We report on the optimization of in situ passivation of ink-based CuIn(S,Se)(2) thin-film solar cells via controlled incorporation of Al2O3 in CuIn(S,Se)(2) films by the addition of Al(NO3)(3) to the molecular ink precursor. For this purpose, the Al/(Al + In) (AAI) metal ratio was varied from 0.05 to 0.30. We observe that the efficiency of the cells made of Al2O3-incorporated CuIn(S,Se)(2) is consistently higher than those without Al2O3, especially due to an improvement in open-circuit voltage (V-OC) and fill factor (FF), for all tested AAI ratios. With an AAI of 0.05, a maximum efficiency of 11.2% and an average efficiency of 8.5% (measured across 18 cells) was achieved, compared to 8.5% maximum efficiency and 6.5% average efficiency for Al-free CuIn(S,Se)(2). Furthermore, we find that cells made of Al2O3-incorporated CuIn(S,Se)(2) with an AAI of 0.2 show a narrow distribution in the photovoltaic performance, indicating higher reproducibility and higher FF. Energy-dispersive X-ray spectroscopy shows that, at AAI = 0.2, Al2O3 is distributed more homogeneously at the surface of the Al2O3-incorporated CISSe. Capacitance-voltage measurements reveal a reduced defect density by incorporation of Al2O3, which could be partly responsible for the higher V-OC. Furthermore, using detailed surface analysis with various X-ray and electron spectroscopy methods, we derive chemical and electronic structure information from the surface. With ultraviolet photoelectron (UPS) and inverse photoemission spectroscopies (IPES), the electronic band gap of the CuIn(S,Se)(2) thin-film surface is found to increase from 1.22 to 1.88 eV (+/- 0.12 eV) with Al2O3 incorporation. This is accompanied by a clear reduction of the conduction band spike at the CdS/CISSe interface due to Al2O3 addition, as derived by both UPS and IPES as well as temperature-dependent V-OC measurements.

In this paper, wide-gap Cu (In,Ga)(S,Se)2 thin-film solar cells are studied in view of their performance, limitations, and opportunities for further optimization. To this end, a wide variety of properties is investigated. This includes the role of gallium gradients, grain size effects, electronic properties, doping metastabilities, and minority carrier lifetime. Particular emphasis is placed on the impact of alkali atoms. A comparison of surface, interface, and grain boundary chemistry shows systematic atomic accumulation and depletion effects. This leads to electronic modifications in the grain boundary regions of the absorber. Heavy alkali treatments also influence the device properties, giving a clear boost of open-circuit voltage. By the combination of different experimental results, this positive open-circuit voltage effect has been explained in terms of reduction of interface recombination. The latter effects are discussed in view of a possible alkali-indium-selenium bond formation at the interface between the absorber and the buffer layer. The properties of a 14.2%-efficient Cu (In,Ga)Se2-based device with [Ga]/([Ga] + [In]) = 0.8 and a wide optical band gap of 1.48 eV are investigated, also in view of further opportunities for improvement.

Using non-classical divalent lanthanide precursors as multi-electron reducing agents, complexes, [K(18-crown-6)(Cp” 2 LnTe 3 )], with Ln = La, Ce, and Nd, Cp” = 1,3-bis-(trimethylsilyl)cyclopentadienyl, were synthesized and investigated to elucidate the mechanism of lanthanide-tellurium bonding and the role of 4 f -element electron density in stabilizing small chalcogenide chains. Density-Functional Theory (DFT) reveals that the frontier molecular orbitals of these complexes are predominantly localized on the [Te 3 ] 2- fragment, while the trivalent lanthanide ions stabilize the tellurium chain through weak but measurable metal-ligand interactions. To experimentally resolve these interactions, we focus on complementary ligand- and metal-centered X-ray spectroscopic approaches. Ln L 3 -edge high-resolution XANES (HR-XANES) and valence-band resonant inelastic X-ray scattering (VB-RIXS) demonstrate that the Ln-Te/C interaction has substantial Ln 5 d orbitals contribution, particularly for the Ln-Te bond, and remains largely constant across the three complexes. DFT-based bond analysis provides a mechanistic interpretation of these trends. The Ln–C interaction exhibits increasing electron density at the bond critical point and a higher delocalization index (QTAIM analysis) from the lighter to the heavier lanthanides, reflecting enhanced Ln 4 f participation within an energy-driven covalency regime. The Ln–Te interaction is predominantly electrostatic, with meaningful orbital contributions arising mainly from Ln 5 d participation within an orbital-overlap driven covalency regime. These results demonstrate that the Ln-C and Ln-Te bonding in the present complexes follow fundamentally distinct covalency mechanisms, which together enable the stabilization and isolation of the small [Te 3 ] 2- fragment chain.

