| The emergence of organic-inorganic halide perovskites has reformed the research status of optoelectronics to a great extent.This field is still experiencing an explosive growth of interest for more than ten years and is even considered to be close to the practical stage due to the intrinsically superior characteristics of halide perovskite materials,including high absorption ability,high mobility,and structural diversity and tenability.However,there are still many controversies about the structure and basic physicochemical properties of halide perovskite materials.The bulk single crystal of halide perovskite can reflect the intrinsic structural characteristics and physicochemical properties of the materials,which is different from the widely used polycrystalline thin film materials,and it is also an ideal carrier to solve the current problems about limit performance and stability.Among them,the MAPbBr3(MA=CH3NH3+)single crystal should be one of the best representatives.Compared with other members of MAPbX3,MAPbBr3 possesses a more suitable band gap and optical absorption ability than MAPbCl3 and higher thermal/chemical stability than MAPbI3.Therefore,this paper focuses on the growth and performance of large-size and high-quality MAPbBr3 single crystal,and studies the crystal growth method,equipment and growth optimization.On the basis of high-quality and large-size single crystal,we further carry out wafer processing,performance characterization,gradient doping and optoelectronic devices of oriented wafer.The main research contents and results are as follows:1.Research on Growth Methods and Equipment.For the research purpose of growing high-quality and large-size MAPbBr3 single crystal,the appropriate crystal growth method was selected,and the new crystal growth equipment was designed and built.However,the size is limited to millimeter scale due to lack of control of the growth process,which normally produces many small crystals in one batch by traditional antisolvent diffusion method.Therefore,based on the traditional antisolvent diffusion method,we developed the settled temperature and controlled antisolvent diffusion system(STCAD).By optimizing the control and optimization of growth factors such as temperature and antisolvent diffusion during the crystal growth process,we have developed three generations of growth equipment(1st,2nd and 3rd STCAD).The size of single crystal grown by 2nd STCAD can reach 50 × 50 × 21 mm3,which is the largest size of such crystals reported at present.In order to solve the macroscopic defects such as inclusions in large-size crystals,MAPbBr3 large-size single crystals with the highest crystalline quality has been grown by the third generation settled temperature and controlled antisolvent diffusion system(3rd STCAD)with the introduction of rotating seed crystals.2.Fabrication of Wafers and Quality Characterization.We designed and established a system called settled temperature and controlled antisolvent diffusion(STCAD)was used to explore the growth conditions of MAPbBr3 single crystal.The properties of the obtained MAPbBr3 single crystal are characterized to clarify its basic parameters,which can provide an important guarantee for the development of new application fields.However,it is difficult to integrate bulk single crystals into functional devices directly.Based on the single crystal grown by STCAD,the orientation,cutting and polishing processes of MAPbBr3 crystal with different orientation were studied for the first time,and the properties of oriented single crystal with different quality were characterized.The results show that the quality of single crystal grown by the method of STCAD is higher than that by inverse temperature crystallization method and other growth methods reported in the literature,especially,the wafer fabricated with the single crystal obtained by 3rd STCAD growth,the half width of the rocking curve is comparable to that of the inorganic crystal.On the one hand,the processing performance of organic-inorganic halide perovskite single crystal is verified.On the other hand,it also conducted in-depth research on the relationship between the structure and function of halide perovskite materials based on wafers with different orientations.The application of the device has laid the material foundation.3.Research on the Anisotropy of Physical Properties of MAPbBr3.At present,much of the structure-function relations of MAPbBr3 are still ambiguous and debated.Thus,a systematic study based on differently oriented single-crystalline wafers is imperative to elucidate anisotropy of both structural and optoelectronic behaviors.In this chapter,based on the prepared high-quality MAPbBr3 oriented single wafer,the structural anisotropy and photoelectric anisotropy are systematically studied.The results of electrical anisotropy show that there are differences in the work function of oriented single crystal.The difference of the density of defect states and carrier transport on different crystal planes should be related to the wafer orientation;The results of optical anisotropy show that the fluorescence lifetime obtained by(111)plane is four times of that obtained by(100)plane,while the fluorescence lifetime obtained by(110)plane is the smallest,which means that the transient fluorescence properties of the oriented single crystal are anisotropic.The optical band gaps revealed no anisotropy on differently oriented wafers,whereas the refractive index and extinction coefficient exhibited obvious anisotropy.The results of structural anisotropy show that there is anisotropy in the orientation of the MA+ static eccentric structure in different domains on different crystal wafer.In addition,this chapter studies the in-plane anisotropy of MAPbBr3 crystals,the equilibrium MA+ orientations in the surrounding Pb-Br cubic cages are deduced to adopt<111>direction with antiparallel MA+ orientation between adjacent domains.The results of light detection anisotropy show that(100)wafer devices show the highest responsivity and EQE under visible light and the highest sensitivity and charge collection capability for X-ray detection;(110)devices take second place and(111)makes the most inferior one.this study,based on the clarification of the debatable intrinsic dipole configuration in the pseudocubic crystal lattice,will provide quantitative information on physicochemical property anisotropy and subsequently facilitate optimization of device performance referring to crystal orientations of halide perovskite crystals.4.Research on Gradient Doping of Halide Ion Exchange Based on Single Crystal Substrate.In view of the wide band gap of MAPbBr3 and the large implantation barrier,halogen ion exchange was used for gradient doping experiments on the basis of MAPbBr3 wafers,which combined the advantages of MAPbBr3 and MAPbI3.Based on the high quality MAPbBr3 single crystal wafer,MAI was used as the source of halide anion source,and the suitable experimental conditions of halide anion gradient doping were investigated by steam diffusion in a double temperature zone tube furnace.After Br-is replaced by I-on the crystal surface,the color of the sample surface changes from red to black.MAPbBr3 single crystals doped with different concentration gradient were measured by XRD and EDS.The band gap of crystal obtained by concentration gradient doping of halide anion is between MAPbBr3 single crystal and pure MAPbI3 single crystal.The fluorescence emission peak of MAPbBr3 crystal with weak intensity at about 529 nm,while the strong fluorescence emission peak of doped crystal appeared at about 740 nm.X-ray photoelectron spectroscopy(XPS)shows that the valence state of I in the doped crystal is-1 valence,and no zero valence of I2 was found.Low temperature DSC showed that there was MAPbI3 phase transition peak in the samples after halide anion conversion.The doped samples begin to lose weight at 300?.The decomposition temperature decreases with the increase of doping degree gradually.These results prove that the exchange degree of doped crystals is controllable,which has a very important guiding significance for the subsequent quantitative analysis of doped crystals. |
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