Resonant x-ray ptychographic nanotomography of kesterite solar cells

Abstract

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.

Magnetism of Kesterite Cu2ZnSnS4 Semiconductor Nanopowders Prepared by Mechanochemically Assisted Synthesis Method

“…The latter aspect stems mainly from the compound’s capability to exist in stable forms with significant non-stoichiometry (e.g., copper rich or copper poor), with variable/interchangeable atom sites in the crystal lattice (e.g., Cu/Zn ordered or disordered polymorphs, kesterite vs. stannite structures), and relatively easily accommodating various structural defects (e.g., vacancies, nanophase segregation) to name the most outstanding [ 6 , 7 , 8 , 9 , 10 , 11 , 12 ]. It is also worth pointing out that the chemistry leading to the quaternary kesterite is by the very nature complex and often leads to detectable amounts of post-synthesis impurities or by-products including the relevant binary and ternary metal sulfides [ 13 , 14 , 15 , 16 ]. All this seems to effect investigations of kesterite by way of making them inadvertently more of the case than clear-cut fundamental studies of the still poorly reproducible syntheses.…”

Multi-modal characterization of kesterite thin-film solar cells: experimental results and numerical interpretation

“…A 1 μm thick lamella was chosen to match the approximate CZTS grain size determined from preliminary experiments on comparable devices. 28,29 Furthermore, the thin sample cross-section allows X-rays to be transmitted and recorded in the far-field regime to derive the differential phase contrast (DPC) using a pixel array detector. 30…”

“…A 1 mm thick lamella was chosen to match the approximate CZTS grain size determined from preliminary experiments on comparable devices. 28,29 Furthermore, the thin sample cross-section allows X-rays to be transmitted and recorded in the far-eld regime to derive the differential phase contrast (DPC) using a pixel array detector. 30 The investigated CZTS thin-lm solar cell was previously characterized by Raman spectroscopy, X-ray diffraction, time-resolved photoluminescence, 3D Xray diffraction 29 and resonant ptychographic tomography.…”

“…30 The investigated CZTS thin-lm solar cell was previously characterized by Raman spectroscopy, X-ray diffraction, time-resolved photoluminescence, 3D Xray diffraction 29 and resonant ptychographic tomography. 28 Fig. 1 (le) shows the device architecture alongside an SEM image of the sample (right) used in the experiment, consisting of a cross-section lamella of ca.…”

Non-destructive determination of phase, size, and strain of individual grains in polycrystalline photovoltaic materials

“… X-ray nanoprobe and forward scattering ptychography methods relying on the use of a synchrotron can reveal the local elemental composition in 3D [23] but does not provide structural information. Moreover, sample must be quite small (<10 µm), and dynamics studies representing bulk conditions are, therefore, excluded.…”