Defect states are crucial in many electronic devices. Oxygen vacancies, for example, are common in transparent conductive oxides and can both be beneficial (e.g., as a dopant to enhance conductivity) as well as detrimental (e.g., leading to reduced device performance by recombination at interfaces). Using soft and hard X-ray photoelectron spectroscopy (PES and HAXPES) with excitation photon energies ranging from 0.1 to 6.3 keV, we study the electronic surface structure of differently processed Ga2O3 samples in a depth-resolved fashion. Specifically, we investigate a Ga2O3 thin film, as used in Cu-(In,Ga)-Se2-based thin-film solar cells, as well as a β-Ga2O3 single crystal before and after a defect-inducing Ar+-ion treatment. Spectra calculations based on density functional theory (DFT) are used to explain the PES and HAXPES valence band signatures as a function of the excitation photon energy. The β-Ga2O3 single crystal spectra can be well described by the DFT calculations. In contrast, the Ga2O3 thin film and Ar+-ion treated β-Ga2O3 show significant spectral broadening of the valence band features and additional spectral intensity close to the valence band maximum, which we assign to defect states. This additional intensity varies as a function of probing depth, suggesting that these defects are mostly localized at the surface. We also discuss the importance of DFT-based spectra calculations to verify the proper determination of valence band maxima using a linear extrapolation.

Abstract S L2,3 X-ray emission spectroscopy (XES) and spectra calculations based on density functional theory are used to study the electronic valence structure of Mn, Fe, Co, and Cu transition metal sulfides, without (MnS, FeS, CoS, and Cu2S) and with (MnS2, FeS2, CoS2, and CuS) sulfur dimers in the crystal structure. We find the upper valence band region in the experimental spectra to be very sensitive to the transition metal, while a splitting in the transitions from the S 3s derived bands indicates the presence of sulfur dimers. This makes S L2,3 XES particularly well suited to the chemical speciation of these compounds. The spectral changes are reproduced by our spectra calculations, which allows for a detailed understanding of the electronic structure based on the band structure and the projected density of states. The splitting of the S 3s derived bands in the dimer compounds is found to increase with decreasing sulfur–sulfur distance, indicating an increase of covalency. The splitting in our experimental XES spectra is found to be smaller than in photoemission data and our spectra calculations, which we attribute to nuclear dynamics on the time scale of the X-ray emission process. In particular, the high sensitivity of S L2,3 XES to the presence of sulfur dimers, together with its suitability for operando studies, makes it a key technique for future studies on a wide range of relevant material systems.

