From molecular salt to layered network: cation-driven tuning of band gap, structure, and charge transport in A3Bi2I9 (A = Cs, Rb) perovskites.

The increasing demand for eco-friendly and stable optoelectronic materials has led to interest in all-inorganic lead-free halide perovskites. This study reports the synthesis of A3Bi2I9 (A = Cs, Rb) perovskites via a solvothermal technique. The materials crystallize in hexagonal and monoclinic structures, with micrometer-sized particles. Optical investigations reveal direct band-gaps of 2.03 eV for Cs3Bi2I9 and 1.90 eV for Rb3Bi2I9. Raman spectroscopy highlights distinct vibrational modes, influenced by their structural differences. Space charge limited current (SCLC) measurements indicate varying threshold voltages and trap densities. Impedance spectroscopy and Jonscher's power law analysis reveal different polaron tunneling mechanisms in each compound. Ultrafast transient absorption spectroscopy shows the formation of self-trapped states upon photoexcitation, linked to lattice distortion and the formation of small polarons, which affect electrical conductivity.