Surface Defects Passivation with Organic Salt for Highly Stable and Efficient Lead-Free Cs₃Sb₂I₉ Perovskite Solar Cells

Poor stability and high toxicity are the primary challenges to the further development of Pb-based perovskite solar cells. To this end, the scientific community has paid increased attention to lead-free perovskites such as cesium antimony iodide (Cs 3 Sb 2 I 9 ), which have demonstrated high stability. However, the charge recombination that arises from deep-level defects in Cs 3 Sb 2 I 9 -based solar cells results in low power conversion efficiencies (PCEs). Therefore, adding passivation or additives such as triiodide ions (I 3 – ) could be viable for achieving homogeneous, compact, and defect-free perovskite films. Herein, we develop a facile approach for eliminating surface defects in Cs 3 Sb 2 I 9 solar cells. Light irradiation of formamidinium iodide (FAI), the generated FA + and I 3 – ions, improves the grain size, crystallinity, and surface morphology of the film, thus decreasing the nonradiative defects in films. Furthermore, a high-quality Cs 3 Sb 2 I 9 -based perovskite-layered phase is achieved at a low temperature (>100 °C). With a hole-transport layer-free architecture (glass/FTO/c-TiO 2 /Cs 3 Sb 2 I 9 /Au, where FTO = fluorine-doped tin oxide), the PCE is boosted from 1.06 to 1.76% with good long-term stability in an ambient environment. Additional physical characterizations evidenced the passivation of detrimental defects, reduction of charge recombination, and efficient charge transfer, which contribute to enhanced device performance. We believe such a facile approach could pave the way for the widespread development of highly stable Pb-free perovskite solar cells.