Metal Halide Perovskites for High-Energy Radiation Detection

Abstract

In this chapter, the fundamental principles of high-energy radiation detection and recent progress in the emerging field of metal halide perovskite (MHP)-based direct and indirect X-ray and γ-ray detectors are discussed. The chapter first introduces the underlying principles of high-energy radiation detection, with emphasis on the key performance metrics. This is followed by a comprehensive summary of the recent progress made in the field of perovskite-based radiation detector technologies. Finally, the chapter ends with an overview of current issues and future perspectives on MHP-based direct and indirect (scintillators) radiation detector technologies.

1 Introduction

High-energy radiation (e.g., X-rays, γ-rays; Fig. 1a) detection plays a vital role in many areas, including scientific research, medical applications, nondestructive evaluation, and national security. Current trends in detector technology offer substantial reductions in size, weight, and power as well as reduced exposure times, broadening the scope for emerging opportunities and applications. Recent advancements in the field of high-energy photon detection, which combine enhanced performance and cost-effective, large-area, and high-throughput manufacturing, could pave the way to new technologies beneficial to our society. Metal halide perovskites (MHPs) represent a promising family of materials for radiation detection. Their attractive physical and chemical properties combine a strong stopping power with a large μ_h/e×τ [mobility (μ_h/e) and lifetime (τ)] product, large linear attenuation coefficients (Fig. 1b), absence of deep traps, and controllable crystallization via scalable solution-based deposition techniques. These features make MHPs ideal for deployment in next-generation high-energy radiation detection technologies. To date, the highest sensitivity reported for MHP-based radiation detector is ~700 mC Gy_air⁻¹ cm⁻² and the lowest detection attained is 0.62 nGys⁻¹ [1, 2].

Fig. 1

(a) The electromagnetic spectrum ranging from infrared (IR) to γ-rays along with their respective wavelengths and photon energies. The visible wavelength range is expanded for clarity. (b) The linear attenuation coefficient of MAPbBr₃, CsPbI₃, CdTe, Se, and TlBr versus photon energy. Figure 1a is reprinted from [3] with the permission of Wiley-VCH GmbH. Figure 1b is reprinted from [4] with the permission of Springer Nature

2 Key Parameters of High-Energy Radiation Detectors

Depending on the detection principle, high-energy radiation detectors are classified into two types: (I) direct radiation detectors and (II) indirect (scintillation) detectors. Direct radiation detectors rely on photoconductive materials that are sensitive to specific high-energy radiation, while indirect detectors work with scintillator materials that convert high-energy X-rays (0.1–100 keV) or γ-rays (0.1–10 MeV) to ultraviolet light (UV) or visible (Vis) (Fig. 1a), which is subsequently detected by a standard photodiode/array.

2.1 Direct Detectors

The functioning principle of direct high-energy radiation detectors is based on the direct interaction of the incoming photons with the sensing material, characteristically a semiconductor, and the immediate generation of an electrical signal (voltage or current).

2.1.1 X-Ray Detectors

The interaction between soft X-ray photons (Fig. 1a) and sensing material depends on the photoelectric absorption of the sensing material. On the other hand, the detection of hard X-ray photons is due to Compton scattering (photon-electron interaction). Important figures of merit (FOM) that regulate the performance of a direct X-ray detector are discussed in the following.

2.1.1.1 Stopping Power

The stopping power is defined as the rate of energy lost per unit of path length (x) by a charged particle with kinetic energy (T_E) in a medium of atomic number Z (Z ∝ ρ, where ρ is the density of the material). It is measured in MeV/cm or J/m and is represented as

$$ {dT}_E/\rho\ dx $$

(1)

2.1.1.2 Ionization Energy (W_±)

The ionization energy (W_±) of the sensing material is another critical parameter. It is defined as the energy required to release an electron-hole pair in the photoconductor. The ionization energy minimum is proportional to the energy bandgap (E_g) of the absorbing material employed, and it can be represented as [5]

$$ {W}_{\pm }=2.2{E}_{\mathrm{g}}+{E}_{\mathrm{photon}} $$

(2)

or

$$ {W}_{\pm }=3{E}_{\mathrm{g}} $$

(3)

2.1.1.3 μ_h/e × τ Product

At any given X-ray dose , high detection sensitivity is needed to generate good-quality images. The sensitivity of X-ray detectors can be enhanced by means of two different methods: (1) increasing the mobility-lifetime (μ_h/e×τ) product as it relates to the carrier drift length (L_D) given as L_D = (k_BTμτ/e)^1/2 and (2) higher applied reverse bias. For efficient direct X-ray detectors , high μ_h/e×τ product and low trap density are essential parameters. The μ_h/e×τ product for a given sensing material can be estimated from modified Hecht formula for photoconductivity (I) [6]:

$$ I=\frac{I_0\mu \tau V}{L^2}\frac{{}^{1-\exp}\left(-\frac{L^2}{\mu \tau V}\right)}{1+\frac{L}{V}\frac{S}{\mu }} $$

(4)

I_o is the saturated photocurrent, L is the material layer thickness, V is the applied bias, and s is the surface recombination velocity.

2.1.1.4 Sensitivity

The following formula can calculate the detector’s sensitivity :

$$ S=\frac{\int \left[{I}_{X-\mathrm{ray}}(t)-{I}_{\mathrm{dark}}\right] dt}{D\times {V}_d} $$

(5)

where I_X-ray and I_dark are the generated currents with and without X-ray irradiation, respectively. D is the dose, and V_d denotes the detector volume.

The linear dynamic range (LDR) of the detector that represents the range of X-ray dose rate under which the sensitivity remains constant is a crucial parameter to describe a detector’s properties. The higher the LDR, the better the resolution.

