Peter Serles

Inverted (pin) perovskite solar cells (PSCs) afford improved operating stability in comparison to their nip counterparts but have lagged in power conversion efficiency (PCE). The energetic losses responsible for this PCE deficit in pin PSCs occur primarily at the interfaces between the perovskite and the charge-transport layers. Additive and surface treatments that use passivating ligands usually bind to a single active binding site: This dense packing of electrically resistive passivants perpendicular to the surface may limit the fill factor in pin PSCs. We identified ligands that bind two neighboring lead(II) ion (Pb 2+ ) defect sites in a planar ligand orientation on the perovskite. We fabricated pin PSCs and report a certified quasi–steady state PCE of 26.15 and 24.74% for 0.05– and 1.04–square centimeter illuminated areas, respectively. The devices retain 95% of their initial PCE after 1200 hours of continuous 1 sun maximum power point operation at 65°C.

Due to the strong in-plane but weak out-of-plane bonding, it is relatively easy to separate nanosheets of two-dimensional (2D) materials from their respective bulk crystals. This exfoliation of 2D materials can yield large 2D nanosheets, hundreds of micrometers wide, that can be as thin as one or a few atomic layers thick. However, the underlying physical mechanisms unique to each exfoliation technique can produce a wide distribution of defects, yields, functionalization, lateral sizes, and thicknesses, which can be appropriate for specific end applications. The five most commonly used exfoliation techniques include micromechanical cleavage, ultrasonication, shear exfoliation, ball milling, and electrochemical exfoliation. In this review, we present an overview of the field of 2D material exfoliation and the underlying physical mechanisms with emphasis on progress over the last decade. The beneficial characteristics and shortcomings of each exfoliation process are discussed in the context of their functional properties to guide the selection of the best technique for a given application. Furthermore, an analysis of standard applications of exfoliated 2D nanosheets is presented including their use in energy storage, electronics, lubrication, composite, and structural applications. By providing detailed insight into the underlying exfoliation mechanisms along with the advantages and disadvantages of each technique, this review intends to guide the reader toward the appropriate batch-scale exfoliation techniques for a wide variety of industrial applications.

2D materials are well-known for their low-friction behavior by modifying the interfacial forces at atomic surfaces. Of the wide range of 2D materials, MXenes represent an emerging material class but their lubricating behavior has been scarcely investigated. Herein, the friction mechanisms of 2D Ti3C2Tx MXenes are demonstrated which are attributed to their surface terminations. We find that Ti3C2Tx MXenes do not exhibit the well-known frictional layer dependence of other 2D materials. Instead, the nanoscale lubricity of 2D MXenes is governed by the termination species resulting from synthesis. Annealing the MXenes demonstrate a 7% reduction in OH termination which translates to a 16–57% reduction of friction in agreement with DFT calculations. Finally, the stability of MXene flakes is demonstrated upon isolation from their aqueous environment. This work indicates that MXenes can provide sustainable lubricity at any thickness which makes them uniquely positioned among 2D material lubricants.