The Cu 2 ZnSnS 4 kesterite is currently among the most promising inorganic, nontoxic, earth-abundant materials for a new generation of solar cells. Interfacial defects and secondary phases present in the kesterite active layer are, however, detrimental to the performance of the device. They are typically probed with techniques that are destructive or limited to the surface, and x-ray diffraction cannot reliably distinguish small amounts of zinc sulfide or copper tin sulfide from kesterite. Conversely, resonant ptychographic tomography, which relies on electron density contrast, overcomes these limitations. Here, we demonstrate how this technique can enable localization and quantification of secondary phases, along with measurements of adherence at the interfacial layers, on complete and functioning devices. In our experiment, we utilize an x-ray energy value far from absorption edges as well as three single energies corresponding to the absorption edges of Cu, Zn, and Sn, to gain elemental sensitivity to these elements and enhance contrast between phases with similar electron density. As a result, we image and identify in the active layer grains of a secondary phase, namely, zinc sulfide, which is not easily discriminated by other standard characterization techniques. In addition, we are able to observe Cu diffused from the active layer into the CdS buffer layer as well as Cu in the form of copper sulfide at their interface. Other relevant morphological features are best resolved off-resonance at the optimal energy for the synchrotron beamline with ∼20 nm resolution.
Strain and interactions at grain boundaries during solid-phase crystallization are known to play a significant role in the functional properties of polycrystalline materials. However, elucidating three-dimensional nanoscale grain morphology, kinetics, and strain under realistic conditions is challenging. Here, we image a single-grain growth during the amorphous-to-polycrystalline transition in technologically relevant transparent conductive oxide film of In2O3:Zr with in situ Bragg coherent X-ray diffraction imaging and transmission electron microscopy. We find that the Johnson-Mehl-Avrami-Kolmogorov theory, which describes the average kinetics of polycrystalline films growth, can be applied to the single grains as well. The quantitative analysis stems directly from imaging results. We elucidate the interface-controlled nature of the single-grain growth in thin films and reveal the surface strains which may be a driving force for anisotropic crystallization rates. Our results bring in situ imaging with coherent X-rays towards understanding and controlling the crystallization processes of transparent conductive oxides and other polycrystalline materials at the nanoscale.
Coherent diffractive imaging (CDI) experiments are adequately simulated assuming the thin sample approximation and using a Fresnel or Fraunhofer wavefront propagator to obtain the diffraction pattern. Although this method is used in wave-based or hybrid X-ray simulators, here the applicability and effectiveness of an alternative approach that is based solely on ray tracing of Huygens wavelets are investigated. It is shown that diffraction fringes of a grating-like source are accurately predicted and that diffraction patterns of a ptychography dataset from an experiment with realistic parameters can be sampled well enough to be retrieved by a standard phase-retrieval algorithm. Potentials and limits of this approach are highlighted. It is suggested that it could be applied to study imperfect or non-standard CDI configurations lacking a satisfactory theoretical formulation. The considerable computational effort required by this method is justified by the great flexibility provided for easy simulation of a large-parameter space.
Scanning probe measurements are an indispensable tool of solar cell research today, and the compatibility with simultaneous acquisition of complementary measurement modes is a particular strength. However, multi-modal data acquisition is often limited by different scan-parameter requirements. As a consequence, the modalities may be assessed subsequently rather than simultaneously. In this instance, image registration serves as a tool to align two-dimensional datasets at nanoscale. Here, we showcase an example of two subsequent scanning Xray microscopy measurements of solar cells with a Cu(In,Ga)Se 2 absorber, the first measurement being optimized for X-ray beam induced current and the second for X-ray fluorescence. We discuss different approaches and pitfalls of image registration and its potential combination with Gaussian filtering. This finally allows us to proceed with the investigation of point-by-point correlations.
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