2.1.1.5 Energy Resolution

The energy resolution is defined as the detector’s ability to accurately determine the incident radiation’s energy [7]. It is derived from the ratio between the FWHM and the photopeak centroid H_max. Thus, the energy resolution (R) is calculated from the following equation:

$$ \boldsymbol{R}\%=\frac{\boldsymbol{FWHM}}{{\boldsymbol{H}}_{\mathbf{max}}} $$

(6)

An additional requirement is that the active layer’s thickness should be at least three times larger than the attenuation length. Direct radiation X-ray detectors with semiconductors operate in current mode, while the magnitude of the produced electrical current is proportional to the incident photon energy.

2.1.2 γ-Ray Detectors

The interactions between γ-ray photons and the sensing element can be described in three mechanisms. (1) Photoelectric process: Complete transfer of energy of the γ-ray photon (with energy between 10 and 500 keV) to electrons. (2) Compton scattering : Part of the γ-ray energy (50 keV to 3 MeV) is lost and is transferred to the electrons. (3) Pair production: An incoming γ-ray photon with energy exceeding 1.022 MeV generates a positron and electron.

The semiconductors with similar characteristics used in X-ray detectors can also serve the purpose of detecting γ-ray photons. The following important characteristics are the prerequisites of semiconductors for high-energy γ-ray detectors.

• 1. Elements with high atomic number (Z): Facilitates large stopping power (see Eq. (1))

• 2. Large bipolar μ_h/e×τ product: Enables efficient detection

• 3. Large bulk resistivity of >10⁹ Ω-cm: Enhances the signal-to-noise ratio (SNR)

The photopeak energy resolution is another FOM parameter, and it is vital as it allows the detector to differentiate between γ-ray photons with various energies. The key FOM parameters of high-energy radiation direct detectors and their significance are listed in Table 1.

Table 1 Some of the crucial direct radiation detectors’ FOM parameters and their significance

2.2 Scintillator Detectors

The ability of scintillators to stop high-energy photons and translate them to lower energy visible photons has found various applications in security and medical imaging [5, 8].

2.2.1 X-Ray Scintillators

The interaction between X-ray photons and the scintillator material can occur via (1) photoelectric absorption, (2) Compton scattering, and (3) pair production [9, 10]. All processes are characterized by absorption coefficients that are eventually determined by the atomic number (Z) of scintillator material and the photon energy. When the photoelectric effect is dominant, and for photon energies far from the absorption edge, the linear absorption coefficient (μ) is given by

$$ {\mu}_{\mathrm{L}}\approx {\rho Z}^n/{E}^{3.5} $$

(7)

where ρ is the material density, E is the photon energy, and n is a constant, which typically varies between 3 and 4. At higher energies, Compton scattering generally occurs. Here, depending on the scattering angle, a part of the photon’s energy is transferred to the electron. The Compton scattering linear absorption coefficient (μ_C) is given by [9, 11]

$$ {\mu}_{\mathrm{C}}\approx \rho /{(E)}^{1/2} $$

(8)

For higher energies, the interaction of the radiation with the matter is governed by the generation of low-energy excitons (pair production) within the scintillating material that eventually recombine and produce visible light. The emitted light can then be detected via different photodetectors coupled with the scintillator element. Inexpensive production and stability are the main strengths of indirect scintillator detectors compared to direct detectors [9, 11].

Light yield (LY) is the most important figure of merit for scintillators that describes the number of electron-hole pairs generated during the ionization process per unit energy and is given in photons per MeV:

$$ \mathrm{LY}={10}^6\ SQ/\left({\beta E}_{\mathrm{g}}\right) $$

(9)

where Q is the luminescence efficiency , S is the efficiency of transport of electron-hole to the optical (emissive) center, and β is a constant with a typical value of 2.5.

2.2.2 γ-Ray Scintillators

Upon interaction of γ-ray photons (with energies higher than 1.02 MeV) with the sensing element of a γ-ray scintillator, pair production occurs. The absorption coefficient (μ_P) is expressed as

$$ {\mu}_{\mathrm{P}}\approx \rho\ Z\ \ln \Big(2E/\left({m}_{\mathrm{e}}{c}^2\right) $$

(10)

where ρ is the material’s density, c is the speed of light, and m_e is the mass of electrons. The phenomenon of scintillation can be divided into three main sub-processes: conversion, energy transfer, and luminescence. Photon absorption (i.e., the first stage of scintillation, which lasts ≈1 ps) is followed by the charge transport and energy transfer steps. The energy of hot electrons and holes is transferred to the luminescence centers, leading to visible light emission via the aforementioned process. Some of the key FOMs of a scintillator are listed in Table 2.

Table 2 Important FOM parameters and their significance of high-energy scintillation detectors

3 Metal Halide Perovskites for Radiation Detectors

Excellent charge carrier mobility, long carrier diffusion length and lifetime, high absorption coefficient, and low exciton binding energies even for polycrystalline films made from solution are some of the properties that forecast the potential of perovskites for optoelectronic applications in general [12, 13]. In addition, the unique physical and chemical properties of strong stopping power, absence of deep traps, large μ_h/e×τ product, and easy crystallization from low-cost solution processes make perovskites suitable for next-generation ionization detection materials. This section focuses on the structure and properties of perovskites that allow them to be superior radiation detection materials.

3.1 Crystal Structure

MHPs with the empirical formula of ABX₃ (e.g., MAPbX₃) are classified as 3D perovskites in which BX₆ octahedra are corner-shared along all three fourfold octahedral axes (Fig. 2). 2D perovskites with a layered structure, organized from octahedra connected along two octahedral axes, can be derived from the 3D structure by slicing along specific crystallographic planes. If 2D perovskites are further sliced perpendicular to the inorganic sheets, octahedra remain connected only along one axis, which can be categorized as 1D perovskites. The extreme case is 0D perovskites, derived by further slicing of 1D structures to form non-connected (i.e., isolated) octahedra or octahedral based clusters. Perovskites can also exist in the form of a superposition of two or more classes; for instance, a framework of 3D and 2D is often called quasi-2D perovskites . It is worth mentioning that the perovskites’ structural stability of all dimensionalities relies on the cationic organic or inorganic sublattice. For the sake of simplicity, the generalized empirical formulas are used for the perovskites with various dimensionalities in Fig. 2. Even though there are multiple stringent requirements in the material selection for ionizing detectors, which narrow down the choice of effective materials substantially, halide perovskites still maintain many relevant advantages due to their compositional and structural flexibility. The main applications of halide perovskites are focused on optoelectronic devices, and obtaining a high-quality active layer is crucial. In general, the MHP-based radiation detectors consist of either thin films or single crystals (SCs). Solution processes such as drop-casting, spin coating, blade coating, spray coating, and centrifugal casting [14] are widely explored as methods to process these materials, are easy to operate, and are compatible with all kinds of perovskite inks.

