Lead‐Free Perovskite Photodetectors Achieved via 2‐Pyridinethiourea Molecular Passivation

ABSTRACT Bismuth based perovskites have emerged as promising alternatives to toxic lead‐based perovskites for photodetectors, owing to their low toxicity and outstanding environmental stability. However, the optoelectronic performance of Bi‐based films is limited by poor crystallinity and high defect density. This work introduces 2‐pyridylthiourea (2‐PTU) as a multifunctional additive. The thiocarbonyl (C═S) group in 2‐PTU molecule acts as a strong Lewis base, forming stable coordinate bonds with undercoordinated Bi 3+ sites in the [BiI 6 ] 3− octahedra. The coordination not only guides the formation of a dense, large‐grained film but also achieves effective defect passivation, reducing the defect density by more than 45%. Consequently, the optimized photodetector exhibits a low dark current density of 1.55 × 10 −8 A cm −2 at 0.1V bias, specific detectivity at 1.7 × 10 12 Jones. The passivated device shows a fast rise time of 20 µs under 365 nm illumination. Furthermore, it maintains 93% of its initial responsivity after 2000 h of storage in a nitrogen atmosphere, and exhibits favorable stability under humid and light‐soaking conditions in air. The demonstrated molecular coordination strategy provides critical insights for controlling crystallization, defects and stability of Bi‐based perovskites, advancing development of lead‐free photodetectors for near‐ultraviolet detection.