Direct Conversion X-Ray Detectors with High Sensitivity at Low Dose Rate Based on All-Inorganic Lead-Free Perovskite Wafers

Cuixian Zhang

Chengdu University of Information and Technology, Chengdu, China.

DOI: 10.4236/detection.2022.92002                

Abstract

Just as lead-based perovskites that are hot in solar cell preparation, Bi-based perovskites have demonstrated excellent performance in direct X-ray detection, especially the Cs₃Bi₂I₉ single crystals (SCs). However, compared with lead-halide perovskites, one challenge for the Cs₃Bi₂I₉ SCs for X-ray detection application is that it is difficult to prepare large-sized and high-quality SCs. Therefore, how to get a large area with a high-quality wafer is also as important as Cs₃Bi₂I₉ growth method research. Here, different anti-solvents are used for the preparation of poly-crystalline powder with the Antisolvents precipitation (A) method, as a control, High-energy ball milling (B) was also used to prepare poly-crystalline powders. The resultant two types of Cs₃Bi₂I₉ wafer exhibit a micro-strain of 1.21 × 10^-3, a resistivity of 5.13 × 10⁸ Ω cm and a microstrain of 1.21 × 10^-3, a resistivity of 2.21 × 10⁹ Ω cm. As a result, an X-ray detector based on the high-quality Cs₃Bi₂I₉ wafer exhibits excellent dose rate linearity, a sensitivity of 588 μC·Gyairs^-1·cm^-2 and a limit of detection (LoD) of 76 nGyair·s^-1.

Keywords

X-Ray Detection, Sensitivity, Cs₃Bi₂I₉

1. Introduction

In recent years, perovskite material (ABX₃), has been widely used in solar cell, visible light detection and Quantum dot luminescent material due to its high absorption coefficient, tunable bandgap and also has achieved a huge success in radiation detection [1] - [7].

In 2015, Yakunin et al. prepared a uniformly dense MAPbI₃ film by spin coating or spraying on the substrate, prepared photodetectors based on the film, and successfully obtained X-ray images [8]. Since then, many researchers have joined in the application of perovskite materials in X-ray detection research, and have achieved very excellent results [9] [10] [11] [12]. But the existence of organic groups affects the stability of organic-inorganic hybrid perovskite materials. And the toxic of lead may cause environmental pollution, potential leakage and other dangers. Therefore, the research on inorganic lead-free perovskite materials is in full swing.

There are many lead-free perovskite materials, but Cs₃Bi₂I₉ is more suitable for X-ray detection because of its very similar structure to the Pb atomic shell structure and its large atomic massBi-based lead-free perovskite materials, including MA₃Bi₂I₉ Cs₃Bi₂I₉, are outstanding in lead-free perovskite class.

In 2020, Zhang et al. grew centimeter-scale Cs₃Bi₂I₉ single crystals by high-temperature solution heating method and shape nucleus control. The sensitivity of the single crystal was as high as 1652.3 μC∙Gyair⁻¹∙cm⁻², and the detection limit was relatively low, reaching 130 nGyair∙s⁻¹ [13]. Sun et al. prepared a 15 mm diameter Cs₃Bi₂I₉ single crystal by Briman hair [14]. The MA₃Bi₂I₉ single crystal sensitivity prepared by Liu et al. reached 872 µC∙Gy⁻¹∙cm⁻² and the detection limit reached a very low 30 nGyair∙s⁻¹ [15]. The above reports are based on photoconductive X-ray detectors based on perovskite materials. In recent years, some X-ray detectors are made of perovskite materials as new scintillator materials, which also have excellent performance [6] [16] [17] [18].

At present, there are many methods to prepare the photosensitive layer of X-ray detector in reported studies. The main methods currently used include spinning coating, scraping coating on the substrate to prepare polycrystalline thick film, melting method, solution method and other methods to grow perovskite single crystal and so on. However, spin coating method such as preparation of polycrystalline film thickness to meet the most X-ray materials of the best absorption thickness, and single crystal while performance is more outstanding, but its growth cycle taking a long time, the solution method of controllability and repeatability is poor, difficult to get the single crystal of large area. How to obtain high-quality large area to detect the cause of the core material? In this paper, piezoelectric ceramics and other experimental preparation methods were used to obtain the detector transport layer by pressing the polycrystalline powder obtained by the anti-solvent directly. In this paper, piezoelectric ceramics and other experimental preparation methods were used to obtain the detector transport layer by pressing the polycrystalline powder obtained by the anti-solvent directly.