Fig. 2

Schematic representations showing the connectivity of BX₆ octahedra in low-dimensional perovskites and their formation by slicing the 3D structure along crystallographic planes

3.2 Properties of Metal Halide Perovskites for Radiation Detection

In recent years, research and development efforts towards next-generation materials to detect high-energy radiation have intensified. The majority of ongoing efforts aim to improve the manufacturability and sensitivity of the detection elements and systems [15]. This is why the advancement and/or unearthing of materials that combine key functionalities with economic production has become an intense area of research, with MHPs currently leading the way [4, 15,16,17]. Due to their high material density (≈4 g cm⁻³) owing to their ability to integrate atoms with high atomic number (Z), e.g., Pb (Z = 82), Cs (Z = 55), Sn (Z = 50), In (Z = 53), and Br (Z = 35), most of the MHPs are attractive for radiation detectors. Note that the scaling of the X-ray absorption strength, which is derived from Z⁴/AE³ (A: atomic mass; E: energy of the high-energy photons), relies on the Z value. Large absorption cross sections, short penetration depths, large μ_h/e×τ products, and short detection times (ns) have propelled MHPs for applications related to high-energy radiation detection.

Tunable and small energy bandgaps of MHPs promote high light yield upon X-ray irradiation (PLQYs: 12,900–250,000 photons/MeV) [18]. The compositional and processing flexibility, high bulk resistivity (≈10⁷ Ω-cm), low charge-trap density, and defect-tolerant nature (i.e., μ_h/e×τ product) are more beneficial attributes of MHPs [19,20,21,22]. Halide perovskites’ stopping power (linear attenuation coefficient of 0.09 cm⁻¹) for γ-rays is approximately two times larger than that of commercially deployed CdTe, further emphasizing the advantages of MHP technology [23]. Furthermore, MHPs also offer extraordinary wide-ranging absorption that spans from the visible to hard X-ray (Fig. 1a), making them an exceptional choice for both high-energy direct detectors and scintillators [24]. Apart from their desired radiation detector characteristics, the toxicity of MHPs is the main hurdle for commercial deployment and needs to be addressed.

4 Advances in the Development of Perovskite X-Ray Detectors

Direct detection of X-ray photons using MHPs offers an efficient, simple, and potentially economic technology for various current and evolving applications. Consequently, detection sensitivities of 55,684μC Gy⁻¹ cm⁻² with a low detection limit down to 36 nGys⁻¹ have been demonstrated and attributed to the perovskites’ superior physical properties [25]. Hence, the application of such a well-performing technology is enormously broad, including flexible and printable large-area X-ray imaging devices as well as futuristic applications such as X-ray photon energy harvesters for powering satellites in space [26, 27]. Two popular forms of MHPs explored for optoelectronic application are polycrystalline thin films and single crystals. For both cases, solution processes have widely been employed to produce these perovskite forms. An overview of the achieved sensitivity values for single- and multi-crystalline MHPs in recent years is given in Fig. 3a and combined with their respective mobility-lifetime products is shown in Fig. 3b.

Fig. 3

(a) An overview of the achieved sensitivity values for single- and polycrystalline MHPs in recent years and (b) combined with their respective mobility-lifetime products. The numbers in the graph indicate the references from which the data have been extracted

4.1 Polycrystalline Lead Halide X-Ray Detectors

4.1.1 3D Perovskites

The conventionally employed semiconductors for X-ray detection, amorphous Se, crystalline Si, and CdTe exhibit large photoconduction upon irradiation with X-ray photons [8, 28, 29]. However, uniform film processing onto arbitrary substrates with other device components, e.g., thin-film transistors (TFTs), is highly challenging. This is an area where halide perovskites could provide essential solutions due to their superb processing versatility. For example, the direct X-ray detectors assembled in p-i-n configuration with thick (10–100μm) crystalline MAPbI₃ (MAPI) layers processed by the spray-coating method have been used to measure the X-ray-induced charges by monitoring the device’s built-in potential (Fig. 4) [24]. Though the detectors show relatively long response times, they exhibit high specific sensitivity (25μC mGy_air⁻¹ cm⁻³). Apart from the improved response, the devices require a high external bias of 80 V to collect the generated charges efficiently due to the thick perovskite layer. Despite the higher film thickness, the reported strategy demonstrated the potential of spray-coated MAPI for the direct conversion of X-ray photons. For X-ray imaging, such as mammography and digital radiography, thin-film transistor (TFT)-based flat-panel detectors (FPD) are widely used. An 830μm thick polycrystalline MAPI photoconductor was successfully fabricated on a conventional TFT-based backplane and addressed the economic and technical challenges associated with the use of single crystals without adversely affecting the detector performance [30].

Fig. 4

(a) Diagrammatic representation of layer stacking of the MAPbI₃-based p-i-n photodiode. (b) Short-circuit X-ray photocurrent as a function of dose rate. Inset: Sensitivity normalized to the active volume for MAPbI₃ layers with different thicknesses. Reprinted from [24] with permission of Springer Nature

For efficient X-ray absorption , the photoconductor layer’s thickness should be approximately three times that of the X-ray attenuation length of the material. In MHPs, this characteristic length is on the order of hundreds of micrometers, which represents a significant technical challenge if one considers the required high structural quality of the layers [22]. In an attempt to tackle this challenge, room-temperature mechanically sintered wafers of microcrystalline MAPI (thickness varying from 0.2 to 1 mm) were fabricated [31] to make an X-ray detector. The resulting layers’ density (≈3.76 g cm⁻³) was quite consistent with MAPI single crystals (≈4.15 g cm⁻³), highlighting the versatility of the sintering process. Upon irradiation of X-rays of 38 keV, the calculated attenuation dept. of the detector (planar-inverted perovskite solar cell architecture) was ≈125μm. At an electric field of 0.2 Vμm⁻¹, the device sensitivity was estimated as 2527 μC Gy_air⁻¹ cm⁻². The μ_h/e×τ product and ionization energy of the perovskite wafer-based detector were comparable with those of commercial CdTe detectors [32].

