材料科学

Accurate optical constants are essential for modelling light propagation, absorption, and ultimately photovoltaic performance in state of the art perovskite solar cells and is especially important for multiple junction or tandem cells. However, available datasets for metal halide perovskites remain sparse, inconsistent in quality, and often suffer from unphysical sub bandgap extinction caused by surface roughness and limitations of conventional ellipsometry fits. Here, we present a comprehensive library of complex refractive indices (n,k) for a technologically relevant set of FA based lead perovskites, spanning bromide compositions from 0 to 100 percent, and mixed Pb Sn perovskites with Sn fractions from 0 to 60 %. Using state of the art fabrication protocols that yield high quality films, we combine variable angle spectroscopic ellipsometry measurements with highly sensitive sub bandgap probes, including photothermal deflection spectroscopy for neat lead based perovskites and Fourier transform photocurrent spectroscopy for Pb Sn alloys, to reconstruct fully zeroed dielectric functions across and below the band edge. The measured data are then stitched and recalculated via a Kramer

Emissive metal halide perovskites (MHPs) have emerged as excellent candidates for next-generation optoelectronics due to their sharp color purity, inexpensive processing, and bandgap tunability. However, the development of violet and ultraviolet light-emitting MHPs has lagged behind due to challenges related to material and device stability, charge carrier transport, tunability into the ultraviolet spectrum, toxicity, and scalability. Here, we review the progress of both violet and ultraviolet MHP nanomaterials and light-emitting diodes, including materials synthesis and device fabrication across various crystal structures and dimensions (e.g., bulk thin films, 2D thin films, nanoplatelets, colloidal nanocrystals, and more) as well as lead-free platforms (e.g., rare-earth metal halide perovskites). By highlighting several pathways to continue the development of violet and ultraviolet light-emitting MHPs while also proposing tactics to overcome their outstanding challenges, we demonstrate the potential of state-of-the-art violet and ultraviolet MHP materials and devices for important applications in public health, 3D printing, nanofabrication, and more.

Monte Carlo simulations are performed on the three-dimensional (3D) Ising model with the 2-1-4 layered perovskite structure as a minimal model for checkerboard charge ordering phenomena in layered perovskite oxides. Due to the interlayer frustration, only 2D long-range order emerges with a finite correlation length along the c axis. Critical exponents of the transition change continuously as a function of the interlayer coupling constant. The interlayer long-range Coulomb interaction decays exponentially and is negligible even between the second-neighbor layers. Instead, monoclinic distortion of a tetragonal unit cell lifts the macroscopic degeneracy to induce a 3D charge ordering. The dimensionality of the charge order in La$_{0.5}$Sr$_{1.5}$MnO$_4$ is discussed from this viewpoint.

Two-dimensional (2D) ferroelectrics and multiferroics have attracted considerable scientific and technological interest in recent years due to the increasing demands for miniaturization and low energy consumption of electronic devices. At present, the research on 2D ferroelectrics and multiferroics is still focused on van der Waals materials, while the known bulk ferroelectric and multiferroic materials are mostly found in perovskite systems. The ability to prepare and transfer 2D perovskite oxides has provided unprecedented opportunities for developing ferroelectrics and multiferroics based on 2D perovskites. In this Perspective, we review the research progress on 2D ferroelectrics and multiferroics in inorganic perovskites in terms of different ferroelectric and magnetoelectric coupling mechanisms. The improper ferroelectricity and novel magnetoelectric coupling mechanisms discovered in 2D perovskites are emphasized, and then, the main challenges and future development direction are put forward.

5d transition-metal-based oxides display emergent phenomena due to the competition between the relevant energy scales of the correlation, bandwidth, and most importantly, the strong spin-orbit coupling (SOC). Starting from the prediction of novel oxide topological insulators in bilayer ABO3 (B = 5d elements) thin-film grown along the (111) direction, 5d-based perovskites (Pv) form a new paradigm in the thin-film community. Here, we reviewed the scientific accomplishments in Pv-SrIrO3 thin films, a popular candidate for observing non-trivial topological phenomena. Although the predicted topological phenomena are unknown, the Pv-SrIrO3 thin film shows many emergent properties due to the delicate interplay between its various degrees of freedom. These observations provide new physical insight and encourage further research on the design of new 5d-based heterostructures or superlattices for the observation of the hidden topological quantum phenomena in strong spin-orbit coupled oxides.

