Sb₂Se₃ film with grain size over 10 μm toward X-ray detection

Abstract

Direct X-ray detectors are considered as competitive next-generation X-ray detectors because of their high spatial resolution, high sensitivity, and simple device configuration. However, their potential is largely limited by the imperfections of traditional materials, such as the low crystallization temperature of α-Se and the low atomic numbers of α-Si and α-Se. Here, we report the Sb₂Se₃ X-ray thin-film detector with a p–n junction structure, which exhibited a sensitivity of 106.3 µC/(Gy_air·cm²) and response time of <2.5 ms. This decent performance and the various advantages of Sb₂Se₃, such as the average atomic number of 40.8 and μτ product (μ is the mobility, and τ is the carrier lifetime) of 1.29 × 10⁻⁵ cm²/V, indicate its potential for application in X-ray detection.

Key words： X-ray detector; Sb₂Se₃; p–n junction; response speed; grain size

1 Introduction

X-ray detection plays an irreplaceable role in various fields, such as medical radiography, materials characterization, and non-destructive testing of chips and constructions [1,2]. There are two main detection approaches: indirect detection and direct detection [3,4]. In the indirect system, scintillators are utilized to convert high-energy X-ray photons into low-energy visible photons that can be detected by photodetectors [5,6]. Conversely, in the direct system, X-ray photons are directly converted into electrical signals by absorber materials, which is beneficial for high sensitivity and good spatial resolution [7,8]. The commonly used amorphous Se-based direct X-ray detectors are restricted by their low atomic numbers (Z = 34), low carrier mobility, and short carrier lifetime [9,10]. Recently, metal halide perovskites were reported as potential direct X-ray detectors, considering the incorporation of high-Z elements (Pb, Bi, etc.) and long carrier lifetime [4,6]. However, the serious ionic migration under an electric field and the toxicity of lead still impede their practical applications [6,11,12].

Antimony selenide (Sb₂Se₃), a non-toxic and stable V–VI group semiconductor with the band gap (E_g) of ~1.1 eV, has a low melting point (~610°C) and a high vapor pressure (3.48 × 10³ Pa at 600°C), which enables the convenient deposition of large-area and uniform films [13,14]. Sb (Z = 51) has a larger atomic number than Se (Z = 34), which is beneficial for the absorption of X-rays. The low band gap also results in a low generation threshold of electron–hole pairs (W = E_g + 1.43) [3]. Besides the advantages in the carrier generation process, the decent μτ product (μ is the mobility, and τ is the carrier lifetime) of Sb₂Se₃ was determined to be ~10⁻⁵ cm²/V. In addition, its one-dimensional structure can suppress the crosstalk between adjacent pixels to strengthen the advantage of direct system in resolution [8]. All these features portend Sb₂Se₃ as an alternative absorber material for application in X-ray detectors.

Hence, in this study, we fabricated a Sb₂Se₃ thin-film X-ray detector. The Sb₂Se₃ film was deposited by the close-spaced sublimation (CSS) method, resulting in a high-quality film with a large grain size of >10 µm. Further, the Sb₂Se₃/TiO₂ p–n junction structure was introduced into the device to efficiently suppress the dark current and achieve a high sensitivity of 106.3 µC/(Gy_air·cm²) under an applied bias of −1 V. Because of the decent μτ product and thin absorber layer, in addition to the built-in electric field in the p–n junction, our Sb₂Se₃ device showed a 3 dB cutoff frequency of >400 Hz, which is higher than the normal requirement of 30 Hz in dynamic digital radiography [15,16]. We believe that our work would pave the way for the potential applications of Sb₂Se₃ in X-ray detection.

2 Experimental details

2.1 Device fabrication

The TiO₂ film was deposited on a cleaned fluorine-doped tin oxide (FTO) conductive glass by the spray pyrolysis method following procedures in Refs. [17,18]. The annealing temperature was set to 550°C. Subsequently, the Sb₂Se₃ layer was deposited in a homemade CSS system, modified from a rapid thermal processing instrument (OTF-1200, Hefei Kejing Materials Technology Co., Ltd.). A cleaned white glass (5 cm × 5 cm) uniformly covered with 0.4 g of Sb₂Se₃ powder (99.999%, Jiangxi Ketai Advanced Materials Co., Ltd.) was placed on an aluminum nitride ceramic plate as the source. The FTO/TiO₂ was face-down mounted above the powder as the substrate and was covered by a graphite block (10 cm × 6 cm × 1 cm) to maintain the temperature. The whole deposition process was conducted under a vacuum of ~5 × 10⁻³ Torr. First, the temperature of the substrate was increased to 400°C in 60 s and maintained for 15 min to ensure a stable high substrate temperature. Thereafter, the temperature of the source was increased to 600°C in 60 s, and the evaporation only continued for 30 s. Finally, the sample was taken out after cooling to room temperature and was subsequently thermally evaporated with an Au electrode (area: 0.09 cm²).

