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Introduction. An important area of research in the field of plasma metal cutting is obtaining a metal cut face characterized by minimal roughness and geometric deviations. It is also important to minimize changes in the structure of the metal under the cutting surface caused by the temperature effects of the plasma jet, including the formation of dross. The solution to the problem of obtaining a quality cut is to optimize the parameters of the cutting process. The plasma arc current and voltage, cutting height and cutting speed are considered to be the main parameters that determine cut quality. However, insufficient attention has been paid to the processes of plasma metal cutting of thicknesses above 20 mm due to the limitations associated with the operation conditions of plasma torches with direct polarity currents. Accordingly, for cutting large thicknesses, the use of a plasma torch operating on currents of reverse polarity seems promising. The aim of this work is to develop the technique of plasma cutting of copper, titanium and aluminum alloy sheets up to 40 mm thick using a plasma torch operating on currents of reverse polarity. Results and discussion. Investigations show that for cutting aluminum alloy (Al 90.9–94.7 %; Cu 3.8–4.9 %; Mg 1.2-1.8 %; Mn 0.3–0.9 %) and titanium alloy (Ti 94.33–97.5 %; Al 1.5–2.5 %; Mn 0.7–2.0 %) it is possible to regulate the cutting speed in a wide range, while for rolled copper (Cu ≥99.96 %) and aluminum alloy with thickness of 40 mm the range of cutting speed regulation is rather narrow. While for aluminum alloy due to excessive precipitation of alloying elements from the solid solution in the heat-affected zone decrease of microhardness is observed, for titanium alloy the microhardness growth due to material hardening is characteristic. Changing the cutting mode parameters allows receiving more homogeneous macrogeometry of a cutting surface, smaller depth of a zone of melting of a material and a heat-affected zone and smaller changes of mechanical properties of a material in a zone of a cut. For the titanium alloy, almost all of the cutting modes used are close to optimum. For alloy aluminum and copper the modes providing the best cutting quality in the considered range of parameters are determined. According to the results of the work it can be concluded that plasma cutting on reverse polarity currents is effective for cutting rolled products of large thicknesses, but the technique requires further development in order to improve the quality of the resulting cut.

Electron beam additive manufacturing from dissimilar metal wires was used to intermix 5, 10 and 15 vol.% of Ti-Al-Mo-Z-V titanium alloy with CuAl9Mn2 bronze on a stainless steel substrate. The resulting alloys were subjected to investigations into their microstructural, phase and mechanical characteristics. It was shown that different microstructures were formed in an alloy containing 5 vol.% titanium alloy, as well as others containing 10 and 15 vol.%. The first was characterized by structural components such as solid solution, eutectic intermetallic compound TiCu2Al and coarse grains of γ1-Al4Cu9. It had enhanced strength and demonstrated steady oxidation wear in sliding tests. The other two alloys also contained large flower-like Ti(Cu,Al)2 dendrites that appeared due to the thermal decomposition of γ1-Al4Cu9. This structural transformation resulted in catastrophic embrittlement of the composite and changing of wear mechanism from oxidative to abrasive.

The regularities of microstructure formation in samples of multiphase composites obtained by additive electron beam manufacturing on the basis of aluminum alloy ER4043 and nickel superalloy Udimet-500 have been studied. The results of the structure study show that a multicomponent structure is formed in the samples with the presence of Cr23C6 carbides, solid solutions based on aluminum -Al or silicon -Si, eutectics along the boundaries of dendrites, intermetallic phases Al3Ni, AlNi3, Al75Co22Ni3, and Al5Co, as well as carbides of complex composition AlCCr, Al8SiC7, of a different morphology. The formation of a number of intermetallic phases present in local areas of the samples was also distinguished. A large amount of solid phases leads to the formation of a material with high hardness and low ductility. The fracture of composite specimens under tension and compression is brittle, without revealing the stage of plastic flow. Tensile strength values are significantly reduced from the initial 142–164 MPa to 55–123 MPa. In compression, the tensile strength values increase to 490–570 MPa and 905–1200 MPa with the introduction of 5% and 10% nickel superalloy, respectively. An increase in the hardness and compressive strength of the surface layers results in an increase in the wear resistance of the specimens and a decrease in the coefficient of friction.