Using perovskite nanostructures as the X-ray sensing layer is another approach, which could address the significant challenges of both inorganic and hybrid MHP-based X-ray detectors, such as the limited temporal resolution, nonuniform sensing, and poor stability. For example, an improved response time (1 s) was reported for CsPbBr₃ nanoparticles (NPs) decorated with reduced graphene oxide (rGO) as an X-ray sensing element [33]. It was postulated that the generated charge carriers in the NPs were quickly transferred to the rGO nanosheets and ultimately to the electrodes. Besides, the scalable synthesis of CsPbBr₃ self-assembled nanosheets (3.1 nm thick) produced by a green synthesis route leads to a high photoluminescence yield, and excellent storage stability is promising for applications as an active material for X-ray imaging screens [34]. The PL quantum yield (PLQY) of cubic phase CsPbBr₃ exhibited a higher value of 68% than the orthorhombic counterparts (18.5%). The green synthesis method adopted in this work could potentially provide a route towards a commercially viable perovskite-based X-ray detector technology.

The detector’s operational stability, which is quite sensitive to the applied electric field, is another technological bottleneck. To enhance the sensitivity of an X-ray detector, a higher electric field is commonly employed, which leads to increased leakage currents and thus a deteriorated detector performance. To overcome this issue, a Schottky-type sandwiched photodetector comprised of Ag/CsPbBr₃/ITO with Ag/perovskite as a rectifying Schottky junction was developed [35]. The device yielded a lower dark current (5 nA/cm²) and a relatively high sensitivity value of 770μC Gy_air⁻¹ cm⁻² upon irradiating with 333.69 nGys⁻¹ dose at 8 V. Further, a functional 4×4 X-ray detector array was also demonstrated to show the applicability for more complex sensor layouts.

4.1.2 Low-Dimensional Perovskites

It is evident from the discussions in the previous section that 3D MHPs encounter various challenges related to operational stability and sensitivity. The strategies implemented to obtain a stable and efficient detector performance would add complexity with adverse effects on the economics of manufacturing. Low-dimensional perovskites with intrinsic chemical and moisture stability could be potential alternatives to eliminate such complicated device processes. For example, solution-processed 1D inorganic halide perovskite CsPbI₃ crystals for X-ray detection show a maximum sensitivity of 2.37 mC Gy⁻¹ cm⁻² with a lowest detectable dose rate of 0.219μGy s⁻¹ (minimum signal used in a regular medical diagnostics is 5.5μGy s⁻¹). These values are far superior to the values obtained from their 3D counterparts [69]. Extremely low dark currents (pA), even at 200 V, resulting from an impressive μ_h/e×τ product and high bulk resistivity, are the preeminent performance characteristics of the device.

High-quality, thick, single crystals that can endure large applied voltages are frequently used to increase the perovskite layers’ resistivity under reverse bias to suppress leakage current. However, developing such single crystals over a large area is quite challenging. A solution-grown 2D Ruddlesden-Popper (RP) phase-layered perovskite film, (BA)₂(MA)₂Pb₃I₁₀ (PbI₃), as the X-ray sensing element was proposed to tackle this issue [36]. Evidently, the device shows various exciting features, such as low dark-current (10⁻⁹ A cm⁻² at zero bias) and low-voltage operation (self-powered devices). The detector configured in p-i-n (ITO/PTAA/(PbI₃)/C₆₀/gold) geometry demonstrated a 10–40-fold higher X-ray absorption coefficient than that of a Si detector (Fig. 5). An excellent X-ray sensitivity of 0.276 CGyair−1 cm⁻³ for 10 keV X-ray photons at zero bias was attributed to the detector’s low dark current. The hysteresis-free operation, short response time (1–10μs), and excellent stability are significant outcomes of the reported detectors. However, when it comes to large-area, flexible/conformable medical imaging X-ray detectors, it is a trade-off between the thickness, stopping power, and mechanical flexibility of a sensing element. Inkjet-printed triple-cation perovskites such as Cs_0.1(FA_0.83MA_0.17)_0.9Pb(Br_0.17I_0.83)₃ with 3.7μm thickness were found to provide the required mechanical flexibility without compromising the performance of the X-ray detector [37]. Along with remarkable characteristics, such as good sensitivity of 59.9μC Gy_air⁻¹ cm⁻² and low operating voltage (0.1 V), the detector also showed enhanced stability under X-ray illumination and accumulative exposure of 4 Gy_air for 1 h without the need of encapsulation. Notably, the sensor displayed outstanding mechanical flexibility even after 500 bending cycles (bending radius: 3 mm).

Fig. 5

(a) Schematic diagram of the 2D RP-based p-i-n thin-film X-ray detector architecture composed of (BA)₂(MA)₂Pb₃I₁₀ (named as Pb3) as an absorbing layer. (b) Signal-to-noise ratio of X-ray-induced charges for 2D RP and a silicon reference detector. Reprinted from [36] with the permission of American Association for the Advancement of Science

When a perovskite film is processed over a large area, developing a homogeneous layer with high X-ray sensitivity is critical. A printable form of 0D CsPbBr₃ quantum-dot formulations that were subsequently used to develop X-ray detectors (Fig. 6a) can promote an X-ray sensing element with desired features [27]. Well-controlled crystallinity of the QDs yielded films with a reduced concentration of surface defects (Fig. 6b). The photogenerated current was modulated from 0.1 to 0.36 nA by varying X-ray intensity ranging from 0.55 to 7.33 mGy_airs⁻¹ (Fig. 6c). Prominently, the detector could sense currents down to 9 pA corresponding to an incident X-ray intensity of 0.0172 mGy_airs⁻¹, with a fast response time of 28 ms (Fig. 6d). The influence of mechanical bending on the performance of the CsPbBr₃ QD-based X-ray detector was evaluated through 200 repeated bending cycles. A small change of only 12% confirmed the mechanical robustness of the detector. This study provided an outstanding demonstration of the durability, bendability, and stability of the printed perovskite X-ray detector technology.