2.2 Growth of Sb₂Se₃ single crystal

The melt growth of the Sb₂Se₃ single crystal was based on a reported method [19]. The internal diameter of our ampoule was 8 mm, and the furnace was a two-zone vertical Bridgman furnace (OTF-1200X-S-VT-BMGH, Hefei Kejing Materials Technology Co., Ltd.). The peak temperature was set to 624.7°C to guarantee the sufficient melting of Sb₂Se₃. The temperature gradient was ~5 °C/cm, and the ampoule moving speed was 117 nm/s. After approximately 7 days, we obtained a Sb₂Se₃ single crystal in the ampoule. The crystal was cut into ~1-mm-thick slices and subsequently evaporated with an Au electrode on both sides after polishing with a 7000-M sandpaper. The thickness of the crystal slice used in the test was measured as 900 µm, and the Au electrode area was 0.04 cm². All the photos are shown in Fig. S1.

2.3 Measurement and characterization

X-ray diffraction (XRD) patterns were measured using an X-ray diffractometer with Cu–Kα radiation (Empyrean, PANalytical B.V.). The surface morphology and cross section of the devices were analyzed by scanning electron microscopy (SEM, FEI Nova NanoSEM450). We used an Au anode X-ray tube (Newton Scientific M237) as the source with a maximum output of 10 W and a X-ray focal spot size of 5 µm. The X-ray source was operated with a constant acceleration voltage of 50 kV. The dose rate was modulated by changing the X-ray tube current and was calibrated with a Radcal ion chamber (model: 10 × 6–180) dosimeter. An electrometer (Keithley 2635 Source Meter) was employed to measure the current–voltage (I–V) curve and the X-ray response current. For the 3dB cutoff frequency (f_{3 dB}) measurement, a bias of −4 V was applied to the device using a low-noise current amplifier (SR570), and the output was connected to the oscilloscope (Keysight DSOS054A). SR570 worked in the high bandwidth mode without requiring any filter. For the X-ray response, an optical chopper with lead plates was used to cut off the X-ray source and provide periodic X-ray exposure. To measure the μτ product, a 532-nm laser was used, and the response was measured by an Agilent B1500A analyzer. All measurements were carried out at room temperature.

3 Results and discussion

The performance of a direct X-ray detector is mainly determined by two processes: X-ray absorption and carrier transportation. X-ray photons have considerably higher energy than visible light photons (more than 10000 times higher); consequently, they mainly interact with inner-shell electrons, whereas the visible photons interact with valance electrons [2,4]. Thus, the X-ray absorption coefficient (α) is highly related to the number of inner electrons and can be approximately defined by the formula, α∝ρ Z⁴/E³ (where ρ is the mass density, Z is the average atomic number, and E is the energy of X-ray photons) [3]. For Sb₂Se₃, the average atomic number is 40.8, and the density is 5.84 g/cm³, which are both larger than those of α-Si (Z = 14, ρ = 2.28 g/cm³) [20,21] and α-Se (Z = 34, ρ = 4.26 g/cm³) [22–24], two main commercial flat-panel X-ray detector materials. The absorption coefficients of these three materials in the radiation energy range of 0.01–100 MeV (Fig. 1(a)) were calculated according to the photon cross section database [25]. Apart from the small region around 0.02 MeV, which is attributed to the resonant absorption at the K-edge of Se atoms, the absorption coefficient of Sb₂Se₃ is higher than that of the other two materials across the range. Figure 1(b) shows the attenuation ratio of these three materials with different film thicknesses under 30 keV X-ray photons, which is the peak of our X-ray source spectrum. Undoubtedly, Sb₂Se₃ films of the same thickness can absorb more X-ray irradiation, because the corresponding absorption coefficient of Sb₂Se₃ is the highest among the three materials.

For the carrier transport process, the μτ product is the key parameter that directly affects the collection of photo-generated carriers. To obtain this parameter, we produced an Sb₂Se₃ single crystal, which is preferred for μτ product measurement [26,27], and cut it into 900-µm-thick slices. Thereafter, we derived the μτ product employing the following modified Hecht equation to fit the photoconductivity, which is widely used in single-crystalline radiation detectors [8,28]:

(1)I={\displaystyle \frac{I_{\mathrm{0}}\mu \tau V}{d^{\mathrm{2}}}}{\displaystyle \frac{\mathrm{1}-\exp (-{\displaystyle \frac{d^{\mathrm{2}}}{\mu \tau V}})}{\mathrm{1}+{\displaystyle \frac{d}{V}}{\displaystyle \frac{s}{\mu }}}},

where I₀ is the saturated photocurrent, V is the bias voltage, and d is the thickness. As shown in Fig. 1(c), the μτ product value of Sb₂Se₃ is calculated as 1.29 × 10⁻⁵ cm²/V.

