Synthesis Of Halide Perovskite Nanocrystals And Photodetection Performance

Photodetectors are used in many areas of current society,such as visible light cameras,mobile devices,vehicle-mounted systems,military applications and security fields.At present,photodetectors is realized by photodiodes and phototransistors made of silicon or III-V compound semiconductors.However,traditional semiconductors face the bottleneck of device density and sensitivity that are difficult to improve for further applications.Thus,it is of great scientific signicance to explore new materials-based photodetectors for the rapid development of new applicationsIn recent years,the inorganic halide perovskite CsPbX₃(X=I,Br,Cl)nanocrystals have attracted much attention from researchers due to their excellent photoelectric properties.Due to the high light absorption coefficient,high mobility(2290cm²V^-1s^-1),continuous adjustablility of the spectrum in the visible light range(400-700 nm),and the low-temperature solution preparation,with the lower cost than the traditional semiconductor prepared in the high-temperature vacuum environment,CsPbX₃ nanocrystals is regarded one of proming materials for future application in the field of visible photodetection.However,related research is still in its infancy.To apply perovskite nanocrystalline optical detectors in practice,the long-term stability in the air environment needs to be improved,and the photodetection performance needs to be further improved through material design and device design.In this paper,the ligand-assisted reprecipitation method(LARP)was used to prepare CsPbBr₃ nanocrystals in an air environment and study its visible photodetection performance.The basic performance(switching ratio,responsivity,specific detectivity and response time)or long-term stability of the photodetector wsa improved through the doping substitution of different atomic positions or the preparation of composite nanomaterials,which providing reference for the development of perovskite nanocrystal photodetector that would be used in practice.The main research results are as follows:(1)Study on preparation and growth mechanism of CsPbBr₃ nanocrystals.Cubic CsPbBr₃ nanocrystals were prepared by LARP method.The average size is 18.8 nm,and the fluorescence quantum yield is 85%.Then,the growth mechanism of CsPbBr₃nanocrystals was studied by changing the ligand concentration ratio,reaction raw material ratio,precursor supply and reaction temperature.The results show that the coordination of Pb²⁺and ligands has a great influence on the growth of nanocrystals.High concentration of ligand leads to high coordination number,which leads to more comprehensive passivation on the surface of nanoparticles,thus preventing the growth of nanocrystals.The directional attachment of CsPbBr₃ unit cells dominates the growth of nanocrystals.The directional attachment of monomers first generates nanosheets,and the nanosheets continue to grow into nanocubes.Finally,the size and morphology of CsPbBr₃ nanocrystals were controlled by adding different halogen acids into the precursors.When HBr and HCl are added,the CsPbBr₃ nanocrystals change from nanocubes to quantum dots.As the acid concentration increases,the size of the quantum dots decreases,and the fluorescence peaks blue shift;when HI is added,the CsPbBr₃ nanocrystals change from nanocubes to nanosheets.(2)CsPbBr₃ nanocrystalline thin film photodetectors are prepared based on CsPbBr₃nanocr-ystalline,and the photodetection performance is optimized through film thickness control and subsequent heat treatment.MSM type photodetector was constructed by spin-coating CsPbBr₃ nanocrystalline films on Au cross finger electrode.The responsivity test results showed that the CsPbBr₃ nanocrystalline photodetector had a higher photoresponse in the blue wavelength band.The effect of different active layer thickness on the performance of the photodetector was studied.The results show that the 247-nm thick CsPbBr₃ nanocrystalline thin film prepared by the two-time spin-coating process has the best photodetection performance.Finally,the effect of annealing temperature on the performance of CsPbBr₃ nanocrystalline thin film photodetector was studied.The results show that when the annealing temperature is 260°C,the switching ratio of CsPbBr₃ nanocrystalline photodetector under 450 nm light illumination is 1474,the responsivity is 7930 m AW^-1,the specific detectivity is 2.39×10¹¹ Jones,and the response time is 1/118 ms.(3)Based on the optimized CsPbBr₃ nanocrystalline photodetector,the long-term stability and photodetection performance were improved by doping at different atomic positions.First,FAPb Br₃ quantum dots were prepared by LARP method.The average diameter of FAPb Br₃ quantum dots decreased from 5.4 nm to 2.3 nm,and the corresponding fluorescence peak blue-shifted from 531 nm to 446 nm by adding HBr to regulate the size of the quantum dots.Then,the mixed cationic Cs_xFA_1-x Pb Br₃nanocrystals were prepared by LARP method.Because the size of FA is larger than that of Cs,the tolerance factor is closer to 1,and the stability of Cs_xFA_1-x Pb Br₃nanocrystals is higher,so there are fewer non-radiative recombination centers produced in the thermal annealing process.The test results show that the Cs_0.84FA_0.16Pb Br₃ nanocrystalline photodetector has the best photodetection performance under 450 nm light illumination,the switching ratio is 3440,the responsivity is 2.70×10⁴ m AW^-1,the specific detectivity is 6.74×10¹¹ Jones,the response time is 3/27 ms.The stability has also been improved.After two months in the air,the detector still retains 80%of the initial photocurrent.Finally,the effect of Sb doping on the structure and properties of CsPbBr₃ nanocrystals was studied.Sb doping increases the schottky potential barrier between CsPbBr₃ nanocrystals and Au,thus reducing the dark current of CsPbBr₃ nanocrystals photodetector,which is very important to improve the specific detectivity.The test results show that when the doping concentration of Sb is 3%.The on-off ratio of Sb-CsPbBr₃ nanocrystalline photodetector under 450 nm light illumination is 48062,the responsivity is 1.41×104m AW^-1,the specific detectivity is 1.82×10¹² Jones,and the response time is 7/30 ms.(4)CsPbBr₃/r GO composites were prepared to improve the photodetection performance by promoting the separation of photogenerated carriers.First,graphene oxide(GO)was prepared by electrolysis method,and then the GO was reduced by Al to produce a few layers of high quality r GO.Then,CsPbBr₃/r GO composites were prepared by LARP method.The results show that the CsPbBr₃ nanosheets grow laterally along the r GO surface.CsPbBr₃ forms chemical bond with r GO,and the Pb-6p and Br-4p orbitals could overlap with the unhybridized 2p orbital in the carbon atom of graphene,which makes the photogenerated charge carriers can be transported more effectively at the interface between CsPbBr₃ and r GO.The separation of photogenerated charge carriers improves the photodetection performance of CsPbBr₃/r GO composites.The results show that the CsPbBr₃/r GO composite photodetector with 0.005 mg r GO content has the best photodetection performance under 450 nm light illumination,with a switch ratio of 3745,a responsivity of2.95×10⁴ m AW^-1,a specific detectivity of 7.34×10¹¹ Jones,and a response time of1/29 ms.