Fig. 6

(a) Schematic of perovskite-based device fabrication procedure via inkjet printing. (b) Photograph of X-ray detector arrays on a 4-in. wafer (top) and TEM image of CsPbBr₃ QDs (bottom). Scale bar: 20 nm. (c) X-ray photocurrents and sensitivity as a function of dose rate with 0.1 V bias voltage. (d) Temporal response of the device under 7.33 mGy_airs⁻¹ dose rates with 0.1 V bias voltage. Reprinted from [27] with the permission of Wiley-VCH GmbH

The processing versatility of MHPs is further expanded by the melt processing method, which is simple, scalable, and cost effective. CsPbBr₃ films deposited onto glass substrates with this method pave the way to large-area, efficient, and low-cost X-ray detectors based on MHPs [38]. Precise control of the cooling rate of the perovskite layer from its melting temperature is the major driving parameter of this process. The subsequent CsPbBr₃ layers yield a specific resistance of 8.5×10⁹ Ω, which remains similar for films with a thickness ranging from 250μm to 1 mm. Moreover, the estimated sensitivity (1450μC Gy_air⁻¹ at 300 V) is analogous to conventional Cd(Zn)Te X-ray detectors and superior to α-Si X-ray detectors.

Further enhancement in sensitivity along with reversible, stable, and fast (5 ms) sensing behavior was reported for CH₃NH₃PbI₂Cl-based planar-inverted perovskite devices [39]. The demonstrated X-ray detector displayed 550% higher sensitivity than the α-Si reference detector. Along with the high sensitivity and large-area processability of MHPs, flexibility is also another attractive property for next-generation X-ray detectors, which strongly relies on the thickness of the sensing element. The integration of an MHP, MAPb(I_0.9Cl_0.1)₃, into a bendable porous nylon membrane is a promising approach to realize real-world flexible X-ray detectors [40]. The device array processed through this method shows the highest μ_h/e×τ product, outstanding sensitivity value of 8696 ± 228μCGy_air⁻¹ cm⁻², and excellent operational stability. The proposed high-performing and large-area flexible X-ray detectors offer a solution to some of the existing challenges in medical and industrial X-ray imaging and detection.

4.2 Single-Crystal-Based X-Ray Detectors

Lead halide single crystals are free of grain boundaries and have been demonstrated with lower defect density, better optoelectronic properties, and higher stability than the polycrystalline thin films. The X-ray sensing and harvesting abilities of a single-crystalline MAPI hybrid halide perovskite were first reported in 2015 [41]. The MAPI single crystals’ stopping power for X-rays was found to be superior to Si-based detectors. Only 110μm thick MAPI was found sufficient to stop soft X-rays (≈30 keV). In contrast, for detectors with Si, this value is about 1 mm. Absorption of X-ray photons in MHP single crystals occurs in the bulk of the perovskite, where a lower number of defect or trap states exist than those at its surface. Hence, stable and hysteresis-free characteristics were observed. The solution-grown MAPbBr₃ single crystals were found to exhibit low defect density, as shown in Fig. 7a. The surface traps on the crystal facets were passivated by UV-O₃ treatment, which resulted in a higher μ_h/e×τ product (1.4 × 10⁻² cm² V⁻¹) and thereby more efficient charge extraction characteristics [42]. A remarkable X-ray photon stopping power, which is higher than that of silicon, CdTe, and MAPbBr₃ detectors, was found (Fig. 7b). Additionally, surface engineering techniques were also invented and employed to reduce the surface trap density, thereby achieving high sensitivity and low dark current. A reduced trap density in thermally annealed (AZO)/MAPbBr₃(crystal)/Au detectors compared to non-annealed devices was found, and the effect was attributed to an efficient interface formation between the AZO anode and the perovskite [43]. The interaction of MA⁺ and Pb⁺² dangling bonds with O²⁻ ions was seen as a possible reason for achieving a low-leakage current (nA), thus enhancing sensitivity (529μC Gy_air⁻¹ cm⁻² at 50 Vcm⁻¹).

Fig. 7

(a) MAPbBr₃ single-crystal radiation detector structure. (b) Attenuation efficiency of CdTe, MAPbI₃, MAPbBr₃, MAPbCl₃, and silicon to 50 keV X-ray photons (in terms of the photoelectric effect) versus thickness. (c) Photograph of As-grown CsPbBr₃ crystals and detector configuration. (d) X-ray response sensitivities of the Al/CsPbBr₃/Au device. Figure 7a and b is reprinted from [42] with permission of Springer Nature, and Fig. 7c and d is reprinted from [45] with the permission of The Royal Society of Chemistry

The integration of perovskite crystals with Si-based readout electronics is a step to ensure efficient transport of X-ray-generated charges from the perovskite. A simple technique to monolithically integrate MAPbBr₃ single crystals onto Si substrates via an NH₃Br-terminating molecular interlayer resulted in tremendous figures of merit of X-ray detector (energy range: 8–50 keV) [44]. An outstanding sensitivity (2.1 × 10⁴μC Gy_air⁻¹ cm⁻² under 8 keV X-ray radiation), ≈1000 times higher than commercial a-Se detectors, is one of the remarkable features. Such high-sensitivity detectors are ideal for medical imaging applications.