While Zintl compounds comprising Bi-based anions of p-block elements have been used as starting materials for larger bismuth-based clusters, their potential to form nanoparticles has not yet been explored. Here, bimetallic nanoparticles are synthesized from the oxidation of bismuth-based pseudo-tetrahedral Zintl anions (InBi3)2-, (Sn2Bi2)2-, (TlBi3)2-, and (Pb2Bi2)2-. The anions rapidly oxidize at ambient conditions to form metallic seeds, with the mild oxidizing agent PVP (polyvinylpyrrolidone), which also serves as a stabilizing agent for nanoparticle growth. Each elemental combination behaves uniquely, resulting in bimetallic nanoparticles of varying forms (i.e., alloyed, core-shell, and Janus-type). The resulting nanoparticles show a relatively narrow size distribution with median diameters of ∼20-25 nm and exhibit ultraviolet (UV) absorption, with spectral features tunable by composition. The morphology and composition were analyzed by scanning transmission electron microscopy (STEM), high-resolution transmission electron microscopy (HRTEM), micro-X-ray fluorescence spectroscopy (µ-XFS), powder X-ray diffraction (PXRD), synchrotron-based hard and soft X-ray photoelectron spectroscopy (HAXPES and PES), and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). This approach demonstrates that binary Zintl anions can serve as versatile molecular precursors for designing heterometallic nanoparticles with controlled composition, morphology, and optical properties.

The light metal Magnesium and the hard metal Molybdenum are among the immiscible metals whose combination is expected to yield interesting material properties. Through forced co-reduction near room temperature in the liquid phase, we are now able to produce MgMo nanoparticles (1.6±0.2 nm) in a 1 : 1, 2 : 1, and 1 : 2 ratio with statistical distribution of Mg and Mo at the atomic level. Using these nanoparticles as shuttles, pressed, bulk-like MgMo pellets can be realized. Beside the formation of the first bimetallic MgMo alloy, we show that its material properties differ significantly from those of the Mg and Mo monometals treated similarly. Although containing 50% Mg, the bimetallic MgMo alloy is characterized by a reactivity and chemical stability similar to Mo, a specific gravity close to Mg, hightemperature crystallization similar to Mo, and an electrical conductivity of the bimetallic MgMo alloy seven orders of magnitude lower than the similarly treated monometallic Mo. Combining previously immiscible metals via nanoparticles as shuttles to new alloys can generally provide powerful access to new metallic materials such as MgMo and beyond.

The role of the equatorial ligands and their influence on the electronic structures and bonding properties of uranyl and other actinyls are not well understood and are thus at the forefront of actinide research. In the study presented here, we found that the good energy match of uranyl(VI) with F- valence orbitals leads to substantial changes in the uranyl electronic structure, compared to uranyl-Cl- and uranyl-Br-. The good energy match between uranyl(VI) and F- likely enhances the stability of the uranyl-F- bond, contributing to the higher U-F- affinity in aqueous solution compared to uranyl-Cl-/Br-, which is also demonstrated for plutonyl(VI). These findings are based on studies of equatorial and axial ligand covalency in three uranyl halides: NaRb8(UO2)5F19·2H2O, Rb2UO2Cl4·2H2O, and Rb2UO2Br4·2H2O. We describe covalent uranium-halide interactions, following the trend Br- ≈ Cl- > F-. Ligand K-edge XANES and DFT (including TDDFT and LFDFT) reveal significant electronic structure differences, with the F-based compound having a uranyl-based HOMO, while Cl- and Br-based compounds show predominant ligand p character in the HOMO. A newly introduced theoretical index evaluates bond covalency. U M4 edge HR-XANES and RIXS exhibit unexpected σ* peak trends not directly correlated with U═O bond lengths but well explained by LFDFT RIXS calculations.

The bulk electronic structure of β-Ga2O3 single crystals was investigated using oxygen K-edge x-ray emission and absorption spectroscopy as well as resonant inelastic soft x-ray scattering. Spectra were obtained for different orientations of the crystal planes with respect to the polarization vector of the incident x-ray beam. The spectra are analyzed with calculations based on density functional theory and using the Bethe–Salpeter equation in the ocean code to take the core–hole interaction into account. These calculations correctly capture all the main features in the experimental spectra, demonstrating the potential of the approach to predict the electronic properties of similar compounds. We find a pronounced anisotropy as a function of the excitation polarization vector and significant differences in the spectral contributions from the inequivalent oxygen atoms of β-Ga2O3.

