Direct X-ray detectors made of zero-dimensional hybrid metal halide perovskite single crystals

Abstract

Zero-dimensional (0D) cadmium(Cd)-based A₂BX₄-type hybrid metal halide perovskites incorporating large sterically hindered cations define a new type of structure, which shows good performance in photoelectric detection because of their unique charge transmission. Herein, we report the synthesis of novel 0D (C₁₉H₁₈P)₂CdCl₄ single crystals (SCs) via a solution natural-volatilization method. Bulk (C₁₉H₁₈P)₂CdCl₄ SCs have sizes of 6 × 5 × 3 mm³. The as-synthesized SCs of (C₁₉H₁₈P)₂CdCl₄ exhibit a direct bandgap of 4.19 eV, possess a high resistivity of 3.73 × 10¹¹ Ω cm, and demonstrate a low stable dark current. The crystal showcases an observed mobility-lifetime (μτ) product on a scale of 10⁻⁴ cm² V⁻¹. Furthermore, an X-ray photoconductor was successfully fabricated based on the SC of (C₁₉H₁₈P)₂CdCl₄, which displays an outstanding sensitivity of ∼143.6 μC Gy_air⁻¹ cm⁻² when biased at 1000 V (333 V mm⁻¹), a response current with exceptional stability, and a stable baseline with the lowest dark current drift of 7.34 × 10⁻⁴ pA cm⁻¹ s⁻¹ V⁻¹. This work introduces a groundbreaking approach using single-crystal metal halide perovskites, opening up new possibilities for exploring optoelectronic applications, which holds the potential to unveil a novel generation of materials for X-ray detection and imaging.

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1. Introduction

X-ray detectors find extensive use in medical imaging, security scrutiny, and nondestructive inspection.^1–9 Semiconductor-based direct X-ray detectors are acknowledged as the upcoming technology for X-ray detection, mainly due to their high sensitivity, high spatial resolution, and simple device configuration.^3,5,10–18 The common semiconductors employed for X-ray detection include α-Se (amorphous selenium), silicon, and CdTe (polycrystalline cadmium telluride).^14–16 These materials face challenges such as low carrier μτ product, low X-ray absorption coefficient, and high fabrication cost, which limit their application in X-ray detectors.

Recently, metal halide perovskites have shown great promise for X-ray detection applications because of their exceptional ability to attenuate X-rays, high μτ products, and the convenience of solution-based fabrication methods.^6,19–22 Huang et al. found that X-ray detectors made of CH₃NH₃PbBr₃ SCs demonstrate a remarkable sensitivity of 21 000 μC Gy_air⁻¹ cm⁻² and an impressive detection limit as low as 38 nGy_air s⁻¹.¹ Gao et al. achieved synergistic strain engineering in 3D perovskites and significantly enhanced the stability of X-ray detectors.²⁰ The structural stability of 0D analogues is significantly higher than that of 3D perovskite SCs, with reduced ionic migration, enhanced self-trapping, and anisotropic charge transport.^23–25 Wu et al. used first-principles calculations to reveal that the ion migration of the perovskite SCs is ranked in the order of 0D MA₃Bi₂I₉ < 2D (PEA)₂PbI₄ < 3D MAPbI₃, which well explained the huge difference in dark current drift values of the 0D MA₃Bi₂I₉ SC (5.0 × 10⁻¹⁰ nA cm⁻¹ s⁻¹ V⁻¹), 2D (PEA)₂PbI₄ SC (1.9 × 10⁻⁸ nA cm⁻¹ s⁻¹ V⁻¹) and 3D MAPbI₃ SC (2.0 × 10⁻³ nA cm⁻¹ s⁻¹ V⁻¹).²⁶ In view of this, we hypothesized that the disconnection of inorganic octahedral entities within hybrid perovskite structures could effectively disrupt ion migration pathways. The 0D structure, characterized by the disconnection of inorganic octahedral (or some other configuration) units in all directions, facilitates this objective. Additionally, 0D compounds display electronic structures that are more localized, resulting in a decrease in the concentration of dark carriers when compared to their 3D or 2D equivalents.^27,28 By reducing ion migration suppression and decreasing dark carrier concentration, we can achieve the desired X-ray detector.

