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Interfacial phenomena critically influence both the performance and long-term stability of perovskite solar cells; hence, optimizing interfaces remains a key challenge for their commercialization. In this study, we investigate the chemical interactions and electronic structure of the buried interface between the FAPbI₃ perovskite absorber and spiro-OMeTAD-based hole transport layer (HTL) in an ITO/SnO₂/FAPbI₃/HTL device using hard X-ray photoelectron spectroscopy. Our results indicate Pb and I ion incorporation in the spiro-OMeTAD HTL. The Pb 4f core level spectra show the formation of new nonperovskite Pb species, and the N 1s spectra exhibit an additional peak, indicating chemical modifications induced by the deposition of spiro-OMeTAD on the perovskite layer. Moreover, the spiro-OMeTAD N 1s peak exhibits a systematic shift toward lower binding energies with increasing HTL thickness, indicating a downward band bending in the spiro-OMeTAD HTL. Overall, these findings provide direct insight into the chemical and electronic interactions near the FAPbI₃/HTL interface and emphasize the importance of optimizing transport layer thickness to achieve favorable energy level alignment and improved device performance.

The development of high-performance, sustainable sodium-ion batteries requires a mechanistic understanding of cathode redox processes and structural stability. Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) is a promising cathode material due to its non-toxicity, high average working voltage, and excellent structural and thermal stability. However, its practical application is hindered by impurity phase formation and low intrinsic electronic conductivity. To address these challenges, Na₄Fe_3-xM_x(PO₄)₂(P₂O₇) (M: Mn, Ni) composites are synthesized via a sol-gel method. A comprehensive characterization approach combining X-ray diffraction (XRD), X-ray absorption spectroscopy (XANES/EXAFS, soft XAS), and resonant inelastic X-ray scattering (RIXS) revealed that low-level substitution of Mn²⁺ and Ni²⁺ into Fe sites suppresses the formation of electrochemically inactive maricite NaFePO₄ and modifies the Fe-O coordination environment. These effects may result in lower ion migration energy barriers and better electrochemical reversibility. Among the doped samples, Mn-NFPP exhibited the best electrochemical performance, delivering a discharge capacity of ∼92 mAh g⁻¹ at 0.1 C and ∼80 mAh g⁻¹ at 2 C, with 99.5 % capacity retention after 100 cycles at 0.1 C. This work provides fundamental insights into the redox mechanism and atomic-scale structure–property relationship of NFPP, guiding the design of high-performance polyanionic cathodes for sodium-ion batteries.

Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) is a promising cathode material for sodium-ion batteries due to its structural stability and low cost, but its performance is often limited by low electronic conductivity and impurity formation. Here, we investigate the effects of Zn²⁺ substitution at Fe sites in NFPP, focusing on how Zn content and calcination atmosphere influence phase purity, local structure, and electrochemical properties. X-ray diffraction (XRD) shows that Zn²⁺ incorporation, particularly at higher concentrations, promotes the formation of electrochemically inactive maricite NaFePO₄ and Na₂ZnP₂O₇, reducing the phase purity and hindering Na⁺ transport. Complementary X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) reveal that Zn does not contribute to redox activity but modifies the Fe─O coordination environment, weakening the overall charge compensation compared to other transition-metal-doped NFPP cathodes. Galvanostatic cycling tests confirm that, despite good cycling stability, all Zn-doped NFPP cathodes exhibit inferior specific capacities compared to pristine NFPP. These results indicate that Zn²⁺ is not an effective capacity-enhancing dopant for NFPP under the studied conditions, while also highlighting the importance of dopant redox activity, structural compatibility, phase purity, and synthesis atmosphere in the design of sodium-ion battery cathodes.

Exploring the conduction mechanism in the chalcogenide perovskite Ba⁢Zr⁢S₃ is of significant interest due to its potential suitability as a top absorber layer in silicon-based tandem solar cells and other optoelectronic applications. Theoretical and experimental studies anticipate native ambipolar doping in Ba⁢Zr⁢S₃, although experimental validation remains limited. This study reveals a transition from highly insulating behavior to 𝑛-type conductivity (approximately 100S/cm) in Ba⁢Zr⁢S₃ thin films through annealing in an S-poor environment. Ba⁢Zr⁢S₃ thin films are synthesized via a two-step process: co-sputtering of Ba-Zr followed by sulfurization at 600⁢^∘⁢C, and subsequent annealing in high vacuum. Ultraviolet-visible spectroscopy measurements reveal a red shift of approximately 100meV in the band gap concurrent with sample-color darkening after vacuum annealing. The increase in defect density from the order of 10¹⁷ to 10²¹ cm⁻³ with vacuum annealing, coupled with the low activation energy (approximately 8meV), and the 𝑛-type character of the defects, strongly suggests that sulfur vacancies (𝑉_S) are responsible for the 𝑛-type doping, in agreement with theoretical predictions. Temperature-dependent Hall measurement shows that phonon scattering governs charge transport at room temperature in Ba⁢Zr⁢S₃ films and that Svacancies are shallow donor defects acting as a weak impurity metal. The shift of the valence-band maximum (VBM) with respect to the Fermi level, quantified by hard x-ray photoelectron spectroscopy (GaK⁢𝛼, 9.25 keV), further corroborates the induced 𝑛 type of conductivity in annealed samples. Our findings indicate that vacuum annealing induces 𝑉_S defects that dominate charge transport, resulting in 𝑛-type conductivity in Ba⁢Zr⁢S₃ under S-poor conditions. These discoveries will inform important design choices, such as selecting appropriate contact materials, for integrating Ba⁢Zr⁢S₃ into solar cell structures or other devices.

