Enhanced Photovoltaic Performance via a Bifunctional Additive in Tin-Based Perovskite Solar Cells

Tin-based perovskites with low toxicity and a narrow band gap have been promising candidates for the fabrication of efficient lead-free perovskite solar cells (PSCs). Nevertheless, the power conversion efficiency (PCE) of tin-based PSCs still lags behind that of their lead counterparts due to the poor film quality induced by uncontrollable crystal growth and Sn2+ self-doping. Herein, we introduce a bifunctional additive, ammonium thiocyanate (NH4SCN), into the precursor solution, which is able to coordinate with SnI2 to effectively control the crystal growth of the FASnI3 perovskite and further passivate the trap states in the perovskite films. Furthermore, the modified FASnI3 perovskite displays an improved film quality, featuring a compact surface morphology with an enlarged grain size, as well as decreased trap density, resulting in the reduction of the Sn4+/Sn2+ proportion in the film. As a result, a considerably enhanced PCE from 4.45 to 8.15% has been demonstrated. More importantly, the NH4SCN-based device exhibits advanced environment stability, retaining over 70% of the initial efficiency after 100 h of exposure to 35 ± 5% relative humidity at room temperature in a dark environment. This finding provides an additive strategy to improve the efficiency and stability of tin-based PSCs.