Investigation Of X-ray/γ-ray Detection Based On Perovskite Single Crystals

X-ray/γ-ray as a common X-ray/γ-ray,due to its high photon energy and strong penetrability,it is widely used in medical diagnosis,radiation therapy,industrial flaw detection,safety detection,space navigation and material analysis of scientific research.in.As a conversion device for converting an X-ray/γ-ray photon signal into an electrical signal,the X-ray/γ-ray detector is a key part of the X-ray/γ-ray application system.In general,both indirect detection and direct detection can be used to detect X-ray/γ-ray signals.An indirect X-ray/γ-ray detector based on a scintillation crystal first converts the X-rayγ-ray into visible light and then converts the visible light signal into an electrical signal by a CMOS or CCD image sensor.The spatial resolution and quantum efficiency of indirect X-ray/γ-ray detectors are limited due to the scattering of visible light generated by X-ray/γ-ray high-energy photons in scintillation crystals.Semiconductor-based direct X-ray/γ-ray detectors directly convert high-energy photon energy into photo-generated carrier signals through semiconductor active materials,making it possible to achieve high spatial resolution and quantum efficiency.However,due to the low atomic number of the constituent elements of semiconductor materials such as silicon,germanium,and selenium,the absorption coefficient of X-ray/γ-ray is low,and it is susceptible to radiation damage.Most direct X-ray imaging devices,such as amorphous selenium,Flat panel detectors,etc.,can only be applied to soft X-ray imaging of lower energy.For hard X-rays/γ-ray with higher photon energy,it is generally detected by using cadmium zinc cadmium single crystal（CdZnTe）or the like.However,these inorganic active crystals are expensive,have low yields,and are difficult to integrate with array readout circuits to form high resolution imaging devices.Therefore,it is an important task for researchers in various countries to explore high-sensitivity detectors for X-ray/γ-ray detection of new active materials.Organic-inorganic lead-halide perovskite（MAPbX3,X=Cl,Br,I）has become a global research hotspot in recent years due to its ease of preparation and high photoelectric conversion efficiency.Because perovskite contains heavy elements（such as lead,iodine,bromine,etc.）,single crystal can be grown by solution method,and it is not easy to produce defects due to X-ray/γ-ray ionizing radiation.It is also considered to be a very suitable X-ray.New semiconductor materials for X-rays/γ-rays detection.However,the current perovskite-based X-ray/γ-ray has a large dark current,a slow response,and insufficient thickness to completely absorb hard X-rays/γ-rays,and the actual distance from the X-ray/γ-ray detector.There is still a big gap in the application.In view of the key problems in the detection of X-rays/γ-rays based on perovskite,this paper focuses on how to prepare high-thickness,large-sized perovskite single crystals;how to improve the carrier mobility-lifetime product of single crystal and reduce defects Density,inhibition of ion movement inside the crystal;how to optimize the device structure to reduce noise,improve response speed,etc.,to carry out innovative research,to explore a new technical road for the development of high-sensitivity,low-dose X-rays/γ-rays detection imaging devices.The specific innovations of this paper are summarized as follows:1.It is proposed to grow a large-sized organic-inorganic lead-halide perovskite single crystal composed of different halogens by variable temperature crystallization.Compared with the already reported inversion crystallization method,the temperature of the growth solution is always in the smooth growth zone,which effectively increases the crystal size of the organic-inorganic lead-halide perovskite single crystal.The results show that the crystal size of the organic-inorganic lead-bromine perovskite single crystal is increased to 30 mm×28 mm×7 mm by the temperature-changing crystallization method.And using a similar method,we have increased the size of the organic-inorganic lead-halide perovskite single crystals of other different halogen components to 10mm×10mm×3mm or more,and the obtained perovskite crystal size is larger than most of the reports reported in the same period.Level.By changing the composition of the halogen,the cut-off wavelength of the absorption spectrum of the organic-inorganic lead-halide perovskite single crystal covers a broad spectrum ranging from ultraviolet to infrared,and the forbidden band width is continuously adjustable from 1.49 eV to 2.89 eV.The lattice constant is continuously adjustable from 5.680 to 6.212?.Through the testing of different crystal orientation structures of organic-inorganic lead-bromine perovskite single crystals,it was found that the organic-inorganic perovskite crystals had the least defects in the（100）crystal plane direction,the carrier mobility was the highest,and the photoelectric response was the largest.The growth method and performance characterization results of these active materials lay the foundation for the fabrication of organic-inorganic lead-halide perovskite single crystal X-rays/γ-rays detectors.The work was published in Crystal research and technology（2017,Vol.52:9,1700115）,Materials Letters（2019,Vol.236,26-29）and Physica Status Solidi A:Applications and Materials Science（in revision）.2.The work found that in MAPbX3 single crystal,reducing the mismatch ratio of halogen atoms and lead atoms is a key factor to improve carrier mobility.The experimental results show that when the ratio of bromine to chlorine in the growth precursor of the