Electrochemical water splitting is an eco-friendly method for large-scale production of high-purity hydrogen (H2) and oxygen (O2), and hence, pioneering the design of efficient and economic bifunctional electrocatalysts is necessary. Here, hexagonal (Cu,Co)Se2 nanoflakes are fabricated to leverage the unique synergy between copper and cobalt for efficient hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). As HER and OER catalysts, (Cu,Co)Se2 nanoflakes demonstrate low overpotentials of 104 and 250 mV, respectively, to reach current densities of 10 mA cm-2 in 1 m KOH solution, with respective Tafel slopes of 117.7 and 61 mV dec-1. The catalyst requires only 403 mV of overpotential to reach 1000 mA cm-2 in OER. XANES analyses reveal average oxidation states of Co as 2.6+ and 3+ post-HER and post-OER, respectively, while Cu remains predominantly in 2+ states. Presence of Cu around Co induces orbital rehybridization through linking Se bonds; establishing a cooperative participation between Co and Cu to facilitate overall charge-transfer processes, eventually enhancing catalytic activity of (Cu,Co)Se2. Further, (Cu,Co)Se2/NF-based overall-water-splitting electrolyzer offers low cell voltage (1.65V at 10 mA cm-2) and high durability even at a high current density (at eta 50) in 1 m KOH, thus demonstrating its commercial application prospects.

Sharp near-infrared-II (NIR-II) emissions are typically achieved through electronic transitions of rare-earth ions, while transitions in transition metal ions are broad due to electron–ligand interactions. An exception is the intra-configurational spin-flip (ICSF) transition like t_2g³ t_2g³ of Mo³⁺ emitting sharp NIR-II emission, but only at cryogenic temperatures under vacuum. The high oxophilicity of Mo³⁺ created defects during the synthesis, quenching the emission at room temperature. Herein, we overcome this issue by synthesizing Mo³⁺− doped Cs₂NaInCl₆ double perovskites in a reducing H₃PO₂ environment. [MoCl₆]³⁻ octahedra are formed, exhibiting ultra-narrow ICSF (²T_1g/²E_g ⁴A_2g) NIR-II emission at 1095 nm in ambient conditions. In addition, a second ICSF ²T_2g ⁴A_2g emission is observed at 700 nm, violating the Kasha's rule. The intensity of ICSF emissions increase with increasing temperature (7–350 K) due to vibronic coupling relaxing the Laporte selection rule. The samples are stable for more than 6 months in ambient conditions, allowing for a detailed study of fundamental photophysics and fabrications of phosphor-converted light emitting diodes. This is the first Mo³⁺–based NIR-II optoelectronic device, opening opportunities for applications like optical fibers and lasing.

Chalcogenide perovskites like BaZrS3 are promising candidates for tandem solar cells due to their stability, nontoxicity, and ideal bandgap of similar to 1.8 eV. However, their thin-film fabrication remains challenging due to the high reactivity of Ba and Zr, leading to oxidation and phase impurities. Here, we report on further development of a two-step physical vapor deposition process that enables BaZrS3 thin films with high crystallinity (FWHM of the XRD peak corresponding to the (121) plane of BaZrS3 decreases by 38%) and exceptionally low oxygen content (3% in bulk), which is difficult to achieve with conventional methods. The process involves cosputtering of Ba-Zr metal precursors followed by thermal treatment in a sulfur-rich atmosphere, with a SnS capping layer protecting the precursor from oxidation during transfer and annealing. A comparative study of cosputtered and layered metal precursors reveals that intermixing of Ba and Zr during deposition is essential for uniform sulfurization and improved crystallinity. Introducing a Zr interlayer effectively prevents reactions between the metal precursor and the substrate, enabling BaZrS3 growth on silicon, quartz, and metallic substrates while minimizing the impurity phase formation. By investigating the reaction mechanism, differential metal reactivity, and detrimental role of ZrO2, we demonstrate that protecting Zr from oxidation is crucial for phase purity and film quality. We demonstrate that slowing down the evaporation of the SnS capping layer using additional SnS vapor during sulfurization effectively blocks oxygen diffusion into the film, resulting in highly crystalline BaZrS3 thin films. These findings provide fundamental insights into BaZrS3 synthesis and offer a scalable approach for integrating chalcogenide perovskites into photovoltaic applications.

