Structural Dynamics and Sharp Band-to-Band Transitions in Halide Perovskites

Solar cells based on halide perovskites (HaPs) have achieved remarkable performance, comparable with conventional, high-quality inorganic semiconductors. However, a variety fundamental aspects in these materials remain enigmatic. For example, an important difference to conventional semiconductors is that HaPs exhibit strong vibrational anharmonicities already at around room temperature, which impact key optoelectronic properties, such as carrier transport, Urbach energy, among others. In this contribution, we investigate the impact that anharmonic lattice dynamics have on the optical absorption of the prototypical HaP, CsPbBr₃. Using molecular dynamics simulations based on density functional theory, we disentangle and better understand the effect of Cs dynamics on the pair-distribution function, as well as its coupling to the Pb-Br framework. Furthermore, we show that large Br displacements occur on planes transversal to the Pb-Br-Pb bonding axis. This <i>transversality</i> results in ultrashort dynamic correlations and sharp joint-density of states at finite temperature, as revealed when we contrast the results for HaPs to the case of PbTe, which shares several important properties with CsPbBr₃, but cannot exhibit any <i>transversality</i>. Finally, we discuss the interrelations of lattice anharmonicity, structural flexibility and bonding mechanisms in HaPs, and how they can enable their outstanding optoelectronic properties, which could provide guidelines for future materials design.<br/><br/><br/>1. X. Zhu, S. Caicedo-Dávila, C. Gehrmann, and D. A. Egger, ACS Appl. Mater. Interfaces 14, 22973 (2022).<br/>2. C. Gehrmann, S. Caicedo-Dávila, X. Zhu, and D. A. Egger, Adv. Sci. 9, 2200706 (2022).