Metal-free Perovskite ABX₃ Single Crystal X-ray Detector

The metal-free perovskites ABX3 material was first discovered 18 years ago with mechanical flexibility,light weight,good biocompatibility and chemical diversity.In the past two years,metal-free perovskites were found to have excellent ferroelectric properties,which greatly promoted the development of molecular ferroelectric field.However,up to now,there are gaps in the metal-free perovskites intrinsic charge transport characteristics and optic-generated carrier behaviors.To solve the above problems,we proposed a controlled nucleation strategy to grow large size and low defect single crystals,systematically studied the structure-charge transport correlation of single crystals,and demonstrated the excellent performance of single crystals in X-ray high-energy radiation detection.In this work,the design of metal-free perovskite materials,the controlled preparation of low defect single crystals,the study of structure-charge transport characteristics and the study of photoelectric properties will promote the development of metal-free perovskite photoelectric field.The specific work is as follows:1.We demonstrate the metal-free perovskite single crystal photoelectric device and reveal the metal-free perovskite intrinsic charge transfer characteristics for the first time.Low defect large single crystal（5×5×2mm3）of DABCO-NH4Br3（DABCO=N-N’diazabicyclo[2.2.2]octonium）was prepared by gradient cooling method.It is found that perovskite has a band gap of 5.1 eV and a large semigap of 16 eV.First principles study shows that halides and organic macromolecules contribute the maximum valence band（VBM）and minimum conduction band（CBM）,respectively.The unusually-large semigap rationalizes extremely long lifetimes approaching the msec regime,leading to very high charge diffusion lengths（tens of μm for both electrons and holes）.The ultralong carrier lifetime further promotes the material to exhibit excellent photoelectric response under the excitation of high-energy X-ray radiation.Therefore,the sensitivity of the X-ray detector based on single crystal reaches 173 μC·Gyair-1·cm-2,which is one of the highest sensitivity of the metal-free X-ray detector at present.This work opens up potential applications of metal-free perovskite in the field of wearable lightweight X-ray high-energy radiation detection.2.We systematically studied the mechanism of charge compound emission fluorescence based on three-dimensional metal-free perovskite MDABCO-NH4I3（MDBACO=Nmethyl-N’-diazabicyclo[2.2.2]octonium）large（4×6×2mm3）single crystals.Large-size metal-free perovskite single crystals with fewer defects were prepared by improving nucleation growth and controlling solvent volatilization at room temperature.It is found that the charge recombination life is as long as 1.03 μs and the charge radiates in triplet form through the self-trapezoidal excitons.The photoluminescence quantum yield（PLQY）is as high as 53%at room temperature.By changing the distortion and band gap of three-dimensional perovskite lattice under high pressure,the crystal luminescence color can be adjusted from orange to yellow to green.The lower defect state and higher X-ray absorption coefficient of MDABCO-NH4I3 single crystal make the MDABCO-NH4Br3 single crystal have better X-ray photoelectric response than DABCO-NH4Br3 single crystal.The X-ray detection sensitivity reached 1997±80 μC Gy·cm-2,which is the highest value in metal-free perovskite detectors.This work will promote the potential application of metal-free perovskite as a biologically compatible light-emitting device.3.Based on dimensionality control,we have designed and fabricated low dimensional metal-free perovskite and its single crystal optoelectronic devices for the first time.One-dimensional metal-free perovskite DABCO-N2H4X3（X is halogen atom）large size（5x35 mm2）single crystal was prepared by replacing NH4+ with N2H4+ at B-site engineering.These crystals are one-dimensional structures with corner-sharing or edgesharing.The maximum charge transfer is achieved along the（hOl）face due to the charge transport anisotropy.Compared with the three-dimensional perovskite structure,the lowdimensional structure has lower halogen ion migration,which effectively reduces the dark state current drift,and the X-ray detector can still work stably under the high operating voltage of 200 V.The high sensitivity（1142±10.28 μC·Gyair-1·cm-2）and low detection limit（2.68 μGy·s-1）of the single crystal X-ray detector were obtained due to the excellent charge transport and low ion transport.This work will promote the development of metalfree perovskites into new low-dimensional semiconductor fields.4.Based on molecular regulation,we have designed and fabricated the hardest metal-free perovskite single crystal and its single crystal optoelectronic devices.Threedimensional metal-free perovskite hmta-NH4Br3 single crystal was prepared by replacing the triethylenediamine molecule with the A-site regulated hmta molecule.Compared with DABCO-NH4I3 crystal,the cage organic molecules in the A site of hmta-NH4Br3 crystal have higher pressure bearing capacity and more hydrogen bond interactions with NH4+ at the B site,which results in the increase of the crystal hardness from 0.78 GPa to 1.5 GPa,and the long-term stability of the crystal structure is also greatly improved.The sensitivity of 150μC·Gyair-1·cm-2 was obtained for the X-ray detector based on the single crystal.This work will promote the development of new metal-free perovskite materials and the potential application of optoelectronic devices in special environments with high stability and high pressure.