CH₃NH₃PbBr₃ Perovskite Single Crystal X-Ray Photon Counting Detection Based on Multi-Layer Electrodes

1. Introduction

X-ray detection technologies play a critical role in a wide range of applications, including medical imaging, industrial non-destructive testing, and security check [1,2,3]. Among various detection modalities, photon counting detection has emerged as a next-generation paradigm due to its superior capabilities in energy resolution, contrast-to-noise ratio, and reduced radiation dose compared to conventional energy-integrating systems [4,5,6,7,8,9]. The success of photon counting detection, however, heavily relies on the availability of high-performance semiconductor materials that exhibit both efficient X-ray absorption and excellent charge transport properties [10].

In recent years, lead halide perovskites have garnered significant attention as promising candidates for X-ray detection, owing to their exceptional optoelectronic properties, including high atomic numbers, large mobility-lifetime (μτ) products, and low-cost solution-processability [11,12,13]. Among kinds of lead halide perovskites, methylammonium lead bromide (MAPbBr₃, MA=CH₃NH₃) stands out as a particularly attractive material for X-ray photon counting applications, as it combines strong X-ray attenuation capability with high resistivity and quick growth speed from solution precursor [14,15,16]. These attributes have enabled MAPbBr3 single crystals based X-ray detectors to achieve impressive sensitivity and lowest detection limits, positioning them as viable alternatives to conventional materials such as amorphous selenium, cadmium zinc telluride, and silicon [17,18,19,20].

Despite these advances, the application of MAPbBr₃ single crystals in X-ray photon counting detection remains challenging, primarily due to issues related to device stability, high dark current, and charge trapping under high electric fields [21,22,23]. The electrode configuration plays a crucial role in governing charge collection efficiency and suppressing dark current in perovskite X-ray detectors. Conventional single-layer metal electrodes such as Au, Ag, Ti and Ga, often suffer from inefficient charge extraction, interfacial reactions with the MAPbBr₃ single crystals, and increased leakage currents, limiting the detector's ability to operate stably under sustained high bias and X-ray irradiation [24,25,26,27]. While previous studies have largely focused on optimizing the active layer composition and interface engineering, relatively little attention has been paid to the rational design of multi-layer electrode architectures tailored for photon counting mode, particularly in the context of MAPbBr₃ single crystal-based devices [5,28].

To address these limitations, we propose the implementation of multi-layer electrode structures employing Au/MAPbBr₃/Au, Au/Ti/MAPbBr₃/Ti/Au and Au/Pt/TiMAPbBr₃/Ti/Pt/Au configurations. These multi-layer architectures are designed to combine the advantages of each metal layer: Ti serves as an adhesion layer, where Ti-N bonds formed at the interface of MAPbBr₃/Ti; Pt functions as a isolating layer to mitigate interfacial reactions and ion migration; while Au ensures high conductivity. By systematically comparing the performance of MAPbBr₃ X-ray detectors with single-layer and multi-layer electrodes, we investigate the influence of electrode architecture on 59.5 keV photon-counting detection and imaging. In this work, low activity ²⁴¹Am radioactive gamma-ray sources were used as low dose monochromatic X-ray source.

Our results demonstrated that the multi-layer electrode strategy, particularly the Au/Pt/Ti/MAPbBr₃/Ti/Pt/Au configuration, significantly enhances both the detection performance and long-term stability of MAPbBr₃-based photon counting X-ray detectors. The improved charge extraction, suppressed dark current, and mitigated interfacial degradation achieved through this rational electrode design offer a promising pathway toward practical and reliable perovskite-based photon counting X-ray detection systems.

2. Materials and Methods

Materials: Lead bromide (PbBr2; 99%) and methylammonium bromide (MABr), were purchased from Sigma Aldrich, USA. Dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) were obtained from Aladdin. Metallic gold were purchased from Chinese reagent, China. All commercial products were used as received.