Although lead halide perovskites have made considerable advancements, their practical application remains constrained due to the utilization of hazardous lead and the well-known characteristics of ion migration. Hence, it is imperative to research a new type of metal halide perovskite for further X-ray-detection applications.^2,8,15,26 Similar to the hybrid lead halide perovskites, cadmium halide perovskites have a tendency to form anionic structures with low dimensions based on either a basic tetrahedron [CdX₄] or octahedron [CdX₆].^29–33 However, the majority of research studies have primarily focused on the regulation of structures by utilizing organic cations’ directional functions to achieve a wide range of structural types. These include 0D [CdX₄]²⁻, 0D [CdX₆]⁴⁻, 1D [CdX₃]⁻, 1D [Cd₃X₈]²⁻, 1D [Cd₄X₁₂]⁴⁻ ribbons and 2D [Cd₃Cl₁₀]⁴⁻ layers.^31,34–37 In contrast to the extensive focus on structural characterizations, there is relatively limited research emphasis on high-energy ray detection, particularly in the area of X-ray detection. The inadequacy prompts us to explore a proficient synthetic approach for fine-tuning the X-ray detection characteristics of 0D hybrid Cd-based perovskites by carefully selecting appropriate organic components as cationic templates.

In this work, a novel 0D Cd-based perovskite was designed by introducing large steric hindrance organic cations (methyltriphenylphosphonium, C₁₉H₁₈P⁺) into cadmium chloride frameworks, and SCs with sizes of 6 × 5 × 3 mm³ were obtained. The crystal structure, optical characteristics, and electrical behavior of the as-grown (C₁₉H₁₈P)₂CdCl₄ SC were explored. Furthermore, the successful fabrication of an X-ray photoconductor was achieved using the SCs of (C₁₉H₁₈P)₂CdCl₄, demonstrating a remarkable ability to directly convert X-ray radiation into electrical signals with a high sensitivity of ∼143.6 μC Gy_air⁻¹ cm⁻² under a bias of 1000 V (333 V mm⁻¹). Moreover, it exhibits an ultrastable response current and maintains a stable baseline with a minimal dark current drift at a rate as low as 7.34 × 10⁻⁴ pA cm⁻¹ s⁻¹ V⁻¹. This study opens up a novel avenue for the utilization of new hybrid halides, particularly in the field of high-energy radiation detectors.

2. Results and discussion

First, we employed a simple solution method to synthesize (C₁₉H₁₈P)₂CdCl₄ SCs. Specifically, the (C₁₉H₁₈P)₂CdCl₄ SC was successfully synthesized by a slow volatilization method, where the crystal growth was meticulously controlled at a constant temperature of 40 °C via a slow evaporation process. To prevent extensive nucleation and ensure optimal crystalline quality, it was imperative to maintain a steady and gradual growth of crystals in the evaporating dish. Our largest SC, measuring 6 × 5 × 3 mm³ in size, was achieved following a few days of growth in the dish (Fig. 1(a)). The flawless crystals underwent a gentle cleaning and drying process within an N₂ glovebox for an extended period, followed by a 2 h annealing treatment at 100 °C to ensure thorough elimination of moisture and alleviation of lattice strain.¹⁰Fig. 1(a) depicts the photograph of a (C₁₉H₁₈P)₂CdCl₄ SC with a well-defined shape. The scanning electron microscopic (SEM) image taken from a top-view perspective reveals a uniformly smooth surface devoid of any discernible grain boundaries (Fig. S1a, ESI†), indicating a flawless standard of the SC, providing additional evidence for the significant benefits offered by the crystal growth technique. The corresponding energy-dispersive spectrum (EDS) analysis reveals a molar ratio of 2.13/0.96/3.85 for P, Cd, and Cl (Fig. S1b, ESI†), which aligns with the stoichiometric ratio of P, Cd, and Cl in (C₁₉H₁₈P)₂CdCl₄. Additionally, the consistency of the (C₁₉H₁₈P)₂CdCl₄ SCs is illustrated by the mapping measurements, as depicted in Fig. S2 (ESI†). The SEM analysis with EDS mapping reveals the homogeneous dispersion of the probed elements across the crystal surface, indicating a pristine surface free from any form of contamination.

Fig. 1 (a) Photograph of a bulk SC of (C₁₉H₁₈P)₂CdCl₄. (b) Powder XRD patterns of (C₁₉H₁₈P)₂CdCl₄. (c) Crystal structure of (C₁₉H₁₈P)₂CdCl₄. (d) (C₁₉H₁₈P)⁺ organic cations and [CdCl₄]²⁻ inorganic anions for (C₁₉H₁₈P)₂CdCl₄.

