Valence and conduction bands engineering in halide perovskites for solar cell applica- tions

We performed ab initio simulations aimed at identifying the atomistic and electronic structure origin of the high valence and conduction band, and band gap tunability of halide perovskites. We found that the two key ingredients are the overlap between bivalent cation and halide atomic orbitals, and the electronic charge of the bottom of the conduction band (CBM) state on the Sn or Pb atoms. In particular, we found that lower gaps are associated to higher negative antibonding overlap, and higher CBM charge on the bivalent cation. Both overlap and CBM charge on Sn/Pb can be tuned by the chemical nature of halide, bi and monovalent cation, as well as the symmetry of crystal structure. On the basis of our results we provide some practical rules to tune the valence band maximum, conduction band minimum, and band gap in this class of materials.