Larger scale dynamics of the catalyst/electrolyte interface can be treated with Molecular Dynamics albeit there is a need for more generalizations of force fields. Monte Carlo and Continuum Modeling techniques are so far not the prominent path for modeling water splitting but interest is growing due to the lower computational cost and the feasibility to compare the modeling outcome directly to experimental data. The future challenges in modeling complex nano-photocatalysts involve combining different methods in a hierarchical way so that resources are spent wisely at each length scale, as well as accounting for excited states chemistry that is important for photocatalysis, a path that will bring devices closer to the theoretical limit of photocatalytic efficiency.

In Situ Formation of Zwitterionic Ligands: Changing the Passivation Paradigms of CsPbBr3 Nanocrystals.

CsPbBr3 nanocrystals (NCs) passivated by conventional lipophilic capping ligands suffer from colloidal and optical instability under ambient conditions, commonly due to the surface rearrangements induced by the polar solvents used for the NC purification steps. To avoid onerous postsynthetic approaches, ascertained as the only viable stability-improvement strategy, the surface passivation paradigms of as-prepared CsPbBr3 NCs should be revisited. In this work, the addition of an extra halide source (8-bromooctanoic acid) to the typical CsPbBr3 synthesis precursors and surfactants leads to the in situ formation of a zwitterionic ligand already before cesium injection. As a result, CsPbBr3 NCs become insoluble in nonpolar hexane, with which they can be washed and purified, and form stable colloidal solutions in a relatively polar medium (dichloromethane), even when longly exposed to ambient conditions. The improved NC stability stems from the effective bidentate adsorption of the zwitterionic ligand on the perovskite surfaces, as supported by theoretical investigations. Furthermore, the bidentate functionalization of the zwitterionic ligand enables the obtainment of blue-emitting perovskite NCs with high PLQYs by UV-irradiation in dichloromethane, functioning as the photoinduced chlorine source.

Unexpected Imidazole Coordination to the Dirhodium Center in a Protein Environment: Insights from X-ray Crystallography and Quantum Chemistry.

X-ray diffraction data demonstrate that the adduct formed upon the reaction of dirhodium(II,II) tetraacetate with RNase A reacts with imidazole, leading to the formation of an unexpected product with the imidazole that binds the dirhodium center at an equatorial site rather than an axial site. The origin of this result has been dissected using quantum-chemical calculations.

Electronic structure and interfacial features of triphenylamine- and phenothiazine-based hole transport materials for methylammonium lead iodide perovskite solar cells.

Recently, great research efforts have been devoted to perovskite solar cells (PSCs) leading to sunlight-to-power conversion efficiencies above 25%. However, several barriers still hinder the full deployment of these devices. Critical issues are related to PCE stability and device lifetimes, which could be improved by targeted engineering of the hole transport material (HTM). Indeed, the HTM is not only responsible for transporting holes and preventing direct contact between the photo-active perovskite and the charge collector layer, but it plays important structural and protective roles too. As alternatives to the widely used yet expensive and unstable Spiro-OMeTAD, organic HTMs based on triphenylamine (TPA) and phenothiazine (PTZ) moieties have been proposed. However, their performances in PSC devices, and in particular their interfacial properties with the most popular methylammonium lead iodide perovskite (MAPI) still need investigations to be fully determined. In this framework, here we report a first-principles study on the structural and the electronic properties of a recently designed TPA and PTZ-based HTM (HTM1) and its interface with the MAPI (001) surface, considering both the PbI2- and the MAI-terminations. We also addressed already known HTM molecular systems to allow for a direct comparison with the recently proposed HTM1: we characterized the molecular parameters and the MAPI/HTM interfacial properties for Spiro-OMeTAD, PTZ1, and PTZ2. Our results suggest that good adhesion properties do not ensure effective and efficient MAPI-HTM hole injection. Despite the theoretical good alignment between HTM1 HOMO and MAPI valence band edge, our results for the mutually polarized interface point out the lack of a sufficient driving force for hole transport. While the hole mobility of HTM1 outperforms those of the other HTM molecules, for this HTM molecule, our findings suggest the application of lead halide perovskite compositions other than MAPI, with substituents that lower its valence band maximum potential value.

