The quick-EXAFS (QEXAFS) method adds time resolution to X-ray absorption spectroscopy (XAS) and allows dynamic structural changes to be followed. A completely new QEXAFS setup consisting of monochromator, detectors and data acquisition system is presented, as installed at the SuperXAS bending-magnet beamline at the Swiss Light Source (Paul Scherrer Institute, Switzerland). The monochromator uses Si(111) and Si(311) channel-cut crystals mounted on one crystal stage, and remote exchange allows an energy range from 4.0 keV to 32 keV to be covered. The spectral scan range can be electronically adjusted up to several keV to cover multiple absorption edges in one scan. The determination of the Bragg angle close to the position of the crystals allows high-accuracy measurements. Absorption spectra can be acquired with fast gridded ionization chambers at oscillation frequencies of up to 50 Hz resulting in a time resolution of 10 ms, using both scan directions of each oscillation period. The carefully developed low-noise detector system yields high-quality absorption data. The unique setup allows both state-of-the-art QEXAFS and stable step-scan operation without the need to exchange whole monochromators. The long-term stability of the Bragg angle was investigated and absorption spectra of reference materials as well as of a fast chemical reaction demonstrate the overall capabilities of the new setup.
Solar cells incorporating metal‐halide perovskite (MHP) semiconductors are continuing to break efficiency records for solution‐processed solar cell devices. Scaling MHP‐based devices to larger area prototypes requires the development and optimization of scalable process technology and ink formulations that enable reproducible coating results. It is demonstrated that the power conversion efficiency (PCE) of small‐area methylammonium lead iodide (MAPbI3) devices, slot‐die coated from a 2‐methoxy‐ethanol (2‐ME) based ink with dimethyl‐sulfoxide (DMSO) used as an additive depends on the amount of DMSO and age of the ink formulation. When adding 12 mol% of DMSO, small‐area devices of high performance (20.8%) are achieved. The effect of DMSO content and age on the thin film morphology and device performance through in situ X‐ray diffraction and small‐angle X‐ray scattering experiments is rationalized. Adding a limited amount of DMSO prevents the formation of a crystalline intermediate phase related to MAPbI3 and 2‐ME (MAPbI3‐2‐ME) and induces the formation of the MAPbI3 perovskite phase. Higher DMSO content leads to the precipitation of the (DMSO)2MA2Pb3I8 intermediate phase that negatively affects the thin‐film morphology. These results demonstrate that rational insights into the ink composition and process control are critical to enable reproducible large‐scale manufacturing of MHP‐based devices for commercial applications.
Secondary phases in Cu2ZnSnS4 (CZTS) are investigated by x-ray absorption spectroscopy. Evaluating the x-ray absorption near edge structure at the sulfur K-edge, we show that secondary phases exhibit sufficiently distinct features to allow their quantitative determination with high accuracy. We are able to quantify the ZnS fraction with an absolute accuracy of ±3%, by applying linear combination analysis using reference spectra. We find that even in CZTS thin films with [Sn]/[Zn] ≈ 1, a significant amount of ZnS can be present. A strong correlation of the ZnS-content with the degradation of the electrical performance of solar cells is observed.
Thin film solar cells with Al/ITO/ZnO/CdS/CZTSSe/Mo‐glass structure are fabricated employing a fast and low‐cost preparation procedure using an aqueous ink deposited by nonpyrolytic spray, followed by high temperature crystallization and selenization steps. Capacitance–voltage measurements on previously reported devices with >8% efficiency under 1 sun irradiation show a charge carrier density of the order of 1017 cm−3. Moreover, admittance spectroscopy indicates the presence of mid‐bandgap defects that are tentatively attributed to a Sn deficit in the film. In order to reduce the number of these deep defects within the active layer of our solar cells, the Sn content is tuned in the precursor ink. Their morphology, elemental composition, crystal phases, capacitance–voltage profiling, admittance, photoluminescence, and photovoltaic performances are characterized. The results indicate that tuning the Sn content offers a strong leverage upon some key properties of the active layer, in particular the grain size, and the charge carrier and defect density. By employing this leverage to optimize the performance of our CZTSSe layers, the cell performances are increased to 10.0% without antireflection coating (ARC) and to 10.8% (on 0.25 cm2) with an ARC.
The structural phases and optoelectronic properties of coevaporated CsPbI3 thin films with a wide range of [CsI]/[PbI2] compositional ratios are investigated using high throughput experimentation and gradient samples. It is found that for CsI‐rich growth conditions, CsPbI3 can be synthesized directly at low temperature into the distorted perovskite γ‐CsPbI3 phase without detectable secondary phases. In contrast, PbI2‐rich growth conditions are found to lead to the non‐perovskite δ‐phase. Photoluminescence spectroscopy and optical‐pump THz‐probe mapping show carrier lifetimes larger than 75 ns and charge carrier (sum) mobilities larger than 60 cm2 V−1 s−1 for the γ‐phase, indicating their suitability for high efficiency solar cells. The dependence of the carrier mobilities and luminescence peak energy on the Cs‐content in the films indicates the presence of Schottky defect pairs, which may cause the stabilization of the γ‐phase. Building on these results, p–i–n type solar cells with a maximum efficiency exceeding 12% and high shelf stability of more than 1200 h are demonstrated, which in the future could still be significantly improved, judging on their bulk optoelectronic properties.
The selenization of Cu-Zn-Sn-S nanocrystals is a promising route for the fabrication of low-cost thin film solar cells. However, the reaction pathway of this process is not completely understood. Here, the evolution of phase formation, grain size, and elemental distributions is investigated during the selenization of Cu-Zn-Sn-S nanoparticle precursor thin films by synchrotron-based in situ energy-dispersive X-ray diffraction and fluorescence analysis as well as by ex situ electron microscopy. The precursor films are heated in a closed volume inside a vacuum chamber under presence of selenium vapor while diffraction and fluorescence signals are recorded. The presented results reveal that during the selenization the cations diffuse to the surface to form large grains on top of the nanoparticle layer and the selenization of the film takes place in two simultaneous reactions: 1) a direct and fast formation of large grained selenides, starting with copper selenide which is subsequently transformed into Cu 2 ZnSnSe 4 ; 2) a slower selenization of the remaining nanoparticles. As a consequence of the initial formation of copper selenides at the surface, the subsequent formation of CZTSe starts under Cu-rich conditions despite an overall Cu-poor composition of the film. The implications of this process path on the film quality is discussed. Additionally, the proposed growth model provides an explanation of the previously observed accumulation of carbon from the nanoparticle precursor beneath the large grained layer.
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