Impact of Annealing-Induced Intermixing on the Electronic Level Alignment at the In₂S₃/Cu(In,Ga)Se₂ Thin-Film Solar Cell Interface

The interface between a nominal In₂S₃ buffer and a Cu­(In,Ga)­Se₂ (CIGSe) thin-film solar cell absorber was investigated by direct and inverse photoemission to determine the interfacial electronic structure. On the basis of a previously reported heavy intermixing at the interface (S diffuses into the absorber; Cu diffuses into the buffer; and Na diffuses through it), we determine here the band alignment at the interface. The results suggest that the pronounced intermixing at the In₂S₃/CIGSe interface leads to a favorable electronic band alignment, necessary for high-efficiency solar cell devices

Finding Correlations of the Oxygen Reduction Reaction Activity of Transition Metal Catalysts with Parameters Obtained from Quantum Mechanics

To facilitate a less empirical approach to developing improved catalysts, it is important to correlate catalytic performance to surrogate properties that can be measured or predicted accurately and quickly, allowing experimental synthesis and testing of catalysts to focus on the most promising cases. Particularly hopeful is correlating catalysis performance to the electronic density of states (DOS). Indeed, there has been success in using just the center of the d-electron density, which in some cases correlates linearly with oxygen atom chemisorption energy, leading to a volcano plot for catalytic performance versus “d-band center”. To test such concepts we calculated the barriers and binding energies for the various reactions and intermediates involved in the oxygen reduction reaction (ORR) for all 12 transition metals in groups 8–11 (Fe–Cu columns). Our results show that the oxygen binding energy can serve as a useful parameter in describing the catalytic activity for pure metals, but it does not necessarily correlate with the d-band center. In addition, we find that the d-band center depends substantially on the calculation method or the experimental setup, making it a much less reliable indicator for ORR activity than the oxygen binding energy. We further examine several surfaces of the same pure metals to evaluate how the d-band center and oxygen binding energy depend on the surface

Using Photoelectron Spectroscopy and Quantum Mechanics to Determine d‑Band Energies of Metals for Catalytic Applications

The valence band structures (VBS) of eight transition metals (Fe, Co, Ni, Cu, Pd, Ag, Pt, Au) were investigated by photoelectron spectroscopy (PES) using He I, He II, and monochromatized Al Kα excitation. The influence of final states, photoionization cross-section, and adsorption of residual gas molecules in an ultrahigh vacuum environment are discussed in terms of their impact on the VBS. We find that VBSs recorded with monochromatized Al Kα radiation are most closely comparable to the ground state density of states (DOS) derived from quantum mechanics calculations. We use the Al Kα-excited PES measurements to correct the energy scale of the calculated ground-state DOS to approximate the “true” ground-state d-band structure. Finally, we use this data to test the d-band center model commonly used to predict the electronic-property/catalytic-activity relationship of metals. We find that a simple continuous dependence of activity on d-band center position is not supported by our results (both experimentally and computationally)

Ion-Solvation-Induced Molecular Reorganization in Liquid Water Probed by Resonant Inelastic Soft X‑ray Scattering

The molecular structure of liquid water is susceptible to changes upon admixture of salts due to ionic solvation, which provides the basis of many chemical and biochemical processes. Here we demonstrate how the local electronic structure of aqueous potassium chloride (KCl) solutions can be studied by resonant inelastic soft X-ray scattering (RIXS) to monitor the effects of the ion solvation on the hydrogen-bond (HB) network of liquid water. Significant changes in the oxygen <i>K-edge</i> emission spectra are observed with increasing KCl concentration. These changes can be attributed to modifications in the proton dynamics, caused by a specific coordination structure around the salt ions. Analysis of the spectator decay spectra reveals a spectral signature that could be characteristic of this structure

Labile or Stable: Can Homoleptic and Heteroleptic PyrPHOS–Copper Complexes Be Processed from Solution?

