Two-Dimensional Layered Structures of Group-V Elements as Transparent

Exotic optoelectronic and transport properties of two-dimensional (2D) materials have made them the focus of several application-oriented studies. This work is a feasibility study of such 2D structures based on group-V elements as passivating/transparent conducting interlayers in photovoltaic applications. We present a detailed first-principles study of the optoelectronic and carrier-transport properties of the two most stable and experimentally synthesized allotropes (alpha and ,B) of As, Sb, and Bi. Monolayers of both allotropes exhibit a band gap for all three elements, which decreases and eventually disappears beyond a critical number of layers (thickness). Interestingly, this transition from semiconducting to metallic behav-ior is found to be very different for As as compared with Sb and Bi. alpha-Arsenene remains semiconducting until the pentalayered structure, while ,B-arsenene becomes metallic beyond the bilayered structure. All other allotropes of Sb and Bi are semiconducting only for a monolayer. The in-plane conductivity of the monolayered structures lies in the range from 104 to 105 S m-1, and increases with increasing layer thick-ness. On the other hand, the monolayers exhibit the lowest reflectivity (5% or less), which increases to more than 25%, 50%, and 40% in the visible region for pentalayers of alpha-and ,B-arsenene, antimonene, and bismuthene, respectively. Trilayered alpha-arsenene, with a figure of merit (T10/Rsh) of approximately 0.15 mS, is a promising candidate as a transparent conducting layer in solar-cell applications. Such com-bined evaluation of 2D materials based on their optoelectronic and transport properties is quite useful for future experimental investigations.