High-Performance Perovskite Light-Emitting Diode with Enhanced Operational Stability Using Lithium Halide Passivation.

Defect passivation has been demonstrated to be effective in improving the radiative recombination of charge carriers in perovskites, and consequently, the device performance of the resultant perovskite light-emitting diodes (LEDs). State-of-the-art useful passivation agents in perovskite LEDs are mostly organic chelating molecules that, however, simultaneously sacrifice the charge-transport properties and thermal stability of the resultant perovskite emissive layers, thereby deteriorating performance, and especially the operational stability of the devices. We demonstrate that lithium halides can efficiently passivate the defects generated by halide vacancies and reduce trap state density, thereby suppressing ion migration in perovskite films. Efficient green perovskite LEDs based on all-inorganic CsPbBr3 perovskite with a peak external quantum efficiency of 16.2 %, as well as a high maximum brightness of 50 270 cd m-2 , are achieved. Moreover, the device shows decent stability even under a brightness of 104  cd m-2 . We highlight the universal applicability of defect passivation using lithium halides, which enabled us to improve the efficiency of blue and red perovskite LEDs.

Beyond Oxidation States: Distinguishing Chemical States of Gallium in Compounds with Multiple Gallium Centers.

The electronic structures of a series of gallium complexes are examined using X-ray absorption spectroscopy (XAS) in combination with ab initio calculations. The chemical states of Ga are strongly affected by the ligands and the bonding environment. For complexes containing multiple gallium sites, we demonstrate that XAS can identify the chemical state of each unique gallium center. A reliable understanding of the chemical nature of the core element in a coordination complex with strong core-ligand interaction can be obtained only when both experimental and theoretical approaches are combined.

Structural evolution of reduced GeOx nanoparticles.

GeOx nanoparticles (NPs) are of growing interest in lithium storage and optoelectronics. GeOx NPs prepared by chemical reduction, exposed to air or retained under N2, then annealed under H2 at various temperatures are studied herein using soft X-ray spectroscopy. We find that fresh and air-exposed GeOx NPs evolve rather differently under annealing. The fresh GeOx NPs start as a very amorphous heterogeneous mixture of GeOx and Ge, and during annealing both the valence band and conduction band edges evolve. In contrast, the air-exposed GeOx NPs initially contain quartz-phase GeO2, and during annealing only the conduction band edge evolves due to increased oxygen vacancies forming unoccupied defect states (the valence band does not change until annealing at high temperture, at which point almost all of the GeO2 is removed). These findings suggest a preparation and annealing strategy that could be used to tailor GeOx NPs for their intended use in lithium storage or optoelectronic applications.

Dissociation of Methylammonium Cations in Hybrid Organic-Inorganic Perovskite Solar Cells.

Organic-inorganic lead perovskites have shown great promise as photovoltaic materials, and within this class of materials (CH3NH3)PbI3-xClx is of particular interest. Herein we use soft X-ray spectroscopy and density functional theory calculations to demonstrate that the methylammonium cations in a typical photovoltaic layer may dissociate into a metastable arrangement of CH3I-Pb2 defects and trapped NH3. The possibility that other metastable configurations of the organic components in (CH3NH3)PbI3-xClx is rarely considered but adds an entirely new dimension in understanding the charge trapping, ionic transport, and structural degradation mechanisms in these materials. Understanding the influence of these other configurations is of critical importance for further improving the performance of these photovoltaics.

Tuning Mixed-Valent Eu(2+) /Eu(3+) in Strontium Formate Frameworks for Multichannel Photoluminescence.

Cooperative performance of mixed-valent Eu(2+) /Eu(3+) in single-compound phosphors offers significant advantages in color rendering and luminescence efficiency, but their synthesis is challenging because of Eu(2+) oxidation. Using the tunable nature of the metal-ion nodes in metal-organic frameworks (MOFs), we present an in situ reduction and crystallization route for preparing MOFs and doping Eu(2+) /Eu(3+) with a controlled ratio. These materials exhibit rich photoluminescence, including intrinsic- and sensitized-emissions of Eu(2+) and Eu(3+) , and long-lived luminescence from charge transfer. Color rendering can be easily achieved by fine-tuning the valence states of Eu. A linear relation between temperature and the intensity ratio of Eu(2+) /Eu(3+) emissions provides outstanding properties for applications as self-calibrated luminescent thermometers with a wide working temperature range. Further incorporation of Tb(3+) into the MOFs results in white light, utilizing all Eu(2+) ,Tb(3+) , and Eu(3+) emissions in a single crystalline lattice.

Linking the HOMO-LUMO gap to torsional disorder in P3HT/PCBM blends.

