Search

Mixed-cation metal halide perovskites have shown remarkable progress in photovoltaic applications with high power conversion efficiencies. However, to achieve large-scale deployment of this technology, efficiencies must be complemented by long-term durability. The latter is limited by external factors, such as exposure to humidity and air, which lead to the rapid degradation of the perovskite materials and devices. In this work, we study the mechanisms causing Cs and formamidinium (FA)-based halide perovskite phase transformations and stabilization during moisture and air exposure. We use in situ X-ray scattering, X-ray photoelectron spectroscopy, and first-principles calculations to study these chemical interactions and their effects on structure. We unravel a surface reaction pathway involving the dissolution of FAI by water and iodide oxidation by oxygen, driving the Cs/FA ratio into thermodynamically unstable regions, leading to undesirable phase transformations. This work demonstrates the interplay of bulk phase transformations with surface chemical reactions, providing a detailed understanding of the degradation mechanism and strategies for designing durable and efficient perovskite materials.

The development of broadband emitters based on metal halide perovskites (MHPs) requires the elucidation of structure-emission property correlations. Herein, we report a combined experimental and theoretical study on a series of novel low-dimensional lead chloride perovskites, including ditopic aromatic cations. Synthesized lead chloride perovskites and their bromide analogues show both narrow and broad photoluminescence emission properties as a function of their cation and halide nature. Structural analysis shows a correlation between the rigidity of the ditopic cations and the lead halide octahedral distortions. Density functional theory calculations reveal, in turn, the pivotal role of octahedral distortions in the formation of self-trapped excitons, which are responsible for the insurgence of broad emission and large Stokes shifts together with a contribution of halide vacancies. For the considered MHP series, the use of conventional octahedral distortion parameters allows us to nicely describe the trend of emission properties, thus providing a solid guide for further materials design.

The outbreak of viral infectious diseases urges airborne droplet and surface disinfection strategies, which may rely on photocatalytic semiconductors. A lipid bilayer membrane generally encloses coronaviruses and promotes the anchoring on the semiconductor surface, where, upon photon absorption, electron-hole pairs are produced, which can react with adsorbed oxygen-containing species and lead to the formation of reactive oxygen species (ROSs). The photogenerated ROSs may support the disruptive oxidation of the lipidic membrane and pathogen death. Density functional theory calculations are employed to investigate adsorption modes, energetics, and electronic structure of a reference phospholipid on anatase TiO 2 nanoparticles. The phospholipid covalently bound on TiO 2 , engaging a stronger adsorption on the (101) than on the (001) surface. The energetically most stable structure involves the formation of four covalent bonds through phosphate and carbonyl oxygen atoms. The adsorbates show a reduction of the band gap compared with standalone TiO 2 , suggesting a significant interfacial coupling.

The novel Zr IV -based perfluorinated metal-organic framework (PF-MOF) [Zr 6 O 4 (OH) 4 ( TFS ) 6 ] ( ZrTFS ) was prepared under solvent-free conditions using the commercially available tetrafluorosuccinic acid ( H 2 TFS ) as a bridging ditopic linker. Since H 2 TFS can be seen as the fully aliphatic and perfluorinated C 4 analogue of fumaric acid, ZrTFS was found to be isoreticular to zirconium fumarate ( MOF-801 ). The structure of ZrTFS was solved and refined from X-ray powder diffraction data. Despite this analogy, the gas adsorption capacity of ZrTFS is much lower than that of MOF-801 ; in the former, the presence of bulky fluorine atoms causes a considerable window size reduction. To have PF-MOFs with more accessible porosity, postsynthetic exchange (PSE) reactions on (defective) MOF-801 suspended in H 2 TFS aqueous solutions were carried out. Despite the different H 2 TFS concentrations used in the PSE process, the exchanges yielded two mixed-linker materials of similar minimal formulae [Zr 6 O 4 (μ 3 -OH) 4 (μ 1 -OH) 2.08 (H 2 O) 2.08 ( FUM ) 4.04 ( HTFS ) 1.84 ] ( PF-MOF1 ) and [Zr 6 O 4 (μ 3 -OH) 4 (μ 1 -OH) 1.83 (H 2 O) 1.83 ( FUM ) 4.04 ( HTFS ) 2.09 ] ( PF-MOF2 ) ( FUM 2- = fumarate), where the perfluorinated linker was found to fully replace the capping acetate in the defective sites of pristine MOF-801 . CO 2 and N 2 adsorption isotherms collected on all samples reveal that both CO 2 thermodynamic affinity (isosteric heat of adsorption at zero coverage, Q st ) and CO 2 /N 2 adsorption selectivity increase with the amount of incorporated TFS 2- , reaching the maximum values of 30 kJ mol -1 and 41 (IAST), respectively, in PF-MOF2 . This confirms the beneficial effect coming from the introduction of fluorinated linkers in MOFs on their CO 2 adsorption ability. Finally, solid-state density functional theory calculations were carried out to cast light on the structural features and on the thermodynamics of CO 2 adsorption in MOF-801 and ZrTFS . Due to the difficulties in modeling a defective MOF, an intermediate structure containing both linkers in the framework was also designed. In this structure, the preferential CO 2 adsorption site is the tetrahedral pore in the "UiO-66-like" structure. The extra energy stabilization stems from a hydrogen bond interaction between CO 2 and a hydroxyl group on the inorganic cluster.

