In the last years, the superb results in terms of the efficiency reached using metal halide perovskites (MHP) as active material in single junction solar cells have been accompanied by growing concerns about their environmental impact, due to the significant presence of water-soluble, toxic lead in these structures [1]. In order to address this issue, tin (Sn)-based and mixed tin-lead (Sn-Pb) perovskites are emerging as interesting substitutes for the widely studied Pb-based perovskites, and furthermore, due to their smaller bandgap, these materials can be implemented as narrow energy gap material in all-perovskite tandem devices with an expected efficiency of about 36%.

However, this thriving solution comes with new issues to overcome, such as the introduction of intrinsic Sn-defects and the Sn(II) oxidation to Sn(IV) that contribute to give rise to photovoltaic performances that are still far behind from those of the lead-based counterparts. With the aim of reducing defect densities related to the fast crystallization of the material and to the Sn(II) oxidation, several passivation strategies have been studied, such as the introduction of bulky organic cations that has been proved to successfully enhance PCE and open-circuit voltage of perovskite solar cells[2,3].

In this work [4] we present a study of the effect of the addition of a small amount of 2,3,4,5,6-pentafluorophenethylammonium cations (5FPEAI) in MA_0.5FA_0.5Sn_0.5Pb_0.5I₃ perovskite, showing an improvement in PCE from 17.47 % to 19.13 % and in Voc from 0.77 V to 0.84 V, respect to the refence device. The addition of 5FPEAI cations induces superior crystallinity of the 3D perovskite grains and a preferential orientation, leading to a more robust perovskite structure. Moreover, the highly crystalline films show larger grains and a reduced density of defect states. Last but not least, the mixed tin-lead perovskite solar cells using the fluorinated cation show a remarkably better thermal stability with respect to the control devices, retaining over 90 % of its initial efficiency after 90 min of thermal stress performed at 85 °C, while we observed only around 66% for the control. Further studies on electroluminescence and photoluminescence under pulsed condition have been performed on air-stable encapsulated devices. We believe that our work is a step further in the optimization of Sn/Pb perovskite solar cells, in the promising path of approaching and overtaking Pb-based devices in terms of performances and stability.