Growth of MAPbBr₃ perovskite single crystals. High-quality MAPbBr₃ perovskite single crystals were grown by inverse temperature crystallization [29,30,31]. Specifically, 1 M MABr and 1 M PbBr₂ were dissolved in DMF solution. The solutions were filtered through a PTFE filter with a 30 μm pore size. The filtrate was then transferred to a culture dish, which was placed on a programmable heating station (IKA-RET control-visc) and the growth temperature is 65 to 85 ℃. For the metal electrodes: electrodes were deposited face of MAPbBr₃ perovskite single crystals by a metal mask under a vacuum of 6×10⁻⁴ Pa.

Characterization of the MAPbBr₃ perovskite single crystals. X-ray diffraction (XRD) patterns were obtained using an X'TRA system with a Cu target (Switzerland). Optical absorption spectra were measured by UV-vis spectroscopy range from 300 to 2000 nm (Lab Tech Bluestar, USA). The ultraviolet photoelectron spectroscopy (UPS) were obtained using a PHI 5000 VersaProbe (Japan). The PL and PL decay processes were measured using a SpectraMax instrument (UK). The kelvin force probe microscopy was using a Dimension Icon (Bruker, German). For the response time measurement: ²⁴¹Am was used as the radioactive source, a high voltage bias (0-50V) was applied on the device, the signal was input into a pre-amplifier(142PC, USA). The response to each alpha particle was traced using an Keysight oscilloscope (DMOX4054A, USA). Characterization of current–voltage were measured using a Keithley 4200SC semiconductor analyzer (USA).

Set-up of the X-ray photon-counting read-out circuits. The pre-amplifier 142PC was purchased from ORTEC (USA). The shaping amplifier and baseline restorer were purchased from CREMAT (USA). The high voltage and low voltage DC source were purchased from DONGWEN HIGH VOLTAGE (China). And the multi-channel analyzer was purchased from AP Techno (Japan).

3. Results

As shown in Figure 1, the energy band of MAPbBr₃ single crystal was investigated. The band gap was fitted of 2.1 eV by Talc plot (Figure S1). And the valance band maximum was measured of 5.7 eV by UPS (Figure S2). And the work function of Au, Pt, Ti were chosen to be 5.1 eV, 5.3 eV and 4.3 eV from published work, respectively. Figure 1b shows the structure of detectors made of MAPbBr₃ single crystals with different electrodes, including Au/MAPbBr₃/Au, Au/Ti/MAPbBr₃/Ti/Au and Au/Pt/Ti/MAPbBr₃/Ti/Pt/Au. Interdigitated electrode was chosen as shown in Figure 1c, the thickness of three type of electrodes was 200 nm. Here, ²⁴¹Am radioactive sources which emit 59.5 keV photons were used as monochromatic X-ray source. As shown in Figure 1d, the 59.5 keV monochromatic X-ray photons would incident into the detectors, while high voltage bias and ground would connect to each electrode, respectively.

Figure 1e shows the current density- voltage characterization of detectors with different electrode structures, the Au/Pt/Ti multi-layer electrodes shows nearly linear behavior, while Au electrode or Au/Ti electrode show back-to-back Schottky behavior. The Au/Pt/Ti multi-layer seems to form more Ohmic contact with MAPbBr₃ single crystal. Further, the dark current density under -50 V bias was investigated in Figure 1f, the Au/Pt/Ti multi-layer device shows lowest dark current of 107 nAcm^-2, which was much lower than Au/Ti device and Au device of 372 nAcm^-2 227~445 nAcm^-2, respectively.

Owning to the low and stable dark current, Au/Pt/Ti multi-layer device was used to detect 59.5 keV photons. As shown in Figure 1g, a 59.5 keV photon induced potential pulse could be clearly observed. While for Au/Ti multi-layer device and Au device, the noise were too large to discriminate the useful signal(Figure S3). And the alpha particles response was shown in Figure S4. Further, we use five ²⁴¹Am radioactive sources as five point X-ray source, a 1 mm thickness Al₂O₃ ceramic sheet was used to filter alpha particles and only 59.5 keV photons could transmit as shown in Figure 1h. Based on the count per second of Au/Pt/Ti multi-layer device in different location, we successfully realize X-ray photon counting image. The five ²⁴¹Am radioactive sources only emit nearly 1000 59.5 keV photons per second, and the photon injection power is only 100 cm^-2s^-1. Based on ${\mathrm{D}}_{\mathrm{air}}=\mathsf{\phi }\text{×}({\displaystyle \frac{{\mathsf{\mu }}_{\mathrm{en}}}{\mathsf{\rho }}})\text{×}\mathrm{E}$, where D_air is the dose rate, $\mathsf{\phi }$ is the photon injection rate, E is the energy of each photon and ${\displaystyle \frac{{\mathsf{\mu }}_{\mathrm{en}}}{\mathsf{\rho }}}$ is the mass energy absorption coefficient, which is 0.0303 cm²g^-1 for air [32]. As a result, the dose rate was only 0.03 nGys^-1, which is much lower to the lowest detectable dose rate in energy-integrating X-ray detectors.

