Multilayered double perovskite ferroelectric for green high-performance self-powered X-ray detection

Abstract

Layered hybrid perovskite ferroelectrics have made significant strides in high-performance X-ray detection, attributed to their polarization-induced large built-in electric fields and excellent carrier mobility. However, most reported layered hybrid perovskite ferroelectrics rely on environmentally hazardous lead halides, which limits their broader application. Recently developed hybrid double perovskites offer promising alternatives for green self-driven X-ray detection. Herein, we explore the bulk photovoltaic effect in a two-dimensional multilayered double perovskite ferroelectric CHMA₂CsAgBiBr₇ (CCAB, CHMA⁺ = cyclohexylmethylammonium) for self-driven X-ray detection. Due to its multilayer structure, CCAB exhibits a large μτ product of 3.3 × 10⁻³ cm² V⁻¹, which is comparable to the three-dimensional double perovskite Cs₂AgBiBr₆. Specifically, the X-ray detector exhibits a photovoltage of 0.84 V under X-ray irradiation, ensuring the capability to convert X-rays into electric signals without bias. Additionally, CCAB exhibits a high sensitivity up to 120 μC Gy⁻¹ cm⁻² and a low detection limit down to 103 nGy s⁻¹ in the self-driven mode. Our work highlights the potential of lead-free multilayered double perovskite ferroelectrics for achieving high-performance self-driven X-ray detection, paving the way for practical applications of layered hybrid perovskite ferroelectrics in this field.

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Introduction

Direct X-ray detectors, which can directly convert X-rays into electrical signals, play an important role in applications such as medical diagnostics, safety inspections, and material analysis.^1–6 In recent years, lead halide perovskites (LHPs) have garnered considerable attention and achieved significant advancements in X-ray detection due to their easy synthesis, high X-ray absorption and excellent carrier mobility.^7,8 For instance, Song et al. developed an X-ray detector based on three-dimensional perovskite single crystals MAPbI₃ (MA = CH₃NH₃⁺) achieving a sensitivity of up to 5.2 × 10⁶ μC Gy⁻¹ cm⁻².⁹ Two-dimensional (2D) multilayer LHPs, which possess a two-dimensional quantum well structure to suppress ion migration and combine the excellent semiconductor properties of three-dimensional halide perovskites, have emerged as a promising option for achieving high-performance X-ray detection.^10–16 For instance, a multilayer perovskite (BDA)(MA)₂Pb₃Br₁₀ (BDA = NH₃C₄H₈NH₃²⁺) demonstrates high X-ray detection performance with a high sensitivity of 1984 μC Gy⁻¹ cm⁻² at 55.6 V mm⁻¹ and a low detection limit of 28.12 nGy s⁻¹ at 22.2 V mm⁻¹.¹⁷ Despite these advances, most of these materials require operation under a high external electric field, resulting in significant ion migration and energy consumption.^18–22 Therefore, developing detectors that can detect X-ray signals without external bias is very urgent.

2D multilayer LHP ferroelectrics which combine spontaneous polarization, excellent absorption coefficients and good charge transport have emerged as promising candidates for realizing high-performance self-driven X-ray detection.²³ Notably, the bulk photovoltaic effect (BPVE) in these materials can create a built-in electric field to spontaneously and efficiently separate photo-generated carriers without the need for external bias, thereby enabling self-driven detection. For instance, Jiang et al. developed a high-performance self-driven X-ray detector utilizing a 2D multilayer LHP ferroelectric (BA)₂(EA)₂Pb₃I₁₀ (BA = NH₃C₄H₉⁺ and EA = CH₃CH₂NH₃⁺), which has a sensitivity of 391 μC Gy⁻¹ cm⁻².²⁴ Despite the superior performance of 2D multilayer lead components, these materials contain toxic lead, posing a significant threat to human health. To address this issue, it is imperative to develop X-ray detection methods utilizing lead-free halide perovskites, thereby achieving environmentally friendly self-driven X-ray detectors with high performance.

