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The light-soaking effect is the observation that under constant illumination the measured power conversion efficiency of certain solar cells changes as a function of time. The theory of the light-soaking in metal halide perovskites is at present incomplete. In this report, we employ steady-state microwave conductivity, a contactless probe of electronic properties of semiconductors, to study the light-soaking effect in metal halide perovskites. By illuminating isolated thin films of two mixed-cation perovskites with AM1.5 solar illumination, we observe a continual increase in photoconductance over a period of many (>12) hours. We can fit the experimentally observed changes in photoconductance to a stretched exponential function, in an analogous manner to bias-stressed thin-film transistors. The information provided in this report should help the community better understand one of the most perplexing open problems in the field of perovskite solar cells and, ultimately, lead to more robust and predictable devices.

Sc has been employed as an electron contact to a number of two-dimensional (2D) materials (e.g. MoS₂, black phosphorous) and has enabled, at times, the lowest electron contact resistance. However, the extremely reactive nature of Sc leads to stringent processing requirements and metastable device performance with no true understanding of how to achieve consistent, high-performance Sc contacts. In this work, WSe₂transistors with impressive subthreshold slope (109 mV dec⁻¹) andI_ON/I_OFF(10⁶) are demonstrated without post-metallization processing by depositing Sc contacts in ultra-high vacuum (UHV) at room temperature (RT). The lowest electron Schottky barrier height (SBH) is achieved by mildly oxidizing the WSe₂in situbefore metallization, which minimizes subsequent reactions between Sc and WSe₂. Post metallization anneals in reducing environments (UHV, forming gas) degrade theI_ON/I_OFFby ~10³and increase the subthreshold slope by a factor of 10. X-ray photoelectron spectroscopy indicates the anneals increase the electron SBH by 0.4–0.5 eV and correspondingly convert 100% of the deposited Sc contacts to intermetallic or scandium oxide. Raman spectroscopy and scanning transmission electron microscopy highlight the highly exothermic reactions between Sc and WSe₂, which consume at least one layer RT and at least three layers after the 400 °C anneals. The observed layer consumption necessitates multiple sacrificial WSe₂layers during fabrication. Scanning tunneling microscopy/spectroscopy elucidate the enhanced local density of states below the WSe₂Fermi level around individual Sc atoms in the WSe₂lattice, which directly connects the scandium selenide intermetallic with the unexpectedly large electron SBH. The interface chemistry and structural properties are correlated with Sc–WSe₂transistor and diode performance. The recommended combination of processing conditions and steps is provided to facilitate consistent Sc contacts to WSe₂.

Electrocatalytic reactions are known to take place at the catalyst/electrolyte interface. Whereas recent studies of size‐dependent activity in nanoparticles and thickness‐dependent activity of thin films imply that the sub‐surface layers of a catalyst can contribute to the catalytic activity as well, most of these studies consider actual modification of the surfaces. In this study, the role of catalytically active sub‐surface layers was investigated by employing atomic‐scale thickness control of the La_0.7Sr_0.3MnO₃(LSMO) films and heterostructures, without altering the catalyst/electrolyte interface. The activity toward the oxygen evolution reaction (OER) shows a non‐monotonic thickness dependence in the LSMO films and a continuous screening effect in LSMO/SrRuO₃heterostructures. The observation leads to the definition of an “electrochemically‐relevant depth” on the order of 10 unit cells. This study on the electrocatalytic activity of epitaxial heterostructures provides new insight in designing efficient electrocatalytic nanomaterials and core‐shell architectures.