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.