Single crystals of MAPbBr₃ were also explored for energy-sensitive X-ray imaging. The sensitivity of the detector configured in p-i-n diode array was found to vary as a function of the photon energy. This phenomenon was ascribed to carriers being created at different depths of the crystal for dissimilar X-ray photon energies [44]. An increased dark current while applying a high bias voltage to improve the device sensitivity is one of the commonly encountered problems for single- and polycrystalline perovskites. MAPbBr₃ crystal X-ray detectors with a Schottky contact were developed that can withstand even high electric fields without increasing the dark current [46]. The X-ray detectors showed a reasonably fast response (76.2μs) and recovery (199.6μs) times, along with good sensitivity (359μC Gy⁻¹ cm⁻² for 50 keV at 200 V). Notably, the higher sensitivity and a threefold faster response were observed for Schottky contact-based detectors than the ohmic device. A similar approach has been employed for inorganic CsPbBr₃ single crystals with Al/CsPbBr₃/Au configuration, and high sensitivity of 1256μC Gy⁻¹ cm⁻² for 80 keV X-ray photons was reported (Fig. 7c). This is almost 60 times higher than for commercial α-Se detectors (Fig. 7d) [45]. Quasi-monocrystalline CsPbBr₃ prepared by the hot-pressed method [25] proved to be highly efficient with the peak sensitivity of 55,684μC Gy_air⁻¹ cm⁻².

Another critical characteristic of MHP-based direct X-ray detectors is their spatial configuration. In 2014 the first single-pixel detector comprising a polycrystalline MAPbI₃ that could record 2D X-ray images was introduced [28]. Even though the scanning time was long, the work inspired the scientific community to develop more X-ray detector arrays with MHPs. Soon after, the first linear detector array (LDA) based on MHP-sensing materials was demonstrated featuring 200μm large pixels [44]. The technology was advanced with the development of 2D arrays [30] that were able to deliver faster imaging with improved spatial resolution. Notably, the 2D imaging arrays were fabricated using similar deposition techniques that are used for printable photovoltaics. Since MHPs allow low-temperature processing, their deposition can directly be carried out onto temperature-sensitive readout electronics, further simplifying the overall manufacturing of the detector arrays.

4.3 Lead-Free Perovskites for X-Ray Detectors

It is clear from the preceding sections that lead perovskites have been assiduously studied, as they have established superior properties and proven to be highly promising for various optoelectronic applications. Despite their fascinating properties and astonishing performances, the cytotoxicity of lead and degradation of lead-based MHP devices in the ambient atmosphere have become major obstacles towards their practical use in direct X-ray detectors [47, 48]. The substitution of monovalent Pb by another metal ion with a similar ionic radius, i.e., tin (Sn), can lead to even better optoelectronic properties than for Pb-based materials, such as longer carrier diffusion length, low exciton binding energy, and narrow bandgap. Another approach is to introduce trivalent metals like bismuth (Bi) and antimony (Sb), which are stable in ambient and inherently more environmentally friendly while maintaining a similar electronic structure as that of perovskites with Pb. For example, inorganic Cs₂AgBiBr₆ perovskite single-crystal X-ray detectors show four orders of magnitude higher sensitivity (20–105μC Gy_air⁻¹ cm⁻²) than commercial a-Se detectors [49, 50]. Along with excellent ambient stability, the detectors also demonstrated high operational stability with the detection limit of 59.7 nGy_airs⁻¹ along with a low dark current (≈9.55 × 10⁻¹⁶ A Hz^−1/2). Post-deposition treatments, such as thermal annealing and rinsing of Cs₂AgBiBr₆ crystals with isopropanol, reduced the number of shallow traps and surface defects [51]. Additionally, the suppression of field-driven ion migration was observed, which helped reduce the vitally important leakage current. With enhanced operational stability, a good sensitivity of 316μGyair⁻¹ cm⁻² resulted from these posttreatments.

Another Pb-free single crystal with exceptional optoelectronic properties is Cs₃Bi₂I₉. A higher X-ray absorption coefficient (compared to inorganic commercial systems, e.g., CdTe and CsI crystals) and the high responsivity make Cs₃Bi₂I₉ single crystals a perfect candidate for X-ray detectors [52]. Some of its characteristic features include a low trap density of 1.4 × 10¹⁰ cm⁻³ (significantly lower than commercial inorganic materials (10¹⁵–10¹⁶ cm⁻³) [53, 54]), the high electrical resistivity of 2.79 × 10¹⁰ Ω cm (higher than any Pb-MHPs), and an excellent thermal (up to 550 °C) as well as moisture stability. The X-ray detectors comprised of Au/Cs₃Bi₂I₉/Au achieved a sensitivity value of 1652.3μC Gy_air⁻¹ cm⁻² and a minimum detection dose of 130 nGy_airs⁻¹. Another material that could open up an opportunity for broadband detection is AgBi₂I₇, with an absorption coefficient for a broad range of X-ray photons, from 0.001 MeV to 10 MeV [55]. The ability of a 0.5 mm thick AgBi₂I₇ crystal to stop 100 keV X-rays demonstrates its great potential. The combination of robust operation and low dark current highlights the advantage of these crystals in next-generation direct X-ray detectors. See also Fig. 3b to compare the sensitivity and mobility-lifetime product of recent Pb-containing and Pb-free MHP-based X-ray detectors.

4.4 Scintillators for X-Ray Detection

Most of the commercial large-area X-ray detectors rely on the use of scintillating elements . The primary object of a scintillator is to convert ionizing radiation into high-efficiency visible photons. As conventional inorganic scintillators’ fabrication process involves high temperatures (up to 1850 °C), recent efforts have focused on developing scintillators that can be fabricated at reduced temperatures, thus facilitating the technological requirements for large-area production. Even though solution-processable MHPs have tremendous potential for direct radiation detection, they displayed lower responsivities at photon energies above 10 keV. This issue can be addressed to some extent by increasing the thickness of the sensing material and by further engineering the perovskite structure and composition. Yet, there still exist several technical challenges. This is the area where scintillators would play a dominant role.

