Passivation Performance in Defect Perovskites Favors Weaker Lewis Donors

Perovskite materials are highly promising for a range of optoelectronic applications, including energy conversion technologies, owing to their high charge-carrier mobilities, adaptability of band gap tuning, and exceptional light-harvesting capabilities. However, defects─especially iodide vacancies─introduced during fabrication often limit performance by altering the local crystal structure and disrupting key electronic features such as the Fermi level, work function, and density of states. In this study, we explored small-molecule passivation of iodide vacancy-defected MAPbI3 surfaces using a series of N-, P-, and O-based benzyl derivatives with varying Lewis base strength. Surprisingly, molecules with formally weaker Lewis basicity, such as phosphonic acid and phosphonate, were found to offer superior passivation performance compared with stronger Lewis donors such as amines and phosphines. This was achieved through shorter Pb–donor bond lengths, stronger surface coordination, and more favorable orbital alignment, resulting in lowered Fermi levels, increased work functions, and suppression of trap states. These findings reveal that passivation effectiveness in defect perovskites does not correlate with classical Lewis base strength but instead depends on orbital energy alignment and bonding interactions. This insight provides a new design principle for surface passivation and advances the development of more stable and efficient perovskite optoelectronic devices.