Enhanced photovoltaic properties of halide perovskites due to multi-centered X–B–X bonding and p–p orbital coupling

Halide perovskites ABX3 are valuable photovoltaic and light-emitting materials due to their favorable optical properties of a direct bandgap and strong optical absorption just above the bandgap. Here, we show that these properties arise mainly from multi-centered X-B-X bonding and B-p X-p coupling. A bonding analysis finds that B–X bonds have shared electrons intermediate between that of ionic and covalent bonding. The B cation site p orbital interacts with left and right lobes of X-p orbitals, forming a special three-centered, two-electron X–B–X bond. This bonding mechanism is shown to give rise to unusually large optical matrix elements, especially in the visible spectrum, with the normalized matrix elements twice that of typical two-center bonds and transition probabilities four times larger. These cause a very sharp increase in optical absorption just above the gap. Experimental values of Urbach energies are found to be small, even if perovskites have disordered structures. The broad valence and conduction bands of the linearly aligned p orbitals cause dispersive band extrema, with small effective masses of both carriers at the R point, indicating a high carrier mobility.