High hole mobility and photovoltaic potential in wrinkled BeP₃ and BeP₄ monolayers

Two-dimensional semiconductors that combine a suitable direct band gap, strong light absorption, and high carrier mobility are promising for nanoscale optoelectronic and photovoltaic applications. Here, using particle-swarm crystal structure prediction combined with first-principles calculations, we identify two stable wrinkled beryllium phosphide monolayers, W-BeP₃ and W-BeP₄. Both structures are dynamically, thermally, and mechanically stable, and are composed of phosphorus-rich networks connected by Be atoms. Hybrid-functional calculations show that W-BeP₃ and W-BeP₄ are direct-gap semiconductors with band gaps of 1.59 and 1.30 eV, respectively, and their direct-gap character is retained under moderate strain. Owing to the anisotropic wrinkled framework and weak band-edge deformation, the two monolayers exhibit highly anisotropic hole transport, with deformation-potential-estimated hole mobilities reaching 1.16 × 10⁵ and 1.11 × 10⁴ cm² V^-1 s^-1 along the preferred transport direction for W-BeP₃ and W-BeP₄, respectively. In addition, within the spectroscopic limited maximum efficiency (SLME) model, their strong visible-light absorption yields theoretical maximum efficiencies of 28.7% and 32.3%, respectively. W-BeP₄ further displays an angle-dependent negative Poisson's ratio, reflecting its unusual mechanical anisotropy. These results suggest that wrinkled Be-P monolayers are promising candidates for anisotropic nanoelectronic and optoelectronic applications and merit further exploration as potential photovoltaic absorber materials.