Halide Perovskites: Advanced Photovoltaic Materials Empowered by a Unique Bonding Mechanism

Outstanding photovoltaic (PV) materials combine a set of advantageous properties including large optical absorption and high charge carrier mobility, facilitated by small effective masses. Halide perovskites (ABX(3), where X = I, Br, or Cl) are among the most promising PV materials. Their optoelectronic properties are governed by the B-X bond, which is responsible for the pronounced optical absorption and the small effective masses of the charge carriers. These properties are frequently attributed to the ns(2) configuration of the B atom, i.e., Pb 6s(2) or Sn 5s(2) ("lone-pair") states. The analysis of the PV properties in conjunction with a quantum-chemical bond analysis reveals a different scenario. The B-X bond differs significantly from ionic, metallic, or conventional 2c-2e covalent bonds. Instead it is better regarded as metavalent, since it shares about one p-electron between adjacent atoms. The resulting sigma-bond, formally a 2c-1e bond, is half-filled, causing pronounced optical absorption. Electron transfer between B and X atoms and lattice distortions open a moderate bandgap resulting in charge carriers with small effective masses. Hence, metavalent bonding explains favorable PV properties of halide perovskites, as summarized in a map for different bond types, which provides a blueprint to design PV materials.