The high conversion efficiency of 49% and a decay time as short as 0.7 ns are the best-reported characteristics of perovskite scintillators for X-rays superior to established inorganic systems [56, 57]. Even so, with the versatile chemistry and structural tunability of MHPs, further improvement is quite possible. A MAPbBr₃-based X-ray scintillator that works in the temperature range of 50–130 K with LY of 90,000 photons/MeV and 1 ns response time is one example [58]. Ultrasensitive X-ray detectors and flexible, large-area X-ray imaging were further enabled by introducing CsPbBr₃ nanocrystals into scintillators [57] (Fig. 8a). Color tunability is an additional advantage of this new class of scintillators (Fig. 8b), including low toxicity, solution synthesis at near-room temperatures, a high emission quantum yield, and fast scintillation response as some of their outstanding attributes. This was followed by exploring CsPbBr₃ perovskite nanocrystal-based X-ray detectors, which are cost effective and readily commercialize [59]. Key figures of merit for this study include a high PLQY of 95% at 550 nm, a short PL decay time of about 2.87 ns, a high spatial resolution (9.8 lp mm⁻¹), and high conversion efficiencies (Fig. 8c and d). Even though the origin of nanocrystal scintillation is still not clear, this approach may hold substantial promise for progressing the X-ray sensing and imaging industry.

Fig. 8

(a) Representation of X-ray-induced luminescence of energy hν (where h is the Planck constant and ν is the frequency). (b) Tunable luminescence spectra of the perovskite QDs under X-ray illumination with a dose rate of 278μGy s⁻¹ at a voltage of 50 kV. (c) X-ray absorption and X-ray absorption efficiency of CsPbBr₃ and conventional GOS scintillators, (d) light output power density and conversion efficiency of CsPbBr₃ PNCs and conventional GOS scintillators. Figure 8a and b is reprinted from [57] with the permission of Springer Nature and Fig. 8c and d is reprinted from [59] with the permission of Wiley-VCH GmbH

For the health and safety concerns of the end user, reducing X-ray dose rates by lowering the detector’s detection limit is vital. A solution-processed MAPbCl₃ single-crystal scintillator was developed to achieve low detection limits [46]. A detection limit of 114.7 nGys⁻¹ at 50 keV X-rays, which was comparable to commercial scintillators based on NaI and CsI [60], was attained.

An attractive route for ultrafast and highly efficient scintillation , also known as “quantum scintillation,” is pursued by integrating low-dimensional perovskites into scintillators, achieving fast light emission due to quantum confinement effects [61]. Scintillators with 2D perovskites (n-C₆H₁₃NH₃)₂PbI₄ that consist of multiple natural quantum well structures demonstrated short decay times of 0.7 ns. Along with the fast response, the visible light (558 nm) emission of 2D scintillators is an additional benefit over other technologies that emit UV light. The large exciton binding energies of 2D perovskites can suppress the detected optical signal losses due to thermal quenching. For example, scintillators with the 2D material (EDBE)PbCl₄ showed reduced thermal effects compared to 3D perovskites and an adequate light yield of 9000 photons/MeV even at room temperature [56]. On the other hand, an environmentally friendly 2D perovskite (C₈H₁₇NH₃)₂SnBr₄ has proven to be efficient, with an excellent absolute PLQY of up to 98% [62]. More importantly, the attained sensitivity was in line with medical safety standards with the emission threshold at 104.23μGys⁻¹. From the discussions so far, it is confirmed that MHPs are very promising scintillator materials in terms of low fabrication costs, nanosecond fast response, low intrinsic trap density, and potentially high light yield.

5 Advance in the Development of Perovskite γ-Ray Detectors

In 2016, the first report appeared on the application of MHPs for the direct detection of γ-rays [63]. Upon exposure with γ-rays (0.96 MeV) from different radioactive sources (¹¹C and ¹³⁷Cs), the 3D perovskite (MAPbI₃, FAPbI₃, and MAPbBr) single-crystal devices revealed an overall current to charge efficiency of 19% (Fig. 9a and b). Furthermore, the potential for single γ-photon counting, which is a highly challenging task, was demonstrated. The requirement to develop MHP-based high-resolution energy spectra γ-ray detectors is a high μ_h/e×τ×E product (E: applied external electric field). However, increasing the electrical field enhances both the dark current/noise ratio and ion migration within the perovskite. This issue was addressed by raising the bulk resistivity of the single crystal (3.6 × 10⁹ Ω-cm) by using large-size dopant-compensated CH₃NH₃PbBr_2.94Cl_0.06 crystals that successfully demonstrated a low operation field (1.8 V mm⁻¹) [23]. The crystals showed high hole and electron mobilities of 560 and 320 cm²V⁻¹ s⁻¹, respectively, resulting in a significantly higher μ_h/e×τ product.

Fig. 9

(a) The attenuation coefficient and corresponding penetration depth of MAPbI₃ and CdTe as a function of photon energy, from soft X-rays to γ radiation. (b) The bias dependence of the photocurrent generated by Cu Kα X-ray radiation (8 keV) in a SC of MAPbI₃ perovskite; the red line indicates a fit with the Hecht model showing a high μτ product of ∼10⁻² cm² V⁻¹. Top inset: Photograph of typical MAPbI₃ perovskite SCs grown from a nonaqueous (retrograde solubility) method, placed on a millimeter ruler. Bottom inset: Schematic of the three-dimensional interconnection of PbI₆-octahedra in a perovskite lattice (green, Pb; yellow, I; blue, MA). (c) p-i-n device structure using MAPbBr_3−xCl_x crystal. (d) Gamma-ray spectra exposed the device under various radioactive sources collected by a multichannel analyzer (MCA) when the detector was operated at −8 V at room temperature. Figure 9a and b is reprinted from [63] with the permission of Springer Nature, and Fig. 9c and d is reprinted from [66] with the permission of Elsevier

