Self-powered flexible all-perovskite X-ray detectors with high sensitivity and fast response

Summary Perovskite materials have demonstrated superior performance in many aspects of optoelectronic applications including X-ray scintillation, photovoltaic, photodetection, and so on. In this work, we demonstrate a self-powered flexible all-perovskite X-ray detector with high sensitivity and fast response, which can be realized by integrating CsPbBr3 perovskite nanocrystals (PNCs) as the X-ray scintillator with a CH3NH3PbI3 perovskite photodetector. The PNCs scintillator exhibits ultra-fast light decay of 2.81 ns, while the perovskite photodetector gives a fast response time of ∼0.3 μs and high-specific detectivity of ∼2.4×1012 Jones. The synergistic effect of these two components ultimately leads to a self-powered flexible all-perovskite X-ray detector that delivers high sensitivity of 600–1,270 μC/mGyaircm3 under X-ray irradiation and fast radiation-to-current response time.


INTRODUCTION
Since Wilhelm Rö ntgen discovered X-rays, extensive studies have been done to find their commercial applications such as crystallography, medical inspection and therapy, non-destructive industrial inspection, security check, and space exploration (Blasse, 1994;Tegze and Faigel, 1996;Rieder et al., 1997). The X-ray has been generally used to obtain the X-ray diffraction patterns of crystals, mammography, intraoral structure, computed tomography (CT), and airport security scan. The emission spectrum of an X-ray source contains a characteristic X-ray with spike shape and Bremsstrahlung X-ray with broad spectrum, where the characteristic X-ray with $30 keV energy is used for mammography and the broad Bremsstrahlung is exploited for chest radiography. The allowed X-ray spectrum to be used for detection also depends on the part of human body for diagnosis because the damage by X-ray irradiation varies with organs such as tissue and bone. Therefore, it is desirable for the X-ray detector to have good performance at a broad X-ray spectrum region for wide application.
The X-ray detectors can be classified as direct and indirect types. The former directly captures the X-ray photoelectrons generated in an X-ray absorption layer such as amorphous Se (a-Se) under applied bias voltage, so it can obtain high resolution. The direct X-ray detector is currently used in relatively soft X-ray detection and imaging such as mammography. The detection of hard X-ray is performed mostly through an indirect scenario (Su et al., 2020). It relies on a scintillator such as thallium doped cesium iodide (CsI:Tl) (Grassmann et al., 1985) and terbium-doped gadolinium oxysulfide (Gd 2 O 2 S:Tb, GOS) (Van Eijk, 2002), to convert X-ray to visible photons, which can be further detected by photodetectors such as Si photodiodes. The indirect type X-ray detectors have captivated most markets today because they are cheaper and more stable than the direct type detectors. However, it is still a great challenge to develop indirect type X-ray detectors with high sensitivity, high resolution, and a fast scan rate in order to minimize radiation exposure to the patient.
In 2015, Yakunin et al. first reported on a direct type CH 3 NH 3 PbI 3 (MAPbI 3 ) perovskite X-ray detector with high sensitivity (25 mC/mGy air cm 3 ) and responsivity (1.9310 4 carriers/photon) (Yakunin et al., 2015). Wei et al. then fabricated a direct type MAPbBr 3 single crystal perovskite X-ray detector with an improved sensitivity of to 80 mC/mGy air cm 2 , which is four folds higher than the sensitivity of a-Se detector (Wei et al., 2016). In 2017, a large-area (50 3 50 cm 2 ) MAPbX 3 (X = Cl, Br or I) perovskite X-ray direct-type detector was fabricated by Kim et al. through a printing process (Kim et al., 2017). The large area device exhibited a sensitivity of 11 mC/mGy air cm 2 and they were able to obtain X-ray images from it. Other than the MAPbI 3 perovskite, CsPbBr 3 based perovskite was also soon realized as an excellent candidate for X-ray detection particularly due to its high Z value coming from the heavy Cs atoms. Pan et al. fabricated direct-type CsPbBr 3 X-ray detectors by employing a hot-pressing method to form a $240 mm thick quasi-monocrystalline CsPbBr 3 film, and reported on an excellent sensitivity of 55.684 mC/mGy air cm 2 (Pan et al., 2019). However, the direct-type perovskite X-ray detectors have been fabricated by depositing very thick crystalline perovskite layer on thin film transistor (TFT) arrays or complementary metal oxide semiconductor (CMOS) arrays, so it is difficult to make flexible X-ray detectors due to the brittle thick perovskite layer.
In contrast, Heo et al. reported an indirect type scintillator-based X-ray detector consisting of CsPbBr 3 perovskite nanocrystals (PNCs), which showed better spatial resolution of 9.8 lp/mm at a modulation transfer function (MTF) of 0.2 and 12.5 lp/mm for a linear line chart, compared to the GOS scintillator-based detector (6.2 lp/mm at MTF = 0.2 and 6.3 lp/mm for a linear line chart) (Heo et al., 2018). The CsPbBr 3 PNCs were dispersed in a relatively rigid poly-methylmethacrylate (PMMA) matrix as the scintillator and the arrayed photodetectors were formed on a rigid substrate, thereby preventing the indirect type X-ray detector from exhibiting flexibility.
Development of portable flexible X-ray detectors with high sensitivity and fast response is very useful for analyzing a structure of curved architecture and to distinguish materials such as soft and hard matters. Currently, the portable chest X-ray has contributed toward detecting lungs infected by the coronavirus disease-19 (COVID-19) (Jacobi et al., 2020). Accordingly, it is important to fabricate a flexible scintillator that can be self-powered in order to develop portable X-ray detectors with high performance. So far, a wealth of studies on flexible perovskite solar cells (Heo et al., 2019;Jung et al., 2019) and photodetectors (Wang and Kim, 2017) have been done, and recently Gill et al. reported that a MAPbI 2 Cl perovskite is $550% more sensitive for X-ray detection than the commonly used a-Si devices (Gill et al., 2018). Hence, here we fabricated a flexible CsPbBr 3 PNCs-based scintillator combined with a self-powered flexible MAPbI 3 perovskite photodetector without requiring external bias voltage. By combining the flexible PNCs-based scintillator and the flexible perovskite photodetector, we could demonstrate all-perovskite flexible self-powered X-ray detectors with high sensitivity and fast response.