The electronic structure of [6,6]-phenyl C61 butyric acid methyl ester (PCBM), poly(3-hexylthiophene) (P3HT), and P3HT/PCBM blends is studied using soft X-ray emission and absorption spectroscopy and density functional theory calculations. We find that annealing reduces the HOMO-LUMO gap of P3HT and P3HT/PCBM blends, whereas annealing has little effect on the HOMO-LUMO gap of PCBM. We propose a model connecting torsional disorder in a P3HT polymer to the HOMO-LUMO gap, which suggests that annealing helps to decrease the torsional disorder in the P3HT polymers. Our model is used to predict the characteristic length scales of the flat P3TH polymer segments in P3HT and P3HT/PCBM blends before and after annealing. Our approach may prove useful in characterizing organic photovoltaic devices in situ or even in operando.

Selective response of mesoporous silicon to adsorbants with nitro groups.

We demonstrate that the electronic structure of mesoporous silicon is affected by adsorption of nitro-based explosive molecules in a compound-selective manner. This selective response is demonstrated by probing the adsorption of two nitro-based molecular explosives (trinitrotoluene and cyclotrimethylenetrinitramine) and a nonexplosive nitro-based aromatic molecule (nitrotoluene) on mesoporous silicon using soft X-ray spectroscopy. The Si atoms strongly interact with adsorbed molecules to form Si-O and Si-N bonds, as evident from the large shifts in emission energy present in the Si L(2,3) X-ray emission spectroscopy (XES) measurements. Furthermore, we find that the energy gap (band gap) of mesoporous silicon changes depending on the adsorbant, as estimated from the Si L(2,3) XES and 2p X-ray absorption spectroscopy (XAS) measurements. Our ab initio molecular dynamics calculations of model compounds suggest that these changes are due to spontaneous breaking of the nitro groups upon contacting surface Si atoms. This compound-selective change in electronic structure may provide a powerful tool for the detection and identification of trace quantities of airborne explosive molecules.

Surface chemical heterogeneous distribution in over-lithiated Li1+xCoO2 electrodes.

In commercial Li-ion batteries, the internal short circuits or over-lithiation often cause structural transformation in electrodes and may lead to safety risks. Herein, we investigate the over-discharged mechanism of LiCoO2/graphite pouch cells, especially spatially resolving the morphological, surface phase, and local electronic structure of LiCoO2 electrode. With synchrotron-based X-ray techniques and Raman mapping, together with spectroscopy simulations, we demonstrate that over-lithiation reaction is a surface effect, accompanied by Co reduction and surface structure transformation to Li2CoO2/Co3O4/CoO/Li2O-like phases. This surface chemical distribution variation is relevant to the depth and exposed crystalline planes of LiCoO2 particles, and the distribution of binder/conductive additives. Theoretical calculations confirm that Li2CoO2-phase has lower electronic/ionic conductivity than LiCoO2-phase, further revealing the critical effect of distribution of conductive additives on the surface chemical heterogeneity evolution. Our findings on such surface phenomena are non-trivial and highlight the capability of synchrotron-based X-ray techniques for studying the spatial chemical phase heterogeneity.

Investigating the luminescence mechanism of Mn-doped CsPb(Br/Cl)3 nanocrystals.

Inorganic lead halide perovskite CsPbX3 (X = Cl, Br, or I) nanocrystals are promising candidate materials for light-emitting devices and optoelectronics. Mn-Doped CsPbX3 is of particular interest, as the Mn-doping introduces an additional emission band, making this material a promising white-light emitter. In this study, Mn-doped CsPb(Br/Cl)3 nanocrystals are prepared at room-temperature and ambient pressure. The chemical environment of Mn, and the luminescence of these nanocrystals are analyzed in detail using X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES) and X-ray excited optical luminescence (XEOL). Although the introduction of Mn does not alter the long-range order of the CsPbX3 crystal, it leads to a local lattice contraction with the bond length of Mn-X much shorter than Pb-X. We also find excitation energy-dependence in both the intensity and wavelength of the perovskite excitonic emission band, while only in intensity of the Mn emission band. Detailed fitting of the XEOL reveals that the perovskite emission band is dual-channel, and it is the excitation energy-dependent intensity variation of these two channels that drives the observed red-shift of the combined emission band. Our findings also confirm that the Mn emission band is driven by exciton-Mn energy transfer and clarify the Mn chemical environment and the luminescence mechanism in Mn-doped CsPb(Br/Cl)3 nanocrystals.

Photocatalytic Polymerization from Amino Acid to Protein by Carbon Dots at Room Temperature.

