Pressure-Driven Band Gap Narrowing in Rb2AgPdCl5: Toward the Shockley–Queisser Limit of Lead-free Double Perovskites

Hydrostatic pressure is an effective tool that can give rise to novel crystal structures and physical properties. This study presents the structural, electronic, and optical properties of electronically one-dimensional (1D) double perovskite Rb2AgPdCl5 (A2BB′X5) under hydrostatic pressure. At ambient pressure, Rb2AgPdCl5 shows a band gap of 2.20 eV (0.65 eV) at the HSE06 + SOC (PBE) level of theory, and effective carrier masses are 0.44 and 0.64 mo (where mo is the rest mass of an electron) for electrons and holes, respectively. Upon applying the hydrostatic pressure, we observe band gap narrowing, accompanied by piezochromism, and a reduction in effective carrier masses. At a relatively low pressure of 9 GPa, Rb2AgPdCl5 achieves the optimum band gap of 1.36 eV, which is close to the optimal value of the Shockley–Queisser limit. The band gap reduction is attributed to the contraction of the metal-halide bond length and the increase in the overlap of atomic orbitals. The decrease in effective carrier masses is attributed to the increase in the width of conduction and valence bands, indicating improved transport of carriers with external pressure. This work elucidates the effects of hydrostatic pressure on the sensitive tuning of the electronic and optical properties of this perovskite family for vivid optoelectronic applications.