  Third-generation photovoltaic (PV) materials combine many advantageous
properties, including a high optical absorption together with a large charge
carrier mobility, facilitated by small effective masses. Halide perovskites
(ABX3, where X is I, Br or Cl) appear to be the most promising third-generation
PV materials at present. Their opto-electronic properties are governed by the
B-X bond. A quantum-chemical bond analysis reveals that this bond differs
significantly from ionic, metallic or covalent bonds. Instead, it is better
regarded as metavalent, since it shares approximately one p-electron between
adjacent atoms. The resulting sigma-bond is half-filled, which causes
pronounced optical absorption. Electron transfer and lattice distortions open a
moderate band gap, resulting in charge carriers with small effective masses.
Hence metavalent bonding explains the favorable PV properties of halide
perovskites. This is summarized in a map for different bond types, which
provides a blueprint to design third-generation PV materials.