A detailed characterization of the impact of a RbF post-deposition treatment (RbF-PDT) on the chemical structure of a wide-gap Cu(In, Ga)Se2 thin-film solar cell absorber surface with a high Ga/(Ga + In) (GGI) ratio of 0.9 is presented. Using synchrotron- and lab-based x-ray photoelectron spectroscopy, as well as x-ray-excited Auger electron spectroscopy, we observe distinct differences to RbF-PDT on absorber surfaces with the common GGI of ∼0.3. In particular, RbF-PDT reduces sodium and oxide content at the surface, while the copper concentration at the surface is not affected. We find no spectral evidence for the formation of a distinct Rb–In–Se surface layer. In addition, we observe that the GGI ratio at the surface is slightly decreased due to a reduction of the Ga and an increase in the In concentration, which may explain the observed improvement in the power conversion efficiency after the PDT (from 6.8% to 7.3%).

Sulfur-based passivation for silicon surfaces using H2S gas is an alternative passivation method to reduce the thermal budget for Si photovoltaics. To understand the impact of the high-quality passivation and an observed passivation efficiency decrease after air exposure, we have studied the chemical surface structure by X-ray photoelectron spectroscopy (XPS), X-ray Auger electron spectroscopy (XAES), and S and Si L2,3 X-ray emission spectroscopy (XES). On the S-passivated silicon surfaces, we find the formation of S-Si bonds, in addition to some Si-O bonds. Upon air exposure, sulfur partially desorbs from the Si surface and an increased presence of Si-O and S-O bonds is observed. We identify that well-defined S-Si bonds are crucial to maintain high-quality surface passivation for Si photovoltaics, which allows further optimization of the fabrication process for S-based passivation on silicon.

The chemical and electronic structure of the CdS/(Ag,Cu)(In,Ga)Se 2 (CdS/ACIGSe) interface for thin‐film solar cells, involving an absorber with a bulk [Ag]/([Ag]+[Cu]) (AAC) ratio of 0.06, a state‐of‐the‐art RbF post‐deposition treatment (PDT), and a chemical‐bath deposited CdS buffer layer, is studied. To gain a detailed and depth‐resolved picture of the CdS/ACIGSe interface, synchrotron‐ and laboratory‐based hard X‐ray, soft X‐ray, and UV photoelectron spectroscopy, inverse photoemission spectroscopy, and X‐ray emission spectroscopy are combined. Compared to the bulk of the absorber, a Cu‐ and Ga‐poor ACIGSe surface is found, with a slightly increased AAC ratio. Strong evidence of a Rb–In–Se species (possibly with some Ag) at the absorber surface is compiled, with a corresponding band gap of 2.79 ± 0.12 eV. This finding is in clear contrast to comparable Ag‐free Cu(In,Ga)Se 2 absorbers with RbF‐PDT. The Rb–In–Se surface species is not removed by the (wet‐chemical) CdS deposition process, while some Se diffuses into the CdS layer and segregates at its surface. The CdS buffer layer shows a band gap of 2.48 ± 0.12 eV, and a cliff (≈ −0.4 eV) is determined in the conduction band alignment at the interface between the Rb–In–Se species and the CdS buffer.

This work reports on the application of sulfur (S)-passivation to passivated emitter and rear contact (PERC) solar cells. The emitter surface was passivated by hydrogen sulfide (H 2 S) gas phase reaction and capped by a hydrogenated amorphous silicon nitride (a-SiN x :H) layer. The sulfur passivation on a symmetrically n + diffused emitter is shown to lead to an emitter saturation current density (J 0n+ ) of 30 fA/cm 2 at R sheet,n+ ≈ 100 Ω/sq. The application of S-passivation to the emitter surface in the PERC cell structure, with the rear surface passivated by an aluminum oxide (Al 2 O 3 )/a-SiN x :H stack, showed a promising implied open-circuit voltage (iV OC ) of 686 mV before metallization. This iV OC was higher than that for the a-SiN x :H or SiO 2 /a-SiN x :H passivated emitter surfaces (675 and 674 mV, respectively) on PERC cells processed in the same run. However, a significant drop in cell V OC is observed for the S-passivated PERC cell after the completion of device fabrication with laser patterning, screen-printed metal contact deposition, and firing. Nonetheless, an efficiency of ∼20% and a V OC of ∼650 mV was achieved with an emitter surface passivated by sulfur. We identified that the 760 °C contact firing process degrades the S-passivation quality. The surface morphology was studied, and a detailed surface analysis was performed to study the causes of the S-passivated surface degradation.

