24378 research outputs found
Sort by
Monitoring Charge Separation of Individual Cells in Perovskite Silicon Tandems via Transient Surface Photovoltage Spectroscopy
No full textIdentification of charge carrier separation processes in perovskite silicon tandem solar cells and recombination at buried interfaces of charge selective contacts are crucial for photovoltaic research. Here, intensity and wavelength dependent transient surface photovoltage tr SPV is used to investigate slot die coated perovskite top layers deposited on n type heterojunction silicon bottom cells. We show that using an appropriate combination of photon energy and or bottom cell polarity, one can individually probe the buried interfaces of the bottom silicon cell or the perovskite s buried interfaces of a tandem solar cell for excitation with higher energy photons, time delays before the onset of a strong SPV signal indicate significant hole minority drift before separation in the silicon bottom cells. Furthermore, symmetric bottom Si heterojunction solar cell stacks can serve to investigate the top perovskite stack including its junction to the bottom cell, unhampered by photovoltages from the silicon substrate. Thus, investigation of the buried interfaces in tandem devices using time resolved surface photovoltage is found to yield valuable information on charge carrier extraction at buried interfaces and demonstrates its unique potential compared to more conventional approaches that rely on photoluminescence decay kinetic
X ray absorption spectroscopy of a coordinatively unsaturated 3d transition metal complex
Get PDFCoordinatively unsaturated transition metal complexes are employed as active catalysts in a wide range of homogeneous chemical reactions, including C H bond activation. Because of their high reactivity and resulting affinity to rapidly bind substrate molecules, they are generally challenging to detect and characterize when prepared photochemically. Here, we generate the 16 electron cationic complex [CpFe CO 2] Cp cyclopentadienyl in the gas phase using electrospray ionization and probe its electronic structure with x ray absorption spectroscopy at the Fe L edge. Supported by multiconfigurational spectrum calculations, the distinct L edge absorption profile of [CpFe CO 2] reveals direct access to a low lying unoccupied Fe 3d derived orbital, which is characteristic of the unsaturated coordination of the complex and of its pronounced affinity to bind even notoriously unreactive moieties such as alkane
Impact and Control of Residual Stresses in Ceramic Packages
No full textAn important challenge in ceramic and hybrid packaging is the joining of different materials within assemblies that demand complex functionality, high performance, and long term reliability. Often, less attention is given to the fact that manufacturing processes such as co firing, film deposition, and soldering generate residual stresses between the bonded materials. These stresses can further evolve due to relaxation effects during subsequent processing steps. Understanding the residual stress state is therefore essential, as it directly impacts the quality and performance of the final product throughout its lifecycle. Effective control of these stresses in industrial production requires reliable detection methods that offer both precision and practical applicability. In this context, three measurement techniques for assessing residual stress in ceramic components are presented and compare
Electronic energy scales of CrX3 X Cl, Br, and I using high resolution x ray scattering
Get PDFChromium trihalides Cr amp; 8290; amp; 119883;3 amp; 119883; Cl, Br, and I have recently become a focal point of research due to their intriguing low temperature, layer dependent magnetic properties that can be manipulated by external stimuli. This makes them essential candidates for spintronic applications. Their magnetic orders are often related to the electronic structure parameters, such as spin orbit coupling SOC , Hund s coupling amp; 119869; amp; 119867; , amp; 119901; amp; 8722; amp; 119889; covalency, and interorbital Coulomb interactions. Accurately determining such parameters is paramount for understanding Cr amp; 8290; amp; 119883;3 physics. We have used high resolution resonant inelastic x ray scattering spectroscopy to study Cr amp; 8290; amp; 119883;3 across phase transition temperatures. Ligand field multiplet calculations were used to determine the electronic structure parameters by incorporating the crystal field interactions in a distorted octahedral orientation with amp; 119862;3 symmetry. These methods provide the most detailed description of Cr amp; 8290; amp; 119883;3 magneto optical and electronic energetic terms to date. The crystal field distortion parameters amp; 119863; amp; 8290; amp; 120590; and amp; 119863; amp; 8290; amp; 120591; were experimentally determined, and the energies of amp; 119889; orbitals have been reported. The spectroscopic measurements reveal an energy separation between spin allowed quartet states and spin forbidden doublet states, which increases upon going from CrCl3 to CrI3. The role of SOC, for Cr 2 amp; 8290; amp; 119901; orbitals, in spin flip excitations has been demonstrated. The determined 10 amp; 8290; amp; 119863; amp; 8290; amp; 119902; values are in good agreement with the spectrochemical series, and Racah amp; 119861; follows the nephelauxetic effect. Such precise measurements offer insights into the energy design of spintronic devices that utilize quantum state tuning within two dimensional magnetic material
The surface chemical bond of a nonbenzenoid aromatic hydrocarbon Acepleiadylene versus pyrene