2. Experimental

Preparation of Cs₃Bi₂I₉ Microcrystalline Powder: cesium iodide (CsI, 99.9%), bismuth triiodide (BiI₃, 99.9%), N, N-dimethylformamide (DMF, 99.5%), dimethyl sulfoxide (DMSO, 99%) and Isopropyl alcohol (IPA, 99.5%) were purchased from Aladin. All materials were used as received without further purification.

Two methods were employed to prepare Cs₃Bi₂I₉ polycrystalline powder, including A method and B method. In A case, CsI and BiI₃ with a mole ration of 3:2 was dissolved in DMF and DMSO component solvent (DMF:DMSO = 3:7). Immediately, the mixture was string at room temperature for 12 h. Add the clarified saturated solution filtered by 0.8 μm PVDF filter into antisolvents isopropyl IPA, then centrifuge the orange turbid liquid, pull off the supernatant liquid, add appropriate amount IPA into centrifuge tube, then repeat the steps several times. Then, the sediment is vacuum-dried for12 hours. As for B method, CsI and BiI₃ with same mole ration was mixed in the agate jar with agate balls, was milling for 8 h. The powders obtained by the above two methods will be used as pressing materials to get the photoelectric conversion layer.

Material Characterization: Crystallization characteristics and phase analysis were characterized by X-ray Diffractometer (XRD: TD-3500, HAO YUAN Instrument). The Optical properties were measured and characterized by UV-VIS spectrometer (UV-2550, Shimadzu). Thermal stability was characterized by Thermogravimetric Analysis instrument (STA 2500 Regulus, Shimadzu).

Fabrication of Devices: The polycrystalline powder obtained previously was pressed into shape at a pressure of 24 tons. The vertical Au/Cs₃Bi₂I₉ wafer/Au devices were fabricated by depositing Au on two opposite surfaces of the slices, and the active area (electrode area) is 1 × 8 mm². For resistivity and X-ray response investigate, 0.8 Pa, 4 min, 6 μA was selected as the best deposition electrode process, Au electrode with thickness of 40 nm was obtained, the thickness of Cs₃Bi₂I₉ wafer-A and wafer-B are 666 μm and 568 μm, respectively.

Device Characterization: The electrical properties and X-ray response of devices were characterized by Agilent (B2912A). A tungsten anode X-ray tube (Series 5000, Oxford Instruments) was used to perpendicularly irradiate devices. The dose rates were regulated by a gas ionization detector dosemeter system (ACCU-GOLD Touch AGD-P-AD, ACCU Digitizer AGDM+ and 10X6-3CT, Radcal). All equipment measurements are made under atmospheric ambient conditions and at room temperature. The test system is shown in Figure 1.

Figure 1. Schematic diagram of test system.

3. Result and Disccution

3.1. Characterizations of Cs₃Bi₂I₉

1) XRD

According to the current research, the photoelectric response of the device is closely related to the crystal quality of the materials. In order to obtain polycrystalline powder materials with excellent crystallization performance, the effects of different anti-solvents on crystallization performance of polycrystalline powder were investigated in the synthesis of polycrystalline powder by antisolvent method. Finally, we choose isopropanol as anti-solvents from ethanol, isopropanol, dichloromethane, n-hexane.

It is easy to find from the XRD pattern by horizontal comparison (Figure 2), the Cs₃Bi₂I₉ microcrystals synthesized by anti-solvent experiments have good crystallinity and are in good agreement with the standard PDF cards, as for Cs₃Bi₂I₉ powder prepared by ball milling method is shown in Figure 2(b), the diffraction peak intensity is lower and the FWHM is large, indicating that the crystallinity is poor. The strongest diffraction intensity is lower than half of the worst strength of polycrystalline powder prepared by anti-solvent method. Vertical comparison, materials are obviously recrystallized after isostatic pessing treatment, the intensity of <006> in two wafer sample were stronger than powder, It can be said that <006> becomes the dominant crystal plane. Both ASP and HEBM dominant crystal faces, the {001} crystallographic plane families’ diffraction peaks of wafer are stronger than powder, meanwhile, the 100 crystallographic plane families’ diffraction peaks are weaker, even the <110> was disappear. As we know Cs₃Bi₂I₉ is a hexagonal crystal system with the densest atoms and lowest energy along the C-axis, so the pressure recrystallizes the polycrystalline powder with energy. Therefore, the densest atoms provide the possibility of better X-ray absorption.