4. Discussion

After multi-layer electrodes deposition, we further confirmed the surface property of MAPbBr₃ single crystals. Since the deposition temperature for metal Pt is much higher than Au, the high temperature vaporized Pt or Au may destroy the surfaces of MAPbBr₃ single crystals. As shown in Figure 2a, the surface property of MAPbBr₃ single crystals was first investigated by the X-ray diffraction. The full width half height (FWHM) for 30.080 degree was 0.2, 0.2 and 0.18 degree for Au, Au/Ti and Au/Pt/Ti device, respectively. The X-ray diffraction shows that the crystallization quality of three MAPbBr₃ single crystals after electrodes deposition was close. Further, photoluminescence of three MAPbBr₃ single crystals after electrodes deposition was investigated in Figure 2b. The peak of the photoluminescence located at 530 nm, and the photoluminescence intensity was nearly equal. And the time-resolved photoluminescence of 530 nm was further investigate in Figure 2c. Through exponential decay fitting, the surface lifetime was 0.32±0.02, 0.37±0.02 and 0.35±0.04 μs and the bulk lifetime was 10.5±3.7, 10.5±2.3 and 14.2±4.5 μs for Au, Au/Ti and Au/Pt/Ti device, respectively. As result, the dark current density and noise behavior were attributed to the different electrodes not to the crystallization quality differences of MAPbBr₃ single crystals.

To further understand the metal- MAPbBr₃ single crystal contact, here, Kelvin Force Microscopy (KFM) was used to measure the contact potential of different electrodes. The KFM results for Au device was shown in Figure 3a, the roughness was 5±1 and 25±3 nm for Au area and MAPbBr₃ single crystal region, respectively. And the contact potential was measured of 0.15±0.02 V, resulted in a energy barrier of 80 mV in Au-MAPbBr₃ interface. Similarly, for Au/Ti device, the roughness was 3±1 and 15±3 nm for Au/Ti area and MAPbBr₃ single crystal region, respectively. Further, the contact potential was measured of 0.15±0.02 V, resulted in a energy barrier of 100 mV in Au/Ti-MAPbBr₃ interface. For the Au/Pt/Ti device, the the roughness was 3±1 and 15±3 nm for Au/Ti area and MAPbBr₃ single crystal region, respectively. What’s interesting, we can not observe clear potential difference between Au/Pt/Ti and MAPbBr₃ single crystal. It also explained the Ohmic behavior as shown in current density-voltage characterization.

As shown in Figure 4a, the response speed for Au/Pt/Ti device, at -50 V bias, the response speed reached 2.9±0.2μs, corresponding to the maximum count rate of 34.4k s^-1. As shown in Figure 4b, the counting stability of Au/Pt/Ti device was 110±15 for 59.5 keV photons for 30 minutes, demonstrating the not bad counting stability. The Au/Pt/Ti device was also very stable. As shown in Figure 4c, the dark current after 6 months (107 nA) maintained almost the same dark current after 3 months(106 nA).

5. Conclusions

In this work, multi-layer electrodes including Au, Au/Ti and Au/Pt/Ti were investigated. Au and Au/Ti would form energy barriers when contact with MAPbBr₃ single crystals, while Au/Pt/Ti could form Ohmic contact with MAPbBr₃ single crystals. Au/Pt/Ti multi-layer electrode shows lowest dark current and noise, which enable 59.5 keV photon counting detection and imaging. This work provided useful information about the electrodes structure for lead halide perovskites based optoelectronic devices.