Synergistic Modulation of Sn²⁺ Oxidation and Perovskite Crystallization Induced by 4-Hydroxypyridine for Stable Lead-Free Solar Cells

Tin perovskites present promising alternatives to lead perovskites, offering comparable optoelectronic properties alongside environmentally friendly characteristics. However, the rapid crystallization and easy oxidation of Sn²⁺ lead to poor film quality, further constraining the device performance. Here, 4-hydroxypyridine (4-HP) is introduced into the tin perovskite precursor for fabrication of high-quality tin perovskite films. 4-HP could modulate the colloidal size of prenucleation perovskite clusters in the precursor, thus inducing fast nucleation and retarding the crystal growth rate of tin perovskite through the formation of chemical interaction between nitrogen of pyridine and Sn²⁺ ions. Furthermore, the hydroxyl group on the pyridine ring contributes to suppressing the oxidation of Sn²⁺. As a result, the power conversion efficiency (PCE) of the devices based on 4-HP increases up to 11.3%. The stability of the unencapsulated devices shows significant improvement, retaining 100% of their initial PCEs after 2000 h of storage in N₂ with 50-100 ppm of O₂. This research presents a novel approach to the synchronized regulation of tin perovskite crystallization and the suppression of Sn²⁺ oxidation.