DFT Investigation of 2D MoSiGeN<sub>4</sub>/MoSe<sub>2</sub> Heterostructures with High Carrier Mobility: Implications\nfor High-Performance Ultrathin Solar Cells

In\nthis study, we developed a Janus MoSiGeN<sub>4</sub>/MoSe<sub>2</sub> van der Waals (vdW) heterostructure and explored its potential\nfor high-performance ultrathin solar cells using the density functional\ntheory (DFT). The structural asymmetry of the Janus MoSiGeN<sub>4</sub> monolayer facilitates distinct Si–Se and Ge–Se interfacial\ncontacts, both exhibiting robust thermal stability. Notably, the heterostructure\nfeaturing a Ge–Se contact interface displays a type-II band\nalignment characterized by the conduction band minimum (CBM) residing\nin the MoSiGeN<sub>4</sub> layer and the valence band maximum (VBM)\nresiding in the MoSe<sub>2</sub> layer. This spatial separation of\nthe CBM and VBM promotes efficient electron–hole separation\nand reduces recombination rates, further enhanced by a favorable internal\nelectric field. The heterostructure also demonstrates optimized carrier\nmobilities exceeding 1 × 10<sup>3</sup> cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> and improved sunlight absorption\ncompared to its individual monolayers. Assuming 100% external quantum\nefficiency, the estimated power conversion efficiency (PCE) reaches\napproximately 20%, attributed to a minimal conduction band offset\n(CBO). Additionally, our investigations into strain effects indicate\nthat the tensile out-of-plane strain optimally tunes the PCE by modulating\nthe CBO. Our findings suggest that the MoSiGeN<sub>4</sub>/MoSe<sub>2</sub> heterostructure with a Ge–Se contact interface holds\nsignificant promise for high-performance ultrathin solar cells and\nprovides valuable theoretical insights for the advancement of next-generation\nphotovoltaic technologies.