Fig.1 (a) Absorption coefficients of Sb₂Se₃, α-Se, and α-Si at an X-ray photon energy range of 0.01–100 MeV. (b) Absorption efficiencies of Sb₂Se₃, α-Se, and α-Si toward 30 keV X-ray photons, as a function of the film thickness. (c) Bias-dependent photocurrent of the Sb₂Se₃ single crystal slice under 532-nm laser light

Full size|PPT slide

Beyond that, the Sb₂Se₃ film is composed of one-dimensional (Sb₄Se₆)_n chains [29]. In this structure, it has strong covalent bonds along the [001] axis, whereas only weak Van der Waals interactions are present in the [100] and [010] axes. Thus, the carrier mobility along [001] has been reported to be 2–4 times larger than that along the [100] or [010] axes [30], which leads to the anisotropic current ratio of ~2.5 measured on an Sb₂Se₃ nanosheet [31]. When the film is [hk1]-oriented (inclined to the substrate) or even [001]-oriented (vertical to the substrate), the crosstalk between adjacent pixels can be effectively reduced, leading to a high spatial resolution.

To build a prototype X-ray detector, we subsequently attempted to deposit a Sb₂Se₃ film because it is easier to fabricate a large-area flat-panel detector than a single crystal. For one, the grain of the Sb₂Se₃ film is expected to be adequately large so that its quality is similar to that of the single crystal. For another, the Sb₂Se₃ film should have a [hk1]-dominant orientation to improve the carrier transportation efficiency and reduce crosstalk. However, the deposition of this optimal Sb₂Se₃ film is significantly challenging. The one-dimensional structure facilitates growth along the chains, as the Sb–Se bond energy along the chains is considerably larger than the Van der Waals force between the chains [32]. Thus, when the film has an [hk1]-dominant orientation, it usually shows rod-like or sheet-like grains rather than compact large grains [33,34]. These rod-like or sheet-like films have roughness of hundreds of nanometers and are unsuitable for application in detectors. As far as we know, the largest grain size of smooth and compact Sb₂Se₃ films ever reported is ~1–2 µm [32,35–37].

Fig.2 (a) Schematic illustration of the homemade close-spaced sublimation (CSS) system. (b) Device architecture of the Sb₂Se₃ heterojunction thin-film X-ray detector

Full size|PPT slide

To overcome this challenge, the CSS method (Fig. 2(a)) was chosen, because the unique structural design of the CSS equipment can promote a high deposition rate, which is essential for increasing the size of [hk1]-oriented grains [32]. First, the equipment has two independent pairs of heaters and thermal couples for controlling the temperatures of the substrate and source, respectively. Therefore, we can achieve a large temperature difference between the source and substrate, which would result in a high deposition rate. Second, the short distance between the source and substrate can effectively prevent the diffusion of Sb₂Se₃ vapor and further improve the deposition rate. In addition, the nucleation site density has been proven to significantly influence the Sb₂Se₃ grain size [38]. Thus, we chose the TiO₂ film as the substrate, on which Phillips et al. obtained a Sb₂Se₃ film with a 2-µm grain size by the CSS method [35]. Our device structure was designed as illustrated in Fig. 2(b). To further enlarge the grain, we increased the annealing temperature of the TiO₂ layer from 450°C to 550°C. The layer annealed at 550°C was more inert and was newly reported to support the fabrication of Sb₂S₃ films with a large grain size of 10 µm [18], which has a similar one-dimensional structure with that of Sb₂Se₃. The results also revealed that the TiO₂ layer annealed at 550°C had a higher crystallinity and smoother surface than that at 450°C (Fig. S2).

Fig.3 Film deposition and characterization. (a) Top-down and (b) cross-sectional scanning electron microscopy (SEM) images of the large grain Sb₂Se₃ film on the TiO₂/FTO substrate (substrate temperature: 400°C, source temperature: 600°C). (c) X-ray diffraction (XRD) pattern of the large grain Sb₂Se₃ film and the corresponding Sb₂Se₃ standard powder diffraction pattern (JCPDS 15-0861). The (hk1) planes inclined to the substrate are signaled in red, whereas the black labels show the parallel (hk0) planes. Top-down SEM images of the Sb₂Se₃ film with source temperatures of (d) 590°C and (e) 610°C (substrate temperature: 400°C)

Full size|PPT slide

During the film deposition process, the substrate temperature was set to 400°C to further decrease the nuclei density. This high substrate temperature can also supply enough energy to promote the bonding process between the substrate and the Sb₂Se₃ chains, which can enhance the formation of [hk1]-oriented nuclei. Subsequently, by setting the source temperature to 600°C to ensure a sufficient deposition rate, we successfully obtained the Sb₂Se₃ film with a grain size of 10–20 µm. As shown in Fig. 3(a), the grains are compact and without pinholes. Further, they are more than ten times larger than the normal Sb₂Se₃ grains. From the cross-sectional SEM image (Fig. 3(b)), it can be observed that the thickness of the film reaches 5–6 µm. There is no discernible grain boundary within the large grains, which indicates that they are not composed of small grains but indeed single large grains. In addition, the corresponding XRD pattern (Fig. 3(c)) supports that the (hk1) peaks are dominant. The three strongest peaks are in order of (141), (061), and (041), which agrees well with the reported common orientation of Sb₂Se₃ films deposited at high substrate temperatures [32].