A powder X-ray diffraction (XRD) pattern of the crystal obtained, depicted in black in Fig. 1(b), exhibits a strong resemblance to the simulated outcome (depicted in red), thereby validating its composition as (C₁₉H₁₈P)₂CdCl₄. To gain insights into the spatial organization of inorganic anion and organic cationic units within the crystal, single-crystal X-ray diffraction was performed. The compound (C₁₉H₁₈P)₂CdCl₄ forms crystals in the P2₁3 space group, with a unit cell of dimensions characterized by the following parameters: a = 15.49955(3) Å, b = 15.49955(3) Å, and c = 15.49955(3) Å, and α = 90°, β = 90°, and γ = 90° (see Table S1 in the ESI†). The crystal structure of the compound is illustrated in Fig. 1(c), featuring a typical hybrid metal halide architecture. It consists of an infinite isolated tetrahedron formed by CdCl₄ units, confirming the (C₁₉H₁₈P)₂CdCl₄ classification as a 0D perovskite. (C₁₉H₁₈P)₂CdCl₄ comprises an organic cation [C₁₉H₁₈P]⁺ and an inorganic structural unit [CdCl₄]²⁻ (Fig. 1(d)). In the context of an organic layer, the cations [C₁₉H₁₈P]⁺ exhibit a globular configuration similar to that of the TMCM⁺ cation.³⁸ The more detailed crystallographic data are summed up in Tables S1–S7 (ESI†). It is worth noting that the bond angles of Cl1–Cd–Cl2 and Cl1–Cd–Cl1¹ are 109.14° and 109.80° respectively (Table S5, ESI†), and the bond angles are close to the perfect tetrahedron (109.5°).

X-ray photoelectron spectroscopy (XPS) was performed to investigate the chemical makeup and molecular arrangement of (C₁₉H₁₈P)₂CdCl₄, as depicted in Fig. 2(a) and (b). The Cl 2p spectrum exhibited the presence of a single chlorine atom in its chemical surroundings, which was accurately represented by a distinct pair of peaks (with approximately 1.6 eV spin–orbit splitting between Cl 2p_3/2 and Cl 2p_1/2). The doublet observed at 198.0 and 199.5 eV in the binding energy spectrum could be ascribed to the presence of Cl–Cd interactions. The Cd 3d spectrum exhibited a pair of prominent peaks at energy levels of 405.3 and 412.0 eV, in line with the Cd–Cl bonds, specifically representing the contributions from 3d_5/2 and 3d_3/2 orbitals.

Fig. 2 X-ray photoelectron spectra of (a) Cl 2p and (b) Cd 3d. (c) Calculation of the optical bandgap by the Tauc method according to the absorbed data in Fig. S3 (ESI†). (d) Electronic band structure of (C₁₉H₁₈P)₂CdCl₄ calculated through the Heyd–Scuseria–Ernzerhof (HSE) functional. (e) Total and partial densities of states of (C₁₉H₁₈P)₂CdCl₄. (f) PL spectra of the (C₁₉H₁₈P)₂CdCl₄ SC.

The optical bandgap of the (C₁₉H₁₈P)₂CdCl₄ crystal was determined by UV-vis absorption spectroscopy, where the absorption edge was observed at approximately 296 nm (Fig. S3, ESI†). By employing the Tauc plot (Fig. 2(c)), we identified the direct bandgap as 4.31 eV. This significant bandgap effectively eliminated the possibility of utilizing it for photovoltaic purposes; however, its interaction with high-energy radiation such as X-rays does not impede the occupation of vacant electron states.³⁹

According to the results of theoretical calculations, it can be observed from Fig. 2(d) that (C₁₉H₁₈P)₂CdCl₄ exhibits a direct bandgap of 4.91 eV, where the valence band maximum (VBM) and conduction band minimum (CBM) are both located at the Γ point. This estimation appears to be higher than the experimental value of 4.31 eV, which is within reasonable limits. It is worth noting that the density of states plots indicate that the VBM primarily originates from Cl 3p orbitals, with a minor contribution from organic and Cd 4d orbitals. For another, the CBM can be attributed mainly to organic sources (Fig. 2(e)). As illustrated in Fig. S4 (ESI†), the onset of absorption for (C₁₉H₁₈P)₂CdCl₄ occurs at approximately 4 eV, falling within the ultraviolet spectrum.

