First-principles study of Y₂CCl₂ and Janus Y₂CClX (X = F, Br, I) MXenes for photovoltaic applications

Two-dimensional (2D) Janus MXenes offer a promising platform for photovoltaic (PV) absorbers, as asymmetric surface terminations can simultaneously tailor band gaps, optical response, and built-in electric field, which promote charge carrier separation. Here, a systematic first-principles study of the symmetric halide-terminated MXene Y₂CCl₂ and the Janus compounds Y₂CClX (X = F, Br, I) using density functional theory has been performed. Hybrid-functional (HSE06) electronic structure calculations identify all compounds as indirect-gap semiconductors with gaps of 1.63 eV (Y₂CCl₂), 1.36 eV (Y₂CClF), 1.57 eV (Y₂CClBr), and 1.21 eV (Y₂CClI). Orbital-resolved density of states, charge-partitioning, and electron-localization function analyses reveal charge transfer from Y toward C and the surface halogens, with Janus functionalization producing pronounced surface asymmetry. Janus functionalization further produces substantial surface asymmetry in the work function, yielding work function differences of 3.09, 1.63, and 2.10 eV for Y₂CClF, Y₂CClBr, and Y₂CClI, respectively, suggesting intrinsic fields that may assist carrier separation. Optical calculations show strong absorption in the visible window with coefficients on the order of 10⁵ cm^- 1. Carrier-transport descriptors reveal smaller electron than hole effective masses across the series, and the screened 2D Mott-Wannier model yields exciton binding energies of 1.09, 1.12, 1.02, and 0.91 eV for Y₂CCl₂, Y₂CClF, Y₂CClBr, and Y₂CClI, respectively. Finally, PV metrics computed within the modified Shockley-Queisser formalism predict maximum efficiencies of 23.10%, 25.66%, 23.92%, and 23.01% for Y₂CCl₂, Y₂CClF, Y₂CClBr, and Y₂CClI, respectively, with Y2CClF emerging as the most favorable absorber due to its near-optimal gap and highest maximum power density. Overall, this study demonstrates that halogen-functionalized Y₂C-based Janus MXenes are fascinating materials for next-generation photovoltaic and optoelectronic devices.