The chemical origins of life have been widely accepted at the present stage. However, the idea that amino acids further react to produce peptides and proteins remains an unsatisfactory explanation, because producing polypeptides via spontaneous reaction of amino acids in solution is extremely difficult. It is also necessary to further answer whether amino acids can form longer peptide chains as well as specific chiral structures and so on under this same reaction mechanism. Carbon dots (CDs) have been intensively researched over the past years due to their unique chemical and physical properties. Here, we demonstrate the photocatalytic polymerization of amino acids into polypeptides and proteins using CDs as a photocatalyst, in which the synthetic conditions required are only room temperature (or as low as -20 °C) and aqueous conditions along with light irradiation, which are very mild and easy to satisfy. We even obtain a protein with tertiary structures, namely an artificial insulin with the biological function to reduce the blood sugar of the laboratory mice. The innovation of using CDs to initiate amino acids to condense into polypeptides is based on strong adsorption (e.g., hydrogen bonding), the acidity of the -OH surface functional groups, and the photogenerated protons/holes, which are the fundamental factors for polypeptide or even ternary structure protein formation by means of capturing and condensing the amino acids as well as forming the S-S bonds.

Alternative Type Two-Dimensional-Three-Dimensional Lead Halide Perovskite with Inorganic Sodium Ions as a Spacer for High-Performance Light-Emitting Diodes.

Two-dimensional (2D) lead halide perovskites with long-chain ammonium halides display high photoluminescence quantum yields (PLQYs), because of their size and dielectric confinement, which hold promise for a high-efficiency and low-cost light-emitting diode (LED). However, the presence of an insulating organic long-chain spacer cation (L) dramatically deteriorates the charge transport properties along the out-of-plane nanoplatelet direction or adjacent nanocrystals, which would limit the device performance of the LED. To overcome this issue, we successfully incorporate small alkaline ions such as sodium (Na+) to replace the long organic molecule. Grazing incidence X-ray diffraction measurements verify 2D layer formation with a preferred crystallite orientation. In addition, the incorporated sodium salt also generates amorphous sodium lead bromide (NaPbBr3) in perovskite as spacers to form a nanocrystal-like halide perovskite film. The PLQY is dramatically improved in the sodium-incorporated film because of its enhanced photoluminescence lifetime. Upon incorporation of a low concentration of an organic additive, this two-dimensional-three-dimensional (2D-3D) perovskite can achieve a compact and uniform film. Therefore, a 2D-3D perovskite achieves a high external quantum efficiency of 15.9% with good operational stability. We develop a type of 2D-3D halide perovskite with various inorganic ions as spacers for promising high-performance optoelectronic devices.

Influence of Eu-substitution on luminescent CH3NH3PbBr3 quantum dots.

We report a series of Eu-substituted methylammonium lead tribromide quantum dots (MAPb1-xEuxBr3 QDs). The crystallinity of these QDs increases with increasing Eu content, while there is only a small change in the lattice constant, and the morphology of the MAPb1-xEuxBr3 QDs is unaffected by the Eu content. This demonstrates that Eu is a suitable element for substituting Pb while retaining the original crystal structure. We observe blue photoluminescence (PL) consistent with Eu2+, which, at high Eu content (x = 0.3), contributes luminescence intensity equal to the green PL of unsubstituted MAPbBr3 QDs. As Eu is a less toxic substitute for Pb, that also provides blue luminescence, MAPb1-xEuxBr3 QDs may prove to be a valuable optoelectronic material.

Prospects for Mitigating Intrinsic Organic Decomposition in Methylammonium Lead Triiodide Perovskite.

Organometallic lead halide perovskites seem to be on the threshold of becoming viable commercial photovoltaics; however, further improvements to the stability of these materials must be made before they can compete with existing photovoltaic technologies. Of the organometallic lead halide perovskites used in photovoltaics, methylammonium lead triiodide perovskite (MAPI) is perhaps the most studied, and understanding how MAPI degrades is crucial for developing strategies to improve stability. We discuss the experimental evidence behind several possible routes for MAPI to degrade into PbI2 and various organics, and how the decomposition path of MAPI may strongly depend on substrate, precursors, intrinsic organic defects, and morphology. Exploring the conditions required for MAPI to degrade according to a particular pathway is important not only from a fundamental materials chemistry perspective, but also for understanding intrinsic instability in MAPI-based photovoltaics and to develop strategies to improve stability.

A chlorine-free protocol for processing germanium.