In order to further understand the optoelectronic characteristics of (C₁₉H₁₈P)₂CdCl₄, photoluminescence (PL) analysis was conducted using an excitation wavelength of 260 nm at room temperature. As shown in Fig. 2(f), a faint and slender emission peak at 338 nm was detected, potentially attributed to robust electron–photon interaction in comparison with an unlike broad band also found in previously reported Cd-based organic–inorganic perovskites.^36,40–43 The fluorescence decay curve of (C₁₉H₁₈P)₂CdCl₄, measured at 338 nm, can be accurately described by a mono-exponential function yielding a lifetime value of 3.8 ns (Fig. S5, ESI†), which is close to that of most of the hybrid lead perovskites such as (C₄H₉NH₃)₂PbCl₄ (2.59–9.68 ns), (2meptH₂)PbBr₄ (2.23 ns), α-(DMEN)PbBr₄ (1.39 ns), (BZA)₂PbCl₄ (3.96 ns), and (4amp)PbBr₄ (2.5 ns).^44–48

In addition, we conducted ultraviolet photoelectron spectroscopy (UPS) on a polycrystalline sample to analyze the energy distribution of (C₁₉H₁₈P)₂CdCl₄. The valence band value relative to the work function can be determined by utilizing the tangent of the cutoff edge (Fig. S6a, ESI†). The valence band maximum (VBM) and conduction band minimum (CBM) energies of (C₁₉H₁₈P)₂CdCl₄ are observed at −6.35 and −2.16 eV, respectively, suggesting that the semiconductor exhibits P-type conductivity (Fig. S6b, ESI†).

It is worth mentioning that the (C₁₉H₁₈P)₂CdCl₄ SC has good stability. As shown in Fig. 3(a), we conducted XRD analysis on the finely ground powders that were kept UNDER ambient conditions for a period of three months to assess their stability. It was observed that (C₁₉H₁₈P)₂CdCl₄ did not undergo any phase transition or decomposition when exposed to air for this duration, indicating its excellent resistance to atmospheric factors. As depicted in Fig. 3(b), the thermogravimetric analyses reveal that (C₁₉H₁₈P)₂CdCl₄ exhibits a temperature of 376 °C at which it experiences a 5% reduction in mass. This finding suggests that the crystal structure of (C₁₉H₁₈P)₂CdCl₄ remains thermally stable even under typical operating conditions for devices. In addition, there is no obvious weight loss until 321 °C. These stability results demonstrate that (C₁₉H₁₈P)₂CdCl₄ exhibits better phase stability compared with other organic–inorganic hybrid materials.^41,49,50

Fig. 3 (a) Powder X-ray diffraction (PXRD) patterns of (C₁₉H₁₈P)₂CdCl₄ recorded after three-month exposure to air. (b) Thermogravimetric (TG) analysis and differential scanning calorimetry (DSC) spectra of the (C₁₉H₁₈P)₂CdCl₄ ground powder. (c) Absorption coefficients of (C₁₉H₁₈P)₂CdCl₄,CdTe and Si semiconductors as a function of photon energy. (d) Attenuation efficiency of (C₁₉H₁₈P)₂CdCl₄,CdTe and Si semiconductors for 50 keV X-ray photon energy versus thickness.

Why is the stability of this type of 0D material good? We think it has something to do with his structure. Its 0D structure is the CdCl₄ tetrahedron structure and the periodic distribution of cations at A-site intervals, as shown in Fig. 1(c). In this way, electrons are transferred in the form of jumping between the tetrahedral structure and the A-site cation, and the direction of transmission is only one, which will lead to weak ion migration and small polarization built-in electric field. In addition, its band gap is about 4.19 eV, which also increases the material's high bias voltage resistance, so the stability of both the material and the device is enhanced. This feature also makes it show good performance in X-ray detection.

The potential of utilizing the hybrid metal halide SC (C₁₉H₁₈P)₂CdCl₄ for direct X-ray detection was assessed. Typically, effective direct X-ray detection necessitates significant absorption of X-ray photons by the active layers. To precisely ascertain the photon absorption capacity of (C₁₉H₁₈P)₂CdCl₄ for X-rays, we extracted the absorption spectra data from NIST's XCOM application, comparing it with traditional semiconductors across various photon energies.⁵¹ Furthermore, it is widely acknowledged that a higher atomic number and greater density have advantageous effects on X-ray attenuation as per the equation α ∝ ρZ⁴/E³, where α is the absorption coefficient, ρ denotes the mass density, Z signifies the atomic number, and E stands for the radiation.⁵²

