Pressure Effects on Structure and Optical Properties in Cesium Lead Bromide Perovskite Nanocrystals

Metal halide perovskites (MHPs) are gaining increasing interest because of their extraordinary performance in optoelectronic devices and solar cells. However, developing an effective strategy for achieving the band-gap engineering of MHPs that will satisfy the practical applications remains a great challenge. In this study, high pressure is introduced to tailor the optical and structural properties of MHP-based cesium lead bromide nanocrystals (CsPbBr₃ NCs), which exhibit excellent thermodynamic stability. Both the pressure-dependent steady-state photoluminescence and absorption spectra experience a stark discontinuity at ∼1.2 GPa, where an isostructural phase transformation regarding the Pbnm space group occurs. The physical origin points to the repulsive force impact due to the overlap between the valence electron charge clouds of neighboring layers. Simultaneous band-gap narrowing and carrier-lifetime prolongation of CsPbBr₃ trihalide perovskite NCs were also achieved as expected, which facilitates the broader solar spectrum absorption for photovoltaic applications. Note that the values of the phase change interval and band-gap red-shift of CsPbBr₃ nanowires are between those for CsPbBr₃ nanocubes and the corresponding bulk counterparts, which results from the unique geometrical morphology effect. First-principles calculations unravel that the band-gap engineering is governed by orbital interactions within the inorganic Pb-Br frame through structural modification. Changes of band structures are attributed to the synergistic effect of pressure-induced modulations of the Br-Pb bond length and Pb-Br-Pb bond angle for the PbBr₆ octahedral framework. Furthermore, the significant distortion of the lead-bromide octahedron to accommodate the Jahn-Teller effect at much higher pressure would eventually lead to a direct to indirect band-gap electronic transition. This study enables high pressure as a robust tool to control the structure and band gap of CsPbBr₃ NCs, thus providing insight into the microscopic physiochemical mechanism of these compressed MHP nanosystems.