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.