2) UV-VIS

The band gap of samples prepared by anti-solvent method is 2.24 eV. It is consistent with the theoretical and the calculated band gap of Cs₃Bi₂I₉ (2.30 eV) [19]. However, it is quite different from the actual reported band gap of Cs₃Bi₂I₉ single crystal material (1.96 eV [13] [20]). The possible reason is that the crystal grains are relatively small. According to the quantum domain effect, the smaller the crystal grains are, the more hypsochromic shift the absorption edge will be, and the band gap will be wider. The band gap width of the sample prepared by ball milling method is 2.3 eV, and the blue shift effect is more obvious.

As Figure 3 shows, the absorption peaks of the samples were 484.6 nm and 487.2 nm respectively, which were basically around 485 nm. The reason for the absorption peaks was typical 0-dimensional [Bi2I9]³⁻ bioctahedral quantum confinement effect, and the corresponding band width was about 2.55 eV [21], which proved that the band gap width of Cs₃Bi₂I₉ polycrystals obtained was also close to the theoretical value At the same time, the transmittance shows that the Cs₃Bi₂I₉ absorbs the red light about 540 nm very obviously, which further confirms

Figure 2. The XRD pattern of Cs₃Bi₂I: (a) ASP; (b) HEBM.

Figure 3. The UV-VIS spectroscopy of two type material.

the experimental feasibility of the method for the synthesis of Cs₃Bi₂I₉ polycrystals.

3) TG-DTA

Thermal analysis technology is that when the physical or chemical states of substances change (such as melting, sublimation, dehydration and polymerization, etc.), thermodynamic properties (specific heat, enthalpy and thermal conductivity, etc.) often change, so the physical and chemical change process of substances can be understood by measuring the change of their thermodynamic properties [22]. The thermogram (DTA curve) obtained by DTA is the temperature as the abscissa and the temperature difference between the sample and reference ΔT as the ordinate, and the different endothermic and exothermic peaks show the different thermal transition states of the sample when heated.

The thermal stability of the Cs₃Bi₂I₉ is confirmed by thermogravimetric analysis (Figure 4), showing no detectable mass loss until 500˚C, confirming good thermal stability of ASP-Cs₃Bi₂I₉. As contrast HBEM-Cs₃Bi₂I₉ were beginning to decompose at 300˚C. It can be clearly seen that the sample prepared by ball milling has decomposition below 400˚C, while the polycrystalline powder obtained by anti-solvent method starts to decompose at 500˚C and above. Among them, Cs₃Bi₂I₉ (Purple curve) prepared by ball milling method does not decompose before 300, and it shows three decomposition steps from 100˚C to 700˚C, corresponding to mass loss of about 70%, 20% and 10% respectively. The temperature of the second step is very close to that of the first step of Cs₃Bi₂I₉. The peak value of the first-order differential curve in Figure 4(b) is 370.3˚C, the sample in Figure 4(a) corresponds to the sample in Figure 2(a). It can be seen that the crystallization is good, the melting decomposition temperature is relatively higher, and the maximum decomposition rate is 564˚C. The whole TAG curve is very consistent with the reported single crystal TAG curve [13].

4) SEM

SEM was used to characterize its micro-structure, in order to observe the properties of powder more directly such as crystallization.

The results obtained from XRD test analysis of the material mentioned above can be explicitly guided. In fact, the crystallization of powder obtained by ASP method is far better than that prepared by HEBM, which is also confirmed by SEM characterization analysis. According to Figure 5, it can be clearly observed that ASP powder and HEBM powder have larger crystal particles and better crystallinity under a scanning electron microscope with a magnification of 2000 times at 5 μm.

By observing the cross section and surface flatness of the thin section by SEM (Figure 6), it was found that although the surface flatness of sample A was worse than that of sample B, the overall density of the material reflected by the cross section seemed to be stronger than that of A. However, it can be easily seen from the SEM figure of cross section that its crystallinity is much stronger than that of sample B, and its grain size is similar to that of powder SEM, which is much larger than that of sample B.