Tin-halide perovskites (THPs) have emerged as promising lead-free perovskites for photovoltaics and photocatalysis applications but still fall short in terms of stability and efficiency with respect to their lead-based counterpart. A detailed understanding of the degradation mechanism of THPs in a water environment is missing. This Letter presents ab initio molecular dynamics (AIMD) simulations to unravel atomistic details of THP/water interfaces comparing methylammonium tin iodide, MASnI 3 , with the lead-based MAPbI 3 . Our results reveal facile solvation of surface tin-iodine bonds in MASnI 3 , while MAPbI 3 remains more robust to degradation despite a larger amount of adsorbed water molecules. Additional AIMD simulations on dimethylammonium tin bromide, DMASnBr 3 , investigate the origins of their unprecedented water stability. Our results indicate the presence of amorphous surface layers of hydrated zero-dimensional SnBr 3 complexes which may protect the inner structure from degradation and explain their success as photocatalysts. We believe that the atomistic details of the mechanisms affecting THP (in-)stability may inspire new strategies to stabilize THPs.

The synthesis and characterization of a family of indene-C 60 adducts obtained via Diels-Alder cycloaddition [4 + 2] are reported. The new C 60 derivatives include indenes with a variety of functional groups. These adducts show lowest unoccupied molecular orbital energy levels to be at the right position to consider these compounds as electron-transporting materials for planar heterojunction perovskite solar cells. Selected derivatives were applied into inverted (p-i-n configuration) perovskite device architectures, fabricated on flexible polymer substrates, with large active areas (1 cm 2 ). The highest power conversion efficiency, reaching 13.61%, was obtained for the 6'-acetamido-1',4'-dihydro-naphtho[2',3':1,2][5,6]fullerene-C 60 ( NHAc-ICMA ). Spectroscopic characterization was applied to visualize possible passivation effects of the perovskite's surface induced by these adducts., Competing Interests: The authors declare no competing financial interest., (© 2021 American Chemical Society.)

First-principles molecular dynamics (FPMD) represents a valuable tool to probe dynamical properties of metal-halide perovskites (MHPs) which are key to their success in optoelectronic devices. Most FPMD studies rely on generalized gradient approximation (GGA) functionals for computational efficiency matters, while hybrid functionals, although computationally demanding, are usually needed to accurately describe structural and electronic properties of MHPs. This Letter reports FPMD simulations on CsPbI 3 based on the hybrid PBE0 functional. Our results demonstrate that PBE0 leads to lattice parameters and phonon modes in excellent agreement with experimental data, while GGA results overestimate the lattice parameter and the electronic band gap and underestimate the phonon energies. Our FPMD results also shed light on anharmonic effects and double-well instabilities in the octahedral tilting, highlighting a lowered free energy barrier for PBE0 and farther separated potential wells. Our results suggest that hybrid functionals are required to accurately describe crystal structure, lattice dynamics, and anharmonicity in MHPs.

We introduce a method for constructing localized excitations and simulating the real time dynamics of excitons at the Many-Body Perturbation Theory Bethe-Salpeter Equation level. We track, on the femto-seconds scale, electron injection from a photoexcited dye into a semiconducting slab. From the time-dependent many-body wave function we compute the spatial evolution of the electron and of the hole; full electron injection is attained within 5 fs. Time-resolved analysis of the electron density and electron-hole interaction energy hints at a two-step charge transfer mechanism through an intermediary partially injected state. We adopt the Von-Neumann entropy for analyzing how the electron and hole entangle. We find that the excitation of the dye-semiconductor model may be represented by a four-level system and register a decrease in entanglement upon electron injection. At full injection, the electron and the hole exhibit only a small degree of entanglement indicative of pure electron and hole states.