Dimethyl sulfoxide (DMSO) is an important polar solvent that derives its unique properties from the lone pair and the strong polar bond at the sulfinyl functional group. To derive the local and symmetry-resolved electronic structure of liquid DMSO, we have used resonant inelastic soft X-ray scattering (RIXS) maps at the S L2,3, C K, and O K edges. The experimental data are compared to calculations of spectra based on density functional theory, which allows a detailed analysis of the molecular orbitals throughout the molecule. In the RIXS maps, we find the signature of molecular-field splitting of the S 2p core levels, vibronic coupling, and ultrafast nuclear dynamics on the time scale of the RIXS process.

Phonons play a crucial role in thermalization and non-radiative recombination losses in semiconductors, impacting the power conversion efficiency of solar cells. To shed light on the lattice dynamics in Cu2ZnSn(S-x,Se1-x)(4) (CZTSSe) thin-film solar cells and validate the extensive number of theoretical studies, we determine the Sn-119-partial phonon density of states (Sn-PDOS) by nuclear inelastic X-ray scattering. CZTSSe-based devices, one with near-stoichiometric and two with off-stoichiometric compositions, are investigated, and the results are correlated with the corresponding power conversion efficiencies (PCEs) of 3.2, 7.6, and 10.6%, respectively. Compared to the near-stoichiometric cell, the main Sn-PDOS peak of the off-stoichiometric cells broadens and slightly shifts to higher energy; this effect is correlated with the type and concentration of the characteristic defects in the studied samples. Furthermore, the Sn-PDOS of the 10.6% device is also obtained under operando (maximum power point) and open-circuit conditions. A comparison of the Sn-PDOS before and after the operando measurements suggests structural changes, likely due to the formation of metastable defects. In agreement with the theoretical studies, the Sn-PDOS of the CZTSSe absorber shows additional peaks compared to CZTSe attributed to coupling of Sn to the vibrations of Se and S atoms. This work paves the way for a further understanding of the lattice dynamics and subsequent enhancement of the PCEs of thin-film solar cells as well as other applied materials and devices containing elements that are M & ouml;ssbauer-active and hence suitable for nuclear inelastic scattering.

The design and first results of a high-transmission soft X-ray spectrometer operated at the X-SPEC double-undulator beamline of the KIT Light Source are presented. As a unique feature, particular emphasis was placed on optimizing the spectrometer transmission by maximizing the solid angle and the efficiencies of spectrometer gratings and detector. A CMOS detector, optimized for soft X-rays, allows for quantum efficiencies of 90% or above over the full energy range of the spectrometer, while simultaneously offering short readout times. Combining an optimized control system at the X-SPEC beamline with continuous energy scans (as opposed to step scans), the high transmission of the spectrometer, and the fast readout of the CMOS camera, enable the collection of entire rapid resonant inelastic soft X-ray scattering maps in less than 1 min. Series of spectra at a fixed energy can be taken with a frequency of up to 5 Hz. Furthermore, the use of higher-order reflections allows a very wide energy range (45 to 2000 eV) to be covered with only two blazed gratings, while keeping the efficiency high and the resolving power E/ΔE above 1500 and 3000 with low- and high-energy gratings, respectively.