Figure 4. The TAG curve of tow method: (a) ASP; (b) HBEM.

Figure 5. The SEM of powder by two methods.

Figure 6. SEM of Cs₃Bi₂I₉ wafer’s cross-section and surface.

3.2. Device Performance of Cs₃Bi₂I₉ Wafer Direct X-Ray Detectors

The performance of devices is affected by many factors, and a lot of useful information has been obtained from the characterization of basic properties of materials. The electrical properties of the materials obtained in the 2 method were characterized. The I-V characteristics and dark current curve were obtained by the semiconductor parameter analyzer, and the dark resistivity and conductivity of the materials were calculated.

As we all know, in semiconductor physics, it is assumed that there are two kinds of carriers, electrons and holes, in semiconductor materials, and the relationship between the current density J and the electric field E can be expressed as [23]

$J=\sigma E=\left(nq{\mu }_n+pq{\mu }_p\right)\left|E\right|$ (3-1)

where σ represents the conductivity of the material, is the reciprocal of the resistivity, n, p respectively represents the number of electronic holes in the semiconductor, q represents the unit charge, and μ_n, μ_p represents the mobility of electrons and holes. According to Equation (4-1), the conductivity of the material can be obtained by taking the derivative of J with respect to E, in unit of W⁻¹×cm⁻¹.

As Figure 7(a) shows, in the electric field intensity range of −30 Vmm⁻¹ to 30 Vmm⁻¹, the current density J and the electric field intensity E show a linear relationship, and the ASP-Cs₃Bi₂I₉ poly-crystalline wafer shows a smaller current density under the same electric field intensity, that is to say, the resistance value of the sample is greater than that of the HEBM-Cs₃Bi₂I₉ sample. The conductivity of the photosensitive element prepared by the two methods is 1.95 × 10⁻¹⁰ W⁻¹∙cm⁻¹ and 4.53 × 10⁻¹¹ W⁻¹¹∙cm⁻¹, respectively.

The test data is not stable enough when the bias voltage is just added to both

Figure 7. (a) The dark current of ASP-Cs₃Bi₂I₉ wafer and HEBM Cs₃Bi₂I₉ wafer; (b) Dark at different electric field intensities.

ends of the device, so the analysis of dark conductance data starts from 20 s. Figure 7(b) shows the devices prepared by two methods. When the applied voltage is 15V, the mean dark current of ASP-Cs₃Bi₂I₉ is about 2.01 × 10⁻¹¹ A, while the dark current of HEBM-Cs₃Bi₂I₉ is about 4.1 × 10⁻¹⁰ A, which also shows that the former has higher resistivity.

As illustrated in Figure 8(a), compared with Cs₂AgBiBr₆, CdTe, MA₃Bi₂I₉, MAPbiI₃, Cs₃Bi₂I₉ exhibited higher absorption coefficient, especially for X-ray photons of 36 - 90 keV, which indicates its potential in medical radiography X-ray detection. As for Figure 8(b), it shows the absorption coefficient corresponding to different materials mentioned above. Figure 8(c) shows the relationship between absorption coefficient of Cs₃Bi₂I₉ material and material thickness, in this way, the absorption efficiency of the sheet material at its current thickness can be easily calculated. The two wafer thicknesses obtained in this work are 589 nm and 535 nm respectively; this means an X-ray absorption efficiency of 100% for 35 KeV. The absorption coefficient datum comes from the NIST database [24].

Sensitivity is a key parameter to characterize the detection efficiency of photodetector. The detection sensitivity S_x of X-ray detector is defined as [25]:

$S_x=\frac{Q}{AX}$ (3-2)

where, A represents the effective irradiation area, in cm²; X represents radiation dose, in unit of G_y; Q is the effective net charge collected by the detector material under X-ray irradiation, which can be expressed as [26]:

$Q={\displaystyle \int {\bar{I}}_{\text{photo}}\left(t\right)\text{d}t}$ (3-3)

where, represents the average photocurrent, and t represents the effective sampling

Figure 8. (a) Spectrum of the relationship between absorption coefficient and photon intensity of different materials; (b) The absorption coefficient corresponding to different materials; (c) The relationship between absorption coefficient and material thickness.

time of the detector; According to Equations (3-2) and (3-3), the sensitivity of the detector can be obtained.