Mn-doped lead halide perovskites exhibit long-lived dopant luminescence and enhanced host excitonic quantum yield. The contention between energy and charge transfer in sensitizing dopant luminescence in Mn-doped perovskites is investigated by state-of-the-art DFT calculations on APbX 3 perovskites (X = Cl, Br, and I). We quantitatively simulate the electronic structure of Mn-doped perovskites in various charge and spin states, providing a structural/mechanistic analysis of Mn sensitization as a function of the perovskite composition. Our analysis supports both energy- and charge-transfer mechanisms, with the latter probably preferred in Mn:CsPbCl 3 due to small energy barriers and avoidance of spin and orbital restrictions. An essential factor determining the dopant luminescence quantum yield in the case of charge transfer is the energetics of intermediate oxidized species, while bandgap resonance can well explain energy transfer. Both aspects are mediated by perovskite host band edge energetics, which is tuned in turn by the nature of the halide X., Competing Interests: The authors declare no competing financial interest., (© 2021 American Chemical Society.)

Low-dimensional metal halide perovskites are being intensively investigated because of their higher stability and chemical versatility in comparison to their 3D counterparts. Unfortunately, this comes at the expense of the electronic and charge transport properties, limited by the reduced perovskite dimensionality. Cation engineering can be envisaged as a solution to tune and possibly further improve the material's optoelectronic properties. In this work, we screen and design new electronically active A-site cations that can promote charge transport across the inorganic layers. We show that hybridization of the valence band electronic states of the perovskite inorganic sublattice and the highest occupied molecular orbitals of the A-site organic cations can be tuned to exhibit a variety of optoelectronic properties. A significant interplay of A-cation size, electronic structure, and steric constraints is revealed, suggesting intriguing means of further tuning the 2D perovskite electronic structure toward achieving stable and efficient solar cell devices.

Water is generally considered an enemy of metal halide perovskites, being responsible for their rapid degradation and, consequently, undermining the long-term stability of perovskite-based solar cells. However, beneficial effects of liquid water have been surprisingly observed, and synthetic routes including water treatments have shown to improve the quality of perovskite films. This suggests that the interactions of water with perovskites and their precursors are far from being completely understood, as water appears to play a puzzling dual role in perovskite precursor solutions. In this context, studying the basic interactions between perovskite precursors in the aqueous environment can provide a deeper comprehension of this conundrum. In this context, it is fundamental to understand how water impacts the chemistry of iodoplumbate perovskite precursor species, PbI x 2- x . Here, we investigate the chemistry of these complexes using a combined experimental and theoretical strategy to unveil their peculiar structural and optical properties and eventually to assign the species present in the solution. Our study indicates that iodide-rich iodoplumbates, which are generally key to the formation of lead halide perovskites, are not easily formed in aqueous solutions because of the competition between iodide and solvent molecules in coordinating Pb 2+ ions, explaining the difficulty of depositing lead iodide perovskites from aqueous solutions. We postulate that the beneficial effect of water when used as an additive is then motivated by its behavior being similar to high coordinative polar aprotic solvents usually employed as additives in one-step perovskite depositions.

We employ a fine-tuned theoretical framework, combining ab initio molecular dynamics (AIMD), density functional theory (DFT), and time-dependent (TD) DFT methods, to investigate the interactions and optical properties of the iodoplumbates within the low coordinative γ-butyrolactone (GBL) solvent environment, widely employed in the perovskite synthesis. We uncover the extent of GBL coordination to PbI 2 investigating its relation to the solvated PbI 2 optical properties. The employed approach has been further validated by comparison with the experimental UV-vis absorption spectrum of PbI 2 in GBL solvent. A comparison with other solvents, commonly employed in the perovskite synthesis, such as N , N -dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) is also reported. The methodology developed in this work can be reasonably extended to the investigation of similar systems.

Metal halide perovskites exhibit impressive optoelectronic properties with applications in solar cells and light-emitting diodes. Co-doping the high-band gap CsPbCl 3 perovskite with Bi and Mn enhances both material stability and luminescence, providing emission on a wide spectral range. To discuss the role of Bi 3+ and Mn 2+ dopants in tuning the CsPbCl 3 perovskite energy levels and their involvement in carrier trapping, we report state-of-the-art hybrid density functional theory calculations, including spin-orbit coupling. We show that co-doping the perovskite with Bi and Mn delivers essentially the sum of the electronic properties of the single dopants, with no significant interaction or the preferential mutual location of them. Furthermore, we identify the structural features and energetics of transitions of electrons trapped at Bi and holes trapped at Mn dopant ions, respectively, and discuss their possible role in determining the optical properties of the co-doped perovskite.