As environmentally friendly alternatives, 2D lead-free multilayer double halide perovskite ferroelectrics are considered promising candidates for green self-driven X-ray detection, owing to their non-toxic, excellent carrier transport properties and possession of spontaneous polarization.²⁵ Xu et al. developed an X-ray detector based on a multilayer double perovskite BA₂CsAgBiBr₇.²⁶ Benefiting from its multi-layer structure, it exhibits excellent carrier transport properties, showing great promise for high-performance self-driven X-ray detection. Similar to 2D multilayer LHP ferroelectrics, 2D multilayer double halide perovskite ferroelectrics have spontaneous polarization-induced bulk photovoltaic effects, which are anticipated to facilitate self-driven X-ray detection. Yao et al. reported a high-curie temperature multilayered hybrid double perovskite (C₆H₅CH₂NH₃)₂CsAgBiBr₇,³⁷ which are anticipated to facilitate self-driven X-ray detection at room temperature. However, due to the limited availability of multilayer double halide perovskite ferroelectrics, no 2D multilayered double halide has been applied to self-driven X-ray detection. Consequently, we posit that utilizing lead-free multilayer perovskite ferroelectrics represents an effective strategy for achieving high-performance self-driven X-ray detection.

In this work, a high-quality single-crystal X-ray detector based on a 2D lead-free double perovskite ferroelectric CHMA₂CsAgBiBr₇ (CCAB, CHMA⁺ is cyclohexylmethylammonium) was constructed. Owing to the merits of its multilayered structure, CCAB has a large μτ product of 3.3 × 10⁻³ cm² V⁻¹ and good X-ray absorption, which facilitates high-performance X-ray detection. Specifically, due to the BPVE of CCAB, the single crystal device can spontaneously separate and transport photogenerated carriers without external bias, thereby imparting self-driven detection capabilities to CCAB. Based on these merits, the CCAB-based device exhibits a great sensitivity of 120 μC Cy⁻¹ cm⁻² and a low detection limit of 103 nGy s⁻¹ at 0 V. Moreover, CCAB achieves a high sensitivity of 3312 μC Gy⁻¹ cm⁻² under a bias of 100 V, outperforming most 2D halide double perovskites. This work successfully achieved high-performance self-driven X-ray detection in multilayer lead-free halide double perovskite ferroelectrics, offering a promising pathway for the development of environmentally friendly self-driven X-ray detectors.

Results and discussion

Single crystals (SC) of CCAB were grown from a hydroiodic acid solution using a slow cooling process (Fig. 1a). Powder X-ray diffraction (PXRD) patterns from simulation and experimental measurement confirm the phase purity (CCDC 2203833, Fig. 1b),²⁷ and the X-ray pattern of the CCAB SC shows well-defined (h00) diffraction peaks, which indicate its high quality and high orientation. The thermogravimetric (TG) curve (Fig. S1†) indicates that the decomposition temperature of CCAB is as high as 555 K, demonstrating its high thermal stability. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the surface morphology of the crystal. As illustrated in Fig. 1c and d, the surface of the crystals is flat and smooth with no obvious defects, which affirms the high quality of the CCAB SC. Through analysis of the AFM data, the root mean square roughness (RMS) of the crystallographic plane was determined to be 0.243 nm, indicating the high quality of the CABB crystal. As depicted in Fig. 1e, CCAB adopts the typical 2D motif and the inorganic layer consists of two octahedral layers composed of alternating AgBr₆ and BiBr₆ octahedra, with the Cs⁺ cation fully encapsulated in a cavity formed by corner-sharing octahedra. The presence of the heavy elements Cs, Bi, and Ag in CCAB is beneficial for X-ray absorption and its bilayer inorganic skeleton facilitates carrier transport, making it a potential candidate for high-performance X-ray detection. Furthermore, CCAB crystallizes in the polar space group Ama2 at room temperature. With the increase in temperature, CCAB further transforms into a middle phase with a centrosymmetric space group of Cmcm (338 K, Fig. S2a†) and finally transforms into a centrosymmetric space group of I4/mmm (378 K, Fig. S2b†). Such a symmetry-breaking form of CCAB obeys the Aizu rule with the notation 4/mmmFmm2 and its ferroelectricity along the c-axis has been demonstrated by the polarization versus electric field hysteresis loops as shown in Fig. S3.† The CCAB-based device shows piezoelectric response along the c-axis with a d₃₃ value of 1.6 pC N⁻¹, while no piezoelectric response is observed along the nonpolar axis (Fig. S4†). The ferroelectric polarization of the CCAB crystal along the c-axis generates a built-in electric field to separate the photogenerated carriers, thus giving the CCAB SC the ability for self-driven detection.