Dye-sensitized solar cells strike back.

Dye-sensitized solar cells (DSCs) are celebrating their 30th birthday and they are attracting a wealth of research efforts aimed at unleashing their full potential. In recent years, DSCs and dye-sensitized photoelectrochemical cells (DSPECs) have experienced a renaissance as the best technology for several niche applications that take advantage of DSCs' unique combination of properties: at low cost, they are composed of non-toxic materials, are colorful, transparent, and very efficient in low light conditions. This review summarizes the advancements in the field over the last decade, encompassing all aspects of the DSC technology: theoretical studies, characterization techniques, materials, applications as solar cells and as drivers for the synthesis of solar fuels, and commercialization efforts from various companies.

Structural and electronic properties of defective 2D transition metal dichalcogenide heterostructures.

We present a first-principles study on the structure-property relationships in MoS2 and WS2 monolayers and their vertically stacked hetero-bilayer, with and without Sulfur vacancies, in order to dissect the electronic features behind their photocatalytic water splitting capabilities. We also benchmark the accuracy of three different exchange-correlation density functionals for both minimum-energy geometries and electronic structure. The best compromise between computational cost and qualitative accuracy is achieved with the HSE06 density functional on top of Perdew-Burke-Ernzerhof minima, including dispersion with Grimme's D3 scheme. This computational approach predicts the presence of mid-gap states for defective monolayers, in accordance with the present literature. For the heterojunction, we find unexpected vacancy-position dependent electronic features: the location of the defects leads either to mid-gap trap states, detrimental for photocatalyst or to a modification of characteristic type II band alignment behavior, responsible for interlayer charge separation and low recombination rates.

Interfacial electronic features in methyl-ammonium lead iodide and p-type oxide heterostructures: new insights for inverted perovskite solar cells.

Perovskite solar cells (PSCs) represent a promising technology for highly efficient sunlight harvesting and its conversion to electricity at convenient costs. However, a few flaws of current devices undermine the long-term stability of PSCs. Some of them concern the interface between the photoactive perovskite and the hole transport layer (HTL), e.g. undesired charge recombination, polarization barriers and oxidation processes. A strategy to solve this problem is to replace the standard organic HTL (e.g. Spiro-OMeTAD) with a solid-state inorganic layer. Being extensively used in p-type dye sensitized solar cells (DSSCs), nickel oxide (NiO) has been the first choice as an inorganic HTL. Despite the great interests in the application of NiO and other p-type oxides in PSCs, there is no available atomistic model of their interface with a halide perovskite. Here, we address this knowledge gap via a thorough first-principles study of the prototypical PSC perovskite methyl-ammonium lead iodide (MAPI) and two inorganic p-type oxides: NiO and CuGaO2. This copper-gallium delafossite oxide is one of the most promising alternatives to NiO in p-type DSSCs, thanks to its wide optical bandgap and low valence band edge. Here, we characterize the properties of both isolated surface slabs and MAPI/HTL heterostructure models. Besides considering MAPI/NiO and MAPI/CuGaO2 interfaces from the pristine materials, we also address the effects of intrinsic and extrinsic p-type defects in both NiO (Ni vacancy, Ni vacancy with Li and Ag doping) and CuGaO2 (Cu vacancy) using more realistic models. Our study reveals the most convenient interfaces in terms of structural affinities and adhesion energies. From the electronic perspective, we present a detailed analysis on band edge alignments, with direct insights into the key functional parameters of PSCs: hole injection driving force and open circuit potential. Our data show how the presence of defects/dopants is crucial for a convenient hole injection in NiO and CuGaO2. These results provide new science-based design principles for further development of p-type oxides in PSC devices.