Transition metal oxides, in particular, 3d or 4d perovskites have provided diverse emergent physics that originates from the coupling of various degrees of freedom such as spin, lattice, charge, orbital, and also disorder. 5d perovskites form a distinct class because they have strong spin-orbit coupling that introduces to the system an additional energy scale that is comparable to bandwidth and Coulomb correlation. Consequent new physics includes novel Jeff = 1/2 Mott insulators, metal-insulator transitions, spin liquids, and topological insulators. After highlighting some of the phenomena appearing in Ruddlesden-Popper iridate series Srn+1IrnO3n+1, we focus on the transport properties of perovskite SrIrO3. Using epitaxial thin films on various substrates, we demonstrate that metal-insulator transitions can be induced in perovskite SrIrO3 by reducing its thickness or by imposing compressive strain. The metal-insulator transition driven by thickness reduction is due to disorder, but the metal-insulator transition driven by compressive strain is accompanied by peculiar non-Fermi liquid behaviors, possibly due to the delicate interplay between correlation, disorder, and spin-orbit cou

Lead-free antiferroelectric perovskite $\rm AgNbO_3$ is nowadays attracting extensive research interests due to its promising applications in energy storage. Although great progress has been made in optimizing the material performance, fundamental questions remain regarding the mechanism stabilizing the antiferroelectric $Pbcm$ phase. Here, combining structural symmetry analysis and first-principles calculations, we identified crucial anharmonic couplings of oxygen octahedra rotations and cation antipolar motions which contribute significantly to lowering the energy of the $Pbcm$ phase. The stabilization of this phase shows close similarities with the stabilization of the $Pbam$ phase in $\rm PbZrO_3$ except that in $\rm AgNbO_3$ the octahedra rotations are the primary distortions while the antipolar cation motions appear to be secondary. The appearance and significant amplitude of the latter are explained from the combination of hybrid-improper and triggered mechanisms.

Bulk chalcogenide perovskite BaZrS3 (BZS), with a direct band gap in visible region, is an important photovoltaic material, albeit with limited applicability owing to its antiferroelectric (AF) nature. Presently, ferroelectric (FE) perovskite-based photovoltaics are attracting enormous attention for environmental stability and better energy conversion efficiency through enhanced charge separation, owing to loss of center of inversion symmetry. We report on antiferroelectric-ferroelectric (AF-FE) phases of BZS thin film, grown with chemical vapor deposition (CVD), using temperature-dependent Raman investigations and first-principles calculations. The origin of FE phases is established from anomalous behavior of A7g ~ 300 cm-1 and B1g5 ~ 420 cm-1 modes, which involves the vibration of atoms at apical site of ZrS6 octahedra. Additionally, below 60 K, B1g1 and B2g2 ( ~ 85 cm-1) modes appear whereas B12g (~ 60 cm-1) disappears to stabilize the Pnma structure against ferroelectricity by local distortion. Here, B2g2 and B1g2 involve vibrations of Ba atoms in AF manner while B1g1 involves, in addition, the rotation of octahedra as well. Our first-principles calculations confirm that FE app

Metal halide perovskites are of great interest for application in semitransparent solar cells due to their tunable bandgap and high performance. However, fabricating high-efficiency perovskite semitransparent devices with high average visible transmittance (AVT) is challenging because of their high absorption coefficient. Here, we adopt a co-evaporation process to fabricate ultrathin CsPbI3 perovskite films. Due to the smooth surface and orientated crystal growth of the evaporated perovskite films, we are able to achieve 10 nm thin films with compact and continuous morphology without pinholes. When integrated into a p-i-n device structure of glass/ITO/PTAA/perovskite/PCBM/BCP/Al/Ag with an optimized transparent electrode, these ultrathin layers result in an impressive open-circuit voltage (VOC) of 1.08 V and a fill factor (FF) of 80%. Consequently, a power conversion efficiency of 3.6% with an AVT above 50% is demonstrated, achieved in the 10 nm semitransparent perovskite solar cells, which is the first report for a perovskite device of 10 nm active layer with higher VOC, FF and AVT. These findings demonstrate that evaporation process is a possible way for compact ultrathin perovsk