Metal halide perovskite quantum wells (PQWs) are quantum and dielectrically confined materials exhibiting strongly bound excitons. The exciton transition dipole moment dictates absorption strength and influences interwell coupling in dipole-mediated energy transfer, a process that influences the performance of PQW optoelectronic devices. Here we use transient reflectance spectroscopy with circularly polarized laser pulses to investigate the optical Stark effect in dimensionally pure single crystals of n = 1, 2, and 3 Ruddlesden-Popper PQWs. From these measurements, we extract in-plane transition dipole moments of 11.1 (±0.4), 9.6 (±0.6) and 13.0 (±0.8) D for n = 1, 2 and 3, respectively. We corroborate our experimental results with density functional and many-body perturbation theory calculations, finding that the nature of band edge orbitals and exciton wave function delocalization depends on the PQW "odd-even" symmetry. This accounts for the nonmonotonic relationship between transition dipole moment and PQW dimensionality in the n = 1-3 range.

Halide perovskites are a strong candidate for the next generation of photovoltaics. Chemical doping of halide perovskites is an established strategy to prepare the highest efficiency and most stable perovskite-based solar cells. In this study, we unveil the doping mechanism of halide perovskites using a series of alkaline earth metals. We find that low doping levels enable the incorporation of the dopant within the perovskite lattice, whereas high doping concentrations induce surface segregation. The threshold from low to high doping regime correlates to the size of the doping element. We show that the low doping regime results in a more n-type material, while the high doping regime induces a less n-type doping character. Our work provides a comprehensive picture of the unique doping mechanism of halide perovskites, which differs from classical semiconductors. We proved the effectiveness of the low doping regime for the first time, demonstrating highly efficient methylammonium lead iodide based solar cells in both n-i-p and p-i-n architectures.

Here we report on the first structural and optical high-pressure investigation of MASnBr 3 (MA = [CH 3 NH 3 ] + ) and CsSnBr 3 halide perovskites. A massive red shift of 0.4 eV for MASnBr 3 and 0.2 eV for CsSnBr 3 is observed within 1.3 to 1.5 GPa from absorption spectroscopy, followed by a huge blue shift of 0.3 and 0.5 eV, respectively. Synchrotron powder diffraction allowed us to correlate the upturn in the optical properties trend (onset of blue shift) with structural phase transitions from cubic to orthorhombic in MASnBr 3 and from tetragonal to monoclinic for CsSnBr 3 . Density functional theory calculations indicate a different underlying mechanism affecting the band gap evolution with pressure, a key role of metal-halide bond lengths for CsSnBr 3 and cation orientation for MASnBr 3 , thus showing the impact of a different A-cation on the pressure response. Finally, the investigated phases, differently from the analogous Pb-based counterparts, are robust against amorphization showing defined diffraction up to the maximum pressure used in the experiments.

A near solvent-free synthetic route for Ce-UiO-66 metal-organic frameworks (MOFs) is presented. The MOFs are obtained by energetically grinding the reagents, cerium ammonium nitrate (CAN) and the carboxylic linkers, in a mortar for a few minutes with the addition of a small amount of acetic acid (AcOH) as a modulator (8.75 equiv, 0.5 mL). The slurry is then transferred into a 2 mL vial and heated at 120 °C for 1 day. The MOFs have been characterized for their composition, crystallinity, and porosity and employed as heterogeneous catalysts for the photo-oxidation reaction of substituted benzylic alcohols to benzaldaldehydes under near-ultraviolet light irradiation. The catalytic performances, such as selectivity, conversion, and kinetics, exceed those of similar systems studied by chemical oxidation using similar Ce-MOFs as a catalyst. Moreover, the MOFs were found to be reusable up to three cycles without loss of activity. Density functional theory (DFT) calculations were used to fully describe the electronic structure of the best performing MOFs and to provide useful information on the catalytic activity experimentally observed.

Recently, several studies have investigated dielectric properties as a possible origin of the exceptional optoelectronic properties of metal halide perovskites (MHPs). In this study we investigated the temperature-dependent dielectric behavior of different MHP films at different frequencies. In the gigahertz regime, dielectric losses in methylammonium-based samples are dominated by the rotational dynamics of the organic cation. Upon increasing the temperature from 160 to 300 K, the rotational relaxation time, τ, decreases from 400 (200) to 6 (1) ps for MAPb-I 3 (-Br 3 ). By contrast, we found negligible temperature-dependent variations in τ for a mixed cation/mixed halide FA 0.85 MA 0.15 Pb(I 0.85 Br 0.15 ) 3 . From temperature-dependent time-resolved microwave conductance measurements we conclude that the dipolar reorientation of the MA cation does not affect charge carrier mobility and lifetime in MHPs. Therefore, charge carriers do not feel the relatively slow-moving MA cations, despite their great impact on the dielectric constants.

