Emission Quenching upon Pressure Correlated to Direct-to-Indirect Band Gap Transition in a Rare Example of N = 2 Ag/Bi Double Halide Perovskite

The emergence of 2D double halide perovskites (DHPs) has led to a more environmentally friendly family of materials, which is promising for a variety of optoelectronic applications. In this work, we introduce a rare example of a 2D bilayer DHP, (Cyst)CsAgBiBr7, where a dication is incorporated between the inorganic layers. This perovskite emits in the deep red region, as a result of a direct band gap. Studies under pressure revealed an enhancement of the photoluminescence (PL), the maximum of which occurs around 0.5 GPa, before its intensity starts to gradually decrease, leading to a plateau, before completely disappearing. For better insight, synchrotron single-crystal X-ray diffraction measurements were performed under pressure in the range of ambient pressure (AP) to 5.63 GPa with a step of ∼0.3 GPa, and crystal structures were solved using the data collected at AP, 0.56, 1.69, and 4.39 GPa. Moreover, first-principle calculations revealed a pressure-induced direct-to-indirect band gap transition due to the relocation of the conduction band minimum (CBM) and the valence band maximum (VBM) for P > 1 GPa, which may account for the changes in the PL spectrum under compression.