3D graphene-like semiconductor Ba2HfTe4 with electronic structure similar to graphene and bandgap close to silicon

Opening the graphene zero bandgap and simultaneously keeping its linear electronic dispersion has been widely accepted as a very challenging scientific puzzle. Moreover, one of the biggest obstacles for all-perovskite tandem solar cells is the lack of perovskite semiconductors with a narrow bandgap (<1.2 eV). Here we examine these two issues and theoretically identify the environmentally friendly perovskite A2HfTe4 (A = Ca, Sr, Ba), especially Ba2HfTe4. This material is a good candidate for three-dimensional graphene-like semiconductors with two-dimensional electronic dimensionality, linear band-edge dispersion, a direct bandgap of 0.77–0.96 eV, carrier effective mass and mobility similar to graphene, visible-light absorption beyond Pb-based perovskites, and power conversion efficiency approaching ∼28%. A2HfTe4 is a promising bottom-cell material that helps achieve all-perovskite tandem solar cells thanks to its narrow bandgap and acts as a potential alternative to the currently popular perovskite-silicon tandem solar cells.