Apart from the performance matrix, the structural stability of MHP single crystals is one of the major pitfalls, in which the phase transformation from cubic to the hexagonal structure within 24 h after the crystal growth was observed [64]. To address this issue, mixed halide perovskite single crystals, Cs_xFA_1−xPbI_3−yBr_y (x = 0–0.1, y = 0–0.6), with various thicknesses ranging from 0.2 to 15 mm, were fabricated to detect γ-ray within the energy range of 0.02–1 MeV [65]. Evidently, the resulting crystals unveiled good stability up to 20 days, which could be extended further to 60 days by substituting Br in place of I. Despite the encouraging progress, there exist uneven energy detection spectra for MHPs. For example, CsPbBr₃ can detect within the 32.3–662 keV energy range while MAPbBr_3-xCl_x detects only high-energy γ-photons (0.1–10 MeV) [23]. A unipolar p-i-p device architecture with MAPbBr_3-xCl_x γ-ray detector was proposed to resolve this issue (Fig. 9c) [66]. It was found that the use of high-work-function contacts effectively blocks the dark noise originating from thermally activated electron injection from the impurities. Thus, efficient pulse collection could be prompted even at higher electrical fields. Consequently, strong electrical pulses were observed when exposing the detector with different energies of gamma-ray photons emitted from various radioactive sources (Fig. 9d).

5.1 Scintillator Detectors for γ-Ray Detection

γ-Ray scintillator detectors are a workhorse for γ-ray spectroscopy that provide high efficiency and, depending on the choice of the scintillator, good energy, and timing resolution. Inorganic γ-ray scintillating materials such as GSO:Ce and LSO:Ge are the most explored systems but often suffer from long decay times (≈40 ns). Hybrid perovskite materials that offer short decay times can be suitable alternatives [4, 66, 67]. However, these are generally unstable with a low material density (1 g/cm³). Hence, the development of materials that address these difficulties is an immediate requirement for further advancing the various imaging technologies. Hybrid 2D perovskite compound ((C₆H₅C₂H₄NH₃)₂PbX₄)-based scintillators with high LY (14,000 photons/MeV), excellent linearity to γ-rays with different energies (122–662 keV), and short decay time (≈11 ns) are showcasing the potential of MHPs as γ-ray scintillators [68], where the natural quantum wells of 2D perovskite promote improved detection characteristics.

6 Summary and Future Perspective

The rise of MHPs as a promising family of materials for application in next-generation X-ray and γ-ray detector technologies has been witnessed from recent reports. Remarkably, the field has skyrocketed since 2017, as proven by the volume of publications and related citations received to date. This global interest stems from the fascinating physical properties that these synthetic perovskites have, which, when combined with unconventional device engineering, can produce detectors with performance features on par with or even better than those of incumbent technologies.

MHPs have been utilized in two types of high-energy radiation detectors, namely direct and scintillator detector technologies. In terms of manufacturability, the simpler one is the direct detector. In contrast, detectors with scintillation systems are bulkier and less portable. Yet, the use of scintillation detectors can address specific difficulties that direct X-ray detectors encountered, so significant ongoing research efforts aim to improve scintillation systems further. Consequently, MHP scintillators have demonstrated the potential to address numerous inadequacies, including manufacturability, sensitivity, response time, and spatial resolution, to the degree that multiple materials are quickly becoming competitive to commercial technologies such as NaI and CsI.

MHPs have also unveiled tremendous potential for direct X-ray detection. The excellent performance of numerous perovskite-based devices reported to date is mainly attributed to the outstanding μ_h/e×τ product (1.1 × 10⁻² cm²V⁻¹), fast response times, long carrier diffusion lengths (up to 175μm), high conversion efficiencies, high sensitivity, emission in the visible region of the electromagnetic spectrum, quantum confinement (low-dimensional perovskites), and characteristically low trap density of states (10⁸ cm⁻³) even in solution-processed polycrystalline systems. Despite the advantages demonstrated by the perovskite-based high-energy radiation detectors so far, there are still many challenges to be considered before translating this technology to commercialization. Primarily, in direct X-ray detectors, the resistivity of the active layers (films, crystals, etc.) needs to be amplified further in order to decrease the dark current. Larger bias voltages can be applied by using thicker active layers that could improve the charge collection efficiency. But this process induces adverse effects on electrical noise (dark current) and operational stability due to the field-induced ion migration—this is particularly true for polycrystalline films. To overcome these bottlenecks, possible approaches include the application of larger bandgap perovskites and/or the use of higher quality single crystals.

The second challenge is to boost the perovskite X-ray detector’s chemical stability towards ambient conditions. While simple device encapsulation could provide a concrete solution for commercial applications, the use of all-inorganic perovskites also shows promise to address this challenge. On the other hand, the simultaneous improvement of LY through enhanced quantum confinement and operational/environmental stability could be attained by developing innovative low-dimension perovskites (e.g., 0D, 1D, 2D). It is clear from the recent reports that the incorporation of such low-dimension perovskite crystals diminishes the ion migration, hence allowing the application of higher bias across the crystals and improving the operational stability of the devices. Increasing the μ_h/e×τ product via engineering the perovskite layer/crystal is another challenging part. The use of indirect bandgap perovskites that display extended carrier lifetimes could be exploited as a potential solution. In sandwich-type direct X-ray detectors, incorporating conducting materials, such as graphene or other 2D materials, in the charge transporting layer could improve the conductivity and increase the charge collection efficiency. The next challenge that needs to be addressed is Pb’s toxicity in most of the MHPs studied to date. To this end, the exchange of Pb with other high-Z elements that are vital for obtaining high X-ray stopping power has already yielded promising results but with plenty of room for further advancement.

Perovskite-based γ-ray detectors share similar challenges as X-ray detectors. Improving the homogeneity and overall quality of the absorbing material will resolve some of these outstanding issues. Despite other existing hurdles, however, the future of perovskite-based X-ray and γ-ray detectors appears very bright. Only time will tell whether MHP-based high-energy radiation detectors will ultimately make it to the commercial stage in fields ranging from medicine and homeland security to portable radiological identification and energy-harvesting devices for space applications.