The performance of lead-halide perovskites in optoelectronic devices is due to a unique combination of factors, including highly efficient generation, transport, and collection of photogenerated charge carriers. The mechanism behind efficient charge generation in lead-halide perovskites is still largely unknown. Here, we investigate the factors that influence the exciton binding energy (E b ) in a series of metal-halide perovskites using accurate first-principles calculations based on solution of the Bethe-Salpeter equation, coupled to ab initio molecular dynamics simulations. We find that E b is strongly modulated by screening from low-energy phonons, which account for a factor ∼2 E b reduction, while dynamic disorder and rotational motion of the organic cations play a minor role. We calculate E b = 15 meV for MAPbI 3 , in excellent agreement with recent experimental estimates. We then explore how different material combinations (e.g., replacing Pb → Pb:Sn→ Sn; and MA → FA → Cs) may lead to different E b values and highlight the mechanisms underlying E b tuning.

Aldehyde oxidase (AOX) is a molibdo-flavoenzyme that has raised great interest in recent years, since its contribution in xenobiotic metabolism has not always been identified before clinical trials, with consequent negative effects on the fate of new potential drugs. The fundamental role of AOX in metabolizing xenobiotics is also due to the attempt of medicinal chemists to stabilize candidates toward cytochrome P450 activity, which increases the risk for new compounds to be susceptible to AOX nucleophile attack. Therefore, novel strategies to predict the potential liability of new entities toward the AOX enzyme are urgently needed to increase effectiveness, reduce costs, and prioritize experimental studies. In the present work, we present the most up-to-date computational method to predict liability toward human AOX (hAOX), for applications in drug design and pharmacokinetic optimization. The method was developed using a large data set of homogeneous experimental data, which is also disclosed as Supporting Information .

Head and neck squamous cell carcinomas (HNSCC) are a diverse group of tumors with high morbidity and mortality that have remained mostly unchanged over the past decades. The epidermal growth factor receptor (EGFR) is often overexpressed and activated in these tumors and strongly contributes to their pathogenesis. Still, EGFR-targeted therapies such as monoclonal antibodies and kinase inhibitors have demonstrated only limited improvements in the clinical outcome of this disease. Here, we take advantage of the extraordinary affinity of EGF for its cognate receptor to specifically target magnetite-containing nanoparticles to HNSCC cells and mediate, in vitro , their cellular upload. On the basis of this, we show efficient accumulation, in vivo , of such nanoparticles in subcutaneous xenograft tumor tissues in sufficient amounts to be able to mediate visualization by magnetic resonance imaging. Overall, our EGF-coated nanosystem may warrant, in the near future, novel and very efficient theranostic approaches to HNSCC., Competing Interests: The authors declare no competing financial interest.

The long diffusion length of charge carriers in the CH 3 NH 3 PbI 3 perovskite is one of the most relevant properties for explaining the high photovoltaic efficiency of perovskite solar cells. As a possible mechanism for the large diffusion length of electrons and holes, several authors suggested a reduced coulomb attraction of the carriers due to the formation of polarons. Here we performed continuous wave far-infrared photoinduced absorption (PIA) experiments on CH 3 NH 3 PbI 3 ; spectral changes are associated with local deformation of the lattice around the photogenerated long-lived charges, a typical signature of photoinduced polarons. Ab initio calculations show confinement of charge carriers at the interface between structural domains characterized by a different tilting of the PbI 6 octahedra. The differential IR spectrum between unperturbed and perturbed simulated structures shows a close pattern to the experimental PIA. Positive and negative charges are confined in different varieties of the perovskites coherent with the low recombination and long diffusion length of photogenerated carriers.

A Rashba/Dresselhaus band splitting has been recently measured in organohalide perovskites and invoked in various experiments as a possible cause for the reduced electron-hole recombination rates observed in this class of materials. In this Perspective, we discuss the interplay of electronic and nuclear degrees of freedom in defining such an effect in realistic methylammonium lead iodide (MAPbI 3 ) models. We distinguish between bulk and surface effects and find that, while a spatially local (in time and space) effect may be at work in the bulk, a "static" band-splitting effect is found at surfaces due to structural distortion. The proposed surface effect is consistent with the low surface recombination reported for MAPbI 3 single crystals and might contribute to the success of organohalide perovskites.

