Sign of the Gap Temperature Dependence in CsPb(Br,Cl)3 Nanocrystals Determined by Cs-Rattler-Mediated Electron-Phonon Coupling.

So far, the striking sign reversal in the near-ambient slope of the gap temperature dependence of colloidal CsPbCl3 perovskite nanocrystals (NCs) compared to its Br counterpart remains unresolved. Pure bromide NCs exhibit a linear gap increase with increasing temperature, to which thermal expansion and electron-phonon interaction equally contribute. In contrast, the temperature slope for the chlorine compound gap is clearly negative. 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 and magnitude due to activation of an anomalous electron-phonon coupling mechanism linked to vibrational modes characterized by synchronous octahedral tilting and Cs rattling. This takes place in the shrunken orthorhombic NC lattice for Cl concentrations exceeding ca. 40%. We have thus clarified a puzzling result directly impacting the optoelectronic properties of lead halide perovskite NCs.