Synthesis, Structure and (Photo)Catalytic Behavior of Ce-MOFs Containing Perfluoroalkylcarboxylate Linkers: Experimental and Theoretical Insights.

Cerium-based Metal-Organic frameworks (Ce-MOFs) are attracting increasing interest due to their similar structural features to zirconium MOFs. The redox behavior of Ce(III/IV) adds a range of properties to the compounds. Recently, perfluorinated linkers have been used in the synthesis of MOFs to introduce new characteristic into the structure. We report the synthesis and structural characterization of Ce(IV)-based MOFs constructed using two perfluorinated alkyl linkers. Their structure, based on hexanuclear Ce6O4(OH)4 12+ clusters linked to each other by the dicarboxylate ions, has been solved ab-initio from X-ray powder diffraction data and refined by the Rietveld method. The crystallization kinetics and the MOF formation mechanism was also invesitigated by Synchrotron radiation with XAS spectroscopies (EXAFS and XANES). The MOFs present the same fcu cubic topology as observed in MOF-801 and UiO-66, and they showed good stability in water at different pH conditions. The electronic structure of these MOFs has been studied by DFT calculations in order to obtain insights into the density of states structure of the reported compounds, resulting in band gaps in the range of 2.8-3.1 eV. Their catalytic properties were tested both thermally and under visible light irradiation for the degradation of methyl orange (MO) dye.

Two-Dimensional Moiré Polaronic Electron Crystals.

Two-dimensional moiré materials have emerged as the most versatile platform for realizing quantum phases of electrons. Here, we explore the stability origins of correlated states in WSe_{2}/WS_{2} moiré superlattices. We find that ultrafast electronic excitation leads to partial melting of the Mott states on timescales 5 times longer than predictions from the charge hopping integrals and that the melting rates are thermally activated, with activation energies of 18±3 and 13±2 meV for the one- and two-hole Mott states, respectively, suggesting significant electron-phonon coupling. A density functional theory calculation of the one-hole Mott state confirms polaron formation and yields a hole-polaron binding energy of 16 meV. These findings reveal a close interplay of electron-electron and electron-phonon interactions in stabilizing the polaronic Mott insulators at transition metal dichalcogenide moiré interfaces.

A universal ligand for lead coordination and tailored crystal growth in perovskite solar cells.

Chemical environment and precursor-coordinating molecular interactions within a perovskite precursor solution can lead to important implications in structural defects and crystallization kinetics of a perovskite film. Thus, the opto-electronic quality of such films can be boosted by carefully fine-tuning the coordination chemistry of perovskite precursors via controllable introduction of additives, capable of forming intermediate complexes. In this work, we employed a new type of ligand, namely 1-phenylguanidine (PGua), which coordinates strongly with the PbI2 complexes in the perovskite precursor, forming new intermediate species. These strong interactions effectively retard the perovskite crystallization process and form homogeneous films with enlarged grain sizes and reduced density of defects. In combination with an interfacial treatment, the resulted champion devices exhibit a 24.6% efficiency with outstanding operational stability. Unprecedently, PGua can be applied in various PSCs with different perovskite compositions and even in both configurations: n-i-p and p-i-n, highlighting the universality of this ligand.

Multifunctional sulfonium-based treatment for perovskite solar cells with less than 1% efficiency loss over 4,500-h operational stability tests.

The stabilization of grain boundaries and surfaces of the perovskite layer is critical to extend the durability of perovskite solar cells. Here we introduced a sulfonium-based molecule, dimethylphenethylsulfonium iodide (DMPESI), for the post-deposition treatment of formamidinium lead iodide perovskite films. The treated films show improved stability upon light soaking and remains in the black α phase after two years ageing under ambient condition without encapsulation. The DMPESI-treated perovskite solar cells show less than 1% performance loss after more than 4,500 h at maximum power point tracking, yielding a theoretical T80 of over nine years under continuous 1-sun illumination. The solar cells also display less than 5% power conversion efficiency drops under various ageing conditions, including 100 thermal cycles between 25 °C and 85 °C and an 1,050-h damp heat test.

Ligand-Induced Chirality in ClMBA2 SnI4 2D Perovskite.

Chiral perovskites possess a huge applicative potential in several areas of optoelectronics and spintronics. The development of novel lead-free perovskites with tunable properties is a key topic of current research. Herein, we report a novel lead-free chiral perovskite, namely (R/S-)ClMBA2 SnI4 (ClMBA=1-(4-chlorophenyl)ethanamine) and the corresponding racemic system. ClMBA2 SnI4 samples exhibit a low band gap (2.12 eV) together with broad emission extending in the red region of the spectrum (∼1.7 eV). Chirality transfer from the organic ligand induces chiroptical activity in the 465-530 nm range. Density functional theory calculations show a Rashba type band splitting for the chiral samples and no band splitting for the racemic isomer. Self-trapped exciton formation is at the origin of the large Stokes shift in the emission. Careful correlation with analogous lead and lead-free 2D chiral perovskites confirms the role of the symmetry-breaking distortions in the inorganic layers associated with the ligands as the source of the observed chiroptical properties providing also preliminary structure-property correlation in 2D chiral perovskites.