As a reference, under the same condition, we fabricated a Sb₂Se₃ film on a TiO₂ layer annealed at 450°C. The film exhibited a sheet-like morphology (Fig. S3) due to the relatively high nuclei density, attributed to the high number of nucleation sites. A similar sheet-like morphology was also obtained when the substrate temperature was decreased to 380°C (Fig. S4), which is also due to the relatively high nuclei density. Conversely, when we slightly changed the source temperature, the film morphology changed significantly. The film deposited at 590°C has noticeably small grains and is discontinuous (Fig. 3(d)). The low source temperature cannot supply sufficient Sb₂Se₃ vapor to complete the re-evaporation of the Sb₂Se₃ film at a high substrate temperature. On the contrary, as Fig. 3(e) shows, the relatively high source temperature (610°C) results in large grains co-existing with many small grains, because the excessive supply of Sb₂Se₃ vapor facilitates the formation of new nuclei. These results highlight the requirements for obtaining an ideal Sb₂Se₃ film for X-ray detection: (a) an inert substrate, (b) a high substrate temperature, and (c) an appropriate source temperature.

Fig.4 X-ray detection performance. (a) Current–voltage (I–V) curve of devices with the structure of Au/Sb₂Se₃/TiO₂/FTO/glass. The anode was connected to the Au side. (b) Device response to X-rays (dose rate: 5.499 µGy_air/s) under a bias of −1 V. (c) Photocurrent of the detector under different dose rates (bias: −1 V). (d) X-ray sensitivity of the detector under different biases (dose rate: 5.499 µGy_air/s). (e) Calculated depletion region width in the Sb₂Se₃/TiO₂ junction versus the bias voltage. (f) Normalized responsivity of the device varies with the input X-ray frequency (bias: −4 V)

Full size|PPT slide

On the basis of the optimized Sb₂Se₃ film, we built a direct X-ray detector and measured its performance. As the I–V curve shows (see Fig. 4(a)), the dark current density is over 10 mA/cm² under a positive bias, whereas under a negative bias, the current density significantly drops to ~10⁻³ mA/cm². The high rectification ratio of ~10000 indicates the good p–n junction quality of our device. The current in the negative bias range is almost a horizontal line, which suggests that there is almost no leaking channel in our device. This provides a stable working state for X-ray detection. As shown in Fig. 4(b), the dark current was ~91.2 nA when the device was operating under −1 V bias. After the X-ray was turned on with a dose rate of 5.499 mGy_air/s, the device obtained a photocurrent of ~143.8 nA. The corresponding sensitivity was calculated to be 106.3 µC/(Gy_air·cm²), which is more than five times that of α-Se X-ray detectors (22 µC/(Gy_air·cm²)) [39]. The stability and radiation hardness of our device were also tested (Fig. S6). After storing in air for more than half a year or exposing to nearly 900 Gy_air X-ray radiation, the dark current and sensitivity of our devices exhibited almost no change.

By adjusting the operating current of the X-ray tube, the response for different X-ray dose rates was also investigated under the same −1 V bias. The results are shown in Fig. 4(c), which suggest that our device has significant linearity in the X-ray dose rate range of 0.5–5.5 mGy_air/s. Furthermore, we tested the sensitivity of the device under different biases ranging from 0 to −10 V. As shown in Fig. 4(d), with the enhancement of the bias, the sensitivity of our device gradually increased and finally reached ~240 µC/(Gy_air·cm²) under −9 V bias, which is 12 times that of α-Se detectors.

Compared with photoconductive detectors, our detector has an internal depletion region, in which carriers are depleted under the built-in electric field. When X-ray irradiation occurs, the electron–hole pairs generated therein are quickly separated and drifted by the built-in electric field. Thus, the carrier recombination can be remarkably decreased, which implies that more photo-generated carriers can be collected by the electrodes. In addition, under the reverse bias, the built-in electric field is further strengthened, and the width of the depletion region expands. Therefore, the stronger the applied reverse bias, the higher the carrier collection efficiency and detection sensitivity of our devices.

To demonstrate the effect of the depletion region, we calculated the depletion region width (X_D) of the device under different biases (V). A common model of an abrupt junction was assumed, in which TiO₂ was considered as heavily doped; thus, the depletion region was all in the Sb₂Se₃ side. The equation is as follows [40]:

(2)X_{\mathrm{D}}\approx {\chi }_{\mathrm{p}}=\sqrt{{\displaystyle \frac{\mathrm{2}{\epsilon }_r{\epsilon }_{\mathrm{0}}(V_{\mathrm{D}}-V)}{q{N}_{\mathrm{A}}}}},

where c_p is the depletion region width in the p-type side; e₀ is the vacuum permittivity, which equals 8.85 × 10⁻¹⁴ F/cm; e_r is the relative permittivity of the p-type material, and here, we use 19 for Sb₂Se₃ [30]; V_D is the contact electric potential difference, and here, we use the reported middle value (0.5 V) for Sb₂Se₃ [41,42]; q is the charge of one electron, which equals 1.6 × 10⁻¹⁴ C; N_A is the acceptor dopant concentration of the p-type material, and here, we use 10¹³ cm⁻³ for Sb₂Se₃ [30]. As Fig. 4(e) shows, with an increase in the applied bias, the depletion region width, and device sensitivity exhibit similar trends, which fully illustrate the importance of the designed p–n junction structure for sensitivity enhancement.