Sodium diffusion in ionic liquid-based electrolytes for Na-ion batteries: the effect of polarizable force fields.

Understanding the transport of sodium ions in ionic liquids is key to designing novel electrolyte materials for sodium-ion batteries. In this work, we combine molecular dynamics simulation and experiments to study how molecular interactions and local ordering affect relevant physico-chemical properties. Ionic transport and local solvation environments are investigated in electrolytes composed of sodium bis(fluorosulfonyl)imide, (Na[FSI]), in N,N-methylpropylpyrrolidinium bis(fluorosulfonyl)imide, [C3C1pyr][FSI], at different salt concentrations. The electrolyte systems are modelled by means of molecular dynamic simulations using a polarizable force field. We show that including polarization effects explicitly in the molecular simulations is required in order to attain a reliable description of the transport properties of sodium in the [C3C1pyr][FSI] electrolyte. The validation of the computational results upon comparison with experimental data allows us to assess the suitability of polarizable force fields in describing and interpreting the structure and dynamics of the sodium salt-ionic liquid system, which is essential to enable the application of IL-based electrolytes in novel energy-storage technologies.

An ab initio study of Cu-based delafossites as an alternative to nickel oxide in photocathodes: effects of Mg-doping and surface electronic features.

CuMO2 delafossites (M = Al, Ga, and Cr) are p-type semiconductor oxides that have been recently proposed as the electrode in p-type dye-sensitized solar cells (p-DSSC) which is an alternative to the standard, low-performing nickel oxide. To assess this potential application of delafossites, we report here a DFT-based investigation of the structural and electronic properties of CuAlO2, CuGaO2 and CuCrO2. In particular, we address the role of Mg doping to obtain the p-type semiconducting character: the substitution of an M3+ cation with Mg2+ is easier with Ga than with Al and Cr, and, in all cases, the hole introduced by Mg2+ leads to the formation of Cu2+ species. Moreover, we address surface electronic features in order to characterize the most exposed delafossite surface termination and, more importantly, to predict the valence band maximum energy value, which determines the p-DSSC open circuit potential. From analysis of all our results, CuGaO2 emerges as the most promising system that can boost the development of new photocathodes for p-DSSCs.

Copper Bipyridyl Redox Mediators for Dye-Sensitized Solar Cells with High Photovoltage.

Redox mediators play a major role determining the photocurrent and the photovoltage in dye-sensitized solar cells (DSCs). To maintain the photocurrent, the reduction of oxidized dye by the redox mediator should be significantly faster than the electron back transfer between TiO2 and the oxidized dye. The driving force for dye regeneration with the redox mediator should be sufficiently low to provide high photovoltages. With the introduction of our new copper complexes as promising redox mediators in DSCs both criteria are satisfied to enhance power conversion efficiencies. In this study, two copper bipyridyl complexes, Cu(II/I)(dmby)2TFSI2/1 (0.97 V vs SHE, dmby = 6,6'-dimethyl-2,2'-bipyridine) and Cu(II/I)(tmby)2TFSI2/1 (0.87 V vs SHE, tmby = 4,4',6,6'-tetramethyl-2,2'-bipyridine), are presented as new redox couples for DSCs. They are compared to previously reported Cu(II/I)(dmp)2TFSI2/1 (0.93 V vs SHE, dmp = bis(2,9-dimethyl-1,10-phenanthroline). Due to the small reorganization energy between Cu(I) and Cu(II) species, these copper complexes can sufficiently regenerate the oxidized dye molecules with close to unity yield at driving force potentials as low as 0.1 V. The high photovoltages of over 1.0 V were achieved by the series of copper complex based redox mediators without compromising photocurrent densities. Despite the small driving forces for dye regeneration, fast and efficient dye regeneration (2-3 µs) was observed for both complexes. As another advantage, the electron back transfer (recombination) rates were slower with Cu(II/I)(tmby)2TFSI2/1 as evidenced by longer lifetimes. The solar-to-electrical power conversion efficiencies for [Cu(tmby)2]2+/1+, [Cu(dmby)2]2+/1+, and [Cu(dmp)2]2+/1+ based electrolytes were 10.3%, 10.0%, and 10.3%, respectively, using the organic Y123 dye under 1000 W m-2 AM1.5G illumination. The high photovoltaic performance of Cu-based redox mediators underlines the significant potential of the new redox mediators and points to a new research and development direction for DSCs.

