Liquid Phase Epitaxial Growth And Photoelectric Properties Of Perovskite MAPbBr₃ And Chalcopyrite ZnSiP₂ Single Crystal Films

With the development of solid-state lasers,photodetectors,X-ray detector,and solar cells,new requirements have been put forward for the gap,resistivity and physical properties of semiconductors.The commercial single-element semiconductors(Si,Ge)and binary compounds Ⅲ-Ⅴ,Ⅱ-Ⅳ semiconductors(GaAs,InP,CdTe,ZnSe,etc)have been difficult to satisfy these requirements.Therefore,exploring new types semiconductor materials is of great significance.In recent years,lead halide perovskites depicted by ABX3 and the ternary chalcopyrites have drawn tremendous attentions due to their excellent photoelectric properties such as high carrier mobility,large absorption coefficients,long carrier lifetime and electron-hole diffusion length.They have shown great performance in photodetectors,lasers,light emitting diodes,solar cells and nuclear radiation detectors.However,it is still very difficult to obtain high-quality,large-area,and uniform perovskite single crystal thin films.Aiming to the shortcomings of the existing technology,we invented a droplet-evaporated epitaxial method for the growth of MAPbBr3-xClx(X=0,0.5,1)single crystal thin films and arrays,which has advantages of low cost,simplicity,efficiency and convenience.The MAPB/CPB heterojunction obtained in this paper shows excellent performance in the field of photodetection and X-ray detection.Besides,the obtained MAPbBr3-xClx single crystal array provides a new idea for the design of flexible X-ray detectors.In addition,the liquid phase epitaxial growth of a ternary chalcopyrite structure ZnSiP2 single crystal film,which is expected as the top material of silicon tandem solar cell,was also studied in this paper.Our work shows ZnSiP2 single crystal and ZnSiP2/Si heterojunction showed outstanding performance in the field of photocatalysis.The contents of this paper are divided into six chapters,the main contents of each chapter are as follows:The first chapter mainly introduces the research background and development of metal halide perovskites and ABC2 type chalcopyrite semiconductors.The growth methods of semiconductor thin films and the applications of new semiconductors in various fields are summarized.Based on these conclusions,the meaning and purpose of this thesis are put forward,and the main research contents of each chapter are briefly described.Chapter two mainly introduces the droplet-evaporated crystallization epitaxial growth of MAPbBr3 single crystal film.We provide a facile low cost heteroepitaxial method for the growth of wafer-scale MAPbBr3 single crystal film on CsPbBr3 substrates.Our growth results show that the morphology and the thickness of the films(1-18μm)are related to the concentration of the precursor solutions.The atomic ratio of Pb and Br in the surface of film is 1:3,and the distribution of Cs element at the interface between film and substrate were analyzed by EPMA.The presence of MA+cations was proved by Raman spectra and infrared spectra.The absorption cut-off edge of the MAPbBr3 single crystal film is 576 nm,corresponding to the band gap of 2.13 eV,and the position of the PL peak is 566 nm.The epitaxial relationship of MAPbBr3(010)//CsPbBr3(010),MAPbBr3[101]//CsPbBr3[200]was authenticated by using XRD,pole figure and TEM.The SCLC results show the low defect density 4.6x1011 cm-3 and high carrier mobility 261.94 cm2 for the as-grown MAPbBr3 film,which are comparable to those of bulk single crystals.The on/off ratio of～113 was achieved with the low dark current of 5.25 μA,which is expected to be applied in the field of photoelectric detection.To the best of our knowledge,this is the first time to demonstrate the growth of the hybrid perovskite/all-inorganic perovskite heterojunction.Chapter three mainly introduces the detection performance of MAPB/CPB heterojunction.The electrical properties of the MAPB/CPB heterojunction were studied for the first time.The solution-grown MAPbBr3 is a p-type semiconductor,and the melt-grown CsPbBr3 is an n-type semiconductor.Therefore,a p-n junction is formed when the two semiconductors are combined,which creates a potential barrier of 1.23 eV and 1.16 eV between the VBM and CBM of the two semiconductors.The hole mobilities of the heterojunction measured by the ToF method are 43.5 cm2 V-1 s-1 at a positive bias,and 6.6 cm2 V-1 s-1 at a negative bias.The optical response performance of Au/MAPB/CPB/Au devices was studied.The dark currents of Au/MAPB/CPB/Au device under negative bias voltage are lower and more stable than that under positive bias voltage,while the on-off ratio