In addition, the p–n junction structure has a significant effect on the response speed of our device. As mentioned above, after applying the reverse bias, the depletion region widens, but the total transportation distance of the photo-generated carriers remains fixed. Thus, the proportion of drift motion under the built-in electric field increases. This can shorten the carrier transit time, because the carrier drift speed in the depletion region is much higher than the diffusion speed in the neutral region. More importantly, because of the high drift speed provided by the built-in electric field, carriers have a low probability to be caught in traps. Thus, the relaxation time of both the rising and falling edges in the response curve can be greatly reduced, resulting in an increased response speed, even with the same trap concentration. We attempted to measure the response speed of our device by varying the frequency of the X-ray signal, which was controlled using a modified chopper with lead chips. However, because of the speed limit of our chopper, the highest X-ray frequency we could achieve was 400 Hz. As shown in Fig. 4(f), at this frequency, the normalized responsivity of our device was 0.775, which is still higher than the 3 dB line (~0.707). Therefore, we conclude that the 3 dB cutoff frequency of our device is higher than 400 Hz, and the corresponding response time is<2.5 ms.

Tab.1 Comparison of several film-based direct X-ray detectors

material	thickness/μm	voltage/V	X-ray energy/keV	sensitivity/(μC·Gyair−1·cm−2)	response speed/ms	Ref.
CsPbBr3	240	1.2	50	55684	92	[43]
MAPbI3	830	200	100	11000	~10	[1]
CdZnTe	300	120	80	1440	NA	[44]
a-Se	200	2000	60	22	NA	[39]
TIPS-pentacene1)	0.1	0.2	35	0.77	NA	[45]
TIPS-pentacene:PS	transistor structure		35	1300	NA	[46]
a-Ga2O3	0.25	50	40	6.8	NA	[47]
MAPbI3	0.6	0	75	1.5	NA	[48]
(BA)2(MA)2Pb3I10	0.47	0.5	10.91	13	NA	[49]
triple cation perovskite2)	0.45	0.4	35	97	NA	[50]
triple cation perovskite3)	3.7	0.1	70	59.9	NA	[51]
Sb2Se3	5	-1	50	106.3	<2.5	this work

Notes: 1) bis (triisopropylsilylethynyl) pentacene; 2) Cs0.05FA0.79MA0.16Pb (I0.8Br0.2)3; 3) Cs0.05FA0.79MA0.16Pb (I0.8Br0.2)3

To identify the potential application field of our Sb₂Se₃ X-ray detector, we compared the parameters of several film-based direct X-ray detectors (Table 1). Although some newly developed perovskite and traditional CdZnTe X-ray detectors have a higher sensitivity than our device due to their high atomic number, our device has a thinner active layer. Thin-film X-ray detectors have been attracting increasing attention [45,52], and researchers are aiming to achieve flexibility, large-area, light weight, low bias, and low cost [47,50]. These detectors demonstrate unique superiority in various applications, such as the detection of cracked oil pipelines, dental radiography, portable security devices, and space exploration missions [51–54]. In this niche application, common materials such as α-Se, organic materials, and metal oxide materials all have lower average atomic numbers than Sb₂Se₃ and result in relatively low sensitivities. Moreover, compared with the perovskite-based thin-film X-ray detectors, our Sb₂Se₃ device has no ionic migration; thus, a relatively high bias can be applied, resulting in a high sensitivity.

4 Conclusions

The X-ray detector application of Sb₂Se₃ was explored, due to its strengths over commercialized α-Se and α-Si, such as its larger atomic number, higher mass density, and lower generation threshold of electron–hole pairs. The inert TiO₂ substrate and high substrate temperature to reduce nucleation density, along with a high source temperature to regulate orientation and morphology, were applied together to modify the CSS process and produce an Sb₂Se₃ film with a grain size of 10–20 µm. This successful experience can be expanded to other low-dimension materials, although the same challenge may be faced in the deposition of large grain films. Meanwhile, the introduction of the p–n junction structure efficiently reduced the dark current and enhanced the device performance. Finally, our Sb₂Se₃ thin-film X-ray detector exhibited a sensitivity of 106.3 µC/(Gy_air·cm²) and a response frequency of >400 Hz. These adequate performances and the unique one-dimensional structure of Sb₂Se₃ demonstrated its significant application potential in X-ray detection.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant Nos. 61725401 and 61904058), the National Key R&D Program of China (No. 2016YFA0204000), the Innovation Fund of Wuhan National Laboratory for Optoelectronics (WNLO), the National Postdoctoral Program for Innovative Talent (No. BX20190127), and China Postdoctoral Science Foundation Project (No. 2019M662623). The authors thank the Analytical and Testing Center of Huazhong University of Science and Technology (HUST) and the facility support of the Center for Nanoscale Characterization and Devices, WNLO-HUST.