Fig. 1 (a) The high-quality crystal image of the CCAB single crystal. (b) XRD measurement of the powder and crystal. (c) The SEM image of CCAB. (d) The AFM image of CCAB. (e) The crystal structure of CCAB. The white color shows the atomic co-occupancy of Ag and Bi.

For efficient X-ray detection, materials must proficiently absorb X-ray photons. Therefore, to further evaluate the efficiency of absorption of X-ray photons by the CCAB SC, the X-ray attenuation efficiency and absorption coefficient of CCAB were calculated using the photon cross-section database. As illustrated in Fig. 2a, the calculated absorption coefficient of CCAB is larger than that of (HIS)₂AgBiBr₈ (HIS = histammonium) and crystalline Si, but comparable to that of α-Se. Conversely, Fig. 2b depicts the thickness of these materials as a function of the 50 keV X-ray photon attenuation efficiency. The CCAB material attenuates over 78% of the X-ray photons at a thickness of 1.0 mm, which is significantly higher than that of Si (≈8.6%). The charge collection of CCAB is another key factor influencing the performance of X-ray detection, which can be evaluated by the carrier mobility lifetime (μτ). Consequently, we derived the μτ product by fitting photocurrent–voltage curves according to the modified Hecht equation:¹

where I is the photocurrent, I₀ is the saturated photocurrent, L is the distance between electrodes, and V is the applied bias. As shown in Fig. 2c, the μτ product of the CCAB SC is up to 3.3 × 10⁻³ cm² V⁻¹ under X-ray irradiation, which is larger than those of the reported lead-free perovskite X-ray detectors, such as (4F-PEA)₂AgBiBr₈ (2.9 × 10⁻⁵ cm² V⁻¹, 4F-PEA = 4-fluorophenethylammonium),²⁸ [(R/S-PPA)₄(n-BA)₆Ag₂Bi₄I₂₄]·2H₂O (2.24 × 10⁻⁴ cm² V⁻¹, R/S-PPA = R/S-1-phenylpropylamine)²⁹ and (4,4-DFPD)₄AgSbI₈ (6.19 × 10⁻⁴ cm² V⁻¹, 4,4-DFPD = 4,4-difluoropiperidinium),³⁰ and much higher than that of the commercial α-Se film detector (≈10⁻⁷ cm² V⁻¹). Additionally, resistivity is also an important parameter for X-ray detectors. The bulk resistivity of the CCAB SC along the c-axis can be calculated as 2.32 × 10¹⁰ Ω cm (Fig. 2d). This value is comparable to those of many lead-free halide double perovskites, such as (I–C₄H₈NH₃)₄AgBiI₈ (3.04 × 10¹⁰ Ω cm)³¹ and (PEA)₄AgBiBr₈ (6.77 × 10¹⁰ Ω cm, PEA = phenethylammonium).²⁸ The high resistivity of the CCAB SC will facilitate the reduction of dark current and suppression of current noise, which is essential for improving the X-ray detection performance.

Fig. 2 (a) The X-ray absorption spectra of CCAB. (b) Attenuation efficiency of CCAB to 50 keV X-ray photons versus thickness. (c) The μτ product of CCAB along the c-axis of the polarization direction. (d) The bulk resistivity of the CCAB SC.

Importantly, CCAB exhibits ferroelectricity, which leads to spontaneous polarization to separate photogenerated carriers, thus enabling self-driven detection. To further investigate the self-driven detection capabilities of CCAB, we fabricated vertical electrodes Ag/CCAB SC/Ag parallel to the polar c-axis direction (Fig. 3a). The absorption onset of CCAB occurs at a wavelength of 569 nm according to the ultraviolet-visible absorption (Fig. S5†); therefore we studied its BPVE under 405 nm light. Under 405 nm illumination, an open-circuit photovoltage of 0.5 V can be obtained from the current–voltage curves of the CCAB SC device (Fig. 3b). Similarly, CCAB exhibits a photovoltage up to 0.85 V under X-ray irradiation as shown in Fig. 3c. Such a bulk photovoltage is larger than many reported bulk photovoltages in lead-free hybrid perovskites, such as (R-MPA)₄AgBiI₈ (0.36 V, R-MPA = R-β-methylphenethylammonium),³² (BZA)₂(R/S-PPA)BiI₆ (0.4 V, BZA = benzylamine) and (R/S-PPA)₂BiI₅ (0.63 V),³³ which has the merit of enabling high-performance self-driven X-ray detection. To further demonstrate the BPVE of the CCAB device, a study was conducted to determine whether the spontaneous polarization of CCAB would result in the corresponding flipping in response to changes in the polarization electric field. As shown in Fig. 3d, the open-circuit voltage and short-circuit current of CCAB have been identified as −0.83 V/+22.5 pA and +0.85 V/−23 pA, respectively, following positive and negative polarization. These results indicate that the BPVE of the CCAB SC originates from spontaneous polarization, thus enabling CCAB with self-driven detection capability. In summary, the good X-ray absorption, large carrier mobility and BPVE of CCAB provide a suitable platform for achieving high-performance self-driven X-ray detection.

Fig. 3 (a) Schematic of the X-ray detector with electrodes oriented parallel to the polar c-axis made from the CCAB single crystal. (b) The current–voltage traces of the CCAB device under 405 nm light illumination. (c) The current–voltage traces of the CCAB device under different X-ray irradiation. (d) Dependence of the photocurrent direction on the polarization direction (the upper and lower parts of the figure correspond to the direction of the current after positive poling and negative poling, respectively).

Hence, we further investigated the response of the CCAB SC detectors to X-rays. Due to the BPVE induced by ferroelectricity, an obvious response of the device to X-rays at 0 V bias can be observed as shown in Fig. 4a. As the X-ray dose increases from 4.35 to 87.66 μGy s⁻¹, the photocurrent density of the CCAB device shows a linear increase from 4.04 to 19.39 nA cm⁻², indicating excellent X-ray response. This device also exhibits a strong X-ray response under external bias voltages from 10 V to 100 V as shown in Fig. S6–S10.† As depicted in Fig. 4b and c, the sensitivity under different bias voltages and doses is obtained by linearly fitting the current density and X-ray dose rate. The high sensitivity of CCAB is up to 120 μC Gy⁻¹ cm⁻² under zero bias, which is higher than those of other lead-free hybrid perovskites such as (BZA)₂(R/S-PPA)BiI₆ (53.2 μC Gy⁻¹ cm⁻² at 0 V bias), (R/S-PPA)₂BiI₅ (0.31 μC Gy⁻¹ cm⁻² at 0 V bias),³³ (R-MPA)₄AgBiI₈ (40 μC Gy⁻¹ cm⁻² at 0 V bias),³²etc. Pleasantly, the sensitivity of CCAB is much higher than that of a conventional inorganic material α-Se film detector (20 μC Gy⁻¹ cm⁻² at 2000 V bias). Moreover, the detector also exhibits excellent X-ray response under external bias. As the external bias voltage increases, the sensitivity increases from 517 μC Gy⁻¹ cm⁻² (10 V bias) to 3312 μC Gy⁻¹ cm⁻² (100 V bias), which is larger than those of most currently available 2D lead-free double perovskite detectors, such as (HIA)₂AgBiBr₈ (118 μC Gy⁻¹ cm⁻², 10 V bias, HIA = histamine),³⁴ (4,4-DFPD)₄AgBiI₈ (188 μC Gy⁻¹ cm⁻², 50 V bias)³⁵ and (4-AP)₂AgBiBr₈ (1117.3 μC Gy⁻¹ cm⁻², 80 V bias, 4-AP = 4-amidinopyridine).³⁶ The detection limit is another critical factor for X-ray detectors. According to The International Union of Pure and Applied Chemistry, the detection limit is the X-ray dose rate when the signal-to-noise (SNR) ratio is equal to 3. The SNR value can be measured using the following equation:

where denotes the average photocurrent, denotes the average dark current, and I_i denotes the measured photocurrent. As illustrated in Fig. 4d, the detection limit is 103 nGy s⁻¹ for the device at zero bias, significantly lower than the current dose rate of the standard medical diagnostic dose (5.5 μGy s⁻¹). The detection limit of the CCAB SC under 100 V bias was also calculated as shown in Fig. S11.† The detection limit under 100 V bias (798 nGy s⁻¹) is higher than that of CCAB under 0 V bias, due to increased dark current and noise resulting from significant ion migration under elevated external bias voltage. In addition, the long-term operational and environmental stability of the device is critical in practical applications. The stability of the dark current is an important parameter of X-ray detectors, which can be calculated from the following equation:where I₀ and I_t are the currents at the start and time t, respectively, E is the electric field, and A is the device area. As shown in Fig. 4e, there is almost no dark current drift in the self-driven mode compared to a dark current drift of 3.7 10⁻⁷ nA cm⁻¹ s⁻¹ V⁻¹ at 10 V bias, which indicates that the device maintains a highly stable baseline at 0 V bias. To evaluate the stability of this device, we assessed the X-ray response of the detector under continuous X-ray radiation with a dose rate of 1.58 mGy s⁻¹. The photocurrent and dark current did not significantly change at a total X-ray dosage of 793 mGy, demonstrating its excellent radiation stability (Fig. 4f). Finally, the environmental stability of the CCAB device is evaluated at 0 V bias. As shown in Fig. 4g, after being exposed to air for three months, PXRD shows that the CCAB crystal exhibited no significant phase change. Moreover, the detector retains exceptional operational stability after three months and the photoresponse to X-ray is only weakly degraded (Fig. 4h). As depicted in Fig. 4i, approximately 92% (111 μC Gy⁻¹ cm⁻²) of the initial sensitivity was still maintained at 0 V bias after three months in air (≈298 K, ≈48.6% RH). This result confirms the excellent environmental stability of the device. This study demonstrates that multilayered double perovskite ferroelectrics with radiation photovoltages are a class of potential materials for high-performance self-driven X-ray detection.

Fig. 4 (a) X-ray response under different dose rates at 0 V bias. (b) Dose rate-dependent X-ray response of the CCAB SC device under different voltages. (c) The sensitivities of devices based on CCAB under different bias voltages. (d) The SNR of the CCAB device at 0 V bias. (e) Dark current drift under 0 and 10 V bias, respectively. (f) Photocurrent stability of the CCAB SC detector under continuous X-ray irradiation. (g) PXRD patterns of the original CCAB sample and after three months. (h) The irradiation response stability is measured after three months at 0 V bias. (i) Comparison of sensitivity of the original CCAB sample and after three months at 0 V bias.

Conclusions

In conclusion, we successfully synthesized high-quality crystals of a 2D lead-free perovskite ferroelectric CHMA₂CsAgBiBr₇ (CCAB) and further constructed a high-performance X-ray detector based on CCAB. Due to the BPVE resulting from the ferroelectricity of CCAB, the single crystal device can drive the separation and transport of charge carriers without bias, thus allowing CCAB to have self-driven detection capability. Furthermore, owing to the merits of its multilayered structure, CCAB has a large μτ product of 3.3 × 10⁻³ cm² V⁻¹, which facilitates the collection of carriers. Under zero bias, CCAB showed a considerable sensitivity of 120 μC Cy⁻¹ cm⁻² and a low detection limit of 103 nGy s⁻¹. Moreover, the sensitivity of CCAB reaches as high as 3312 μC Gy⁻¹ cm⁻² under 100 V bias, which is higher than those of most 2D halide double perovskites. This work successfully achieved high-performance self-driven X-ray detection in multilayered lead-free halide double perovskite ferroelectrics, offering a promising pathway for developing environmentally friendly self-driven X-ray detectors.

Data availability

The data supporting this article have been included as part of the ESI,† including additional computational details and experimental details, materials and methods, crystal morphology, crystal structure data, PXRD patterns, the TG curve, basic photoelectric properties, and X-ray detection performance.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22435005, 22193042, 22125110, and U21A2069), the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (ZDBS-LY-SLH024), and the Youth Innovation Promotion of Chinese Academy of Sciences (2019301, Y202069, and 2020307).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi03059a

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