Promoting oxygen vacancy formation and p-type conductivity in SrTiO3via alkali metal doping: a first principles study.

Strontium titanate (SrTiO3, STO) is a prototypical perovskite oxide, widely exploited in many technological applications, from catalysis to energy conversion devices. In the context of solid-oxide fuel cells, STO has been recently applied as an epitaxial substrate for nano-sized layers of mixed ion-electron conductive catalysts with enhanced electrochemical performances. To extend the applications of such heterogeneous nano-cathodes in real devices, also the STO support should be active for both electron transport and oxide diffusion. To this end, we explored using first-principles calculations the strategy of doping of STO at the Sr site with sodium and potassium. These two ions fit in the perovskite structure and induce holes in the STO valence band, so as to obtain the desired p-type electronic conduction. At the same time, the doping with alkali ions also promotes the formation of oxygen vacancies in STO, a prerequisite for effective oxide diffusion. Analysis of electron density rearrangements upon defect formation allows relating the favorable vacancy formation energies to an improved electronic delocalization over the oxide sub-lattice, as observed in closely related materials (e.g. Sr2Fe1.5Mo0.5O6). Overall, our results suggest the alkali-doped STO as a new potential substrate material in nanoscale heterogeneous electrodes for solid oxide electrochemical cells.

Stability of melamine-exfoliated graphene in aqueous media: quantum-mechanical insights at the nanoscale.

In recent experiments, melamine (1,3,5-triazine-2,4,6-triamine) has been proposed as an effective exfoliating agent to obtain high quality graphene from graphite. After washing out the melamine in excess, small amounts (ppm) are still needed to stabilize the dispersion of graphene flakes in aqueous media. To understand the origin of this behaviour, we investigated the melamine-graphene-water system and the fundamental interactions that determine its structure and energetics. To disentangle the subtle interplay of hydrogen-bonding and dispersive forces we used state-of-the-art ab initio calculations based on density functional theory. First, we focused on the case of water molecules interacting with melamine-graphene assemblies at different melamine coverages. We found that water-melamine interactions provide the driving force for washing off the melamine from graphene. Then, we addressed the interaction of single and double layers of water molecules with the graphene surface in the presence of an adsorbed melamine molecule. We found that this melamine acts as a non-covalent anchor for keeping a number of water molecules conveniently close to the graphene surface, thus helping its stabilization in aqueous media. Our analysis helps understanding how competing weak forces can lead to a stable graphene water suspension thanks to small amounts of adsorbed melamine. From our results, we derive simple indications on how the water-graphene interfacial properties can be tuned via non-covalent adsorption of small functional molecules with H-bond donor/acceptor groups. These new hints can be helpful to prepare stable graphene dispersions in water and so to unlock graphene potential in aqueous environments.

Effective scheme for partitioning covalent bonds in density-functional embedding theory: From molecules to extended covalent systems.

Density-functional embedding theory provides a general way to perform multi-physics quantum mechanics simulations of large-scale materials by dividing the total system's electron density into a cluster's density and its environment's density. It is then possible to compute the accurate local electronic structures and energetics of the embedded cluster with high-level methods, meanwhile retaining a low-level description of the environment. The prerequisite step in the density-functional embedding theory is the cluster definition. In covalent systems, cutting across the covalent bonds that connect the cluster and its environment leads to dangling bonds (unpaired electrons). These represent a major obstacle for the application of density-functional embedding theory to study extended covalent systems. In this work, we developed a simple scheme to define the cluster in covalent systems. Instead of cutting covalent bonds, we directly split the boundary atoms for maintaining the valency of the cluster. With this new covalent embedding scheme, we compute the dehydrogenation energies of several different molecules, as well as the binding energy of a cobalt atom on graphene. Well localized cluster densities are observed, which can facilitate the use of localized basis sets in high-level calculations. The results are found to converge faster with the embedding method than the other multi-physics approach ONIOM. This work paves the way to perform the density-functional embedding simulations of heterogeneous systems in which different types of chemical bonds are present.

