Synergistic Modulation\nof Sn<sup>2+</sup> Oxidation\nand Perovskite Crystallization Induced by 4‑Hydroxypyridine\nfor Stable Lead-Free Solar Cells

Tin perovskites present promising alternatives to lead\nperovskites,\noffering comparable optoelectronic properties alongside environmentally\nfriendly characteristics. However, the rapid crystallization and easy\noxidation of Sn<sup>2+</sup> lead to poor film quality, further constraining\nthe device performance. Here, 4-hydroxypyridine (4-HP) is introduced\ninto the tin perovskite precursor for fabrication of high-quality\ntin perovskite films. 4-HP could modulate the colloidal size of prenucleation\nperovskite clusters in the precursor, thus inducing fast nucleation\nand retarding the crystal growth rate of tin perovskite through the\nformation of chemical interaction between nitrogen of pyridine and\nSn<sup>2+</sup> ions. Furthermore, the hydroxyl group on the pyridine\nring contributes to suppressing the oxidation of Sn<sup>2+</sup>.\nAs a result, the power conversion efficiency (PCE) of the devices\nbased on 4-HP increases up to 11.3%. The stability of the unencapsulated\ndevices shows significant improvement, retaining 100% of their initial\nPCEs after 2000 h of storage in N<sub>2</sub> with 50–100 ppm\nof O<sub>2</sub>. This research presents a novel approach to the synchronized\nregulation of tin perovskite crystallization and the suppression of\nSn<sup>2+</sup> oxidation.