organic-inorganic lead-halide perovskite single crystal is less than 5:1,a large amount of halogen vacancies exist in the grown crystal,resulting in the crystal being an N-type semiconductor.When the halogen ratio continues to decrease,the halogen vacancies increase,causing the resistivity to decrease with the increase of the forbidden band width;when the ratio of bromine to chlorine in the growth precursor of the organic-inorganic lead-halide perovskite single crystal is greater than 5:1 There are a large number of lead element vacancies,resulting in the entire crystal being a P-type semiconductor.And the defects caused by these vacancies will reduce the effect of carrier transport in it,its carrier mobility,carrier lifetime,when the ratio of bromine to chlorine in the growth precursor of organic-inorganic lead-halide perovskite single crystal is 5:1,the highest value is 170.5 cm²V^-1s^-1（electron）,181.2 cm²V^-1s^-1（hole）,4.1microseconds;the defect density is at least 1.9×10⁹ cm^-3.This work was published in Materials Letters（2019,Vol.:236,26-29）and Physica Status Solidi A:Applications and Materials Science（Revision）.3.Prepare the PN junction and PIN junction on the perovskite crystal and use its internal barrier to suppress the dark current of the detection unit,reduce the noise current detected by X-ray and X-ray,and improve the detection by a higher bias electric field.responding speed.1)On the crystal face of the perovskite crystal,we spin-coated a layer of P-type poly-TPD on the upper surface of the intrinsic perovskite crystal as an electron blocking layer,and then vapor-deposited N-type Fuller on the back side.The olefin,PCBM acts as a hole blocking layer to form a PIN structure.Thereafter,gold electrodes and silver electrodes were deposited on both sides by an electron beam evaporation stage.When the PIN junction operates at a reverse bias of 600 Vcm^-1 field strength,since the electron blocking layer and the hole blocking layer can block the injection current of the electrode,the dark state current density is compared to that based on the perovskite single crystal.The guiding structure was reduced from 130μAcm^-2 to120 nA cm^-2,which was reduced by about 1000 times.At a field strength of 10 Vcm^-1,the response speed is as high as 26μs（down to 37%）.The X-ray detection sensitivity for 30 keV is as high as 23.6μCmGy^-1_（64）（88）^-2.It is 40 times the sensitivity of photoconductive X-ray detection reported in the same period.Based on a large size MAPbBr3 crystal of 30 mm×28mm×7 mm,we propose a signal extraction algorithm for different energy photons,separating low energy（<30 keV）from a multi-color X-ray single exposure imaging signal from 30 keV to100 keV.)and high energy（>80 keV）X-ray photon images.This work explored a new technological path for the development of low-dose X-ray subtraction imaging,published in Physica Status Solidi:Rapid Research Letters（2018,Vol.12,Issue:10,1800380）.2)It is found that we enter the precursor fluid by selenium doping,grow perovskite crystals by variable temperature crystallization,and grow into P-type semiconductors at lower temperatures.As the temperature rises,the selenium atom begins to replace the halogen atom and combines with Pb²⁺to form an N-type semiconductor.On the surface of the experiment,when the doping concentration of selenium is 0.4 gL^-1,the PN junction dark state current density formed inside the perovskite single crystal is the smallest,and the dark state current density is low at 250 Vcm^-1 field strength.Up to 4 nA cm^-2.The response time is reduced from26μs to 3μs when the same dark state current density is maintained with the PIN junction.The detection sensitivity of the PN junction to 60 keV X-ray photons is increased to 21μCmGy^-1_（64）（88）^-2,which is higher than the detection sensitivity of the PIN junction formed by evaporation(8μCmGy^-1_（64）（88）^-2)improved nearly 3 times.We performed high-activity gamma ray detection on the doped PN junction,and the detection sensitivity of 1.25 MeV gamma ray was as high as 41μCGy^-1_（64）（88）^-2.This work is published in Advanced Electronic Materials（2018,Vol.4,Issue:11,1800237）,IEEE Translation on Electronic Devices（2019,Vol.66,Issue:1,485-490）.4.The solution method epitaxial growth technique is proposed.The intrinsic layer and the P-type layer are epitaxially grown on the N-type perovskite single crystal to form a PIN junction.Compared with the previously proposed X-ray detection of the PIN junction prepared by spin-coating evaporation.The lattice of the epitaxial growth layer is matched to improve the interface problem in the spin-coating PIN junction.It is found that the solution-based epitaxial growth technique is based on the PIN junction of a perovskite single crystal.By real-time regulation of the components in the growth solution,any kind of organic-inorganic lead-halide perovskite single crystal can pass through the growth buffer layer.Epitaxial growth with a mismatch ratio of less than 1%is achieved.Further,by controlling the ratio of the lead element and the halogen in the precursor liquid,it is possible to quantitatively control the multi-sub-and minority-sub-concentration of the epitaxial layer to grow a cascading perovskite single crystal.The average carrier mobility of the cascaded PIN-junction perovskite single crystal reaches over 200 cm²V^-1s^-1,and the sensitivity to 140 keV hard X-ray detection reaches 1.58μCmGy^-1_（64）（88）^-2.The realization of solution-based epitaxial growth technology has greatly expanded the application prospects of organic-inorganic lead-halide perovskite single crystal devices.The paper is currently in the review of the journal Nature Communications.