DFT Investigation of 2D MoSiGeN₄/MoSe₂ Heterostructures with High Carrier Mobility: Implications for High-Performance Ultrathin Solar Cells

In this study, we developed a Janus MoSiGeN₄/MoSe₂ van der Waals (vdW) heterostructure and explored its potential for high-performance ultrathin solar cells using the density functional theory (DFT). The structural asymmetry of the Janus MoSiGeN₄ monolayer facilitates distinct Si–Se and Ge–Se interfacial contacts, both exhibiting robust thermal stability. Notably, the heterostructure featuring a Ge–Se contact interface displays a type-II band alignment characterized by the conduction band minimum (CBM) residing in the MoSiGeN₄ layer and the valence band maximum (VBM) residing in the MoSe₂ layer. This spatial separation of the CBM and VBM promotes efficient electron–hole separation and reduces recombination rates, further enhanced by a favorable internal electric field. The heterostructure also demonstrates optimized carrier mobilities exceeding 1 × 10³ cm² V^–1 s^–1 and improved sunlight absorption compared to its individual monolayers. Assuming 100% external quantum efficiency, the estimated power conversion efficiency (PCE) reaches approximately 20%, attributed to a minimal conduction band offset (CBO). Additionally, our investigations into strain effects indicate that the tensile out-of-plane strain optimally tunes the PCE by modulating the CBO. Our findings suggest that the MoSiGeN₄/MoSe₂ heterostructure with a Ge–Se contact interface holds significant promise for high-performance ultrathin solar cells and provides valuable theoretical insights for the advancement of next-generation photovoltaic technologies.