under positive bias voltage is higher.Benefited from the p-n junction characteristics,the Au/MAPB/CPB/Au device heterostructure demonstrated better photodetection and X-ray detection performances than that of traditional Au/CPB/Au device.Ultra-high sensitivity of 31155 μC Gy-1cm2 was achieved under positive bias of 125V mm-1,which is 1500 times higher than that of a-Se detectors(20 μC Gy-1 cm2 at field of 10 V μm-1),100 times higher than that of Cd1-xZnxTe(318 μC Gy-1 cm-2),and close to the record sensitivity(55684 μC Gy-1 cm-2)of hot-pressed CsPbBr3 quasi-monocrystalline film detector.Furthermore,the MAPB/CPB detector can tolerate an electrical field of 125 V mm-1 and the X-ray sensitivity will be higher as the voltage continues to increase.Chapter four mainly introduces the growth of MAPbBr3-xClx(X=0、0.5、1)single crystal films and arrays.The mixed halogen MAPbBr3-xClx(X=0.5,1)single crystal films were successfully grown by droplet-evaporated epitaxial method.The doping of Cl element was confirmed by EDS,mapping and XRD.Inspired by the growth of single crystal thin films,a simple,fast and efficient droplet-evaporated epitaxial method was designed to grow a large area of perovskite MAPbBr3-XClx(X=0,0.5,1)single crystal array.In addition,the Sn-mixed MAPb0.5Sn0.5Br3 single crystal array was also obtained by this method.These arrays with uniform size are orderly and in the same direction.The sizes of array can be adujusted by controlling the size of mask.The arrays with Mo mask have a certain degree of toughness,which can be kept intact without falling off when bend at 45°.Besides,the materials of the masks used above can be replaced with more flexible materials,such as parylene or polyimide,which is expected to realize the manufacture of flexible X-ray detectors.The MAPbBr3 single crystal array obtained on the flexible PI mask shows great flexibility.Thus,these single crystal arrays can fabricate the flexible X-ray detectors,that is significative in imaging non-flat objects or imaging objects in a narrow space.Chapter five mainly introduces the liquid phase epitaxial growth of ZnSiP2/Si heterojunction and the applications of ZnSiP2 single crystal and ZnSiP2/Si heterojunction in photoelectrochemical water splitting hydrogen generation.A continuous epitaxial ZnSiP2 single crystal film with a thickness of 3 μm,satisfied stoichiometric ratio,was successfully grown on Si single crystal substrate by the liquid phase epitaxy method for the first time.The surface morphology,thickness,composition,and the orientation of the film were characterized by SEM,EDS,EPMA,and XRD.According to the results of XRD-2θ scan and HRXRD-φ scan,the ZnSiP2 film with the same in-plane and out-of-plane orientation was confirmed.The lattice fringes and diffraction spots after the Fourier transform at ZnSiP2/Si interface were observed by the HRTEM and the selected area electron diffraction(SAED).The epitaxial relationship between ZnSiP2 film and Si substrate is determined to be ZnSiP2(100)/Si(001),ZnSiP2<112>/Si<111>.The optical and electrical properties of the samples were tested by ultraviolet-visible-near-infrared diffuse reflection spectrum and Hall effect measurement system.The results show that the existence of ZnSiP2 single crystal film does enhance light absorption.The electrical conductivity σ,Hall mobility μH and carrier concentrations n of the ZnSiP2 film was 1.38×10-5 Ω-1 cm-1,94.5 cm-2 V-1 s-1 and 9.11×109 cm-3,respectively.In addition,the electrical properties of the ZnSiP2 thin film can be adjusted by doping.The Hall mobility and carrier concentration increase significantly as the doping concentration(Cu or Se)increases,while the resistivity ρ reduces significantly.ZnSiP2 single crystal used as a photoanode to produce oxygen had a lower initial potential.In addition,the current density has reached 3.5 mA cm-2 when the bias voltage is only 0.8V,which is higher than other photodegradation electrode materials,such as ZnO and Ta3N5.The photoelectric conversion efficiency reaches the maximum(2.6%)when the bias voltage is 0.3V.Compared with Ta3Ns,ZnSiP2 has higher conversion efficiency and lower bias voltage required to achieve the highest conversion efficiency.When the heterojunction is used as a photoanode to generate oxygen,both the photoelectric conversion efficiency and the dark current stability of ZnSiP2/Si(n-type)are better than that of ZnSiP2/Si(p-type),indicating that the heterojunction band structure formed by n-type Si and ZnSiP2 is more favorable to the photolysis of water.In Chapter six,we summarized the main conclusions and main innovations of this paper,as well as the work to be carried out in the following.