Oxygen transport in perovskite-type solid oxide fuel cell materials: insights from quantum mechanics.

CONSPECTUS: Global advances in industrialization are precipitating increasingly rapid consumption of fossil fuel resources and heightened levels of atmospheric CO2. World sustainability requires viable sources of renewable energy and its efficient use. First-principles quantum mechanics (QM) studies can help guide developments in energy technologies by characterizing complex material properties and predicting reaction mechanisms at the atomic scale. QM can provide unbiased, qualitative guidelines for experimentally tailoring materials for energy applications. This Account primarily reviews our recent QM studies of electrode materials for solid oxide fuel cells (SOFCs), a promising technology for clean, efficient power generation. SOFCs presently must operate at very high temperatures to allow transport of oxygen ions and electrons through solid-state electrolytes and electrodes. High temperatures, however, engender slow startup times and accelerate material degradation. SOFC technologies need cathode and anode materials that function well at lower temperatures, which have been realized with mixed ion-electron conductor (MIEC) materials. Unfortunately, the complexity of MIECs has inhibited the rational tailoring of improved SOFC materials. Here, we gather theoretically obtained insights into oxygen ion conductivity in two classes of perovskite-type materials for SOFC applications: the conventional La1-xSrxMO3 family (M = Cr, Mn, Fe, Co) and the new, promising class of Sr2Fe2-xMoxO6 materials. Using density functional theory + U (DFT+U) with U-J values obtained from ab initio theory, we have characterized the accompanying electronic structures for the two processes that govern ionic diffusion in these materials: (i) oxygen vacancy formation and (ii) vacancy-mediated oxygen migration. We show how the corresponding macroscopic oxygen diffusion coefficient can be accurately obtained in terms of microscopic quantities calculated with first-principles QM. We find that the oxygen vacancy formation energy is a robust descriptor for evaluating oxide ion transport properties. We also find it has a direct relationship with (i) the transition metal-oxygen bond strength and (ii) the extent to which electrons left behind by the departing oxygen delocalize onto the oxygen sublattice. Design principles from our QM results may guide further development of perovskite-based MIEC materials for SOFC applications.

Structure and energy level alignment at the dye-electrode interface in p-type DSSCs: new hints on the role of anchoring modes from ab initio calculations.

p-type dye-sensitized solar cells (DSSCs) represent the complementary photocathodes to the well-studied n-type DSSCs (Grätzel cells), but their low performances have hindered the development of convenient tandem solar cells based on cost-effective n- and p-type DSSCs. Because of their low efficiencies, experimental investigations highlighted the role of hole-electron transport processes at the dye-electrode interface. However, the effects of the dye anchoring groups on interfacial electronic features are still unclear. We report here a first principles study of a benchmark p-type DSSC model, namely the widely used Coumarin-based dye C343 adsorbed on the p-NiO surface. Together with the original carboxylic acid, we test the alternative phosphonic acid as the anchoring group. We investigate binding energies, structural features and electronic energy level alignments: our results highlight that these properties are highly sensitive to the binding modes. In particular, both the chemical nature of the anchoring group and the binding mode strongly affect the thermodynamic driving force for the dye-electrode hole injection process. From analysis of the electronic densities, we find that favorable driving forces are correlated with small values of the interfacial electrostatic dipole that is formed upon dye adsorption. From our results, we derive new hints for improving open circuit potential and the hole injection process in p-type DSSCs based on NiO electrodes.