Microstructural and photoconversion efficiency enhancement of compact films of lead-free perovskite derivative Rb3Sb2I9

While lead-based perovskites have held center stage in photovoltaic and optoelectronic research over the past decade, their toxicity has raised significant concerns, spurring the search for lead-free alternatives with similar potential. While a number of lead-free antimony-/bismuth-based compounds have been proposed, they have typically exhibited limited charge extraction efficiency, which has prompted the widespread adoption of a mesoporous device architecture. With a focus on compact films of Rb3Sb2I9-an emerging lead-free two-dimensional perovskite derivative-this study presents two strategies to enhance their microstructure: one relying on the reduction of the supersaturation level during the annealing step, and the other involving high-temperature annealing in an SbI3 atmosphere. Both strategies lead to a considerable improvement in film morphology and microstructure, with a twofold increase in apparent grain size. Such high-quality compact films deliver a pronounced rise in external quantum efficiency, as well as in short-circuit photocurrent under solar illumination-all this without the aid of a mesoporous architecture for charge extraction. Hall effect and photocurrent-power characterization show that this performance improvement results from an increase in charge carrier mobility and a reduction in the number of recombination centers. The microstructural improvement, photoconversion efficiency boost, and mechanistic insight provide valuable indications on the status and prospects of Rb3Sb2I9 and related derivatives-as relevant to the future exploration of these compounds for lead-free top-cells in tandem photovoltaics, indoor photovoltaics, and other optoelectronic application domains.