RESULTS AND DISCUSSION
Fabrication and X-Ray scintillation of flexible CsPbBr 3 PNCs film Figure 1A is a transmission electron microscopy (TEM) image of CsPbBr 3 PNCs, which were synthesized by previously reported procedures involving solution chemistry (Heo et al., 2018). The synthesized CsPbBr 3 PNCs had $10 nm-sized nanocubes or nanobars and were uniformly dispersed. The inset TEM image indicates that the CsPbBr 3 PNCs have a cubic crystal structure exposing the {100} facet. Further characterizations of CsPbBr 3 PNCs such as X-ray diffraction (XRD) pattern, UV-visible absorption spectrum, static photoluminescent (PL) spectrum, and dynamic transient PL spectrum are shown in Figure S1. The XRD pattern in Figure S1A confirms that the synthesized CsPbBr 3 PNCs have a cubic phase, which is consistent with the TEM results. The UV-visible absorption and static PL spectrum in Figure S1B indicate that the CsPbBr 3 PNCs have an on-set absorption band edge at a wavelength of $510 nm and a strong single PL peak at a wavelength of $520 nm-wavelength with a full width at half maximum (FWHM) value of $20 nm. The dynamic transient PL spectrum in Figure S1C indicates that the CsPbBr 3 PNCs have an average PL life-time of 2.81 ns (t 1 = 0.42 ns (48.77%), t 2 = 5.16 ns (51.23%)).
Figures 1B and 1C are photographs of the flexible CsPbBr 3 PNCs scintillator film, which was made by mixing the CsPbBr 3 PNCs with poly-dimethylsiloxane (PDMS) resin and a curing agent, subsequently followed by the degassing and curing processes at 60 C for 12 hr at N 2 atmosphere, under room light ( Figure 1B) and X-ray (90 keV, 1 mA) irradiation ( Figure 1C). The strong blue-green PL emission of the PNCs scintillator film implies that the CsPbBr 3 PNCs can emit PL under a low X-ray dose. The average PLQY (PL quantum yield) of the scintillator film was 63.5 G 3.57% as shown in Figure S1D.  Figure 1F) under X-ray exposure (tube voltage = 90 keV, tube current = 1 mA). It should be noted that here r is the outer bending radius. If we consider the thickness of the PNCs scintillator film (thickness = 1.5 mm), the inner bending radius is much smaller than the outer r. Figure 1G shows bending stabilities of the PNCs scintillator films. All samples maintained their PL intensities irrespective of the repeated bending cycles up to 1,000 cycles. Therefore, the CsPbBr 3 PNCs scintillator film has excellent bending stability and sufficient flexibility because its matrix is made by super flexible PDMS rubber.  Figure 1H) and under a fixed tube current of 1 mA ( Figure 1I). Both PL spectra show a strong emission of green light at a wavelength of $533 nm while their intensities gradually increased as either the tube current or tube voltage increased. Evidently, the PL intensities are strongly dependent on the dose rate of Data with error bars are represented as mean +/À standard deviation.