The chalcogenide perovskite BaZrS3 is a semiconductor that exhibits a high absorption coefficient and is composed of earth-abundant elements, making it a promising candidate for sustainable optoelectronic devices. To integrate BaZrS3 thin films into devices, one needs to obtain clean surfaces and characterize them thoroughly. Herein, we report a sputtering-annealing method to produce clean surfaces of prefabricated BaZrS3 thin films with varying metal ratios (Ba-rich, stoichiometric, and Zr-rich). This method combines Ar sputtering with high-temperature annealing (600 and 750 °C) in ultra-high vacuum. Depth-profiling via photoelectron spectroscopy with soft (950 eV) and hard X-rays (6.6 keV) confirms that this processing route substantially mitigates undesired surface oxidation of the films, revealing predominantly core level peaks characteristic of the BaZrS3 perovskite. As a drawback, the sputtering process also produces Zr0, which persists in the Zr-rich sample even after the annealing treatment. In contrast, the Ba-rich and stoichiometric BaZrS3 samples converge to similar surface compositions free of Zr0, and the low roughness of the Ba-rich thin film indicates its preference for device integration. While the thermal treatment modifies the surface chemistry, the bulk characteristics, e.g., nominal metal-ratio, orthorhombic structure, and crystallite sizes, remain unaffected. However, high-temperature annealing affects band realignment with respect to the Fermi level, resulting in n-type doping characteristics. By correlating the experimentally measured valence band to the density functional theory calculated molecular orbital picture, we assign the valence band features to specific elemental orbitals and their interactions. The proposed cleaning procedure has the potential to advance the application of BaZrS3 in layered devices, such as photovoltaic cells and photodetectors.

Aqueous processing offers an environmentally friendly alternative to conventional organic solvent-based methods for producing Ni-rich cathodes for lithium-ion batteries (LIBs). However, interaction with water can drive phase reconstruction at the cathode surface region, increasing activation polarization and charge transfer resistance, which can partially lower specific capacity. This study investigates the beneficial effect of Ti(iv) incorporation on single-crystalline Ni-rich LiNi0.8Mn0.1Co0.1O2 (SNMC) cathodes and demonstrates that the resulting Ti-rich, Ni-depleted surface layer enhances overall structural stability. Specifically, high-resolution scanning transmission electron microscopy images reveal that titanium suppresses undesirable rock-salt phase formation during aqueous processing, electrochemical impedance spectroscopy shows that Ti-incorporated SNMC cathodes exhibit lower charge transfer resistance compared to the conventional SNMC electrodes, and soft X-ray photoelectron spectroscopy exhibits a thinner cathode electrolyte interphase (CEI) layer with Ti incorporation. Improved cycling capacity was evidenced down to the atomic scale by utilizing operando X-ray absorption spectroscopy measurements, revealing stronger changes in the formal valence of all transition metals (or higher redox activity/charge compensation) during battery cycling for Ti-incorporated SNMC electrodes compared to the conventional SNMC electrodes. These results highlight that controlled surface modification using titanium can effectively stabilize the surface structure and reduce charge transfer resistance, offering a viable strategy to improve the structural and electrochemical performance of aqueous-processed Ni-rich cathode materials.

Chalcogenide perovskites exhibit optoelectronic properties that position them as potential materials in the field of photovoltaics. We report a detailed investigation into the electronic structure and chemical properties of polycrystalline BaZrS3 perovskite powder by X-ray photoelectron spectroscopy, complemented by an analysis of its long- and short-range geometric structures using X-ray diffraction and X-ray absorption spectroscopy. The results obtained for the powdered BaZrS3 are compared to similar measurements on a sputtered polycrystalline BaZrS3 thin film prepared through rapid thermal processing. While bulk characterization confirms the good quality of the powder, depth-profiling achieved by photoelectron spectroscopy utilizing Al K-alpha (1.487 keV) and Ga K-alpha (9.25 keV) radiations shows that, regardless of the fabrication method, the oxidation effects extend beyond 10 nm from the sample surface, with zirconium oxides specifically distributing deeper than the oxidized sulfur species. A hard X-ray photoelectron spectroscopy study on the powder and thin film detects signals with minimal contamination contributions and allows for the determination of the valence band maximum position with respect to the Fermi level. Based on these measurements, we establish a correlation between the experimental valence band spectra and the theoretical density of states derived from density functional theory calculations, thereby discerning the orbital constituents involved. Our analysis provides an improved understanding of the electronic structure of BaZrS3 developed through different synthesis protocols by linking it to material geometry, surface chemistry, and the nature of doping. This methodology can thus be adapted for describing electronic structures of chalcogenide perovskite semiconductors in general, a knowledge that is significant for interface engineering and, consequently, for device integration.