One of the most viable renewable energies is solar power because of its versatility, reliability, and abundance. In the market, a majority of the solar panels are made from silicon wafers. These solar panels have an efficiency of 26.4 percent and can last more than 25 years. The perovskite solar cell is a relatively new type of solar technology that has a similar maximum efficiency and much cheaper costs, the only downside is that it is less stable and the most efficient type uses lead. The name perovskite refers to the crystal structure with an ABX3 formula of the perovskite layer of the cell. All materials possess a property called a band gap. The smaller the band gap the more conductive the material, but this does not necessarily mean that the smaller the band gap the better the solar cell. The Shockley-Queisser limit provides the optimal band gap in terms of efficiency for a single junction solar cell which is 1.34 eV for single junction cells. This research focuses on tuning the band gap of lead-free perovskites through B-site cation replacement. Through this investigation, the optical band gaps of tin and lead perovskites were re-established. However, the copper-based perovsk

Organic-inorganic hybrid perovskites (OIHPs) have recently emerged as groundbreaking semiconductor materials owing to their remarkable properties. Transmission electron microscopy (TEM), as a very powerful characterization tool, has been widely used in perovskite materials for structural analysis and phase identification. However, the perovskites are highly sensitive to electron beams and easily decompose into PbX2 (X= I, Br, Cl) and metallic Pb. The electron dose of general high-resolution TEM is much higher than the critical dose of MAPbI3, which results in universal misidentifications that PbI2 and Pb are incorrectly labeled as perovskite. The widely existed mistakes have negatively affected the development of perovskite research fields. Here misidentifications of the best-known MAPbI3 perovskite are summarized and corrected, then the causes of mistakes are classified and ascertained. Above all, a solid method for phase identification and practical strategies to reduce the radiation damage for perovskite materials have also been proposed. This review aims to provide the causes of mistakes and avoid misinterpretations in perovskite research fields in the future.

Light-emitting diodes (LEDs) based on halide perovskite nanocrystals have attracted extensive attention due to their considerable luminescence efficiency, wide color gamut, high color purity, and facile material synthesis. Since the first demonstration of LEDs based on MAPbBr3 nanocrystals were reported in 2014, the community has witnessed a rapid development in their performances. In this review, we provide a historical perspective of the development of LEDs based on halide perovskite nanocrystals and then present a comprehensive survey of current strategies to high-efficiency lead-based perovskite nanocrystals LEDs, including synthesis optimization, ion doping/alloying and shell coating. We then review the basic characteristics and emission mechanisms of lead-free perovskite and perovskite-related nanocrystals emitters in environmentally friendly LEDs, from the standpoint of different emission colors. Finally, we cover the progress in LED applications and provide an outlook of the opportunities and challenges for future developments in this field.

Metal halide perovskites have recently emerged as promising materials for the next generation of optoelectronic devices owing to their remarkable intrinsic properties. In the growth of perovskite crystals, the substrates are essential and play a vital role. Herein, substrate engineering in the growth of perovskite crystals have been reviewed. Particularly, various modified strategies and corresponding mechanism based on the substrate engineering applied to the optimization of thickness, nucleation and growth rate are highlighted. Then the alterable adhesion to substrates will also be discussed. Furthermore, applying the structural coherence of epitaxial crystals with substrate, scalable perovskite single-crystalline thin films have been obtained and can be transferred onto arbitrary substrates. Substrate engineering also can stabilize the desired perovskite phases by modulating the strain between crystals and substrates. Finally, several key challenges and related solutions in the growth of perovskite crystals based on substrate engineering are proposed. This review aims to guide the future of substrate engineering in perovskite crystals for various optoelectronic applications.

We use the density functional perturbation theory to determine for the first time the pressure evolution of the Raman intensities for a mineral, the two high-pressure structures of MgSiO3 perovskite and post-perovskite. At high pressures, the Raman powder spectra reveals three main peaks for the perovskite structure and one main peak for the post-perovskite structure. Due to the large differences in the spectra of the two phases Raman spectroscopy can be used as a good experimental indication of the phase transition.

Perovskite solar cells employing self assembled monolayers such as Me-4PACz as hole transport layer has been reported to demonstrate high device efficiency. However, the poor perovskite wetting on the Me-4PACz caused by poor perovskite ink interaction with the underlying Me-4PACz presents significant challenges for fabricating efficient perovskite devices. A triple co-solvent system comprising of dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and N-methyl-2-pyrrolidone (NMP) is employed to improve the perovskite ink-substrate interaction and obtain a uniform perovskite layer. In comparison to DMF, DMSO-based inks, the inclusion of NMP shows considerably higher binding energies of the perovskite ink with Me-4PACz as revealed by density-functional theory calculations. With the optimized triple co-solvent ratio, the perovskite devices deliver high power conversion efficiencies of >20%, 19.5% and ~18.5% for active areas of 0.16 cm2, 0.72 cm2 and 1.08 cm2 respectively. Importantly, this perovskite ink-substrate interaction approach is universal and helps in obtaining a uniform layer and high photovoltaic device performance for other perovskite compositions such as MAPbI3, FAMAPbI

