Two-Dimensional Mg₁M₂X₄ (M = Ga, In; X = Se, Te) Semiconductors for High-Performance Photovoltaic Photodetectors

Finding two-dimensional (2D) semiconductors with exceptional electronic and optical properties is an ongoing hot topic in advanced optoelectronics. Here, by means of first-principles calculations coupled with the nonequilibrium Green’s function method, we have identified four 2D Mg₁M₂X₄ (M = Ga, In; X = Se, Te) semiconductors. These 2D crystals are predicted to be feasibly exfoliated and exhibit robust thermodynamic, dynamical, and thermal stability. Importantly, their optimal band gaps (0.82-1.67 eV), remarkable optical absorption coefficients (∼2 × 10⁵ cm^-1), appropriate exciton binding energies (0.37-0.64 eV), and favorable electron mobilities (98.7-245.3 cm²/V s) highlight the potential of 2D Mg₁M₂X₄ semiconductors for photovoltaic applications. Moreover, the absorption spectra undergo significant renormalization at 300 K as a result of exciton-phonon interaction. A comprehensive analysis of carrier scattering mechanisms reveals that polar optical phonon scattering is the dominant mechanism limiting mobility at or above 300 K. Given these outstanding electronic and optical properties, we further explored the photovoltaic performance of the constructed p-n junctions. These devices exhibit type-II (staggered) band alignments, with maximum responsivities (R_max) of 76, 111, 57, and 85 mA/W for the Mg₁M₂X₄-based photovoltaic photodetectors, respectively. These values are comparable to or higher than the maximum responsivity (R_max = 65 mA/W) observed in the widely studied monolayer MoS₂ p-n junction. Thus, the 2D Mg₁M₂X₄ semiconductors offer promising prospects for applications in next-generation nanoscale photovoltaic devices.