Heteroepitaxial passivation of Cs2AgBiBr6 wafers with suppressed ionic migration for X-ray imaging

Abstract

X-ray detectors are broadly utilized in medical imaging and product inspection. Halide perovskites recently demonstrate excellent performance for direct X-ray detection. However, ionic migration causes large noise and baseline drift, limiting the detection and imaging performance. Here we largely eliminate the ionic migration in cesium silver bismuth bromide (Cs₂AgBiBr₆) polycrystalline wafers by introducing bismuth oxybromide (BiOBr) as heteroepitaxial passivation layers. Good lattice match between BiOBr and Cs₂AgBiBr₆ enables complete defect passivation and suppressed ionic migration. The detector hence achieves outstanding balanced performance with a signal drifting one order of magnitude lower than all previous studies, low noise (1/f noise free), a high sensitivity of 250 µC Gy _air^-1 cm^-2, and a spatial resolution of 4.9 lp mm^-1. The wafer area could be easily scaled up by the isostatic-pressing method, together with the heteroepitaxial passivation, strengthens the competitiveness of Cs₂AgBiBr₆-based X-ray detectors as next-generation X-ray imaging flat panels.

Fig. 1

Isostatic-pressing method to prepare Cs₂AgBiBr₆ wafers. a Schematic illustration of the isostatic-pressing process, while Cs₂AgBiBr₆ powders were firstly modeled into a pie shape and then subsequently subjected to a pressure of 200 MPa through a hydraulic press, and the additional annealing process could enhance the crystallinity and grain growth. b As-prepared Cs₂AgBiBr₆ wafers with tunable sizes and the diameters are 5, 3, and 1 cm from left to right. c Top–down scanning electron microscopy (SEM) of the wafer. d Cross-sectional SEM image of the wafer and the inset is a higher resolution image, demonstrating the grain size is larger than 100 μm

Fig. 2

Structural determination of BiOBr-passivated Cs₂AgBiBr₆ wafer. a XRD spectra of the wafers with/without BiOBr passivation. b Cross-sectional SEM image of the BiOBr-passivated Cs₂AgBiBr₆ wafer, the inset is the top view. c TEM image of the selected region containing both BiOBr and Cs₂AgBiBr₆, the inset is the EDS result to verify the presence of Bi, O, Br, Cs, and Ag elements. d SAED pattern of the selected region in c, and the inset is the epitaxial growth model. The corresponding diffraction spots from Cs₂AgBiBr₆ and BiOBr are labeled accordingly, and the zone axis is [001]. e Epitaxial growth direction between BiOBr and Cs₂AgBiBr₆. f Crystal structure of Cs₂AgBiBr₆ and BiOBr viewed from [001] directions and the related diffraction pattern

Fig. 3

Optical and electrical properties of Cs₂AgBiBr₆ wafers with/without BiOBr passivation. a Schematic illustration of the suppressed ionic migrations by BiOBr passivation. b The resistivity of the wafers and the inset is the statistical result. c Arrhenius plots of the temperature dependence of kT versus 1000/T, while k is the ionic migration rate (s⁻¹) and is proportional to the ionic conductivity. The fitting curve gives the activation energy of ionic migration, also known as the ion diffusion barriers. d Calculated energy profile along the ionic migration path for V_Br (Br⁻ vacancies) in bulk Cs₂AgBiBr₆, in-plane migration for surface of Cs₂AgBiBr₆ without and with BiOBr passivation. Inset: migration path of V_Br with the Br⁻ highlighted in green (bulk), red (surface without BiOBr passivation), and blue (surface with BiOBr passivation) color, respectively. e Calculated energy profile of the out-of-plane ionic migration path for the V_Br in BiOBr-passivated Cs₂AgBiBr₆. Inset: Migration path of V_Br with the Br⁻ highlighted in green, and the V_Br migration direction is from BiOBr to Cs₂AgBiBr₆. f Bias-dependent photoconductivity of the wafers and the derived μτ value. g The time-dependent photoluminescence of the wafers. h Layered decomposed density of states for Cs₂AgBiBr₆/BiOBr heterostructures

Fig. 4

Performance of Cs₂AgBiBr₆ wafer X-ray detector. a Device response to X-rays (138.7 μGy_air s⁻¹) under an electric field of 0.1 V μm⁻¹. b X-ray sensitivity under different electric fields. c The response of the detectors toward stacked ITO glass coverslips. d Modulation transfer function for the fabricated detector and the inset is the line pair card for edge spread function measurement. e Measured dark current noise at various frequencies

Fig. 5

Imaging applications of Cs₂AgBiBr₆ wafer X-ray detector. a The fabricated multi-pixel wafer-based detector. b Mapping of photocurrent (left) and dark current (right) for the wafer with 6 × 6 pixels as the region of interest. c The Schematic illustration of the imaging process, and the X-ray image (top) and the optical image (bottom) of ‘HUST’ symbol. d Optical image and X-ray image of the heart-shaped logo obtained by the linear detector array, the dose rate for imaging is 138 μGy_air s⁻¹, and the scanning mode, as well as the linear detector array is shown at the bottom