Tin halide perovskites (THP) are one of the most promising and less toxic alternatives to lead-based perovskites. These materials have gained increasing interest in recent decades due to their exceptional optoelectronic properties, structure tunability and straightforward processing, that when included in solar cells allow the obtainment of performances up to silicon-based ones. Despite the outstanding properties of THPs, one of the main disadvantages of this class of materials is the high concentration of defects associated to Sn (II) instability and intrinsic tendency to oxidise over time. The consequent self p-doping of the perovskite leads to increased free-charge recombination rate and reduced efficiency of solar cells. Engineering the surface characteristics of tin perovskites is one of the key strategies to address both stability and performance enhancement of solar cells, an example is the use of additives, such as tin halides or hydrazine, the latter being a potent reducing agent able to compensate for tin vacancies.

Most of the explored strategies are solvent-based. Herein, we investigated for the first time an innovative use of plasma as a solvent-free, reproducible and scalable approach, ^1,2 to gently modify the perovskite surface and reduce surface defects. A preliminary study focused on nitrogen-based plasma treatment, applied to a DMSO-free FASnI₃ perovskite surface ³. The mild nature of the plasma process enabled subtle surface modifications, effectively suppressing the natural tendency of tin (II) to oxidize, as confirmed by the XPS analysis performed on aged films. Subsequently, we extended this approach to a FASnI_2.7Br0.₃ perovskite, employing a plasma generated from a mixture of N₂ and H₂ gases. Owing to the reducing character of hydrogen-based plasma, we observed a notable enhancement in device performance, accompanied by increased photoluminescence and reduced non-radiative recombination. The reactive hydrogen species generated within the plasma interact with the perovskite surface, mitigating carrier losses associated with self-doping, thereby contributing to improved device efficiency.

These studies establish the basis for a novel application of plasma technology to enhance tin-based perovskite solar cells, offering an approach that is not only effective but also readily scalable for industrial implementation.