HfX_2 (X = Cl, Br, I) Monolayer and Type II Heterostructures with Promising Photovoltaic Characteristics

Two-dimensional (2D) materials and their corresponding van der Waals (vdW) heterostructures are considered as promising candidates for highly efficient solar cell applications. A series of 2D HfX_2 (X = Cl, Br, I) monolayers are proposed, via first-principle calculations. The vibrational phonon spectra and molecular dynamics simulation results indicate that HfX_2 monolayers possess dynamical and thermodynamical stability. Moreover, their electronic structure shows that their Heyd–Scuseria–Ernzerhof(HSE06)-based band values (1.033–1.475 eV) are suitable as donor systems for excitonic solar cells (XSCs). The material's significant visible-light absorbing capability (\sim10^5 cm^-1) and superior power conversion efficiency (\sim20%) are demonstrated by establishing a reasonable type II vdW heterostructure. This suggests the significant potential of HfX_2 monolayers as a candidate material for XSCs.