International audience; In Cu(In,Ga)Se 2 (CIGS) thin-film solar cells, laterally inhomogeneous distributions of point defects may induce electrostatic potential fluctuations and thus reduce the open-circuit voltage (V oc). In the present work, we investigate possible origins of fluctuating potentials and estimate the amplitude of fluctuations and V oc losses in solar cells with various [Ga] in the CIGS absorber, with different buffer layers and with different durations of an RbF postdeposition treatment (PDT). Electron-beam-induced current measurements were employed to study the local difference in the width of the space-charge region (w SCR). It is shown that the amplitude of fluctuations in the w SCR depends significantly on the choice of buffer system and on the duration of the RbF PDT. In addition, energy-dispersive X-ray spectroscopy and cat-hodoluminescence measurements reveal that band-gap fluctuations do not have substantial impact on the device performance. Finally, some of the investigated cells were exposed to light soaking, which was found to be a means to reduce the detected electrostatic potential fluctuations and also to increase the effective electron diffusion length in the CIGS absorber for a part of the investigated cells.

Erratum

Thin-film solar cells based on polycrystalline absorbers have reached very high conversion efficiencies of up to 23-25%. In order to elucidate the limiting factors that need to be overcome for even higher efficiency levels, it is essential to investigate microscopic origins of loss mechanisms in these devices. In the present work, a high efficiency (21% without anti-reflection coating) copper indium gallium diselenide (CIGSe) solar cell is characterized by means of a correlative microscopy approach and corroborated by means of photoluminescence spectroscopy. The values obtained by the experimental characterization are used as input parameters for two-dimensional device simulations, for which a real microstructure was used. It can be shown that electrostatic potential and lifetime fluctuations exhibit no substantial impact on the device performance. In contrast, nonradiative recombination at random grain boundaries can be identified as a significant loss mechanism for CIGSe solar cells, even for devices at a very high performance level., Achieving higher efficiencies for thin-film solar cells always requires identification of the limiting factors. Here Krause et al. show that inhomogeneously distributed net doping or lifetime have little impact while recombination at grain boundaries is one of the main loss mechanisms for high performance Cu(In,Ga)Se2 solar cells.

High-efficiency Cu(In,Ga)Se2 (CIGS) thin-film solar cells are based on polycrystalline CIGS absorber layers, which contain grain boundaries, stacking faults, and dislocations. While planar defects in CIGS layers have been investigated extensively, little is still known about the impact of dislocations on optoelectronic properties of CIGS absorbers. Herein, evidence for an optoelectronic inactivity of dislocations in these thin films is given, in contrast to the situation at grain boundaries. This unique behavior is explained by the extensive elemental redistribution detected around dislocation cores, which is connected with the dislocation strain field, probably leading to a shift of defect states toward the band edges.

Thin film solar cells based on Cu In,Ga Se2 absorber layers have reached conversion efficiencies of well above 20 . One key of this success is the incorporation of alkali metals such as Na and K into the surface and the volume of the Cu In,Ga Se2 thin film. The present work discusses the impact of Na and K on the grain boundary GB properties in Cu In,Ga Se2 thin films, i.e., on the barriers for charge carriers, amp; 934;b, and on the recombination velocities at the GBs, sGB. The authors first revise the physics connected with these two quantities as well as their impact on the device performance, and then provide values for the barrier heights and recombination velocities from the literature. The authors measured sGB values by means of cathodoluminescence analysis of Na K free CIGSe layers as well as on CIGSe layers on Mo sapphire substrates which were submitted to only NaF or only KF PDTs. Overall, passivating effects on GBs by neither Na nor K can be confirmed. The GB recombination velocities seem to remain on the same order of magnitude, in average about 103 104 cm s, irrespective of whether Cu In,Ga Se2 thin films are Na K free or Na K containing