The calculated average atomic number of (C₁₉H₁₈P)₂CdCl₄ at 50 keV, which corresponds to the peak intensity of the X-ray source, is 15.3. This value exceeds that of Si (14.0). The software “AutoZeff” was utilized to determine the effective atomic number.⁵³ As shown in Fig. 3(c), we acquired the absorption coefficient for (C₁₉H₁₈P)₂CdCl₄, CdTe, and Si across an extensive range of photon energies from 0.001 to 100 MeV based on data from the database containing cross-section information for photons. Irrespective of the resonant absorption occurring at the K, L, and M edges, it can be observed that the absorption coefficient of (C₁₉H₁₈P)₂CdCl₄ is greater than that of Si across all energy ranges. This observation aligns following the trend exhibited by average atomic numbers.

Fig. 3(d) illustrates the energy-dependent attenuation coefficient, covering a broad spectrum spanning from 0.001 to 100 meV, as per the photon cross-section database provided by the National Institute of Standards and Technology.⁵¹ The (C₁₉H₁₈P)₂CdCl₄ compound exhibits comparable attenuation efficiencies to other substances such as CdTe and Si. A 13 mm crystal of (C₁₉H₁₈P)₂CdCl₄ can absorb approximately 95% of 50 keV X-ray photons, which is slightly lower than that of CdTe but considerably higher than that of Si with an equivalent thickness (approximately 73%). The increased X-ray attenuation in the (C₁₉H₁₈P)₂CdCl₄ crystal semiconductor can be attributed to the higher Z value of Cd.

The lateral structure of Au/(C₁₉H₁₈P)₂CdCl₄/Au was utilized to measure photoconductivity, which was based on the 0D (C₁₉H₁₈P)₂CdCl₄ SC. The I–V curves exhibit linear and symmetrical characteristics, suggesting a strong ohmic connection between the Au electrode and the SC (Fig. 4(a)). Upon exposure to X-ray photoexcitation at a dose rate of 580 μGy s⁻¹, the photocurrent demonstrates a significant increase from 13 pA (dark) to 402 pA (light), resulting in an impressive photocurrent on/off ratio of ∼31. These findings highlight its potential as a highly efficient material for X-ray detection. As illustrated in Fig. S7 (ESI†), the resistivity of (C₁₉H₁₈P)₂CdCl₄ was calculated to be 3.73 × 10¹¹ Ω cm, which is at least three orders significantly greater than that of CH₃NH₃PbBr₃ as well as various other perovskite materials containing lead (10⁷–10⁸ Ω cm).^1,54

Fig. 4 (a) Single-crystal devices of (C₁₉H₁₈P)₂CdCl₄ with lateral electrodes subjected to an X-ray source for the dark current and photocurrent. (b) Photoconductivity measurement of the (C₁₉H₁₈P)₂CdCl₄ device. (c) Impact of X-ray irradiation on the device performance examined at different dose rates, indicated by blue markings representing the dose rate (unit: μGy_air s⁻¹). The bias voltage applied is 1000 V. (d) Relationship between the dose rate and response current density is examined at different bias voltages. The sensitivity of X-ray detection can be determined by calculating the slope of the fitted line. (e) X-ray detection sensitivity of the (C₁₉H₁₈P)₂CdCl₄ SC device at different voltage levels. (f) Curve of 3 on (1 min)-off (2 min) cycles for the (C₁₉H₁₈P)₂CdCl₄ SC X-ray detector. The applied bias voltage was 1000 V (field of 333 V mm⁻¹) and the dose rate was 580 μGy_air s⁻¹.

The high capacity for attenuating X-rays ensures effective absorption of a significant number of X-ray photons despite the thinness of the material, thereby facilitating the retrieval of charge carriers.³² Inspired by the encouraging photoconductive properties and X-ray with a dose rate of 410 μGy_air s⁻¹, and measurements taken absorption capability, we proceeded to evaluate the detection efficiency of an X-ray source using a vertical-type (C₁₉H₁₈P)₂CdCl₄ SC detector. Charge collection plays a crucial part in the detection of X-rays, and it is determined by factors such as carrier mobility (μ), carrier lifetime (τ), and trap density. The crucial parameter for evaluating the properties of carriers (such as mobility and drift length) in X-ray detection is μτ, which indicates a semiconductor's ability to extract charge from deep within. For high is preferable to have a high μτ value and low trap density. In this work, we obtained the μτ products by analyzing the photoconductivity data with a customized version of the Hecht equation:¹⁰

where I₀ represents the fully saturated photocurrent, L denotes the thickness measurements, V signifies an applied bias voltage, τ refers to the carrier lifetime duration, and s indicates the surface recombination velocity. The curve of the fitting is illustrated in Fig. 4(b). The (C₁₉H₁₈P)₂CdCl₄ SC has μτ products of 8.53 × 10⁻⁴ cm² V⁻¹, which exhibits similar values to those observed for TMCM-CdCl₃ (1.42 × 10⁻⁴ cm² V⁻¹) and CdZnTe (9.1 × 10⁻³ cm² V⁻¹) SCs.^49,55

