In view of the diverse functional groups left on the MXene during the etching process, we computationally investigated the effects of surface-group types on the structural, electronic and optical properties of Sc2CT2 (T = -O, -OH, -F) MXenes. For all geometries of the Sc2CT2 MXenes, the geometry I of Sc2CT2, which has the functional groups locating above the opposite-side Sc atoms, are lowest-energy structure. Accordingly, the energetically favorable Sc2CF2-I, Sc2CO2-I and Sc2C(OH)2-I were selected for further evaluation of the photocatalytic activities. We found that the Sc2CO2-I is metallic, while Sc2CF2-I and Sc2C(OH)2 are semiconductors with visible-light absorptions and promising carrier mobilities. Compared with the Sc2C(OH)2-I, the Sc2CF2-I has not only more suitable band gap (1.91 eV), but also the higher redox capability of photo-activated carriers, which should have better photocatalytic performance.

The ternary approach using a smaller band gap acceptor as the near-infrared (NIR) absorber to increase the short-circuit current density (J sc) usually decreases the open-circuit voltage (V oc). In this contribution, we report a small-molecule acceptor, IN-4F, which has a reduced band gap and a higher LUMO level than IT-4F, hence enabling the concurrent increase in the J sc and V oc when using IT-4F as the acceptor guest of the host binary of PM6:IT-4F. IN-4F was judiciously designed by fusing benzodithiophene (BDT) and thieno­[2′,3′:4,5]­thieno to make a larger π-system so as to upshift the LUMO level and reduce the optical band gap and, meanwhile, by substituting the BDT-4,8 positions with trialkylsilylthiophene chains to downshift the HOMO level to match the deep HOMO of PM6. Again, the structural similarity between IN-4F and IT-4F makes the nanoscaled homogeneous fine film morphology and the π–π stacking patterns both well kept; hence, the fill factor (FF) is well maintained. The IN-4F-based binary solar cell shows 13.1% efficiency, and its ternary solar cell blended with IT-4F supplies 14.9% efficiency. Again, the use of IN-4F as the guest acceptor of the PM6:Y6 system enables the increase in V oc due to its higher LUMO level, the increase in J sc because of the increase in charge mobilities, and the maintenance of FF, affording 16.3% efficiency. This work demonstrates that the π-system extending and the trialkylsilylthiophene chain substitution can be an effective strategy to synthesize a nonfullerene acceptor guest to realize a ternary material system, which enables to increase V oc from its entanglement with J sc (an issue of the current material approach).

By employing the density functional theory computations, we predict that the appropriate strain modification and surface fluorination of the monolayer blue phosphorene result in promising 2D topological insulators (TIs), which have sizable bulk gaps (284−554 meV) and great potential to be characterized and utilized at room temperature. The nontrivial topological phase was further convinced by the s−p band inversion, topological invariant ν = 1, and edge states with a single Dirac-type crossing at the Γ (or K) point. Our computational results provide some theoretical clues for the design of 2D TIs that can be experimentally realized from earth-abundant elements.

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A novel 2D boron-based Dirac cone material: a HfB2 monolayer.