Direct Observation of 2D Electrostatics and Ohmic Contacts in Template-Grown Graphene/WS₂ Heterostructures

Large-area two-dimensional (2D) heterojunctions are promising building blocks of 2D circuits. Understanding their intriguing electrostatics is pivotal but largely hindered by the lack of direct observations. Here graphene–WS₂ heterojunctions are prepared over large areas using a seedless ambient-pressure chemical vapor deposition technique. Kelvin probe force microscopy, photoluminescence spectroscopy, and scanning tunneling microscopy characterize the doping in graphene–WS₂ heterojunctions as-grown on sapphire and transferred to SiO₂ with and without thermal annealing. Both p–n and n–n junctions are observed, and a flat-band condition (zero Schottky barrier height) is found for lightly n-doped WS₂, promising low-resistance ohmic contacts. This indicates a more favorable band alignment for graphene–WS₂ than has been predicted, likely explaining the low barriers observed in transport experiments on similar heterojunctions. Electrostatic modeling demonstrates that the large depletion width of the graphene–WS₂ junction reflects the electrostatics of the one-dimensional junction between two-dimensional materials

Polypyridyl Iron Complex as a Hole-Transporting Material for Formamidinium Lead Bromide Perovskite Solar Cells

An efficient hole-transporting material (HTM) is indispensable for high-performing perovskite solar cells (PSCs), which have recently emerged as a breakthrough photovoltaic technology. Here, we demonstrate the capacity of the transition metal complex (6,6′-bis­(1,1-di­(pyridin-2-yl)­ethyl)-2,2′-bipyridine)-iron­(II/III) trifluoromethanesulfonate ([Fe­(bpyPY4)]­(OTf)_2+<i>x</i>) to act as an additive-free, solution-processable HTM in PSCs based on the formamidinium lead bromide absorber. State-of-the-art physical methods have been employed to characterize [Fe­(bpyPY4)]­(OTf)_2+<i>x</i> and, in particular, to demonstrate its significantly higher conductivity compared to that of the conventional HTM spiro-OMeTAD. A maximum power conversion efficiency of 2.2% was obtained for a device employing [Fe­(bpyPY4)]­(OTf)_2+<i>x</i>, which is the first evidence of the applicability as a HTM in a PSC of a solid material in which conductivity is provided by a redox transformation of a transition metal