Luminescent Cu­(I) complexes are interesting candidates as dopants in organic light-emitting diodes (OLEDs). However, open questions remain regarding the stability of such complexes in solution and therefore their suitability for solution processing. Since the emission behavior of Cu­(I) emitters often drastically differs between bulk and thin film samples, it cannot be excluded that changes such as partial decomposition or formation of alternative emitting compounds upon processing are responsible. In this study, we present three particularly interesting candidates of the recently established copper–halide–(diphenylphosphino)­pyridine derivatives (PyrPHOS) family that do not show such changes. We compare single crystals, amorphous bulk samples, and neat thin films in order to verify whether the material remains stable upon processing. Solid-state nuclear magnetic resonance (MAS ³¹P NMR) was used to investigate the electronic environment of the phosphorus atoms, and X-ray absorption spectroscopy at the Cu K edge provides insight into the local electronic and geometrical environment of the copper­(I) metal centers of the samples. Our results suggest thatunlike other copper­(I) complexesthe copper–halide–PyrPHOS clusters are significantly more stable upon processing and retain their initial structure upon quick precipitation as well as thin film processing

Direct Observation of Reactant, Intermediate, and Product Species for Nitrogen Oxide-Selective Catalytic Reduction on Cu-SSZ-13 Using <i>In Situ</i> Soft X‑ray Spectroscopy

Catalytic processes have supported the development of myriad beneficial technologies, yet our fundamental understanding of the complex interactions between reaction intermediates and catalyst surfaces is still largely undefined for many reactions. Experimental analyses have generally been limited to investigation of catalyst materials or a subset of functional groups as indirect probes of the critical surface-bound intermediate species and reaction mechanisms. A more direct approach is to probe the intermediate species themselves, but this requires direct study of the local chemical environment of light elements. In this work, we use soft X-ray emission spectroscopy (XES) and a custom-designed in situ reactor cell to directly observe and characterize the electronic structure of reactant, intermediate, and product species under reaction conditions. Specifically, we employ N K XES to probe the interaction of various nitrogen species with a Cu-SSZ-13 catalyst during selective catalytic reduction of nitrogen oxides (NO and NO2) by ammonia (NH3-SCR), a reaction that is critical for the removal of NOx pollutants formed in combustion reactions. This work reveals a novel spectral feature for all spectra measured with flowing NO gas present, which we attribute to the interaction of NO with the catalyst. We find that introducing both NO and O2 gases (compared to only NO) increases the interaction of NO with Cu-SSZ-13. Adsorption of NH3 leads to a more pronounced spectral signal compared to NO adsorption. For the standard NH3-SCR reaction, we observe a strong N2 signal, comprising 30% of the total spectral intensity. These results demonstrate the vast potential of this technique to provide direct, novel insights into the complex interactions between reaction intermediates and the active sites of catalysts, which may guide advanced knowledge-based optimization of these processes

Formation of a KInSe Surface Species by NaF/KF Postdeposition Treatment of Cu(In,Ga)Se₂ Thin-Film Solar Cell Absorbers

A NaF/KF postdeposition treatment (PDT) has recently been employed to achieve new record efficiencies of Cu­(In,Ga)­Se₂ (CIGSe) thin film solar cells. We have used a combination of depth-dependent soft and hard X-ray photoelectron spectroscopy as well as soft X-ray absorption and emission spectroscopy to gain detailed insight into the chemical structure of the CIGSe surface and how it is changed by different PDTs. Alkali-free CIGSe, NaF-PDT CIGSe, and NaF/KF-PDT CIGSe absorbers grown by low-temperature coevaporation have been interrogated. We find that the alkali-free and NaF-PDT CIGSe surfaces both display the well-known Cu-poor CIGSe chemical surface structure. The NaF/KF-PDT, however, leads to the formation of bilayer structure in which a KInSe species covers the CIGSe compound that in composition is identical to the chalcopyrite structure of the alkali-free and NaF-PDT absorber