Due to its excellent X-ray absorption, high μτ product, and thermal stability, (C₁₉H₁₈P)₂CdCl₄ shows great potential for X-ray detection. Here, we employed a vertical photoconductive detector structure and fabricated (C₁₉H₁₈P)₂CdCl₄ SC (6 × 5 × 3 mm³) X-ray detectors with the device configuration of Au/(C₁₉H₁₈P)₂CdCl₄/Au. The operational principle of our designed direct Au/(C₁₉H₁₈P)₂CdCl₄/Au detection device when exposed to X-ray radiation can be characterized as a process involving the conversion of light signals into electrical ones.^4,15 The X-ray photons are absorbed by the active material, leading to the generation of high-energy electrons. These high-energy electrons have a tendency to dissipate their energy through the creation of additional electron–hole pairs. The energy required for the creation of an electron–hole pair, as determined by the empirical model W = 2E_g + 1.43, was measured to be 10.05 eV in this study, where E_g represents the bandgap of (C₁₉H₁₈P)₂CdCl₄.⁵⁶

For our device based on photoconductivity, the amplification of photocurrent is achieved by capturing photoexcited electrons in shallow defects. During their capture, the photoexcited holes can repeatedly move between the electrodes.⁵⁷ The X-rays were produced by an X-ray tube (NEWTON SCIENTIFIC Model M237) with a gold target, operating at an accelerating voltage of 50 kV and a power output of 10 W. The photocurrent of the SC device in Fig. 4(c) can be adjusted from 195 to 901 pA by rising the dose rate of X-ray, while maintaining a stable dark current around 200 pA, up to a bias voltage of 1000 V (results at other bias voltages are presented in Fig. S8, ESI†). The rise and fall time of the detector reaches 0.637 s and 0.431 s, respectively (Fig. S9, ESI†). Compared to the crystal structure of 3D, 2D or 1D perovskites, 0D (C₁₉H₁₈P)₂CdCl₄ hybrid metal halide perovskites consist of isolated metal halide [CdCl₄]²⁻ tetrahedral units. This arrangement leads to more localized electronic structures, which is consistent with the previously calculated electronic band structure (Fig. 2(d)), resulting in higher activation energy. Consequently, ion migration is effectively suppressed and the operational stability of X-ray detectors under high working bias is enhanced. Moreover, the dark current density was estimated to be approximately 20 nA cm⁻², obtained through dividing the current value with the active area of Au electrodes (approximately 0.01 cm²), which is comparable to the values of Cs₃Bi₂I₉ (18 nA cm⁻²) and much lower than those of CsPbBr₃ (170 nA cm⁻²) SCs.^58,59

The X-ray tube current is varied from 30 to 150 μA in order to modify the X-ray dose rates, which range between 124 and 621 μGy_air s⁻¹. Fig. 4(d) illustrates the correlation between the dose rates and X-ray response current densities under various biased voltages. The relationship between the generated current density and X-ray dose rate shows a nearly linear trend for different applied voltages. Fig. 4(e) presents the X-ray detection sensitivities calculated at different applied voltages.