Interface recombination at the absorber buffer interface impedes the efficiency of a solar cell with an otherwise excellent absorber. The internal voltage or the quasi-Fermi level splitting (qFLs) measures the quality of the absorber. Interface recombination reduces the open circuit voltage (VOC) with respect to the qFLs. The present work explores a facile sulfur-based post-deposition treatment (S-PDT) to passivate the interface of CuInS2 thin films grown under Cu-rich conditions, which show excellent qFLs values, but much lower VOCs. The CuInS2 absorbers are treated in three different S-containing solutions at 80 oC. Absolute calibrated photoluminescence and current-voltage measurements demonstrate a reduction of the deficit between qFLs and VOC in the best S-PDT device by almost one third compared to the untreated device. Analysis of temperature dependence of the open-circuit voltage shows increased activation energy for the dominant recombination path, indicating less interface recombination. In addition, capacitance transient measurements reveal the presence of slow metastable defects in the untreated solar cell. The slow response is considerably reduced by the S-PDT, suggesting passivation of these slow metastable defects. The results demonstrate the effectiveness of solution based S-treatment in passivating defects, presenting a promising strategy to explore and reduce defect states near the interface of chalcogenide semiconductors.

The highest conversion efficiencies for Cu(In,Ga)Se$_{\mathbf {2}}$ (CIGS) solar cells have reached levels of more than 22%. The present case study was performed on a CIGS device with stacking sequence ZnO:Al/(Zn,Mg)O/CdS/Cu(In,Ga)Se$_{\mathbf {2}}$/Mo/glass, of which the co-evaporated CIGS absorber underwent a RbF postdeposition treatment, exhibiting a high conversion efficiency of almost 21% without anti-reflection coating. Electron backscatter diffraction, energy-dispersive X-ray spectrometry (EDX), electron-beam-induced current (EBIC), and cathodoluminescence (CL) measurements were performed on cross-sectional specimens in order to get insight to microstructural, compositional, electrical, and optoelectronic properties. It was found that the average grain size of the CIGS layer was about $0.5 {\mu } \mathrm {m}$. The recombination velocities extracted from CL profiles across grain boundaries were about 5x10$^{\mathbf {3}}$ cm/s in average. From the EBIC results, the width of the space charge region, $w _{\mathbf {SCR}}$, and the minority-carrier diffusion length in the quasi-neutral region, $L _{\mathbf {D}}$, were determined. EDX elemental distribution maps did not show any substantial change in composition in the lateral direction; only the Ga/In gradients perpendicular to the substrate were detected, which can be correlated well with the peak shifts visible in the hyperspectral CL map. In contrast, $w _{\mathbf {SCR}}$ and $L _{\mathbf {D}}$ were found to be inhomogeneously distributed along the $p {-} n$junction, indicating lateral fluctuations in the net doping and the carrier lifetime. The present contribution discusses these fluctuations as possible origin for the limited open-circuit voltage of the CIGS solar cell.

By extracting profiles perpendicular to the substrate from images obtained by electron-beam-induced current measurements on Cu(In,Ga)Se 2 (CIGS) solar cells, fluctuations in the width of the space-charge region as well as in the diffusion length in the quasi-neutral region were studied. Devices with various solution-grown buffer layers as well as different [Ga]/([Ga]+[In]) ratios in the CIGS absorber were investigated. Complementary analyses by means of energy-dispersive X-ray spectrometry, and cathodoluminescence provide additional insight into the local optoelectronic and compositional properties of investigated cells.

The continuous improvement of Cu(In,Ga)Se2 (CIGSe) solar cells relies considerably on advanced characterization of individual layers in the solar-cell stacks as well as of completed CIGSe devices. The present contribution provides an overview of corresponding efforts performed by various research groups at Helmholtz-Zentrum Berlin fur Materialien und Energie GmbH. In-situ growth monitoring of CIGSe absorber layers by means of energy-dispersive X-ray spectrometry and light scattering is described, as well as structural analyses by means of X-ray and neutron diffraction. In addition, the characterization of surfaces and interfaces by soft X-ray and electron spectroscopy, the microscopic analysis by means of correlative electron microscopy, and optoelectronic characterization by optical spectroscopy are highlighted. The present contribution shows which substantial efforts in a research network are necessary in order to obtain deeper insight into materials properties and potentially limiting factors for the device performance, as well as to be able to control these factors during the solar-cell production.