Printable organometallic perovskite enables large-area, low-dose X-ray imaging

Abstract

Medical X-ray imaging procedures require digital flat detectors operating at low doses to reduce radiation health risks1,2. Solution-processed organic–inorganic hybrid perovskites have characteristics that make them good candidates for the photoconductive layer of such sensitive detectors3,4,5,6,7. However, such detectors have not yet been built on thin-film transistor arrays because it has been difficult to prepare thick perovskite films (more than a few hundred micrometres) over large areas (a detector is typically 50 centimetres by 50 centimetres). We report here an all-solution-based (in contrast to conventional vacuum processing) synthetic route to producing printable polycrystalline perovskites with sharply faceted large grains having morphologies and optoelectronic properties comparable to those of single crystals. High sensitivities of up to 11 microcoulombs per air KERMA of milligray per square centimetre (μC mGyair−1 cm−2) are achieved under irradiation with a 100-kilovolt bremsstrahlung source, which are at least one order of magnitude higher than the sensitivities achieved with currently used amorphous selenium or thallium-doped cesium iodide detectors. We demonstrate X-ray imaging in a conventional thin-film transistor substrate by embedding an 830-micrometre-thick perovskite film and an additional two interlayers of polymer/perovskite composites to provide conformal interfaces between perovskite films and electrodes that control dark currents and temporal charge carrier transportation. Such an all-solution-based perovskite detector could enable low-dose X-ray imaging, and could also be used in photoconductive devices for radiation imaging, sensing and energy harvesting.

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Figure 1: Building an X-ray detector based on hybrid perovskites.
Figure 2: MPC perovskite basic properties.
Figure 3: Optoelectronic characterization of MPC thick films.
Figure 4: Diode MPC device characterization.

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Acknowledgements

N.-G.P. acknowledges partial financial support from the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT Future Planning (MSIP) of Korea under contracts NRF-2012M3A6A7054861 and NRF-2014M3A6A7060583 (Global Frontier R&D Program on Center for Multiscale Energy System) and NRF-2016M3D1A1027664 (Future Materials Discovery Program). Y.C.K., I.T.H. and S.Y.L. are indebted to all the members of SAIT (especially H. Kim and Y. Kim) and the Healthcare & Medical Equipment Division of Samsung Electronics (S. M. Yoon) for their help with the material characterization (SEM, X-ray diffraction, time-resolved photoluminescence and time-of-flight spectroscopy), X-ray detector fabrication, tape-automated bonding ROIC for the detector driving for its characterization, imaging processes and interpretation.

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Contributions

I.T.H. initialized this project. Y.C.K. and N.-G.P. organized the collaboration. Y.S.C. and S.Y.L. planned and performed thermal and optoelectronic analysis. Y.C.K. and K.H.K. performed the synthesis of the MCP and PI-composites, and their analysis. D.-Y.S and D.-N.J. performed adduct film synthesis and analysis. D.-Y.S. and J.-Y.S. performed the impedance analysis. Y.C.K. and K.H.K. performed X-ray analysis, detector fabrication and imaging. Y.C.K., I.T.H. and N.G.P. wrote the manuscript. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to In Taek Han or Nam-Gyu Park.

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The authors declare no competing financial interests.

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Reviewer Information Nature thanks J. A. Rowlands and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 MPC X-ray detector.

a, Calculated mass attenuation coefficient for a-Se, CdZnTe and MAPbI3, and two normalized bremsstrahlung X-ray spectra used in medical applications as a function of X-ray energy. The linear attenuation coefficient (in cm−1) can be obtained by multiplying mass attenuation and mass density (a.u., arbitrary units). b, An X-ray spectrum of a 3-mm-filtered 100-kVp tungsten source in 1 mGyair (black line), the absorption by 830-μm-thick MPC film (blue line), and the resulting absorbed spectrum by the MPC film (red line). The integral of the absorbed spectrum equals the total deposited energy (Eabs) in the MPC film of about 2.91 × 1011 keV cm−2 mGyair−1. The W± is defined as EabsS−1, where S is the X-ray sensitivity. There are various loss mechanisms that reduce the sensitivity in the pixelated detector. Provided that all the specific loss factors are understood and corrected by further study, the panel sensitivities should coincide with those measured from simple diodes. At this stage this discrepancy is probably mainly due to the incomplete charge collection in the gap between pixels by the limited geometric fill factor‚ imperfect MPC contact on the pixel electrode, and a possible reduction in the effective applied bias on the MPC layer caused by the thicker interlayers formed in the processing of large-area detectors (rather than smaller diodes). c, The MTF of an MPC detector and a conventional a-Se direct converting detector with a same pixel pitch of 70 μm. The inset shows a magnified 100-kVp X-ray image of a resolution phantom. The MTF presented here was measured under 15 μGyair of dose (dose rate of 1.5 mGyair s−1 for 10 ms exposure time). d, A smartphone X-ray image at dose rate 1 mGyair s−1 with exposure time 10 ms (10 μGyair of dose).

Extended Data Figure 2 Optoelectronic properties of MPC film.

a, Photoluminescence spectra of the MPC and the thin adduct MAPbI3 film using an excitation source wavelength of 505 nm. b, Photoluminescence lifetime of the MPC film. The fitted recombination lifetime is 1.052 μs. c, Full time-of-flight current behaviour for both charge carriers: left for hole and right for electron. d, Nyquist plots of impedance (Z) spectra for the 400-μm-thick MPC film measured under 1 Sun and 0.1 Sun irradiation conditions. The inset shows the equivalent circuit for analysis, where Rs is the series resistance of the MPC, Rrec is the recombination resistance related to the recombination current, and CPE is the chemical capacitance represented as a constant phase element. Here, the lifetime is estimated by multiplying Rrec and CPE.

Extended Data Figure 3 Interlayers of PI–perovskite composites.

a, Additional images of interlayer films and their precursor solutions. Scale bar for the upper middle image, 30 μm. b, Morphology changes in PI-MAPbBr3 film with decreasing precursor concentration of MAPbBr3. All scale bars, 200 μm.

Extended Data Figure 4 Temporal X-ray response characterization.

a, Rise and fall time response dependent on the applied bias. b, ‘Lag’ measurement as a function of time after each X-ray pulse signal. c, ‘Ghosting’ measured using the sensitivity change for each X-ray pulse signal. Lag can be defined as [Idark(t) – Idark(0)]Sn−1, and ghosting as SnS1−1, where Idark(t) is the dark current measured at time t after each exposure, Idark(0) is the initial dark current level before any exposures, S1 is the X-ray sensitivity for the first exposure, and Sn is the sensitivity of the nth exposure, respectively.

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Kim, Y., Kim, K., Son, DY. et al. Printable organometallic perovskite enables large-area, low-dose X-ray imaging. Nature 550, 87–91 (2017). https://doi.org/10.1038/nature24032

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