The BeP monolayer exhibits ultra-high and highly anisotropic carrier mobility and 29.3% photovoltaic efficiency.

Two-dimensional materials with a combination of a moderate bandgap, highly anisotropic carrier mobility, and a planar structure are highly desirable for nanoelectronic devices. This study predicts a planar BeP monolayer with hexagonal symmetry that meets the aforementioned desirable criteria using the CALYPSO method and first-principles calculations. Calculations of electronic properties demonstrate that the hexagonal BeP monolayer is an intrinsic semiconductor with a direct band gap of approximately 0.94 eV and this direct bandgap characteristic is maintained under strain. The mobilities of hexagonal BeP are electron-dominated, reaching ∼10 cm V s, which is two orders of magnitude higher than the mobility of holes. The high carrier mobility results from the small deformation potential constant, which arises from the unique decoupling behavior of electrons in the valence and conduction bands. Furthermore, our calculations reveal that the photovoltaic efficiency of hex-BeP is as high as 29.3%, which is comparable to those of well-known thin-film solar cell absorbers, thanks to its high visible light absorption coefficient of ∼10 cm and its direct bandgap feature.