When Electrons in Metal Halide Perovskites "Hear" the Sound of Cationic Rattlers

The band gap is a fundamental quantum mechanical property of the electronic band structure of crystalline solids and corresponds to an energy range of forbidden states that separates the so-called valence and conduction bands. Electrons residing in the valence band contribute to the crystal's bonding, while conduction electrons contribute to the electrical conductivity of the material. In semiconductors, band gap energies typically fall within the visible spectral range, thus playing a crucial role in their optoelectronic properties. For example, the band gap determines the color of light-emitting devices or the energy conversion efficiency of solar cells. The dependence on temperature of the gap is also fundamental and technologically important.Most metal halide perovskites exhibit, at around room temperature, a linear increase in the gap with increasing temperature. This is roughly due to an equal footing of the effects of thermal expansion and the influence of the coupling between band electrons and the quantized crystal vibrations: the phonons. CsPbCl3 perovskite nanocrystals (NCs) constitute a blatant exception, because of the striking sign reversal in the near-ambient slope of the gap temperature dependence. By combining temperature- and pressure dependent photoluminescence on a series of CsPb(Br1-xClx)3 NCs, we unravel the origin of such inversion. The electron-phonon interaction is solely responsible, undergoing a sudden change in sign at high Cl concentrations due to activation of an anomalous electron-phonon coupling mechanism linked to (mainly acoustic) vibrational modes characterized by synchronous tilting of the PbCl6 octahedra and Cs rattlers (see the figurative representation of this coupling in the artwork of Fig. 1). We have thus clarified a puzzling result directly impacting the optoelectronic properties of lead halide perovskite NCs. Given the relevance of the gap temperature dependence for the optoelectronic properties of perovskite NCs, its correct assessment is key for the advancement of emergent photovoltaics and efficient light emission and/or sensing devices, for instance.