Photoelectric Properties And X-ray Detection Investigation Of Molecular Perovskites

X-ray detectors are widely used in the fields of medical imaging,industrial flaw inspection and daily security checks.In recent years,halide perovskites have been considered as a promising candidate for X-ray detection due to high X-ray absorption,large resistivity,excellent charge transport,and low-cost solution processability.However,most extensively studied halide perovskites contain highly toxic elements such as lead,which poses a threat to living organisms and the environment,to some extent limiting their further applications.As a newly-developed member of the perovskite family,molecular perovskites are structurally non-toxic and lightweight.Therefore,they not only inherit the chemical diversity and structural adjustability of perovskite materials,but also have the advantages of biocompatibility and light weight.At present,molecular perovskites have shown excellent ferroelectric properties.However,there is still a gap in the study of their intrinsic physical properties,charge transport and photogenerated carrier behavior,not to mention the lack of exploration in device applications.In view of the above problems,this paper focused on the molecular perovskites,regulated their structures and properties through A-,B-and X-site engineering,systematically studied the influence of engineering at each site on the molecular perovskite crystal structure,energy band structure,intrinsic physical properties,charge transport and photogenerated carrier behavior,as well as showed their application potential in X-ray detectors.The specific research work is as follows:1.By X-site engineering,a series of DABCO-NH4X3(DABCO=N-N’diazabicyclo[2.2.2]octonium,X=Cl,Br,I)metal-free molecular perovskites were synthesized,high-quality and large-sized single crystals of this series were grown at room temperature for the first time.The effect of halide-modulated molecular assembly behavior on the crystal structure,energy band structure,intrinsic physical properties,and final optoelectronic performance of DABCO-based metal-free molecular perovskite series was systematically studied.It is found that the crystal structures of metal-free perovskite DABCO-NH4X3 evolved from one-dimensional(1D)hexagonal perovskite structure to three-dimensional(3D)cubic perovskite structure,and the energy band structures evolved from indirect band gap to direct band gap when the Xsite halide transited from Cl to Br and then to I.Characterizations showed single crystals of this kind also had wide band gap(～5eV).high resistance(～109 Ω·cm)and excellent charge extraction ability(μτ～10-3-10-2 cm2·V-1),which was conducive to the fabrication of high-performance X-ray detectors.On this basis,their responses under X-ray excitation were preliminarily explored.The planar X-ray detector based on DABCONH4I3 single crystal achieved the highest detection sensitivity of 567 μC·Gyair-1cm-2 among the three,and realized a well-defined X-ray image,implying good potential for X-ray imaging.This work unveiled the modulated role of halides in X-site engineering of molecular perovskites and expanded the application range of molecular perovskites.2.By A-site engineering,a new stable one-dimensional(1D)metal-free molecular perovskite rac-3APD-NH413(racemic-3APD=racemic-3-aminopiperidinium)was designed by introducing the same amount of chiral isomeric molecular cations S3APD2+and R-3APD2+.The successful growth of high-quality and large-sized rac3APD-NH4I3 single crystal enabled systematical investigations on its crystal structure,photoelectric properties and carrier dynamics.It is found that rac-3 APD-NH4I3 adopted one-dimensional perovskite structure and emitted warm orange fluorescence under(UV)irradiation.Benefitted from strong quantum confinement effect of one-dimensional octahedra chains,photo luminescence quantum yield(PLQY)reached up to 41.6%.Wide photoluminescence(PL)spectrum and large Stokes shift showed that the photoluminescence was from self-trapped excitons.Finally,according to the structureproperty relationship,a planar X-ray detector was fabricated on the(002)plane with excellent charge transport,and the detection sensitivity was increased to 771 μC·Gyair1 cm-2,with a low detection limit and excellent X-ray irradiation stability.This work is based on the A-site engineering strategy,innovatively extended the study of metal-free molecular perovskites to photoluminescence and dynamic behavior,and provided a more comprehensive research paradigm of molecular perovskite-based X-ray detectors.3.By B-site engineering,with DABCO-NH4Br3 as the prototype,a metal-based molecular perovskite DABCO-CsBr3,was designed by replacing NH4+ with Cs+.This metal-based molecular perovskite built a link between traditional metal halide perovskites and metal-free molecular perovskites.DABCO-CsBr3 inherited the environmental friendliness and photoelectric properties of the metal-free molecular perovskites,but with enhanced X-ray absorption capability.The sensitivity of the asfabricated single crystal X-ray detector was increased to 1345 μC·Gyair-1 cm-2.Nanoindentation experiments found that the Young’s modulus of DABCO-CsBr3 was smaller than that of DABCO-NH4Br3,close to that of organic-inorganic hybrid perovskites,suitable for flexible fabrication.Finally,a molecular-perovskite-based flexible X-ray detector was demonstrated based on DABCO-CsBr3/poly(vinylidene fluoride)(PVDF)composite by combining DABCO-CsBr3 with polymer PVDF,with a high sensitivity of 106.7 μC·Gyair-1 cm-2.This work innovatively adopted a B-site engineering strategy to optimize the X-ray detection performance of molecular perovskite,and demonstrated their prospects for the next-generation eco-friendly and wearable optoelectronic devices.