Based on the results of the nearly linear fit, it can be estimated that the Cd-based single-crystal device has an X-ray detection sensitivity of ∼ 62.3 μC Gy_air⁻¹ cm⁻² when biased at 200 V (with a field strength of 66.67 V mm⁻¹), and ∼ 143.6 μC Gy_air⁻¹ cm⁻² when biased at 1000 V (with a field strength of 333 V mm⁻¹). These values are similar to those reported for bismuth-based and Cd-based perovskite TMCM-CdCl₃ devices (128.9 ± 4.64 μC Gy_air⁻¹ cm⁻² when biased at a field strength of 10 V mm⁻¹),⁴⁹ as well as double perovskite Cs₂AgBiBr₆ X-ray detectors (∼105 μC Gy_air⁻¹ cm⁻² when biased at a field strength of 25 V mm⁻¹).¹¹ Given the operational efficiency of the amorphous selenium X-ray detector commonly employed in commercial applications (20 μC Gy_air⁻¹ cm⁻² at 10 000 V mm⁻¹), our X-ray detector's relatively favorable outcomes highlight the potential application of hybrid Cd-based metal halide perovskites in X-ray detection.¹⁷ In addition, the photocurrent response of a (C₁₉H₁₈P)₂CdCl₄ SC X-ray detector to ambient air radiation was observed when biased at a field strength of 333 V mm⁻¹ (see Fig. 4(f)). It is evident that upon activation of the X-ray, there is a rapid increase in the photocurrent up to its maximum value of 629 pA. After the radiation is deactivated, the current quickly decreases to a low value of 9 pA known as the dark current. Additionally, no noticeable degradation in photocurrent is observed when subjected to air testing for three cycles. The dark current remained stable throughout the measurements, and there was no noticeable drift in current observed vertically. The stable outputs of current suggest that the ion migration were found to be negligible in 0D (C₁₉H₁₈P)₂CdCl₄ SCs. The remarkable stability of our device aligns with the excellent air storage stability observed in (C₁₉H₁₈P)₂CdCl₄ SCs, as shown in Fig. 3(a).

We also examined the baseline drift observed in the manufactured detectors. A bias voltage of 1000 V was sustained for a duration of 120 seconds, during which the baseline current exhibited greater stability in the (C₁₉H₁₈P)₂CdCl₄ device than in the CsPbBr₃ device (Fig. S10a, ESI†).¹⁰ We determined the present deviation D between the 10th and 110th s by employing the following equation:

where t represents the time period, E denotes the strength of the electric field, and J_t and J₀ refer to the current densities at the initial and final locations, respectively. The calculated current drift of the (C₁₉H₁₈P)₂CdCl₄ device was 7.34 × 10⁻⁴ pA cm⁻¹ s⁻¹ V⁻¹, which is significantly smaller by five orders of magnitude than that of CsPbBr₃ (14.9 pA cm⁻¹ s⁻¹ V⁻¹) and on par with Rb₃Bi₂I₉ (1.82 × 10⁻⁴ pA cm⁻¹ s⁻¹ V⁻¹). Besides, the (C₁₉H₁₈P)₂CdCl₄ device exhibited a significantly lower fluctuation standard variance in the time range of 60 to 80 s, measuring merely 0.33 pA (Fig. S10b, ESI†), which was notably smaller than that of CsPbBr₃ and Rb₃Bi₂I₉.¹⁰ The stability of both dark current drift and fluctuation in the (C₁₉H₁₈P)₂CdCl₄ device was found to be higher than that of CsPbBr₃. This indicates that the (C₁₉H₁₈P)₂CdCl₄ device holds great potential for bright applications in X-ray detection.

3. Conclusions

In summary, we have successfully fabricated a novel Cd-based hybrid metal halide perovskite (C₁₉H₁₈P)₂CdCl₄ large crystal and demonstrated its potential for X-ray detection. This crystal reaching dimensions of up to 6 × 5 × 3 mm³ can be acquired from aqueous solutions under ambient conditions, which proves advantageous for the utilization of this material in optoelectronic applications. The crystal exhibited a direct band gap of 4.19 eV, indicating its P-type semiconductor characteristics, and the product of carrier lifetime and mobility achieved a value on the order of 10⁻⁴ cm² V⁻¹. Due to the high product of carrier lifetime and mobility, we can create an exceptionally effective X-ray detector based on crystals, which exhibits a sensitivity of ∼143.6 μC Gy_air⁻¹ cm⁻². Moreover, it demonstrates an exceptionally stable response current and maintains a steady baseline with the lowest drift in dark current at 7.34 × 10⁻⁴ pA cm⁻¹ s⁻¹ V⁻¹. This work showcases the promising capabilities of molecular perovskites in detecting X-rays with high efficiency, thanks to their exceptional charge transport properties. Additionally, it opens up a novel avenue for investigating functional materials in optoelectronics research. Moreover, our findings provide an opportunity to explore reliable and enduring X-ray detectors.

4. Experimental section

4.1 Materials

Chemicals. Methyltriphenylphosphonium chloride (AR, 98%) and cadmium(II) chloride (CdCl₂) (AR, 99.99%) were purchased from INNOCHEM. Hydrochloric acid (HCl) (AR, 37 wt%) was purchased from Acros. All reagents were used as received, without further purification.