Replacing molecular chlorine and hydrochloric acid with less energy- and risk-intensive reagents would markedly improve the environmental impact of metal manufacturing at a time when demand for metals is rapidly increasing. We describe a recyclable quinone/catechol redox platform that provides an innovative replacement for elemental chlorine and hydrochloric acid in the conversion of either germanium metal or germanium dioxide to a germanium tetrachloride substitute. Germanium is classified as a "critical" element based on its high dispersion in the environment, growing demand, and lack of suitable substitutes. Our approach replaces the oxidizing capacity of chlorine with molecular oxygen and replaces germanium tetrachloride with an air- and moisture-stable Ge(IV)-catecholate that is kinetically competent for conversion to high-purity germanes.

The influence of the I/Cl ratio on the performance of CH3NH3PbI(3-x)Cl(x)-based solar cells: why is CH3NH3I : PbCl2 = 3 : 1 the "magic" ratio?

Methylammonium lead trihalide (CH3NH3PbI(3-x)Cl(x)) perovskites are usually synthesized from two precursors, CH3NH3I and PbCl2 at a ratio of 3 : 1. It was found that a slight adjustment of the I/Cl ratio in the precursor mixture plays a strong effect on solar cell performance. In this study, perovskites made with different I/Cl ratios were comparatively studied. In combination with X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) measured at the Pb L3-edge, we demonstrate that the device performance can be directly correlated to the change in the coordination environment of Pb.

Iodomethane-Mediated Organometal Halide Perovskite with Record Photoluminescence Lifetime.

Organometallic lead halide perovskites are excellent light harvesters for high-efficiency photovoltaic devices. However, as the key component in these devices, a perovskite thin film with good morphology and minimal trap states is still difficult to obtain. Herein we show that by incorporating a low boiling point alkyl halide such as iodomethane (CH3I) into the precursor solution, a perovskite (CH3NH3PbI3-xClx) film with improved grain size and orientation can be easily achieved. More importantly, these films exhibit a significantly reduced amount of trap states. Record photoluminescence lifetimes of more than 4 µs are achieved; these lifetimes are significantly longer than that of pristine CH3NH3PbI3-xClx films. Planar heterojunction solar cells incorporating these CH3I-mediated perovskites have demonstrated a dramatically increased power conversion efficiency compared to the ones using pristine CH3NH3PbI3-xClx. Photoluminescence, transient absorption, and microwave detected photoconductivity measurements all provide consistent evidence that CH3I addition increases the number of excitons generated and their diffusion length, both of which assist efficient carrier transport in the photovoltaic device. The simple incorporation of alkyl halide to enhance perovskite surface passivation introduces an important direction for future progress on high efficiency perovskite optoelectronic devices.

Reduced GeO2 Nanoparticles: Electronic Structure of a Nominal GeOx Complex and Its Stability under H2 Annealing.

A nominal GeOx (x ≤ 2) compound contains mixtures of Ge, Ge suboxides, and GeO2, but the detailed composition and crystallinity could vary from material to material. In this study, we synthesize GeOx nanoparticles by chemical reduction of GeO2, and comparatively investigate the freshly prepared sample and the sample exposed to ambient conditions. Although both compounds are nominally GeOx, they exhibit different X-ray diffraction patterns. X-ray absorption fine structure (XAFS) is utilized to analyse the detailed structure of GeOx. We find that the two initial GeOx compounds have entirely different compositions: the fresh GeOx contains large amorphous Ge clusters connected by GeOx, while after air exposure; the Ge clusters are replaced by a GeO2-GeOx composite. In addition, the two GeOx products undergo different structural rearrangement under H2 annealing, producing different intermediate phases before ultimately turning into metallic Ge. In the fresh GeOx, the amorphous Ge remains stable, with the GeOx being gradually reduced to Ge, leading to a final structure of crystalline Ge grains connected by GeOx. The air-exposed GeOx on the other hand, undergoes a GeO2→GeOx→Ge transition, in which H2 induces the creation of oxygen vacancies at intermediate stage. A complete removal of oxides occurs at high temperature.

Self-Alignment of the Methylammonium Cations in Thin-Film Organometal Perovskites.

A comparative study of the electronic structure of methylammonium (CH3NH3) in organometallic lead triiodide perovskite (CH3NH3PbI3) thin films synthesized using either one- or two-step deposition protocols is performed using angle-resolved C K-edge soft X-ray absorption spectroscopy (XAS) and model calculations. We find that our XAS measurements can be accurately related to the ground-state unoccupied orbitals using a simple crystal field model. We further find that films made by the one-step deposition protocol exhibit angle-dependent features, indicating long-range alignment of the CH3NH3 molecules, although the angle-dependency decreases as the film thickness increases. No angle-dependency was observed in the films made via the two-step deposition method.