Synergistic Role of Water and Oxygen Leads to Degradation in Formamidinium-Based Halide Perovskites.

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.

Origin of Broad Emission Induced by Rigid Aromatic Ditopic Cations in Low-Dimensional Metal Halide Perovskites.

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.

Modeling the Interaction of Coronavirus Membrane Phospholipids with Photocatalitically Active Titanium Dioxide.

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 TiO2 nanoparticles. The phospholipid covalently bound on TiO2, 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 TiO2, suggesting a significant interfacial coupling.

Air- and water-stable and photocatalytically active germanium-based 2D perovskites by organic spacer engineering.

There is increasing interest in the role of metal halide perovskites for heterogeneous catalysis. Here, we report a Ge-based 2D perovskite material that shows intrinsic water stability realized through organic cation engineering. Incorporating 4-phenylbenzilammonium (PhBz) we demonstrate, by means of extended experimental and computational results, that PhBz2GeBr4 and PhBz2GeI4 can achieve relevant air and water stability. The creation of composites embedding graphitic carbon nitride (g-C3N4) allows a proof of concept for light-induced hydrogen evolution in an aqueous environment by 2D Ge-based perovskites thanks to the effective charge transfer at the heterojunction between the two semiconductors.

Increased CO2 Affinity and Adsorption Selectivity in MOF-801 Fluorinated Analogues.

The novel ZrIV-based perfluorinated metal-organic framework (PF-MOF) [Zr6O4(OH)4(TFS)6] (ZrTFS) was prepared under solvent-free conditions using the commercially available tetrafluorosuccinic acid (H2TFS) as a bridging ditopic linker. Since H2TFS can be seen as the fully aliphatic and perfluorinated C4 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 H2TFS aqueous solutions were carried out. Despite the different H2TFS concentrations used in the PSE process, the exchanges yielded two mixed-linker materials of similar minimal formulae [Zr6O4(µ3-OH)4(µ1-OH)2.08(H2O)2.08(FUM)4.04(HTFS)1.84] (PF-MOF1) and [Zr6O4(µ3-OH)4(µ1-OH)1.83(H2O)1.83(FUM)4.04(HTFS)2.09] (PF-MOF2) (FUM2- = fumarate), where the perfluorinated linker was found to fully replace the capping acetate in the defective sites of pristine MOF-801. CO2 and N2 adsorption isotherms collected on all samples reveal that both CO2 thermodynamic affinity (isosteric heat of adsorption at zero coverage, Qst) and CO2/N2 adsorption selectivity increase with the amount of incorporated TFS2-, 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 CO2 adsorption ability. Finally, solid-state density functional theory calculations were carried out to cast light on the structural features and on the thermodynamics of CO2 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 CO2 adsorption site is the tetrahedral pore in the "UiO-66-like" structure. The extra energy stabilization stems from a hydrogen bond interaction between CO2 and a hydroxyl group on the inorganic cluster.

Stability of Tin- versus Lead-Halide Perovskites: Ab Initio Molecular Dynamics Simulations of Perovskite/Water Interfaces.

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, MASnI3, with the lead-based MAPbI3. Our results reveal facile solvation of surface tin-iodine bonds in MASnI3, while MAPbI3 remains more robust to degradation despite a larger amount of adsorbed water molecules. Additional AIMD simulations on dimethylammonium tin bromide, DMASnBr3, investigate the origins of their unprecedented water stability. Our results indicate the presence of amorphous surface layers of hydrated zero-dimensional SnBr3 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.

First-Principles Molecular Dynamics in Metal-Halide Perovskites: Contrasting Generalized Gradient Approximation and Hybrid Functionals.

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 CsPbI3 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.

Tuning structural isomers of phenylenediammonium to afford efficient and stable perovskite solar cells and modules.

Organic halide salt passivation is considered to be an essential strategy to reduce defects in state-of-the-art perovskite solar cells (PSCs). This strategy, however, suffers from the inevitable formation of in-plane favored two-dimensional (2D) perovskite layers with impaired charge transport, especially under thermal conditions, impeding photovoltaic performance and device scale-up. To overcome this limitation, we studied the energy barrier of 2D perovskite formation from ortho-, meta- and para-isomers of (phenylene)di(ethylammonium) iodide (PDEAI2) that were designed for tailored defect passivation. Treatment with the most sterically hindered ortho-isomer not only prevents the formation of surficial 2D perovskite film, even at elevated temperatures, but also maximizes the passivation effect on both shallow- and deep-level defects. The ensuing PSCs achieve an efficiency of 23.9% with long-term operational stability (over 1000 h). Importantly, a record efficiency of 21.4% for the perovskite module with an active area of 26 cm2 was achieved.

