Growth And Device Of Centimeter-scale Tunable Component MAPb(Cl_xBr_1-x)₃ Single Crystals For Radiation Detection

Organic-inorganic hybrid perovskite single crystal materials have attracted extensive attention because of their long carrier diffusion length,high carrier mobility,wide spectral absorption range,low defect density and excellent photoelectric performance.Compared with perovskite thin film,perovskite single crystal has a lower defect density due to the absence of grain boundaries,improving the its optoelectronic propertie and the application performance of the device.In this paper,we explore and optimise the growth process of MAPb Cl₃ single crystals to prepare centimetre-scale single crystals for detectors.The corresponding surface structures and growth mechanisms of MAPb Cl₃ single crystals under different crystal growth processes are discussed.On the basis of MAPb Cl₃ crystal,the X-site element doping is carried out,and the band gap width of the material is tuned by doping different proportion of bromine atoms,so as to prepare the component adjustable MAPb(Cl_xBr_1-x)₃crystal.Furthermore,the influence of mixed halogen doping on the structure and photoelectric properties of the crystal material is studied.Finally,a simple planar detector with a sandwich structure is fabricated to analyze its electrical properties and photostability.Firstly,based on the traditional inverse temperature crystallization method,high-quality seed crystals were introduced to tune the crystal nucleation and growth processes.The seed-induced inverse temperature crystallization method is used to fabricate a centimeter-sized（size:11mm×12mm×2mm）MAPb Cl₃ single crystal.It shows that the MAPb Cl₃ single crystals are cubic crystal system with space group Pm m,prepared by the traditional inverse temperature crystallization method and seed-induced inverse temperature crystallization method,respectively.Surface morphology and growth mechanism were invstigated by polarizing microscope.It is found that the growth of MAPb Cl₃ single crystal by the traditional inverse temperature crystallization method follows a two-dimensional nucleation step growth mechanism,while the growth of MAPb Cl₃ single crystal by seed-induced inverse temperature crystallization method follows a step horizontal growing mechanism of smooth interface and its growth direction（that is,step movement direction）is along the outward normal direction of the step.XPS analysis of its electronic structure reveals that the Cl 2p spectrum consists of Cl 2p_3/2 and Cl 2p_1/2 spin-orbit split peaks,indicating that Cl is the-1 valence reduction state;the Pb 4f spectrum corresponds to Pb 4f_7/2 and 4f_5/2 spin-orbit split peaks,indicating that Pb is the+2 valence oxidation state.Subsequently,the structural symmetry of MAPb Cl₃ crystal is studied.We found that the crystal undergoes a orthorhombic（Pnma）-tetragonal（I4/mcm）phase transition at 160K.However the tetragonal phase structure is unstable with a very narrow temperature range,and then transforms to the cubic（Pm m）structure.In addition,it is found that the bandgap width of crystals grown by the above-mentioned both methods is slightly larger than the theoretical values calculated by first-principles,suggesting a correlation of the internal crystal symmetry and defects.On the basis of he continuous tunability of the chemical composition of perovskite single crystals,mixed-halide MAPb(Cl_xBr_1-x)₃ perovskite single crystals with fine-tunable bandgap are grown using the inverse temperature crystallization method.It is found that halide doping causes the diffraction peaks of the crystal to shift towards lower angles with decreasing Cl content,and the structural changes of the crystal are mainly related to the internal vibrations of the inorganic octahedral cage[Pb X6]^4-.As the Cl concentration gradually increases,the UV-visible absorption cutoff edge of the MAPb(Cl_xBr_1-x)₃ single crystal shifts from 517 nm to 438 nm,which is due to the smaller atomic number of Cl than Br,leading to a decrease in unit cell volume and a blue shift in the absorption cutoff edge.TG-DSC results show that the starting decomposition temperature of the MAPb(Cl_xBr_1-x)₃ crystal is above 300℃,indicating a good thermal stability.Finally,the above-mentional crystals are fabricated into a simple planar detector structure of Pt/MAPb(Cl_xBr_1-x)₃/Pt.The results show that the resistivity of MAPb(Cl_xBr_1-x)₃ crystals can all reach 10⁸Ω·cm order of magnitudes.Moreover,the double halide crystal with the composition of MAPb(Cl_0.23Br_0.77)₃ exhibits the largest on/off ratio of K=4.8.