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iScience 24, 102927, August 20, 2021 3 iScience Article the irradiated X-rays, implying that the flexible PNCs scintillator film can respond to a broad X-ray photon energy spectrum while exhibiting a linear response with X-ray dose rate. To exactly measure the correlation of tube current vs PL intensity and tube voltage vs PL intensity, we measured again the PL intensities with a photodetector as shown in Figure 1J, because the indirect type X-ray detector would also measure the PL intensity originating from the scintillator film.
Self-powered flexible MAPbI 3 perovskite photodetector with fast response iScience Article 510 nm-wavelength laser. The J-V curves clearly show that the current density at zero bias potential gradually increases with increasing intensity of the irradiated light. The responsivity (R) and specific detectivity (D*) can be written as follows: where J ph is the photocurrent density, J d is the dark current density, and P is the incident light intensity, and: The calculated R and D* values relative to light intensity for the self-powered MAPbI 3 perovskite photodetector were plotted in Figure 2C, as J ph , J d and P are obtainable in Figure 2B, and S n is $4.0 3 10 À12 A/Hz 0.5 from Figure 2D. The calculated R and D* of the self-powered detector were $0.35 A/W and $2.4310 12 Jones, respectively. The log(current density)-log(light intensity) plot in Figure 2E indicates that the linear dynamic range (LDR) of the perovskite photodetector is $158 dB. The signal attenuation (10log(I3I 0 À1 )) with frequency was shown in Figure 2F, which indicates that the 3 dB penalty frequency of the perovskite photodetector is $5 MHz. This implies that the self-powered MAPbI 3 perovskite photodetector can process information up to the MHz frequency range. To confirm this, we acquired a 1 MHz signal through the perovskite photodetector as shown in Figure 2G. The magnified signal show that the rising time (t r ) and decay time (t d ) of the response signals of perovskite photodetector are 0.30 ms and 0.31 ms, respectively. Accordingly, the perovskite photodetector can effectively acquire real-time information.
To fabricate a flexible self-powered X-ray detector, both the PNCs scintillator and perovskite photodetector must have sufficient mechanical flexibility and bending stability. The mechanical flexibility and stability of the PNCs scintillator were confirmed in the previous section. Similarly, the mechanical flexibility and stability of the perovskite photodetector are shown in Figures 2H-2K. A photograph of a flexible perovskite photodetector (size = 2.54 3 2.54 cm 2 , each active area = 0.16 cm 2 ) composed of poly(ethylene terephthalate) (PET)/ITO/PEDOT:PSS/MAPbI 3 /PCBM/Al is shown in Figure 2H. The J-V curves of the flexible perovskite photodetector bent to specific bending radii (r = N (flat), 6, 4, and 2 mm) under dark and photo conditions (1 mW/cm 2 ) are shown in Figure 2I. Apparently, the J-V curves indicate that the J ph and J d of the bent photodetector are almost constant irrespective of bending radius due to its small active area. This implies that the flexible perovskite photodetector can acquire certain information signals without significant distortion under bent circumstances. To check the bending stability of the perovskite detector, the J ph and J d of the flexible perovskite photodetector were measured with respect to the repeated bending cycles as shown in Figure 2J. Interestingly, the J ph of the repeatedly bent perovskite photodetector was almost constant regardless of the repeated bending cycles, while J d was slightly degraded with repeated bending cycles. The degradation amount of J d was also slightly increased as the bending radius decreased. The responsivities of the flexible perovskite photodetectors with respect to the repeated bending cycles are shown in Figure 2K, and the inset shows photographs of flexible perovskite photodetectors bent to r = 6, 4, and 2 mm. Since R is a function of (J ph À J d )P À1 , the responsivities of the flexible perovskite photodetector were almost constant up to a repeated bending test of 1,000 cycles but very slightly deteriorated with decreasing bending radius due to the increasing J d .
Device performance of self-powered all-perovskite X-Ray detectors Figure 3A is the schematic device structure of the self-powered all-perovskite X-ray detector composed of carbon fiber reinforced polymer (CFRP) film/CsPbBr 3 PNCs scintillator film/substrate (glass for rigid device or PET for flexible device)/ITO/PEDOT:PSS/MAPbI 3 perovskite/PCBM/Al. Figure 3B shows the current densities detected by the photodetector with respect to the X-ray dose rate. The current density of perovskite photodetector linearly increases with increasing X-ray dose rate. Under a fixed tube current of 1 mA, the all-perovskite X-ray detector acquired current density values of 0.017-0.199 mA/cm 2 as the tube voltage increased from 30 to 120 keV, whereas under a fixed tube voltage of 90 keV, the detector current density values ranged from 0.041 to 0.174 mA/cm 2 as the tube current increased from 0.25 to 1 mA.