We have developed a colloidal synthesis of nearly monodisperse nanocrystals of pure Cs 4 PbX 6 (X = Cl, Br, I) and their mixed halide compositions with sizes ranging from 9 to 37 nm. The optical absorption spectra of these nanocrystals display a sharp, high energy peak due to transitions between states localized in individual PbX 6 4- octahedra. These spectral features are insensitive to the size of the particles and in agreement with the features of the corresponding bulk materials. Samples with mixed halide composition exhibit absorption bands that are intermediate in spectral position between those of the pure halide compounds. Furthermore, the absorption bands of intermediate compositions broaden due to the different possible combinations of halide coordination around the Pb 2+ ions. Both observations are supportive of the fact that the [PbX 6 ] 4- octahedra are electronically decoupled in these systems. Because of the large band gap of Cs 4 PbX 6 (>3.2 eV), no excitonic emission in the visible range was observed. The Cs 4 PbBr 6 nanocrystals can be converted into green fluorescent CsPbBr 3 nanocrystals by their reaction with an excess of PbBr 2 with preservation of size and size distributions. The insertion of PbX 2 into Cs 4 PbX 6 provides a means of accessing CsPbX 3 nanocrystals in a wide variety of sizes, shapes, and compositions, an important aspect for the development of precisely tuned perovskite nanocrystal inks.

Only a selected group of two-dimensional (2D) lead-halide perovskites shows a peculiar broad-band photoluminescence. Here we show that the structural distortions of the perovskite lattice can determine the defectivity of the material by modulating the defect formation energies. By selecting and comparing two archetype systems, namely, (NBT) 2 PbI 4 and (EDBE)PbI 4 perovskites (NBT = n-butylammonium and EDBE = 2,2-(ethylenedioxy)bis(ethylammonium)), we find that only the latter, subject to larger deformation of the Pb-X bond length and X-Pb-X bond angles, sees the formation of V F color centers whose radiative decay ultimately leads to broadened PL. These findings highlight the importance of structural engineering to control the optoelectronic properties of this class of soft materials.

The role of chlorine doping in CH 3 NH 3 PbI 3 represents an important open issue in the use of hybrid perovskites for photovoltaic applications. In particular, even if a positive role of chlorine doping on perovskite film formation and on material morphology has been demonstrated, an inherent positive effect on the electronic and photovoltaic properties cannot be excluded. Here we carried out periodic density functional theory and Car-Parrinello molecular dynamics simulations, going down to ∼1% doping, to investigate the effect of chlorine on CH 3 NH 3 PbI 3 . We found that such a small doping has important effects on the dynamics of the crystalline structure, both with respect to the inorganic framework and with respect to the cation libration motion. Together, we observe a dynamic spatial localization of the valence and conduction states in separated spatial material regions, which takes place in the 10 -1 ps time scale and which could be the key to ease of exciton dissociation and, likely, to small charge recombination in hybrid perovskites. Moreover, such localization is enhanced by chlorine doping, demonstrating an inherent positive role of chlorine doping on the electronic properties of this class of materials.

Compositional engineering of a mixed cation/mixed halide perovskite in the form of (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 is one of the most effective strategies to obtain record-efficiency perovskite solar cells. However, the perovskite self-organization upon crystallization and the final elemental distribution, which are paramount for device optimization, are still poorly understood. Here we map the nanoscale charge carrier and elemental distribution of mixed perovskite films yielding 20% efficient devices. Combining a novel in-house-developed high-resolution helium ion microscope coupled with a secondary ion mass spectrometer (HIM-SIMS) with Kelvin probe force microscopy (KPFM), we demonstrate that part of the mixed perovskite film intrinsically segregates into iodide-rich perovskite nanodomains on a length scale of up to a few hundred nanometers. Thus, the homogeneity of the film is disrupted, leading to a variation in the optical properties at the micrometer scale. Our results provide unprecedented understanding of the nanoscale perovskite composition.

The presence of a Rashba band-splitting mechanism mediated by spin-orbit coupling and breaking of inversion symmetry has been suggested as a possible cause for the reduced recombination rates observed in organohalide perovskites. Here, we investigate the interplay of electronic and nuclear degrees of freedom in defining the Rashba splitting in realistic MAPbI3 models. Our simulations disclose a "dynamical Rashba effect", allowing for a quantification of its magnitude under thermal conditions. We find that even in globally centrosymmetric structures the dynamics of the coupled inorganic-organic degrees of freedom give rise to a spatially local Rashba effect which fluctuates on the subpicosecond time scale typical of the methylammonium cation dynamics. This effect is progressively quenched in globally centrosymmetric structures, likely representing the MAPbI3 perovskite at room temperature, on increasing the probed spatial scale up to 32 MAPbI3 units (∼3 nm size) because of the incoherent nuclear thermal motion mediated by the disorder of the organic cations.

We report a colloidal synthesis approach to CsPbBr3 nanoplatelets (NPLs). The nucleation and growth of the platelets, which takes place at room temperature, is triggered by the injection of acetone in a mixture of precursors that would remain unreactive otherwise. The low growth temperature enables the control of the plate thickness, which can be precisely tuned from 3 to 5 monolayers. The strong two-dimensional confinement of the carriers at such small vertical sizes is responsible for a narrow PL, strong excitonic absorption, and a blue shift of the optical band gap by more than 0.47 eV compared to that of bulk CsPbBr3. We also show that the composition of the NPLs can be varied all the way to CsPbBr3 or CsPbI3 by anion exchange, with preservation of the size and shape of the starting particles. The blue fluorescent CsPbCl3 NPLs represent a new member of the scarcely populated group of blue-emitting colloidal nanocrystals. The exciton dynamics were found to be independent of the extent of 2D confinement in these platelets, and this was supported by band structure calculations.

