| Recently,halide perovskites with the general formula ABX3(where A = CH3NH3,CH(NH2)2,Cs;B = Pb,Sn,Ge;and X = Cl,Br,I)have attracted intense attention due to the inspiring optoelectronic properties,such as large light absorption coefficient,high carrier mobility and long carrier diffusion length.Combining with the merits of high defect tolerance,facile synthesis and low cost,this family of compounds show great potential in the applications of optoelectronic devices including solar cells,light-emitting diodes,photodetectors,X-ray and y-ray detectors and lasers.Compared with the organic-inorganic hybrid perovskites,the all-inorganic CsPbBr3 possesses improved thermal and moisture stability due to the replacement of the volatized organic cation by inorganic Cs+,without deteriorating the optoelectronic properties.The content of this paper is divided into five chapters:In the first chapter,the research background and development status of metal halide perovskite materials are summarized,and then the crystal structure,growth methods and application prospects of all-inorganic metal halide perovskite materials are introduced in detail.Based on these summaries,the topic basis,aims and main research contents of this paper are put forward.In the second chapter,the preparation and purification process of CsPbBr3 polycrystals are introduced,and high-purity CsPbBr3 polycrystals have been obtained successfully CsPbBr3 polycrystals were prepared by solution method and solid method,respectively.And the results of powder XRD showed that all the products we obtained were CsPbBr3.CsPbBr3 polycrystals obtained by this two methods were both purified by directional solidification using the Bridgeman method.In the process of purification,the surface of solution-method-prepared-CsPbBr3 was enriched with white matter,which was analyzed to be CsPb2Br5 by powder XRD.After repeated purification,the color of solid-method-prepared-CsPbBr3 gradually changed from brown to orange.The GDMS test showed that the impurities in orange CsPbBr3 polycrystals were significantly reduced than those in brown CsPbBr3 polycrystals,indicating the impurities were continuously discharged in the process of repeated purification and the purification effect was good.Therefore,compared with the solution method,the solid method is more suitable for the synthesis of CsPbBr3 polycrystals.Chapter 3 mainly introduces the exploration and optimization of the CsPbBr3 single crystal growth,as well as the directional processing of the CsPbBr3 crystals.After the explorations of crystal growth,a high quality CsPbBr3 single crystal with a size of?30×100 mm3 and good transparency has been successfully obtained.The main factors that affecting the quality of CsPbBr3 crystals were summarized:1.Synthesis method and purification of CsPbBr3 polycrystals;2.Temperature gradient of the gradient zone;3.Crystal cooling process.It is necessary that high purity CsPbBr3 polycrystals,suitable temperature gradient and slow cooling rate to obtain large size and high quality CsPbBr3 single crystals.CsPbBr3 crystals were roughly polished by 7000 mesh sandpaper firstly,and then finely polished by silk cloth,alumina polishing powder and absolute ethyl alcohol.Finally,chemical etching polishing was adopted to eliminate the scratches on the surface of CsPbBr3 crystals with hydrobromic acid solution,then the crystals were cleaned by absolute ethyl alcohol in the ultrasonic device for 5 minutes to remove the residues.The surface roughness of the polished CsPbBr3 wafer was tested by atomic force microscopy,and the average surface roughness was 10.74 nm,indicating that the crystal polishing effect was good.Chapter 4 mainly characterizes the stability,thermal,optical and electrical properties of CsPbBr3 crystal,and studies the anisotropy photoresponse performance of CsPbBr3 crystal.CsPbBr3 crystal was exposed to room temperature condition for 6 months,and no change in appearance and composition was observed.The room temperature density of CsPbBr3 is about 4.84 g·cm-3,Vickers hardness is 25.63 Kg·mm-2 and Mohs hardness is 1.99;The cut-off edge of ultraviolet absorption of CsPbBr3 crystal is 560 nm,the room temperature bandgap is 2.25 eV,and the transmittance of CsPbBr3 crystal is over 80%in the spectrum range of 1.2-20 ?m.The melting point of CsPbBr3 crystal is 567 ? and the decomposition point is about 570 ?.And there are two phase transition points around 87 ? and 130 ?.The specific heat of CsPbBr3 crystal is about 0.15 J·g-1·K-1 in the temperature range of 30 ? to 89 ?,and the value changes to 0.18 J·g-1·K-1 at the phase transition temperature about 89 ?.In the temperature range of 90 ? to 132 ?,the specific heat of CsPbBr3 crystal decreases to about 0.15 J·g-1·K-1.At the phase transition temperature 132?,the value reaches about 0.35 J·g-1·K-1 and then decreases to 0.14 J·g-1·K-1.In the temperature range of 25 ? to 90 ?,the thermal expansion coefficients along a axis,b axis and c axis of CsPbBr3 crystal are linear with temperature.At 91 ?,the value along a axis and b axis increase,while the value along c axis decreases.After 131 ?,the thermal expansion coefficients along a axis,b axis and c axis of CsPbBr3 crystal are almost equal.The work function of CsPbBr3 crystal is 4.22 eV,the relative permittivity along a axis,b axis and c axis of CsPbBr3 crystal are 22.9,23.94 and 23.2,respectively.The trap density along a axis,b axis and c axis of CsPbBr3 crystal are 1.65 × 1010 cm-3,6.31 × 1010 cm-3 and 1.08 × 109 cm-3,respectively.The carrier mobility along a axis,b axis and c axis of CsPbBr3 crystal are 3.54 cm2·V-1·s-1,11.61 cm2·V-1·s-1 and 1.62 cm2·V-1·s-1,respectively.Irradiated by the light with the optical power density of 1 mW·cm-2 under 10 V bias,the responsivity and external quantum efficiency of CsPbBr3(100)plane are 4.45 A-W'1 and 1037%,respectively.The responsivity and external quantum efficiency of(010)plane are 1.44 A·W-1 and 336%,respectively and the value of(001)plane are 5.83 A·W-1 and 1360%,respectively.The(001)plane of CsPbBr3 crystal exhibits better photoresponse performance.The responsivity and external quantum efficiency are four times higher than those of(010)plane and slightly higher than those of(100)plane.The fifth chapter summarizes the main conclusions and innovations of this paper,and puts forward the future work to be carried out. |
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