References

1 Kim Y C, Kim K H, Son D Y, Jeong D N, Seo J Y, Choi Y S, Han I T, Lee S Y, Park N G. Printable organometallic perovskite enables large-area, low-dose X-ray imaging. Nature, 2017, 550(7674): 87–91

PMID

2 Chen Q, Wu J, Ou X, Huang B, Almutlaq J, Zhumekenov A A, Guan X, Han S, Liang L, Yi Z, Li J, Xie X, Wang Y, Li Y, Fan D, Teh D B L, All A H, Mohammed O F, Bakr O M, Wu T, Bettinelli M, Yang H, Huang W, Liu X. All-inorganic perovskite nanocrystal scintillators. Nature, 2018, 561(7721): 88–93

PMID

3 Pan W, Wu H, Luo J, Deng Z, Ge C, Chen C, Jiang X, Yin W J, Niu G, Zhu L, Yin L, Zhou Y, Xie Q, Ke X, Sui M, Tang J. Cs₂AgBiBr₆ single-crystal X-ray detectors with a low detection limit. Nature Photonics, 2017, 11(11): 726–732

4 Wei H, Huang J. Halide lead perovskites for ionizing radiation detection. Nature Communications, 2019, 10(1): 1066

PMID

5 Yang B, Yin L, Niu G, Yuan J H, Xue K H, Tan Z, Miao X S, Niu M, Du X, Song H, Lifshitz E, Tang J. Lead-free halide Rb₂CuBr₃ as sensitive X-ray scintillator. Advanced Materials, 2019, 31(44): 1904711

PMID

6 Gao L, Yan Q F. Recent advances in lead halide perovskites for radiation detectors. Solar RRL, 2020, 4(2): 1900210

7 Wei W, Zhang Y, Xu Q, Wei H T, Fang Y J, Wang Q, Deng Y H, Li T, Gruverman A, Cao L, Huang J S. Monolithic integration of hybrid perovskite single crystals with heterogenous substrate for highly sensitive X-ray imaging. Nature Photonics, 2017, 11(5): 315–321

8 Zhuang R Z, Wang X J, Ma W B, Wu Y H, Chen X, Tang L H, Zhu H M, Liu J Y, Wu L L, Zhou W, Liu X, Yang Y. Highly sensitive X-ray detector made of layered perovskite-like (NH₄)₃Bi₂I₉ single crystal with anisotropic response. Nature Photonics, 2019, 13(9): 602–608

9 Huang H Y, Abbaszadeh S. Recent developments of amorphous selenium-based X-ray detectors: a review. IEEE Sensors Journal, 2020, 20(4): 1694–1704

10 Zhu M, Niu G, Tang J. Elemental Se: fundamentals and its optoelectronic applications. Journal of Materials Chemistry C, Materials for Optical and Electronic Devices, 2019, 7(8): 2199–2206

11 Jeong D N, Yang J M, Park N G. Roadmap on halide perovskite and related devices. Nanotechnology, 2020, 31(15): 152001

PMID

12 Cheng X, Yang S, Cao B Q, Tao X T, Chen Z L. Single crystal perovskite solar cells: development and perspectives. Advanced Functional Materials, 2020, 30(4): 1905021

13 Zeng K, Xue D J, Tang J. Antimony selenide thin-film solar cells. Semiconductor Science and Technology, 2016, 31(6): 063001

14 Xue D J, Shi H J, Tang J. Recent progress in material study and photovoltaic device of Sb₂Se₃. Acta Physica Sinica, 2015, 64(3): 038406 (in Chinese)

15 Yaffe M J, Rowlands J A. X-ray detectors for digital radiography. Physics in Medicine and Biology, 1997, 42(1): 1–39

PMID

16 Kim H K, Cunningham I A, Yin Z, Cho G. On the development of digital radiography detectors: a review. International Journal of Precision Engineering and Manufacturing, 2008, 9(4): 86–100

17 Chen C, Zhao Y, Lu S, Li K, Li Y, Yang B, Chen W, Wang L, Li D, Deng H, Yi F, Tang J. Accelerated optimization of TiO₂/Sb₂Se₃ thin film solar cells by high-throughput combinatorial approach. Advanced Energy Materials, 2017, 7(20): 1700866

18 Deng H, Zeng Y, Ishaq M, Yuan S, Zhang H, Yang X, Hou M, Farooq U, Huang J, Sun K, Webster R, Wu H, Chen Z, Yi F, Song H, Hao X, Tang J. Quasiepitaxy strategy for efficient full-inorganic Sb₂S₃ solar cells. Advanced Functional Materials, 2019, 29(31): 1901720

