Control Of Morphology,Crystal Phase And Optoelectronic Properties Of CsPbI₃ Nanocrystals Via Fluorinated Ligands

All-inorganic perovskite nanocrystals（CsPbX₃ NCs,X=Cl,Br,I）are new type of semiconductor materials attracting much attention in recent years.Their excellent properties in photoelectric conversion,luminescence,and photocatalysis make them widely used in the field of display and photovoltaic.However,due to the ionic nature of perovskites results in their poor stability in polar solvents.Especially for CsPbI₃ NCs with a small tolerance factor（t）,they are quite unstable in storage.A large number of defects form resulting from their ligands shedding,which induces the lattice distortion and phase transition to yellow phase with poor optical properties.Therefore,the poor stability of CsPbI₃ NCs has restricted the application and development of red perovskite.Herein,this thesis carries out a detailed study on the above issues:（1）Improving the stability of CsPbI₃ NCs using NFSI ligands post-treatment.N-fluorobenzene sulfonimide（NFSI）ligand was added into the prepared CsPbI₃ NCs to induce the growth of preferred orientation of CsPbI₃ NCs,resulting in morphological transformation.The halogen-rich environment can passivate the surface vacancies and improve the stability of the nanocrystals.The photoluminescence（PL）spectra of CsPbI₃ NCs were blue shifted to 25 nm,and X-ray photoelectron spectroscopy（XPS）results showed that the binding energy of Pb²⁺increased,while the binding energy of I^-decreased.This indicates that the F^-released by NFSI partially replaces I^-and combines with Pb²⁺to form Pb-F bond with higher bond energy.Additionly,to verify the stability of CsPbI₃ NCs,we conducted the stability tests in extreme environments:1)Thermal stability test:the standard and post-treatment samples were placed on a 100°C heating table,and the thermal stability of NFSI post-treatment sample was increased by 182.9%as compared to the standard sample;2)UV stability test:the standard and NFSI post-treated samples were placed in the UV light box for continuous irradiation,and the anti-UV stability of the post-treated sample increased by 888.9%.（2）Efficient and stableγ-CsPbI₃ NRs synthesized by using NFSI ligand.γ-CsPbI₃ NRs were prepared by using NFSI.X-ray diffraction（XRD）result demonstrated the phase transition,and transmission electron microscopy（TEM）iamges further confirmed the morphologic transformation from NCs to nanorods（NRs）.The XPS results showed that F^-partially replaced I^-to form Pb-F bond with larger bond energy and shorter bond length,which led the lattice distortion and phase transition toγphase.At the same time,the theoretical calculation results showed that F^-could inhibit the growth ofγ-CsPbI₃ NCs（220）crystal surface,making the preferred orientation growth perpendicular to the（202）crystal surface,thus inducing their morphology evolution.The polarization response ofγ-CsPbI₃ NRs film was tested.The maximum polarization degree of the NRs film was 0.63,the average polarization degree was 0.45,and the polarization visualization was realized.Finally,we verified the stability ofγ-CsPbI₃ NRs:1)Adding water into the sample and shaking,γ-CsPbI₃ NRs still maintained good luminous properties,whileα-CsPbI₃ NCs exhibited no PL properties;2)The phase stability ofγ-CsPbI₃ NRs could be up to 113 days,while the phase stability ofα-CsPbI₃ NCs was only 69 days.（3）TBAF Passivation of CsPbI₃ NCs to improve their stability and optoelectronic properties.Tetrabutylammonium fluoride（TBAF）was added as a fluorine reagent to post-treat CsPbI₃ NCs.Due to the strong binding of TBAF to CsPbI₃ NCs,the stability and optoelectronic properties of CsPbI₃NCs were significantly improved:1)the thermal stability of the sample with TBAF post-treated 15min was improved by 190.0%as compared to the standard sample at 100°C;2)the polar solution resistance of the sample with 20 min post-treatment was improved by 4500.0%as compared to the standard sample;3)the UV stability of the sample with 15 min post-treatment was improved by 171.0%under continuous UV lamp irradiation;4)the long-term stability of the sample with post-treatment20 min was improved by 169%in the condition of 10°C～25°C and 60%～85%relative humidity.Finally,the red light device was prepared by TBAF passivated CsPbI₃ NCs.The maximum luminance of the device was 503 cd/m² and the maximum EQE was 12.78%.