The cobalt cubium Co4O4(OAc)4(py)4(ClO4) (1A(+)) containing the mixed valence [Co4O4](5+) core is shown by multiple spectroscopic methods to react with hydroxide (OH(-)) but not with water molecules to produce O2. The yield of reaction products is stoichiometric (>99.5%): 41A(+) + 4OH(-) → O2 + 2H2O + 41A. By contrast, the structurally homologous cubium Co4O4(trans-OAc)2(bpy)4(ClO4)3, 1B(ClO4)3, produces no O2. EPR/NMR spectroscopies show clean conversion to cubane 1A during O2 evolution with no Co(2+) or Co3O4 side products. Mass spectrometry of the reaction between isotopically labeled μ-(16)O(bridging-oxo) 1A(+) and (18)O-bicarbonate/water shows (1) no exchange of (18)O into the bridging oxos of 1A(+), and (2) (36)O2 is the major product, thus requiring two OH(-) in the reactive intermediate. DFT calculations of solvated intermediates suggest that addition of two OH(-) to 1A(+) via OH(-) insertion into Co-OAc bonds is energetically favored, followed by outer-sphere oxidation to intermediate [1A(OH)2](0). The absence of O2 production by cubium 1B(3+) indicates the reactive intermediate derived from 1A(+) requires gem-1,1-dihydoxo stereochemistry to perform O-O bond formation. Outer-sphere oxidation of this intermediate by 2 equiv of 1A(+) accounts for the final stoichiometry. Collectively, these results and recent literature (Faraday Discuss., doi:10.1039/C5FD00076A and J. Am. Chem. Soc. 2015, 137, 12865-12872) validate the [Co4O4](4+/5+) cubane core as an intrinsic catalyst for oxidation of hydroxide by an inner-sphere mechanism.

In the pursuit to better understand the mechanisms of perovskite solar cells we performed Raman and photoluminescence measurements of free-standing CH3NH3PbI3 films, comparing dark with working conditions. The films, grown on a glass substrate and sealed by a thin glass coverslip, were measured subsequent to dark and white-light pretreatments. The extremely slow changes we observe in both the Raman and photoluminescence cannot be regarded as electronic processes, which are much faster. Thus, the most probable explanation is of slow photoinduced structural changes. The CH3NH3PbI3 transformation between the dark and the light structures is reversible, with faster rates for the changes under illumination. The results seem to clarify several common observations associated with solar cell mechanisms, like performance improvement under light soaking. More important is the call for solar-cell-related investigation of CH3NH3PbI3 to take the photoinduced structural changes into consideration when measuring and interpreting the results.

The role of chloride in the MAPbI3-xClx perovskite is still limitedly understood, albeit subjected of much debate. Here, we present a combined angle-resolved X-ray photoelectron spectroscopy (AR-XPS) and first-principles DFT modeling to investigate the MAPbI3-xClx/TiO2 interface. AR-XPS analyses carried out on ad hoc designed bilayers of MAPbI3-xClx perovskite deposited onto a flat TiO2 substrate reveal that the chloride is preferentially located in close proximity to the perovskite/TiO2 interface. DFT calculations indicate the preferential location of chloride at the TiO2 interface compared to the bulk perovskite due to an increased chloride-TiO2 surface affinity. Furthermore, our calculations clearly demonstrate an interfacial chloride-induced band bending, creating a directional "electron funnel" that may improve the charge collection efficiency of the device and possibly affecting also recombination pathways. Our findings represent a step forward to the rationalization of the peculiar properties of mixed halide perovskite, allowing one to further address material and device design issues.

Photoconductivity measurements of CH3NH3PbI3 deposited between two dielectric-protected Au electrodes show extremely slow response. The CH3NH3PbI3, bridging a gap of ∼2000 nm, was subjected to a DC bias and cycles of 5 min illumination and varying dark duration. The approach to steady -state photocurrent lasted tens of seconds with a strong dependence on the dark duration preceding the illumination. On the basis of DFT calculations, we propose that under light + bias the methylammonium ions are freed to rotate and align along the electric field, thus modifying the structure of the inorganic scaffold. While ions alignment is expected to be fast, the adjustment of the inorganic scaffold seems to last seconds as reflected in the extremely slow photoconductivity response. We propose that under working conditions a modified, photostable, perovskite structure is formed, depending on the bias and illumination parameters. Our findings seem to clarify the origin of the well-known hysteresis in perovskite solar cells.

