A direct-band-gap planar BeN₂ monolayer with high mobility and bilayer-enhanced photovoltaic efficiency

Two-dimensional (2D) materials featuring direct and tunable band gaps, high carrier mobility, and strong visible-light absorption have attracted considerable attention as promising platforms for next-generation nanoelectronic and optoelectronic applications. In this work, we identify a planar BeN₂ monolayer as the global minimum using the CALYPSO method, exhibiting the above-mentioned desirable electronic and optical properties. Structurally, the BeN₂ monolayer consists of edge-sharing five-, six-, and seven-membered rings (denoted as 567-BeN₂) and displays robust thermal, dynamical, and mechanical stability. Electronically, 567-BeN₂ is a direct-band-gap semiconductor with a band gap of 1.82 eV and exhibits an electron mobility of approximately 10⁴ cm² V^-1 s^-1, which is about 16 times higher than the hole mobility along the x direction. Optically, 567-BeN₂ shows strong visible-light absorption, with an absorption coefficient of approximately 10⁴ cm^-1, leading to a predicted photovoltaic efficiency of 24.8%, outperforming crystalline Si within the same theoretical framework. Furthermore, bilayer stacking reduces the band gap to 1.13 eV while preserving the direct-gap character, boosts the electron mobility by up to an order of magnitude, enhances visible-light absorption, and increases the photovoltaic efficiency to as high as approximately 31%. Together, these properties position 567-BeN₂ as a highly promising candidate for efficient, lightweight, and tunable 2D optoelectronic and photovoltaic devices.