The hybridization state in solids often defines the critical chemical and physical properties of a compound. However, it is difficult to spectroscopically detect and evaluate hybridization beyond just general fingerprint signatures. Here, the valence-band hybridization of metal d-derived bands (short: "metal d bands") in selected metal sulphides is studied with a combined spectroscopic and theoretical approach to derive deeper insights into the fundamental nature of such compounds. The valence bands of the studied sulphides are comprised of hybrid bands derived from the metal d, S 3s, and S 3p states. Employing S K and L2,3 X-ray emission spectroscopy and spectra calculations based on density functional theory, the degree of hybridization (i.e., the covalency) of these bands can be directly probed as a function of their relative energies. We find that the relative intensity of the "metal d band" features in the spectra scales with the inverse square of the energy separation to the respective sulfur-derived bands, which can be analytically derived from a simple two-orbital model. This study demonstrates that soft X-ray emission spectroscopy is a powerful tool to study valence state hybridization, in particular in combination with hard X-ray emission spectroscopy, promising a broad impact in many research fields. Valence-band hybridization in sulphides is studied with X-ray spectroscopy in experiment and theory. The admixture of the "atomic" states to the hybrid bands scales with their inverse energy separation, which we describe by a simple two-state model.

The chemical and electronic structure of the front contact i-ZnO/InxSy:Na interface for Cu(In,Ga)(S,Se)(2)-based thin-film solar cells is investigated using a combination of x-ray and electron spectroscopies. Upon i-ZnO sputter deposition on the InxSy:Na buffer layer, we find an intermixed heterojunction and the formation of InOx and Na2SO4. The window layer is shown to consist of a mixture of Zn(OH)(2) and ZnO, with decreasing relative Zn(OH)(2) content for thicker window layers. Moreover, we observe diffusion of sodium to the surface of the window layer. We derive electronic surface band gaps of the i-ZnO and InxSy:Na layers of 3.86 +/- 0.18 eV and 2.60 +/- 0.18 eV, respectively, and find a largely flat conduction band alignment at the i-ZnO/InxSy:Na interface.

Interfacial engineering has fueled recent development of p-i-n perovskite solar cells (PSCs), with self-assembled monolayer-based hole-transport layers (SAM-HTLs) enabling almost lossless contacts for solution-processed PSCs, resulting in the highest achieved power conversion efficiency (PCE) to date. Substrate interfaces are particularly crucial for the growth and quality of co-evaporated PSCs. However, adoption of SAM-HTLs for co-evaporated perovskite absorbers is complicated by the underexplored interaction of such perovskites with phosphonic acid functional groups. In this work, we highlight how exposed phosphonic acid functional groups impact the initial phase and final bulk crystal structures of co-evaporated perovskites and their resultant PCE. The explored surface interaction is mediated by hydrogen bonding with interfacial iodine, leading to increased formamidinium iodide adsorption, persistent changes in perovskite structure, and stabilization of bulk α-FAPbI3, hypothesized as being due to kinetic trapping. Our results highlight the potential of exploiting substrates to increase control of co-evaporated perovskite growth.

It is the purpose of this presentation to demonstrate how the unique capabilities of novel x-ray spectroscopy methods (in particular element-specific soft x-ray emission spectroscopy, XES) can be employed to derive important insights into PEC and other solar devices from the viewpoint of sulfur. Of course, other elements will also play a role, as will delocalized valence and conduction bands, but sulfur is unique and deserves to be celebrated. The presentation will include results from lab-based x-ray and UV photoelectron spectroscopy, inverse photoelectron spectroscopy, and x-ray-excited Auger electron spectroscopy, complemented by XES and soft x-ray absorption spectroscopy using high-brilliance synchrotron radiation. A particularly powerful approach is their resonant combination, Resonant Inelastic soft X-ray Scattering (RIXS), best displayed in a “RIXS map”. In the presentation, we will discuss the impact of sulfur on the electronic level alignment at surfaces and interfaces, and how to go about learning more about local chemical bonding (hybridization) in the vicinity of sulfur atoms, using a large variety of compounds as examples – sulfides, sulfates, chalcopyrites, ... The presentation will thus also include a bit of a discussion of cutting-edge instrumentation recently developed for studying buried layers and materials in in situ and operando environments, such as the novel X-SPEC beamline and its unique endstations at the KIT Light Source in Karlsruhe, Germany.