4.2 Synthesis of (C₁₉H₁₈P)₂CdCl₄ single crystals

Synthesis. The synthesis of (C₁₉H₁₈P)₂CdCl₄ crystals was conducted based on the previous study, incorporating certain modifications for optimization purposes:⁴⁹

A mixture of deionized water (55 mL) and HCl (5 mL) was stirred at room temperature overnight, followed by the addition of a methyltriphenylphosphonium chloride solution (10 mmol, 3.13 g). Subsequently, a solution of CdCl₂ (5 mmol, 0.92 g) was added. Here, hydrochloric acid was used to create an acidic setting. The solution was then allowed to naturally evaporate at room temperature. After several days, colorless, transparent SCs were grown from the solution in the form of parallel epipedic shapes.

4.3 Device fabrication with (C₁₉H₁₈P)₂CdCl₄ single crystals

Prior to the fabrication of the device, the crystal was subjected to multiple rounds of tape lifting in order to eliminate any contaminants and imperfections on its surface. To fabricate vertically aligned photodetectors, square-shaped gold (Au) electrodes measuring 1 mm on each side and with a thickness of 80 nm were thermally evaporated onto the upper and lower surfaces of (C₁₉H₁₈P)₂CdCl₄ crystals using an already prepared shadow mask. The separation between the two Au electrodes measured 3 mm.

4.4 Material property characterization of (C₁₉H₁₈P)₂CdCl₄

Structure determination. The SCXRD experiments were conducted using a Rigaku XtaLAB PRO diffractometer equipped with Cu K_α radiation at λ = 1.54178 Å. To account for absorption effects, empirical corrections based on spherical harmonics using the SCALE3 ABSPACK scaling algorithm were employed during data integration. Crystal structure determination involved initial phasing using the SHLEXT program's Intrinsic Phasing method, followed by refinement utilizing full-matrix least-squares fitting of F² values using the SHLEXL program. For complementary analysis, PXRD measurements employing Cu Kα radiation at λ = 1.54178 Å were performed using a Rigaku SmartLab-SE diffractometer. Additionally, simulated powder diffraction patterns derived from CIF files facilitated further investigation through VESTA.

Other material characterization. The XRD patterns were acquired using an X-ray diffractometer (model 38 x'pert pro MPD) operating at a scanning speed of 10° min⁻¹ and a step size of 0.02°. X-ray photoelectron spectroscopy (XPS) was conducted using a photoelectron spectrometer (Thermo Fisher Scientific), specifically the ESCALAB250Xi model. The ultraviolet photoelectron spectra (UPS) were recorded using a Thermo Scientific Escalab 250 Xi instrument with monochromatic Al Kα radiation (hν = 1486.7 eV). The TG-DSC curve was acquired by simultaneously employing a thermal analyzer (PerkinElmer Diamond TG/DSC6300) in an Ar environment, operating at a heating rate of 10 K min⁻¹. The UV-visible-near infrared absorption spectra were recorded using a spectrophotometer (PerkinElmer Lambda 35). PL spectra were recorded using a laser confocal Raman microspectroscopy system (LabRAM HR800) equipped with a 280 nm Xenon lamp for excitation. To conduct time-resolved photoluminescence (TRPL) measurements, a picosecond laser with a wavelength of 280 nm was utilized as the source of excitation. The determination of corresponding lifetimes involved fitting single-exponential curves.

4.5 DFT calculations

DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP) 6.1 software employing the projection-augmented wave (PAW) approach.⁶⁰ The energy cutoff for plane waves was chosen as 400 eV. The structural optimization utilized the Heyd–Scuseria–Ernzerhof (HSE) hybrid functional, with a mixing parameter of 25%.^61,62Γ-centered k-meshes were employed to sample the Brillouin zones, with a k-spacing of 0.2 Å⁻¹. The crystal structures underwent complete relaxation until the total force on each atom reached a value lower than 0.03 eV Å⁻¹.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (62134003, 62074068, 62074066, 62204092, 12050005), Major State Basic Research Development Program of China (2021YFB3201000), China Postdoctoral Science Foundation (2023T160242, 2022M710054), Fund for the Natural Science Foundation of Hubei Province (2021CFA036, 2020CFA034), the Shenzhen Basic Research Program (JCYJ20200109115212546).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2332882. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4tc00594e

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