19 Hobson T D C, Hutter O S, Birkett M, Veal T D, Durose K. Growth and characterization of Sb₂S₃ single crystals for fundamental studies. In: Proceedings of 2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC & 34th EU PVSEC). Waikoloa: IEEE, 2018, 0818–0822

20 Moy J P. Large area X-ray detectors based on amorphous silicon technology. Thin Solid Films, 1999, 337(1–2): 213–221

21 Jung N, Alving P L, Busse F, Conrads N, Meulenbrugge H M, Ruetten W, Schiebel U, Weibrecht M, Wieczorek H. Dynamic X-ray imaging system based on an amorphous silicon thin-film array. Physics of Medical Imaging, 1998, 3336: 396–407

22 Zhao W, Law J, Waechter D, Huang Z, Rowlands J A. Digital radiology using active matrix readout of amorphous selenium: detectors with high voltage protection. Medical Physics, 1998, 25(4): 539–549

PMID

23 Zhao W, Rowlands J A. Digital radiology using active matrix readout of amorphous selenium: theoretical analysis of detective quantum efficiency. Medical Physics, 1997, 24(12): 1819–1833

PMID

24 Kasap S, Frey J B, Belev G, Tousignant O, Mani H, Laperriere L, Reznik A, Rowlands J A. Amorphous selenium and its alloys from early xeroradiography to high resolution X-ray image detectors and ultrasensitive imaging tubes. Physica Status Solidi (B), 2009, 246(8): 1794–1805

25 XCOM. Photon Cross Sections Database: NIST Standard Reference Database 8 (NIST, 2013). Available at physics.nist.gov/PhysRefData/Xcom/html/xcom1.html

26 Yakunin S, Dirin D N, Shynkarenko Y, Morad V, Cherniukh I, Nazarenko O, Kreil D, Nauser T, Kovalenko M V. Detection of gamma photons using solution-grown single crystals of hybrid lead halide perovskites. Nature Photonics, 2016, 10(9): 585–589

27 Wei H, Fang Y, Mulligan P, Chuirazzi W, Fang H H, Wang C, Ecker B R, Gao Y, Loi M A, Cao L, Huang J. Sensitive X-ray detectors made of methylammonium lead tribromide perovskite single crystals. Nature Photonics, 2016, 10(5): 333–339

28 Ji C M, Wang S S, Wang Y X, Chen H X, Li L N, Sun Z H, Sui Y, Wang S A, Luo J H. 2D hybrid perovskite ferroelectric enables highly sensitive X-ray detection with low driving voltage. Advanced Functional Materials, 2020, 30(5): 1905529

29 Zhou Y, Wang L, Chen S, Qin S, Liu X, Chen J, Xue D, Luo M, Cao Y, Cheng Y, Sargent E H, Tang J. Thin-film Sb₂Se₃ photovoltaics with oriented one-dimensional ribbons and benign grain boundaries. Nature Photonics, 2015, 9(6): 409–415

30 Chen C, Bobela D C, Yang Y, Lu S, Zeng K, Ge C, Yang B, Gao L, Zhao Y, Beard M C, Tang J. Characterization of basic physical properties of Sb₂Se₃ and its relevance for photovoltaics. Frontiers of Optoelectronics, 2017, 10(1): 18–30

31 Zhao M, Su J, Zhao Y, Luo P, Wang F, Han W, Li Y, Zu X, Qiao L, Zhai T. Sodium-mediated epitaxial growth of 2D ultrathin Sb₂Se₃ flakes for broadband photodetection. Advanced Functional Materials, 2020, 30(13): 1909849

32 Kondrotas R, Zhang J, Wang C, Tang J. Growth mechanism of Sb₂Se₃ thin films for photovoltaic application by vapor transport deposition. Solar Energy Materials and Solar Cells, 2019, 199: 16–23

33 Li Z, Liang X, Li G, Liu H, Zhang H, Guo J, Chen J, Shen K, San X, Yu W, Schropp R E I, Mai Y. 9.2%-efficient core-shell structured antimony selenide nanorod array solar cells. Nature Communications, 2019, 10(1): 125

PMID

34 Yuan C, Jin X, Jiang G, Liu W, Zhu C. Sb₂Se₃ solar cells prepared with selenized dc-sputtered metallic precursors. Journal of Materials Science Materials in Electronics, 2016, 27(9): 8906–8910

35 Phillips L J, Savory C N, Hutter O S, Yates P J, Shiel H, Mariotti S, Bowen L, Birkett M, Durose K, Scanlon D O, Major J D. Current enhancement via a TiO₂ window layer for CSS Sb₂Se₃ solar cells: Performance limits and high V_OC. IEEE Journal of Photovoltaics, 2019, 9(2): 544–551

36 Wen X, Chen C, Lu S, Li K, Kondrotas R, Zhao Y, Chen W, Gao L, Wang C, Zhang J, Niu G, Tang J. Vapor transport deposition of antimony selenide thin film solar cells with 7.6% efficiency. Nature Communications, 2018, 9(1): 2179

