The Study Of The Relationship Between Structure And Properties Of Organic Lead Halide Perovskite Via Solid State Nuclear Magnetic Resonance

Organolead halide perovskites have excellent optoelectronic properties,such as high optical absorption coefficient,long carrier lifetime and diffusion length,and low defect density,and are one of the hottest materials in the photovoltaic field in recent years.However,perovskite has natural structural instability.Lighting,dopants,etc.may have a great impact on macroscopic properties such as long-term stability,thermal stability and optoelectronic properties of perovskite,which seriously hinders the market application of such materials.Therefore,in-depth study of the microstructure transformation mechanism of perovskite under the action of different factors is of great significance for fundamentally understanding and solving the structural instability of perovskite.Solid-state nuclear magnetic resonance technology is a powerful analytical and detection technology for studying the atomic-level microstructure and atomic dynamics in solid matter.Organic lead halide perovskite are organic-inorganic framework structures composed of multiple elements.By selectively detecting the NMR signals of different elements,the structural information of the lattice and organic components can be obtained,respectively.For example,the sensitive properties of the chemical shift of²⁰⁷Pbto its bonding structure can detect fine changes in the crystal framework structure;using ~2H NMR as an atom probe can detect information such as molecular dynamics and structural phase transitions of cations in perovskites;Solid-state NMR technology can not only obtain ordered phase structure information in crystalline materials,but also detect short-range disordered structures in materials,which provides an important help for the atomic-level detection of short-range structures caused by passivation/doping processes.In this dissertation,the structural transformation phenomena of several types of perovskite materials under different conditions are deeply studied by using solid-state NMR technology.The correlation mechanism between these structural transitions and the macroscopic properties of the materials is discussed at the molecular level.The main research contents of this paper are as follows:(1)A novel optical in situ solid-state NMR technology was developed.The transient structure of MAPbI₃ perovskite induced by light was studied by using this technology,and the molecular mechanism of the transient structure change was revealed.Firstly,the kinetic model and orientation information of methylamine cation in MAPbI₃ were established by a series of temperature-variable deuterium spectroscopy and single crystal deuterium spectroscopy measurements.On this basis,the in situ NMR and XRD devices were used to detect for the first time that the dynamic orientation angle of MA cations would undergo an ultra-slow continuous change under illumination,and at the same time,the inorganic lattice would slightly expand under illumination,revealing that transient steady-state structures of MAPbI₃ perovskites under illumination.DFT calculations predict that transient steady-state structures may lead to the change of the band gap and absorbance of the perovskite.(2)The structure and long-term stability of MAPbI₃ after the introduction of chloride ion were studied,and the relationship between the long-term structural stability and photovoltaic properties of the double-halide perovskite was revealed at the molecular level.First,the double halide perovskite crystal MAPbI_3-xCl_x was successfully synthesized.Using ~2H NMR and ²⁰⁷PbNMR combined with other characterization methods,it is proved that a small amount of chloride ions can enter the lattice to form the I/Cl alloying phase,resulting in a phase transition from tetragonal to cubic crystal and a cation kinetic orientation transition from ordered to disordered.Time-dependent structural testing found that the alloy structure reverts to the original undoped state within tens of days,including the reverse recovery of the crystalline phase and cation kinetics,suggesting that the metastable properties of I/Cl alloying structure in MAPbI_3-xCl_x.(3)The structural changes of MAPbBr₃ upon incorporation of DMA cations were studied,and the relationship between the lattice structure changes induced by the incorporation of DMA cations and the corresponding changes of photovoltaic properties was revealed at the molecular level.First,a series of double-cation perovskite single crystals MA_1-xDMA_xPbBr₃ were synthesized.The actual maximum content of DMA was as high as 70%.Subsequently,a series of MA_1-xDMA_xPbBr₃ perovskites were subjected to structural analysis and characterization of their optoelectronic properties.The experiments showed that fine-tuning the relative ratio of DMA to MA would lead to two-stage variation trends in lattice and photocurrent.A small amount of DMA incorporated into the lattice does not cause significant lattice distortion,while the rapid reorientation and relatively large size of DMA cations limit the movement space of MA cations,suppress lattice fluctuations,and reduce electron-phonon coupling strength,which in turn facilitates the charge carrier collection process.However,high-concentration DMA incorporation leads to severe lattice distortion,which in turn causes the degradation of optoelectronic properties.(4)The structural changes of FAPbI₃ upon introduction of Br and DMA ions are studied respectively.The unique advantages of micro doping of DMA in synchronously improving the structural stability and photovoltaic properties of FAPbI₃ are revealed from the molecular level.First,a novel double-cation perovskite FA_1-xDMA_xPbI₃ was synthesized,and the stability and optical properties were compared with the traditional double-halogen perovskite FAPb(I_1-yBr_y)₃.Introduction of DMA in FAPbI₃ enhances the interaction between the cation and the inorganic lattice,which in turn improves the long-term stability,thermal stability,and photovoltaic properties of the material,while the band gap remains constant.Introduction of Br helps to improve the long-term stability of the material,but it will reduce the thermal stability of the material and increase the band gap.