Real Space-Real Time Evolution of Excitonic States Based on the Bethe-Salpeter Equation Method.

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.

Cation Engineering for Resonant Energy Level Alignment in Two-Dimensional Lead Halide Perovskites.

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.

Ligand-engineered bandgap stability in mixed-halide perovskite LEDs.

Lead halide perovskites are promising semiconductors for light-emitting applications because they exhibit bright, bandgap-tunable luminescence with high colour purity1,2. Photoluminescence quantum yields close to unity have been achieved for perovskite nanocrystals across a broad range of emission colours, and light-emitting diodes with external quantum efficiencies exceeding 20 per cent-approaching those of commercial organic light-emitting diodes-have been demonstrated in both the infrared and the green emission channels1,3,4. However, owing to the formation of lower-bandgap iodide-rich domains, efficient and colour-stable red electroluminescence from mixed-halide perovskites has not yet been realized5,6. Here we report the treatment of mixed-halide perovskite nanocrystals with multidentate ligands to suppress halide segregation under electroluminescent operation. We demonstrate colour-stable, red emission centred at 620 nanometres, with an electroluminescence external quantum efficiency of 20.3 per cent. We show that a key function of the ligand treatment is to 'clean' the nanocrystal surface through the removal of lead atoms. Density functional theory calculations reveal that the binding between the ligands and the nanocrystal surface suppresses the formation of iodine Frenkel defects, which in turn inhibits halide segregation. Our work exemplifies how the functionality of metal halide perovskites is extremely sensitive to the nature of the (nano)crystalline surface and presents a route through which to control the formation and migration of surface defects. This is critical to achieve bandgap stability for light emission and could also have a broader impact on other optoelectronic applications-such as photovoltaics-for which bandgap stability is required.

Decoding ultrafast polarization responses in lead halide perovskites by the two-dimensional optical Kerr effect.

The ultrafast polarization response to incident light and ensuing exciton/carrier generation are essential to outstanding optoelectronic properties of lead halide perovskites (LHPs). A large number of mechanistic studies in the LHP field to date have focused on contributions to polarizability from organic cations and the highly polarizable inorganic lattice. For a comprehensive understanding of the ultrafast polarization response, we must additionally account for the nearly instantaneous hyperpolarizability response to the propagating light field itself. While light propagation is pivotal to optoelectronics and photonics, little is known about this in LHPs in the vicinity of the bandgap where stimulated emission, polariton condensation, superfluorescence, and photon recycling may take place. Here we develop two-dimensional optical Kerr effect (2D-OKE) spectroscopy to energetically dissect broadband light propagation and dispersive nonlinear polarization responses in LHPs. In contrast to earlier interpretations, the below-bandgap OKE responses in both hybrid CH3NH3PbBr3 and all-inorganic CsPbBr3 perovskites are found to originate from strong hyperpolarizability and highly anisotropic dispersions. In both materials, the nonlinear mixing of anisotropically propagating light fields results in convoluted oscillatory polarization dynamics. Based on a four-wave mixing model, we quantitatively derive dispersion anisotropies, reproduce 2D-OKE frequency correlations, and establish polarization-dressed light propagation in single-crystal LHPs. Moreover, our findings highlight the importance of distinguishing the often-neglected anisotropic light propagation from underlying coherent quasiparticle responses in various forms of ultrafast spectroscopy.

Water-Stable DMASnBr3 Lead-Free Perovskite for Effective Solar-Driven Photocatalysis.

Water-stable metal halide perovskites could foster tremendous progresses in several research fields where their superior optical properties can make differences. In this work we report clear evidence of water stability in a lead-free metal halide perovskite, namely DMASnBr3 , obtained by means of diffraction, optical and X-ray photoelectron spectroscopy. Such unprecedented water-stability has been applied to promote photocatalysis in aqueous medium, in particular by devising a novel composite material by coupling DMASnBr3 to g-C3 N4 , taking advantage from the combination of their optimal photophysical properties. The prepared composites provide an impressive hydrogen evolution rate >1700 µmol g-1 h-1 generated by the synergistic activity of the two composite costituents. DFT calculations provide insight into this enhancement deriving it from the favorable alignment of interfacial energy levels of DMASnBr3 and g-C3 N4 . The demonstration of an efficient photocatalytic activity for a composite based on lead-free metal halide perovskite in water paves the way to a new class of light-driven catalysts working in aqueous environments.

Designing New Indene-Fullerene Derivatives as Electron-Transporting Materials for Flexible Perovskite Solar Cells.

The synthesis and characterization of a family of indene-C60 adducts obtained via Diels-Alder cycloaddition [4 + 2] are reported. The new C60 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 cm2). 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-C60 (NHAc-ICMA). Spectroscopic characterization was applied to visualize possible passivation effects of the perovskite's surface induced by these adducts.

Energy vs Charge Transfer in Manganese-Doped Lead Halide Perovskites.

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 APbX3 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:CsPbCl3 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.