Pressure-Tailored Band Gap Engineering and Structure Evolution of Cubic Cesium Lead Iodide Perovskite Nanocrystals

Metal halide perovskites (MHPs) have attracted increasing research attention given the ease of solution processability with excellent optical absorption and emission qualities. However, effective strategies for engineering the band gap of MHPs to satisfy the requirements of practical applications are difficult to develop. Cubic cesium lead iodide (alpha-CsPbI3), a typical MHP with an ideal band gap of 1.73 eV, is an intriguing optoelectric material owing to the approaching Shockley-Queisser limit. Here, we carried out a combination of in situ photoluminescence, absorption, and angle-dispersive synchrotron X-ray diffraction spectra to investigate the pressure-induced optical and structural changes of alpha-CsPbI3 nanocrystals (NCs). The alpha-CsPbI3 NCs underwent a phase transition from cubic (alpha) to orthorhombic phase and subsequent amorphization upon further compression. The structural changes with octahedron distortion to accommodate the Jahn-Teller effect were strongly responsible for the optical variation with the increase of pressure. First-principles calculations reveal that the band-gap engineering is governed by orbital interactions within the inorganic Pb-I frame through the structural modification. Our high-pressure studies not only established structure- property relationships at the atomic scale of alpha-CsPbI3 NCs, but also provided significant clues in optimizing photovoltaic performance, thus facilitating the design of novel MHPs with increased stimulus-resistant capability.