While nature provides a plethora of perovskite materials, only a few exhibits large ferroelectricity and possibly multiferroicity. The majority of perovskite materials have the non-polar CaTiO$_3$(CTO)structure, limiting the scope of their applications. Based on effective Hamiltonian model as well as first-principles calculations, we propose a general thin-film design method to stabilize the functional BiFeO$_3$(BFO)-type structure, which is a common metastable structure in widespread CaTiO$_3$-type perovskite oxides. It is found that the improper antiferroelectricity in CTO-type perovskite and ferroelectricity in BFO-type perovskite have distinct dependences on mechanical and electric boundary conditions, both of which involve oxygen octahedral rotation and tilt. The above difference can be used to stabilize the highly polar BFO-type structure in many CTO-type perovskite materials.

The fabrication of metal halide perovskite films using the solvent-engineering method is increasingly common. In this method, the crystallisation of the perovskite layer is triggered by the application of an antisolvent during the spin-coating of a perovskite precursor solution. Herein, we introduce the current state of understanding of the processes involved in the crystallisation of perovskite layers formed by solvent engineering, focusing in particular on the role of antisolvent properties and solvent-antisolvent interactions. By considering the impact of the Hansen solubility parameters, we propose guidelines for selecting the appropriate antisolvent and outline open questions and future research directions for the fabrication of perovskite films by this method.

We demonstrate four and two-terminal perovskite-perovskite tandem solar cells with ideally matched bandgaps. We develop an infrared absorbing 1.2eV bandgap perovskite, $FA_{0.75}Cs_{0.25}Sn_{0.5}Pb_{0.5}I_3$, that can deliver 14.8 % efficiency. By combining this material with a wider bandgap $FA_{0.83}Cs_{0.17}Pb(I_{0.5}Br_{0.5})_3$ material, we reach monolithic two terminal tandem efficiencies of 17.0 % with over 1.65 volts open-circuit voltage. We also make mechanically stacked four terminal tandem cells and obtain 20.3 % efficiency. Crucially, we find that our infrared absorbing perovskite cells exhibit excellent thermal and atmospheric stability, unprecedented for Sn based perovskites. This device architecture and materials set will enable 'all perovskite' thin film solar cells to reach the highest efficiencies in the long term at the lowest costs.

Perovskite materials are at the forefront of modern materials science due to their exceptional structural, electronic, and optical properties. The controlled fabrication of perovskite nanostructures is crucial for enhancing their performance, stability, and scalability, directly impacting their applications in next-generation devices such as solar cells, LEDs, and sensors. Here, we present a novel, ligand-free approach to synthesize perovskite nanocrystals (NCs) with average sizes up to 100 nm, using femtosecond pulsed laser ablation (PLA) in ambient air without additional liquid media. We demonstrate this method for both organic-inorganic (methylamino lead) hybrid perovskites (MAPbX3, X = Cl, Br, I) and fully inorganic lead-free double perovskites (Cs2AgBiX6, X = Cl, Br), achieving high-purity NCs without stabilizing ligands - a critical advancement over conventional chemical synthesis methods. By tailoring laser parameters, we systematically elucidate the influence of perovskite composition (halide type, organic vs. inorganic cation, single versus double perovskite structure) on the ablation process and the resulting nanocrystal properties. Transmission electron microscopy and X-

The prototypical antiferroelectric perovskite $\rm PbZrO_3$ (PZO) has garnered considerable attentions in recent years due to its significance in technological applications and fundamental research. Many unresolved issues in PZO are associated with large length- and time-scales, as well as finite temperatures, presenting significant challenges for first-principles density functional theory studies. Here, we introduce a deep learning interatomic potential of PZO, enabling investigation of finite-temperature properties through large-scale atomistic simulations. Trained using an elaborately designed dataset, the model successfully reproduces a large number of phases, in particular, the recently discovered 80-atom antiferroelectric $Pnam$ phase and ferrielectric $Ima2$ phase, providing precise predictions for their structural and dynamical properties. Using this model, we investigated phase transitions of multiple phases, including $Pbam$/$Pnam$, $Ima2$ and $R3c$, which show high similarity to the experimental observation. Our simulation results also highlight the crucial role of free-energy in determining the low-temperature phase of PZO, reconciling the apparent contradiction: $Pbam$