PMID

37 Tang R, Zheng Z H, Su Z H, Li X J, Wei Y D, Zhang X H, Fu Y Q, Luo J T, Fan P, Liang G X. Highly efficient and stable planar heterojunction solar cell based on sputtered and post-selenized Sb₂Se₃ thin film. Nano Energy, 2019, 64: 103929

38 Li K, Chen C, Lu S, Wang C, Wang S, Lu Y, Tang J. Orientation engineering in low-dimensional crystal-structural materials via seed screening Sb₂Se₃. Advanced Materials, 2019, 31(44): 1903914

39 Kasap S O. X-ray sensitivity of photoconductors: application to stabilized a-Se. Journal of Physics D, Applied Physics, 2000, 33(21): 2853–2865

40 Cousins P J, Neuhaus D H, Cotter J E. Experimental verification of the effect of depletion-region modulation on photoconductance lifetime measurements. Journal of Applied Physics, 2004, 95(4): 1854–1858

41 Li K, Kondrotas R, Chen C, Lu S, Wen X, Li D, Luo J, Zhao Y, Tang J. Improved efficiency by insertion of Zn_1−xMg_xO through sol-gel method in ZnO/Sb₂Se₃ solar cell. Solar Energy, 2018, 167: 10–17

42 Wang C, Lu S, Li S, Wang S, Lin X, Zhang J, Kondrotas R, Li K, Chen C, Tang J. Efficiency improvement of flexible Sb₂Se₃ solar cells with non-toxic buffer layer via interface engineering. Nano Energy, 2020, 71: 104577

43 Pan W, Yang B, Niu G, Xue K H, Du X, Yin L, Zhang M, Wu H, Miao X S, Tang J. Hot-pressed CsPbBr₃ quasi-monocrystalline film for sensitive direct X-ray detection. Advanced Materials, 2019, 31(44): 1904405

PMID

44 Tokuda S, Adachi S, Sato T, Yoshimuta T, Nagata H, Uehara K, Izumi Y, Teranuma O, Yamada S. Experimental evaluation of a novel CdZnTe flat-panel X-ray detector for digital radiography and fluoroscopy. In: Proceedings of SPIE 4320, Medical Imaging 2001: Physics of Medical Imaging. San Diego: SPIE, 2001, 4320: 140–147

45 Basiricò L, Ciavatti A, Cramer T, Cosseddu P, Bonfiglio A, Fraboni B. Direct X-ray photoconversion in flexible organic thin film devices operated below 1 V. Nature Communications, 2016, 7(1): 13063

PMID

46 Temiño I, Basiricò L, Fratelli I, Tamayo A, Ciavatti A, Mas-Torrent M, Fraboni B. Morphology and mobility as tools to control and unprecedentedly enhance X-ray sensitivity in organic thin-films. Nature Communications, 2020, 11(1): 2136

PMID

47 Liang H L, Cui S J, Su R, Guan P F, He Y H, Yang L H, Chen L M, Zhang Y H, Mei Z X, Du X L. Flexible X-ray detectors based on amorphous Ga₂O₃ thin films. ACS Photonics, 2019, 6(2): 351–359

48 Yakunin S, Sytnyk M, Kriegner D, Shrestha S, Richter M, Matt G J, Azimi H, Brabec C J, Stangl J, Kovalenko M V, Heiss W. Detection of X-ray photons by solution-processed lead halide perovskite. Nature Photonics, 2015, 9(7): 444–449

PMID

49 Tsai H, Liu F, Shrestha S, Fernando K, Tretiak S, Scott B, Vo D T, Strzalka J, Nie W. A sensitive and robust thin-film X-ray detector using 2D layered perovskite diodes. Science Advances, 2020, 6(15): eaay0815

PMID

50 Basiricò L, Senanayak S P, Ciavatti A, Abdi-Jalebi MFraboni BSirringhaus H. Detection of X-rays by solution-processed cesium-containing mixed triple cation perovskite thin films. Advanced Functional Materials, 2019, 29(34): 1902346

51 Mescher H, Schackmar F, Eggers H, Abzieher T, Zuber M, Hamann E, Baumbach T, Richards B S, Hernandez-Sosa G, Paetzold U W, Lemmer U. Flexible inkjet-printed triple cation perovskite X-ray detectors. ACS Applied Materials & Interfaces, 2020, 12(13): 15774–15784

PMID

52 Budil K S, Perry T S, Bell P M, Hares J D, Miller P L, Peyser T A, Wallace R, Louis H, Smith D E. The flexible X-ray imager. Review of Scientific Instruments, 1996, 67(2): 485–488

53 Kuo T T, Wu C M, Lu H H, Chan I, Wang K, Leou K C. Flexible X-ray imaging detector based on direct conversion in amorphous selenium. Journal of Vacuum Science & Technology A, Vacuum, Surfaces, and Films, 2014, 32(4): 041507

54 Sun H, Zhao B, Yang D, Wangyang P, Gao X, Zhu X. Flexible X-ray detector based on sliced lead iodide crystal. Phyica Status Solidi (RRL)-Rapid Research Letters, 2017, 11(2): 1600397