Bandgap engineering of a lead-free defect perovskite Cs₃Bi₂I₉ through trivalent doping of Ru³

Inorganic defect halide compounds such as Cs₃Bi₂I₉ have been regarded as promising alternatives to overcome the instability and toxicity issues of conventional perovskite solar cells. However, their wide indirect bandgaps and deep defect states severely limit their photoelectronic conversion efficiency when implemented in devices. Trivalent cation substitution has been proposed by previous calculations allowing the engineering of their band structures, but experimental evidences are still lacking. Herein we use the trivalent cation Ru³⁺ to partially replace Bi³⁺ in Cs₃Bi₂I₉, and reveal their structural and optoelectronic properties, as well as the environmental stability. The Ru-doped Cs₃Bi₂I₉ shows a decreasing bandgap with the increasing doping levels and an overall up-shift of band structure, owing to the dopant-induced defect states and thus enhanced phonon-electron coupling. As a result, upon Ru³⁺ doping, the narrowed bandgap and the upward shift of the band structures might facilitate and broaden their applications in optoelectronic devices.