The ratio of detector effective signal to noise signal is signal-to-noise ratio (SNR), which is one another of the key parameters to evaluate the overall performance of detector [27]. SNR for X-ray imaging devices is expressed as the ratio of device signal current to noise current, namely [20]:

$\text{SNR}=\frac{I_{\text{singal}}}{I_{\text{noise}}}$ (3-4)

$I_{\text{singal}}={\bar{I}}_{\text{photo}}-{\bar{I}}_{\text{dark}}$ (3-5)

$I_{\text{noise}}=\sqrt{\frac{1}{N}{\displaystyle {\sum }_i^N{\left(I_i-{\bar{I}}_{\text{photo}}\right)}^2}}$ (3-6)

where I_signal is the optical gain, that is, the mean difference between photocurrent and dark current, Ī_dark represents the average dark current, Ī_photo represents the average photocurrent, and I_noise is the standard deviation of the photocurrent of the detector.

As shown in Figure 9, different radiation dose rates (7.6 μGyS⁻¹ - 396 μGyS⁻¹)

Figure 9. J-E curves at different dose rates. (a)ASP-Cs₃Bi₂I₉; (b) HEBM Cs₃Bi₂I₉.

were measured respectively. The J-E curve of the detector was obtained by the preparation of polycrystalline powder by HEBM and ASP, and the optical gain of the device was visualized under different irradiation intensiities. The slope of the J-E curve, in general, and the device linearity are both good, contrast in the section on the dark conductivity, two kinds of method, the resistivity of ASP sample is greater than HEBM sample material, and the former under X-ray irradiation of photocurrent density larger instead, which further confirmed the samples prepared by the solvent method for better crystallization properties, thus gain more light. The density of J-E curve can directly reflect the size of gain. The denser (blue curves) means the increase of the same irradiation measure, the smaller the optical gain is, and vice versa (red curves).

Figure 10(a) shows the device response diagram prepared by the two methods at 2 s irradiation switch, 15 V bias voltage and 315.5 μGyS⁻¹ dose rate. It can be clearly seen that the response signal current of black curve (ASP) is greater than that of red curve (HEBM), while the photocurrent of the latter shows a very obvious noise signal (curve burr). Combined with the performance characterization of materials prepared by the two methods described above, the reason for the poor response performance of HEBM-Cs₃Bi₂I₉ detector is still that the crystallization performance of core layer material is lower, and the probability of recombination of carriers in the material body is higher, showing stronger noise signal. Figure 10(b) and Figure 10(c) show that under the same test bias light measured by conductivity with the increase of irradiation dose rate increased significantly, consistent with the J-E Bright field test results, photocurrent increased with the increase of irradiation dose rate ladder type at the same time also presents the ladder type growth obviously, and irradiation dose rate according to the linear increase response between current linear fitting are also presents linear basically. The response linearity of the device is high.

Figure 10. The response characteristics of 2 type Cs₃Bi₂I₉ detectors under X-ray radiation. (a) 2 s on-off; (b) dose-rate: 7.6 μGys⁻¹ - 31.3 μGys⁻¹; (c) dose-rate: 79 μGys⁻¹ - 396 μGys⁻¹.

Table 1. Important factors have been reported and compared with this work.

4. Conclusions

In this paper, two methods of anti-solvent and ball milling were used to synthesize Cs₃Bi₂I₉. In addition, the basic properties of the materials were tested and analyzed, and it was found that ASP method had better crystallinity. Although HBEM's crystallinity was not good enough, it was better than the crystal uniformity, which made the wager surface smoother and more conducive to obtaining good ohmic contact. The X-ray detector was fabricated by pressing polycrystalline powders obtained from two methods mentioned above.

When testing and analyzing the electrical properties of 2 types of materials, it is found that the resistivity of ASP material is significantly higher than that of HBEM, which is also the reason why the former has lower dark current under the same electric field intensity, which is conducive to signal discrimination of the device. Finally, the response performance was analyzed for 2 devices, and important device parameters including SNR and sensitivity were obtained. In order to more conveniently explain the advantages and disadvantages of this working device, some typical device parameter pairs are selected to report, for example, see Table 1.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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