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.

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.

We report the low-frequency resonant Raman spectrum of methylammonium lead-iodide, a prototypical perovskite for solar cells applications, on mesoporous Al2O3. The measured spectrum assignment is assisted by DFT simulations of the Raman spectra of suitable periodic and model systems. The bands at 62 and 94 cm(-1) are assigned respectively to the bending and to the stretching of the Pb-I bonds, and are thus diagnostic modes of the inorganic cage. We also assign the librations of the organic cations at 119 and 154 cm(-1). The broad, unstructured 200-400 cm(-1) features are assigned to the torsional mode of the methylammonium cations, which we propose as a marker of the orientational disorder of the material. Our study provides the basis to interpret the Raman spectra of organohalide perovskites, which may allow one to further understand the properties of this important class of materials in relation to their full exploitation in solar cells.

To unveil the mechanisms controlling photovoltaic conversion in high-performing perovskite-based mesostructured solar cells, we focus on the key role played by the mesoporous oxide/perovskite interface. We employ several spectroscopic techniques to design a complete scenario and corroborate our results with first principle density functional theory calculations. In particular Stark spectroscopy, a powerful tool allowing interface-sensitive analysis is employed to prove the existence of oriented permanent dipoles, consistent with the hypothesis of an ordered perovskite layer, close to the oxide surface. The existence of a structural order, promoted by specific local interactions, could be one of the decisive reasons for highly efficient carriers transport within perovskite films.

We report a combined experimental and computational investigation to understand the nature of the interactions between cobalt redox mediators and TiO(2) surfaces sensitized by ruthenium and organic dyes, and their impact on the performance of the corresponding dye-sensitized solar cells (DSSCs). We focus on different ruthenium dyes and fully organic dyes, to understand the dramatic loss of efficiency observed for the prototype Ru(II) N719 dye in conjunction with cobalt electrolytes. Both N719- and Z907-based DSSCs showed an increased lifetime in iodine-based electrolyte compared to the cobalt-based redox shuttle, while the organic D21L6 and D25L6 dyes, endowed with long alkoxy chains, show no significant change in the electron lifetime regardless of employed electrolyte and deliver a high photovoltaic efficiency of 6.5% with a cobalt electrolyte. Ab initio molecular dynamics simulations show the formation of a complex between the cobalt electrolyte and the surface-adsorbed ruthenium dye, which brings the [Co(bpy)(3)](3+) species into contact with the TiO(2) surface. This translates into a high probability of intercepting TiO(2)-injected electrons by the oxidized [Co(bpy)(3)](3+) species, lying close to the N719-sensitized TiO(2) surface. Investigation of the dye regeneration mechanism by the cobalt electrolyte in the Marcus theory framework led to substantially different reorganization energies for the high-spin (HS) and low-spin (LS) reaction pathways. Our calculated reorganization energies for the LS pathways are in excellent agreement with recent data for a series of cobalt complexes, lending support to the proposed regeneration pathway. Finally, we systematically investigate a series of Co(II)/Co(III) complexes to gauge the impact of ligand substitution and of metal coordination (tris-bidentate vs bis-tridentate) on the HS/LS energy difference and reorganization energies. Our results allow us to trace structure/property relations required for further development of cobalt electrolytes for DSSCs.

Solid-state dye-sensitized solar cells were fabricated using an organic dye, 2-cyanoacrylic acid-4-(bis-dimethylfluoreneaniline)dithiophene (JK2), which exhibits more than 1 V open-circuit potential (V(oc)). To scrutinize the origin of high voltage in these cells, transient V(oc) decay measurements and density functional theroy calculations of the interacting dye/semiconductor surface were performed. A negative conduction band shift was observed due to the favorable dipolar field exerted by the JK2 sensitizer to the TiO(2) surface, at variance with heteroleptic Ru(II)-dyes for which an opposite dipole effect was found, providing an increased V(oc).

The allylic amination of acetates and carbonates affords dehydro-beta-aminoesters, which are useful precursors of biologically active compounds. The uncatalyzed reaction proceeds via a S(N)2' mechanism. On the other hand, under palladium-catalyzed conditions, the reaction shows a strong solvent-dependent regiocontrol, affording exclusively one of the two possible regioisomers with complete transfer of chirality from the substrates to the products.

Zinc-promoted hydroxyallylation of alpha-amidoalkyl arylsulfones 4 using 3-bromo-propenyl methyl carbonate 5 proceeds smoothly in DMF at room temperature to afford high yields of differentially protected anti-1,2-amino alcohols 6.