No full textDefects play a critical role in the performance of carbon based opto electronic materials, because the materials interaction with metal electrodes can strongly depend on the topology of the amp; 960; electron system. However, the direct investigation of defects is difficult due to their typically low density. To address this issue, we use a molecular model system comparing the polycyclic aromatic hydrocarbon pyrene with its isomer acepleiadylene regarding their interaction with a Cu 111 surface. Acepleiadylene serves as a model defect with a nonbenzenoid nonalternant topology, while pyrene represents an ideal benzenoid alternant structure. We find that acepleiadylene forms a stronger bond to the metal surface than pyrene. This is evidenced by a higher molecule surface bond energy, significant adsorption induced changes in electronic structure studied via photoelectron and X ray absorption spectroscopies , and a potentially lower adsorption height according to non contact atomic force microscopy . The stronger bond of acepleiadylene is linked to its smaller gap between the highest occupied and the lowest unoccupied orbitals HOMO LUMO gap , bringing the LUMO closer to the metal s Fermi energy and resulting in stronger hybridization with the metal s electronic states. Density functional theory calculations support our findings, suggesting that nonbenzenoid, nonalternant structural elements can enhance the bonding between graphene based materials and metal electrodes. Additionally, these results highlight the potential of nonbenzenoid molecular organic semiconductors as alternatives to their benzenoid counterpart
X ray analysis of capillary and melt pool dynamics using laser beam oscillation welding for aluminum die casts
Get PDFThe knowledge of underlying physical phenomena and the understanding of parameter dependencies are essential for the development of stable welding processes. Quantitative data based on a sensor based approach are necessary to draw conclusions about the resulting quality. These are at the same time key factors for the optimization of industrial processes. The paper refers to this in the context of laser welding technology using superimposed dynamic beam oscillation of aluminum alloys. However, particularly die cast aluminum is characterized by limited weldability due to entrapped gases, which lead to defects like high porosity and blow out formation along the weld seam. For the welding trails, a novel laser processing head was designed. The optics enables a synchronized and fast beam deflection in all three spatial directions. Therefore, a galvanometer scanner for x y oscillation was combined with a piezo driven mirror for the beam movement in the z direction. By means of in situ synchrotron x ray imaging synchronized with recordings of acoustic process emissions and high speed imaging of the melt pool, deep insights into the interactions of the laser beam and material could be investigated. The collected data show a clear correlation between oscillation parameters frequency and amplitude and the resulting weld seam quality. Furthermore, audio signal information provides an explicit link for different vapor capillary formations as well as melt flow fluctuations. The adapted process dynamics for dynamic beam oscillation welding enable a significant quality improvement and open up the possibility for efficient and safe process monitorin
Metastable Crystalline Cobalt Iron Oxide Nano Flakes with Antiferromagnetic Ferrimagnetic Composition Mosaicity
Get PDFBy thermal decomposition of a crystalline hydroxycarbonate precursor with a Co Fe ratio of 2 1, crystals with alternating ferrimagnetic and antiferromagnetic nano domains were synthesized using a facile synthetic approach that combined bottom up co precipitation of the precursor with a self assembled top down nano structuring during spinel formation. Due to the miscibility gap of the spinel phase diagram at this composition, a topotactic segregation into CoFe2 O 4 like and Co 3 O 4 like domains takes place at 400 C, giving rise to porous crystalline nano flakes with spatial compositional fluctuations on a scale of approximately 5 nm. Experimental methods and density functional theory showed that the metastable nature of this interface rich material is manifested in the unexpectedly low lattice parameter of the iron rich domains, which can be explained by the compressive strain executed on this phase due to mosaicity. Investigations of the magnetic properties revealed an exchange bias effect, due to this unique microstructure, which is typically known for thin films or core shell nanoparticles. Treatment at temperatures higher than 450 C causes this microstructure to break down, the lattice strain to relax, and finally leads to properties expected for the thermodynamically stable phases according to the phase diagra
Hydrogen Diffusion in Ti3C2 MXenes
Get PDFFor energy storage in 2D MXenes using hydrogen, it is crucial to understand the relevant diffusion mechanisms. Density functional theory was used to determine hydrogen migration barriers, hopping frequencies, enthalpy and entropy of vacancy formation, and the diffusion coefficient of various possible migration paths. The results show that H diffusion is primarily governed by interstitial diffusion. For all investigated diffusion paths, the diffusion coefficients and prefactors, D0, the corresponding activation energy, E, and the hopping frequencies are calculated from the ab initio approac
Direct observation of the on site oxygen 2p two hole Coulomb energy in La2CuO4
Get PDFElectron correlation in functional materials has remained a challenge with strong deviations of electronic structure from mean field approaches. In high temperature superconductors the electron electron and hole hole interaction energies are essential in the underlying pairing mechanisms. For cuprates, oxygen holes have been considered of central importance for superconductivity. In La2CuO4 the site specific oxygen 2p hole hole Coulomb energy has been determined by Auger photoelectron coincidence spectroscopy. This experimental approach allows to separate the different oxygen sites, i.e. the lattice oxygen, and distinguish from otherwise overlapping signal from surface oxygen. Values of 6.3 0.2 eV for oxygen in the Cu O planes and an upper limit of 9.2 0.2 eV for apical oxygen are found to be on the high energy side of reported computational values and narrows the range of experimentally reported values. Additionally, a much reduced hybridization in La2CuO4 as compared to CuO is found in O 2p hybridization strength