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iScience 24, 102927, August 20, 2021 5 iScience Article The X-ray sensitivity (S) can be expressed as follows (Thirimanne et al., 2018): where J x-ray (t), J dark , D, and A t stand for current density generated by X-ray irradiation during time (t), dark current density without X-ray irradiation, X-ray dose, and thickness of active region.
When calculating S in an active area basis, the thickness term represented by A t can be neglected. Here, we fixed the active area, the thickness of PNCs scintillator, and the thickness of the perovskite photodetector to 0.16 cm 2 , 0.15 cm, and 4 3 10 À5 cm, respectively. Overall, the self-powered all-perovskite X-ray detector exhibited sensitivity values of 130-140 mC/mGy air cm 2 in an active area basis and 880 to 960 mC/mGy air cm 3 in an active volume basis, when the tube voltage was fixed to 90 keV, compared with the sensitivity values ranging from 100 to 210 mC/mGy air cm 2 in an active area basis and from 660 to 1,370 mC/mGy air cm 3 in an . Self-powered all-perovskite X-ray detector (A-F) (A) Schematic device structure of the self-powered all-perovskite X-ray photodetector, (B) current densities of rigid glass substrate-based device as a function of X-ray dose rate, (C) the corresponding X-ray sensitivities with dose rate, (D) X-ray transmittance of the MAPbI 3 perovskite photodetector with various tube current at 90 keV tube voltage and various tube voltage at 1 mA tube current, (E) the corresponding current densities of perovskite photodetector with dose rate, and (F) output signals of the self-powered all-perovskite X-ray detector under in response to rectangular input X-ray signals with a 50 ms time interval.
(G-L) (G) Photograph of the flexible substrate-based self-powered all-perovskite X-ray detector, (H) current densities with respect to dose rate, (I) the corresponding X-ray sensitivities with dose rate, and output signals of the flexible X-ray detector with bending radii of (J) 6, (K) 4, and (L) 2 mm under exposure of rectangular input X-ray signals with a 50 ms time interval. Data with error bars are represented as mean +/À standard deviation.

