Methylhydrazine Lone-Pair Engineering for Polar Lead-Free Perovskite Enables Self-Powered X-Ray Detection

Abstract

Lead-free A₃Bi₂I₉-type perovskites demonstrate excellent performance in direct X-ray detection owing to their high bulk resistivity and reduced ion migration. However, the reported centrosymmetric A₃Bi₂I₉ can only operate with external voltage, inevitably resulting in energy consumption and bulky monolithic circuits, limiting their further development. Herein, exploiting the methylhydrazine (MHy) cation with 2s ² lone-pair electrons (LPEs), a chiral-polarity perovskite MHy₃Bi₂I₉ are obtained and explored its self-powered X-ray detection properties. Where MHy forms the strong hydrogen bond interaction with the inorganic framework, resulting in the asymmetric Bi₂I₉ unit. Meanwhile, the 2s ² LPEs contribute to generating MHy dipole moments, leading to spontaneous polarization. On the one hand, spontaneous polarization acts as a driving force to realize the X-ray-generated carriers' separation and transport to acquire self-powered detection ability. On the other hand, the reduced noise current and dark current under zero bias further increase the signal-to-noise ratio and lower the detection limit. Notably, the MHy₃Bi₂I₉ single-crystal-based detector displays a considerable sensitivity (106 μC Gy^-1 cm^-2) and an ultralow detection limit (55 nGy s^-1) in self-powered mode. Herein, new insights for constructing polar lead-free perovskite and realizing unprecedented A₃Bi₂I₉-type self-powered X-ray detectors are provided.

Keywords: lead‐free; lone‐pair engineering; low detection limit; polar A3Bi2I9‐type perovskite; self‐powered X‐ray detection.

Figure 1

a) The crystal structure of typical A‐cation (Cs, MA, and FA) in A₃Bi₂I₉‐type perovskite. b) The MHy cation with 2s ² lone‐pair electrons. The room‐temperature crystal structure of c) Cs₃Bi₂I₉, d) MA₃Bi₂I₉, and e) FA₃Bi₂I₉, which crystalized in the centrosymmetric space group P6₃/mmc. f) Exploiting the MHy as A‐cation, MHy₃Bi₂I₉ possesses a strong hydrogen bond interaction and crystalizes in the chiral‐polar space group P6₁ at room temperature. The dotted line refers to the hydrogen bond length between the adjacent layers.

Figure 2

a) The Hirshfeld surface of the Bi₂I₉ unit in MHy₃Bi₂I₉. b,c) The fingerprint plots of MHy₃Bi₂I₉ indicate the strong hydrogen bond interaction. The inorganic unit of d) MHy₃Bi₂I₉, and e) Cs₃Bi₂I₉, MA₃Bi₂I₉, and FA₃Bi₂I₉. The gray arrows represent the direction of the BiI₆ dipole moment. The red arrow in MHy₃Bi₂I₉ refers to the direction of Bi₂I₉ net dipole moment, which points to the c‐axis.

Figure 3

a) The 6‐fold roto‐inversion operation of MHy. H atoms are omitted for clarity. b) In the unit cell, the MHy dipole moment (light‐red arrow) contributes to the spontaneous polarization (P _s, red arrow) along the c‐axis.

Figure 4

a) The thermogravimetric analysis experiment of MHy₃Bi₂I₉. b) The second harmonic generation (SHG) test of MHy₃Bi₂I₉ and KH₂PO₄. c) Piezoelectric measurement and the corresponding d ₃₃ value. d) The temperature‐dependent pyroelectricity current and polarization curves.

Figure 5

a) The absorption coefficient of MHy₃Bi₂I₉, Cs₃Bi₂I₉, MA₃Bi₂I₉, and Si as a function of X‐ray photon energy. b) Attenuation efficiency versus thickness curves of MHy₃Bi₂I₉, Cs₃Bi₂I₉, MA₃Bi₂I₉, and Si. c) I–V curves of MHy₃Bi₂I₉ SC‐based detector in the dark and under X‐ray irradiation. d) The photo‐responses of the MHy₃Bi₂I₉ SC‐based detector under increased X‐ray dose rate at 454 V mm⁻¹ electric field (top) and 0 V bias (bottom). e) Under different external fields, the current density as a function of the X‐ray dose rate, and the slope of fitting lines corresponding to the sensitivity. f) The SNR of MHy₃Bi₂I₉ SC‐based detector under 0 V and 454 V mm⁻¹ electric field. g) The current response cycle of the detector under a low X‐ray dose rate to verify the reliability of the detection limit. h) The photocurrent of the detector under continuous X‐ray irradiation indicates its remarkable irradiation stability. i) The comparison of sensitivity and detection limit between MHy₃Bi₂I₉ and other lead‐free detectors.