An Investigation On Photodetection Properties Of Lead-Free All-Inorganic CsSnBr₃ Perovskite

In recent years,environmentally friendly lead-free all-inorganic perovskites have gradually become the forefront of the new generation of semiconductor research due to its excellent optoelectronic properties,such as strong optical absorption coefficient,low defect density,and long diffusion length.Among many lead-free all-inorganic perovskites（such as Sn-based,Bi-based,Sb-based,and double-perovskite）,Sn-based perovskite has become one of the candidate semiconductor materials for optoelectronic devices due to its narrow band gap and low exciton binding energy.Due to the easy oxidation of Sn²⁺to Sn⁴⁺ and the formation of Sn defects,it is difficult to grow highquality Sn-based lead-free all-inorganic perovskite,which seriously hinders its application in the field of optoelectronic devices.Developing the growth technology of high crystal quality,exploring the mechanism of controlling carrier transport behavior by factors such as morphology,grain boundaries,and thickness,and broadening the functions of optoelectronic devices are currently the research focus of Sn-based leadfree all-inorganic perovskite.In this thesis,lead-free all-inorganic perovskite CsSnBr₃ was used as the research object.CsSnBr₃ films with adjustable grain size,CsSnBr₃ microplatelets with adjustable thickness,and CsSnBr₃ nanoparticles were grown on rigid and flexible substrates by chemical vapor deposition（CVD）method.By adjusting the source region temperature of CVD,the grain size of CsSnBr₃ films was controlled;The larger the grain size of the thin film,the less the density of grain boundary defects,resulting in a smaller probability of non-radiative recombination,and the higher the responsivity and detectivity of the corresponding photodetector.On the contrary,the smaller the grain size of CsSnBr₃ thin films,the lower performance of their photodetectors.CsSnBr₃ microplatelets without grain boundary defects and with adjustable thickness were grown using fluorocrystalline mica as the growth substrates for CVD;The thicker the microplatelets,the less the density of surface defects,resulting in a lower probability of non-radiative recombination,and the higher the responsivity and detectivity of the corresponding photodetector.On the contrary,the thinner the CsSnBr₃ microplatelets,the lower performance of their photodetector.When combined with narrow band gapⅢ-Ⅴ semiconductor nanowires,it successfully broadens the photodetection range of CsSnBr₃ to the near infrared band.When using flexible transparent ITO as a growth substrate,a 360°omnidirectional flexible self-powered photodetector was successfully constructed.The specific research content is as follows:（1）Lead-free all-inorganic CsSnBr₃ perovskite with controllable morphology,grain boundaries,size,and thickness were grown using a low pressure and high vacuum CVD method.CsSnBr₃ thin films with adjustable grain boundary size were grown on ITO,CsSnBr₃ microplatelets with adjustable thickness were grown on mica substrates,and CsSnBr₃ nanoparticles with adjustable size were grown on narrow band gap III-V semiconductor GaSb nano wires.（2）The mechanism of controlling the carrier transport in CsSnBr₃ photodetectors by the film grain size,microplatelets thickness,and nanoparticle size was systematically studied.Firstly,the effect of grain boundary defects in CsSnBr₃ thin films on the performance of photodetectors was systematically studied.CsSnBr₃ thin films with adjustable grain size were grown by changing the source temperature of low-pressure CVD.The higher the growth temperature,the larger the grain size of CsSnBr₃ thin films,and the less grain boundary defects.Fewer grain boundary defects reduce the rate of non-radiative recombination and extend the lifetime of photogenerated carriers,resulting in a device with detectivity of 5.8 × 10⁷ Jones and response time of 5/12 ms.In order to minimize the adverse impact of grain boundary defects on the performance of photodetectors,CsSnBr₃ microplatelets without grain boundary defects were grown by using fluorocrystalline mica as a substrate,and their thickness related photodetection performance was systematically studied.Thin CsSnBr₃ microplatelets exhibit excellent nonlinear optical response;The thick CsSnBr₃ microplatelets has outstanding photoelectric performance,with detectivity of 1.22 ×109 Jones and response time of 3/10 ms,all of which are superior to the photodetection performance of CsSnBr₃ thin film.This is because the thick CsSnBr₃ microplatelets have fewer trapped states on its surface,which is beneficial for photodetection.However,for the thick CsSnBr₃ microplatelets,more defects in the crystal would induce more scattering and energy loss,which is not conducive to nonlinear optical response.At present,broad band photodetection has been widely studied because it can bring more detection information.Due to band gap limitation,CsSnBr₃ perovskite can only detect visible light,so it is particularly important to broaden its photodetection range to infrared.In order to broaden the photodetection range of CsSnBr₃,its nanoparticles were in-situ deposited on narrow bandgap GaSb nanowires（band gap～0.7 8 eV,corresponding cut-off wavelength～15 90 nm）by CVD method.The surface Fermi level pinning effect promotes the formation of metal-independent Ohmic contacts for the high-speed GaSb nanowires（NWs）electronic devices.However,the resulting large dark current limits its next-generation optoelectronic application.CsSnBr₃ with broad bandgaps and high work functions are adopted to decorate the surfaces of GaSb NWs,demonstrating the success in the construction of Schottky contacts by surface engineering.Benefiting from the expected Schottky barrier,the dark current is reduced as 2 pA,the Ilight/Idark ratio is improved as 103 and the response time is reduced by more than 5 times.Furthermore,Schottky-contacted parallel array GaSb NWs photodetector is also fabricated by the contact printing technology,showing a higher photocurrent and a low dark current of 15 pA,along with the good infrared photodetection ability for concealed target.The above results indicate that optimizing the grain size of thin films,adjusting the thickness of micrometers,and combining nanoparticles with narrow band gap semiconductors are of positive significance for improving the photodetection performance and broadening the detection range of CsSnBr₃ photodetectors.（3）A 360° omnidirectional self-powered photodetector has been successfully developed by combining CsSnBr₃ perovskite with a transparent semiconductor substrate to construct a semiconductor Schottky junction.The implementation of 360° omnidirectional self-powered photodetectors is the core of advancing the next generation of portable and intelligent photodetector systems.However,due to the weak intrinsic light absorption of materials or their sensitivity to incident angles,the preparation of high-performance omnidirectional photodetectors remains challenging.A photodetector with a vertical structure（ITO/CsSnBr₃/Ni）on a transparent conductive substrate ITO was fabricated.Due to the difference in the work functions of the electrodes on both sides,the device can detect optical signals with an incident angle of 20° without external bias,exhibiting multi-angle omnidirectional photodetection capabilities.In order to verify the universality of this structure,PbI₂ microplatelets on flexible ITO were prepared by solution method.The results show that the detection angle of this flexible photodetector with a structure ITO/PbI₂/Ni can be further increased to 5° incidence angle,which is better than the reported mainstream 360°omnidirectional photodetector.The above results have reference significance for broadening the application range of lead-free all-inorganic perovskite photodetectors and realizing their multi scene functional applications.