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We investigate the limit of X-ray detection at room temperature on rare-earth molecular films using lanthanum and a pyridine-based dicarboxamide organic linker as a model system. Synchrotron X-ray scanning tunneling microscopy is used to probe the molecules with different coverages on a HOPG substrate. X-ray-induced photocurrent intensities are measured as a function of molecular coverage on the sample allowing a correlation of the amount of La ions with the photocurrent signal strength. X-ray absorption spectroscopy shows cogent M4,5 absorption edges of the lanthanum ion originated by the transitions from the 3d3/2 and 3d5/2 to 4f orbitals. X-ray absorption spectra measured in the tunneling regime further reveal an X-ray excited tunneling current produced at the M4,5 absorption edge of La ion down to the ultimate atomic limit at room temperature., Comment: 19 pages, 4 figures

Perovskite solar cells have shown remarkable efficiencies beyond 22%, through organic and inorganic cation alloying. However, the role of alkali-metal cations is not well-understood. By using synchrotron-based nano-X-ray fluorescence and complementary measurements, we show that when adding RbI and/or CsI the halide distribution becomes homogenous. This homogenization translates into long-lived charge carrier decays, spatially homogenous carrier dynamics visualized by ultrafast microscopy, as well as improved photovoltaic device performance. We find that Rb and K phase-segregate in highly concentrated aggregates. Synchrotron-based X-ray-beam-induced current and electron-beam-induced current of solar cells show that Rb clusters do not contribute to the current and are recombination active. Our findings bring light to the beneficial effects of alkali metal halides in perovskites, and point at areas of weakness in the elemental composition of these complex perovskites, paving the way to improved performance in this rapidly growing family of materials for solar cell applications., Comment: updated author metadata

The role electron-transport layers (ETLs) play in perovskite solar cells (PSCs) is still widely debated. Conduction band alignment at the perovskite/ETL interface has been suggested to be an important role for the performance of the solar cells. However, little is known about the effects of work-function shifts on the solar-cell performance, and specifically, the open-circuit voltage (V-OC). Here, the effects of surface modification of SnO2 ETLs using polar phosphonic acids are investigated, including the effects on work function, surface energy, device performance, and device stability in inert atmosphere. The phosphonic acid modifications did not have a large effect on V-OC; however, a sharp decrease in the overall device performance was found, mostly due to reduced fill factors. When exposed to conditions of low oxygen concentration, the phosphonic acid surface modified devices yielded current-voltage (J-V) curves with considerably lower hysteresis than those based on unmodified SnO2. This suggests that this modification method may be valuable for achieving stabilized power conversion efficiency without hysteresis.

Impurity contamination in thin-film solar cells remains an uncertain risk due to the little-known impact of impurities on recombination. Building upon previous work, in which we intentionally contaminated lead halide perovskite (LHP) solar cells with iron, we further examine the distribution and charge state of iron-induced defects in LHP films using synchrotron-based X-ray techniques. X-ray absorption measurements suggest that iron-rich regions, which form among iron feedstock concentrations that exceed 100 ppm, most closely resemble the chemistry of Fe2O3. Iron distributed within the bulk may form a mix of Fe2+and Fe3+, the latter of which is not expected to be recombination active, potentially allowing LHPs to incorporate more iron than traditional semiconductors. X-ray beam induced current measurements show little correlation between the presence of iron-rich regions and charge collection, which further suggests low recombination activity at these sites. These results further elucidate the recombination behavior caused by iron incorporation in LHP films, revealing insight into how inhomogeneous incorporation of impurities may mitigate photovoltaic performance degradation., National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (award number DMR-1419807), United States. Department of Energy. Office of Science User Facility (Contract No. DE-AC02-06CH11357), National Science Foundation (U.S.) (NSF EECS Award No. 1541959), Martin Family Society of Fellows for Sustainability, National Science Foundation (U.S.). Graduate Research Fellowship (Grant No. 1122374), National Science Foundation (U.S.). (CA No. EEC-1041895)

The high capital expenditure (capex) necessary to manufacture crystalline silicon PV modules negatively affects the levelized cost of electricity (¢/kWh) and critically impacts the rate at which the PV industry can scale up. Wafer, cell, and module fabrication with thin free-standing silicon wafers is one key to reduce capex. Thin wafers reduce capex associated with silicon refining and wafer fabrication, which together sum to 58% of the total capex of silicon module manufacturing. In addition, thin wafers directly and significantly reduce variable costs. However, introducing 50 μm thin free-standing wafers into today's manufacturing lines result in cracking, creating a yield-based disincentive. Due to the brittle nature of silicon, wafer breakage is the major concern due to the high stress that is induced during processes in manufacturing lines. In this paper, we describe an improved method for edge micro-crack detection that can help enable low-capex, thin free-standing Si wafers. We present a method of detecting and measuring cracks along wafer edges by using a dark-field IR scattering imaging technique which enables detection of edge cracks at the micron scale. Keywords: Capex; polysilicon; thin free-standing wafer; edge crack detection; IR scattering; dark-field imaging, United States. Department of Energy (Grant DE-EE0007535)