Local Strain Engineering in Atomically Thin MoS₂

Controlling the bandstructure through local-strain engineering is an exciting avenue for tailoring optoelectronic properties of materials at the nanoscale. Atomically thin materials are particularly well-suited for this purpose because they can withstand extreme nonhomogeneous deformations before rupture. Here, we study the effect of large localized strain in the electronic bandstructure of atomically thin MoS₂. Using photoluminescence imaging, we observe a strain-induced reduction of the direct bandgap and funneling of photogenerated excitons toward regions of higher strain. To understand these results, we develop a nonuniform tight-binding model to calculate the electronic properties of MoS₂ nanolayers with complex and realistic local strain geometries, finding good agreement with our experimental results

Perovskite/Perovskite Tandem Solar Cells in the Substrate Configuration with Potential for Bifacial Operation

Perovskite/perovskite tandem solar cells have recently exceeded the record power conversion efficiency (PCE) of single-junction perovskite solar cells. They are typically built in the superstrate configuration, in which the device is illuminated from the substrate side. This limits the fabrication of the solar cell to transparent substrates, typically glass coated with a transparent conductive oxide (TCO), and adds constraints because the first subcell that is deposited on the substrate must contain the wide-bandgap perovskite. However, devices in the substrate configuration could potentially be fabricated on a large variety of opaque and inexpensive substrates, such as plastic and metal foils. Importantly, in the substrate configuration the narrow-bandgap subcell is deposited first, which allows for more freedom in the device design. In this work, we report perovskite/perovskite tandem solar cells fabricated in the substrate configuration. As the substrate we use TCO-coated glass on which a solution-processed narrow-bandgap perovskite solar cell is deposited. All of the other layers are then processed using vacuum sublimation, starting with the charge recombination layers, then the wide-bandgap perovskite subcell, and finishing with the transparent top TCO electrode. Proof-of-concept tandem solar cells show a maximum PCE of 20%, which is still moderate compared to those of best-in-class devices realized in the superstrate configuration yet higher than those of the corresponding single-junction devices in the substrate configuration. As both the top and bottom electrodes are semitransparent, these devices also have the potential to be used as bifacial tandem solar cells

Electronic Structure and Charge Transport Properties of a Series of 3,6-(Diphenyl)‑<i>s</i>‑tetrazine Derivatives: Are They Suitable Candidates for Molecular Electronics?

Optoelectronic and charge-transport related properties of a series of 3,6-diphenyl-<i>s</i>-tetrazine derivatives, including F, Cl, Br, and CN substituents, have been analyzed. The molecular structure and electronic properties of the new fluorine-containing derivative, bis­(3,6-difluorophenyl)-<i>s</i>-tetrazine, were explored by spectroscopic, electrochemical, and theoretical methods. The effects of the substituent on the pristine compound have been assessed from a theoretical perspective, showing that the fluorinated and brominated derivatives have the highest predicted electron mobilities, whereas the cyano derivative is foreseen to undergo the most efficient electron injection process