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6 iScience 24, 102927, August 20, 2021 iScience Article active volume basis, when the tube current was otherwise fixed at 1 mA, as shown in Figure 3C. The correlations between transmittance and dose rate of PNCs scintillator are shown in Figure S3. The constant sensitivity under a fixed tube voltage irrespective of the variation in tube current is attributed to the constant absorptivity of the CsPbBr 3 PNCs scintillator as shown in Figure S3B . In contrast, the decreasing sensitivity under fixed tube current with increasing tube voltage is caused by the decreasing absorptivity of PNCs scintillator with dose rate as shown in Figure S3C.
The $400-nm-thick MAPbI 3 perovskite photodetector itself is also responsible for X-ray detection, behaving similarly to a direct type X-ray detector. The X-ray absorbance of the perovskite photodetector was below 10% as shown in Figure 3D, thus generating a current density of 1-7 nA/cm 2 as shown in Figure 3E. Considering that the net generated current density by the all-perovskite X-ray detector under the same X-ray irradiation ranged from 0.017 to 0.174 mA/cm 2 , the contribution of the perovskite photodetector to the detection signal can be neglected. Figure 3F shows the response time of the selfpowered all-perovskite X-ray detector, indicating that the output signals exhibit no time delay in response to the rectangular input X-ray signal pulses with a 50 ms time interval, irradiating upon the detector. This fast response of the X-ray detector can be attributed to a very short PL life-time of the CsPbBr 3 PNCs scintillator film (2.81 ns) and quick response time of MAPbI 3 perovskite photodetector ($0.3 ms).
Finally, we fabricated a flexible self-powered all-perovskite X-ray detector composed of CsPbBr 3 PNCs scintillator/PET/ITO/PEDOT:PSS/MAPbI 3 /PCBM/Al and its photograph was shown in Figure 3G. The current densities generated by the X-ray irradiation are shown in Figure 3H, which exhibits similar results to the rigid glass based all-perovskite X-ray detector. Additionally, the X-ray sensitivity dependences on the dose rate for the flexible X-ray detector are shown in Figure 3I. The X-ray sensitivity ranged from 120 to 130 mC/ mGy air cm 2 in an active area basis and from 800 to 870 mC/mGy air cm 3 in an active volume basis under a fixed tube voltage of 90 keV, and ranged from 90 to 190 mC/mGy air cm 2 in an active area basis and from 600 to 1,270 mC/mGy air cm 3 in an active volume basis under a fixed tube current of 1 mA. It is important to check if the X-ray signals of the flexible all-perovskite X-ray detector were distorted during the bending stages, so we acquired output signals of the flexible all-perovskite X-ray detector at bending radii of r = 6 (Figures 3J), 4 ( Figure 3K), and 2 mm ( Figure 3L), respectively, when under irradiation of rectangular input X-ray pulse signals with 50 ms time interval. It can be seen that the flexible self-powered perovskite X-ray detector did not exhibit signal distortion, which is a combined result of both the flexible CsPbBr 3 PNCs scintillator ( Figure 1G) and the flexible MAPbI 3 perovskite photodetector ( Figures 2I-2K) not showing significant signal distortion under the same bending conditions (r = 6, 4, and 2 mm). This implies that the flexible detector can acquire X-ray images without distortion when attached to curved or bent objects. A comparison chart of sensitivities from this work and among various X-ray detectors from other works can be seen in Figure S4 and Table S1.

Conclusion
In summary, we fabricated self-powered flexible all-perovskite X-ray detectors with high sensitivity and fast response time. The flexible X-ray detector was fabricated by combining a CsPbBr 3 PNCs scintillator and an MAPbI 3 perovskite photodetector in order to take advantage of both indirect and direct-type Xray detectors. The flexible PNCs X-ray scintillator was made by dispersing PNCs in PDMS matrix, and exhibited super-flexibility and quick response (PL life-time = 2.81 ns). The PNCs scintillator efficiently emitted PL under X-ray irradiation with a broad X-ray energy spectrum (30-120 keV) and low dose. Meanwhile, the MAPbI 3 perovskite photodetector exhibited photodetection parameters of R = $0.35 A/W, D* = $ 2.4310 12 Jones, LDR = $158 dB, and a 3 dB signal response frequency of $5 MHz (response time = $0.3 ms). Accordingly, the self-powered flexible all-perovskite X-ray detector that integrates a PNCs scintillator and a perovskite photodetector showed a high sensitivity range of 600-1,270 mC/ mGy air cm 3 under X-ray irradiation (tube current = 1 mA, and tube voltage 30-120 keV) without the need of applying bias voltage and a very quick response time. In addition, the all-perovskite X-ray detector maintained its performance under severe bending conditions (r = 6, 4, and 2 mm) and even after a rigorous repeated bending test of 1,000 cycles. We believe that the flexible all-perovskite X-ray detector holds potential as a versatile X-ray detector which can be applicable to broad applications such as mammography, CT, medical inspection, security check, and industrial inspection, largely attributed to its high sensitivity, high resolution, fast response time, and versatility by simply switching among different PNCs scintillators suitable for specific applications.

Limitations of the study
For the X-ray experiments, a specialized lead chamber and the supervision of an authorized specialist with a certification of X-ray source handling are necessary. Please see the caution in STAR Methods section.

STAR+METHODS
Detailed methods are provided in the online version of this paper and include the following: