Epitaxial BiFeO3 (BFO) thin films grown on SrTiO3 (001) and Nb: SrTiO3 (001) substrates were subjected to post-deposition sulfurization by sulfur vapor at 550 °C. This leads to a drastic change of bandgap from 2.68 to 1.72 eV, accompanied with an increase in the absorption coefficient of the film. The energy level diagram was determined for the films before and after sulfurization using Kelvin probe and ambient pressure photoemission spectroscopy. In addition, surface photovoltage (SPV) measurements indicate that due to the reduction of the bandgap the photovoltaic response is drastically improved after sulfurization. Ab initio Density Functional Theory calculations predict that substituting O by S in BFO leads to a reduction of the bandgap. However, sulfurization leads to a severe degradation of the quality of the films so that ferroelectricity and epitaxy are no longer present. Therefore, the structural nature of the sulfurized phase obtained is unknown. Still, these results highlight the potential of sulfur incorporation into oxides to achieve lower bandgap material.

Emerging stable, non-toxic and lead-free perovskites based on earth-abundant elements is a central challenge for optoelectronic and photovoltaic applications. Chalcogenide and oxychalcogenide perovskites offer a promising route toward this goal, but their integration into solar-cell architectures is still limited by relatively wide bandgaps and modest carrier transport. Herein, we report Ti doped CaZrO3-xSx (CZOS) thin films grown on LaAlO3(001) substrate by direct pulsed laser deposition (PLD). Across CaZr1-xTix(O,S)3 films with Ti concentration ranging from 0 to10%, the optical bandgap is reduced by ~550 meV, reaching 1.9 eV at 10% Ti. The evolution of the electronic energy levels reveals the combined effects of Ti incorporation and variations in the O/S ratio across the doping series. In parallel, the carrier density is tuned toward a range relevant for a photovoltaic semiconductor absorber, while the electron mobility is improved with Ti content, approaching around 30 cm2 V-1 s-1 at the highest doping levels. This combination of direct thin-film processing, bandgap tunability, and enhanced charge-transport properties places Ti-doped CZOS thin films a promising lead-free semiconductor absorber, particularly for tandem or indoor solar cell applications.

Organic-inorganic halide perovskites are naturally unstable when exposed to various environmental conditions, and the presence of lead raises significant ecological concerns. Herein, calcium zirconium oxysulfide, CaZrO3-xSx, a lead-free oxychalcogenide perovskite, is investigated as a potential alternative for optoelectronic and photovoltaic applications. The environmental aging of CaZrO3-xSx thin films was systematically investigated under various external stress conditions, including humidity, temperature, and illumination. Structural and optical investigations indicate that the film retains its crystallinity and remains optically responsive up to 350 degrees C, without detectable degradation or phase transformation. Under high (80%) humidity conditions, the film remains stable for a few days before gradually converting into the oxide phase. The demonstrated environmental and thermal stability of CaZrO3-xSx highlights its potential as a promising, intrinsically stable, and lead-free material for durable photovoltaic and optoelectronic applications.

We aim to identify alternative perovskite candidates for optoelectronic and photovoltaic applications that would combine several key requirements, such as stability, low toxicity, scalability, abundance, and efficacy. Among the most promising candidates are chalcogenide and oxysulfide perovskites, such as CaZrS3 or CaZrO3-xSx. While several methods to grow such materials involve post-sulfurization of precursors, in this work, we investigate the direct growth of CaZrO3-xSx films by pulsed laser deposition. Growth conditions were explored on various substrates, including LaAlO3(001) (LAO), SrTiO3(001) (STO), SrTiO3:Nb(001) (Nb:STO), YAlO3(001) (YAO), and Al2O3(1-102) (ALO), over a temperature range from 600 degrees C to 900 degrees C. The resulting films were systematically characterized in terms of their structural, optical and electrical properties, as well as their semiconducting energy levels. The most promising results were obtained on LAO substrates at a growth temperature of 800 degrees C, yielding improved crystallinity and a bandgap of 2.4 eV, the lowest among the investigated films. This film also exhibited a very high and promising surface photovoltage signal, reaching 900 mV at 0.2 sun. However, to further improve the material's suitability for photovoltaic integration, particularly in single junction or tandem architectures, additional bandgap tuning via doping may be necessary to enhance electrical transport and broaden absorption.

The imprint effect, one of the most critical parameters in ferroelectrics, manifests as an asymmetry in polarization reversal. The precise control of imprint is essential for reliable implementations of ferroelectric materials in many applications. In this study, we demonstrate that imprint can be tuned in Bi2FeCrO6 (BFCO) thin films by changing the nature of the underlying layer. Specifically, BFCO(111) films deposited on Nb-doped SrTiO3 (Nb:STO) and SrRuO3 (SRO) exhibit distinct imprint characteristics. Piezoresponse force microscopy reveals a consistent imprint difference of about 1 V between BFCO/Nb:STO and BFCO/SRO, which correlates with the work function difference between the two substrates. These findings underscore a generalizable mechanism for imprint control in ferroelectric heterostructures and hold particular relevance for applications demanding fine polarization management.

Among ferroelectric oxides, Bi2FeCrO6 (BFCO) stands out due to its high polarization and relatively low bandgap of 1.5 eV, which make it promising for optoelectronic applications based on visible-light absorption. Its properties such as bandgap width, polarization switching bias, and imprint of the ferroelectric hysteresis loop can be largely modulated by inducing different types of defects in the structure such as Fe (Cr) antisite defects or oxygen vacancies. In this work, we demonstrate that Bi vacancies can also play a key role in the properties of epitaxial BFCO thin films grown by pulsed laser deposition. Notably, we show that increasing the deposition temperature enhances the concentration of Bi vacancies, which, in turn, significantly impacts both the transport and ferroelectric properties. The film shows a p-type character, which is consistent with density functional theory (DFT) calculations on BFCO containing Bi vacancies. In extreme cases, the lack of Bi can lead to nonmiscible highly Bi-deficient phases with high conductivities that prevent the establishment of an electric field across the film, thereby hindering its polarization control.

Silicon clathrates, which exhibit a direct or near-direct bandgap, represent a promising alternative to conventional diamond cubic silicon for optoelectronic applications. They combine the elemental abundance and nontoxicity of Si with a more favorable bandgap and higher optical absorption at room temperature. Their open cage-like frameworks allow for reversible guest atom incorporation with minimal lattice distortion, making them attractive not only for optoelectronics but also for energy storage applications. However, conventional synthesis methods typically require the use of sodium as a structure-directing agent to stabilize the metastable clathrate lattice during thermal decomposition. The resulting sodium incorporation degrades the optoelectronic properties and complicates device integration. Herein, a nontoxic, scalable, vacuum-annealing approach is utilized that effectively removes sodium from silicon clathrates films without the use of hazardous chemicals, such as iodine, HF, or SF6. Optoelectronic characterization reveals a significant improvement in semiconducting behavior, including enhanced surface photovoltage response, marking a critical step toward developing sodium-free silicon clathrate thin films as efficient light absorbers. This vacuum annealing process is compatible with large-area substrates and offers a key step toward a safer, and more sustainable approach to integrating exotic Si phases into the next-generation optoelectronic devices.

BiFeO3 (BFO) epitaxial thin films were grown on SrTiO3 (001) and Nb:SrTiO3 (001) substrates using pulsed laser deposition (PLD). The structural, optical and ferroelectric properties were characterized by X-ray diffraction (XRD), UV-visible spectroscopy, spectroscopic ellipsometry and piezoresponse force microscopy (PFM), confirming the good quality of the BFO films. Sulfur (S) was incorporated in the films using the ion implantation technique with three different doses, aiming for BiFeS0.03O3 (1 % mol. S), BiFeS0.09O3 (3 % mol. S), BiFeS0.3O3 (10 % mol. S), compositions. While XRD shows that the perovskite structure is preserved with a change in lattice parameters, spectroscopic ellipsometry indicates that the optical properties of the highest dose sample are strongly altered. The energy diagrams of the samples could be established with the use of a scanning Kelvin probe system combined with Ambient-Pressure Photoemission Spectroscopy (APS). Results indicated that the presence of S in BFO did not significantly modify the bandgap of the films, while the valence band level (ionization energy) is increased by about 0.3 eV. This shift leads to a strong improvement of Surface Photovoltage properties of the films (maximum from 10 mV to 137 mV), forecasting that devices based on such S-doped films would provide strongly enhanced photovoltaic performance. While the highest dose sample does not display ferroelectric properties anymore, the intermediate dose leads to ferroelectricity comparable or even better to that of the pristine samples. This paves the way for high photoferroelectric effects in S-doped BFO films.

Although silicon clathrates were discovered about 60 years ago, there has been little research on diverse applications of such materials beyond thermoelectrics. With a direct bandgap of about 1.7 eV and given the advantages of the silicon element such as abundance, nontoxicity and stability, silicon clathrates hold potential for use in photovoltaics and optoelectronics. Additionally, due to their unique cage structure that can store and release sodium atoms with minimal lattice parameter changes, they are promising for battery applications. However, issues like nonhomogeneity, defects, and poor density in clathrate films have hindered such applications. We provide in this work substantial pathways to mitigate such issues with the use of SF6 etching and thermal press annealing, enabling an improvement of the optoelectronic properties, by a factor of 7 as observed by the surface photovoltage technique. The photovoltage response of above 200 mV at 0.2 sun being above key photovoltaic thin film absorbers such as CIGS and rivaling III-V semiconductors such as GaAs.

In this study we investigate for the first time the effect of hydrogenation on the properties of type II silicon clathrate films (SiCL). These clathrates are an alternative form of silicon based on a cage structure. It can be either emptied or filled with sodium atoms, leading to a metallic or semiconducting behavior with a tunable direct bandgap of 1.6–1.8 eV. There are a wide range of potential applications for such materials such as in electronics, optoelectronics, photovoltaics, batteries, spintronics or hydrogen (H) storage. However, the role of H in such materials remains largely unexplored and is not well understood experimentally. In this work, we hydrogenate the clathrates films using a H plasma with a substrate temperature of 400 °C. We evaluate the H content in the films by Time-of-Flight Secondary Ion Mass Spectrometry and Elastic Recoil Detection Analysis. The latter indicates a SiH _0.006 molar concentration before and SiH _0.070 after hydrogenation. Such a H content within the SiCL films is too low for practical hydrogen storage applications. Nevertheless, the incorporated H plays the role of dopant, leading to a reduction in the work function by around 0.3 eV. This demonstrates that even a modest hydrogen uptake can significantly enhance the electronic properties of silicon clathrates.

YbxEryZnO thin films with a low concentration (x = 5%, y = 0, 1, 3%) were made on glass substrates using the spray pyrolysis method. The films were characterized through the use of specific techniques to investigate their structural, optical, and electrical properties. The XRD structural analysis of the films revealed that they are polycrystalline with a hexagonal wurtzite structure and a preferential orientation in the (002) direction. The optical characterization of the co-doped layers in the range of 200 to 800 nm revealed that co-doping had a significant impact on the values of transmission. A well-defined peak in the infrared domain centered around 980 nm was observed in photoluminescence measurements. This peak signifies the transition between the electronic levels 2F5/2 (ground state) and 2F7/2 (excited state), proving that photons are efficiently transferred between the ZnO matrix and the Yb3+ ion. All layers exhibited n-type conduction and an electrical resistivity decrease to 6.0 × 10−2 Ω cm according to Hall effect measurements at room temperature.

Silicon clathrates (SiCL) are a class of cage-structured materials with tuneable optoelectronic properties, offering significant potential for photovoltaic applications. Among them, type II SiCL (NaxSi136) are particularly attractive due to their unique ability to reversibly accommodate guest atoms without compromising the crystal structure. Traditional synthesis methods, such as synthesis under extreme pressure or fabrication under glove boxes, although essential for probing the intrinsic properties of clathrates under well-controlled conditions, are not well-suited for scaling up and are often energy-intensive and require specialized equipment. To overcome these limitations, a scalable, glovebox-free synthesis method based on two thermal decomposition steps was developed to produce silicon clathrate films. Post-synthesis treatment, including thermal pressing and reactive ion etching, was employed to enhance the electrical properties of the material. Phase formation and crystallinity are confirmed by X-ray diffraction and Raman spectroscopy; morphology is assessed by scanning electron microscopy (SEM); and optoelectronic properties are evaluated by photoluminescence. This approach provides a reproducible and scalable route to fabricate type II silicon clathrate films, with promising potential for integration into optoelectronic devices.

Silicon clathrates are a unique class of materials with a cage-like structure that offer significant advantages for optoelectronic applications, particularly in indoor photovoltaics (IPV). Their direct bandgaps and tunable electronic properties from metal-like to semiconductor-like behavior enable a wide range of optoelectronic functionalities. In this study, a photovoltaic device is demonstrated based on semiconducting silicon clathrate films (SCF) synthesized from crystalline silicon (c-Si) wafers, with NiOx incorporated as hole selective contact. XPS and Auger spectroscopy confirmed the incorporation of Na within the SCF framework, in agreement with the type-II clathrate structure revealed by XRD analysis. Optical and electronic measurements indicate that the SCF exhibits intrinsic-like behavior with a bandgap of 1.72 eV. Integration of SCF with NiOx resulted in rectification and a clear photovoltaic response under illumination, confirming device functionality. Band alignment analysis suggested efficient hole transport and secondary charge generation from the c-Si substrate. These results highlight the importance of selective contact engineering for efficient charge extraction and demonstrate the potential for further optimization of SCF photovoltaic devices.

Accurate characterization of polarization switching in ferroelectric thin films is critical for their effective integration into functional devices. Among the key parameters used to evaluate the polarization switching behavior, the bias switching voltages and the imprint can easily be obtained from ferroelectric polarization-driven hysteresis loops obtained by piezoresponse force microscopy (PFM). Although local "point & shoot" PFM measurements can provide reliable results on atomically flat films, their application to rough surfaces requires extensive data collection over numerous points, making the process tedious and time consuming. In order to obtain faster results and reduce artifacts, we propose here a rapid and effective method based on readily averaged hysteresis loop measurements. In particular, we demonstrate in the case of rough Bi2FeCrO6 ferroelectric films a spatially averaged off-field hysteresis loop acquisition method that mitigates morphology-induced inhomogeneities and electrostatic field effects. This methodology enables the accurate extraction of ferroelectric switching parameters regardless of the film roughness, offering a robust and efficient alternative for characterizing a wide range of ferroelectric thin films.

The two primary forms of silicon clathrates are Type I (Na8Si46) and Type II (NaxSi136). Both forms have been investigated for diverse silicon-based optoelectronic applications owing to their unique characteristics and cage-like structure. Determining the energy levels of these clathrates is crucial for device manufacturing, particularly because Type I clathrates display metallic characteristics and Type II clathrates are either semiconductors or metal depending on sodium “x” content. In this study, we established the energy levels of both Type I and Type II silicon clathrates. This offers crucial information for customizing these materials to meet specific optoelectronic device application requirements.

SrTiO3 (STO) is a well‐known perovskite oxide often used, among others, as a crystalline substrate for epitaxial deposition. However, its indirect bandgap of 3.25 eV is too high for solar applications. Tentative experiments to reduce its bandgap are very welcome. To this end, sulfurization of STO substrates using ion implantation is performed. The simulated profile is controlled by time‐of‐flight secondary‐ion mass spectrometry. S is at least partially inserted into the STO lattice, as evidenced by X‐ray diffraction and X‐ray photoelectron spectroscopy. UV–vis spectroscopy indicates a drastic reduction of the bandgap of S:STO from 3.25 eV (indirect) down to 2.14 eV for 10% S:STO, a trend also observed by spectroscopic ellipsometry. The results are confronted to ab initio calculations. Besides bandgap, the valence band level, that is the ionization energy, is determined by ambient‐pressure photoemission spectroscopy so that the complete energy diagrams could be plotted. This work paves the way for tuning the bandgap of perovskites in a highly controlled manner for solar energy applications.

Due to their original optoelectronic properties and cage-like structure, type-II silicon clathrate (NaxSi136) films are investigated as a new silicon-based material for energy applications. In view of the devices and applications, understanding the energy levels of these clathrates is crucial and needs to be investigated as they vary strongly with Na concentration. In this study, we determined the optical bandgap energy, work function, ionization energy, and surface photovoltage for a wide range of compositions ranging from Na0.1Si136 to Na23Si136. Our findings reveal that the semiconducting properties and potential as solar cell absorbers are dramatically dependent on the Na concentration, with nearly Na-free cages showing the most promise for device applications.

The controlled switching of spontaneous polarization in ferroelectrics by applying an external electric field is essential for many device operations. Oxygen vacancy defects commonly found in oxide ferroelectrics offer a tempting means for polarization switching regulation at an atomic scale. This study presents a method involving a series of annealing cycles under various environments to modulate the amount of oxygen vacancies in ferroelectric Bi2FeCrO6 thin films. By performing a piezoresponse force microscopy study after each annealing cycle, it is shown that a high content of oxygen vacancies induces specific fingerprints on polarization hysteresis loops. Reducing oxygen vacancy density by annealing in oxygen-rich environments results in hysteresis loops being wider and displaying less imprint, while increasing oxygen vacancy density by annealing in oxygen-poor environments results in thinner and more imprinted hysteresis loops. The process is largely reversible, constituting an alternative method for polarization switching control at specific electric fields. Ferroelectric phase-field calculations and energy level diagrams obtained through ellipsometry, Kelvin probe, and photoemission spectroscopy indicate a combined switching mechanism based on a doping effect due to oxygen vacancies and a built-in electric field originating from the interface. Our study is helpful for designing ferroelectric films with a precise imprint or, more generally, for understanding the impact of charged defects on the polarization states in ferroelectric films.

Rare-earth substituted multiferroic Bi1-xRExFeO3 powders with chemical compositions (x = 0.05, 0.15, 0.20 and RE = Nd3+; Eu3+) are synthesized by sol-gel method. The powder X-ray diffraction results manifest that samples are in single phase (R3c) for x < 0.15; the structural phase transformation from the distorted perovskite R3c to the ideal perovskite (Pm3 m) was made for BFO-Nd at x = 0.15 and for BFO-Eu (BEFO) at x = 0.20. Theoretical analyses allowed us to propose & UGamma;4 (k = 0, 0, 0) as the irreducible representation associated to the distortion of the ideal perovskite Pm3 m to the R3c distorted one, where the displacement of Fe atom along [0 0 1] axis is considered as so the parameter order & eta; = (0; 0; & eta;3), which drives the transition Pm3 m -R3c. Dielectric properties of Bi1-xRExFeO3 change noticeably with concentration and nature of RE-doping. The increase of Nd and Eu concentration decreases clearly the dielectric temperature anomalies corresponding to the ferroelectricparaelectric phase transitions, which are more shifting towards lower temperature when the dopant is Eu. Magnetic measurements indicate that antiferromagnetic BFO-Nd (BNFO) powders became ferromagnetic at (0.15 & LE;x & LE; 0.2), while the BFO-Eu (BEFO, x & LE; 0.15) compounds, antiferromagnetic with a small remanant magnetization, become weak ferromagnetic when x = 0.20.