Weak X‐Ray to Visible Lights Detection Enabled by a 2D Multilayered Lead Iodide Perovskite with Iodine‐Substituted Spacer

Broadband photodetectors (PDs) with low detection limits hold significant importance to next‐generation optoelectronic devices. However, simultaneously detecting broadband (i.e., X‐ray to visible regimes) and weak lights in a single semiconducting material remains highly challenging. Here, by alloying iodine‐substituted short‐chain cations into the 3D FAPbI(3) (FA = formamidine), a new 2D bilayered lead iodide hybrid perovskite, (2IPA)(2)FAPb(2)I(7) (1, 2IPA = 2‐iodopropylammonium), that enables addressing this challenge is reported. Such a 2D multilayered structure and lead iodide composition jointly endow 1 with a minimized dark current (6.04 pA), excellent electrical property, and narrow bandgap (2.03 eV), which further gives it great potential for detecting broadband weak lights. Consequently, its high‐quality single crystal PDs exhibit remarkable photoresponses to weak ultraviolet–visible lights (377–637 nm) at several tens of nW cm(−2) with high responsivities (>10(2) mA W(−1)) and significant detectivities (>10(12) Jones). Moreover, 1 has an excellent X‐ray detection performance with a high sensitivity of 438 µC Gy(−1) cm(−2) and an ultralow detection limit of 20 nGy s(−1). These exceptional attributes make 1 a promising material for broadband weak lights detection, which also sheds light on future explorations of high‐performance PDs based on 2D hybrid perovskites.